http://www.mdpi.com/journal/energies Latest open access articles published in Energies at http://www.mdpi.com/journal/energies http://www.mdpi.com/1996-1073/6/5/2709 This paper presents a model-based cell-health-conscious thermal energy management method. An Arrhenius equation-based mathematical model is firstly identified to quantify the effect of temperature on the cell lifetime of a Nickel Metal Hydride (NiMH) battery pack. The cell aging datasets collected under multiple ambient temperatures are applied to extract the Arrhenius equation parameters. The model is then used as an assessment criterion and guidance for the thermal management design of battery packs. The feasibility and applicability of a pack structure with its cooling system, is then evaluated, and its design problems are studied by a computational fluid dynamics (CFD) analysis. The performance and eligibility of the design method is validated by both CFD simulations and experiments. Energies 2013-05-22 6 5 Article 10.3390/en6052709 2709 2725 1996-1073 2013-05-22 doi: 10.3390/en6052709 Yalian Yang Xiaosong Hu Datong Qing Fangyuan Chen http://www.mdpi.com/1996-1073/6/5/2686 In the first part of this work, combined heat and power (CHP) criteria pertaining to energy, exergy, environmental (pollutant emission) and economic aspects, have been investigated and compared. Although the constraints in legislation usually refer to energy efficiency, primary energy savings and greenhouse gas savings, other criteria should also be taken into account in order to obtain a better evaluation of a cogeneration plant. Here particular attention has been paid to saving indexes for both an individual CHP-unit and for a CHP-system, that is the complete system with all the cogeneration units and the auxiliary plants necessary to cover the users’ demand. Five indexes, named potential indexes, have been introduced to evaluate the cogeneration potential: one for energy saving, one for exergy, two for environmental aspects (global and local scale) and one for economic aspects. Finally, some indexes analysed in the paper have been applied to a case study concerning a district heating cogeneration system, and the different behaviour of the energy-exergy, environmental and economic aspects has been discussed. Energies 2013-05-22 6 5 Article 10.3390/en6052686 2686 2708 1996-1073 2013-05-22 doi: 10.3390/en6052686 Marco Torchio http://www.mdpi.com/1996-1073/6/5/2663 The promotion of hybrid and electric vehicles (EVs) has been proposed as one promising solution for reducing transport energy consumption and mitigating vehicular emissions in China. In this study, the energy and environmental impacts of hybrid and EVs during 2010–2020 were evaluated through an energy conversion analysis and a life cycle assessment (LCA), and the per-kilometer energy consumptions of gasoline, coal, natural gas (NG), oil, biomass, garbage and electricity for EVs and HEVs were estimated. Results show that the EVs and HEVs can reduce the energy consumption of vehicles by national average ratios of 17%–19% and 30%–33%, respectively. The study also calculated the detailed emission factors of SO2, NOX, VOC, CO, NH3, PM10, PM2.5, OC, EC, CO2, N2O, CH4, Pb and Hg. It is indicated that the HEVs can bring significant reductions of NOX, VOC and CO emissions and lesser decreases of SO2 and CO2 for a single vehicle. The EVs could decrease many of the VOC, NH3, CO and CO2 emissions, but increase the SO2, NOX and particles by 10.8–13.0, 2.7–2.9 and 3.6–11.5 times, respectively. In addition, the electricity sources had significant influence on energy consumption (EC) and emissions. A high proportion of coal-fired energy resulted in large ECs and emission factors. The total energy consumption and pollutants emission changes in 2015 and 2020 were also calculated. Based on the energy use and emission analysis of HEVs and EVs, it is suggested that EVs should be promoted in the regions with higher proportions of hydropower, natural gas-fired power and clean energy power, while HEVs can be widely adopted in the regions with high coal-fired power ratios. This is to achieve a higher energy consumption reduction and pollutant emission mitigation. Moreover, the results can also provide scientific support for the total amount control of regional air pollutants in China. Energies 2013-05-22 6 5 Article 10.3390/en6052663 2663 2685 1996-1073 2013-05-22 doi: 10.3390/en6052663 Jianlei Lang Shuiyuan Cheng Ying Zhou Beibei Zhao Haiyan Wang Shujing Zhang http://www.mdpi.com/1996-1073/6/5/2644 This paper compares the greenhouse gas (GHG) emissions of natural gas (NG)- based fuels to the GHG emissions of electric vehicles (EVs) powered with NG-to-electricity in China. A life-cycle model is used to account for full fuel cycle and use-phase emissions, as well as vehicle cycle and battery manufacturing. The reduction of life-cycle GHG emissions of EVs charged by electricity generated from NG, without utilizing carbon dioxide capture and storage (CCS) technology can be 36%–47% when compared to gasoline vehicles. The large range change in emissions reduction potential is driven by the different generation technologies that could in the future be used to generate electricity in China. When CCS is employed in power plants, the GHG emission reductions increase to about 71%–73% compared to gasoline vehicles. It is found that compressed NG (CNG) and liquefied NG (LNG) fuels can save about 10% of carbon as compared to gasoline vehicles. However, gas-to-liquid (GTL) fuel made through the Fischer-Tropsch method will likely lead to a life-cycle GHG emissions increase, potentially 3%–15% higher than gasoline, but roughly equal to petroleum-based diesel. When CCS is utilized, the GTL fueled vehicles emit roughly equal GHG emissions to petroleum-based diesel fuel high-efficient hybrid electric vehicle from the life-cycle perspective. Energies 2013-05-22 6 5 Article 10.3390/en6052644 2644 2662 1996-1073 2013-05-22 doi: 10.3390/en6052644 Xunmin Ou Xiliang Zhang Xu Zhang Qian Zhang http://www.mdpi.com/1996-1073/6/5/2624 This paper proposes a new model for short-term forecasting of electric energy production in a photovoltaic (PV) plant. The model is called HIstorical SImilar MIning (HISIMI) model; its final structure is optimized by using a genetic algorithm, based on data mining techniques applied to historical cases composed by past forecasted values of weather variables, obtained from numerical tools for weather prediction, and by past production of electric power in a PV plant. The HISIMI model is able to supply spot values of power forecasts, and also the uncertainty, or probabilities, associated with those spot values, providing new useful information to users with respect to traditional forecasting models for PV plants. Such probabilities enable analysis and evaluation of risk associated with those spot forecasts, for example, in offers of energy sale for electricity markets. The results of spot forecasting of an illustrative example obtained with the HISIMI model for a real-life grid-connected PV plant, which shows high intra-hour variability of its actual power output, with forecasting horizons covering the following day, have improved those obtained with other two power spot forecasting models, which are a persistence model and an artificial neural network model. Energies 2013-05-22 6 5 Article 10.3390/en6052624 2624 2643 1996-1073 2013-05-22 doi: 10.3390/en6052624 Claudio Monteiro Tiago Santos L. Fernandez-Jimenez Ignacio Ramirez-Rosado M. Terreros-Olarte http://www.mdpi.com/1996-1073/6/5/2605 Broiler production in modern poultry farms commonly uses mechanical ventilation systems. This mechanical ventilation requires an amount of electric energy and a high level of investment in technology. Nevertheless, broiler production is affected by periodic problems of mortality because of thermal stress, thus being crucial to explore the ventilation efficiency. In this article, we analyze a cross-mechanical ventilation system focusing on air velocity distribution. In this way, two methodologies were used to explore indoor environment in livestock buildings: Computational Fluid Dynamics (CFD) simulations and direct measurements for verification and validation (V&V) of CFD. In this study, a validation model using a Generalized Linear Model (GLM) was conducted to compare these methodologies. The results showed that both methodologies were similar in results: the average of air velocities values were 0.60 ± 0.56 m s−1 for CFD and 0.64 ± 0.54 m s−1 for direct measurements. In conclusion, the air velocity was not affected by the methodology (CFD or direct measurements), and the CFD simulations were therefore validated to analyze indoor environment of poultry farms and its operations. A better knowledge of the indoor environment may contribute to reduce the demand of electric energy, increasing benefits and improving the thermal comfort of broilers. Energies 2013-05-21 6 5 Article 10.3390/en6052605 2605 2623 1996-1073 2013-05-21 doi: 10.3390/en6052605 Eliseo Bustamante Fernando-Juan García-Diego Salvador Calvet Fernando Estellés Pedro Beltrán Antonio Hospitaler Antonio Torres http://www.mdpi.com/1996-1073/6/5/2583 Battery model identification is very important for reliable battery management as well as for battery system design process. The common problem in identifying battery models is how to determine the most appropriate mathematical model structure and parameterized coefficients based on the measured terminal voltage and current. This paper proposes a novel semiparametric approach using the wavelet-based partially linear battery model (PLBM) and a recursive penalized wavelet estimator for online battery model identification. Three main contributions are presented. First, the semiparametric PLBM is proposed to simulate the battery dynamics. Compared with conventional electrical models of a battery, the proposed PLBM is equipped with a semiparametric partially linear structure, which includes a parametric part (involving the linear equivalent circuit parameters) and a nonparametric part [involving the open-circuit voltage (OCV)]. Thus, even with little prior knowledge about the OCV, the PLBM can be identified using a semiparametric identification framework. Second, we model the nonparametric part of the PLBM using the truncated wavelet multiresolution analysis (MRA) expansion, which leads to a parsimonious model structure that is highly desirable for model identification; using this model, the PLBM could be represented in a linear-in-parameter manner. Finally, to exploit the sparsity of the wavelet MRA representation and allow for online implementation, a penalized wavelet estimator that uses a modified online cyclic coordinate descent algorithm is proposed to identify the PLBM in a recursive fashion. The simulation and experimental results demonstrate that the proposed PLBM with the corresponding identification algorithm can accurately simulate the dynamic behavior of a lithium-ion battery in the Federal Urban Driving Schedule tests. Energies 2013-05-21 6 5 Article 10.3390/en6052583 2583 2604 1996-1073 2013-05-21 doi: 10.3390/en6052583 Dazhong Mu Jiuchun Jiang Caiping Zhang http://www.mdpi.com/1996-1073/6/5/2562 Wave energy conversion is a clean electric power production technology. During operation there are no emissions in the form of harmful gases. However there are unsolved issues considering environmental impacts such as: electromagnetism; the artificial reef effect and underwater noise. Anthropogenic noise is increasing in the oceans worldwide and wave power will contribute to this sound pollution in the oceans; but to what extent? The main purpose of this study was to examine the noise emitted by a full scale operating Wave Energy Converter (WEC) in the Lysekil project at Uppsala University in Sweden. A minor review of the hearing capabilities of fish and marine mammals is presented to aid in the conclusions of impact from anthropogenic sound. A hydrophone was deployed to the seabed in the Lysekil research site park at distance of 20 and 40 m away from two operational WECs. The measurements were performed in the spring of 2011. The results showed that the main noise was a transient noise with most of its energy in frequencies below 1 kHz. These results indicate that several marine organisms (fish and mammals) will be able to hear the operating WECs of a distance of at least 20 m. Energies 2013-05-21 6 5 Article 10.3390/en6052562 2562 2582 1996-1073 2013-05-21 doi: 10.3390/en6052562 Kalle Haikonen Jan Sundberg Mats Leijon http://www.mdpi.com/1996-1073/6/5/2541 This paper presents a coordinated control method for a doubly-fed induction generator (DFIG)-based wind-power generation system with a series grid-side converter (SGSC) under distorted grid voltage conditions. The detailed mathematical models of the DFIG system with SGSC are developed in the multiple synchronous rotating reference frames. In order to counteract the adverse effects of the voltage harmonics upon the DFIG, the SGSC generates series compensation control voltages to keep the stator voltage sinusoidal and symmetrical, which allows the use of the conventional vector control strategy for the rotor-side converter (RSC), regardless of grid voltage harmonics. Meanwhile, two control targets for the parallel grid-side converter (PGSC) are identified, including eliminating the oscillations in total active and reactive power entering the grid or suppressing the fifth- and seventh-order harmonic currents injected to the grid. Furthermore, the respective PI-R controller in the positive synchronous reference frame for the SGSC voltage control and PGSC current control have been developed to achieve precise and rapid regulation of the corresponding components. Finally, the proposed coordinated control strategy has been fully validated by the simulation results of a 2 MW DFIG-based wind turbine with SGSC under distorted grid voltage conditions. Energies 2013-05-17 6 5 Article 10.3390/en6052541 2541 2561 1996-1073 2013-05-17 doi: 10.3390/en6052541 Jun Yao Qing Li Zhe Chen Aolin Liu http://www.mdpi.com/1996-1073/6/5/2521 This paper studies the change in Energy Intensity (EI) of the main economic activities in the EU15 countries, which represents approximately 45% of their final energy consumption. The purpose is, first, to measure the different patterns between the countries by establishing differentiated typologies, and second, to investigate those reasons that explain the different trends by country. To attain our objective, the changes in EI are decomposed into their structural and efficiency components for EU15 countries for the period 1991–2005. Results reveal four different typologies for this set of countries, and show the importance of identifying those economic activities which, due to their special impact, are key to reducing energy consumption. The changes in the structural component are due mainly to a transformative process in which the importance of industry in the economy as a whole drops, while the opposite holds for services. However, the changes in the efficiency component do not seem to be linked to this same process. It does not appear as though the services sector resulted in a more efficient use of final energy. We have detected significant evidence of convergence for EI in the service sector that would help to understand the recent worsen evolution of EI in this sector (and in overall EI) of Southern European countries. It can also be concluded that an analysis of global EI change without distinguishing among its components can result in misleading conclusions and in improperly conceived Energy Policies. Energies 2013-05-16 6 5 Article 10.3390/en6052521 2521 2540 1996-1073 2013-05-16 doi: 10.3390/en6052521 Gustavo Marrero Francisco Ramos-Real http://www.mdpi.com/1996-1073/6/5/2501 There has been a resurgence of interest in the development of vertical axis wind turbines which have several inherent attributes that offer some advantages for offshore operations, particularly their scalability and low over-turning moments with better accessibility to drivetrain components. This paper describes an aerodynamic performance model for vertical axis wind turbines specifically developed for the design of a novel offshore V-shaped rotor with multiple aerodynamic surfaces. The model is based on the Double-Multiple Streamtube method and includes a number of developments for alternative complex rotor shapes. The paper compares predicted results with measured field data for five different turbines with both curved and straight blades and rated powers in the range 100–500 kW. Based on these comparisons, the paper proposes modifications to the Gormont dynamic stall model that gives improved predictions of rotor power for the turbines considered. Energies 2013-05-15 6 5 Article 10.3390/en6052501 2501 2520 1996-1073 2013-05-15 doi: 10.3390/en6052501 Andrew Shires http://www.mdpi.com/1996-1073/6/5/2481 The future success of wave energy in the renewable energy mix depends on the technical advancements of the specific components and