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Energies, Volume 8, Issue 3 (March 2015) , Pages 1547-2294

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Open AccessEditorial
Energies Best Paper Awards for 2015
Energies 2015, 8(3), 2292-2294; https://doi.org/10.3390/en8032292
Received: 12 March 2015 / Accepted: 13 March 2015 / Published: 23 March 2015
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Abstract
We are pleased to announce the Energies Best Papers awards for 2015. Energies is an open access journal of energy-related scientific research, technology development, and policy and management studies. It publishes reviews, regular research articles, and communications in a number of fields related [...] Read more.
We are pleased to announce the Energies Best Papers awards for 2015. Energies is an open access journal of energy-related scientific research, technology development, and policy and management studies. It publishes reviews, regular research articles, and communications in a number of fields related to the procurement, conversion and final uses of energy.[...] Full article
Open AccessCorrection
Menéndez, R.P.; Martínez, J.A.; Prieto, M.J.; Barcia, L.A.; Sánchez, J.M.M. A Novel Modeling of Molten-Salt Heat Storage Systems in Thermal Solar Power Plants. Energies 2014, 7, 6721–6740
Energies 2015, 8(3), 2290-2291; https://doi.org/10.3390/en8032290
Received: 19 January 2015 / Revised: 6 February 2015 / Accepted: 11 February 2015 / Published: 23 March 2015
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Abstract
The authors wish to make the following corrections to this paper [1]: Figure 8 on page 6730 is replaced as follows:[...] Full article
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Open AccessArticle
Optimal Dispatch Strategy of a Virtual Power Plant Containing Battery Switch Stations in a Unified Electricity Market
Energies 2015, 8(3), 2268-2289; https://doi.org/10.3390/en8032268
Received: 12 October 2014 / Revised: 28 January 2015 / Accepted: 16 March 2015 / Published: 23 March 2015
Cited by 18 | Viewed by 4456 | PDF Full-text (928 KB) | HTML Full-text | XML Full-text
Abstract
A virtual power plant takes advantage of interactive communication and energy management systems to optimize and coordinate the dispatch of distributed generation, interruptible loads, energy storage systems and battery switch stations, so as to integrate them as an entity to exchange energy with [...] Read more.
A virtual power plant takes advantage of interactive communication and energy management systems to optimize and coordinate the dispatch of distributed generation, interruptible loads, energy storage systems and battery switch stations, so as to integrate them as an entity to exchange energy with the power market. This paper studies the optimal dispatch strategy of a virtual power plant, based on a unified electricity market combining day-ahead trading with real-time trading. The operation models of interruptible loads, energy storage systems and battery switch stations are specifically described in the paper. The virtual power plant applies an optimal dispatch strategy to earn the maximal expected profit under some fluctuating parameters, including market price, retail price and load demand. The presented model is a nonlinear mixed-integer programming with inter-temporal constraints and is solved by the fruit fly algorithm. Full article
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Open AccessArticle
Modeling Innovative Power Take-Off Based on Double-Acting Hydraulic Cylinders Array for Wave Energy Conversion
Energies 2015, 8(3), 2230-2267; https://doi.org/10.3390/en8032230
Received: 28 January 2015 / Revised: 27 February 2015 / Accepted: 2 March 2015 / Published: 20 March 2015
Cited by 4 | Viewed by 2657 | PDF Full-text (5232 KB) | HTML Full-text | XML Full-text
Abstract
One of the key systems of a Wave Energy Converter for extraction of wave energy is the Power Take-Off (PTO) device. This device transforms the mechanical energy of a moving body into electrical energy. This paper describes the model of an innovative PTO [...] Read more.
One of the key systems of a Wave Energy Converter for extraction of wave energy is the Power Take-Off (PTO) device. This device transforms the mechanical energy of a moving body into electrical energy. This paper describes the model of an innovative PTO based on independently activated double-acting hydraulic cylinders array. The model has been developed using a simulation tool, based on a port-based approach to model hydraulics systems. The components and subsystems used in the model have been parameterized as real components and their values experimentally obtained from an existing prototype. In fact, the model takes into account most of the hydraulic losses of each component. The simulations show the flexibility to apply different restraining torques to the input movement depending on the geometrical configuration and the hydraulic cylinders on duty, easily modified by a control law. The combination of these two actions allows suitable flexibility to adapt the device to different sea states whilst optimizing the energy extraction. The model has been validated using a real test bench showing good correlations between simulation and experimental tests. Full article
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Open AccessArticle
Modelling of a Solid Oxide Fuel Cell CHP System Coupled with a Hot Water Storage Tank for a Single Household
Energies 2015, 8(3), 2211-2229; https://doi.org/10.3390/en8032211
Received: 3 November 2014 / Revised: 4 March 2015 / Accepted: 12 March 2015 / Published: 20 March 2015
Cited by 4 | Viewed by 2760 | PDF Full-text (1101 KB) | HTML Full-text | XML Full-text
Abstract
In this paper a solid oxide fuel cell (SOFC) system for cogeneration of heat and power integrated with a stratified heat storage tank is studied. The use of a storage tank with thermal stratification allows one to increase the annual operating hours of [...] Read more.
In this paper a solid oxide fuel cell (SOFC) system for cogeneration of heat and power integrated with a stratified heat storage tank is studied. The use of a storage tank with thermal stratification allows one to increase the annual operating hours of CHP: heat can be produced when the request is low (for instance during the night), taking advantage of thermal stratification to increases the heat recovery performance. A model of the SOFC system is presented to estimate the energy required to meet the average electric energy demand of the residence. Two fuels are considered, namely syngas produced by gasification and natural gas. The tank model considers the temperature gradients over the tank height. The results of the numerical simulation are used to size the SOFC system and storage heat tank to provide energy for a small household using two different fuels. In particular it was shown that in the case of syngas, due to larger system heat output, a larger tank volume was required in order to accumulate unused heat over the night. The detailed description of the tank model will be useful to energy system modelers when sizing hot water tanks. Problem formulation is reported also using a Matlab script. Full article
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Open AccessArticle
Experimental Analysis of Cool Traditional Solar Shading Systems for Residential Buildings
Energies 2015, 8(3), 2197-2210; https://doi.org/10.3390/en8032197
Received: 9 February 2015 / Revised: 10 March 2015 / Accepted: 12 March 2015 / Published: 20 March 2015
Cited by 12 | Viewed by 2295 | PDF Full-text (2441 KB) | HTML Full-text | XML Full-text
Abstract
In recent years there has been a growing interest in the development and thermal-energy analysis of passive solutions for reducing building cooling needs and thus improving indoor thermal comfort conditions. In this view, several studies were carried out about cool roofs and cool [...] Read more.
