Energies
http://www.mdpi.com/journal/energies
Latest open access articles published in Energies at http://www.mdpi.com/journal/energies<![CDATA[Energies, Vol. 7, Pages 8446-8464: Comparison Study on Two Model-Based Adaptive Algorithms for SOC Estimation of Lithium-Ion Batteries in Electric Vehicles]]>
http://www.mdpi.com/1996-1073/7/12/8446
State of charge (SOC) estimation is essential to battery management systems in electric vehicles (EVs) to ensure the safe operations of batteries and providing drivers with the remaining range of the EVs. A number of estimation algorithms have been developed to get an accurate SOC value because the SOC cannot be directly measured with sensors and is closely related to various factors, such as ambient temperature, current rate and battery aging. In this paper, two model-based adaptive algorithms, including the adaptive unscented Kalman filter (AUKF) and adaptive slide mode observer (ASMO) are applied and compared in terms of convergence behavior, tracking accuracy, computational cost and estimation robustness against parameter uncertainties of the battery model in SOC estimation. Two typical driving cycles, including the Dynamic Stress Test (DST) and New European Driving Cycle (NEDC) are applied to evaluate the performance of the two algorithms. Comparison results show that the AUKF has merits in convergence ability and tracking accuracy with an accurate battery model, while the ASMO has lower computational cost and better estimation robustness against parameter uncertainties of the battery model.Energies2014-12-17712Article10.3390/en7128446844684641996-10732014-12-17doi: 10.3390/en7128446Yong TianBizhong XiaMingwang WangWei SunZhihui Xu<![CDATA[Energies, Vol. 7, Pages 8427-8445: Comparative Analysis of Power Plant Options for Enhanced Geothermal Systems (EGS)]]>
http://www.mdpi.com/1996-1073/7/12/8427
Enhanced geothermal systems (EGS) extract heat from underground hot dry rock (HDR) by first fracturing the HDR and then circulating a geofluid (typically water) into it and bringing the heated geofluid to a power plant to generate electricity. This study focuses on analysis, examination, and comparison of leading geothermal power plant configurations with a geofluid temperature from 200 to 800 °C, and also analyzes the embodied energy of EGS surface power plants. The power generation analysis is focused on flash type cycles for using subcritical geofluid (&lt;374 °C) and expansion type cycles for using supercritical geofluid (&gt;374 °C). Key findings of this study include: (i) double-flash plants have 24.3%–29.0% higher geofluid effectiveness than single-flash ones, and 3%–10% lower specific embodied energy; (ii) the expansion type plants have geofluid effectiveness &gt; 750 kJ/kg, significantly higher than flash type plants (geofluid effectiveness &lt; 300 kJ/kg) and the specific embodied energy is lower; (iii) to increase the turbine outlet vapor fraction from 0.75 to 0.90, we include superheating by geofluid but that reduces the geofluid effectiveness by 28.3%; (iv) for geofluid temperatures above 650 °C, double-expansion plants have a 2% higher geofluid effectiveness and 5%–8% lower specific embodied energy than single-expansion ones.Energies2014-12-17712Article10.3390/en7128427842784451996-10732014-12-17doi: 10.3390/en7128427Mengying LiNoam Lior<![CDATA[Energies, Vol. 7, Pages 8411-8426: Kinetic Modeling of Ethanol Batch Fermentation by Escherichia Coli FBWHR Using Hot-Water Sugar Maple Wood Extract Hydrolyzate as Substrate]]>
http://www.mdpi.com/1996-1073/7/12/8411
A recombinant strain of Escherichia coli FBWHR was used for ethanol fermentation from hot-water sugar maple wood extract hydrolyzate in batch experiments. Kinetic studies of cell growth, sugar utilization and ethanol production were investigated at different initial total sugar concentrations of wood extract hydrolyzate. The highest ethanol concentration of 24.05 g/L was obtained using an initial total sugar concentration of 70.30 g/L. Unstructured models were developed to describe cell growth, sugar utilization and ethanol production and validated by comparing the predictions of model and experimental data. The results from this study could be expected to provide insights into the process performance, optimize the process and aid in the design of processes for large-scale production of ethanol fermentation from woody biomass.Energies2014-12-16712Article10.3390/en7128411841184261996-10732014-12-16doi: 10.3390/en7128411Yang WangShijie Liu<![CDATA[Energies, Vol. 7, Pages 8396-8410: Optimal Sizing and Control of Battery Energy Storage System for Peak Load Shaving]]>
http://www.mdpi.com/1996-1073/7/12/8396
Battery Energy Storage System (BESS) can be utilized to shave the peak load in power systems and thus defer the need to upgrade the power grid. Based on a rolling load forecasting method, along with the peak load reduction requirements in reality, at the planning level, we propose a BESS capacity planning model for peak and load shaving problem. At the operational level, we consider the optimal control policy towards charging and discharging power with two different optimization objectives: one is to diminish the difference between the peak load and the valley load, the other is to minimize the daily load variance. Particularly, the constraint of charging and discharging cycles, which is an important issue in practice, is taken into consideration. Finally, based on real load data, we provide simulation results that validate the proposed optimization models and control strategies.Energies2014-12-15712Article10.3390/en7128396839684101996-10732014-12-15doi: 10.3390/en7128396Chao LuHanchen XuXin PanJie Song<![CDATA[Energies, Vol. 7, Pages 8374-8395: How Technology Can Improve the Efficiency of Excavator-Based Cable Harvesting for Potential Biomass Extraction—A Woody Productivity Resource and Cost Analysis for Ireland]]>
http://www.mdpi.com/1996-1073/7/12/8374
Two cable logging systems were reviewed to compare the efficiency of potential biomass extraction from remote forest sites in Ireland based on productive machine hour (PMH) and unit cost of operation (€/m3). Three operational scenarios (SC) were analysed where SC I was a three man crew operation (choker setter, the carriage operator and unhooking chokers). SC II was a variation of this with a two man crew operation. SC III was operating radio controlled chokers there was a two man crew (choker setter and carriage operator). The study aims to assess how operations in Ireland perform against previous known cable studies to determine whether the cost of timber extraction on remote forest sites inaccessible for mechanised felling, has a future given the increased demand for wood fibre in Ireland, both from the sawmilling industries and the wood for energy sector. The volume per PMH was recorded at 17.97 for SC I, 15.09 for SC II and 20.58 m3 for SC III. The difference in productivity versus SC III remote controlled chokers is 5.49 m3/PMH for SC II crew and 2.61 m3/PMH for SC I. The decrease in total volume extracted from SCs I and II versus SC III was recorded at 15.69 m3 (15%) and 32.97 m3 (36%) product respectively. In value terms, the unit cost (€/m3) varied from 6.29 (SC I) to 6.43 (SC II) to 4.57 (SC III). When looking at the production unit costs of normal wood energy supply chains in Ireland, the figures are similar ranging from 3.17 €/m3 to 8.01 €/m3. The value of the end product of course will always determine which market the eventually goes to but given that cable log wood fibre has been unthinned and unmaintained then the biomass sector may be an ever increasing demand point in the search for increased woody biomass given that the unit costs can be competitive with other wood energy supply chains.Energies2014-12-15712Article10.3390/en7128374837483951996-10732014-12-15doi: 10.3390/en7128374Ger DevlinRadomír Klvač<![CDATA[Energies, Vol. 7, Pages 8355-8373: Using the Direct Search Method for Optimal Dispatch of Distributed Generation in a Medium-Voltage Microgrid]]>
http://www.mdpi.com/1996-1073/7/12/8355
This paper proposes a simple and efficient approach for the optimal dispatch in a medium-voltage microgrid (MG) with various types of distributed generation (DG). The fuel costs generated by these DGs are determined using quadratic and linear functions dependent on the types of DGs. Instead of using the traditional Lagrange multiplier method for power system economic dispatch, the proposed direct search method (DSM) approach is able to handle several inequality constraints without introducing any multipliers and furthermore it can solve the non-derivative problems or the fuel cost functions being much more complicated. Accordingly, the DSM is proposed for determining the optimal dispatch of MGs with various types of DG to minimize generation costs under grid-tied and autonomous operations. Results demonstrate that the proposed DSM is a highly suitable and simple approach to determining the optimal dispatch in medium-voltage MGs with various types of DG.Energies2014-12-12712Article10.3390/en7128355835583731996-10732014-12-12doi: 10.3390/en7128355Wei-Tzer HuangKai-Chao YaoChun-Ching Wu<![CDATA[Energies, Vol. 7, Pages 8338-8354: Atmospheric Dispersion of Radioactivity from Nuclear Power Plant Accidents: Global Assessment and Case Study for the Eastern Mediterranean and Middle East]]>
http://www.mdpi.com/1996-1073/7/12/8338
We estimate the contamination risks from the atmospheric dispersion of radionuclides released by severe nuclear power plant accidents using the ECHAM/Modular Earth Submodel System (MESSy) atmospheric chemistry (EMAC) atmospheric chemistry-general circulation model at high resolution (50 km). We present an overview of global risks and also a case study of nuclear power plants that are currently under construction, planned and proposed in the Eastern Mediterranean and Middle East, a region prone to earthquakes. We implemented continuous emissions from each location, making the simplifying assumption that all potential accidents release the same amount of radioactivity. We simulated atmospheric transport and decay, focusing on 137Cs and 131I as proxies for particulate and gaseous radionuclides, respectively. We present risk maps for potential surface layer concentrations, deposition and doses to humans from the inhalation exposure of 131I. The estimated risks exhibit seasonal variability, with the highest surface level concentrations of gaseous radionuclides in the Northern Hemisphere during winter.Energies2014-12-12712Article10.3390/en7128338833883541996-10732014-12-12doi: 10.3390/en7128338Theodoros ChristoudiasYiannis ProestosJos Lelieveld<![CDATA[Energies, Vol. 7, Pages 8317-8337: Energy Consumption of a Battery Electric Vehicle with Infinitely Variable Transmission]]>
http://www.mdpi.com/1996-1073/7/12/8317
Battery electric vehicles (BEVs) represent a possible sustainable solution for personal urban transportation. Presently, the most limiting characteristic of BEVs is their short range, mainly because of battery technology limitations. A proper design and control of the drivetrain, aimed at reducing the power losses and thus increasing BEV range, can contribute to make the electrification of urban transportation a convenient choice. This paper presents a simulation-based comparison of the energy efficiency performance of six drivetrain architectures for BEVs. Although many different drivetrain and transmission architectures have been proposed for BEVs, no literature was found regarding BEVs equipped with infinitely variable transmissions (IVTs). The analyzed drivetrain configurations are: single- (1G) and two-speed (2G) gear drives, half toroidal (HT) and full toroidal (FT) continuously variable transmissions (CVTs), and infinitely variable transmissions (IVTs) with two different types of internal power flow (IVT-I and IVT-II). An off-line procedure for determining the most efficient control action for each drivetrain configuration is proposed, which allows selecting the optimal speed ratio for each operating condition. The energy consumption of the BEVs is simulated along the UDC (Urban Driving Cycle) and Japanese 10-15 driving cycle, with a backward facing approach. In order to achieve the lowest energy consumption, a trade-off between high transmission efficiency and flexibility in terms of allowed speed ratios is required.Energies2014-12-12712Article10.3390/en7128317831783371996-10732014-12-12doi: 10.3390/en7128317Francesco BottiglioneStefano De PintoGiacomo MantriotaAldo Sorniotti<![CDATA[Energies, Vol. 7, Pages 8294-8316: Part-Load Performance of aWet Indirectly Fired Gas Turbine Integrated with an Organic Rankine Cycle Turbogenerator]]>
http://www.mdpi.com/1996-1073/7/12/8294
Over the last years, much attention has been paid to the development of efficient and low-cost power systems for biomass-to-electricity conversion. This paper aims at investigating the design- and part-load performance of an innovative plant based on a wet indirectly fired gas turbine (WIFGT) fueled by woodchips and an organic Rankine cycle (ORC) turbogenerator. An exergy analysis is performed to identify the sources of inefficiencies, the optimal design variables, and the most suitable working fluid for the organic Rankine process. This step enables to parametrize the part-load model of the plant and to estimate its performance at different power outputs. The novel plant has a nominal power of 250 kW and a thermal efficiency of 43%. The major irreversibilities take place in the burner, recuperator, compressor and in the condenser. Toluene is the optimal working fluid for the organic Rankine engine. The part-load investigation indicates that the plant can operate at high efficiencies over a wide range of power outputs (50%–100%), with a peak thermal efficiency of 45% at around 80% load. While the ORC turbogenerator is responsible for the efficiency drop at low capacities, the off-design performance is governed by the efficiency characteristics of the compressor and turbine serving the gas turbine unit.Energies2014-12-11712Review10.3390/en7128294829483161996-10732014-12-11doi: 10.3390/en7128294Leonardo PierobonTuong-Van NguyenAndrea MazzuccoUlrik LarsenFredrik Haglind<![CDATA[Energies, Vol. 7, Pages 8279-8293: Fatigue Load Estimation through a Simple Stochastic Model]]>
http://www.mdpi.com/1996-1073/7/12/8279
We propose a procedure to estimate the fatigue loads on wind turbines, based on a recent framework used for reconstructing data series of stochastic properties measured at wind turbines. Through a standard fatigue analysis, we show that it is possible to accurately estimate fatigue loads in any wind turbine within one wind park, using only the load measurements at one single turbine and the set of wind speed measurements. Our framework consists of deriving a stochastic differential equation that describes the evolution of the torque at one wind turbine driven by the wind speed. The stochastic equation is derived directly from the measurements and is afterwards used for predicting the fatigue loads for neighboring turbines. Such a framework could be used to mitigate the financial efforts usually necessary for placing measurement devices in all wind turbines within one wind farm. Finally, we also discuss the limitations and possible improvements of the proposed procedure.Energies2014-12-11712Article10.3390/en7128279827982931996-10732014-12-11doi: 10.3390/en7128279Pedro LindIván HerráezMatthias WächterJoachim Peinke<![CDATA[Energies, Vol. 7, Pages 8264-8278: Modeling of the Electrical and Thermal Behaviors of an Ultracapacitor]]>
http://www.mdpi.com/1996-1073/7/12/8264
This paper reports a modeling methodology to predict the electrical and thermal behaviors of a 2.7 V/650 F ultracapacitor (UC) cell from LS Mtron Ltd. (Anyang, Korea). The UC cell is subject to the charge/discharge cycling with constant-current between 1.35 V and 2.7 V. The charge/discharge current values examined are 50, 100, 150, and 200 A. A three resistor-capacitor (RC) parallel branch model is employed to calculate the electrical behavior of the UC. The modeling results for the variations of the UC cell voltage as a function of time for various charge/discharge currents are in good agreement with the experimental measurements. A three-dimensional thermal model is presented to predict the thermal behavior of the UC. Both of the irreversible and reversible heat generations inside the UC cell are considered. The validation of the three-dimensional thermal model is provided through the comparison of the modeling results with the experimental infrared (IR) image at various charge/discharge currents. A zero-dimensional thermal model is proposed to reduce the significant computational burden required for the three-dimensional thermal model. The zero-dimensional thermal model appears to generate the numerical results accurate enough to resolve the thermal management issues related to the UC for automotive applications without relying on significant computing resources.Energies2014-12-10712Article10.3390/en7128264826482781996-10732014-12-10doi: 10.3390/en7128264Jeongbin LeeJaeshin YiDaeyong KimChee ShinKyung-Seok MinJongrak ChoiHa-Young Lee<![CDATA[Energies, Vol. 7, Pages 8236-8263: International Diffusion of Renewable Energy Innovations: Lessons from the Lead Markets for Wind Power in China, Germany and USA]]>
http://www.mdpi.com/1996-1073/7/12/8236
The international diffusion of environmental innovations is getting increasing attention as an opportunity to improve competitiveness. Especially in the energy sector, countries use policy support to this end. A recent goal in this context is the formation of “lead markets”, which represents the idea that countries can build up first-mover advantages that will increase their competitiveness. Taking the lead in international diffusion of a particular innovation benefits a country’s industry through creating increasing returns of technological development and stimulating exports to expanding international markets. Interaction between national and international forces affecting renewable energy innovation and its diffusion has received fairly little attention so far. Here, we investigate the formation of lead markets for wind power technologies in China, Germany and the USA to see whether policy support of renewable energy innovation is capable of improving competitiveness. An extension of the current lead market framework is developed to include supply side factors and technology policy issues. The comparative analysis of lead market potential for wind power indicates a high level of internationalization of the industry with countries holding lead positions in specific parts the supply chain. Competitive advantages were built upon policy support but tended to shift among countries.Energies2014-12-10712Article10.3390/en7128236823682631996-10732014-12-10doi: 10.3390/en7128236Juliana LacerdaJeroen van den Bergh<![CDATA[Energies, Vol. 7, Pages 8217-8235: Advanced Energy Harvesting from Macroalgae—Innovative Integration of Drying, Gasification and Combined Cycle]]>
http://www.mdpi.com/1996-1073/7/12/8217