systems, on the grid access availability and, ultimately, on the economical profitability of the investment. Small and remote islands represent an ideal framework for wave energy exploitation, due both to resource availability and to the current high cost of electricity that mostly relies on diesel generation. Energy storage can be the enabling technology to match the intermittent power generation from waves to the energy needs of the local community. In this paper real data from La Palma, in the Canary Islands, are used as a basis for the considered test case. As a first step the study quantifies the expected power production from Wave Energy Converter (WEC) arrays, based on data from the Lifesaver point absorber developed by Fred. Olsen. Then, a stochastic optimization approach is applied to evaluate the convenience of energy storage introduction for reducing the final cost of energy and to define the corresponding optimal rating of the storage devices. Energies 2013-05-15 6 5 Article 10.3390/en6052481 2481 2500 1996-1073 2013-05-15 doi: 10.3390/en6052481 Elisabetta Tedeschi Jonas Sjolte Marta Molinas Maider Santos http://www.mdpi.com/1996-1073/6/5/2468 A complete set of scaling criteria for gas hydrate reservoir of five-spot well system case is derived from the 3D governing equations, involving the mass balance equation, the energy balance equation, the kinetic model, the endothermic model and the phase equilibrium model. In the scaling criteria, the key parameters of the experiment are the water/gas production rates, the water injection rate, and the production time. By using the scaling criteria, the experimental results can be enlarged to a field scale. Therefore, the experimental results and the scaling criteria could be used to evaluate the hydrate dissociation strategies and the gas production potential of the hydrate reservoir. Energies 2013-05-13 6 5 Article 10.3390/en6052468 2468 2480 1996-1073 2013-05-13 doi: 10.3390/en6052468 Yi Wang Chun-Gang Xu Xiao-Sen Li Gang Li Zhao-Yang Chen http://www.mdpi.com/1996-1073/6/5/2446 Combined Cooling, Heating and Power (CCHP) systems have been widely used in different kinds of buildings to make better use of fuels because of their high overall efficiency. This paper presents a mathematical analysis of a CCHP system in comparison to a Heating, Ventilation and Air Conditioning (HVAC) system. The operation strategies following electric load (FEL), thermal load (FTL) and a hybrid electric-thermal load (FHL) are proposed and investigated in this study. Criteria, namely primary energy saving (PES), exergy efficiency (ηexergy), and CO2 emission reduction (CER) are defined to evaluate the performances of CCHP systems for a hypothetical building located in Dalian (China). The results indicate that: (1) a new mathematical foundation is established to find whether the recovered thermal energy and the amount of electricity generated by the power generation unit (PGU) are enough to provide the energy required; (2) the CCHP system does not always perform better than a HVAC system from an instantaneous perspective, especially in FTL mode; (3) the CCHP system in FEL operation mode can be seen as a suitable energy system in Dalian from the annual performance perspective. Furthermore, a sensitivity analysis is presented in order to show how the performances vary due to the changes of various technical variables. Energies 2013-05-08 6 5 Article 10.3390/en6052446 2446 2467 1996-1073 2013-05-08 doi: 10.3390/en6052446 Miao Li Hailin Mu Huanan Li http://www.mdpi.com/1996-1073/6/5/2428 In the present study, a fuel cell driven ground source heat pump (GSHP) system is applied in a community building and heat pump system performance is analyzed by computational methods. Conduction heat transfer between the brine pipe and ground is analyzed by TEACH code in order to predict the performance of the heat pump system. The predicted coefficient of performance (COP) of the heat pump system and the energy cost were compared with the variation of the location of the objective building, the water saturation rate of the soil, and the driven powers of the heat pump system. Compared to the late-night electricity driven system, a significant reduction of energy cost can be accomplished by employing the fuel cell driven heat pump system. This is due to the low cost of electricity production of the fuel cell system and to the application of the recovered waste heat generated during the electricity production process to the heating of the community building. Energies 2013-05-07 6 5 Article 10.3390/en6052428 2428 2445 1996-1073 2013-05-07 doi: 10.3390/en6052428 Jae-Ki Byun Dong-Hwa Jeong Young-Don Choi Jong-Keun Shin http://www.mdpi.com/1996-1073/6/5/2407 This paper presents current research on Danish municipalities’ use of Energy Service Companies (ESCO) as a way to improve the standard of public buildings and to increase energy efficiency. In recent years more and more municipalities have used ESCO-contracts to retrofit existing public buildings, and to make them more energy efficient. At the moment 30 municipalities (of the 98 municipalities in Denmark) are involved in, or preparing, ESCO contracts. Nevertheless, ESCO-contracting still faces many challenges on the Danish market, as there is a widespread skepticism towards the concept amongst many stakeholders. The purpose of this paper is to discuss the various experience gained so far by municipalities use of ESCO-contracting, the different approached to ESCO-contracting being used in practice, as well as the different viewpoints drivers and barriers behind the development. The strong growth in ESCO-contracts reflects that the ESCO-concept fits well with a number of present problems that municipalities are facing, as well as a flexible adaptation to the local context in different municipalities. Energies 2013-05-06 6 5 Article 10.3390/en6052407 2407 2427 1996-1073 2013-05-06 doi: 10.3390/en6052407 Jesper Jensen Susanne Nielsen Jesper Hansen http://www.mdpi.com/1996-1073/6/5/2386 In the underground coal gasification (UCG) process, cavity growth with crack extension inside the coal seam is an important phenomenon that directly influences gasification efficiency. An efficient and environmentally friendly UCG system also relies upon the precise control and evaluation of the gasification zone. This paper presents details of laboratory studies undertaken to evaluate structural changes that occur inside the coal under thermal stress and to evaluate underground coal-oxygen gasification simulated in an ex-situ reactor. The effects of feed temperature, the direction of the stratified plane, and the inherent microcracks on the coal fracture and crack extension were investigated using some heating experiments performed using plate-shaped and cylindrical coal specimens. To monitor the failure process and to measure the microcrack distribution inside the coal specimen before and after heating, acoustic emission (AE) analysis and X-ray CT were applied. We also introduce a laboratory-scale UCG model experiment conducted with set design and operating parameters. The temperature profiles, AE activities, product gas concentration as well as the gasifier weight lossess were measured successively during gasification. The product gas mainly comprised combustible components such as CO, CH4, and H2 (27.5, 5.5, and 17.2 vol% respectively), which produced a high average calorific value (9.1 MJ/m3). Energies 2013-05-03 6 5 Article 10.3390/en6052386 2386 2406 1996-1073 2013-05-03 doi: 10.3390/en6052386 Faqiang Su Takuya Nakanowataru Ken-ichi Itakura Koutarou Ohga Gota Deguchi http://www.mdpi.com/1996-1073/6/5/2362 In hydropower, the exploitation of small power sources requires the use of small turbines that combine efficiency and economy. Banki-Michell turbines represent a possible choice for their simplicity and for their good efficiency under variable load conditions. Several experimental and numerical tests have already been designed for examining the best geometry and optimal design of cross-flow type machines, but a theoretical framework for a sequential design of the turbine parameters, taking full advantage of recently expanded computational capabilities, is still missing. To this aim, after a review of the available criteria for Banki-Michell parameter design, a novel two-step procedure is described. In the first step, the initial and final blade angles, the outer impeller diameter and the shape of the nozzle are selected using a simple hydrodynamic analysis, based on a very strong simplification of reality. In the second step, the inner diameter, as well as the number of blades and their shape, are selected by testing single options using computational fluid dynamics (CFD) simulations, starting from the suggested literature values. Good efficiency is attained not only for the design discharge, but also for a large range of variability around the design value. Energies 2013-04-29 6 5 Article 10.3390/en6052362 2362 2385 1996-1073 2013-04-29 doi: 10.3390/en6052362 Vincenzo Sammartano Costanza Aricò Armando Carravetta Oreste Fecarotta Tullio Tucciarelli http://www.mdpi.com/1996-1073/6/5/2338 Large-eddy simulation is used to study the influence of free-atmosphere stratification on the structure of atmospheric boundary-layer flow inside and above very large wind farms, as well as the power extracted by the wind turbines. In the simulations, tuning-free Lagrangian scale-dependent dynamic models are used to model the subgrid-scale turbulent fluxes, while the turbine-induced forces are parameterized with an actuator-disk model. It is shown that for a given surface cover (with and without turbines) thermal stratification of the free atmosphere reduces the entrainment from the flow above compared with the unstratified case, leading to lower boundary-layer depth. Due to the fact that in very large wind farms vertical energy transport associated with turbulence is the only source of kinetic energy, lower entrainment leads to lower power production by the wind turbines. In particular, for the wind-turbine arrangements considered in the present work, the power output from the wind farms is reduced by about 35% when the potential temperature lapse rate in the free atmosphere increases from 1 to 10 K/km (within the range of values typically observed in the atmosphere). Moreover, it is shown that the presence of the turbines has significant effect on the growth of the boundary layer. Inspired by the obtained results, a simple one-dimensional model is developed to account for the effect of free-atmosphere stability on the mean flow and the power output from very large wind farms. Energies 2013-04-29 6 5 Article 10.3390/en6052338 2338 2361 1996-1073 2013-04-29 doi: 10.3390/en6052338 Mahdi Abkar Fernando Porté-Agel http://www.mdpi.com/1996-1073/6/5/2319 Based on the logarithmic mean Divisia index (LMDI) approach, this paper presents a decomposition analysis of China’s energy-related industrial CO2 emissions from 1985 to 2007, as well as a comparative analysis of differential influences of various factors on six sectors. Via the decomposition, five categories of influencing factors are included: (1) Per capita GDP (PCG) was the largest positive driving factor for industrial CO2 emissions growth for all sectors in China, with the largest cumulative contribution value; Population (P), economic structure (YS) and energy structure (ES) also played a positive driving role, but with weak contributions. As the only negative inhibiting factor, energy intensity (EI) significantly reduced the energy-related CO2 emissions from industrial sectors. Meanwhile, CO2 emissions reduction based on the efficiency of energy use still held a large space. (2) Various influencing factors imposed differential impacts on CO2 emissions of six sectors. Energies 2013-04-25 6 5 Article 10.3390/en6052319 2319 2337 1996-1073 2013-04-25 doi: 10.3390/en6052319 Liang Chen Zhifeng Yang Bin Chen http://www.mdpi.com/1996-1073/6/4/2305 This paper compares two optimal energy management methods for parallel hybrid electric vehicles using an Automatic Manual Transmission (AMT). A control-oriented model of the powertrain and vehicle dynamics is built first. The energy management is formulated as a typical optimal control problem to trade off the fuel consumption and gear shifting frequency under admissible constraints. The Dynamic Programming (DP) and Pontryagin’s Minimum Principle (PMP) are applied to obtain the optimal solutions. Tuning with the appropriate co-states, the PMP solution is found to be very close to that from DP. The solution for the gear shifting in PMP has an algebraic expression associated with the vehicular velocity and can be implemented more efficiently in the control algorithm. The computation time of PMP is significantly less than DP. Energies 2013-04-22 6 4 Article 10.3390/en6042305 2305 2318 1996-1073 2013-04-22 doi: 10.3390/en6042305 Zou Yuan Liu Teng Sun Fengchun Huei Peng http://www.mdpi.com/1996-1073/6/4/2291 Zirconium dioxide has received particular attention as a fuel matrix because of its ability to form a solid solution with transuranic elements, natural radiation stability and desirable mechanical properties. However, zirconium dioxide has a lower coefficient of thermal conductivity than uranium dioxide and this presents an obstacle to the deployment of these fuels in commercial reactors. Here we show that axial doping of a zirconium dioxide based fuel with erbium reduces power peaking and fuel temperature. Full core simulations of a modified AP1000 core were done using MCNPX 2.7.0. The inert matrix fuel contained 15 w/o transuranics at its beginning of life and constituted 28% of the assemblies in the core. Axial doping reduced power peaking at startup by more than ~23% in the axial direction and reduced the peak to average power within the core from 1.80 to 1.44. The core was able to remain critical between refueling while running at a simulated 2000 MWth on an 18 month refueling cycle. The results show that the reactor would maintain negative core average reactivity and void coefficients during operation. This type of fuel cycle would reduce the overall production of transuranics in a pressurized water reactor by 86%. Energies 2013-04-22 6 4 Article 10.3390/en6042291 2291 2304 1996-1073 2013-04-22 doi: 10.3390/en6042291 Geoff Recktenwald Mark Deinert http://www.mdpi.com/1996-1073/6/4/2262 The paper outlines energy management concepts and the smart grid evolution. The necessity of considering energy management as a crucial innovation in load supplying to permit a more powerful penetration of renewable energy usage at the building and city level and to perform energy savings and CO2 emissions reduction is pointed out. The driving factors to enhance the current power distribution are presented, and the benefits concerning smart grids are underlined. In the paper, a specific energy management analysis is reported by considering all the electric value chain, and the demand-side management and distributed on site control actions are described. To verify the benefit of energy management control actions, a house simulator and a grid simulator are here presented and the results discussed in three different scenarios. Moreover, in the paper, the evaluation of ecological benefits are reported, and a cost benefit analysis of the energy management system is performed. Results pointed out that with the standard control actions, the system is not economic for the end user, and only by using energy management systems with renewable energy, in site production remunerative energy savings can be reached. Finally the evolution of smart grids is presented, focusing on potential benefits and technical problems. The active grids, microgrids and virtual utility are described, and final consideration on hypothetical scenarios is presented in the conclusion. Energies 2013-04-22 6 4 Article 10.3390/en6042262 2262 2290 1996-1073 2013-04-22 doi: 10.3390/en6042262 Rosario Miceli http://www.mdpi.com/1996-1073/6/4/2242 Aeroelastic instability problems have become an increasingly important issue due to the increased use of larger horizontal axis wind turbines. To maintain these large structures in a stable manner, the blade design process should include studies on the dynamic stability of the wind turbine blade. Therefore, fluid-structure interaction analyses of the large-scaled wind turbine blade were performed with a focus on dynamic stability in this study. A finite element method based on the large deflection beam theory is used for structural analysis considering the geometric nonlinearities. For the stability analysis, a proposed aerodynamic approach based on Greenberg’s extension of Theodorsen’s strip theory and blade element momentum method were employed in conjunction with a structural model. The present methods proved to be valid for estimations of the aerodynamic responses and blade behavior compared with numerical results obtained in the previous studies. Additionally, torsional stiffness effects on the dynamic stability of the wind turbine blade were