In recent years there has been a growing interest in the development and thermal-energy analysis of passive solutions for reducing building cooling needs and thus improving indoor thermal comfort conditions. In this view, several studies were carried out about cool roofs and cool coatings, producing acknowledged mitigation effects on urban heat island phenomenon. The purpose of this work is to investigate the thermal-energy performance of cool louvers of shutters, usually installed in residential buildings, compared to dark color traditional shading systems. To this aim, two full-scale prototype buildings were continuously monitored under summer conditions and the role of the cool shutter in reducing the overheating of the shading system and the energy requirements for cooling was analyzed. After an in-lab optical analysis of the cool coating, showing a huge solar reflectance increase with respect to the traditional configuration, i.e., by about 75%, field monitoring results showed that the cool shutter is able to decrease the indoor air temperature up to 2 °C under free floating conditions. The corresponding energy saving was about 25%, with even much higher peaks during very hot summer conditions. Full article
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Open AccessArticle
Estimation of CO2 Transport Costs in South Korea Using a Techno-Economic Model
Energies 2015, 8(3), 2176-2196; https://doi.org/10.3390/en8032176
Received: 16 December 2014 / Revised: 5 March 2015 / Accepted: 16 March 2015 / Published: 19 March 2015
Cited by 3 | Viewed by 2238 | PDF Full-text (1406 KB) | HTML Full-text | XML Full-text
Abstract
In this study, a techno–economic model was used to calculate the costs of CO2 transport and specify the major equipment required for transport in order to demonstrate and implement CO2 sequestration in the offshore sediments of South Korea. First, three different [...] Read more.
In this study, a techno–economic model was used to calculate the costs of CO2 transport and specify the major equipment required for transport in order to demonstrate and implement CO2 sequestration in the offshore sediments of South Korea. First, three different carbon capture and storage demonstration scenarios were set up involving the use of three CO2 capture plants and one offshore storage site. Each transport scenario considered both the pipeline transport and ship transport options. The temperature and pressure conditions of CO2 in each transport stage were determined from engineering and economic viewpoints, and the corresponding specifications and equipment costs were calculated. The transport costs for a 1 MtCO2/year transport rate were estimated to be US$33/tCO2 and US$28/tCO2 for a pipeline transport of ~530 km and ship transport of ~724 km, respectively. Through the economies of scale effect, the pipeline and ship transport costs for a transport rate of 3 MtCO2/year were reduced to approximately US$21/tCO2 and US$23/tCO2, respectively. A CO2 hub terminal did not significantly reduce the cost because of the short distance from the hub to the storage site and the small number of captured sources. Full article
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Open AccessArticle
The Impact of a Mild Sub-Critical Hydrothermal Carbonization Pretreatment on Umbila Wood. A Mass and Energy Balance Perspective
Energies 2015, 8(3), 2165-2175; https://doi.org/10.3390/en8032165
Received: 8 October 2014 / Revised: 24 December 2014 / Accepted: 3 March 2015 / Published: 19 March 2015
Cited by 2 | Viewed by 2241 | PDF Full-text (419 KB) | HTML Full-text | XML Full-text
Abstract
Over the last years, the pretreatment of biomass as a source of energy has become one of the most important steps of biomass conversion. In this work the effect of a mild subcritical hydrothermal carbonization of a tropical woody biomass was studied. Results [...] Read more.
Over the last years, the pretreatment of biomass as a source of energy has become one of the most important steps of biomass conversion. In this work the effect of a mild subcritical hydrothermal carbonization of a tropical woody biomass was studied. Results indicate considerable change in carbon content from 52.78% to 65.1%, reduction of oxygen content from 41.14% to 28.72% and ash slagging and fouling potential. Even though decarboxylation, decarbonylation and dehydration reactions take place, dehydration is the one that prevails. The mass and energy balance was affected by the treatment conditions than the severity of the treatment. Full article
(This article belongs to the Special Issue Bioenergy and Biorefining)
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Open AccessArticle
Load Frequency Control in Isolated Micro-Grids with Electrical Vehicles Based on Multivariable Generalized Predictive Theory
Energies 2015, 8(3), 2145-2164; https://doi.org/10.3390/en8032145
Received: 16 October 2014 / Accepted: 4 March 2015 / Published: 18 March 2015
Cited by 34 | Viewed by 4091 | PDF Full-text (533 KB) | HTML Full-text | XML Full-text
Abstract
In power systems, although the inertia energy in power sources can partly cover power unbalances caused by load disturbance or renewable energy fluctuation, it is still hard to maintain the frequency deviation within acceptable ranges. However, with the vehicle-to-grid (V2G) technique, electric vehicles [...] Read more.