State-of-the-art integrated macroalgae utilization processes, consisting of drying, gasification, and combined cycle, are proposed and their performance with respect to energy efficiency are evaluated. To achieve high exergy efficiency, the integration is performed through two main principles: exergy recovery and process integration. Initially, the energy involved in one process is recirculated intensively through exergy elevation and effective heat coupling. Furthermore, the unrecoverable energy from one process will be utilized in the other processes through process integration. As the result, the total exergy destruction from the whole integrated processes can be minimized significantly leading to significant improvement in energy efficiency. The first analysis relates to the performance of integrated drying process, especially the influence of target moisture content to energy consumption. Furthermore, the influences of gasification fluidization velocity to the total generated power and power generation efficiency are also calculated. As the results of study, the proposed integrated-processes proved a very high energy efficiency. A positive energy harvesting with the total power generation efficiency of about 40% could be achieved.Energies2014-12-10712Article10.3390/en7128217821782351996-10732014-12-10doi: 10.3390/en7128217Muhammad AzizTakuya OdaTakao Kashiwagi<![CDATA[Energies, Vol. 7, Pages 8201-8216: Thermal Performance Evaluation of Two Thermal Energy Storage Tank Design Concepts for Use with a Solid Particle Receiver-Based Solar Power Tower]]>
http://www.mdpi.com/1996-1073/7/12/8201
This paper presents the results of an extensive study of two thermal energy storage (TES) systems. The goal of the research is to make solar energy cost-competitive with other forms of electricity. A small-scale TES system was first built. The inner to outer layers were made of firebrick (FB), autoclaved aerated concrete (AAC) and reinforced concrete brick (CB). The experiments were conducted at temperatures of up to 1000 °C for sustained periods of time. AAC was found to be prone to cracking at temperatures exceeding 900 °C; as a result, AAC was eliminated from the second TES system. The second, larger-scale TES system was subsequently built of multiple layers of readily available materials, namely, insulating firebrick (IFB), perlite concrete (PC), expansion joint (EJ), and CB. All of the surfaces were instrumented with thermocouples to estimate the heat loss from the system. The temperature was maintained at approximately 800 °C to approximate steady state conditions closely. The steady state heat loss was determined to be approximately 4.4% for a day. The results indicate that high-temperature TES systems can be constructed of readily available materials while meeting the heat loss requirements for a falling particle receiver system, thereby contributing to reducing the overall cost of concentrating solar power systems.Energies2014-12-09712Article10.3390/en7128201820182161996-10732014-12-09doi: 10.3390/en7128201Abdelrahman El-LeathySheldon JeterHany Al-AnsarySaid Abdel-KhalikJonathan RoopMatthew GolobSyed DanishAbdulaziz AlrishedEldwin DjajadiwinataZeyad Al-Suhaibani<![CDATA[Energies, Vol. 7, Pages 8178-8200: Reliability-Based Structural Optimization of Wave Energy Converters]]>
http://www.mdpi.com/1996-1073/7/12/8178
More and more wave energy converter (WEC) concepts are reaching prototypelevel. Once the prototype level is reached, the next step in order to further decrease thelevelized cost of energy (LCOE) is optimizing the overall system with a focus on structuraland maintenance (inspection) costs, as well as on the harvested power from the waves.The target of a fully-developed WEC technology is not maximizing its power output,but minimizing the resulting LCOE. This paper presents a methodology to optimize thestructural design of WECs based on a reliability-based optimization problem and the intentto maximize the investor’s benefits by maximizing the difference between income (e.g., fromselling electricity) and the expected expenses (e.g., structural building costs or failure costs).Furthermore, different development levels, like prototype or commercial devices, may havedifferent main objectives and will be located at different locations, as well as receive varioussubsidies. These points should be accounted for when performing structural optimizationsof WECs. An illustrative example on the gravity-based foundation of the Wavestar deviceis performed showing how structural design can be optimized taking target reliability levelsand different structural failure modes due to extreme loads into account.Energies2014-12-09712Article10.3390/en7128178817882001996-10732014-12-09doi: 10.3390/en7128178Simon AmbühlMorten KramerJohn Sørensen<![CDATA[Energies, Vol. 7, Pages 8150-8177: Numerical and Theoretical Investigations Concerning the Continuous-Surface-Curvature Effect in Compressor Blades]]>
http://www.mdpi.com/1996-1073/7/12/8150
Though the importance of curvature continuity on compressor blade performances has been realized, there are two major questions that need to be solved, i.e., the respective effects of curvature continuity at the leading-edge blend point and the main surface, and the contradiction between the traditional theory and experimental observations in the effect of those novel leading-edge shapes with smaller curvature discontinuity and sharper nose. In this paper, an optimization method to design continuous-curvature blade profiles which deviate little from datum blades is proposed, and numerical and theoretical analysis is carried out to investigate the continuous-curvature effect on blade performances. The results show that the curvature continuity at the leading-edge blend point helps to eliminate the separation bubble, thus improving the blade performance. The main-surface curvature continuity is also beneficial, although its effects are much smaller than those of the blend-point curvature continuity. Furthermore, it is observed that there exist two factors controlling the leading-edge spike, i.e., the curvature discontinuity at the blend point which dominates at small incidences, and the nose curvature which dominates at large incidences. To the authors’ knowledge, such mechanisms have not been reported before, and they can help to solve the sharp-leading-edge paradox.Energies2014-12-09712Article10.3390/en7128150815081771996-10732014-12-09doi: 10.3390/en7128150Yin SongChun-Wei GuYao-Bing Xiao<![CDATA[Energies, Vol. 7, Pages 8132-8149: Effects of Canola Oil Biodiesel Fuel Blends on Combustion, Performance, and Emissions Reduction in a Common Rail Diesel Engine]]>
http://www.mdpi.com/1996-1073/7/12/8132
In this study, we investigated the effects of canola oil biodiesel (BD) to improve combustion and exhaust emissions in a common rail direct injection (DI) diesel engine using BD fuel blended with diesel. Experiments were conducted with BD blend amounts of 10%, 20%, and 30% on a volume basis under various engine speeds. As the BD blend ratio increased, the combustion pressure and indicated mean effective pressure (IMEP) decreased slightly at the low engine speed of 1500 rpm, while they increased at the middle engine speed of 2500 rpm. The brake specific fuel consumption (BSFC) increased at all engine speeds while the carbon monoxide (CO) and particulate matter (PM) emissions were considerably reduced. On the other hand, the nitrogen oxide (NOx) emissions only increased slightly. When increasing the BD blend ratio at an engine speed of 2000 rpm with exhaust gas recirculation (EGR) rates of 0%, 10%, 20%, and 30%, the combustion pressure and IMEP tended to decrease. The CO and PM emissions decreased in proportion to the BD blend ratio. Also, the NOx emissions decreased considerably as the EGR rate increased whereas the BD blend ratio only slightly influenced the NOx emissions.Energies2014-12-05712Article10.3390/en7128132813281491996-10732014-12-05doi: 10.3390/en7128132Sam YoonMin KimHan KimNag Choi<![CDATA[Energies, Vol. 7, Pages 8116-8131: Ground Return Current Behaviour in High Voltage Alternating Current Insulated Cables]]>
http://www.mdpi.com/1996-1073/7/12/8116
The knowledge of ground return current in fault occurrence plays a key role in the dimensioning of the earthing grid of substations and of cable sealing end compounds, in the computation of rise of earth potential at substation sites and in electromagnetic interference (EMI) on neighbouring parallel metallic conductors (pipes, handrails, etc.). Moreover, the ground return current evaluation is also important in steady-state regime since this stray current can be responsible for EMI and also for alternating current (AC) corrosion. In fault situations and under some assumptions, the ground return current value at a substation site can be computed by means of k-factors. The paper shows that these simplified and approximated approaches have a lot of limitations and only multiconductor analysis can show the ground return current behaviour along the cable (not only the two end values) both in steady-state regime and in short circuit occurrence (e.g., phase-to-ground and phase-to-phase-to-ground). Multiconductor cell analysis (MCA) considers the cable system in its real asymmetry without simplified and approximated hypotheses. The sensitivity of ground return current on circuit parameters (cross-bonding box resistances, substation earthing resistances, soil resistivity) is presented in the paper. Energies2014-12-03712Article10.3390/en7128116811681311996-10732014-12-03doi: 10.3390/en7128116Roberto BenatoSebastian SessaFabio GuglielmiErtugrul PartalNasser Tleis<![CDATA[Energies, Vol. 7, Pages 8095-8115: Improved Control Strategy for Microgrid Ultracapacitor Energy Storage Systems]]>
http://www.mdpi.com/1996-1073/7/12/8095
Ultracapacitors (UCs), with their features of high power density and high current charge-discharge, have become the best choice for dynamic power compensation to improve the stability of microgrids and are increasingly being applied in microgrids. This paper presents the control of an energy storage system (ESS) based on ultracapacitors in the context of grid-connected microgrids. The ESS is composed of DC/AC and DC/DC converters tied by a dc link. An improved dynamic model for the ESS is proposed. Based on the proposed model a Proportional-Integral-Resonant (PIR) DC link voltage controller is proposed to maintain the DC link voltage through the charging-discharging control of ultracapacitors, capable of working properly under all operating conditions. An extra double frequency component is injected into the UC current by a R controller to dynamically compensate for DC instantaneous power and double frequency AC instantaneous power due to unbalanced grid conditions and disturbances. This feature maintains the DC link voltage constant under unbalanced conditions and increases the degrees of freedom of the DC/AC converter and thus facilitates the application of UCs in microgrids. Simulation and experimental results verify the effectiveness of the proposed control strategy. Energies2014-12-02712Article10.3390/en7128095809581151996-10732014-12-02doi: 10.3390/en7128095Xiaobo DouXiangjun QuanZaijun WuMinqiang HuJianlong SunKang YangMinhui Xu<![CDATA[Energies, Vol. 7, Pages 8076-8094: A Novel Data-Driven Fast Capacity Estimation of Spent Electric Vehicle Lithium-ion Batteries]]>
http://www.mdpi.com/1996-1073/7/12/8076
Fast capacity estimation is a key enabling technique for second-life of lithium-ion batteries due to the hard work involved in determining the capacity of a large number of used electric vehicle (EV) batteries. This paper tries to make three contributions to the existing literature through a robust and advanced algorithm: (1) a three layer back propagation artificial neural network (BP ANN) model is developed to estimate the battery capacity. The model employs internal resistance expressing the battery’s kinetics as the model input, which can realize fast capacity estimation; (2) an estimation error model is established to investigate the relationship between the robustness coefficient and regression coefficient. It is revealed that commonly used ANN capacity estimation algorithm is flawed in providing robustness of parameter measurement uncertainties; (3) the law of large numbers is used as the basis for a proposed robust estimation approach, which optimally balances the relationship between estimation accuracy and disturbance rejection. An optimal range of the threshold for robustness coefficient is also discussed and proposed. Experimental results demonstrate the efficacy and the robustness of the BP ANN model together with the proposed identification approach, which can provide an important basis for large scale applications of second-life of batteries.Energies2014-12-01712Article10.3390/en7128076807680941996-10732014-12-01doi: 10.3390/en7128076Caiping ZhangJiuchun JiangWeige ZhangYukun WangSuleiman SharkhRui Xiong<![CDATA[Energies, Vol. 7, Pages 8056-8075: Influence of Different Rotor Teeth Shapes on the Performance of Flux Switching Permanent Magnet Machines Used for Electric Vehicles]]>
http://www.mdpi.com/1996-1073/7/12/8056
This paper investigated a 12-slot/11-pole flux switching permanent magnet (FSPM) machine used for electric vehicles (EVs). Five novel rotor teeth shapes are proposed and researched to reduce the cogging torque and torque ripple of the FSPM machine. These rotor teeth shapes are notched teeth, stepped teeth, eccentric teeth, combination of notched and stepped teeth, and combination of notched and eccentric teeth. They are applied on the rotor and optimized, respectively. The influences of different rotor teeth shapes on cogging torque, torque ripple and electromagnetic torque are analyzed by the 2-D finite-element method (FEM). Then, the performance of FSPMs with different rotor teeth shapes are compared and evaluated comprehensively from the points of view of cogging torque, torque ripple, electromagnetic torque, flux linkage, back electromotive force (EMF), and so on. The results show that the presented rotor teeth shapes, especially the combination of stepped and notched teeth, can greatly reduce the cogging torque and torque ripple with only slight changes in the average electromagnetic torque.Energies2014-12-01712Article10.3390/en7128056805680751996-10732014-12-01doi: 10.3390/en7128056Jing ZhaoYashuang YanBin LiXiangdong LiuZhen Chen<![CDATA[Energies, Vol. 7, Pages 8036-8055: PID Controller Design for UPS Three-Phase Inverters Considering Magnetic Coupling]]>
http://www.mdpi.com/1996-1073/7/12/8036
In three-phase inverters used in uninterruptible power supplies (UPSs), three-limb inductors and three-limb transformers are commonly used in consideration of cost and size. However, magnetic coupling exists between the three phases of the inverter, which can result in complex models. When instantaneous feedback control strategies are introduced to achieve high quality output waveforms, the transient analysis of the closed-loop inverters becomes difficult. In this paper, the phenomenon of magnetic coupling in three-phase inverters due to three-limb inductors and three-limb transformers is analyzed. A decoupled dynamic model is derived based on the instantaneous symmetrical components transformation, which comprises three decoupled equivalent circuits of instantaneous symmetrical components. Analyses based on this model indicate that magnetic coupling may have a significant impact on the performance of three-phase inverters under unbalanced load conditions and transient responses. For three-phase inverters in UPSs with Proportional-Integral-Differential (PID) closed-loop control strategies, the interactive influence between instantaneous closed-loop regulation and magnetic coupling is researched. Finally, a method of reliability analysis and PID controller design for inverters with magnetic coupling is derived. Simulation and experiment results validate the model and conclusions.Energies2014-11-28712Article10.3390/en7128036803680551996-10732014-11-28doi: 10.3390/en7128036Yu ZhangMinying LiYong Kang<![CDATA[Energies, Vol. 7, Pages 8008-8035: Preliminary Design of Compact Condenser in an Organic Rankine Cycle System for the Low Grade Waste Heat Recovery]]>
http://www.mdpi.com/1996-1073/7/12/8008
The aim of this paper is to present a thermodynamic cycle for the production of electrical power in the 2–5 kW range, suitable for all types of thermally propelled vehicles. The sensible heat recovered from the exhaust gases feeds the energy recovery system, which is able to produce sufficient power to sustain the air conditioning system or other auxiliaries. The working fluids R134a and R245fa have been used in the ORC system, and the systems are simulated by CAMEL-ProTM software. The cycles are generated starting from the same heat source: the exhaust gas of a typical 2.0 L Diesel engine (or from a small size turbine engine). The design of the condenser has been performed to obtain a very compact component, evaluating the heat exchanger tube and fins type design. Through empirical formulas, the area of heat exchange, the heat required to exchange and the pressure drop in the element have been calculated. A commercial software package is used to build the model of the condenser, then a thermal and mechanical analysis and a CFD analysis are realized to estimate the heat exchange. Finally the evaluations, the possible future studies and possible improvements of the system are shown.Energies2014-11-28712Article10.3390/en7128008800880351996-10732014-11-28doi: 10.3390/en7128008Roberto CapataErasmo Zangrillo<![CDATA[Energies, Vol. 7, Pages 7994-8007: Corner Separation Control by Boundary Layer Suction Applied to a Highly Loaded Axial Compressor Cascade]]>
http://www.mdpi.com/1996-1073/7/12/7994
Control of corner separation has attracted much interest due to its improvement of performance and energy utilization in turbomachinery. Numerical studies have been performed under both design and off-design flow conditions to investigate the effects of boundary layer suction (BLS) on corner separation in a highly loaded compressor cascade. Two new BLS slot configurations are proposed and a total of five suction slot configurations were studied and compared. Averaged static pressure rise, exit loss coefficient, passage blockage and flow turning angle have been given and compared systematically over a range of operation incidence angles. Distributions of significant loss removal, blade loading, exit deviation and total pressure loss at 3 degree and 7 degree incidence have also been studied. Under the same suction mass flows of 0.7% of the inlet mass flows, the pitchwise suction slot on the endwall shows a better optimal performance over the whole operation incidence among single suction slots. By using of the new proposed compound slot configuration with one spanwise slot on the blade suction side and one pitchwise slot on the endwall, the maximum reduction of total pressure loss at 7 degree incidence can be 39.4%.Energies2014-11-27712Article10.3390/en7127994799480071996-10732014-11-27doi: 10.3390/en7127994Yangwei LiuJinjing SunLipeng Lu<![CDATA[Energies, Vol. 7, Pages 7955-7993: U.S. Energy Transitions 1780–2010]]>
http://www.mdpi.com/1996-1073/7/12/7955
Economic and social factors compel large-scale changes in energy systems. An ongoing transition in the United States is driven by environmental concerns, changing patterns of energy end-use, constraints on petroleum supply. Analysis of prior transitions shows that energy intensity in the U.S. from 1820 to 2010 features a declining trend when traditional energy is included, in contrast to the “inverted U-curve” seen when only commercial energy is considered. This analysis quantifies use of human and animal muscle power, wind and water power, biomass, harvested ice, fossil fuels, and nuclear power, with some consumption series extending back to 1780. The analysis reaffirms the importance of innovation in energy conversion technologies in energy transitions. An increase in energy intensity in the early 20th century is explained by diminishing returns to pre-electric manufacturing systems, which produced a transformation in manufacturing. In comparison to similar studies for other countries, the U.S. has generally higher energy intensity.Energies2014-11-27712Article10.3390/en7127955795579931996-10732014-11-27doi: 10.3390/en7127955Peter O'ConnorCutler Cleveland<![CDATA[Energies, Vol. 7, Pages 7936-7954: State of the Art and Future Trends in Grid Codes Applicable to Isolated Electrical Systems]]>