investigated. It is demonstrated that the damping is considerably influenced by variations of the torsional stiffness. Also, in normal operating conditions, the destabilizing phenomena were observed to occur with low torsional stiffness. Energies 2013-04-19 6 4 Article 10.3390/en6042242 2242 2261 1996-1073 2013-04-19 doi: 10.3390/en6042242 Min-Soo Jeong In Lee Seung-Jae Yoo Kwang-Choon Park http://www.mdpi.com/1996-1073/6/4/2221 Due to the variable nature of wind resources, the increasing penetration level of wind power will have a significant impact on the operation and planning of the electric power system. Energy storage systems are considered an effective way to compensate for the variability of wind generation. This paper presents a detailed production cost simulation model to evaluate the economic value of compressed air energy storage (CAES) in systems with large-scale wind power generation. The co-optimization of energy and ancillary services markets is implemented in order to analyze the impacts of CAES, not only on energy supply, but also on system operating reserves. Both hourly and 5-minute simulations are considered to capture the economic performance of CAES in the day-ahead (DA) and real-time (RT) markets. The generalized network flow formulation is used to model the characteristics of CAES in detail. The proposed model is applied on a modified IEEE 24-bus reliability test system. The numerical example shows that besides the economic benefits gained through energy arbitrage in the DA market, CAES can also generate significant profits by providing reserves, compensating for wind forecast errors and intra-hour fluctuation, and participating in the RT market. Energies 2013-04-18 6 4 Article 10.3390/en6042221 2221 2241 1996-1073 2013-04-18 doi: 10.3390/en6042221 Yang Gu James McCalley Ming Ni Rui Bo http://www.mdpi.com/1996-1073/6/4/2204 Frequency Domain Spectroscopy (FDS) is an effective tool allowing assessing the condition of oil-paper insulation system in power equipment. However, results from these measurements are known to be greatly influenced by various parameters, including insulation aging, moisture content, and insulation geometry/volume, together with environmental condition such as temperature. In this contribution, a series of experiments have been performed under controlled laboratory conditions. The dielectric response of the oil impregnated paper, along with the degree of polymerization and moisture content, were monitored. Since dielectric parameters are geometry dependent, poles (independent of the geometry) which depends on resistivity and permittivity, were considered to assess the condition of the insulation. From the investigations performed on new and aged samples, it is shown that poles (P) can be regarded as insulation aging indicator. It is also shown that a per unit value based on the Dielectric Dissipation Factor (DDF), measured in the frequency range from 1 to 1000 Hz can be correlated to moisture content in the insulation paper. Energies 2013-04-18 6 4 Article 10.3390/en6042204 2204 2220 1996-1073 2013-04-18 doi: 10.3390/en6042204 Yazid Hadjadj Fethi Meghnefi Issouf Fofana Hassan Ezzaidi http://www.mdpi.com/1996-1073/6/4/2191 Tidal stream speeds in straits are accelerated because of geographic and bathymetric features. For instance, narrow channels and shallows can cause high tidal stream energy. In this study, water level and tidal current were simulated using a three-dimensional semi-implicit Eulerian-Lagrangian finite-element model to investigate the complex tidal characteristics in the Taiwan Strait and to determine potential locations for harnessing tidal stream energy. The model was driven by nine tidal components (M2, S2, N2, K2, K1, O1, P1, Q1, and M4) at open boundaries. The modeling results were validated with the measured data, including water level and tidal current. Through the model simulations, we found that the highest tidal currents occurred at the Penghu Channel in the Taiwan Strait. The Penghu Channel is an appropriate location for the deployment of a tidal turbine array because of its deep and flat bathymetry. The impacts of energy extraction on hydrodynamics were assessed by considering the momentum sink approach. The simulated results indicate that only minimal impacts would occur on water level and tidal current in the Taiwan Strait if a turbine array (55 turbines) was installed in the Penghu Channel. Energies 2013-04-18 6 4 Article 10.3390/en6042191 2191 2203 1996-1073 2013-04-18 doi: 10.3390/en6042191 Wei-Bo Chen Wen-Cheng Liu Ming-Hsi Hsu http://www.mdpi.com/1996-1073/6/4/2175 A statistical experimental design was employed to optimize factors that affect the production of hydrogen from the glucose contained in pineapple waste extract by anaerobic mixed cultures. Results from Plackett-Burman design indicated that substrate concentration, initial pH and FeSO4 concentration had a statistically significant (p ≤ 0.05) influence on the hydrogen production potential (Ps) and the specific hydrogen production rate (SHPR). The path of steepest ascent was undertaken to approach the optimal region of these three significant factors which was then optimized using response surface methodology (RSM) with central composite design (CCD). The presence of a substrate concentration of 25.76 g-total sugar/L, initial pH of 5.56, and FeSO4 concentration of 0.81 g/L gave a maximum predicted Ps of 5489 mL H2/L, hydrogen yield of 1.83 mol H2/mol glucose, and SHPR of 77.31 mL H2/g-volatile suspended solid (VSS) h. A verification experiment indicated highly reproducible results with the observed Ps and SHPR being only 1.13% and 1.14% different from the predicted values. Energies 2013-04-18 6 4 Article 10.3390/en6042175 2175 2190 1996-1073 2013-04-18 doi: 10.3390/en6042175 Alissara Reungsang Chakkrit Sreela-or http://www.mdpi.com/1996-1073/6/4/2149 Battery management systems (BMS) are a key element in electric vehicle energy storage systems. The BMS performs several functions concerning to the battery system, its key task being balancing the battery cells. Battery cell unbalancing hampers electric vehicles’ performance, with differing individual cell voltages decreasing the battery pack capacity and cell lifetime, leading to the eventual failure of the total battery system. Quite a lot of cell balancing topologies have been proposed, such as shunt resistor, shuttling capacitor, inductor/transformer based and DC energy converters. The shuttling capacitor balancing systems in particular have not been subject to much research efforts however, due to their perceived low balancing speed and high cost. This paper tries to fill this gap by briefly discussing the shuttling capacitor cell balancing topologies, focusing on the single switched capacitor (SSC) cell balancing and proposing a novel procedure to improve the SSC balancing system performance. This leads to a new control strategy for the SSC system that can decrease the balancing system size, cost, balancing time and that can improve the SSC balancing system efficiency. Energies 2013-04-18 6 4 Article 10.3390/en6042149 2149 2174 1996-1073 2013-04-18 doi: 10.3390/en6042149 Mohamed Daowd Mailier Antoine Noshin Omar Peter van den Bossche Joeri van Mierlo http://www.mdpi.com/1996-1073/6/4/2130 In the deregulated energy market, the accuracy of load forecasting has a significant effect on the planning and operational decision making of utility companies. Electric load is a random non-stationary process influenced by a number of factors which make it difficult to model. To achieve better forecasting accuracy, a wide variety of models have been proposed. These models are based on different mathematical methods and offer different features. This paper presents a new two-stage approach for short-term electrical load forecasting based on least-squares support vector machines. With the aim of improving forecasting accuracy, one more feature was added to the model feature set, the next day average load demand. As this feature is unknown for one day ahead, in the first stage, forecasting of the next day average load demand is done and then used in the model in the second stage for next day hourly load forecasting. The effectiveness of the presented model is shown on the real data of the ISO New England electricity market. The obtained results confirm the validity advantage of the proposed approach. Energies 2013-04-18 6 4 Article 10.3390/en6042130 2130 2148 1996-1073 2013-04-18 doi: 10.3390/en6042130 Miloš Božić Miloš Stojanović Zoran Stajić Dragan Tasić http://www.mdpi.com/1996-1073/6/4/2110 Short-term load forecasting (STLF) in buildings differs from its broader counterpart in that the load to be predicted does not seem to be stationary, seasonal and regular but, on the contrary, it may be subject to sudden changes and variations on its consumption behaviour. Classical STLF methods do not react fast enough to these perturbations (i.e., they are not robust) and the literature on building STLF has not yet explored this area. Hereby, we evaluate a well-known post-processing method (Learning Window Reinitialization) applied to two broadly-used STLF algorithms (Autoregressive Model and Support Vector Machines) in buildings to check their adaptability and robustness. We have tested the proposed method with real-world data and our results state that this methodology is especially suited for buildings with non-regular consumption profiles, as classical STLF methods are enough to model regular-profiled ones. Energies 2013-04-17 6 4 Article 10.3390/en6042110 2110 2129 1996-1073 2013-04-17 doi: 10.3390/en6042110 Cruz Borges Yoseba Penya Iván Fernández Juan Prieto Oscar Bretos http://www.mdpi.com/1996-1073/6/4/2084 A digital H∞ controller for a permanent magnet synchronous generator (PMSG) based wind energy conversion system (WECS) is presented. Wind energy is an uncertain fluctuating resource which requires a tight control management. So, it is still an exigent task for the control design engineers. The conventional proportional-integral (PI) control is not ideal during high turbulence wind velocities, and the nonlinear behavior of the power converters. These are raising interest towards the robust control concepts. The robust design is to find a controller, for a given system, such that the closed-loop system becomes robust that assurance high-integrity and fault tolerant control system, robust H∞ control theory has befallen a standard design method of choice over the past two decades in industrial control applications. The robust H∞ control theory is also gaining eminence in the WECS. Due to the implementation complexity for the continuous H∞ controller, and availability of the high speedy micro-controllers, the design of a sample-data or a digital H∞ controller is very important for the realistic implementation. But there isn’t a single research to evaluate the performance of the digital H∞ controller for the WECS. In this paper, the proposed digital H∞ controller schemes comprise for the both generator and grid interactive power converters, and the control performances are compared with the conventional PI controller and the fuzzy controller. Simulation results confirm the efficacy of the proposed method Energies 2013, 6 2085 which are ensured the WECS stabilities, mitigate shaft stress, and improving the DC-link voltage and output power qualities. Energies 2013-04-16 6 4 Article 10.3390/en6042084 2084 2109 1996-1073 2013-04-16 doi: 10.3390/en6042084 Abdul Howlader Naomitsu Urasaki Atsushi Yona Tomonobu Senjyu Ahmed Saber http://www.mdpi.com/1996-1073/6/4/2065 Energy and environmental effects of wheat-based fuel, produced continuously by a Blenke cascade system, were assessed. Two scenarios: (1) no-co-products utilization scenario; and (2) co-products utilization scenario, were compared. A Life Cycle Assessment (LCA) model was used for analysis. The scope covered a cradle-to-gate inventory. The results from energy analysis showed, that wheat-based ethanol has a positive average net energy value (NEV), NEV = 3.35 MJ/kg ethanol with an average net energy ratio (NER), NER = 1.14 MJ/MJ fossils for scenario 1, while for scenario 2, NEV = 20 MJ/kg ethanol with NER = 3.94 MJ/MJ fossils. The environmental performance analysis indicated that in scenario 1, the strongest contribution to environmental impacts was from the ethanol conversion stage; whereas in scenario 2, it was from wheat production stage. The use of a continuous fermentation system based on Blenke cascade is a promising technology that increases wheat based bio-ethanol’s energy benefits. In addition, the calculated parameters show the potential to significantly reduce emissions level. Energies 2013-04-11 6 4 Article 10.3390/en6042065 2065 2083 1996-1073 2013-04-11 doi: 10.3390/en6042065 Jean Ntihuga Thomas Senn Peter Gschwind Reinhard Kohlus http://www.mdpi.com/1996-1073/6/4/2052 We entrapped lipase from Pseudomonas cepacia in polyallylamine-mediated biomimetic silica, and then applied entrapped lipase to the synthesis of biodiesel with soybean oil or waste cooking oil as a feedstock. The effects of reaction temperature, substrate molar ratio (methanol/oil) and n-hexane content (w/w of oil) were evaluated using response surface methodology (RSM) combined with Box-Behnken design. The optimal reaction conditions for soybean oil were 43.6 °C, substrate molar ratio of 4.3%, and 75% n-hexane. The predicted and experimental values of biodiesel conversion were 79% and 76%, respectively. The optimal reaction conditions for waste cooking oil were 43.3 °C, substrate molar ratio of 5%, and 38% n-hexane. The predicted and experimental values of conversion were 68% and 67%, respectively. The conversion efficiency remained the same even after 1-month storage of entrapped lipase at 4 °C or room temperature. Energies 2013-04-10 6 4 Article 10.3390/en6042052 2052 2064 1996-1073 2013-04-10 doi: 10.3390/en6042052 I-Ching Kuan Chia-Chi Lee Bing-Hong Tsai Shiow-Ling Lee Wei-Ting Lee Chi-Yang Yu http://www.mdpi.com/1996-1073/6/4/2031 Sufficiently accurate and low-cost estimation of tidal velocities is of importance when evaluating a potential site for a tidal energy farm. Here we suggest and evaluate a model to calculate the tidal velocity in fjord entrances. The model is compared with tidal velocities from Acoustic Doppler Current Profiler (ADCP) measurements in the tidal channel Skarpsundet in Norway. The calculated velocity value from the model corresponded well with the measured cross-sectional average velocity, but was shown to underestimate the velocity in the centre of the channel. The effect of this was quantified by calculating the kinetic energy of the flow for a 14-day period. A numerical simulation using TELEMAC-2D was performed and validated with ADCP measurements. Velocity data from the simulation was used as input for calculating the kinetic energy at various locations in the channel. It was concluded that the model presented here is not accurate enough for assessing the tidal energy resource. However, the simplicity of the model was considered promising in the use of finding sites where further analyses can be made. Energies 2013-04-09 6 4 Article 10.3390/en6042031 2031 2051 1996-1073 2013-04-09 doi: 10.3390/en6042031 Emilia Lalander Paul Thomassen Mats Leijon http://www.mdpi.com/1996-1073/6/4/2007 The accurate state of charge (SOC) estimation of the LiFePO4 battery packs used in robot applications is required for better battery life cycle, performance, reliability, and economic issues. In this paper, a new SOC estimation method, “Modified ECE + EKF”, is proposed. The method is the combination of the modified Equivalent Coulombic Efficiency (ECE) method and the Extended Kalman Filter (EKF) method. It is based on the zero-state hysteresis battery model, and adopts the EKF method to correct the initial value used in the Ah counting method. Experimental results show that the proposed technique is superior to the traditional techniques, such as ECE + EKF and ECE + Unscented Kalman Filter (UKF), and the accuracy of estimation is within 1%. Energies 2013-04-08 6 4 Article 10.3390/en6042007 2007 2030 1996-1073 2013-04-08 doi: 10.3390/en6042007 Ming-Hui Chang Han-Pang Huang Shu-Wei Chang http://www.mdpi.com/1996-1073/6/4/1993 This article proposes a simple and reliable damping strategy for wave powerfarm operation of small-scale point-absorber converters. The strategy is based on passiverectification onto a constant DC-link, making it very suitable for grid integration of the farm.A complete model of the system has been developed in Matlab Simulink, and uses real sitedata as input. The optimal constant DC-voltage is evaluated as a function of the significantwave height and energy period of the waves. The total energy output of the WEC is derivedfor one year of experimental site data. The energy output is compared for two cases, onewhere the optimal DC-voltage is determined and held constant at half-hour basis throughoutthe year, and one where a selected value of the DC-voltage is kept constant throughout theyear regardless of sea state. Energies 