In power systems, although the inertia energy in power sources can partly cover power unbalances caused by load disturbance or renewable energy fluctuation, it is still hard to maintain the frequency deviation within acceptable ranges. However, with the vehicle-to-grid (V2G) technique, electric vehicles (EVs) can act as mobile energy storage units, which could be a solution for load frequency control (LFC) in an isolated grid. In this paper, a LFC model of an isolated micro-grid with EVs, distributed generations and their constraints is developed. In addition, a controller based on multivariable generalized predictive control (MGPC) theory is proposed for LFC in the isolated micro-grid, where EVs and diesel generator (DG) are coordinated to achieve a satisfied performance on load frequency. A benchmark isolated micro-grid with EVs, DG, and wind farm is modeled in the Matlab/Simulink environment to demonstrate the effectiveness of the proposed method. Simulation results demonstrate that with MGPC, the energy stored in EVs can be managed intelligently according to LFC requirement. This improves the system frequency stability with complex operation situations including the random renewable energy resource and the continuous load disturbances. Full article
(This article belongs to the Special Issue Microgrids)
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Open AccessArticle
Design of an LLC Resonant Converter for Driving Multiple LED Lights Using Current Balancing of Capacitor and Transformer
Energies 2015, 8(3), 2125-2144; https://doi.org/10.3390/en8032125
Received: 3 November 2014 / Accepted: 12 March 2015 / Published: 18 March 2015
Cited by 7 | Viewed by 3571 | PDF Full-text (2265 KB) | HTML Full-text | XML Full-text
Abstract
In this study, to achieve the constant current drive and brightness control without a separate pulse width modulation (PWM) converter, a single converter is designed and verified by experiment under the condition of a multiple LED light load with different output voltage (Vf) [...] Read more.
In this study, to achieve the constant current drive and brightness control without a separate pulse width modulation (PWM) converter, a single converter is designed and verified by experiment under the condition of a multiple LED light load with different output voltage (Vf) characteristics. In the case of the input of 140 Watt class level, the proposed converter can drive two voltage type 95 Vdc (300 mA) light emitting diode (LED) lights loads and 120 Vdc (300 mA) LED lights loads simultaneously. In addition, to improve commercial compatibility, the proposed converter is operated in a wide range of the input voltage 90~264 Vac; also, the Power Factor Correction (PFC) circuit with the input power factor of more than 0.9 is added. In order to maximize the power conversion efficiency, a LLC resonant converter is applied to the PFC block with the output voltage of 380 Vdc and to a DC-DC conversion block. Finally, reliability of the proposed converter is verified through total harmonic distortion (THD) and electromagnetic interference (EMI) tests. Full article
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Open AccessArticle
Thermo-Economic Evaluation of Organic Rankine Cycles for Geothermal Power Generation Using Zeotropic Mixtures
Energies 2015, 8(3), 2097-2124; https://doi.org/10.3390/en8032097
Received: 30 January 2015 / Revised: 4 March 2015 / Accepted: 11 March 2015 / Published: 17 March 2015
Cited by 30 | Viewed by 3853 | PDF Full-text (470 KB) | HTML Full-text | XML Full-text
Abstract
We present a thermo-economic evaluation of binary power plants based on the Organic Rankine Cycle (ORC) for geothermal power generation. The focus of this study is to analyse if an efficiency increase by using zeotropic mixtures as working fluid overcompensates additional requirements regarding [...] Read more.
We present a thermo-economic evaluation of binary power plants based on the Organic Rankine Cycle (ORC) for geothermal power generation. The focus of this study is to analyse if an efficiency increase by using zeotropic mixtures as working fluid overcompensates additional requirements regarding the major power plant components. The optimization approach is compared to systems with pure media. Based on process simulations, heat exchange equipment is designed and cost estimations are performed. For heat source temperatures between 100 and 180 °C selected zeotropic mixtures lead to an increase in second law efficiency of up to 20.6% compared to pure fluids. Especially for temperatures about 160 °C, mixtures like propane/isobutane, isobutane/isopentane, or R227ea/R245fa show lower electricity generation costs compared to the most efficient pure fluid. In case of a geothermal fluid temperature of 120 °C, R227ea and propane/isobutane are cost-efficient working fluids. The uncertainties regarding fluid properties of zeotropic mixtures, mainly affect the heat exchange surface. However, the influence on the determined economic parameter is marginal. In general, zeotropic mixtures are a promising approach to improve the economics of geothermal ORC systems. Additionally, the use of mixtures increases the spectrum of potential working fluids, which is important in context of present and future legal requirements considering fluorinated refrigerants. Full article
(This article belongs to the Special Issue Organic Rankine Cycle (ORC))
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Open AccessArticle
Fabrication and Test of an Air-Breathing Microfluidic Fuel Cell
Energies 2015, 8(3), 2082-2096; https://doi.org/10.3390/en8032082
Received: 18 January 2015 / Revised: 23 February 2015 / Accepted: 5 March 2015 / Published: 16 March 2015
Cited by 5 | Viewed by 2310 | PDF Full-text (3094 KB) | HTML Full-text | XML Full-text
Abstract
An air-breathing direct formic acid microfluidic fuel cell, which had a self-made anode electrode of 10 mg/cm2 Pd loading and 6 mg/cm2 Nafion content, was fabricated and tested. The microfluidic fuel cell was achieved by bonding a PDMS microchannel that was [...] Read more.
An air-breathing direct formic acid microfluidic fuel cell, which had a self-made anode electrode of 10 mg/cm2 Pd loading and 6 mg/cm2 Nafion content, was fabricated and tested. The microfluidic fuel cell was achieved by bonding a PDMS microchannel that was fabricated by a soft-lithography process and a PMMA sheet that was machined by a CO2 laser for obtaining 50 through holes of 0.5 mm in diameter. Formic acid of 0.3 M, 0.5 M, and 1.0 M, mixed with 0.5-M H2SO4, was supplied at a flow rate ranging from 0.1 to 0.7 mL/min as fuel. The maximum power density of the fuel cell fed with 0.5-M HCOOH was approximately 31, 32.16, and 31 mW/cm2 at 0.5, 0.6, and 0.7 mL/min, respectively. The simultaneous recording of the flow in the microchannel and the current density of the fuel cell at 0.2 V, within a 100-s duration, showed that the period and amplitude of each unsteady current oscillation were associated with the bubble resident time and bubble dimension, respectively. The effect of bubble dimension included the longitudinal and transverse bubble dimension, and the distance between two in-line bubbles as well. Full article
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Open AccessArticle
Radiological Impacts and Regulation of Rare Earth Elements in Non-Nuclear Energy Production
Energies 2015, 8(3), 2066-2081; https://doi.org/10.3390/en8032066
Received: 4 December 2014 / Revised: 4 March 2015 / Accepted: 5 March 2015 / Published: 13 March 2015
Cited by 8 | Viewed by 2288 | PDF Full-text (471 KB) | HTML Full-text | XML Full-text
Abstract
Energy industries account for a significant portion of total rare earth usage, both in the US and worldwide. Rare earth minerals are frequently collocated with naturally occurring radioactive material, imparting an occupational radiological dose during recovery. This paper explores the extent to which [...] Read more.