http://www.mdpi.com/1996-1073/7/12/7936
Isolated electrical systems lack electrical interconnection to other networks and are usually placed in geographically isolated areas—mainly islands or locations in developing countries. Until recently, only diesel generators were able to assure a safe and reliable supply in exchange for very high costs for fuel transportation and system operation. Transmission system operators (TSOs) are increasingly seeking to replace traditional energy models based on large groups of conventional generation units with mixed solutions where diesel groups are held as backup generation and important advantages are provided by renewable energy sources. The grid codes determine the technical requirements to be fulfilled by the generators connected in any electrical network, but regulations applied to isolated grids are more demanding. In technical literature it is rather easy to find and compare grid codes for interconnected electrical systems. However, the existing literature is incomplete and sparse regarding isolated grids. This paper aims to review the current state of isolated systems and grid codes applicable to them, specifying points of comparison and defining the guidelines to be followed by the upcoming regulations.Energies2014-11-27712Review10.3390/en7127936793679541996-10732014-11-27doi: 10.3390/en7127936Julia MerinoPatricio Mendoza-ArayaCarlos Veganzones<![CDATA[Energies, Vol. 7, Pages 7915-7935: A Generalized Approach for the Steady-State Analysis of Dual-Bridge Resonant Converters]]>
http://www.mdpi.com/1996-1073/7/12/7915
In this paper, a dual-bridge DC/DC resonant converter with a generalized series and parallel resonant tank is analyzed. A general approach based on Fundamental Harmonic Approximation is used to find the universal steady-state solutions. The analysis results for particular resonant tank configurations are exemplified with several typical resonant tank configurations respectively. The corresponded soft-switching conditions are discussed too. To illustrate the usefulness of the generalized approach, a dual-bridge (LC)(L)-type resonant converter working in below resonance mode is designed based on the analysis results. Finally, simulation and experimental plots of the design example are included to evaluate the validity and the accuracy of the proposed analysis approach.Energies2014-11-27712Article10.3390/en7127915791579351996-10732014-11-27doi: 10.3390/en7127915Gao-Yuan HuXiaodong LiBo-Yue Luan<![CDATA[Energies, Vol. 7, Pages 7893-7914: Experimental and Numerical Study of the Aerodynamic Characteristics of an Archimedes Spiral Wind Turbine Blade]]>
http://www.mdpi.com/1996-1073/7/12/7893
A new type of horizontal axis wind turbine adopting the Archimedes spiral blade is introduced for urban-use. Based on the angular momentum conservation law, the design formula for the blade was derived using a variety of shape factors. The aerodynamic characteristics and performance of the designed Archimedes wind turbine were examined using computational fluid dynamics (CFD) simulations. The CFD simulations showed that the new type of wind turbine produced a power coefficient (Cp) of approximately 0.25, which is relatively high compared to other types of urban-usage wind turbines. To validate the CFD results, experimental studies were carried out using a scaled-down model. The instantaneous velocity fields were measured using the two-dimensional particle image velocimetry (PIV) method in the near field of the blade. The PIV measurements revealed the presence of dominant vortical structures downstream the hub and near the blade tip. The interaction between the wake flow at the rotor downstream and the induced velocity due to the tip vortices were strongly affected by the wind speed and resulting rotational speed of the blade. The mean velocity profiles were compared with those predicted by the steady state and unsteady state CFD simulations. The unsteady CFD simulation agreed better with those of the PIV experiments than the steady state CFD simulations.Energies2014-11-26712Article10.3390/en7127893789379141996-10732014-11-26doi: 10.3390/en7127893Kyung KimHo JiYoon KimQian LuJoon BaekRinus Mieremet<![CDATA[Energies, Vol. 7, Pages 7878-7892: An Active Power Sharing Method among Distributed Energy Sources in an Islanded Series Micro-Grid]]>
http://www.mdpi.com/1996-1073/7/12/7878
Active power-sharing among distributed energy sources (DESs) is not only an important way to realize optimal operation of micro-grids, but also the key to maintaining stability for islanded operation. Due to the unique configuration of series micro-grids (SMGs), the power-sharing method adopted in an ordinary AC, DC, and hybrid AC/DC system cannot be directly applied into SMGs. Power-sharing in one SMG with multiple DESs involves two aspects. On the one hand, capacitor voltage stability based on an energy storage system (ESS) in the DC link must be complemented. Actually, this is a problem of power allocation between the generating unit and the ESS in the DES; an extensively researched, similar problem has been grid-off distributed power generation, for which there are good solutions. On the other hand, power-sharing among DESs should be considered to optimize the operation of a series micro-grid. In this paper, a novel method combining master control with auxiliary control is proposed. Master action of a quasi-proportional resonant controller is responsible for stability of the islanded SMG; auxiliary action based on state of charge (SOC) realizes coordinated allocation of load power among the source. At the same time, it is important to ensure that the auxiliary control does not influence the master action.Energies2014-11-26712Article10.3390/en7127878787878921996-10732014-11-26doi: 10.3390/en7127878Wei-Man YangXing-Gui WangXiao-Ying LiZheng-Ying Liu<![CDATA[Energies, Vol. 7, Pages 7857-7877: The Application of Discontinuous Galerkin Methods in Conjugate Heat Transfer Simulations of Gas Turbines]]>
http://www.mdpi.com/1996-1073/7/12/7857
The performance of modern heavy-duty gas turbines is greatly determined by the accurate numerical predictions of thermal loading on the hot-end components. The purpose of this paper is: (1) to present an approach applying a novel numerical technique—the discontinuous Galerkin (DG) method—to conjugate heat transfer (CHT) simulations, develop the engineering-oriented numerical platform, and validate the feasibility of the methodology and tool preliminarily; and (2) to utilize the constructed platform to investigate the aerothermodynamic features of a typical transonic turbine vane with convection cooling. Fluid dynamic and solid heat conductive equations are discretized into explicit DG formulations. A centroid-expanded Taylor basis is adopted for various types of elements. The Bassi-Rebay method is used in the computation of gradients. A coupled strategy based on a data exchange process via numerical flux on interface quadrature points is simply devised. Additionally, various turbulence Reynolds-Averaged-Navier-Stokes (RANS) models and the local-variable-based transition model γ-Reθ are assimilated into the integral framework, combining sophisticated modelling with the innovative algorithm. Numerical tests exhibit good consistency between computational and analytical or experimental results, demonstrating that the presented approach and tool can handle well general CHT simulations. Application and analysis in the turbine vane, focusing on features around where there in cluster exist shock, separation and transition, illustrate the effects of Bradshaw’s shear stress limitation and separation-induced-transition modelling. The general overestimation of heat transfer intensity behind shock is conjectured to be associated with compressibility effects on transition modeling. This work presents an unconventional formulation in CHT problems and achieves its engineering applications in gas turbines.Energies2014-11-26712Article10.3390/en7127857785778771996-10732014-11-26doi: 10.3390/en7127857Zeng-Rong HaoChun-Wei GuXiao-Dong Ren<![CDATA[Energies, Vol. 7, Pages 7847-7856: Isolation and Characterization of New Temperature Tolerant Microalgal Strains for Biomass Production]]>
http://www.mdpi.com/1996-1073/7/12/7847
Microalgae exhibit great potential for biomass production. Although microalgae display an enormous biodiversity, surprisingly only 15 species are used for large scale production processes worldwide. The implementation of new production strains with good process-oriented properties, especially fast growth rate and heat resistance, could improve production efficiency and reduce costs. In this study 130 environmental samples collected in Germany, Spain, Italy and Portugal were investigated for fast growing thermotolerant photosynthetic species. Isolates were characterized and identified on a molecular level. In total 21 of the isolated freshwater strains were able to grow at 40 °C. Additionally, 13 of those 21 strains are able to grow at 45 °C. The highest growth rate at room temperature was 1.16 per day (isolate T306A), compared to 0.053 per day at 45 °C (isolate Sp13). In three thermotolerant strains pigment production was induced. Molecular identification by 18S rDNA sequencing revealed that the isolates were all chlorophytes belonging to four different families.Energies2014-11-25712Article10.3390/en7127847784778561996-10732014-11-25doi: 10.3390/en7127847Franziska BleekeVincent RwehumbizaDominik WinckelmannGerd Klöck<![CDATA[Energies, Vol. 7, Pages 7816-7846: Performance Analyses of Renewable and Fuel Power Supply Systems for Different Base Station Sites]]>
http://www.mdpi.com/1996-1073/7/12/7816
Base station sites (BSSs) powered with renewable energy sources have gained the attention of cellular operators during the last few years. This is because such “green” BSSs impose significant reductions in the operational expenditures (OPEX) of telecom operators due to the possibility of on-site renewable energy harvesting. In this paper, the green BSSs power supply system parameters detected through remote and centralized real time sensing are presented. An implemented sensing system based on a wireless sensor network enables reliable collection and post-processing analyses of many parameters, such as: total charging/discharging current of power supply system, battery voltage and temperature, wind speed, etc. As an example, yearly sensing results for three different BSS configurations powered by solar and/or wind energy are discussed in terms of renewable energy supply (RES) system performance. In the case of powering those BSS with standalone systems based on a fuel generator, the fuel consumption models expressing interdependence among the generator load and fuel consumption are proposed. This has allowed energy-efficiency comparison of the fuel powered and RES systems, which is presented in terms of the OPEX and carbon dioxide (CO2) reductions. Additionally, approaches based on different BSS air-conditioning systems and the on/off regulation of a daily fuel generator activity are proposed and validated in terms of energy and capital expenditure (CAPEX) savings. Energies2014-11-25712Communication10.3390/en7127816781678461996-10732014-11-25doi: 10.3390/en7127816Josip LorinczIvana BuleMilutin Kapov<![CDATA[Energies, Vol. 7, Pages 7794-7815: Analyzing the Performance of a Dual Loop Organic Rankine Cycle System for Waste Heat Recovery of a Heavy-Duty Compressed Natural Gas Engine]]>
http://www.mdpi.com/1996-1073/7/11/7794
A dual loop organic Rankine cycle (DORC) system is designed to recover waste heat from a heavy-duty compressed natural gas engine (CNGE), and the performance of the DORC–CNGE combined system is simulated and discussed. The DORC system includes high-temperature (HT) and low-temperature (LT) cycles. The HT cycle recovers energy from the exhaust gas emitted by the engine, whereas the LT cycle recovers energy from intake air, engine coolant, and the HT cycle working fluid in the preheater. The mathematical model of the system is established based on the first and second laws of thermodynamics. The characteristics of waste heat energy from the CNGE are calculated according to engine test data under various operating conditions. Moreover, the performance of the DORC–CNGE combined system is simulated and analyzed using R245fa as the working fluid. Results show that the maximum net power output and the maximum thermal efficiency of the DORC system are 29.37 kW and 10.81%, respectively, under the rated power output condition of the engine. Compared with the original CNG engine, the maximum power output increase ratio and the maximum brake specific fuel consumption improvement ratio are 33.73% and 25%, respectively, in the DORC–CNGE combined system.Energies2014-11-21711Article10.3390/en7117794779478151996-10732014-11-21doi: 10.3390/en7117794Baofeng YaoFubin YangHongguang ZhangEnhua WangKai Yang<![CDATA[Energies, Vol. 7, Pages 7773-7793: System Integration of the Horizontal-Axis Wind Turbine: The Design of Turbine Blades with an Axial-Flux Permanent Magnet Generator]]>
http://www.mdpi.com/1996-1073/7/11/7773
In designing a horizontal-axis wind turbine (HAWT) blade, system integration between the blade design and the performance test of the generator is important. This study shows the aerodynamic design of a HAWT blade operating with an axial-flux permanent magnet (AFPM) generator. An experimental platform was built to measure the performance curves of the AFPM generator for the purpose of designing the turbine blade. An in-house simulation code was developed based on the blade element momentum (BEM) theory and was used to lay out the geometric shape of the turbine blade, including the pitch angle and chord length at each section. This simulation code was combined with the two-dimensional (2D) airfoil data for predicting the aerodynamic performance of the designed blades. In addition, wind tunnel experiments were performed to verify the simulation results for the various operating conditions. By varying the rotational speeds at four wind speeds, the experimental and simulation results for the mechanical torques and powers presented good agreement. The mechanical power of the system, which maximizes at the best operating region, provided significant information for designing the HAWT blade.Energies2014-11-21711Article10.3390/en7117773777377931996-10732014-11-21doi: 10.3390/en7117773Chi-Jeng BaiWei-Cheng WangPo-Wei ChenWen-Tong Chong<![CDATA[Energies, Vol. 7, Pages 7746-7772: Dynamics and Control of Lateral Tower Vibrations in Offshore Wind Turbines by Means of Active Generator Torque]]>
http://www.mdpi.com/1996-1073/7/11/7746
Lateral tower vibrations of offshore wind turbines are normally lightly damped, and large amplitude vibrations induced by wind and wave loads in this direction may significantly shorten the fatigue life of the tower. This paper proposes the modeling and control of lateral tower vibrations in offshore wind turbines using active generator torque. To implement the active control algorithm, both the mechanical and power electronic aspects have been taken into consideration. A 13-degrees-of-freedom aeroelastic wind turbine model with generator and pitch controllers is derived using the Euler–Lagrangian approach. The model displays important features of wind turbines, such as mixed moving frame and fixed frame-defined degrees-of-freedom, couplings of the tower-blade-drivetrain vibrations, as well as aerodynamic damping present in different modes of motions. The load transfer mechanisms from the drivetrain and the generator to the nacelle are derived, and the interaction between the generator torque and the lateral tower vibration are presented in a generalized manner. A three-dimensional rotational sampled turbulence field is generated and applied to the rotor, and the tower is excited by a first order wave load in the lateral direction. Next, a simple active control algorithm is proposed based on active generator torques with feedback from the measured lateral tower vibrations. A full-scale power converter configuration with a cascaded loop control structure is also introduced to produce the feedback control torque in real time. Numerical simulations have been carried out using data calibrated to the referential 5-MW NREL (National Renewable Energy Laboratory) offshore wind turbine. Cases of drivetrains with a gearbox and direct drive to the generator are considered using the same time series for the wave and turbulence loadings. Results show that by using active generator torque control, lateral tower vibrations can be significantly mitigated for both gear-driven and direct-driven wind turbines, with modest influence on the smoothness of the power output from the generator.Energies2014-11-21711Article10.3390/en7117746774677721996-10732014-11-21doi: 10.3390/en7117746Zili ZhangSøren NielsenFrede BlaabjergDao Zhou<![CDATA[Energies, Vol. 7, Pages 7732-7745: A Grid Voltage Measurement Method for Wind Power Systems during Grid Fault Conditions]]>
http://www.mdpi.com/1996-1073/7/11/7732
Grid codes in many countries require low-voltage ride-through (LVRT) capability to maintain power system stability and reliability during grid fault conditions. To meet the LVRT requirement, wind power systems must stay connected to the grid and also supply reactive currents to the grid to support the recovery from fault voltages. This paper presents a new fault detection method and inverter control scheme to improve the LVRT capability for full-scale permanent magnet synchronous generator (PMSG) wind power systems. Fast fault detection can help the wind power systems maintain the DC-link voltage in a safe region. The proposed fault detection method is based on on-line adaptive parameter estimation. The performance of the proposed method is verified in comparison to the conventional voltage measurement method defined in the IEC 61400-21 standard.Energies2014-11-20711Article10.3390/en7117732773277451996-10732014-11-20doi: 10.3390/en7117732Cheol-Hee YooIl-Yop ChungHyun-Jae YooSung-Soo Hong<![CDATA[Energies, Vol. 7, Pages 7717-7731: Point Estimate Method for Voltage Unbalance Evaluation in Residential Distribution Networks with High Penetration of Small Wind Turbines]]>
http://www.mdpi.com/1996-1073/7/11/7717
Voltage unbalance (VU) in residential distribution networks (RDNs) is mainly caused by load unbalance in three phases, resulting from network configuration and load-variations. The increasing penetration of distributed generation devices, such as small wind turbines (SWTs), and their uneven distribution over the three phases have introduced difficulties in evaluating possible VU. This paper aims to provide a three-phase probabilistic power flow method, point estimate method to evaluate the VU. This method, considering the randomness of load switching in customers’ homes and time-variation in wind speed, is shown to be capable of providing a global picture of a network’s VU degree so that it can be used for fast evaluation. Applying the 2m + 1 scheme of the proposed method to a generic UK distribution network shows that a balanced SWT penetration over three phases reduces the VU of a RDN. Greater unbalance in SWT penetration results in higher voltage unbalance factor (VUF), and cause VUF in excess of the UK statutory limit of 1.3%.Energies2014-11-20711Article10.3390/en7117717771777311996-10732014-11-20doi: 10.3390/en7117717Chao LongMohamed FarragDonald HepburnChengke Zhou<![CDATA[Energies, Vol. 7, Pages 7694-7716: High Performance Reduced Order Models for Wind Turbines with Full-Scale Converters Applied on Grid Interconnection Studies]]>