2013-04-08 6 4 Article 10.3390/en6041993 1993 2006 1996-1073 2013-04-08 doi: 10.3390/en6041993 Venugopalan Kurupath Rickard Ekström Mats Leijon http://www.mdpi.com/1996-1073/6/4/1961 An overtopping model specifically suited for Wave Dragon is needed in order to improve the reliability of its performance estimates. The model shall be comprehensive of all relevant physical processes that affect overtopping and flexible to adapt to any local conditions and device configuration. An experimental investigation is carried out to update an existing formulation suited for 2D draft-limited, low-crested structures, in order to include the effects on the overtopping flow of the wave steepness, the 3D geometry of Wave Dragon, the wing reflectors, the device motions and the non-rigid connection between platform and reflectors. The study is carried out in four phases, each of them specifically targeted at quantifying one of these effects through a sensitivity analysis and at modeling it through custom-made parameters. These are depending on features of the wave or the device configuration, all of which can be measured in real-time. Instead of using new fitting coefficients, this approach allows a broader applicability of the model beyond the Wave Dragon case, to any overtopping WEC or structure within the range of tested conditions. Predictions reliability of overtopping over Wave Dragon increased, as the updated model allows improved accuracy and precision respect to the former version. Energies 2013-04-03 6 4 Article 10.3390/en6041961 1961 1992 1996-1073 2013-04-03 doi: 10.3390/en6041961 Stefano Parmeggiani Jens Kofoed Erik Friis-Madsen http://www.mdpi.com/1996-1073/6/4/1944 Increasingly strict energy policies, rising energy prices, and a desire for a positive corporate image currently serve as incentives for multinational corporations to reduce their plants’ energy consumption. This paper quantitatively investigates and discusses the value of a traditional north-light roof using a complete building energy simulation and optimization framework. The findings indicate that the north-light system yields positive building energy performance for several climate zones, including: (i) Humid Subtropical; (ii) Semiarid Continental; (iii) Mediterranean; and (iv) Subtropical Highland. In the Subtropical Highland climate zone, for example, the building energy consumption of a north-light roof is up to 54% less than that of a conventional flat roof. Based on these positive findings, this paper further presents an optimization framework that alters the north-light roof shape to further improve its energy performance. To quantitatively guarantee a high probability of finding satisfactory designs while reducing the computational processing time, ordinal optimization is introduced into the scheme. The Subtropical Highland case study shows further energy building consumption reduction of 26% for an optimized north-light roof shape. The presented evaluation and optimization framework could be used in designing a plant with integrated north-lights roof that aim at energy efficiency while maintaining environmental occupant comfort levels. Energies 2013-04-03 6 4 Article 10.3390/en6041944 1944 1960 1996-1073 2013-04-03 doi: 10.3390/en6041944 Sigrid Adriaenssens Hao Liu Mariam Wahed Qianchuan Zhao http://www.mdpi.com/1996-1073/6/4/1930 The current study is focused on controlling sulphur emissions by pre-treating coal to remove sulphur prior to combustion using the oxydesulphurisation technique. Three British coals were chosen for the study. Working with these coals gave a better insight to the oxydesulphurisation reactions for pyritic and organic sulphur. Effect of air and oxygen pressure in a fixed time interval on sulphur removal was studied by series of experimental runs at various temperatures. Heating value recoveries were significant. Increase in oxygen content is reported as a measure of carbon loss. The coal samples were analyzed according to British standard methods. Experimental results demonstrate that the sulphur removal was enhanced with the increase in air pressure, particularly up to 100 bars, with only a small decrease in calorific value at a particular temperature. Increasing temperature was witnessed to be more important in sulphur removal than increasing pressure. Energies 2013-04-03 6 4 Article 10.3390/en6041930 1930 1943 1996-1073 2013-04-03 doi: 10.3390/en6041930 Moinuddin Ghauri Abrar Inayat Muhammad Bashir Salmiaton Ali Keith Cliffe http://www.mdpi.com/1996-1073/6/4/1918 The accurate forecasting of energy production from renewable sources represents an important topic also looking at different national authorities that are starting to stimulate a greater responsibility towards plants using non-programmable renewables. In this paper the authors use advanced hybrid evolutionary techniques of computational intelligence applied to photovoltaic systems forecasting, analyzing the predictions obtained by comparing different definitions of the forecasting error. Energies 2013-04-03 6 4 Article 10.3390/en6041918 1918 1929 1996-1073 2013-04-03 doi: 10.3390/en6041918 Emanuele Ogliari Francesco Grimaccia Sonia Leva Marco Mussetta http://www.mdpi.com/1996-1073/6/4/1902 Wind power causes fluctuations in power systems and introduces issues concerning system stability and power quality because of the lack of controllability of its discontinuous and intermittent resources. This paper presents a coordinated control strategy for solid oxide fuel cells (SOFCs) and superconducting magnetic energy storage (SMES) to match the intermittent wind power generation and compensate for the rapid load changes. An optimal H∞ control method, where the weighting function selection is expressed as an optimization problem, is proposed to mitigate tie-line power fluctuations and the mixed-sensitivity approach is used to deal with the interference suppression. Simulation results show that the proposed method significantly improves the smoothing effect of wind power fluctuations. Compared with the conventional control method, the proposed method has better anti-interference performance in various operating situations. Energies 2013-04-03 6 4 Article 10.3390/en6041902 1902 1917 1996-1073 2013-04-03 doi: 10.3390/en6041902 Ning Zhang Wei Gu Haojun Yu Wei Liu http://www.mdpi.com/1996-1073/6/4/1887 Electric load forecasting is an important issue for a power utility, associated with the management of daily operations such as energy transfer scheduling, unit commitment, and load dispatch. Inspired by strong non-linear learning capability of support vector regression (SVR), this paper presents a SVR model hybridized with the empirical mode decomposition (EMD) method and auto regression (AR) for electric load forecasting. The electric load data of the New South Wales (Australia) market are employed for comparing the forecasting performances of different forecasting models. The results confirm the validity of the idea that the proposed model can simultaneously provide forecasting with good accuracy and interpretability. Energies 2013-04-02 6 4 Article 10.3390/en6041887 1887 1901 1996-1073 2013-04-02 doi: 10.3390/en6041887 Guo-Feng Fan Shan Qing Hua Wang Wei-Chiang Hong Hong-Juan Li http://www.mdpi.com/1996-1073/6/4/1863 The paper presents the results of an experimental study identifying the response of a 1.5 MW Wave Dragon to extreme conditions typical of the DanWEC test center. The best strategies allowing for a reduction in the extreme mooring tension have also been investigated, showing that this is possible by increasing the surge natural period of the system. The most efficient strategy in doing this is to provide the mooring system with a large horizontal compliance (typically in the order of 100 s), which shall be therefore assumed as design configuration. If this is not possible, it can also be partly achieved by lowering the floating level to a minimum (survivability mode) and by adopting a negative trim position. The adoption of the design configuration would determine in a 100-year storm extreme mooring tensions in the order of 0.9 MN, 65% lower than the worst case experienced in the worst case configuration. At the same time it would lead to a reduction in the extreme motion response, resulting in heave and pitch oscillation heights of 7 m and 19° and surge excursion of 12 m. Future work will numerically identify mooring configurations that could provide the desired compliance. Energies 2013-04-02 6 4 Article 10.3390/en6041863 1863 1886 1996-1073 2013-04-02 doi: 10.3390/en6041863 Stefano Parmeggiani Jens Kofoed Erik Friis-Madsen http://www.mdpi.com/1996-1073/6/4/1843 The availability of compressed air in combination with downsizing and turbocharging is a promising approach to improve the fuel economy and the driveability of internal combustion engines. The compressed air is used to boost and start the engine. It is generated during deceleration phases by running the engine as a piston compressor. In this paper, a camshaft-driven valve is considered for the control of the air exchange between the tank and the combustion chamber. Such a valve system is cost-effective and robust. Each pneumatic engine mode is realized by a separate cam. The air mass transfer in each mode is analyzed. Special attention is paid to the tank pressure dependence. The air demand in the boost mode is found to increase with the tank pressure. However, the dependence on the tank pressure is small in the most relevant operating region. The air demand of the pneumatic start shows a piecewise continuous dependence on the tank pressure. Finally, a tank sizing method is proposed which uses a quasi-static simulation. It is applied to a compact class vehicle, for which a tank volume of less than 10 L is sufficient. A further reduction of the tank volume is limited by the specifications imposed on the pneumatic start. Energies 2013-03-28 6 4 Article 10.3390/en6041843 1843 1862 1996-1073 2013-03-28 doi: 10.3390/en6041843 Christoph Voser Christopher Onder Lino Guzzella http://www.mdpi.com/1996-1073/6/4/1821 The study of the heat transfer of solar air heaters with a new design using an absorbing plate with fins and baffles, which facilitate the recycling of flowing air, is reported. The mathematical formulation and analytical analysis for such a recyclic baffled double-pass solar air heater were developed theoretically. The performance of the device was studied experimentally as well. The theoretical predicted and experimental results were compared with another design, i.e., a downward-type single-pass solar air heater without recycle and double-pass operations reported in our previous work. Significant improvement in heat-transfer efficiency is achieved with the baffle and fin design due to the recycling heating and the extended heat transfer area. The effects of mass flow rate and recycle ratio on the heat-transfer efficiency enhancement as well as on the power consumption increment are also discussed. Energies 2013-03-28 6 4 Article 10.3390/en6041821 1821 1842 1996-1073 2013-03-28 doi: 10.3390/en6041821 Chii Ho Hsuan Chang Rei Wang Chun Lin http://www.mdpi.com/1996-1073/6/4/1802 The paper aims to contribute to the use of electric double layer capacitor (EDLC) sets for boosting voltages of contact lines in urban and suburban railway traction systems. Different electrical configurations of contact lines are considered and investigated. For each of them, proper mathematical models are suggested to evaluate the electrical performances of the contact lines. They give rise, also, to sample design procedures for the sizing of the most appropriate energy storage systems, to be distributed along the lines, for boosting line voltages and avoiding undesired voltage drops. A numerical example based on the “Cumana” suburban Naples railway network is presented to give an idea of the weights and sizes of electric double layer capacitors needed to boost the voltage of a sample contact line. In particular, three different EDLC systems, for a overall installed energy of 9.6 kWh, have been placed nearby the stations presenting the highest voltage drops during the most representative situation of trains’ service. The new voltage drop is equal to 32% of that obtained in absence of EDLCs. Energies 2013-03-27 6 4 Article 10.3390/en6041802 1802 1820 1996-1073 2013-03-27 doi: 10.3390/en6041802 Diego Iannuzzi Enrico Pagano Pietro Tricoli http://www.mdpi.com/1996-1073/6/3/1794 Carrot pomace, a major agricultural waste from the juice industry, was used as a feedstock for bioethanol production by fermentation with the thermotolerant yeast Kluyveromyces marxianus. Treatment of the carrot pomace with AccelleraseTM 1000 and pectinase at 50 °C for 84 h, resulted in conversion of 42% of its mass to fermentable sugars, mainly glucose, fructose, and sucrose. Simultaneous saccharification and fermentation (SSF) at 42 °C was performed on 10% (w/v) carrot pomace; the concentration of ethanol reached 18 g/L and the yield of ethanol from carrot pomace was 0.18 g/g. The highest ethanol concentration of 37 g/L was observed with an additional charge of 10% supplemented to the original 10% of carrot pomace after 12 h; the corresponding yield was 0.185 g/g. Our results clearly demonstrated the potential of combining a SSF process with thermotolerant yeast for the production of bioethanol using carrot pomace as a feedstock. Energies 2013-03-21 6 3 Article 10.3390/en6031794 1794 1801 1996-1073 2013-03-21 doi: 10.3390/en6031794 Chi-Yang Yu Bo-Hong Jiang Kow-Jen Duan http://www.mdpi.com/1996-1073/6/3/1764 We examine technological progress in the US and Canada to answer the question: has the efficiency (e.g., the edible energy efficiency, or EEE) for producing agricultural products in the US and Canada increased in recent decades? Specifically, we determined the energy efficiency of agriculture at the farm gate in recent decades by dividing the outputs (the total annual crop and animal output in energy units minus the feed used for animal production and the grain used for ethanol production) by the energy inputs: all the energy used by the nation to produce food (the energy used to generate and apply the fertilizer, pesticides, seed and to operate machinery) minus the energy inputs to produce grain for ethanol. Our data comes primarily from national and international agricultural censuses. Our study found that the energy efficiency of US agriculture has more than doubled from 0.8:1 in 1970 to 2.2:1 by 2000, then increased more slowly to 2.3:1 by 2009. The energy efficiency of the agricultural sector in Canada has not changed appreciably since 1980, and has varied about a mean of 2:1 from 1981 to 2009. Our study found that EEE improvements in the US could be attributable in part to advancements in crop production per hectare, and lower direct fuel consumption, but also a greater proportion of less energy-intensive corn and changes to the diet of livestock (e.g., increased use of meals and other by-products which have increased the availability of grain). Thus increases due to technological progress alone for the last several decades appear small, less than one percent a year. Energies 2013-03-19 6 3 Article 10.3390/en6031764 1764 1793 1996-1073 2013-03-19 doi: 10.3390/en6031764 Abbe Hamilton Stephen Balogh Adrienna Maxwell Charles Hall http://www.mdpi.com/1996-1073/6/3/1746 Downsizing and turbocharging is a widely used approach to reduce the fuel consumption of spark ignited engines while retaining the maximum power output. However, a substantial loss in drivability must be expected due to the occurrence of the so-called turbo lag. The turbo lag results from the additional inertia that the turbocharger adds to the system. Supplying air by an additional valve, the boost valve, to the intake manifold can be used to overcome the turbo lag. This turbo lag compensationmethod is referred to as intakemanifold boosting. The aims of this study are to show the effectiveness of intake manifold boosting on a turbocharged spark-ignited engine and to show that intake manifold boosting can be used as an enabler of strong downsizing. Guidelines for the dimensioning of the boost valve are given and a control strategy is presented. The trade-off between additional fuel consumption and the consumption of pressurized air during the turbo lag compensation is discussed. For a load step at 2000 rpm the rise time can be reduced from 2.8 s to 124ms, requiring 11.8 g of pressurized air. The transient performance is verified experimentally by means of load steps at various engine speeds to various engine loads. Energies 2013-03-13 6 3 Article 10.3390/en6031746 1746 1763 1996-1073 2013-03-13 doi: 10.3390/en6031746 Norbert Zsiga Christoph Voser Christopher Onder Lino Guzzella http://www.mdpi.com/1996-1073/6/3/1731 This study presents an experimental investigation of a piston engine driven by compressed air. The compressed air engine was a modified 100 cm3 internal combustion engine obtained from a motorcycle manufacturer. The experiments in this study used a test bench to examine the power performance