Energy industries account for a significant portion of total rare earth usage, both in the US and worldwide. Rare earth minerals are frequently collocated with naturally occurring radioactive material, imparting an occupational radiological dose during recovery. This paper explores the extent to which rare earths are used by various non-nuclear energy industries and estimates the radiological dose which can be attributed to these industries on absolute and normalized scales. It was determined that typical rare earth mining results in an occupational collective dose of approximately 0.0061 person-mSv/t rare earth elements, amounting to a total of 330 person-mSv/year across all non-nuclear energy industries (about 60% of the annual collective dose from one pressurized water reactor operated in the US, although for rare earth mining the impact is spread out over many more workers). About half of the collective dose from non-nuclear energy production results from use of fuel cracking catalysts for oil refining, although given the extent of the oil industry, it is a small dose when normalized to the energy equivalent of the oil that is used annually. Another factor in energy industries’ reliance on rare earths is the complicated state of the regulation of naturally occurring radiological materials; correspondingly, this paper also explores regulatory and management implications. Full article
(This article belongs to the Special Issue Advances in Nuclear Reactor and Fuel Cycle Technologies)
Open AccessArticle
Integration of a Water Scrubbing Technique and Two-Stage Pressurized Anaerobic Digestion in One Process
Energies 2015, 8(3), 2048-2065; https://doi.org/10.3390/en8032048
Received: 23 December 2014 / Revised: 13 February 2015 / Accepted: 15 February 2015 / Published: 13 March 2015
Cited by 9 | Viewed by 2411 | PDF Full-text (998 KB) | HTML Full-text | XML Full-text
Abstract
Two-stage pressurized anaerobic digestion is a promising technology. This technology integrates in one process biogas production with upgrading and pressure boosting for grid injection. To investigate whether the efficiency of this novel system could be further increased, a water scrubbing system was integrated [...] Read more.
Two-stage pressurized anaerobic digestion is a promising technology. This technology integrates in one process biogas production with upgrading and pressure boosting for grid injection. To investigate whether the efficiency of this novel system could be further increased, a water scrubbing system was integrated into the methanogensis step. Therefore, six leach-bed reactors were used for hydrolysis/acidification and a 30-L pressurized anaerobic filter operated at 9 bar was adopted for acetogenesis/methanogenesis. The fermentation liquid of the pressurized anaerobic filter was circulated periodically via a flash tank, operating at atmospheric pressure. Due to the pressure drop, part of dissolved carbon dioxide was released from the liquid phase into the flash tank. The depressurized fermentation liquid was then recycled to the pressurized reactor. Three different flow rates (0 L·day−1, 20 L·day−1 and 40 L·day−1) were tested with three repetitions. As the daily recycled flashed liquid flow was increased from 0 to 40 L, six times as much as the daily feeding, the methane content in the biogas increased from 75 molar percent (mol%) to 87 mol%. The pH value of the substrate in the methane reactor rose simultaneously from 6.5 to 6.7. The experimental data were verified by calculation. Full article
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Open AccessArticle
A Varied VSVM Strategy for Balancing the Neutral-Point Voltage of DC-Link Capacitors in Three-Level NPC Converters
Energies 2015, 8(3), 2032-2047; https://doi.org/10.3390/en8032032
Received: 6 January 2015 / Revised: 18 February 2015 / Accepted: 28 February 2015 / Published: 13 March 2015
Cited by 8 | Viewed by 2466 | PDF Full-text (823 KB) | HTML Full-text | XML Full-text
Abstract
In the research field of multilevel converters, three-level NPC (neutral-point-clamped) converters, which unfortunately may cause the deviation of the neutral-point voltage of DC-link capacitors, are widely discussed. Theoretically, virtual space vector modulation (VSVM) could guarantee the balance control of the neutral-point voltage. However, [...] Read more.
In the research field of multilevel converters, three-level NPC (neutral-point-clamped) converters, which unfortunately may cause the deviation of the neutral-point voltage of DC-link capacitors, are widely discussed. Theoretically, virtual space vector modulation (VSVM) could guarantee the balance control of the neutral-point voltage. However, there still exist some uncontrollable space vector regions. Based on VSVM, this paper proposes a varied virtual space vector modulation (VVSVM) method for three-level NPC converters. Under complete modulation conditions, this method can control the balance of the neutral-point voltage of DC-link capacitors by adjusting the duty cycle of small vectors and regulating the current generated by virtual medium vectors. Compared with commonly used VSVM methods and mixed modulation strategies, this method is simpler and more practical. The effectiveness and validity of this method are verified by simulations and experiments. Full article
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Open AccessArticle
New TA Index-Based Rollover Prevention System for Electric Vehicles
Energies 2015, 8(3), 2008-2031; https://doi.org/10.3390/en8032008
Received: 20 November 2014 / Revised: 12 February 2015 / Accepted: 13 February 2015 / Published: 13 March 2015
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Abstract
In addition to clean transportation and energy savings, electric vehicles can inherently offer better performance in the field of active safety and dynamic stability control, thanks to the superior fast and accurate control characteristics of electric motors. With the novel wheel status parameter [...] Read more.