http://www.mdpi.com/1996-1073/7/11/7694
Wind power has achieved technological evolution, and Grid Code (GC) requirements forced wind industry consolidation in the last three decades. However, more studies are necessary to understand how the dynamics inherent in this energy source interact with the power system. Traditional energy production usually contains few high power unit generators; however, Wind Power Plants (WPPs) consist of dozens or hundreds of low-power units. Time domain simulations of WPPs may take too much time if detailed models are considered in such studies. This work discusses reduced order models used in interconnection studies of synchronous machines with full converter technology. The performance of all models is evaluated based on time domain simulations in the Simulink/MATLAB environment. A detailed model is described, and four reduced order models are compared using the performance index, Normalized Integral of Absolute Error (NIAE). Models are analyzed during wind speed variations and balanced voltage dip. During faults, WPPs must be able to supply reactive power to the grid, and this characteristic is analyzed. Using the proposed performance index, it is possible to conclude if a reduced order model is suitable to represent the WPPs dynamics on grid studies.Energies2014-11-20711Article10.3390/en7117694769477161996-10732014-11-20doi: 10.3390/en7117694Heverton PereiraAllan CupertinoRemus TeodorescuSelênio Silva<![CDATA[Energies, Vol. 7, Pages 7673-7693: A Mine-Based Uranium Market Clearing Model]]>
http://www.mdpi.com/1996-1073/7/11/7673
Economic analysis and market simulation tools are used to evaluate uranium (U) supply shocks, sale or purchase of uranium stockpiles, or market effects of new uranium mines or enrichment technologies. This work expands on an existing U market model that couples the market for primary U from uranium mines with those of secondary uranium, e.g., depleted uranium (DU) upgrading or highly enriched uranium (HEU) down blending, and enrichment services. This model accounts for the interdependence between the primary U supply on the U market price, the economic characteristics of each individual U mine, sources of secondary supply, and the U enrichment market. This work defines a procedure for developing an aggregate supply curve for primary uranium from marginal cost curves for individual firms (Uranium mines). Under this model, market conditions drive individual mines’ startup and short- and long-term shutdown decisions. It is applied to the uranium industry for the period 2010–2030 in order to illustrate the evolution of the front end markets under conditions of moderate growth in demand for nuclear fuel. The approach is applicable not only to uranium mines but also other facilities and reactors within the nuclear economy that may be modeled as independent, decision-making entities inside a nuclear fuel cycle simulator.Energies2014-11-19711Article10.3390/en7117673767376931996-10732014-11-19doi: 10.3390/en7117673Aris AuzansErich SchneiderRobert FlanaganAlan Tkaczyk<![CDATA[Energies, Vol. 7, Pages 7640-7672: Metaheuristic Algorithms Applied to Bioenergy Supply Chain Problems: Theory, Review, Challenges, and Future]]>
http://www.mdpi.com/1996-1073/7/11/7640
Bioenergy is a new source of energy that accounts for a substantial portion of the renewable energy production in many countries. The production of bioenergy is expected to increase due to its unique advantages, such as no harmful emissions and abundance. Supply-related problems are the main obstacles precluding the increase of use of biomass (which is bulky and has low energy density) to produce bioenergy. To overcome this challenge, large-scale optimization models are needed to be solved to enable decision makers to plan, design, and manage bioenergy supply chains. Therefore, the use of effective optimization approaches is of great importance. The traditional mathematical methods (such as linear, integer, and mixed-integer programming) frequently fail to find optimal solutions for non-convex and/or large-scale models whereas metaheuristics are efficient approaches for finding near-optimal solutions that use less computational resources. This paper presents a comprehensive review by studying and analyzing the application of metaheuristics to solve bioenergy supply chain models as well as the exclusive challenges of the mathematical problems applied in the bioenergy supply chain field. The reviewed metaheuristics include: (1) population approaches, such as ant colony optimization (ACO), the genetic algorithm (GA), particle swarm optimization (PSO), and bee colony algorithm (BCA); and (2) trajectory approaches, such as the tabu search (TS) and simulated annealing (SA). Based on the outcomes of this literature review, the integrated design and planning of bioenergy supply chains problem has been solved primarily by implementing the GA. The production process optimization was addressed primarily by using both the GA and PSO. The supply chain network design problem was treated by utilizing the GA and ACO. The truck and task scheduling problem was solved using the SA and the TS, where the trajectory-based methods proved to outperform the population-based methods.Energies2014-11-19711Review10.3390/en7117640764076721996-10732014-11-19doi: 10.3390/en7117640Krystel Castillo-Villar<![CDATA[Energies, Vol. 7, Pages 7620-7639: Saturated Adaptive Output-Feedback Power-Level Control for Modular High Temperature Gas-Cooled Reactors]]>
http://www.mdpi.com/1996-1073/7/11/7620
Small modular reactors (SMRs) are those nuclear fission reactors with electrical output powers of less than 300 MWe. Due to its inherent safety features, the modular high temperature gas-cooled reactor (MHTGR) has been seen as one of the best candidates for building SMR-based nuclear plants with high safety-level and economical competitive power. Power-level control is crucial in providing grid-appropriation for all types of SMRs. Usually, there exists nonlinearity, parameter uncertainty and control input saturation in the SMR-based plant dynamics. Motivated by this, a novel saturated adaptive output-feedback power-level control of the MHTGR is proposed in this paper. This newly-built control law has the virtues of having relatively neat form, of being strong adaptive to parameter uncertainty and of being able to compensate control input saturation, which are given by constructing Lyapunov functions based upon the shifted-ectropies of neutron kinetics and reactor thermal-hydraulics, giving an online tuning algorithm for the controller parameters and proposing a control input saturation compensator respectively. It is proved theoretically that input-to-state stability (ISS) can be guaranteed for the corresponding closed-loop system. In order to verify the theoretical results, this new control strategy is then applied to the large-range power maneuvering control for the MHTGR of the HTR-PM plant. Numerical simulation results show not only the relationship between regulating performance and control input saturation bound but also the feasibility of applying this saturated adaptive control law practically.Energies2014-11-19711Article10.3390/en7117620762076391996-10732014-11-19doi: 10.3390/en7117620Zhe Dong<![CDATA[Energies, Vol. 7, Pages 7602-7619: Exergy Destruction in Pipeline Flow of Surfactant-Stabilized Oil-in-Water Emulsions]]>
http://www.mdpi.com/1996-1073/7/11/7602
Exergy destruction in adiabatic pipeline flow of surfactant-stabilized oil-in-water emulsions is investigated in five different diameter pipes. The dispersed-phase (oil droplets) concentration of the emulsions is varied from 0% to 55.14% vol. The emulsions are Newtonian in that the viscosity is independent of the shear rate. For a given emulsion and pipe diameter, the exergy destruction rate per unit pipe length increases linearly with the increase in the Reynolds number on a log-log scale in both laminar and turbulent regimes. However the slope in the turbulent regime is higher. The exergy destruction rate increases with the increase in the dispersed-phase concentration of emulsion and decreases with the increase in the pipe diameter. New models are developed for the prediction of exergy destruction rate in pipeline flow of surfactant-stabilized oil-in-water emulsions. The models are based on the single-phase flow equations. The experimental data on exergy destruction in pipeline flow of emulsions shows excellent agreement with the predictions of the proposed models.Energies2014-11-18711Article10.3390/en7117602760276191996-10732014-11-18doi: 10.3390/en7117602Rajinder Pal<![CDATA[Energies, Vol. 7, Pages 7586-7601: Three-Dimensional Modeling of the Thermal Behavior of a Lithium-Ion Battery Module for Hybrid Electric Vehicle Applications]]>
http://www.mdpi.com/1996-1073/7/11/7586
This paper reports a modeling methodology to predict the effects of operating conditions on the thermal behavior of a lithium-ion battery (LIB) module. The potential and current density distributions on the electrodes of an LIB cell are predicted as a function of discharge time based on the principle of charge conservation. By using the modeling results of the potential and current density distributions of the LIB cell, the non-uniform distribution of the heat generation rate in a single LIB cell within the module is calculated. Based on the heat generation rate in the single LIB cell determined as a function of the position on the electrode and time, a three-dimensional thermal modeling of an LIB module is performed to calculate the three-dimensional velocity, pressure, and temperature distributions within the LIB module as a function of time at various operating conditions. Thermal modeling of an LIB module is validated by the comparison between the experimental measurements and the modeling results. The effect of the cooling condition of the LIB module on the temperature rise of the LIB cells within the module and the uniformity of the distribution of the cell temperatures are analyzed quantitatively based on the modeling results.Energies2014-11-18711Article10.3390/en7117586758676011996-10732014-11-18doi: 10.3390/en7117586Jaeshin YiBoram KooChee Shin<![CDATA[Energies, Vol. 7, Pages 7568-7585: Recycle Effect on Device Performance of Wire Mesh Packed Double-Pass Solar Air Heaters]]>
http://www.mdpi.com/1996-1073/7/11/7568
A new device for inserting an absorber plate to divide a flat-plate channel into two subchannels to conduct double-pass wire mesh packed operations was developed. The proposed wire mesh packed device improves the heat transfer efficiency substantially as compared that to flat-plate single-pass and double-pass operations using the same working dimensions, and the improvement of device performance was investigated experimentally and theoretically. Good agreement between the theoretical prediction and the measured values from the experimental results was achieved. Considerable heat transfer improvement was obtained employing wire mesh packed double-pass operations under the absorber plate with external recycle. The influences of recycle ratio on the heat transfer efficiency and the power consumption increase were also discussed.Energies2014-11-18711Article10.3390/en7117568756875851996-10732014-11-18doi: 10.3390/en7117568Chii-Dong HoChun-Sheng LinTz-Jin YangChun-Chieh Chao<![CDATA[Energies, Vol. 7, Pages 7555-7567: Introducing Textiles as Material of Construction of Ethanol Bioreactors]]>
http://www.mdpi.com/1996-1073/7/11/7555
The conventional materials for constructing bioreactors for ethanol production are stainless and cladded carbon steel because of the corrosive behaviour of the fermenting media. As an alternative and cheaper material of construction, a novel textile bioreactor was developed and examined. The textile, coated with several layers to withstand the pressure, resist the chemicals inside the reactor and to be gas-proof was welded to form a 30 L lab reactor. The reactor had excellent performance for fermentative production of bioethanol from sugar using baker’s yeast. Experiments with temperature and mixing as process parameters were performed. No bacterial contamination was observed. Bioethanol was produced for all conditions considered with the optimum fermentation time of 15 h and ethanol yield of 0.48 g/g sucrose. The need for mixing and temperature control can be eliminated. Using a textile bioreactor at room temperature of 22 °C without mixing required 2.5 times longer retention time to produce bioethanol than at 30 °C with mixing. This will reduce the fermentation investment cost by 26% for an ethanol plant with capacity of 100,000 m3 ethanol/y. Also, replacing one 1300 m3 stainless steel reactor with 1300 m3 of the textile bioreactor in this plant will reduce the fermentation investment cost by 19%.Energies2014-11-18711Article10.3390/en7117555755575671996-10732014-11-18doi: 10.3390/en7117555Osagie OsadolorPatrik LennartssonMohammad Taherzadeh<![CDATA[Energies, Vol. 7, Pages 7535-7554: Numerical Study on Heat Transfer Deterioration of Supercritical n-Decane in Horizontal Circular Tubes]]>
http://www.mdpi.com/1996-1073/7/11/7535
In order to obtain a deeper understanding of the regenerative cooling process of scramjet engines, in this paper, a numerical investigation on the supercritical convective heat transfer of n-decane in horizontal circular tubes was conducted, based on a complete set of conservation equations and the Renormalization group (RNG) k–ε turbulence model with enhanced wall treatment. The present study mainly focuses on the heat transfer deterioration (HTD) phenomenon, including the mechanism and critical conditions for the onset of HTD. Moreover, the applicability of some conventional heat transfer empirical correlations was analyzed and compared, thus providing guidance for the Nusselt number predictions in the cooling channels. Results indicate that under the compositive conditions of low pressure and high heat flux, two types of HTD phenomena could occur when the wall and bulk fluid temperatures are near the pseudo-critical temperature, owing to the abnormal distributions of near-wall turbulent kinetic energy and radial velocity, respectively. Increasing the pressure would effectively alleviate and eliminate the HTD. A comparison of numerical results with those obtained with different empirical expressions shows that the Bae-Kim expression provides the best agreement, especially when HTD occurs. Furthermore, a new correction for critical heat flux of HTD has been successfully developed.Energies2014-11-18711Article10.3390/en7117535753575541996-10732014-11-18doi: 10.3390/en7117535Yanhong WangSufen LiMing Dong<![CDATA[Energies, Vol. 7, Pages 7519-7534: A Recursive Solution for Power-Transmission Loss in DC-Powered Networks]]>
http://www.mdpi.com/1996-1073/7/11/7519
This article presents a recursive solution to the power-transmission loss in DC-powered networks. In such a network, the load cannot be modeled as a fixed equivalent resistance value, since the switching regulator may draw more or less current based on the actual supply voltage to meet the power demand. Although the power-transmission loss itself is simply I2 RL, I, in turn, depends on the load’s supply voltage, which, in turn, depends on I, making it impossible to derive a closed-form solution by classical resistive network analysis in general. The proposed approach is to first derive a closed-form solution to I in the one-node topology using the quadratic formula. Next, we extend our solution to a locally daisy-chained (LDC) network, where the network is readily decomposable into stages, such that the solution combines the closed-form formula for the current stage with the recursive solution for the subsequent stages. We then generalize the LDC topology to trees. In practice, the solution converges quickly after a small number of iterations. It has been validated on real-life networks, such as power over controller area network (PoCAN).Energies2014-11-18711Article10.3390/en7117519751975341996-10732014-11-18doi: 10.3390/en7117519Sehwan KimPai Chou<![CDATA[Energies, Vol. 7, Pages 7499-7518: Game-Theoretic Energy Management for Residential Users with Dischargeable Plug-in Electric Vehicles]]>
http://www.mdpi.com/1996-1073/7/11/7499
The plug-in electric vehicle (PEV) has attracted more and more attention because of the energy crisis and environmental pollution, which is also the main shiftable load of the residential users’ demand side management (DSM) system in the future smart grid (SG). In this paper, we employ game theory to provide an autonomous energy management system among residential users considering selling energy back to the utility company by discharging the PEV’s battery. By assuming all users are equipped with smart meters to execute automatic energy consumption scheduling (ECS) and the energy company can adopt adequate pricing tariffs relating to time and level of energy usage, we formulate an energy management game, where the players are the residential users and the strategies are their daily schedules of household appliance use. We will show that the Nash equilibrium of the formulated energy management game can guarantee the global optimization in terms of minimizing the energy costs, where the depreciation cost of PEV’s battery because of discharging and selling energy back is also considered. Simulation results verify that the proposed game-theoretic approach can reduce the total energy cost and individual daily electricity payment. Moreover, since plug-in electric bicycles (PEBs) are currently widely used in China, simulation results of residential users owing household appliances and bidirectional energy trading of PEBs are also provided and discussed.Energies2014-11-18711Article10.3390/en7117499749975181996-10732014-11-18doi: 10.3390/en7117499Bingtuan GaoWenhu ZhangYi TangMingjin HuMingcheng ZhuHuiyu Zhan<![CDATA[Energies, Vol. 7, Pages 7483-7498: CFD Study on Aerodynamic Power Output Changes with Inter-Turbine Spacing Variation for a 6 MW Offshore Wind Farm]]>
http://www.mdpi.com/1996-1073/7/11/7483
This study examined the aerodynamic power output change of wind turbines with inter-turbine spacing variation for a 6 MW wind farm composed of three sets of 2 MW wind turbines using computational fluid dynamics (CFD). The wind farm layout design is becoming increasingly important as the use of wind energy is steadily increasing. Among the many wind farm layout design parameters, the inter-turbine spacing is a key factor in the initial investment cost, annual energy production and maintenance cost. The inter-turbine spacing should be determined to maximize the annual energy production and minimize the wake effect, turbulence effect and fatigue load during the service lifetime of wind turbines. Therefore, some compromise between the aerodynamic power output of wind turbines and the inter-turbine spacing is needed. An actuator disc model with the addition of a momentum source was not used, and instead, a full 3-dimensional model with a tower and nacelle was used for CFD analysis because of its great technical significance. The CFD analysis results, such as the aerodynamic power output, axial direction wind speed change, pressure drop across the rotor of wind turbine, and wind speed deficit due to the wake effect with inter-turbine spacing variation, were studied. The results of this study can be applied effectively to wind farm layout design and evaluation.Energies2014-11-18711Article10.3390/en7117483748374981996-10732014-11-18doi: 10.3390/en7117483Nak ChoiSang NamJong JeongKyung Kim<![CDATA[Energies, Vol. 7, Pages 7454-7482: Analysis of Future Vehicle Energy Demand in China Based on a Gompertz Function Method and Computable General Equilibrium Model]]>