and pressure/temperature variations of the compressed air engine at pressures ranging from 5 to 9 bar (absolute pressure). The engine was modified from a 4-stroke to a 2-stroke engine using a cam system driven by a crankshaft and the intake and exhaust valves have a small lift due to this modification. The highest power output of 0.95 kW was obtained at 9 bar and 1320 rpm. The highest torque of 9.99 N·m occurred at the same pressure, but at 465 rpm. The pressure-volume (P-V) diagram shows that cylinder pressure gradually increases after the intake valve opens because of the limited lift movement of the intake valve. Similar situations occurred during the exhaust process, restricting the power output of the compressed air engine. The pressure and temperature variation of the air at engine inlet and outlet were recorded during the experiment. The outlet pressure increased from 1.5 bar at 500 rpm to 2.25 bar at 2000 rpm, showing the potential of recycling the compressed air energy by attaching additional cylinders (split-cycle engine). A temperature decrease (from room temperature to 17 °C) inside the cylinder was observed. It should be noted that pressures higher than that currently employed can result in lower temperatures and this can cause poor lubrication and sealing issues. The current design of a compressed air engine, which uses a conventional cam mechanism for intake and exhaust, has limited lift movement during operation, and has a restricted flow rate and power output. Fast valve actuation and a large lift are essential for improving the performance of the current compressed air engine. This study presents a power output examination with the pressure and temperature measurements of a piston-type compressed air engine to be installed in compact vehicles as the main or auxiliary power system. Energies 2013-03-12 6 3 Article 10.3390/en6031731 1731 1745 1996-1073 2013-03-12 doi: 10.3390/en6031731 Chih-Yung Huang Cheng-Kang Hu Chih-Jie Yu Cheng-Kuo Sung http://www.mdpi.com/1996-1073/6/3/1718 Spinel LiMn2O4 is an appealing candidate cathode material for Li-ion rechargeable batteries, but it suffers from severe capacity fading, especially at higher temperature (55 °C) during discharging/charging. In recent years, many attempts have been made to synthesize modified LiMn2O4. This paper reviews the recent research on the preparation and doping modes of doped LiMn2O4 for modifying the LiMn2O4. We firstly compared preparation methods for doped spinel LiMn2O4, such as solid state reactions and solution synthetic methods. Then we mainly discuss doping modes reported in recent years, such as bulk doping, surface doping and combined doping. A comparison of different doping modes is also provided. The research shows that the multiple-ion doping and combined doping modes of LiMn2O4 used in Li-ion battery are excellent for improving different aspects of the electrochemical performance which holds great promise in the future. From this paper, we also can see that spinel LiMnO4 as an attractive candidate cathode material for Li-ion batteries. Energies 2013-03-12 6 3 Review 10.3390/en6031718 1718 1730 1996-1073 2013-03-12 doi: 10.3390/en6031718 Qiuling Liu Shengping Wang Haibo Tan Zhigao Yang Jian Zeng http://www.mdpi.com/1996-1073/6/3/1700 Constant power loads (CPLs), interfaced through active rectifiers can be used for improving the stability of induction generator (IG)-based wind turbines under balanced grid voltage dips by providing the reactive power. Under asymmetrical grid faults, the negative sequence voltage produces additional generator torque oscillations and reduces the lifetime of the installed equipment. This article explores the possibility of using a CPL for mitigation of unbalanced voltage dips in an AC distribution system in addition to consuming a constant active power. Unbalanced fault mitigation as an ancillary service by the load itself could greatly increase the stability and performance of the overall power system. A CPL control structure, capable of controlling the positive and negative sequence of the grid voltage is suggested. The simulation results clearly indicate the effects of compensating the positive and negative sequence of the grid voltage on the performance of IG based wind turbines. The maximum Fault Ride Through (FRT) enhancement has been given priority and is done by the compensation of positive sequence voltage. The remaining CPL current capacity is used to compensate the negative sequence voltage in order to reduce the additional torque ripples in the IG. Energies 2013-03-12 6 3 Article 10.3390/en6031700 1700 1717 1996-1073 2013-03-12 doi: 10.3390/en6031700 Nadeem Jelani Marta Molinas http://www.mdpi.com/1996-1073/6/3/1669 This paper analyzes and simulates the Li-ion battery charging process for a solar powered battery management system. The battery is charged using a non-inverting synchronous buck-boost DC/DC power converter. The system operates in buck, buck-boost, or boost mode, according to the supply voltage conditions from the solar panels. Rapid changes in atmospheric conditions or sunlight incident angle cause supply voltage variations. This study develops an electrochemical-based equivalent circuit model for a Li-ion battery. A dynamic model for the battery charging process is then constructed based on the Li-ion battery electrochemical model and the buck-boost power converter dynamic model. The battery charging process forms a system with multiple interconnections. Characteristics, including battery charging system stability margins for each individual operating mode, are analyzed and discussed. Because of supply voltage variation, the system can switch between buck, buck-boost, and boost modes. The system is modeled as a Markov jump system to evaluate the mean square stability of the system. The MATLAB based Simulink piecewise linear electric circuit simulation tool is used to verify the battery charging model. Energies 2013-03-11 6 3 Article 10.3390/en6031669 1669 1699 1996-1073 2013-03-11 doi: 10.3390/en6031669 Jaw-Kuen Shiau Chien-Wei Ma http://www.mdpi.com/1996-1073/6/3/1657 It is shown that carbon fuel cell technology can be combined with that of high temperature steam electrolysis by the incorporation of carbon fuel at the cell anode, with the resulting reduction of the required electrolysis voltage by around 1 V. The behaviour of the cell current density and applied voltage are shown to be connected with the threshold of electrolysis and the main features are compared with theoretical results from the literature. The advantage arises from the avoidance of efficiency losses associated with electricity generation using thermal cycles, as well as the natural separation of the carbon dioxide product stream for subsequent processing. Energies 2013-03-11 6 3 Communication 10.3390/en6031657 1657 1668 1996-1073 2013-03-11 doi: 10.3390/en6031657 Bruce Ewan Olalekan Adeniyi http://www.mdpi.com/1996-1073/6/3/1632 Percolation theory is generalized to predict the effective properties of specific solid oxide fuel cell composite electrodes, which consist of a pure ion conducting material (e.g., YSZ or GDC) and a mixed electron and ion conducting material (e.g., LSCF, LSCM or CeO2). The investigated properties include the probabilities of an LSCF particle belonging to the electron and ion conducting paths, percolated three-phase-boundary electrochemical reaction sites, which are based on different assumptions, the exposed LSCF surface electrochemical reaction sites and the revised expressions for the inter-particle ionic conductivities among LSCF and YSZ materials. The effects of the microstructure parameters, such as the volume fraction of the LSCF material, the particle size distributions of both the LSCF and YSZ materials (i.e., the mean particle radii and the non-dimensional standard deviations, which represent the particle size distributions) and the porosity are studied. Finally, all of the calculated results are presented in non-dimensional forms to provide generality for practical application. Based on these results, the relevant properties can be easily evaluated, and the microstructure parameters and intrinsic properties of each material are specified. Energies 2013-03-11 6 3 Article 10.3390/en6031632 1632 1656 1996-1073 2013-03-11 doi: 10.3390/en6031632 Daifen Chen Huanhuan He Donghui Zhang Hanzhi Wang Meng Ni http://www.mdpi.com/1996-1073/6/3/1618 Dried spent yeast (DSY) and its hydrolysate (DSYH) were used as low-cost nitrogen supplements to improve ethanol production from sweet sorghum juice by Saccharomyces cerevisiae NP01 under very high gravity (VHG) fermentation (280 g·L−1 of total sugar) conditions. The supplemented DSY and DSYH concentrations were 11, 16 and 21 g·L−1, corresponding to a yeast extract nitrogen content of 6, 9 and 12 g·L−1, respectively. The initial yeast cell concentration for ethanol fermentation was approximately 5 × 107 cells·mL−1. The fermentation was carried out in single batch mode at 30 °C in 1-L air-locked bottles with an agitation rate of 100 rpm. Ethanol production from the juice with and without yeast extract (9 g·L−1) was also performed as control treatments. The results showed that DSY at 21 g·L−1gave the highest ethanol concentration (PE, 107 g·L−1) and yield (Yp/s, 0.47 g·g−1). The use of DSYH at the same DSY concentration improved ethanol productivity (Qp), but not PE and Yp/s. The ethanol production efficiencies of the juice under DSY and DSYH supplementations were markedly higher than those without nutrient supplementation. However, the PE and Qp values of the juice containing 21 g·L−1 of DSY was approximately 7 g·L−1 and 0.62 g·L−1·h−1 lower than those under the presence of yeast extract (9 g·L−1), respectively. At the end of the single batch fermentation under the optimum DSY concentration, the sugar consumption was approximately 80%. Therefore in the repeated-batch fermentation, the initial total sugar was reduced to 240 g·L−1. The results showed that the system could be carried out at least 20 successive batches with the average PE, Yp/s and Qp of 95 g·L−1, 0.46 g·g−1 and 1.45 g·L−1·h−1, respectively. Energies 2013-03-11 6 3 Article 10.3390/en6031618 1618 1631 1996-1073 2013-03-11 doi: 10.3390/en6031618 Sureerat Suwanapong Naulchan Khongsay Lakkana Laopaiboon Prasit Jaisil Pattana Laopaiboon http://www.mdpi.com/1996-1073/6/3/1604 A membrane bioreactor was developed to counteract the inhibition effect of furfural in ethanol production. Furfural, a major inhibitor in lignocellulosic hydrolyzates, is a highly toxic substance which is formed from pentose sugars released during the acidic degradation of lignocellulosic materials. Continuous cultivations with complete cell retention were performed at a high dilution rate of 0.5 h−1. Furfural was added directly into the bioreactor by pulse injection or by addition into the feed medium to obtain furfural concentrations ranging from 0.1 to 21.8 g L−1. At all pulse injections of furfural, the yeast was able to convert the furfural very rapidly by in situ detoxification. When injecting 21.8 g L−1 furfural to the cultivation, the yeast converted it by a specific conversion rate of 0.35 g g−1 h−1. At high cell density, Saccharomyces cerevisiae could tolerate very high furfural levels without major changes in the ethanol production. During the continuous cultures when up to 17.0 g L−1 furfural was added to the inlet medium, the yeast successfully produced ethanol, whereas an increase of furfural to 18.6 and 20.6 g L−1 resulted in a rapidly decreasing ethanol production and accumulation of sugars in the permeate. This study show that continuous ethanol fermentations by total cell retention in a membrane bioreactor has a high furfural tolerance and can conduct rapid in situ detoxification of medium containing high furfural concentrations. Energies 2013-03-11 6 3 Article 10.3390/en6031604 1604 1617 1996-1073 2013-03-11 doi: 10.3390/en6031604 Päivi Ylitervo Carl Franzén Mohammad Taherzadeh http://www.mdpi.com/1996-1073/6/3/1590 The mechanism of hydrothermal liquefaction of cypress was investigated by examining the effects of temperature and retention time on the characteristics of the solid residues remaining after liquefaction. The solid residues were divided into acid-soluble and acid-insoluble residues. Results showed the polymerization reactions also mainly occurred at low temperatures. The reactive fragments transformed into acid-insoluble solid residue in the form of carbon and oxygen through polymerization reactions. The process of cellulose degradation consists of two steps: an initial hydrolysis of the more solvent- accessible amorphous region and a later hydrolytic attack on the crystalline portion. Hemicelluloses were decomposed into small compounds during the initial stage of the cypress liquefaction process, and then these compounds may rearrange through polymerization to form acid-insoluble solid residues above 240 °C. The higher heating value of the solid residues obtained from liquefaction at 260–300 °C was 23.4–26.3 MJ/kg, indicating that they were suitable for combustion as a solid fuel. Energies 2013-03-11 6 3 Article 10.3390/en6031590 1590 1603 1996-1073 2013-03-11 doi: 10.3390/en6031590 Hua-Min Liu Ming-Fei Li Sheng Yang Run-Cang Sun http://www.mdpi.com/1996-1073/6/3/1568 Catalytic refining of bio-oil by reacting with olefin/alcohol over solid acids can convert bio-oil to oxygen-containing fuels. Reactivities of groups of compounds typically present in bio-oil with 1-octene (or 1-butanol) were studied at 120 °C/3 h over Dowex50WX2, Amberlyst15, Amberlyst36, silica sulfuric acid (SSA) and Cs2.5H0.5PW12O40 supported on K10 clay (Cs2.5/K10, 30 wt. %). These compounds include phenol, water, acetic acid, acetaldehyde, hydroxyacetone, d-glucose and 2-hydroxymethylfuran. Mechanisms for the overall conversions were proposed. Other olefins (1,7-octadiene, cyclohexene, and 2,4,4-trimethylpentene) and alcohols (iso-butanol) with different activities were also investigated. All the olefins and alcohols used were effective but produced varying product selectivities. A complex model bio-oil, synthesized by mixing all the above-stated model compounds, was refined under similar conditions to test the catalyst’s activity. SSA shows the highest hydrothermal stability. Cs2.5/K10 lost most of its activity. A global reaction pathway is outlined. Simultaneous and competing esterification, etherfication, acetal formation, hydration, isomerization and other equilibria were involved. Synergistic interactions among reactants and products were determined. Acid-catalyzed olefin hydration removed water and drove the esterification and acetal formation equilibria toward ester and acetal products. Energies 2013-03-11 6 3 Article 10.3390/en6031568 1568 1589 1996-1073 2013-03-11 doi: 10.3390/en6031568 Zhijun Zhang Charles Pittman Shujuan Sui Jianping Sun Qingwen Wang http://www.mdpi.com/1996-1073/6/3/1554 A novel pumped hydro combined with compressed air energy storage (PHCA) system is proposed in this paper to resolve the problems of bulk energy storage in the wind power generation industry over an area in China, which is characterised by drought and water shortages. Thermodynamic analysis of the energy storage system, which focuses on the pre-set pressure, storage volume capacity, water air volume ratio, pump performance, and water turbine performance of the storage system, is also presented. This paper discovers how such parameters affect the performance of the whole system. The ideal performance of this novel system has the following advantages: a simple, highly effective and low cost structure, which is comparable to the efficiency of a traditional pumped hydro storage system. Research results show a great solution to the current storage constraints encountered in the development of the wind power industry in China, which have been widely recognised as a bottleneck in the wind energy storage industry. Energies 2013-03-11 6 3 Article 10.3390/en6031554 1554 1567 1996-1073 2013-03-11 doi: 10.3390/en6031554 Huanran Wang Liqin Wang Xinbing Wang Erren Yao http://www.mdpi.com/1996-1073/6/3/1527 Suitability of energy systems based on Superconducting Magnetic Energy Storage (SMES) has been widely tested in the field of wind energy, being able to supply power in cases such as low wind speeds or voltage dips, and to store energy when there are surpluses. This article analyzes and compares the performance of three SMES-based systems that differ in the topology of power converter: a two-level Voltage Source Converter (VSC), a three-level VSC and a two-level Current Source Converter (CSC). Their performance has been improved by means of an appropriate modulation strategy. To obtain a high reliability and accuracy, a co-simulation between MATLAB/Simulink® (running the control system) and PSIM® (running the power system) has been executed. Energies 2013-03-06 6 3 Article 10.3390/en6031527 1527 1553 1996-1073 2013-03-06 doi: 10.3390/en6031527 Ana Rodríguez Francisco Huerta Emilio Bueno Francisco Rodríguez http://www.mdpi.com/1996-1073/6/3/1497 Research