In addition to clean transportation and energy savings, electric vehicles can inherently offer better performance in the field of active safety and dynamic stability control, thanks to the superior fast and accurate control characteristics of electric motors. With the novel wheel status parameter TA for electric vehicles proposed by the authors in an earlier publication, a new TA index (TAI)-based rollover prevention method is presented in this paper to improve the driving performance of EVs equipped with in-wheel motors. A three-level electric vehicle control structure is used to analyze the effective control steps for rollover prevention with the newly proposed TAI method. The simulation is conducted using an in-house developed electric vehicle dynamic model. The simulation results prove the feasibility of using TAI to detect rollover. The experiment uses an electric vehicle equipped with four in-wheel motors in the authors’ research lab. The vehicle parameter and performance data are imported to CarSim, which is industrial standard vehicle dynamic analysis software to run the rollover test. The experimental results also demonstrate that TAI is an effective method of rollover prevention. Full article
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Open AccessArticle
Impact of Organic Loading Rate on Psychrophilic Anaerobic Digestion of Solid Dairy Manure
Energies 2015, 8(3), 1990-2007; https://doi.org/10.3390/en8031990
Received: 21 November 2014 / Revised: 24 February 2015 / Accepted: 3 March 2015 / Published: 13 March 2015
Cited by 7 | Viewed by 2230 | PDF Full-text (522 KB) | HTML Full-text | XML Full-text
Abstract
Increasing the feed total solids to anaerobic digester improves the process economics and decreases the volume of liquid effluent from current wet anaerobic digestion. The objective of this study was to develop a novel psychrophilic (20 °C) anaerobic digestion technology of undiluted cow [...] Read more.
Increasing the feed total solids to anaerobic digester improves the process economics and decreases the volume of liquid effluent from current wet anaerobic digestion. The objective of this study was to develop a novel psychrophilic (20 °C) anaerobic digestion technology of undiluted cow feces (total solids of 11%–16%). Two sets of duplicate laboratory-scale sequence batch bioreactors have been operated at organic loading rates (OLR) of 6.0 to 8.0 g total chemical oxygen demand (TCOD) kg−1 inoculum day−1 (d−1) during 210 days. The results demonstrated that the process is feasible at treatment cycle length (TCL) of 21 days; however, the quality of cow feces rather than the OLR had a direct influence on the specific methane yield (SMY). The SMY ranged between 124.5 ± 1.4 and 227.9 ± 4.8 normalized liter (NL) CH4 kg−1 volatile solids (VS) fed d−1. Substrate-to-inoculum mass ratio (SIR) was 0.63 ± 0.05, 0.90 ± 0.09, and 1.06 ± 0.07 at OLR of 6.0, 7.0, and 8.0 g TCOD kg−1 inoculum d−1, respectively. No volatile fatty acids (VFAs) accumulation has been observed which indicated that hydrolysis was the rate limiting step and VFAs have been consumed immediately. Bioreactors performance consistency in terms of the level of SMYs, VFAs concentrations at end of the TCL, pH stability and volatile solids reduction indicates a stable and reproducible process during the entire operation. Full article
(This article belongs to the collection Bioenergy and Biofuel)
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Open AccessArticle
Energy’s Shadow Price and Energy Efficiency in China: A Non-Parametric Input Distance Function Analysis
Energies 2015, 8(3), 1975-1989; https://doi.org/10.3390/en8031975
Received: 10 November 2014 / Revised: 12 January 2015 / Accepted: 3 March 2015 / Published: 13 March 2015
Cited by 6 | Viewed by 2456 | PDF Full-text (1150 KB) | HTML Full-text | XML Full-text
Abstract
This paper extends prior research on energy inefficiency in China by utilizing a unique shadow price framework allocation in 30 Chinese provinces. We estimate the shadow price for energy input using the framework of production, and use the ratio of the shadow price [...] Read more.
This paper extends prior research on energy inefficiency in China by utilizing a unique shadow price framework allocation in 30 Chinese provinces. We estimate the shadow price for energy input using the framework of production, and use the ratio of the shadow price to the market price to describe energy utilization. Using Chinese provincial-level data from 1998 to 2011, the results of the analysis reveal that shadow prices in China have grown rapidly during the sample period, which signifies that China has improved its performance in energy utilization since 1998. However, there are eighteen provinces whose shadow prices are lower than market prices. This result suggests that energy utilization is at a low level in these provinces and can be improved by a reallocation of inputs. Full article
(This article belongs to the Special Issue Energy Policy and Climate Change) Printed Edition available
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Open AccessArticle
Slit Wall and Heat Transfer Effect on the Taylor Vortex Flow
Energies 2015, 8(3), 1958-1974; https://doi.org/10.3390/en8031958
Received: 30 November 2014 / Revised: 3 February 2015 / Accepted: 3 March 2015 / Published: 12 March 2015
Cited by 5 | Viewed by 2167 | PDF Full-text (1787 KB) | HTML Full-text | XML Full-text
Abstract
The Taylor vortex flow in the plain model with a constant temperature gradient effect was studied by experimental measurement, and the preliminary features of Taylor vortex flow affected by heat transfer process were obtained. This flow field in the plain model was also [...] Read more.
The Taylor vortex flow in the plain model with a constant temperature gradient effect was studied by experimental measurement, and the preliminary features of Taylor vortex flow affected by heat transfer process were obtained. This flow field in the plain model was also studied by numerical simulation. The reliability of numerical simulation was verified by comparing the numerical results with the experimental ones. To study the slit wall effect on this flow regime under the same temperature gradient conditions, another three models with different slit numbers (6, 9 and 12) were considered. The vortex pairs were found to have a motion along the axial direction. Because of the existence of the temperature gradient, the axial flow in the annulus gap was enhanced, but the radial velocity near the inner cylinder was found to be weakened. The heat flux generated by the inner cylinder was also compared among different models, and it was found that the heat flux generated by the 6-slit model was increased by 4.5% compared to that of the plain model, and the 12-slit model generated the maximum heat flux, which has the best heat transfer ability. Full article
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Open AccessArticle
Ex-Post Critical Evaluations of Energy Policies in Malaysia from 1970 to 2010: A Historical Institutionalism Perspective
Energies 2015, 8(3), 1936-1957; https://doi.org/10.3390/en8031936
Received: 20 January 2015 / Revised: 17 February 2015 / Accepted: 28 February 2015 / Published: 12 March 2015
Cited by 2 | Viewed by 2652 | PDF Full-text (686 KB) | HTML Full-text | XML Full-text
Abstract
Ex-post evaluations of energy policies in Malaysia between 1970 and 2010 were conducted. The developments of energy policies in Malaysia were traced from the early 1970s with the introduction of the country’s first energy-related policy all the way to 2010 with the country’s [...] Read more.