http://www.mdpi.com/1996-1073/7/11/7454
This paper presents a model for the projection of Chinese vehicle stocks and road vehicle energy demand through 2050 based on low-, medium-, and high-growth scenarios. To derive a gross-domestic product (GDP)-dependent Gompertz function, Chinese GDP is estimated using a recursive dynamic Computable General Equilibrium (CGE) model. The Gompertz function is estimated using historical data on vehicle development trends in North America, Pacific Rim and Europe to overcome the problem of insufficient long-running data on Chinese vehicle ownership. Results indicate that the number of projected vehicle stocks for 2050 is 300, 455 and 463 million for low-, medium-, and high-growth scenarios respectively. Furthermore, the growth in China’s vehicle stock will increase beyond the inflection point of Gompertz curve by 2020, but will not reach saturation point during the period 2014–2050. Of major road vehicle categories, cars are the largest energy consumers, followed by trucks and buses. Growth in Chinese vehicle demand is primarily determined by per capita GDP. Vehicle saturation levels solely influence the shape of the Gompertz curve and population growth weakly affects vehicle demand. Projected total energy consumption of road vehicles in 2050 is 380, 575 and 586 million tonnes of oil equivalent for each scenario.Energies2014-11-18711Article10.3390/en7117454745474821996-10732014-11-18doi: 10.3390/en7117454Tian WuMengbo ZhangXunmin Ou<![CDATA[Energies, Vol. 7, Pages 7434-7453: 3D Geothermal Modelling of the Mount Amiata Hydrothermal System in Italy]]>
http://www.mdpi.com/1996-1073/7/11/7434
In this paper we build a subsurface model that helps in visualizing and understanding the structural framework, geology and their interactions with the Mt. Amiata geothermal system. Modelling in 3D provides the possibility to interpolate the geometry of structures and is an effective way of understanding geological features. The 3D modelling approach appears to be crucial for further progress in the reconstruction of the assessment of the geothermal model of Mt. Amiata. Furthermore, this model is used as the basis of a 3D numerical thermo-fluid-dynamic model of the existing reservoir(s). The integration between borehole data and numerical modelling results allows reconstructing the temperature distribution in the subsoil of the Mt. Amiata area.Energies2014-11-17711Article10.3390/en7117434743474531996-10732014-11-17doi: 10.3390/en7117434Paolo FulignatiPaola MarianelliAlessandro SbranaValentina Ciani<![CDATA[Energies, Vol. 7, Pages 7415-7433: Operation of Steam Turbines under Blade Failures during the Summer Peak Load Periods]]>
http://www.mdpi.com/1996-1073/7/11/7415
This paper presents a discussion of practical experience related to the study of a cogeneration system where one of the four steam units occurs a failure of the low-pressure blades during peak load times of the summer months in Taiwan in the year 2007. This study investigates various scenarios consisting of shutting down the damaged unit for repairs and having continued operation of the unit by removing the low-pressure blades and replacing the stationary blade ring with buffer boards. Based on the simulation results, the numerical model has reflected strong agreement with the critical decisions made to operate the damaged unit continuously in a time of the blade failure.Energies2014-11-17711Article10.3390/en7117415741574331996-10732014-11-17doi: 10.3390/en7117415Chien-Hsing LeeShih-Cheng HuangChia-An ChangBin-Kwie Chen<![CDATA[Energies, Vol. 7, Pages 7395-7414: Dynamic Simulation of a CPV/T System Using the Finite Element Method]]>
http://www.mdpi.com/1996-1073/7/11/7395
The aim of this paper is the determination of a concentrating thermo-photovoltaic (CPV/T) system dynamic model by means of the finite element method (FEM). The system consist of triple-junction InGaP/InGaAs/Ge (indium-gallium phosphide/indium-gallium-arsenide/germanium) solar cells connected to a metal core printed circuit board (MCPCB) placed on a coil circuit used for the thermal energy recovery. In particular, the main aim is to determine the fluid outlet temperature. It is evaluated corresponding both to a constant cell temperature equal to 120 °C, generally representing the maximum operating temperature, and to cell temperature values instantly variable with the direct normal irradiation (DNI). Hence, an accurate DNI analysis is realized adopting the Gordon-Reddy statistical model. Using an accurate electric model, the cell temperature and efficiency are determined together with the CPV/T module electric and thermal powers. Generally, the CPV system size is realized according to the user electric load demand and, then, it is important to evaluate the necessary minimum concentration ratio (Cmin), the limit of CPV system applicability, in order to determine the energy convenience profile. The fluid outlet temperature can be then obtained by the FEM analysis to verify if a CPV/T system can be used in solar heating and cooling applications.Energies2014-11-14711Article10.3390/en7117395739574141996-10732014-11-14doi: 10.3390/en7117395Carlo RennoMichele De Giacomo<![CDATA[Energies, Vol. 7, Pages 7368-7394: Thermodynamic Analysis of a Ship Power Plant Operating with Waste Heat Recovery through Combined Heat and Power Production]]>
http://www.mdpi.com/1996-1073/7/11/7368
The goal of this research is to study a cogeneration plant for combined heat &amp; power (CHP) production that utilises the low-temperature waste energy in the power plant of a Suezmax-size oil tanker for all heating and electricity requirements during navigation. After considering various configurations, a standard propulsion engine operating at maximum efficiency and a CHP Plant with R245fa fluid using a supercritical organic Rankine cycle (ORC) is selected. All the ship heat requirements can be covered by energy of organic fluid after expansion in the turbine, except feeder-booster heating. Hence, an additional quantity of working fluid may be heated using an after Heat Recovery Steam Generator (HRSG) directed to the feeder-booster module. An analysis of the obtained results shows that the steam turbine plant does not yield significant fuel savings. However, a CHP plant with R245fa fluid using supercritical ORC meets all of the demands for electrical energy and heat while burning only a small amount of additional fuel in HRSG at the main engine off-design operation.Energies2014-11-14711Article10.3390/en7117368736873941996-10732014-11-14doi: 10.3390/en7117368Mirko GrljušićVladimir MedicaNikola Račić<![CDATA[Energies, Vol. 7, Pages 7348-7367: Design Optimization of Heat Wheels for Energy Recovery in HVAC Systems]]>
http://www.mdpi.com/1996-1073/7/11/7348
Air to air heat exchangers play a crucial role in mechanical ventilation equipment, due to the potential primary energy savings both in case of refurbishment of existing buildings or in case of new ones. In particular, interest in heat wheels is increasing due to their low pressure drop and high effectiveness. In this paper a detailed optimization of design parameters of heat wheels is performed in order to maximize sensible effectiveness and to minimize pressure drop. The analysis is carried out through a one dimensional lumped parameters heat wheel model, which solves heat and mass transfer equations, and through appropriate correlations to estimate pressure drop. Simulation results have been compared with experimental data of a heat wheel tested in specific facilities, and good agreement is attained. The device optimization is performed through the variation of main design parameters, such as heat wheel length, channel base, height and thickness and for different operating conditions, namely the air face velocity and the revolution speed. It is shown that the best configurations are achieved with small channel thickness and, depending on the required sensible effectiveness, with appropriate values of wheel length and channel base and height.Energies2014-11-14711Article10.3390/en7117348734873671996-10732014-11-14doi: 10.3390/en7117348Stefano De AntonellisManuel IntiniCesare JoppoloCalogero Leone<![CDATA[Energies, Vol. 7, Pages 7330-7347: Electronic Power Transformer Control Strategy in Wind Energy Conversion Systems for Low Voltage Ride-through Capability Enhancement of Directly Driven Wind Turbines with Permanent Magnet Synchronous Generators (D-PMSGs)]]>
http://www.mdpi.com/1996-1073/7/11/7330
This paper investigates the use of an Electronic Power Transformer (EPT) incorporated with an energy storage system to smooth the wind power fluctuations and enhance the low voltage ride-through (LVRT) capability of directly driven wind turbines with permanent magnet synchronous generators (D-PMSGs). The decoupled control schemes of the system, including the grid side converter control scheme, generator side converter control scheme and the control scheme of the energy storage system, are presented in detail. Under normal operating conditions, the energy storage system absorbs the high frequency component of the D-PMSG output power to smooth the wind power fluctuations. Under grid fault conditions, the energy storage system absorbs the redundant power, which could not be transferred to the grid by the EPT, to help the D-PMSG to ride through low voltage conditions. This coordinated control strategy is validated by simulation studies using MATLAB/Simulink. With the proposed control strategy, the output wind power quality is improved and the D-PMSG can ride through severe grid fault conditions.Energies2014-11-14711Article10.3390/en7117330733073471996-10732014-11-14doi: 10.3390/en7117330Hui HuangChengxiong MaoJiming LuDan Wang<![CDATA[Energies, Vol. 7, Pages 7305-7329: An Integrated Energy-Efficient Operation Methodology for Metro Systems Based on a Real Case of Shanghai Metro Line One]]>
http://www.mdpi.com/1996-1073/7/11/7305
Metro systems are one of the most important transportation systems in people’s lives. Due to the huge amount of energy it consumes every day, highly-efficient operation of a metro system will lead to significant energy savings. In this paper, a new integrated Energy-efficient Operation Methodology (EOM) for metro systems is proposed and validated. Compared with other energy saving methods, EOM does not incur additional cost. In addition, it provides solutions to the frequent disturbance problems in the metro systems. EOM can be divided into two parts: Timetable Optimization (TO) and Compensational Driving Strategy Algorithm (CDSA). First, to get a basic energy-saving effect, a genetic algorithm is used to modify the dwell time of each stop to obtain the most optimal energy-efficient timetable. Then, in order to save additional energy when disturbances happen, a novel CDSA algorithm is formulated and proposed based on the foregoing method. To validate the correctness and effectiveness of the energy-savings possible with EOM, a real case of Shanghai Metro Line One (SMLO) is studied, where EOM was applied. The result shows that a significant amount of energy can be saved by using EOM.Energies2014-11-13711Article10.3390/en7117305730573291996-10732014-11-13doi: 10.3390/en7117305Cheng GongShiwen ZhangFeng ZhangJianguo JiangXinheng Wang<![CDATA[Energies, Vol. 7, Pages 7282-7304: An Optimization Model for Large–Scale Wind Power Grid Connection Considering Demand Response and Energy Storage Systems]]>
http://www.mdpi.com/1996-1073/7/11/7282
To reduce the influence of wind power output uncertainty on power system stability, demand response (DRPs) and energy storage systems (ESSs) are introduced while solving scheduling optimization problems. To simulate wind power scenarios, this paper uses Latin Hypercube Sampling (LHS) to generate the initial scenario set and constructs a scenario reduction strategy based on Kantorovich distance. Since DRPs and ESSs can influence the distribution of demand load, this paper constructs a joint scheduling optimization model for wind power, ESSs and DRPs under the objective of minimizing total coal cost, and constraints of power demand and supply balance, users’ demand elasticity, thermal units’ startup-shutdown, thermal units’ output power climbing and wind power backup service. To analyze the influences of ESSs and DRPs on system wind power consumption capacity, example simulation is made in a 10 thermal units system with a 1000 MW wind farm and 400 MW energy storage systems under four simulation scenarios. The simulation results show that the introduction of DRPs and ESSs could promote system wind power consumption capacity with significantly economic and environment benefits, which include less coal consumption and less pollutant emission; and the optimization effect reaches the optimum when DRPs and ESSs are both introduced.Energies2014-11-13711Article10.3390/en7117282728273041996-10732014-11-13doi: 10.3390/en7117282Zhongfu TanHuanhuan LiLiwei JuYihang Song<![CDATA[Energies, Vol. 7, Pages 7266-7281: Performance Analysis of Cold Energy Recovery from CO2 Injection in Ship-Based Carbon Capture and Storage (CCS)]]>
http://www.mdpi.com/1996-1073/7/11/7266
Carbon capture and storage (CCS) technology is one of the practical solutions for mitigating the effects of global warming. When captured CO2 is injected into storage sites, the CO2 is subjected to a heating process. In a conventional CO2 injection system, CO2 cold energy is wasted during this heating process. This study proposes a new CO2 injection system that takes advantage of the cold energy using the Rankine cycle. The study compared the conventional system with the new CO2 injection system in terms of specific net power consumption, exergy efficiency, and life-cycle cost (LCC) to estimate the economic effects. The results showed that the new system reduced specific net power consumption and yielded higher exergy efficiency. The LCC of the new system was more economical. Several cases were examined corresponding to different conditions, specifically, discharge pressure and seawater temperature. This information may affect decision-making when CCS projects are implemented.Energies2014-11-12711Article10.3390/en7117266726672811996-10732014-11-12doi: 10.3390/en7117266Hwalong YouYoungkyun SeoCheol HuhDaejun Chang<![CDATA[Energies, Vol. 7, Pages 7245-7265: Evaluation of Artificial Neural Network-Based Temperature Control for Optimum Operation of Building Envelopes]]>
http://www.mdpi.com/1996-1073/7/11/7245
This study aims at developing an indoor temperature control method that could provide comfortable thermal conditions by integrating heating system control and the opening conditions of building envelopes. Artificial neural network (ANN)-based temperature control logic was developed for the control of heating systems and openings at the building envelopes in a predictive and adaptive manner. Numerical comparative performance tests for the ANN-based temperature control logic and conventional non-ANN-based counterpart were conducted for single skin enveloped and double skin enveloped buildings after the simulation program was validated by comparing the simulation and the field measurement results. Analysis results revealed that the ANN-based control logic improved the indoor temperature environment with an increased comfortable temperature period and decreased overshoot and undershoot of temperatures outside of the operating range. The proposed logic did not show significant superiority in energy efficiency over the conventional logic. The ANN-based temperature control logic was able to maintain the indoor temperature more comfortably and with more stability within the operating range due to the predictive and adaptive features of ANN models.Energies2014-11-12711Article10.3390/en7117245724572651996-10732014-11-12doi: 10.3390/en7117245Jin MoonJi-Hyun LeeSooyoung Kim<![CDATA[Energies, Vol. 7, Pages 7223-7244: Naturally-Forced Slug Flow Expander for Application in a Waste-Heat Recovery Cycle]]>
http://www.mdpi.com/1996-1073/7/11/7223
This paper investigates a slug-flow expander (SFE) for conversion of high-pressure gas/vapor into kinetic energy of liquid slugs. The energy transfer from high-pressure to kinetic energy is quantified using thrust plate measurements. Non-dimensional thrust data is used to quantify performance by normalizing measured thrust by thrust for the same water flow rate at zero air flow rate. A total of 13 expander configurations are investigated and geometries with the shortest cavity length and the smallest exit diameter are found to result in the largest non-dimensional thrust increase. Results show that thrust augmentation increases with the initiation of slug flow in the SFE. The analysis performed on the normalized thrust readings suggested that as the water and air flow were increased to critical conditions, the liquid slugs produced by the SFE augmented the thrust measurements. The final performance evaluation was based on linear regression of the normalized thrust measurements where slug flow was generated for each SFE architecture. Greater magnitudes of the slope from the linear regression indicated the propensity of the SFE to augment thrust. This analysis confirmed that for the SFE configurations over the range of values investigated, the SFE increased thrust up to three times its original value at no air flow. Given the inherent multiphase nature of the slug-flow expander, application to systems involving expansion of wetting fluids (water as part of a waste-heat recovery system) or air with water droplet formation (as part of a compressed-air energy storage system) could be considered.Energies2014-11-10711Article10.3390/en7117223722372441996-10732014-11-10doi: 10.3390/en7117223Ben de WittRon Hugo<![CDATA[Energies, Vol. 7, Pages 7194-7222: Macroalgae-Derived Biofuel: A Review of Methods of Energy Extraction from Seaweed Biomass]]>
http://www.mdpi.com/1996-1073/7/11/7194
The potential of algal biomass as a source of liquid and gaseous biofuels is a highly topical theme, but as yet there is no successful economically viable commercial system producing biofuel. However, the majority of the research has focused on producing fuels from microalgae rather than from macroalgae. This article briefly reviews the methods by which useful energy may be extracted from macroalgae biomass including: direct combustion, pyrolysis, gasification, trans-esterification to biodiesel, hydrothermal liquefaction, fermentation to bioethanol, fermentation to biobutanol and anaerobic digestion, and explores technical and engineering difficulties that remain to be resolved.Energies2014-11-07711Review10.3390/en7117194719472221996-10732014-11-07doi: 10.3390/en7117194John MilledgeBenjamin SmithPhilip DyerPatricia Harvey<![CDATA[Energies, Vol. 7, Pages 7178-7193: Lorenz Wind Disturbance Model Based on Grey Generated Components]]>
http://www.mdpi.com/1996-1073/7/11/7178