in marine current energy, including tidal and ocean currents, has undergone significant growth in the past decade. The horizontal-axis marine current turbine is one of the machines used to harness marine current energy, which appears to be the most technologically and economically viable one at this stage. A number of large-scale marine current turbines rated at more than 1 MW have been deployed around the World. Parallel to the development of industry, academic research on horizontal-axis marine current turbines has also shown positive growth. This paper reviews previous research on horizontal-axis marine current turbines and provides a concise overview for future researchers who might be interested in horizontal-axis marine current turbines. The review covers several main aspects, such as: energy assessment, turbine design, wakes, generators, novel modifications and environmental impact. Future trends for research on horizontal-axis marine current turbines are also discussed. Energies 2013-03-06 6 3 Review 10.3390/en6031497 1497 1526 1996-1073 2013-03-06 doi: 10.3390/en6031497 Kai-Wern Ng Wei-Haur Lam Khai-Ching Ng http://www.mdpi.com/1996-1073/6/3/1478 The present work investigates the matching of an advanced small scale Combined Heat and Power (CHP) Rankine cycle plant with end-user thermal and electric load. The power plant consists of a concentrated solar power field co-powered by a biomass furnace to produce steam in a Rankine cycle, with a CHP configuration. A hotel was selected as the end user due to its high thermal to electric consumption ratio. The power plant design and its operation were modelled and investigated by adopting transient simulations with an hourly distribution. The study of the load matching of the proposed renewable power technology and the final user has been carried out by comparing two different load tracking scenarios, i.e., the thermal and the electric demands. As a result, the power output follows fairly well the given load curves, supplying, on a selected winter day, about 50 GJ/d of thermal energy and the 6 GJ/d of electric energy, with reduced energy dumps when matching the load. Energies 2013-03-06 6 3 Article 10.3390/en6031478 1478 1496 1996-1073 2013-03-06 doi: 10.3390/en6031478 Domenico Borello Alessandro Corsini Franco Rispoli Eileen Tortora http://www.mdpi.com/1996-1073/6/3/1456 he accurate forecasting of carbon dioxide (CO2) emissions from fossil fuel energy consumption is a key requirement for making energy policy and environmental strategy. In this paper, a novel quantum harmony search (QHS) algorithm-based discounted mean square forecast error (DMSFE) combination model is proposed. In the DMSFE combination forecasting model, almost all investigations assign the discounting factor (β) arbitrarily since β varies between 0 and 1 and adopt one value for all individual models and forecasting periods. The original method doesn’t consider the influences of the individual model and the forecasting period. This work contributes by changing β from one value to a matrix taking the different model and the forecasting period into consideration and presenting a way of searching for the optimal β values by using the QHS algorithm through optimizing the mean absolute percent error (MAPE) objective function. The QHS algorithm-based optimization DMSFE combination forecasting model is established and tested by forecasting CO2 emission of the World top‒5 CO2 emitters. The evaluation indexes such as MAPE, root mean squared error (RMSE) and mean absolute error (MAE) are employed to test the performance of the presented approach. The empirical analyses confirm the validity of the presented method and the forecasting accuracy can be increased in a certain degree. Energies 2013-03-06 6 3 Article 10.3390/en6031456 1456 1477 1996-1073 2013-03-06 doi: 10.3390/en6031456 Hong Chang Wei Sun Xingsheng Gu http://www.mdpi.com/1996-1073/6/3/1439 Reconfiguration, by exchanging the functional links between the elements of the system, represents one of the most important measures which can improve the operational performance of a distribution system. The authors propose an original method, aiming at achieving such optimization through the reconfiguration of distribution systems taking into account various criteria in a flexible and robust approach. The novelty of the method consists in: the criteria for optimization are evaluated on active power distribution systems (containing distributed generators connected directly to the main distribution system and microgrids operated in grid-connected mode); the original formulation (Pareto optimality) of the optimization problem and an original genetic algorithm (based on NSGA-II) to solve the problem in a non-prohibitive execution time. The comparative tests performed on test systems have demonstrated the accuracy and promptness of the proposed algorithm. Energies 2013-03-06 6 3 Article 10.3390/en6031439 1439 1455 1996-1073 2013-03-06 doi: 10.3390/en6031439 Bogdan Tomoiagă Mircea Chindriş Andreas Sumper Antoni Sudria-Andreu Roberto Villafafila-Robles http://www.mdpi.com/1996-1073/6/3/1421 A novel artificial cobweb routing algorithm (ACRA) for routing the tree-type physical topology of a low-voltage distribution network in a smart grid is proposed and analyzed in this paper. The establishment, maintenance and reconstruction of the route are presented. The artificial cobweb routing algorithm is shown to have broad general applicability for power line communication. To provide a theoretical foundation for further research, the communication delay of the network is calculated accurately. Simulation analysis of the communication delay and throughputs, which were based on Opnet14.5, demonstrate the accuracy of the theoretical calculation. For the performance evaluation of ACRA, a test-bed that includes PLC nodes with the ACRA is set up in a noisy environment. Experimental results show the feasibility of the ACRA algorithm. These indicate that ACRA is effective for guaranteeing Quality of Service (QoS) and reliability in power line communication. Energies 2013-03-05 6 3 Article 10.3390/en6031421 1421 1438 1996-1073 2013-03-05 doi: 10.3390/en6031421 Liang Zhang Xiaosheng Liu Yan Zhou Dianguo Xu http://www.mdpi.com/1996-1073/6/3/1409 China has witnessed a fast economic growth in the recent two decades. However, the heavy energy exploitation seems to show a negative relation to regional economic growth. Thus, the issue is whether the energy production is a curse or blessing for the regional economic growth in China. The present study deploys a comprehensive approach to rigorously prove the validity of a proposed panel data model that includes a second generation panel unit root test and panel cointegration and a spatial panel model. The results from the second generation panel unit root test and panel cointegration allowing for cross-sectional dependences show the differenced series are stationary and there exists a cointegration relationship among these variables for all sub-regions. The results from the spatial panel data model support the conjecture of the spatial dependent and show that there is a “resource curse” only for the Western region and Central region in China. Energies 2013-03-05 6 3 Article 10.3390/en6031409 1409 1420 1996-1073 2013-03-05 doi: 10.3390/en6031409 Yaobin Liu http://www.mdpi.com/1996-1073/6/3/1385 Electricity is indispensable and of strategic importance to national economies. Consequently, electric utilities make an effort to balance power generation and demand in order to offer a good service at a competitive price. For this purpose, these utilities need electric load forecasts to be as accurate as possible. However, electric load depends on many factors (day of the week, month of the year, etc.), which makes load forecasting quite a complex process requiring something other than statistical methods. This study presents an electric load forecast architectural model based on an Artificial Neural Network (ANN) that performs Short-Term Load Forecasting (STLF). In this study, we present the excellent results obtained, and highlight the simplicity of the proposed model. Load forecasting was performed in a geographic location of the size of a potential microgrid, as microgrids appear to be the future of electric power supply. Energies 2013-03-05 6 3 Article 10.3390/en6031385 1385 1408 1996-1073 2013-03-05 doi: 10.3390/en6031385 Luis Hernandez Carlos Baladrón Javier Aguiar Belén Carro Antonio Sanchez-Esguevillas Jaime Lloret http://www.mdpi.com/1996-1073/6/3/1365 The design of a four-bed three-stage adsorption cycle has been proposed to reduce the volume of the six-bed three-stage adsorption cycle. A simulation model for the proposed innovative cycle was developed to analyse the influence of cycle time on the system performance identifying the specific cooling power (SCP) and coefficient of performance (COP). A particle swarm optimization (PSO) technique was used to optimize the cycle time enabling us to maximize the SCP. PSO results showed that the optimal cycle time was decreased with heat source temperature and SCP value was proportional to heat source temperature. It was found that the proposed cycle could be driven by waste heat as low as 40 °C, along with coolant at 30 °C. Comparative study of optimized result indicated that the proposed cycle increased the performance significantly over a whole range of temperatures from 40 to 70 °C and reduced two adsorbent beds, compared to the six-bed three-stage cycle. Energies 2013-03-05 6 3 Article 10.3390/en6031365 1365 1384 1996-1073 2013-03-05 doi: 10.3390/en6031365 Abul Rahman Yuki Ueda Atsushi Akisawa Takahiko Miyazaki Bidyut Saha http://www.mdpi.com/1996-1073/6/3/1344 The objective of the present work is to perform an evaluation of the performance provided by various technologies for wave energy conversion in the Portuguese continental coastal environment. The wave climate in the target area is first analyzed using the results from three years of simulations with a wave prediction system based on numerical models. Based on the above data, diagrams for the bivariate distributions of the sea states occurrences, defined by the significant wave height and the energy period, are designed for both winters and whole years. On this basis, the output of five different technologies for the conversion of wave energy is assessed in some relevant locations from the Portuguese nearshore. According to the results obtained, the Portuguese continental coastal environment appears to be appropriate for the wave energy extraction. At the same time, the present work shows that the output of the wave energy conversion devices does not depend only on the average wave energy but is also dependent on the distribution of the wave energy among the sea states of different periods. For this reason, a good agreement between the characteristics of the power matrices of the wave energy converters operating in a certain place and the diagrams for the bivariate distributions of the sea states occurrences corresponding to the considered location represents a key issue in selecting the most appropriate technology for wave energy conversion. Energies 2013-03-05 6 3 Article 10.3390/en6031344 1344 1364 1996-1073 2013-03-05 doi: 10.3390/en6031344 Dina Silva Eugen Rusu Carlos Soares http://www.mdpi.com/1996-1073/6/3/1329 This paper presents a new, accurate load forecasting technique robust to fluctuations due to unusual load behavioral changes in buildings, i.e., the potential for small commercial buildings with heterogeneous stores. The proposed scheme is featured with two functional components: data classification by daily characteristics and automatic forecast model switching. The scheme extracts daily characteristics of the input load data and arranges the load data into weekday and weekend data. Forecasting is conducted based on a selected model among ARMAX (autoregressive moving average with exogenous variable) models with the processed input data. Kalman filtering is applied to estimate model parameters. The model-switching scheme monitors the accumulated error and substitutes a backup load model for the currently working model, when the accumulated error exceeds a threshold value, to reduce the increased bias error due to the change in the consumption pattern. This switching reinforces the limited performance of parameter estimation given a fixed structure and, thus, forecasting capability. The study results demonstrate that the proposed scheme is reasonably accurate and even robust to changes in the electricity use patterns. It should help improve the performance for building control systems for energy efficiency. Energies 2013-03-05 6 3 Article 10.3390/en6031329 1329 1343 1996-1073 2013-03-05 doi: 10.3390/en6031329 Jaeyeong Yoo Kyeon Hur http://www.mdpi.com/1996-1073/6/3/1314 The harmonic impact caused by wind turbines should be carefully investigated before wind turbines are interconnected. However, the harmonic currents of wind turbines are not easily predicted due to the variations of wind speed. If the harmonic current outputs can be predicted accurately, the harmonic impact of wind turbines and wind farms for power grids can be analyzed efficiently. Therefore, this paper analyzes the harmonic current characteristics of wind turbines and investigates the feasibility of developing harmonic current predictors. Field measurement, data sorting, and analysis are conducted for wind turbines. Two harmonic current predictors are proposed based on the measured harmonic data. One is the Auto-Regressive and Moving Average (ARMA)-based harmonic current predictor, which can be used for real-time prediction. The other is the stochastic harmonic current predictor considering the probability density distributions of harmonic currents. It uses the measured harmonic data to establish the probability density distributions of harmonic currents at different wind speeds, and then uses them to implement a long-term harmonic current prediction. Test results use the measured data to validate the forecast ability of these two harmonic current predictors. The ARMA-based predictor obtains poor performance on some harmonic orders due to the stochastic characteristics of harmonic current caused by the variations of wind speed. Relatively, the prediction results of stochastic harmonic current predictor show that the harmonic currents of a wind turbine in long-term operation can be effectively analyzed by the established probability density distributions. Therefore, the proposed stochastic harmonic current predictor is helpful in predicting and analyzing the possible harmonic problems during the operation of wind turbines and wind farms. Energies 2013-03-04 6 3 Article 10.3390/en6031314 1314 1328 1996-1073 2013-03-04 doi: 10.3390/en6031314 Jen-Hao Teng Rong-Ceng Leou Chuo-Yean Chang Shun-Yu Chan http://www.mdpi.com/1996-1073/6/3/1298 Lowering the exergy content of heat required for heating purposes decreases the primary energy consumption. District heating systems are often an important link between facilities that generate heat with low exergy content and consumers. Exergetic efficiency of heat distribution is an important performance criterion in heat supply to consumers. It can serve as a criterion for optimization, towards a more sustainable distribution-network design and operation. This paper presents a methodology for an exergy-based distribution-network analysis in a district heating system. Criteria for performance evaluations are defined. They can be used to evaluate heat supply to different points in the network, or individual system components. A case study is performed on an existing district heating system. Energetic and exergetic efficiencies of supply lines are analyzed. Exergy destructions and exergy losses are studied. Large differences in efficiency of heat supply to different points in the network are discovered. Over-dimensioned parameters of the distribution network are investigated. Energies 2013-03-04 6 3 Article 10.3390/en6031298 1298 1313 1996-1073 2013-03-04 doi: 10.3390/en6031298 Andrej Ljubenko Alojz Poredoš Tatiana Morosuk George Tsatsaronis http://www.mdpi.com/1996-1073/6/3/1266 This study estimates the fiscal, energy, and environmental tradeoffs involved in supplying California’s future energy needs. An integrated framework is developed whereby an econometric forecasting system of California energy demand is coupled with engineering-economic models of energy supply, and economic impacts are estimated using input-output models of the California economy. A baseline scenario in which California relies on imported electricity to meet future demand is then compared against various energy supply development scenarios over the forecast horizon (2012–2035). The results indicate that if California implements its renewable portfolio standard (RPS), there will be a substantial net cost in terms of value added, employment, and state tax revenues because the economic benefits of building capacity are outweighed by higher energy prices. Although carbon emissions fall, the