Ex-post evaluations of energy policies in Malaysia between 1970 and 2010 were conducted. The developments of energy policies in Malaysia were traced from the early 1970s with the introduction of the country’s first energy-related policy all the way to 2010 with the country’s first endeavour towards a biobased energy system. Analyses revealed that many of the policies were either: (1) directly responding to changes in global/domestic socioeconomic and political events, or (2) provided visions to guide developments of the energy sector in alignment with the country’s growth agenda. Critical examinations of the country’s actual energy consumptions during these 40 years were also conducted to evaluate the efficacy of these energy-related policies. Three noteworthy successes in Malaysia’s energy landscape are: (1) the formation of PETRONAS as the national oil and gas company; (2) reduction of country’s over-reliance on oil as a single source of energy by significantly growing the production and use of natural gas in a short span of time; and (3) creation of a thriving oil and gas value chain and ecosystem in the country. However, the country is still critically dependent on scarce petroleum resources, despite having an abundance of renewable reserves. Progress towards renewable energy has been too little and too slow. Full article
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Open AccessReview
Heat Recovery from High Temperature Slags: A Review of Chemical Methods
Energies 2015, 8(3), 1917-1935; https://doi.org/10.3390/en8031917
Received: 3 December 2014 / Revised: 13 January 2015 / Accepted: 5 March 2015 / Published: 12 March 2015
Cited by 38 | Viewed by 3097 | PDF Full-text (652 KB) | HTML Full-text | XML Full-text
Abstract
Waste heat recovery from high temperature slags represents the latest potential way to remarkably reduce the energy consumption and CO2 emissions of the steel industry. The molten slags, in the temperature range of 1723–1923 K, carry large amounts of high quality energy. [...] Read more.
Waste heat recovery from high temperature slags represents the latest potential way to remarkably reduce the energy consumption and CO2 emissions of the steel industry. The molten slags, in the temperature range of 1723–1923 K, carry large amounts of high quality energy. However, the heat recovery from slags faces several fundamental challenges, including their low thermal conductivity, inside crystallization, and discontinuous availability. During past decades, various chemical methods have been exploited and performed including methane reforming, coal and biomass gasification, and direct compositional modification and utilization of slags. These methods effectively meet the challenges mentioned before and help integrate the steel industry with other industrial sectors. During the heat recovery using chemical methods, slags can act as not only heat carriers but also as catalysts and reactants, which expands the field of utilization of slags. Fuel gas production using the waste heat accounts for the main R&D trend, through which the thermal heat in the slag could be transformed into high quality chemical energy in the fuel gas. Moreover, these chemical methods should be extended to an industrial scale to realize their commercial application, which is the only way by which the substantial energy in the slags could be extracted, i.e., amounting to 16 million tons of standard coal in China. Full article
(This article belongs to the Special Issue Recent Advances in Coal Combustion and Gasification)
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Open AccessArticle
Hierarchical Load Tracking Control of a Grid-Connected Solid Oxide Fuel Cell for Maximum Electrical Efficiency Operation
Energies 2015, 8(3), 1896-1916; https://doi.org/10.3390/en8031896
Received: 11 December 2014 / Revised: 12 February 2015 / Accepted: 4 March 2015 / Published: 11 March 2015
Cited by 4 | Viewed by 2270 | PDF Full-text (1184 KB) | HTML Full-text | XML Full-text
Abstract
Based on the benchmark solid oxide fuel cell (SOFC) dynamic model for power system studies and the analysis of the SOFC operating conditions, the nonlinear programming (NLP) optimization method was used to determine the maximum electrical efficiency of the grid-connected SOFC subject to [...] Read more.
Based on the benchmark solid oxide fuel cell (SOFC) dynamic model for power system studies and the analysis of the SOFC operating conditions, the nonlinear programming (NLP) optimization method was used to determine the maximum electrical efficiency of the grid-connected SOFC subject to the constraints of fuel utilization factor, stack temperature and output active power. The optimal operating conditions of the grid-connected SOFC were obtained by solving the NLP problem considering the power consumed by the air compressor. With the optimal operating conditions of the SOFC for the maximum efficiency operation obtained at different active power output levels, a hierarchical load tracking control scheme for the grid-connected SOFC was proposed to realize the maximum electrical efficiency operation with the stack temperature bounded. The hierarchical control scheme consists of a fast active power control and a slower stack temperature control. The active power control was developed by using a decentralized control method. The efficiency of the proposed hierarchical control scheme was demonstrated by case studies using the benchmark SOFC dynamic model. Full article
(This article belongs to the Special Issue Microgrids)
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Open AccessArticle
Experimental and Numerical Investigation of Termination Impedance Effects in Wireless Power Transfer via Metamaterial
Energies 2015, 8(3), 1882-1895; https://doi.org/10.3390/en8031882
Received: 8 December 2014 / Revised: 16 February 2015 / Accepted: 26 February 2015 / Published: 9 March 2015
Cited by 16 | Viewed by 2574 | PDF Full-text (16671 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents an investigation of the transmitted power in a wireless power transfer system that employs a metamaterial. Metamaterials are a good means to transfer power wirelessly, as they are composed of multiple inductively-coupled resonators. The system can be designed and matched [...] Read more.