In order to meet the needs of wind speed prediction in wind farms, we consider the influence of random atmospheric disturbances on wind variations. Considering a simplified fluid convection mode, a Lorenz system can be employed as an atmospheric disturbance model. Here Lorenz disturbance is defined as the European norm of the solutions of the Lorenz equation. Grey generating and accumulated generating models are employed to explore the relationship between wind speed and its related disturbance series. We conclude that a linear or quadric polynomial generating model are optimal through the verification of short-term wind speed prediction in the Sotavento wind farm. The new proposed model not only greatly improves the precision of short-term wind speed prediction, but also has great significance for the maintenance and stability of wind power system operation.Energies2014-11-07711Article10.3390/en7117178717871931996-10732014-11-07doi: 10.3390/en7117178Yagang ZhangJingyun YangKangcheng WangYinding Wang<![CDATA[Energies, Vol. 7, Pages 7166-7177: A Polygeneration System Based on Multi-Input Chemical Looping Combustion]]>
http://www.mdpi.com/1996-1073/7/11/7166
This paper proposes a polygeneration system based on a multi-input chemical looping combustion system, which generates methanol and electricity, through the use of natural gas and coal. In this system, the chemical looping hydrogen (CLH) production system and the coal-based methanol production system are integrated. A high quality fuel, natural gas, is used to improve the conversion ratio of coal. The Gibbs energy of the two kinds of fuels is fully used. Benefitting from the chemical looping process, 27% CO2 can be captured without energy penalty. With the same outputs of methanol and electricity, the energy savings ratio of the new system is about 12%. Based on the exergy analyses, it is disclosed that the integration of synthetic utilization of natural gas and coal plays a significant role in reducing the exergy destruction of the new system. The promising results obtained in this paper may lead to a clean coal technology that will utilize natural gas and coal more efficiently and economically.Energies2014-11-06711Article10.3390/en7117166716671771996-10732014-11-06doi: 10.3390/en7117166Xiaosong ZhangSheng LiHongguang Jin<![CDATA[Energies, Vol. 7, Pages 7147-7165: Post Feed-in Scheme Photovoltaic System Feasibility Evaluation in Italy: Sicilian Case Studies]]>
http://www.mdpi.com/1996-1073/7/11/7147
Thanks to national energy policies, over recent years the Italian photovoltaic (PV) sector has undergone an extraordinary growth, also affecting the primary sector. In this context, Mediterranean greenhouses are well-adapted to photovoltaic systems because they represent one of the most energy-intensive sectors in agriculture. The Italian feed-in scheme ended at the beginning of 2013, making it necessary to investigate the feasibility of photovoltaic systems devoid of any electricity production-related incentives. In this paper, production cost and profitability analyses of photovoltaic electricity have been conducted, considering Mediterranean solar greenhouses in which, thanks to net metering, all the electricity produced by photovoltaic panels is self-consumed. Our results showed that grid parity is already reached for Sicilian PV systems with a capacity greater than 50 kW. Moreover, net present value, internal rate of return and discounted payback time all demonstrate the high economic convenience of all the photovoltaic investments analyzed, due to the huge savings on energy expenditures.Energies2014-11-05711Article10.3390/en7117147714771651996-10732014-11-05doi: 10.3390/en7117147Riccardo SquatritoFilippo SgroiSalvatore TudiscaAnna TrapaniRiccardo Testa<![CDATA[Energies, Vol. 7, Pages 7125-7146: Uncertainties in Life Cycle Greenhouse Gas Emissions from Advanced Biomass Feedstock Logistics Supply Chains in Kansas]]>
http://www.mdpi.com/1996-1073/7/11/7125
To meet Energy Independence and Security Act (EISA) cellulosic biofuel mandates, the United States will require an annual domestic supply of about 242 million Mg of biomass by 2022. To improve the feedstock logistics of lignocellulosic biofuels in order to access available biomass resources from areas with varying yields, commodity systems have been proposed and designed to deliver quality-controlled biomass feedstocks at preprocessing “depots”. Preprocessing depots densify and stabilize the biomass prior to long-distance transport and delivery to centralized biorefineries. The logistics of biomass commodity supply chains could introduce spatially variable environmental impacts into the biofuel life cycle due to needing to harvest, move, and preprocess biomass from multiple distances that have variable spatial density. This study examines the uncertainty in greenhouse gas (GHG) emissions of corn stover logistics within a bio-ethanol supply chain in the state of Kansas, where sustainable biomass supply varies spatially. Two scenarios were evaluated each having a different number of depots of varying capacity and location within Kansas relative to a central commodity-receiving biorefinery to test GHG emissions uncertainty. The first scenario sited four preprocessing depots evenly across the state of Kansas but within the vicinity of counties having high biomass supply density. The second scenario located five depots based on the shortest depot-to-biorefinery rail distance and biomass availability. The logistics supply chain consists of corn stover harvest, collection and storage, feedstock transport from field to biomass preprocessing depot, preprocessing depot operations, and commodity transport from the biomass preprocessing depot to the biorefinery. Monte Carlo simulation was used to estimate the spatial uncertainty in the feedstock logistics gate-to-gate sequence. Within the logistics supply chain GHG emissions are most sensitive to the transport of the densified biomass, which introduces the highest variability (0.2–13 g CO2e/MJ) to life cycle GHG emissions. Moreover, depending upon the biomass availability and its spatial density and surrounding transportation infrastructure (road and rail), logistics can increase the variability in life cycle environmental impacts for lignocellulosic biofuels. Within Kansas, life cycle GHG emissions could range from 24 g CO2e/MJ to 41 g CO2e/MJ depending upon the location, size and number of preprocessing depots constructed. However, this range can be minimized through optimizing the siting of preprocessing depots where ample rail infrastructure exists to supply biomass commodity to a regional biorefinery supply system.Energies2014-11-04711Article10.3390/en7117125712571461996-10732014-11-04doi: 10.3390/en7117125Long NguyenKara CaffertyErin SearcySabrina Spatari<![CDATA[Energies, Vol. 7, Pages 7105-7124: Design of High Performance Permanent-Magnet Synchronous Wind Generators]]>
http://www.mdpi.com/1996-1073/7/11/7105
This paper is devoted to the analysis and design of high performance permanent-magnet synchronous wind generators (PSWGs). A systematic and sequential methodology for the design of PMSGs is proposed with a high performance wind generator as a design model. Aiming at high induced voltage, low harmonic distortion as well as high generator efficiency, optimal generator parameters such as pole-arc to pole-pitch ratio and stator-slot-shoes dimension, etc. are determined with the proposed technique using Maxwell 2-D, Matlab software and the Taguchi method. The proposed double three-phase and six-phase winding configurations, which consist of six windings in the stator, can provide evenly distributed current for versatile applications regarding the voltage and current demands for practical consideration. Specifically, windings are connected in series to increase the output voltage at low wind speed, and in parallel during high wind speed to generate electricity even when either one winding fails, thereby enhancing the reliability as well. A PMSG is designed and implemented based on the proposed method. When the simulation is performed with a 6 Ω load, the output power for the double three-phase winding and six-phase winding are correspondingly 10.64 and 11.13 kW. In addition, 24 Ω load experiments show that the efficiencies of double three-phase winding and six-phase winding are 96.56% and 98.54%, respectively, verifying the proposed high performance operation.Energies2014-11-04711Article10.3390/en7117105710571241996-10732014-11-04doi: 10.3390/en7117105Chun-Yu HsiaoSheng-Nian YehJonq-Chin Hwang<![CDATA[Energies, Vol. 7, Pages 7094-7104: Experimental Investigation of Heat Transfer and Pressure Drop Characteristics of H-type Finned Tube Banks]]>
http://www.mdpi.com/1996-1073/7/11/7094
H-type finned tube heat exchanger elements maintain a high capacity for heat transfer, possess superior self-cleaning properties and retain the ability to effect flue gas waste heat recovery in boiler renovations. In this paper, the heat transfer and pressure drop characteristics of H-type finned tube banks are studied via an experimental open high-temperature wind tunnel system. The effects of fin width, fin height, fin pitch and air velocity on fin efficiency, convective heat transfer coefficient, integrated heat transfer capacity and pressure drop are examined. The results indicate that as air velocity, fin height and fin width increase, fin efficiency decreases. Convective heat transfer coefficient is proportional to fin pitch, but inversely proportional to fin height and fin width. Integrated heat transfer capacity is related to fin efficiency, convective heat transfer coefficient and finned ratio. Pressure drop increases with the increase of fin height and fin width. Finally, predictive correlations of fin efficiency, Nusselt number and Euler Number are developed based on the experimental data.Energies2014-11-04711Article10.3390/en7117094709471041996-10732014-11-04doi: 10.3390/en7117094Heng ChenYungang WangQinxin ZhaoHaidong MaYuxin LiZhongya Chen<![CDATA[Energies, Vol. 7, Pages 7067-7093: Preliminary Design and Simulation of a Turbo Expander for Small Rated Power Organic Rankine Cycle (ORC)]]>
http://www.mdpi.com/1996-1073/7/11/7067
Nowadays, the Organic Rankine Cycle (ORC) system, which operates with organic fluids, is one of the leading technologies for “waste energy recovery”. It works as a conventional Rankine Cycle but, as mentioned, instead of steam/water, an organic fluid is used. This change allows it to convert low temperature heat into electric energy where required. Large numbers of studies have been carried out to identify the most suitable fluids, system parameters and the various configurations. In the present market, most ORC systems are designed and manufactured for the recovery of thermal energy from various sources operating at “large power rating” (exhaust gas turbines, internal combustion engines, geothermal sources, large melting furnaces, biomass, solar, etc.); from which it is possible to produce a large amount of electric energy (30 kW ÷ 300 kW). Such applications for small nominal power sources, as well as the exhaust gases of internal combustion engines (car sedan or town, ships, etc.) or small heat exchangers, are very limited. The few systems that have been designed and built for small scale applications, have, on the other hand, different types of expander (screw, scroll, etc.). These devices are not adapted for placement in small and restricted places like the interior of a conventional car. The aim of this work is to perform the preliminary design of a turbo-expander that meets diverse system requirements such as low pressure, small size and low mass flow rates. The expander must be adaptable to a small ORC system utilizing gas of a diesel engine or small gas turbine as thermal source to produce 2–10 kW of electricity. The temperature and pressure of the exhaust gases, in this case study (400–600 °C and a pressure of 2 bar), imposes a limit on the use of an organic fluid and on the net power that can be produced. In addition to water, fluids such as CO2, R134a and R245fa have been considered. Once the operating fluids has been chosen, the turbine characteristics (dimensions, input and output temperature, pressure ratio, etc.) have been calculated and an attempt to find the “nearly-optimal” combination has been carried out. The detailed design of a radial expander is presented and discussed. A thermo-mechanical performance study was carry out to verify structural tension and possible displacement. On the other hand, preliminary CFD analyses have been performed to verify the effectiveness of the design procedure.Energies2014-11-03711Article10.3390/en7117067706770931996-10732014-11-03doi: 10.3390/en7117067Roberto CapataGustavo Hernandez<![CDATA[Energies, Vol. 7, Pages 7041-7066: An Efficient, Scalable Time-Frequency Method for Tracking Energy Usage of Domestic Appliances Using a Two-Step Classification Algorithm]]>
http://www.mdpi.com/1996-1073/7/11/7041
Load monitoring is the practice of measuring electrical signals in a domestic environment in order to identify which electrical appliances are consuming power. One reason for developing a load monitoring system is to reduce power consumption by increasing consumers’ awareness of which appliances consume the most energy. Another example of an application of load monitoring is activity sensing in the home for the provision of healthcare services. This paper outlines the development of a load disaggregation method that measures the aggregate electrical signals of a domestic environment and extracts features to identify each power consuming appliance. A single sensor is deployed at the main incoming power point, to sample the aggregate current signal. The method senses when an appliance switches ON or OFF and uses a two-step classification algorithm to identify which appliance has caused the event. Parameters from the current in the temporal and frequency domains are used as features to define each appliance. These parameters are the steady-state current harmonics and the rate of change of the transient signal. Each appliance’s electrical characteristics are distinguishable using these parameters. There are three Types of loads that an appliance can fall into, linear nonreactive, linear reactive or nonlinear reactive. It has been found that by identifying the load type first and then using a second classifier to identify individual appliances within these Types, the overall accuracy of the identification algorithm is improved.Energies2014-10-31711Article10.3390/en7117041704170661996-10732014-10-31doi: 10.3390/en7117041Paula MeehanConor McArdleStephen Daniels<![CDATA[Energies, Vol. 7, Pages 7017-7040: A Model Predictive Control Approach for Fuel Economy Improvement of a Series Hydraulic Hybrid Vehicle]]>
http://www.mdpi.com/1996-1073/7/11/7017
This study applied a model predictive control (MPC) framework to solve the cruising control problem of a series hydraulic hybrid vehicle (SHHV). The controller not only regulates vehicle velocity, but also engine torque, engine speed, and accumulator pressure to their corresponding reference values. At each time step, a quadratic programming problem is solved within a predictive horizon to obtain the optimal control inputs. The objective is to minimize the output error. This approach ensures that the components operate at high efficiency thereby improving the total efficiency of the system. The proposed SHHV control system was evaluated under urban and highway driving conditions. By handling constraints and input-output interactions, the MPC-based control system ensures that the system operates safely and efficiently. The fuel economy of the proposed control scheme shows a noticeable improvement in comparison with the PID-based system, in which three Proportional-Integral-Derivative (PID) controllers are used for cruising control.Energies2014-10-31711Article10.3390/en7117017701770401996-10732014-10-31doi: 10.3390/en7117017Tri-Vien VuChih-Keng ChenChih-Wei Hung<![CDATA[Energies, Vol. 7, Pages 6930-7016: A Review of Methodological Approaches for the Design and Optimization of Wind Farms]]>
http://www.mdpi.com/1996-1073/7/11/6930
This article presents a review of the state of the art of the Wind Farm Design and Optimization (WFDO) problem. The WFDO problem refers to a set of advanced planning actions needed to extremize the performance of wind farms, which may be composed of a few individual Wind Turbines (WTs) up to thousands of WTs. The WFDO problem has been investigated in different scenarios, with substantial differences in main objectives, modelling assumptions, constraints, and numerical solution methods. The aim of this paper is: (1) to present an exhaustive survey of the literature covering the full span of the subject, an analysis of the state-of-the-art models describing the performance of wind farms as well as its extensions, and the numerical approaches used to solve the problem; (2) to provide an overview of the available knowledge and recent progress in the application of such strategies to real onshore and offshore wind farms; and (3) to propose a comprehensive agenda for future research.Energies2014-10-29711Review10.3390/en7116930693070161996-10732014-10-29doi: 10.3390/en7116930José Herbert-AceroOliver ProbstPierre-Elouan RéthoréGunner LarsenKrystel Castillo-Villar<![CDATA[Energies, Vol. 7, Pages 6897-6929: A Heuristic Rule-Based Passive Design Decision Model for Reducing Heating Energy Consumption of Korean Apartment Buildings]]>
http://www.mdpi.com/1996-1073/7/11/6897
This research presents an evaluative energy model for estimating the energy efficiency of the design choices of architects and engineers in the early design phase. We analyze the effects of various parameters with different characteristics in various combinations for building energy consumption. With this analysis, we build a database that identifies a set of heuristic rules for energy-efficient building design to facilitate the design of sustainable apartment housing. Perturbation studies are based on a sensitivity analysis used to identify the thermal influence of the input design parameters on various simulation outputs and compare the results to a reference case. Energy sensitivity weight factors are obtained from an extensive sensitivity study using building energy simulations. The results of the energy sensitivity study summarized in a set of heuristic rules for evaluating architectural features are estimated through case studies of Korean apartment buildings. This study offers valuable guidelines for developing energy-efficient residential housing in Korea and will help architects in considering appropriate design schemes and provide a ready reference to generalized test cases for both architects and engineers so that they can zero in on a set of effective design solutions.Energies2014-10-29711Article10.3390/en7116897689769291996-10732014-10-29doi: 10.3390/en7116897Dongjun SuhSeongju Chang<![CDATA[Energies, Vol. 7, Pages 6886-6896: Ambiguity Reduction by Objective Model Selection, with an Application to the Costs of the EU 2030 Climate Targets]]>
http://www.mdpi.com/1996-1073/7/11/6886
I estimate the cost of meeting the EU 2030 targets for greenhouse gas emission reduction, using statistical emulators of ten alternative models. Assuming a first-best policy implementation, I find that total and marginal costs are modest. The statistical emulators allow me to compute the risk premiums, which are small, because the EU is rich and the policy impact is small. The ensemble of ten models allows me to compute the ambiguity premium, which is small for the same reason. I construct a counterfactual estimate of recent emissions without the climate policy and use that to test the predictive skill of the ten models. The models that show the lowest cost of emission reduction also have the lowest skill for Europe in recent times.Energies2014-10-28711Article10.3390/en7116886688668961996-10732014-10-28doi: 10.3390/en7116886Richard Tol<![CDATA[Energies, Vol. 7, Pages 6856-6885: Simplified Analysis of the Electric Power Losses for On-Shore Wind Farms Considering Weibull Distribution Parameters]]>