cost per ton of avoided emissions is well above market prices. Building out natural gas fired generation capacity also leads to losses compared to the baseline, although the impacts are relatively minor. Meanwhile, a strategy of replacing imported crude oil and natural gas with domestic production using indigenous resources increases gross state product, employment, and tax revenues, with minimal impact on carbon emissions. This option could, therefore, help mitigate the costs of California meeting its RPS commitment. Energies 2013-03-04 6 3 Article 10.3390/en6031266 1266 1297 1996-1073 2013-03-04 doi: 10.3390/en6031266 Timothy Considine Edward Manderson http://www.mdpi.com/1996-1073/6/3/1250 Carbon Capture and Storage (CCS) is a very innovative and promising solution for greenhouse gases (GHG) reduction, i.e., capturing carbon dioxide (CO2) at its source and storing it indefinitely to avoid its release to the atmosphere. This paper investigates a set of key issues in the development of specific rules for the application of Life Cycle Assessment (LCA) to CCS. The following LCA-based information are addressed in this work: definition of service type, definition of functional unit, definition of system boundaries, choice of allocation rules, choice of selected Life Cycle Inventory (LCI) results or other selected parameters for description of environmental performance. From a communication perspective, the specific rules defined in this study have been developed coherently with the requirements of a type III environment label scheme, the International EPD® System, according to the ISO 14025 standard. Energies 2013-03-04 6 3 Article 10.3390/en6031250 1250 1265 1996-1073 2013-03-04 doi: 10.3390/en6031250 Carlo Strazza Adriana Del Borghi Michela Gallo http://www.mdpi.com/1996-1073/6/3/1233 The formation of methane hydrate in two significantly different media was investigated, using silica gel as an artificial medium and loess as a natural medium. The methane hydrate formation was observed through the depletion of water in the matrix, measured via the matrix potential and the relationship between the matrix potential and the water content was determined using established equations. The velocity of methane hydrate nucleation slowed over the course of the reaction, as it relied on water transfer to the hydrate surfaces with lower Gibbs free energy after nucleation. Significant differences in the reactions in the two types of media arose from differences in the water retention capacity and lithology of media due to the internal surface area and pore size distributions. Compared with methane hydrate formation in silica gel, the reaction in loess was much slower and formed far less methane hydrate. The results of this study will advance the understanding of how the properties of the environment affect the formation of gas hydrates in nature. Energies 2013-03-01 6 3 Article 10.3390/en6031233 1233 1249 1996-1073 2013-03-01 doi: 10.3390/en6031233 Peng Zhang Qingbai Wu Yuzhong Yang http://www.mdpi.com/1996-1073/6/3/1217 Producing electricity from waste gas is an after treatment for waste gas while recovering the energy content. This paper addresses the methodology to calculate the effect that waste gas energy recovery has on lowering the impact of climate change. Greenhouse gases are emitted while burning the waste gas. However, a thorough study should include the production of the feedstock as well as the production of the infrastructure. A framework is developed to calculate the environmental impact of electricity production from waste gas with a life cycle approach. The present paper has a twofold purpose: to assess the climate change impact of generating electricity with blast furnace gas (BFG) as a waste gas from the steel industry; and to establish a sensitivity assessment of the environmental implications of different allocation rules. Energies 2013-03-01 6 3 Article 10.3390/en6031217 1217 1232 1996-1073 2013-03-01 doi: 10.3390/en6031217 Maarten Messagie Fayçal Boureima Jan Mertens Javier Sanfelix Cathy Macharis Joeri Mierlo http://www.mdpi.com/1996-1073/6/3/1198 An experimental platform was designed and built for testing the thermal performance of a water/steam cavity receiver. The experimental platform was utilized to investigate the start-up performance and operation characteristics of the receiver. The electrical heating mode was chosen to simulate the non-uniform distribution of heat flux on the surface of absorber tubes inside the cavity. During start-up the temperature rise rate and the mass flow rate are considered as control variables. A couple of start-up curves under different working pressures were finally obtained. The results showed that the receiver performed at relatively low thermal efficiencies. The main reason for the low thermal efficiency was attributed to the low steam mass flow rate, which causes a high proportional heat loss. In order to study the relationship between thermal efficiency and mass flow rate, a computational model for evaluating the thermal performance of a cavity receiver was built and verified. This model couples three aspects of heat transfer: the radiative heat transfer inside the receiver, the flow boiling heat transfer inside the absorber tubes and the convection heat transfer around the receiver. The water/steam cavity receiver of the experimental platform was studied numerically. The curve of thermal efficiency versus mass flow rate was obtained to show that the thermal efficiency increases with increasing mass flow rate within a certain range, and the increase is more remarkable at low mass flow rates. The purpose of the present study was to determine an appropriate mass flow rate for the receiver of the experimental platform to ensure its efficient operation. Energies 2013-02-25 6 3 Article 10.3390/en6031198 1198 1216 1996-1073 2013-02-25 doi: 10.3390/en6031198 Nan Tu Jinjia Wei Jiabin Fang http://www.mdpi.com/1996-1073/6/3/1181 This paper presents an improved control strategy for both the rotor side converter (RSC) and grid side converter (GSC) of a doubly fed induction generator (DFIG)-based wind turbine (WT) system to enhance the low voltage ride through (LVRT) capability. Within the proposed control strategy, the RSC control introduces transient feed-forward compensation terms to mitigate the high frequency harmonic components and reduce the surge in the rotor currents. The proposed GSC control scheme also introduces a compensation term reflecting the instantaneous variation of the output power of the rotor side converter with consideration of the instantaneous power of grid filter impendence to keep the dc-link voltage nearly constant during the grid faults. To provide precise control, non-ideal proportional resonant (PR) controllers for both the RSC and GSC current regulation are employed to further improve dynamic performance. Simulations performed in Matlab/Simulink verify the effectiveness of the proposed control strategy. Energies 2013-02-25 6 3 Article 10.3390/en6031181 1181 1197 1996-1073 2013-02-25 doi: 10.3390/en6031181 Zaijun Wu Chanxia Zhu Minqiang Hu http://www.mdpi.com/1996-1073/6/2/1178 To better recognize the outstanding papers in the area of energy technologies and applications published in Energies, we announce the institution of an annual award. We are therefore pleased to announce the first “Energies Best Paper Award” for 2013. Nominations were made by the Editor-in-Chief and Editorial Board members from all papers published in 2009. The awards are issued to reviews and full research articles separately. Energies 2013-02-22 6 2 Editorial 10.3390/en6021178 1178 1180 1996-1073 2013-02-22 doi: 10.3390/en6021178 Ophelia Han http://www.mdpi.com/1996-1073/6/2/1165 Effective utilization of scarce water resources has presented a significant challenge to respond to the needs created by rapid economic growth in China. In this study, the efficiency of the joint operation of the Three Gorges and Gezhouba cascade hydropower plants in terms of power generation was evaluated on the basis of a precise simulation-optimization technique. The joint operation conditions of the Three Gorges and Gezhouba hydropower plants between 2004 and 2010 were utilized in this research in order to investigate the major factors that could affect power output of the cascade complex. The results showed that the current power output of the Three Gorges and Gezhouba cascade complex had already reached around 90% of the maximum theoretical value. Compared to other influencing factors evaluated in this study, the accuracy of hydrological forecasts and flood control levels can have significant impact on the power generating efficiency, whereas the navigation has a minor influence. This research provides a solid quantitative-based methodology to assess the operation efficiency of cascade hydropower plants, and more importantly, proposes potential methods that could improve the operation efficiency of cascade hydropower plants. Energies 2013-02-22 6 2 Article 10.3390/en6021165 1165 1177 1996-1073 2013-02-22 doi: 10.3390/en6021165 Ying Zheng Xudong Fu Jiahua Wei http://www.mdpi.com/1996-1073/6/2/1142 Power-level control is a crucial technique for the safe, stable and efficient operation of modular high temperature gas-cooled nuclear reactors (MHTGRs), which have strong inherent safety features and high outlet temperatures. The current power-level controllers of the MHTGRs need measurements of both the nuclear power and the helium temperature, which cannot provide satisfactory control performance and can even induce large oscillations when the neutron sensors are in error. In order to improve the fault tolerance of the control system, it is important to develop a power-level control strategy that only requires the helium temperature. The basis for developing this kind of control law is to give a state-observer of the MHTGR a relationship that only needs the measurement of helium temperature. With this in mind, a novel nonlinear state observer which only needs the measurement of helium temperature is proposed. This observer is globally convergent if there is no disturbance, and has the L2 disturbance attenuation performance if the disturbance is nonzero. The separation principle of this observer is also proven, which denotes that this observer can recover the performance of both globally asymptotic stabilizers and L2 disturbance attenuators. Then, a new dynamic output feedback power-level control strategy is established, which is composed of this observer and the well-built static state-feedback power-level control based upon iterative dissipation assignment (IDA-PLC). Finally, numerical simulation results show the high performance and feasibility of this newly-built dynamic output feedback power-level controller. Energies 2013-02-22 6 2 Article 10.3390/en6021142 1142 1164 1996-1073 2013-02-22 doi: 10.3390/en6021142 Zhe Dong http://www.mdpi.com/1996-1073/6/2/1125 This study analyzed the environmental impacts of the materials phase of a net-zero energy building. The Center for Sustainable Landscapes (CSL) is a three-story, 24,350 square foot educational, research, and administrative office in Pittsburgh, PA, USA. This net-zero energy building is designed to meet Living Building Challenge criteria. The largest environmental impacts from the production of building materials is from concrete, structural steel, photovoltaic (PV) panels, inverters, and gravel. Comparing the LCA results of the CSL to standard commercial structures reveals a 10% larger global warming potential and a nearly equal embodied energy per square feet, largely due to the CSL’s PV system. As a net-zero energy building, the environmental impacts associated with the use phase are expected to be very low relative to standard structures. Future studies will incorporate the construction and use phases of the CSL for a more comprehensive life cycle perspective. Energies 2013-02-21 6 2 Article 10.3390/en6021125 1125 1141 1996-1073 2013-02-21 doi: 10.3390/en6021125 Cassandra Thiel Nicole Campion Amy Landis Alex Jones Laura Schaefer Melissa Bilec http://www.mdpi.com/1996-1073/6/2/1101 Based on Game Theory and Multi-objective optimization problems (MOP), Game Optimization Theory (GOT) is discussed in this paper. Optimization Stability Analysis (OSA), Distance Entropy Multi-Objective Particle Swarm Optimization (DEMPSO) and Fuzzy Multi-weights Decision-making Method (FMW) are proposed as well. Game Optimization Theory, which is a comprehensive system, could not only handle multi-objective optimization problems effectively, but also could offset the disadvantages of traditional optimization theories, such as lack of framework and the insufficient consideration of relevant elements. In this paper GOT is used for the first time in solving the distribution systems planning (DSP) issue by implementing distributed generation. The proposed model integrates costs, losses, and voltage index to achieve optimal size and site of distributed generation. The model allows minimizing total system costs, system power losses and maximizing voltage improvement. A detailed DSP example is used for verifying the effectiveness and reasonableness of GOT in this context. Energies 2013-02-21 6 2 Article 10.3390/en6021101 1101 1124 1996-1073 2013-02-21 doi: 10.3390/en6021101 Ran Li Huizhuo Ma Feifei Wang Yihe Wang Yang Liu Zenghui Li http://www.mdpi.com/1996-1073/6/2/1068 This paper presents a power system using a high step-up converter for dc load applications. The high step-up converter adopts a boost converter with interleaved mode and a coupled inductor to raise its powering ability and increase its step-up voltage ratio, respectively. In order to increase conversion efficiency, an active clamp circuit is introduced into the proposed one to provide soft-switching features to reduce switching losses. Moreover, switches in the converter and active clamp circuit are integrated with a synchronous switching technique to reduce circuit complexity and component counts, resulting in a lower cost and smaller volume. A perturb and observe method is adopted to extract the maximum power from photovoltaic (PV) arrays. Furthermore, a microchip associated with PWM IC is used to implement maximum power point tracking operation, voltage regulation and power management. Finally, a prototype PV power system with 400 V/6 A has been implemented for verifying the feasibility of the proposed PV power system. It is shown to be suitable for PV energy conversion applications when the duty ratios of switches in the dc/dc converter are less than 0.5. Energies 2013-02-21 6 2 Article 10.3390/en6021068 1068 1100 1996-1073 2013-02-21 doi: 10.3390/en6021068 Sheng-Yu Tseng Hung-Yuan Wang http://www.mdpi.com/1996-1073/6/2/1050 Compressed Air Energy Storage is recognized as a promising technology for applying energy storage to grids which are more and more challenged by the increasing contribution of renewable such as solar or wind energy. The paper proposes a medium-size ground-based CAES system, based on pressurized vessels and on a multiple-stage arrangement of compression and expansion machinery; the system includes recovery of heat from the intercoolers, and its storage as sensible heat in two separate (hot/cold) water reservoirs, and regenerative reheat of the expansions. The CAES plant parameters were adapted to the requirements of existing equipment (compressors, expanders and heat exchangers). A complete exergy analysis of the plant was performed. Most component cost data were procured from the market, asking specific quotations to the industrial providers. It is thus possible to calculate the final cost of the electricity unit (kWh) produced under peak-load mode, and to identify the relative contribution between the two relevant groups of capital and component inefficiencies costs. Energies 2013-02-19 6 2 Article 10.3390/en6021050 1050 1067 1996-1073 2013-02-19 doi: 10.3390/en6021050 Francesco Buffa Simon Kemble Giampaolo Manfrida Adriano Milazzo http://www.mdpi.com/1996-1073/6/2/1023 This paper addresses recycling of waste engine oils treated using acetic acid. A recycling process was developed which eventually led to comparable results with some of the conventional methods. This gives the recycled oil the potential to be reused in cars’ engines after adding the required additives. The advantage of using the acetic acid is that it does not react or only reacts slightly with base oils. The recycling process takes place at room temperature. It has been shown that base oils and oils’ additives are slightly affected by the acetic acid. Upon adding 0.8 vol% of acetic acid to the used oil, two layers were separated, a transparent dark red colored oil and a black dark sludge at the bottom of the container. The base oils resulting from other recycling methods were compared to the results of this paper. The comparison showed that the recycled oil produced by acetic acid treatment is comparable to those recycled by the other conventional methods. Energies 2013-02-19 6 2 Article 10.3390/en6021023 1023 1049 1996-1073 2013-02-19 