This paper presents an investigation of the transmitted power in a wireless power transfer system that employs a metamaterial. Metamaterials are a good means to transfer power wirelessly, as they are composed of multiple inductively-coupled resonators. The system can be designed and matched simply through magneto-inductive wave theory, particularly when the receiver inductor is located at the end of the metamaterial line. However, the power distribution changes significantly in terms of transmitted power, efficiency and frequency if the receiver inductor slides along the line. In this paper, the power distribution and transfer efficiency are analysed, studying the effects of a termination impedance in the last cell of the metamaterial and improving the system performance for the resonant frequency and for any position of the receiver inductor. Furthermore, a numerical characterisation is presented in order to support experimental tests and to predict the performance of a metamaterial composed of spiral inductor cells with very good accuracy. Full article
(This article belongs to the Special Issue Wireless Power Transfer)
Open AccessArticle
Nanoparticle Filtration Characteristics of Advanced Metal Foam Media for a Spark Ignition Direct Injection Engine in Steady Engine Operating Conditions and Vehicle Test Modes
Energies 2015, 8(3), 1865-1881; https://doi.org/10.3390/en8031865
Received: 18 November 2014 / Revised: 13 February 2015 / Accepted: 28 February 2015 / Published: 9 March 2015
Cited by 11 | Viewed by 2810 | PDF Full-text (1909 KB) | HTML Full-text | XML Full-text
Abstract
In this study, the particle formation and reduction characteristics at the engine-out position, after a three-way catalyst (TWC) and a metal foam gasoline particulate filter (GPF), were evaluated for a gasoline direct-injection (GDI) engine under part-load operating conditions. The vehicle tests were performed [...] Read more.
In this study, the particle formation and reduction characteristics at the engine-out position, after a three-way catalyst (TWC) and a metal foam gasoline particulate filter (GPF), were evaluated for a gasoline direct-injection (GDI) engine under part-load operating conditions. The vehicle tests were performed under the Federal Test Procedure-75 (FTP-75) and the Highway Fuel Economy Test (HWFET) modes. Particle number (PN) concentrations, size distributions, and the filtering efficiency with the GPF were evaluated with a condensation particle counter (CPC) and a differential mobility spectrometer (DMS500). Under steady engine operating conditions, the PN concentrations at the engine-out position were 9.7 × 105–2.5 × 106 N/cc. While, the PN concentrations after the GPF were 9.2 × 104–3.5 × 105 N/cc, and the PN was reduced by 77%–96%. The PN filtering efficiency with the GPF-GDI vehicle reached approximately 58% in the FTP-75 and 62% in the HWFET mode. The PN concentration of the GPF-GDI vehicle was significantly reduced to 3.95 × 1011 N/km for the FTP-75 and 8.86 × 1010 N/km for the HWFET mode. The amount of nucleation mode particles below 23 nm was substantially reduced with the GPF-GDI vehicle. The fuel economy, CO2, and regulated emissions of the GPF-GDI vehicle were equivalent to those of the base GDI vehicle under the vehicle certification modes. Full article
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Open AccessArticle
A Linearized Large Signal Model of an LCL-Type Resonant Converter
Energies 2015, 8(3), 1848-1864; https://doi.org/10.3390/en8031848
Received: 10 January 2015 / Revised: 27 February 2015 / Accepted: 27 February 2015 / Published: 5 March 2015
Cited by 5 | Viewed by 2209 | PDF Full-text (689 KB) | HTML Full-text | XML Full-text
Abstract
In this work, an LCL-type resonant dc/dc converter with a capacitive output filter is modeled in two stages. In the first high-frequency ac stage, all ac signals are decomposed into two orthogonal vectors in a synchronous rotating d–q frame using multi-frequency modeling. In [...] Read more.
In this work, an LCL-type resonant dc/dc converter with a capacitive output filter is modeled in two stages. In the first high-frequency ac stage, all ac signals are decomposed into two orthogonal vectors in a synchronous rotating d–q frame using multi-frequency modeling. In the dc stage, all dc quantities are represented by their average values with average state-space modeling. A nonlinear two-stage model is then created by means of a non-linear link. By aligning the transformer voltage on the d-axis, the nonlinear link can be eliminated, and the whole converter can be modeled by a single set of linear state-space equations. Furthermore, a feedback control scheme can be formed according to the steady-state solutions. Simulation and experimental results have proven that the resulted model is good for fast simulation and state variable estimation. Full article
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Open AccessArticle
Balancing Control Strategy for Li-Ion Batteries String Based on Dynamic Balanced Point
Energies 2015, 8(3), 1830-1847; https://doi.org/10.3390/en8031830
Received: 20 November 2014 / Revised: 23 January 2015 / Accepted: 6 February 2015 / Published: 4 March 2015
Cited by 10 | Viewed by 2329 | PDF Full-text (626 KB) | HTML Full-text | XML Full-text
Abstract
The Li-ion battery is becoming the optimal choice for the Electric Vehicle’s (EV) power supply. In order to protect the Li-ion battery from charging damage and to prolong the battery’s life, a special control strategy based on the dynamic balanced point along with [...] Read more.
The Li-ion battery is becoming the optimal choice for the Electric Vehicle’s (EV) power supply. In order to protect the Li-ion battery from charging damage and to prolong the battery’s life, a special control strategy based on the dynamic balanced point along with a non-dissipative equalizer is presented. The main focus of the proposed control strategy is to insure that the individual cell of a battery pack will be rapidly, efficiently and simultaneously balanced, by adjusting equalizing current of each cell dynamically. In this paper, a model of a four series connected Li-ion batteries pack has been established to evaluate the proposed control strategy. Superior performance is demonstrated by the simulation and experiment. Full article
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Open AccessArticle
Changes in Carbon Electrode Morphology Affect Microbial Fuel Cell Performance with Shewanella oneidensis MR-1
Energies 2015, 8(3), 1817-1829; https://doi.org/10.3390/en8031817
Received: 19 November 2014 / Revised: 8 January 2015 / Accepted: 11 February 2015 / Published: 4 March 2015
Cited by 11 | Viewed by 3445 | PDF Full-text (2262 KB) | HTML Full-text | XML Full-text
Abstract
The formation of biofilm-electrodes is crucial for microbial fuel cell current production because optimal performance is often associated with thick biofilms. However, the influence of the electrode structure and morphology on biofilm formation is only beginning to be investigated. This study provides insight [...] Read more.