http://www.mdpi.com/1996-1073/7/11/6856
Electric power losses are constantly present during the service life of wind farms and must be considered in the calculation of the income arising from selling the produced electricity. It is typical to estimate the electrical losses in the design stage as those occurring when the wind farm operates at rated power, nevertheless, it is necessary to determine a method for checking if the actual losses meet the design requirements during the operation period. In this paper, we prove that the electric losses at rated power should not be considered as a reference level and a simple methodology will be developed to analyse and foresee the actual losses in a set period as a function of the wind resource in such period, defined according to the Weibull distribution, and the characteristics of the wind farm electrical infrastructure. This methodology facilitates a simple way, to determine in the design phase and to check during operation, the actual electricity losses.Energies2014-10-28711Article10.3390/en7116856685668851996-10732014-10-28doi: 10.3390/en7116856Antonio Colmenar-SantosSevero Campíez-RomeroLorenzo Enríquez-GarciaClara Pérez-Molina<![CDATA[Energies, Vol. 7, Pages 6837-6855: A New System to Estimate and Reduce Electrical Energy Consumption of Domestic Hot Water in Spain]]>
http://www.mdpi.com/1996-1073/7/11/6837
Energy consumption rose about 28% over the 2001 to 2011 period in the Spanish residential sector. In this environment, domestic hot water (DHW) represents the second highest energy demand. There are several methodologies to estimate DHW consumption, but each methodology uses different inputs and some of them are based on obsolete data. DHW energy consumption estimation is a key tool to plan modifications that could enhance this consumption and we decided to update the methodologies. We studied DHW consumption with data from 10 apartments in the same building during 18 months. As a result of the study, we updated one chosen methodology, adapting it to the current situation. One of the challenges to improve efficiency of DHW use is that most of people are not aware of how it is consumed in their homes. To help this information to reach consumers, we developed a website to allow users to estimate the final electrical energy needed for DHW. The site uses three estimation methodologies and chooses the best fit based on information given by the users. Finally, the application provides users with recommendations and tips to reduce their DHW consumption while still maintaining the desired comfort level.Energies2014-10-24711Article10.3390/en7116837683768551996-10732014-10-24doi: 10.3390/en7116837Alberto Gutierrez-EscolarAna Castillo-MartinezJose Gomez-PulidoJose-Maria Gutierrez-MartinezZlatko Stapic<![CDATA[Energies, Vol. 7, Pages 6825-6836: Sustainability Criteria and Indicators for the Bio-Based Economy in Europe: State of Discussion and Way Forward]]>
http://www.mdpi.com/1996-1073/7/11/6825
There is a strong interest in the EU to promote the bioeconomy sector within the EU 2020 strategy. It is thus necessary to assure a sound sustainability framework. This paper reviews international and European sustainability initiatives mainly for biomass for bioenergy. The basic and advanced sustainability indicators are identified and described with particular attention to those points without agreement between stakeholders. Based on the state of the discussion, some suggestions to enhance the sustainable development of the bioeconomy sector are proposed.Energies2014-10-24711Article10.3390/en7116825682568361996-10732014-10-24doi: 10.3390/en7116825Uwe FritscheLeire Iriarte<![CDATA[Energies, Vol. 7, Pages 6823-6824: Correction: Legros Arnaud, Guillaume Ludovic, Diny Mouad, Zaïdi Hamid and Lemort Vincent. Comparison and Impact of Waste Heat Recovery Technologies on Passenger Car Fuel Consumption in a Normalized Driving Cycle. Energies 2014, 7, 5273–5290]]>
http://www.mdpi.com/1996-1073/7/10/6823
We have found the following error in the authors’ information in this article, which was recently published in Energies [1]. On page 5273, the names of the authors are incorrect; they should be changed from:[...]Energies2014-10-23710Correction10.3390/en7106823682368241996-10732014-10-23doi: 10.3390/en7106823Arnaud LegrosLudovic GuillaumeMouad DinyHamid ZaïdiVincent Lemort<![CDATA[Energies, Vol. 7, Pages 6798-6822: Insight into Rotational Effects on a Wind Turbine Blade Using Navier–Stokes Computations]]>
http://www.mdpi.com/1996-1073/7/10/6798
Rotational effects are known to influence severely the aerodynamic performance of the inboard region of rotor blades. The underlying physical mechanisms are however far from being well understood. The present work addresses this problem using Reynolds averaged Navier–Stokes computations and experimental results of the MEXICO (Model Experiments in Controlled Conditions) rotor. Four axisymmetric inflow cases with wind speeds ranging from pre-stall to post-stall conditions are computed and compared with pressure and particle image velocimetry (PIV) experimental data, obtaining, in general, consistent results. At low angles of attack, the aerodynamic behavior of all of the studied blade sections resembles the one from the corresponding 2D airfoils. However, at high angles of attack, rotational effects lead to stall delay and/or lift enhancement at inboard positions. Such effects are shown to occur only in the presence of significant radial flows. Interestingly, the way in which rotational effects influence the aerodynamics of the MEXICO blades differs qualitatively in certain aspects from the descriptions found in the literature about this topic. The presented results provide new insights that are useful for the development of advanced and physically-sound correction models.Energies2014-10-21710Article10.3390/en7106798679868221996-10732014-10-21doi: 10.3390/en7106798Iván HerráezBernhard StoevesandtJoachim Peinke<![CDATA[Energies, Vol. 7, Pages 6783-6797: Study on the Optimal Charging Strategy for Lithium-Ion Batteries Used in Electric Vehicles]]>
http://www.mdpi.com/1996-1073/7/10/6783
The charging method of lithium-ion batteries used in electric vehicles (EVs) significantly affects its commercial application. This paper aims to make three contributions to the existing literature. (1) In order to achieve an efficient charging strategy for lithium-ion batteries with shorter charging time and lower charring loss, the trade-off problem between charging loss and charging time has been analyzed in details through the dynamic programing (DP) optimization algorithm; (2) To reduce the computation time consumed during the optimization process, we have proposed a database based optimization approach. After off-line calculation, the simulation results can be applied to on-line charge; (3) The novel database-based DP method is proposed and the simulation results illustrate that this method can effectively find the suboptimal charging strategies under a certain balance between the charging loss and charging time.Energies2014-10-21710Article10.3390/en7106783678367971996-10732014-10-21doi: 10.3390/en7106783Shuo ZhangChengning ZhangRui XiongWei Zhou<![CDATA[Energies, Vol. 7, Pages 6765-6782: A Zero Input Current Ripple ZVS/ZCS Boost Converter with Boundary-Mode Control]]>
http://www.mdpi.com/1996-1073/7/10/6765
In this paper, in order to achieve zero ripple conditions, the use of a ripple mirror (RM) circuit for the boost converter is proposed. The operation modes are studied and steady-state analyses performed to show the merits of the proposed converter. It is found that the proposed RM circuit technique can provide much better flexibility than the two-phase interleaved boost converter for locating the zero ripple operating point in the design stage. In addition, the choice of using a boundary-mode control is mainly based on the consideration of achieving both ZVS (zero voltage switching)/ZCS (zero current switching) soft-switching and constant on-time control for the converter. To verify the performance of the proposed converter, a 48 V input and 200 W/200 V output prototype is constructed. Experimental results verify the effectiveness of the proposed converter.Energies2014-10-20710Article10.3390/en7106765676567821996-10732014-10-20doi: 10.3390/en7106765Ching-Ming LaiMing-Ji YangShih-Kun Liang<![CDATA[Energies, Vol. 7, Pages 6741-6764: Assessment of Energy, Environmental and Economic Performance of a Solar Desiccant Cooling System with Different Collector Types]]>
http://www.mdpi.com/1996-1073/7/10/6741
Desiccant-based air handling units can achieve reductions in greenhouse gas emissions and energy savings with respect to conventional air conditioning systems. Benefits are maximized when they interact with renewable energy technologies, such as solar collectors. In this work, experimental tests and data derived from scientific and technical literature are used to implement a model of a solar desiccant cooling system, considering three different collector technologies (air, flat-plate and evacuated collectors). Simulations were then performed to compare the energy, environmental and economic performance of the system with those of a desiccant-based unit where regeneration thermal energy is supplied by a natural gas boiler, and with those of a conventional air-handling unit. The only solution that allows achieving the economic feasibility of the solar desiccant cooling unit consists of 16 m2 of evacuated solar collectors. This is able to obtain, with respect to the reference system, a reduction of primary energy consumption and of the equivalent CO2 emissions of 50.2% and 49.8%, respectively, but with a payback time of 20 years.Energies2014-10-20710Article10.3390/en7106741674167641996-10732014-10-20doi: 10.3390/en7106741Giovanni AngrisaniCarlo RoselliMaurizio SassoFrancesco Tariello<![CDATA[Energies, Vol. 7, Pages 6721-6740: A Novel Modeling of Molten-Salt Heat Storage Systems in Thermal Solar Power Plants]]>
http://www.mdpi.com/1996-1073/7/10/6721
Many thermal solar power plants use thermal oil as heat transfer fluid, and molten salts as thermal energy storage. Oil absorbs energy from sun light, and transfers it to a water-steam cycle across heat exchangers, to be converted into electric energy by means of a turbogenerator, or to be stored in a thermal energy storage system so that it can be later transferred to the water-steam cycle. The complexity of these thermal solar plants is rather high, as they combine traditional engineering used in power stations (water-steam cycle) or petrochemical (oil piping), with the new solar (parabolic trough collector) and heat storage (molten salts) technologies. With the engineering of these plants being relatively new, regulation of the thermal energy storage system is currently achieved in manual or semiautomatic ways, controlling its variables with proportional-integral-derivative (PID) regulators. This makes the overall performance of these plants non optimal. This work focuses on energy storage systems based on molten salt, and defines a complete model of the process. By defining such a model, the ground for future research into optimal control methods will be established. The accuracy of the model will be determined by comparing the results it provides and those measured in the molten-salt heat storage system of an actual power plant.Energies2014-10-17710Article10.3390/en7106721672167401996-10732014-10-17doi: 10.3390/en7106721Rogelio MenéndezJuan MartínezMiguel PrietoLourdes BarciaJuan Sánchez<![CDATA[Energies, Vol. 7, Pages 6689-6720: Seismic Velocity/Temperature Correlations and a Possible New Geothermometer: Insights from Exploration of a High-Temperature Geothermal System on Montserrat, West Indies]]>
http://www.mdpi.com/1996-1073/7/10/6689
In 2013, two production wells were drilled into a geothermal reservoir on Montserrat, W.I. (West Indies) Drilling results confirmed the main features of a previously developed conceptual model. The results confirm that below ~220 °C there is a negative correlation between reservoir temperature and seismic velocity anomaly. However, above ~220 °C there is a positive correlation. We hypothesise that anomalous variations in seismic velocity within the reservoir are controlled to first order by the hydrothermal mineral assemblage. This study suggests a new geophysical thermometer which can be used to estimate temperatures in three dimensions with unprecedented resolution and to indicate the subsurface fluid pathways which are the target of geothermal exploitation.Energies2014-10-17710Article10.3390/en7106689668967201996-10732014-10-17doi: 10.3390/en7106689Graham RyanEylon Shalev<![CDATA[Energies, Vol. 7, Pages 6665-6688: Electromagnetic Analysis and Design of Switched Reluctance Double-Rotor Machine for Hybrid Electric Vehicles]]>
http://www.mdpi.com/1996-1073/7/10/6665
The double-rotor machine is a kind of multiple input and output electromechanical energy transducer with two electrical ports and two mechanical ports, which is an ideal transmission system for hybrid electric vehicles and has a series of advantages such as integration of power and energy, high efficiency and compaction. In this paper, a switched reluctance double-rotor machine (SRDRM) is proposed for hybrid electric vehicles, while no conductor or PM in the middle rotor. This machine not only inherits the merits of switched reluctance machine, such as simple salient rotor structure, high reliability and wide speed range, but also can avoid the outer rotor’s cooling problem effectively. By using an equivalent magnetic circuit model, the function of middle rotor yoke is analyzed. Electromagnetic analyses of the SRDRM are performed with analytical calculations and 2-D finite element methods, including the effects of main parameters on performance. Finally, a 4.4 kW prototype machine is designed and manufactured, and the tests are performed, which validate the proposed design method.Energies2014-10-16710Article10.3390/en7106665666566881996-10732014-10-16doi: 10.3390/en7106665Shouliang HanShumei CuiLiwei SongChing Chan<![CDATA[Energies, Vol. 7, Pages 6645-6664: Methods for Risk-Based Planning of O&M of Wind Turbines]]>
http://www.mdpi.com/1996-1073/7/10/6645
In order to make wind energy more competitive, the big expenses for operation and maintenance must be reduced. Consistent decisions that minimize the expected costs can be made based on risk-based methods. Such methods have been implemented for maintenance planning for oil and gas structures, but for offshore wind turbines, the conditions are different and the methods need to be adjusted accordingly. This paper gives an overview of various approaches to solve the decision problem: methods with decision rules based on observed variables, a method with decision rules based on the probability of failure, a method based on limited memory influence diagrams and a method based on the partially observable Markov decision process. The methods with decision rules based on observed variables are easy to use, but can only take the most recent observation into account, when a decision is made. The other methods can take more information into account, and especially, the method based on the Markov decision process is very flexible and accurate. A case study shows that the Markov decision process and decision rules based on the probability of failure are equally good and give lower costs compared to decision rules based on observed variables.Energies2014-10-16710Article10.3390/en7106645664566641996-10732014-10-16doi: 10.3390/en7106645Jannie NielsenJohn Sørensen<![CDATA[Energies, Vol. 7, Pages 6620-6644: Real-Time Wavelet-Based Coordinated Control of Hybrid Energy Storage Systems for Denoising and Flattening Wind Power Output]]>
http://www.mdpi.com/1996-1073/7/10/6620
Since the penetration level of wind energy is continuously increasing, the negative impact caused by the fluctuation of wind power output needs to be carefully managed. This paper proposes a novel real-time coordinated control algorithm based on a wavelet transform to mitigate both short-term and long-term fluctuations by using a hybrid energy storage system (HESS). The short-term fluctuation is eliminated by using an electric double-layer capacitor (EDLC), while the wind-HESS system output is kept constant during each 10-min period by a Ni-MH battery (NB). State-of-charge (SOC) control strategies for both EDLC and NB are proposed to maintain the SOC level of storage within safe operating limits. A ramp rate limitation (RRL) requirement is also considered in the proposed algorithm. The effectiveness of the proposed algorithm has been tested by using real time simulation. The simulation model of the wind-HESS system is developed in the real-time digital simulator (RTDS)/RSCAD environment. The proposed algorithm is also implemented as a user defined model of the RSCAD. The simulation results demonstrate that the HESS with the proposed control algorithm can indeed assist in dealing with the variation of wind power generation. Moreover, the proposed method shows better performance in smoothing out the fluctuation and managing the SOC of battery and EDLC than the simple moving average (SMA) based method.Energies2014-10-16710Article10.3390/en7106620662066441996-10732014-10-16doi: 10.3390/en7106620Tran TrungSeon-Ju AhnJoon-Ho ChoiSeok-Il GoSoon-Ryul Nam<![CDATA[Energies, Vol. 7, Pages 6593-6619: Control Strategies to Smooth Short-Term Power Fluctuations in Large Photovoltaic Plants Using Battery Storage Systems]]>
http://www.mdpi.com/1996-1073/7/10/6593
The variations in irradiance produced by changes in cloud cover can cause rapid fluctuations in the power generated by large photovoltaic (PV) plants. As the PV power share in the grid increases, such fluctuations may adversely affect power quality and reliability. Thus, energy storage systems (ESS) are necessary in order to smooth power fluctuations below the maximum allowable. This article first proposes a new control strategy (step-control), to improve the results in relation to two state-of-the-art strategies, ramp-rate control and moving average. It also presents a method to quantify the storage capacity requirements according to the three different smoothing strategies and for different PV plant sizes. Finally, simulations shows that, although the moving-average (MA) strategy requires the smallest capacity, it presents more losses (2–3 times more) and produces a much higher number of cycles over the ESS (around 10 times more), making it unsuitable with storage technologies as lithium-ion. The step-control shown as a better option in scenery with exigent ramp restrictions (around 2%/min) and distributed generation against the ramp-rate control in all ESS key aspects: 20% less of capacity, up to 30% less of losses and a 40% less of ageing. All the simulations were based on real PV production data, taken every 5 s in the course of one year (2012) from a number of systems with power outputs ranging from 550 kW to 40 MW.Energies2014-10-16710Article10.3390/en7106593659366191996-10732014-10-16doi: 10.3390/en7106593Javier MarcosIñigo de la ParraMiguel GarcíaLuis Marroyo<![CDATA[Energies, Vol. 7, Pages 6571-6592: Modeling and Control of a Parallel Waste Heat Recovery System for Euro-VI Heavy-Duty Diesel Engines]]>
http://www.mdpi.com/1996-1073/7/10/6571