doi: 10.3390/en6021023 Ihsan Hamawand Talal Yusaf Sardasht Rafat http://www.mdpi.com/1996-1073/6/2/1009 Hydrate formation/dissociation of natural gas in (diesel oil + water) emulsion systems containing 3 wt% anti-agglomerant were performed for five water cuts: 5, 10, 15, 20, and 25 vol%. The natural gas solubilities in the emulsion systems were also examined. The experimental results showed that the solubility of natural gas in emulsion systems increases almost linearly with the increase of pressure, and decreases with the increase of water cut. There exists an initial slow hydrate formation stage for systems with lower water cut, while rapid hydrate formation takes place and the process of the gas-liquid dissolution equilibrium at higher water cut does not appear in the pressure curve. The gas consumption amount due to hydrate formation at high water cut is significantly higher than that at low water cut. Fractional distillation for natural gas components also exists during the hydrate formation process. The experiments on hydrate dissociation showed that the dissociation rate and the amount of dissociated gas increase with the increase of water cut. The variations of temperature in the process of natural gas hydrate formation and dissociation in emulsion systems were also examined. Energies 2013-02-18 6 2 Article 10.3390/en6021009 1009 1022 1996-1073 2013-02-18 doi: 10.3390/en6021009 Chang-Sheng Xiang Bao-Zi Peng Huang Liu Chang-Yu Sun Guang-Jin Chen Bao-Jiang Sun http://www.mdpi.com/1996-1073/6/2/988 The Smart Grid has the potential to bring significant value to the various stakeholders of the electricity market. A methodology for the evaluation of the smart grid benefits is required to facilitate the decision making by quantifying the benefits expected from a smart grid project. The present paper proposes a generic framework to assess these expected benefits taking into account the regulatory, business and technical challenges focusing particularly on Distributed Systems Operators (DSOs) and end users. An indicative study case is presented where the proposed cost-benefit approach assesses the expected value of DSOs from the Smart Grid and determines whether and under what conditions such an investment should be initiated. Energies 2013-02-13 6 2 Review 10.3390/en6020988 988 1008 1996-1073 2013-02-13 doi: 10.3390/en6020988 Spiros Livieratos Vasiliki-Emmanouela Vogiatzaki Panayotis Cottis http://www.mdpi.com/1996-1073/6/2/973 High-technology industries provide opportunities for economic growth, but also raise concerns because of their energy-demanding nature. This paper provides an integrated evaluation of both economic benefits and energy efficiency of high-technology industries based on the real data from one of the globally recognized high-technology industrial clusters, the national science parks in Taiwan. A nation-wide industrial Input-Output Analysis is conducted to demonstrate the positive effects of science parks on national economic developments and industrial upgrades. The concept of energy intensity and an energy-efficient economy index are applied to an integrated assessment of the relationship between economic growth and energy consumption. The proposed case study suggests that economic and energy efficiency objectives can be simultaneously achieved by the development of high-technology industries, while three energy policy implications are considered. First, a nation-wide macro viewpoint is needed and high-technology industries should be considered as parts of the national/regional economies by governmental agencies. Second, a proper industrial clustering mechanism and the shared environmental facilities supported by the government, such as planned land and road usage, electricity and water supply, telecommunications system, sewerage system and wastewater treatments, can improve energy efficiency of high-technology industries. Third, the governmental policies on the taxing and management system in science parks would also direct energy-efficient economy of high-technology industries. Energies 2013-02-13 6 2 Article 10.3390/en6020973 973 987 1996-1073 2013-02-13 doi: 10.3390/en6020973 Min-Ren Yan Kuo-Ming Chien http://www.mdpi.com/1996-1073/6/2/953 System cost reductions and development of standardised plug-and-function systems are some of the most important goals for solar heating technology development. Retrofitting hot water boilers in single-family houses when installing solar collectors has the potential to significantly reduce both material and installation costs. In this study, the TRNSYS simulation models of the retrofitting solar thermal system were validated against measurements. Results show that the validated models are in good agreement with measurements. On an annual basis a deviation of 2.5% out of 1099 kWh was obtained between the auxiliary energy from results and from the simulation model for a complete system. Using the validated model a system optimization was carried out with respect to control strategies for auxiliary heating, heat losses and volume of auxiliary storage. A sensitivity analysis was carried out regarding different volumes of retrofitted hot water boiler, DHW profiles and climates. It was estimated that, with adequate improvements, extended annual solar fractions of 60%, 78% and 81% can be achieved for Lund (Sweden), Lisbon (Portugal) and Lusaka (Zambia), respectively. The correspondent collector area was 6, 4 and 3 m2, respectively. The studied retrofitted system achieves a comparable performance with conventional solar thermal systems with the potential to reduce the investment cost. Energies 2013-02-12 6 2 Article 10.3390/en6020953 953 972 1996-1073 2013-02-12 doi: 10.3390/en6020953 Luis Bernardo http://www.mdpi.com/1996-1073/6/2/934 The partial discharge (PD) process in liquid dielectrics is influenced by several factors. Although the PD current contains the information representing the discharge process during the PD event, it is difficult to determine the detailed dynamics of what is happening in the bulk of the liquid. In this paper, a microscopic model describing the dynamics of the charge carriers is implemented. The model consists of drift-diffusion equations of electrons, positive and negative ions coupled with Poisson’s equation. The stochastic feature of PD events is included in the equation. First the model is validated through comparison between the calculated PD current and experimental data. Then experiments are conducted to study the effects of the amplitude of the applied voltage, gap distance and electrode type on the PD process. The PD currents under each condition are recorded. Simulations based on the model have been conducted to analyze the dynamics of the PD events under each condition, and thus explain the mechanism of how these factors influence the PD events. The space charge generated in the PD process is revealed as the main reason affecting the microscopic process of the PD events. Energies 2013-02-08 6 2 Article 10.3390/en6020934 934 952 1996-1073 2013-02-08 doi: 10.3390/en6020934 Wenxia Sima Chilong Jiang Paul Lewin Qing Yang Tao Yuan http://www.mdpi.com/1996-1073/6/2/921 This paper presents an integration design and implementation of an air motor and a DC servo motor which utilizes a magnetic powder brake to integrate these two motors together. The dynamic model of the air/electric hybrid system will be derived and eventually leads to successful ECE-40 driving cycle tests with a FPGA-based speed controller. The testing results obtained by using the proposed experimental platform indicate that the total air consumption is about 256 L under air motor mode and the electric charge consumption is about 530 coulombs under DC servo motor mode. In a hybrid mode, the current reduction of the battery is about 18.5%, and then the service life of the battery can be improved. Furthermore, a prototype is built with a proportional-integral (PI) speed controller based on a field-programmable gate array (FPGA) in order to facilitate the entire analysis of the velocity switch experiment. Through the modular methodology of FPGA, the hybrid power platform can successfully operate under ECE-40 driving cycle with the PI speed controller. The experimental data shows that the chattering ranges of the air motor within ±1 km/h and ±0.2 km/h under DC servo motor drive. Therefore, the PI speed controller based on FPGA is successfully actualized. Energies 2013-02-08 6 2 Article 10.3390/en6020921 921 933 1996-1073 2013-02-08 doi: 10.3390/en6020921 Yean-Ren Hwang Shih-Yao Huang http://www.mdpi.com/1996-1073/6/2/900 Modeling of supermarket refrigeration systems for supervisory control in the smart grid is presented in this paper. A modular modeling approach is proposed in which each module is modeled and identified separately. The focus of the work is on estimating the power consumption of the system while estimating the cold reservoir temperatures as well. The models developed for each module as well as for the overall integrated system are validated by real data collected from a supermarket in Denmark. The results show that the model is able to estimate the actual electrical power consumption with a high fidelity. Moreover a simulation benchmark is introduced based on the produced model for demand-side management in smart grid. Finally, a potential application of the proposed benchmark in direct control of the power/energy consumption is presented by a simple simulation example. Energies 2013-02-08 6 2 Article 10.3390/en6020900 900 920 1996-1073 2013-02-08 doi: 10.3390/en6020900 S. Shafiei Henrik Rasmussen Jakob Stoustrup http://www.mdpi.com/1996-1073/6/2/884 Optimization of nutrient supplements i.e., yeast extract (1, 3 and 5 g·L−1), dried spent yeast (DSY: 4, 12 and 20 g·L−1) and osmoprotectant (glycine: 1, 3 and 5 g·L−1) to improve the efficiency of ethanol production from a synthetic medium under very high gravity (VHG) fermentation by Saccharomyces cerevisiae NP 01 was performed using a statistical method, an L9 (34) orthogonal array design. The synthetic medium contained 280 g·L−1 of sucrose as a sole carbon source. When the fermentation was carried out at 30 °C, the ethanol concentration (P), yield (Yp/s) and productivity (Qp) without supplementation were 95.3 g·L−1, 0.49 g·g−1 and 1.70 g·L−1·h−1, respectively. According to the orthogonal results, the order of influence on the P and Qp values were yeast extract > glycine > DSY, and the optimum nutrient concentrations were yeast extract, 3; DSY, 4 and glycine, 5 g·L−1, respectively. The verification experiment using these parameters found that the P, Yp/s and Qp values were 119.9 g·L−1, 0.49 g g−1 and 2.14 g·L−1·h−1, respectively. These values were not different from those of the synthetic medium supplemented with 9 g·L−1 of yeast extract, indicating that DSY could be used to replace some amount of yeast extract. When sweet sorghum juice cv. KKU40 containing 280 g·L−1 of total sugar supplemented with the three nutrients at the optimum concentrations was used as the ethanol production medium, the P value (120.0 g·L−1) was not changed, but the Qp value was increased to 2.50 g·L−1·h−1. Energies 2013-02-08 6 2 Article 10.3390/en6020884 884 899 1996-1073 2013-02-08 doi: 10.3390/en6020884 Pachaya Chan-u-tit Lakkana Laopaiboon Prasit Jaisil Pattana Laopaiboon http://www.mdpi.com/1996-1073/6/2/871 Carbonization is a newly developed process that converts sewage sludge to biocoal, a type of solid biomass that can partially substitute for coal during power generation. This study presents an assessment of the environmental effects of various sewage sludge treatment processes, including carbonization, direct landfills, co-incineration with municipal solid waste, and mono-incineration in Taiwan. This assessment was conducted using the life cycle assessment software SimaPro 7.2 and the IMPACT2002+ model. Results show that carbonization is the best approach for sewage sludge treatment, followed in descending order by co-incineration with municipal solid waste, direct landfills, and mono-incineration. The carbonization process has noticeable positive effects in the environmental impact categories of terrestrial ecotoxicity, aquatic ecotoxicity, land occupation, ionizing radiation, aquatic eutrophication, non-renewable energy, and mineral extraction. For the emission quantity of greenhouse gases, landfilling has the greatest impact (296.9 kg CO2 eq./t sludge), followed by mono-incineration (232.2 kg CO2 eq./t sludge) and carbonization (146.1 kg CO2 eq./t sludge). Co-incineration with municipal solid waste has the benefit of reducing green house gas emission (–15.4 kg CO2 eq./t sludge). In the aspect of energy recovery, sewerage sludge that has been pretreated by thickening, digestion, and dewatering still retains a high moisture content, and thus requires a significant amount of energy use when used as a substitute solid fuel. Therefore, the carbonization of sewage sludge would be a more sustainable option if the energy delivery and integration processes are made more efficient. Energies 2013-02-07 6 2 Article 10.3390/en6020871 871 883 1996-1073 2013-02-07 doi: 10.3390/en6020871 Ning-Yi Wang Chun-Hao Shih Pei-Te Chiueh Yu-Fong Huang http://www.mdpi.com/1996-1073/6/2/839 Efficient high speed propulsion requires exploiting the cooling capability of the cryogenic fuel in the propulsion cycle. This paper presents the numerical model of a combined cycle engine while in air turbo-rocket configuration. Specific models of the various heat exchanger modules and the turbomachinery elements were developed to represent the physical behavior at off-design operation. The dynamic nature of the model allows the introduction of the engine control logic that limits the operation of certain subcomponents and extends the overall engine operational envelope. The specific impulse and uninstalled thrust are detailed while flying a determined trajectory between Mach 2.5 and 5 for varying throttling levels throughout the operational envelope. Energies 2013-02-07 6 2 Article 10.3390/en6020839 839 870 1996-1073 2013-02-07 doi: 10.3390/en6020839 Victor Fernandez-Villace Guillermo Paniagua http://www.mdpi.com/1996-1073/6/2/807 The viscosity of slag and the thermal conductivity of ash deposits are among two of the most important constitutive parameters that need to be studied. The accurate formulation or representations of the (transport) properties of coal present a special challenge of modeling efforts in computational fluid dynamics applications. Studies have indicated that slag viscosity must be within a certain range of temperatures for tapping and the membrane wall to be accessible, for example, between 1,300 °C and 1,500 °C, the viscosity is approximately 25 Pa·s. As the operating temperature decreases, the slag cools and solid crystals begin to form. Since slag behaves as a non-linear fluid, we discuss the constitutive modeling of slag and the important parameters that must be studied. We propose a new constitutive model, where the stress tensor not only has a yield stress part, but it also has a viscous part with a shear rate dependency of the viscosity, along with temperature and concentration dependency, while allowing for the possibility of the normal stress effects. In Part I, we reviewed, identify and discuss the key coal ash properties and the operating conditions impacting slag behavior. Energies 2013-02-07 6 2 Review 10.3390/en6020807 807 838 1996-1073 2013-02-07 doi: 10.3390/en6020807 Mehrdad Massoudi Ping Wang http://www.mdpi.com/1996-1073/6/2/784 In the entrained-flow gasifiers used in integrated gasification combined cycle (IGCC) plants, the majority of mineral matter transforms to liquid slag on the wall of the gasifier and flows out the bottom. However, a small fraction of the mineral matter is entrained (as fly ash) with the raw syngas out of the gasifier to downstream processing. This molten/sticky fly ash could cause fouling of the syngas cooler. To improve gasification availability through better design and operation of the gasification process, a better understanding of slag behavior and the characteristics of the slagging process is needed. Char/ash properties, gas compositions in the gasifier, the gasifier wall structure, fluid dynamics, and plant operating conditions (mainly temperature and oxygen/carbon ratio) all affect slagging behavior. Because coal has varying ash content and composition, different operating conditions are required to maintain the slag flow and limit problems downstream. In Part I, we review the main types and the operating conditions of entrained-flow gasifiers and coal properties used in IGCC plants; we identify and discuss the key coal ash properties and the operating conditions impacting slag behavior; finally, we summarize the coal quality criteria and the operating conditions in entrained-flow gasifiers. In Part II, we discuss the constitutive modeling related to the rheological studies of slag flow. Energies 2013-02-07 6 2 Review 10.3390/en6020784 784 806 1996-1073 2013-02-07 doi: 10.3390/en6020784 Ping Wang Mehrdad Massoudi