The formation of biofilm-electrodes is crucial for microbial fuel cell current production because optimal performance is often associated with thick biofilms. However, the influence of the electrode structure and morphology on biofilm formation is only beginning to be investigated. This study provides insight on how changing the electrode morphology affects current production of a pure culture of anode-respiring bacteria. Specifically, an analysis of the effects of carbon fiber electrodes with drastically different morphologies on biofilm formation and anode respiration by a pure culture (Shewanella oneidensis MR-1) were examined. Results showed that carbon nanofiber mats had ~10 fold higher current than plain carbon microfiber paper and that the increase was not due to an increase in electrode surface area, conductivity, or the size of the constituent material. Cyclic voltammograms reveal that electron transfer from the carbon nanofiber mats was biofilm-based suggesting that decreasing the diameter of the constituent carbon material from a few microns to a few hundred nanometers is beneficial for electricity production solely because the electrode surface creates a more relevant mesh for biofilm formation by Shewanella oneidensis MR-1. Full article
(This article belongs to the Special Issue Bioenergy and Biorefining)
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Open AccessArticle
Evaluation of Novel Inoculation Strategies for Solid State Anaerobic Digestion of Yam Peelings in Low-Tech Digesters
Energies 2015, 8(3), 1802-1816; https://doi.org/10.3390/en8031802
Received: 23 December 2014 / Revised: 9 February 2015 / Accepted: 15 February 2015 / Published: 4 March 2015
Cited by 4 | Viewed by 2022 | PDF Full-text (806 KB) | HTML Full-text | XML Full-text
Abstract
The operation of household scale anaerobic digesters is typically based on diluted animal dung, requiring stabled livestock and adequate water availability. This excludes many rural households in low-income countries from the benefits of a domestic biogas digester. Solid state anaerobic digestion (SSAD) can [...] Read more.
The operation of household scale anaerobic digesters is typically based on diluted animal dung, requiring stabled livestock and adequate water availability. This excludes many rural households in low-income countries from the benefits of a domestic biogas digester. Solid state anaerobic digestion (SSAD) can be operated with low process water demands, but the technology involves operational challenges, as e.g., risk of process acidification or low degradation rates. This study aimed at developing simple methods to perform SSAD of yam peelings in low-tech applications by testing different inoculation strategies and evaluating the necessity of dung addition as a supportive biomass. In initial lab scale trials 143 ± 4 mL CH4/g VS (volatile solids) were obtained from a mixture of yam peelings and dung digested in a multi-layer-inoculated batch reactor. In a consecutive incubation cycle in which adapted inoculum was applied, bottom inoculated digesters loaded without dung reached a yield of 140 ± 16 mL CH4/g VS. This indicates that SSAD of yam peelings is possible with simple inoculation methods and dung addition is unnecessary after microbial adaptation. A comparison with a conventional fixed dome digester indicated that SSAD can reduce process water demand and the digester volume necessary to supply a given biogas demand. Full article
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Open AccessArticle
Microgrid Stability Controller Based on Adaptive Robust Total SMC
Energies 2015, 8(3), 1784-1801; https://doi.org/10.3390/en8031784
Received: 19 December 2014 / Revised: 14 February 2015 / Accepted: 25 February 2015 / Published: 4 March 2015
Cited by 12 | Viewed by 2378 | PDF Full-text (928 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a microgrid stability controller (MSC) in order to provide existing distributed generation units (DGs) the additional functionality of working in islanding mode without changing their control strategies in grid-connected mode and to enhance the stability of the microgrid. Microgrid operating [...] Read more.
This paper presents a microgrid stability controller (MSC) in order to provide existing distributed generation units (DGs) the additional functionality of working in islanding mode without changing their control strategies in grid-connected mode and to enhance the stability of the microgrid. Microgrid operating characteristics and mathematical models of the MSC indicate that the system is inherently nonlinear and time-variable. Therefore, this paper proposes an adaptive robust total sliding-mode control (ARTSMC) system for the MSC. It is proved that the ARTSMC system is insensitive to parametric uncertainties and external disturbances. The MSC provides fast dynamic response and robustness to the microgrid. When the system is operating in grid-connected mode, it is able to improve the controllability of the exchanged power between the microgrid and the utility grid, while smoothing the DGs’ output power. When the microgrid is operating in islanded mode, it provides voltage and frequency support, while guaranteeing seamless transition between the two operation modes. Simulation and experimental results show the effectiveness of the proposed approach. Full article
(This article belongs to the Special Issue Microgrids)
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Open AccessArticle
MV and LV Residential Grid Impact of Combined Slow and Fast Charging of Electric Vehicles
Energies 2015, 8(3), 1760-1783; https://doi.org/10.3390/en8031760
Received: 16 December 2014 / Revised: 15 February 2015 / Accepted: 17 February 2015 / Published: 3 March 2015
Cited by 10 | Viewed by 2955 | PDF Full-text (1095 KB) | HTML Full-text | XML Full-text
Abstract
This article investigates the combined low voltage (LV) and medium voltage (MV) residential grid impact for slow and fast electric vehicle (EV) charging, for an increasing local penetration rate and for different residential slow charging strategies. A realistic case study for a Flemish [...] Read more.
This article investigates the combined low voltage (LV) and medium voltage (MV) residential grid impact for slow and fast electric vehicle (EV) charging, for an increasing local penetration rate and for different residential slow charging strategies. A realistic case study for a Flemish urban distribution grid is used, for which three residential slow charging strategies are modeled: uncoordinated charging, residential off-peak charging, and EV-based peak shaving. For each slow charging strategy, the EV hosting capacity is determined, with and without the possibility of fast charging, while keeping the grid within its operating limits. The results show that the distribution grid impact is much less sensitive to the presence of fast charging compared to the slow charging strategy. EV-based peak shaving results in the lowest grid impact, allowing for the highest EV hosting capacity. Residential off-peak charging has the highest grid impact, due the load synchronization effect that occurs, resulting in the lowest EV hosting capacity. Therefore, the EV users should be incentivized to charge their EVs in a more grid-friendly manner when the local EV penetration rate becomes significant, as this increases the EV hosting capacity much more than the presence of fast charging decreases it. Full article
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