This paper presents the modeling and control of a waste heat recovery systemfor a Euro-VI heavy-duty truck engine. The considered waste heat recovery system consists of two parallel evaporators with expander and pumps mechanically coupled to the engine crankshaft. Compared to previous work, the waste heat recovery system modeling is improved by including evaporator models that combine the finite difference modeling approach with a moving boundary one. Over a specific cycle, the steady-state and dynamic temperature prediction accuracy improved on average by 2% and 7%. From a control design perspective, the objective is to maximize the waste heat recovery system output power.However, for safe system operation, the vapor state needs to be maintained before the expander under highly dynamic engine disturbances. To achieve this, a switching model predictive control strategy is developed. The proposed control strategy performance is demonstrated using the high-fidelity waste heat recovery system model subject to measured disturbances from an Euro-VI heavy-duty diesel engine. Simulations are performed usinga cold-start World Harmonized Transient cycle that covers typical urban, rural and highway driving conditions. The model predictive control strategy provides 15% more time in vaporand recovered thermal energy than a classical proportional-integral (PI) control strategy. In the case that the model is accurately known, the proposed control strategy performance can be improved by 10% in terms of time in vapor and recovered thermal energy. This is demonstrated with an offline nonlinear model predictive control strategy.Energies2014-10-14710Article10.3390/en7106571657165921996-10732014-10-14doi: 10.3390/en7106571Emanuel FeruFrank WillemsBram de JagerMaarten Steinbuch<![CDATA[Energies, Vol. 7, Pages 6558-6570: Does a Change in Price of Fuel Affect GDP Growth? An Examination of the U.S. Data from 1950–2013]]>
http://www.mdpi.com/1996-1073/7/10/6558
We examined data on fuel consumption and costs for the years 1950 through 2013, along with economic and population data, to determine the percent of U.S. gross domestic product (GDP) spent each year on fuels, including fossil fuels and nuclear ore, and the growth of the economy. We found that these variables are inversely correlated. This suggests that the availability and cost of energy is a significant determinant of economic performance. We believe this relation is consistent with analyses based on the energy return on investment (EROI) concept in that increasingly scarce, and hence expensive, fuels are a drag on economic growth. The best-fitting linear equation relating the percent of GDP (energy cost share) and year-over-year (YoY) GDP change variables suggests that a threshold exists in the vicinity of 4%; if the percent of GDP spent on fuels is greater than this, poorer economic performance has been likely. Currently, about 5% of GDP is spent on fuels; most of this is for liquids. Continued weak economic performance appears likely unless improvements in energy efficiency, on the order of a factor of 3 for liquid fuels, and/or a more rapid adoption of renewable or nuclear energy sources can be achieved, provided that the EROI of these new sources proves to be sufficiently high.Energies2014-10-14710Article10.3390/en7106558655865701996-10732014-10-14doi: 10.3390/en7106558Michael AucottCharles Hall<![CDATA[Energies, Vol. 7, Pages 6549-6557: Development and Characterization of an Electrically Rechargeable Zinc-Air Battery Stack]]>
http://www.mdpi.com/1996-1073/7/10/6549
An electrically rechargeable zinc-air battery stack consisting of three single cells in series was designed using a novel structured bipolar plate with air-breathing holes. Alpha-MnO2 and LaNiO3 severed as the catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The anodic and cathodic polarization and individual cell voltages were measured at constant charge-discharge (C-D) current densities indicating a uniform voltage profile for each single cell. One hundred C-D cycles were carried out for the stack. The results showed that, over the initial 10 cycles, the average C-D voltage gap was about 0.94 V and the average energy efficiency reached 89.28% with current density charging at 15 mA·cm−2 and discharging at 25 mA·cm−2. The total increase in charging voltage over the 100 C-D cycles was ~1.56% demonstrating excellent stability performance. The stack performance degradation was analyzed by galvanostatic electrochemical impedance spectroscopy. The charge transfer resistance of ORR increased from 1.57 to 2.21 Ω and that of Zn/Zn2+ reaction increased from 0.21 to 0.34 Ω after 100 C-D cycles. The quantitative analysis guided the potential for the optimization of both positive and negative electrodes to improve the cycle life of the cell stack.Energies2014-10-13710Article10.3390/en7106549654965571996-10732014-10-13doi: 10.3390/en7106549Hongyun MaBaoguo WangYongsheng FanWeichen Hong<![CDATA[Energies, Vol. 7, Pages 6527-6548: Transient Thunderstorm Downbursts and Their Effects on Wind Turbines]]>
http://www.mdpi.com/1996-1073/7/10/6527
The International Electrotechnical Commission (IEC) Standard 61400-1 for the design of wind turbines does not explicitly address site-specific conditions associated with anomalous atmospheric events or conditions. Examples of off-standard atmospheric conditions include thunderstorm downbursts, hurricanes, tornadoes, low-level jets, etc. The simulation of thunderstorm downbursts and associated loads on a utility-scale wind turbine is the focus of this study. Since the problem has not received sufficient attention, especially in terms of design, we thus focus in this paper on practical aspects. A wind field model that incorporates component non-turbulent and turbulent parts is described and employed in inflow simulations. The non-turbulent part is based on an available analytical model with some modifications, while the turbulent part is simulated as a stochastic process using standard turbulence power spectral density functions and coherence functions whose defining parameters are related to the downburst characteristics such as the storm translation velocity. Available information on recorded downbursts is used to define two storm scenarios that are studied. Rotor loads are generated using stochastic simulation of the aeroelastic response of a model of a utility-scale 5-MW turbine. An illustrative single storm simulation and the associated turbine response are used to discuss load characteristics and to highlight storm-related and environmental parameters of interest. Extensive simulations for two downbursts are then conducted while varying the storm’s location and track relative to the turbine. Results suggest that wind turbine yaw and pitch control systems clearly influence overall system response. Results also highlight the important effects of both the turbulence as well as the downburst mean wind profiles on turbine extreme loads.Energies2014-10-13710Article10.3390/en7106527652765481996-10732014-10-13doi: 10.3390/en7106527Hieu NguyenLance Manuel<![CDATA[Energies, Vol. 7, Pages 6509-6526: Opportunities for Energy Crop Production Based on Subfield Scale Distribution of Profitability]]>
http://www.mdpi.com/1996-1073/7/10/6509
Incorporation of dedicated herbaceous energy crops into row crop landscapes is a promising means to supply an expanding biofuel industry while benefiting soil and water quality and increasing biodiversity. Despite these positive traits, energy crops remain largely unaccepted due to concerns over their practicality and cost of implementation. This paper presents a case study for Hardin County, Iowa, to demonstrate how subfield decision making can be used to target candidate areas for conversion to energy crop production. Estimates of variability in row crop production at a subfield level are used to model the economic performance of corn (Zea mays L.) grain and the environmental impacts of corn stover collection using the Landscape Environmental Analysis Framework (LEAF). The strategy used in the case study integrates switchgrass (Panicum virgatum L.) into subfield landscape positions where corn grain is modeled to return a net economic loss. Results show that switchgrass integration has the potential to increase sustainable biomass production from 48% to 99% (depending on the rigor of conservation practices applied to corn stover collection), while also improving field level profitability of corn. Candidate land area is highly sensitive to grain price (0.18 to 0.26 $·kg−1) and dependent on the acceptable subfield net loss for corn production (ranging from 0 to −1000 $·ha−1) and the ability of switchgrass production to meet or exceed this return. This work presents the case that switchgrass may be economically incorporated into row crop landscapes when management decisions are applied at a subfield scale within field areas modeled to have a negative net profit with current management practices.Energies2014-10-13710Article10.3390/en7106509650965261996-10732014-10-13doi: 10.3390/en7106509Ian BonnerKara CaffertyDavid MuthMark TomerDavid JamesSarah PorterDouglas Karlen<![CDATA[Energies, Vol. 7, Pages 6492-6508: Prognostics of Lithium-Ion Batteries Based on Battery Performance Analysis and Flexible Support Vector Regression]]>
http://www.mdpi.com/1996-1073/7/10/6492
Accurate prediction of the remaining useful life (RUL) of lithium-ion batteries is important for battery management systems. Traditional empirical data-driven approaches for RUL prediction usually require multidimensional physical characteristics including the current, voltage, usage duration, battery temperature, and ambient temperature. From a capacity fading analysis of lithium-ion batteries, it is found that the energy efficiency and battery working temperature are closely related to the capacity degradation, which account for all performance metrics of lithium-ion batteries with regard to the RUL and the relationships between some performance metrics. Thus, we devise a non-iterative prediction model based on flexible support vector regression (F-SVR) and an iterative multi-step prediction model based on support vector regression (SVR) using the energy efficiency and battery working temperature as input physical characteristics. The experimental results show that the proposed prognostic models have high prediction accuracy by using fewer dimensions for the input data than the traditional empirical models.Energies2014-10-10710Article10.3390/en7106492649265081996-10732014-10-10doi: 10.3390/en7106492Shuai WangLingling ZhaoXiaohong SuPeijun Ma<![CDATA[Energies, Vol. 7, Pages 6477-6491: Voltage Control Scheme with Distributed Generation and Grid Connected Converter in a DC Microgrid]]>
http://www.mdpi.com/1996-1073/7/10/6477
Direct Current (DC) microgrids are expected to become larger due to the rapid growth of DC energy sources and power loads. As the scale of the system expends, the importance of voltage control will be increased to operate power systems stably. Many studies have been performed on voltage control methods in a DC microgrid, but most of them focused only on a small scale microgrid, such as a building microgrid. Therefore, a new control method is needed for a middle or large scale DC microgrid. This paper analyzes voltage drop problems in a large DC microgrid and proposes a cooperative voltage control scheme with a distributed generator (DG) and a grid connected converter (GCC). For the voltage control with DGs, their location and capacity should be considered for economic operation in the systems. Accordingly, an optimal DG allocation algorithm is proposed to minimize the capacity of a DG for voltage control in DC microgrids. The proposed methods are verified with typical load types by a simulation using MATLAB and PSCAD/EMTDC.Energies2014-10-10710Article10.3390/en7106477647764911996-10732014-10-10doi: 10.3390/en7106477Jong-Chan ChoiHo-Yong JeongJin-Young ChoiDong-Jun WonSeon-Ju AhnSeung-il Moon<![CDATA[Energies, Vol. 7, Pages 6459-6476: Performance Evaluation of an Anti-Lock Braking System for Electric Vehicles with a Fuzzy Sliding Mode Controller]]>
http://www.mdpi.com/1996-1073/7/10/6459
Traditional friction braking torque and motor braking torque can be used in braking for electric vehicles (EVs). A sliding mode controller (SMC) based on the exponential reaching law for the anti-lock braking system (ABS) is developed to maintain the optimal slip value. Parameter optimizing is applied to the reaching law by fuzzy logic control (FLC). A regenerative braking algorithm, in which the motor torque is taken full advantage of, is adopted to distribute the braking force between the motor braking and the hydraulic braking. Simulations were carried out with Matlab/Simulink. By comparing with a conventional Bang-bang ABS controller, braking stability and passenger comfort is improved with the proposed SMC controller, and the chatting phenomenon is reduced effectively with the parameter optimizing by FLC. With the increasing proportion of the motor braking torque, the tracking of the slip ratio is more rapid and accurate. Furthermore, the braking distance is shortened and the conversion energy is enhanced.Energies2014-10-09710Article10.3390/en7106459645964761996-10732014-10-09doi: 10.3390/en7106459Jingang GuoXiaoping JianGuangyu Lin<![CDATA[Energies, Vol. 7, Pages 6434-6458: An Improved Genetic Algorithm for Optimal Stationary Energy Storage System Locating and Sizing]]>
http://www.mdpi.com/1996-1073/7/10/6434
The application of a stationary ultra-capacitor energy storage system (ESS) in urban rail transit allows for the recuperation of vehicle braking energy for increasing energy savings as well as for a better vehicle voltage profile. This paper aims to obtain the best energy savings and voltage profile by optimizing the location and size of ultra-capacitors. This paper firstly raises the optimization objective functions from the perspectives of energy savings, regenerative braking cancellation and installation cost, respectively. Then, proper mathematical models of the DC (direct current) traction power supply system are established to simulate the electrical load-flow of the traction supply network, and the optimization objections are evaluated in the example of a Chinese metro line. Ultimately, a methodology for optimal ultra-capacitor energy storage system locating and sizing is put forward based on the improved genetic algorithm. The optimized result shows that certain preferable and compromised schemes of ESSs’ location and size can be obtained, acting as a compromise between satisfying better energy savings, voltage profile and lower installation cost.Energies2014-10-09710Article10.3390/en7106434643464581996-10732014-10-09doi: 10.3390/en7106434Bin WangZhongping YangFei LinWei Zhao<![CDATA[Energies, Vol. 7, Pages 6412-6433: A Real-Time Sliding Mode Control for a Wind Energy System Based on a Doubly Fed Induction Generator]]>
http://www.mdpi.com/1996-1073/7/10/6412
In this paper, a real time sliding mode control scheme for a variable speed wind turbine that incorporates a doubly feed induction generator is described. In this design, the so-called vector control theory is applied, in order to simplify the system electrical equations. The proposed control scheme involves a low computational cost and therefore can be implemented in real-time applications using a low cost Digital Signal Processor (DSP). The stability analysis of the proposed sliding mode controller under disturbances and parameter uncertainties is provided using the Lyapunov stability theory. A new experimental platform has been designed and constructed in order to analyze the real-time performance of the proposed controller in a real system. Finally, the experimental validation carried out in the experimental platform shows; on the one hand that the proposed controller provides high-performance dynamic characteristics, and on the other hand that this scheme is robust with respect to the uncertainties that usually appear in the real systems.Energies2014-10-09710Article10.3390/en7106412641264331996-10732014-10-09doi: 10.3390/en7106412Oscar BarambonesJose CortajarenaPatxi AlkortaJose de Durana<![CDATA[Energies, Vol. 7, Pages 6401-6411: Experimental Investigation and Discussion on the Mechanical Endurance Limit of Nafion Membrane Used in Proton Exchange Membrane Fuel Cell]]>
http://www.mdpi.com/1996-1073/7/10/6401
As a solution of high efficiency and clean energy, fuel cell technologies, especially proton exchange membrane fuel cell (PEMFC), have caught extensive attention. However, after decades of development, the performances of PEMFCs are far from achieving the target from the Department of Energy (DOE). Thus, further understanding of the degradation mechanism is needed to overcome this obstacle. Due to the importance of proton exchange membrane in a PEMFC, the degradation of the membrane, such as hygrothermal aging effect on its properties, are particularly necessary. In this work, a thick membrane (Nafion N117), which is always used as an ionic polymer for the PEMFCs, has been analyzed. Experimental investigation is performed for understanding the mechanical endurance of the bare membranes under different loading conditions. Tensile tests are conducted to compare the mechanical property evolution of two kinds of bare-membrane specimens including the dog-bone and the deeply double edge notched (DDEN) types. Both dog-bone and DDEN specimens were subjected to a series of degradation tests with different cycling times and wide humidity ranges. The tensile tests are repeated for both kinds of specimens to assess the strain-stress relations. Furthermore, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Scanning electron microscope (SEM) observation and water absorption measurement were conducted to speculate the cause of this variation. The initial cracks along with the increasing of bound water content were speculated as the primary cause.Energies2014-10-09710Article10.3390/en7106401640164111996-10732014-10-09doi: 10.3390/en7106401Yang XiaoChongdu Cho<![CDATA[Energies, Vol. 7, Pages 6382-6400: Performance Evaluation of a Continuous Operation Adsorption Chiller Powered by Solar Energy Using Silica Gel and Water as the Working Pair]]>
http://www.mdpi.com/1996-1073/7/10/6382
In the present study, dynamic analysis and performance evaluation of a solar-powered continuous operation adsorption chiller are introduced. The adsorption chiller uses silica gel and water as the working pair. The developed mathematical model represents the heat and mass transfer within the reactor coupled with the energy balance of the collector plate and the glass cover. Moreover, a non-equilibrium adsorption kinetic model is taken into account by using the linear driving force equation. The variation of solar radiation, wind speed, and atmospheric temperature along a complete cycle are considered for a more realistic simulation. Based on the case studied and the baseline parameters, the chiller is found to acquire a coefficient of performance of 0.402. The average thermal efficiency of the solar collector is estimated to be 62.96% and the average total efficiency approaches a value of 50.91%. Other performance parameters obtained are 363.8 W and 1.82 W/kg for the cooling capacity and the specific cooling power of the chiller, respectively. Furthermore, every 1 kg of silica gel inside the adsorption reactor produces a daily chilled water mass of 3 kg at a temperature of 10 ◦C. In addition, the cooling system harnesses 25.35% of the total available solar radiation and converts it to a cooling effect.Energies2014-10-09710Article10.3390/en7106382638264001996-10732014-10-09doi: 10.3390/en7106382Hassan Hassan