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Energies, Volume 7, Issue 2 (February 2014), Pages 498-1097

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Editorial

Jump to: Research, Review, Other

Open AccessEditorial Acknowledgement to Reviewers of Energies in 2013
Energies 2014, 7(2), 1066-1075; doi:10.3390/en7021066
Received: 24 February 2014 / Accepted: 24 February 2014 / Published: 24 February 2014
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Abstract The editors of Energies would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2013. [...] Full article
Open AccessEditorial Energies Best Paper Award 2014
Energies 2014, 7(2), 1095-1097; doi:10.3390/en7021095
Received: 13 February 2014 / Accepted: 19 February 2014 / Published: 24 February 2014
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Abstract
Last year, Energies (ISSN 1996-1073) decided to award a prize to outstanding papers in the area of energy technologies and applications published in Energies. In 2014, the Prize Awarding Committee has decided to also grant awards in the category of “best review
[...] Read more.
Last year, Energies (ISSN 1996-1073) decided to award a prize to outstanding papers in the area of energy technologies and applications published in Energies. In 2014, the Prize Awarding Committee has decided to also grant awards in the category of “best review papers”. [...] Full article
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Research

Jump to: Editorial, Review, Other

Open AccessArticle Low Voltage Ride-through in DFIG Wind Generators by Controlling the Rotor Current without Crowbars
Energies 2014, 7(2), 498-519; doi:10.3390/en7020498
Received: 13 November 2013 / Revised: 17 January 2014 / Accepted: 20 January 2014 / Published: 23 January 2014
Cited by 7 | PDF Full-text (1160 KB) | HTML Full-text | XML Full-text
Abstract
Among all the different types of electric wind generators, those that are based on doubly fed induction generators, or DFIG technology, are the most vulnerable to grid faults such as voltage sags. This paper proposes a new control strategy for this type of
[...] Read more.
Among all the different types of electric wind generators, those that are based on doubly fed induction generators, or DFIG technology, are the most vulnerable to grid faults such as voltage sags. This paper proposes a new control strategy for this type of wind generator, that allows these devices to withstand the effects of a voltage sag while following the new requirements imposed by grid operators. This new control strategy makes the use of complementary devices such as crowbars unnecessary, as it greatly reduces the value of currents originated by the fault. This ensures less costly designs for the rotor systems as well as a more economic sizing of the necessary power electronics. The strategy described here uses an electric generator model based on space-phasor theory that provides a direct control over the position of the rotor magnetic flux. Controlling the rotor magnetic flux has a direct influence on the rest of the electrical variables enabling the machine to evolve to a desired work point during the transient imposed by the grid disturbance. Simulation studies have been carried out, as well as test bench trials, in order to prove the viability and functionality of the proposed control strategy. Full article
(This article belongs to the Special Issue Wind Turbines 2014)
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Open AccessArticle Remaining Useful Life Prediction of Lithium-Ion Batteries Based on the Wiener Process with Measurement Error
Energies 2014, 7(2), 520-547; doi:10.3390/en7020520
Received: 6 December 2013 / Revised: 13 January 2014 / Accepted: 17 January 2014 / Published: 23 January 2014
Cited by 29 | PDF Full-text (743 KB) | HTML Full-text | XML Full-text
Abstract
Remaining useful life (RUL) prediction is central to the prognostics and health management (PHM) of lithium-ion batteries. This paper proposes a novel RUL prediction method for lithium-ion batteries based on the Wiener process with measurement error (WPME). First, we use the truncated normal
[...] Read more.
Remaining useful life (RUL) prediction is central to the prognostics and health management (PHM) of lithium-ion batteries. This paper proposes a novel RUL prediction method for lithium-ion batteries based on the Wiener process with measurement error (WPME). First, we use the truncated normal distribution (TND) based modeling approach for the estimated degradation state and obtain an exact and closed-form RUL distribution by simultaneously considering the measurement uncertainty and the distribution of the estimated drift parameter. Then, the traditional maximum likelihood estimation (MLE) method for population based parameters estimation is remedied to improve the estimation efficiency. Additionally, we analyze the relationship between the classic MLE method and the combination of the Bayesian updating algorithm and the expectation maximization algorithm for the real time RUL prediction. Interestingly, it is found that the result of the combination algorithm is equal to the classic MLE method. Inspired by this observation, a heuristic algorithm for the real time parameters updating is presented. Finally, numerical examples and a case study of lithium-ion batteries are provided to substantiate the superiority of the proposed RUL prediction method. Full article
Open AccessArticle Characterization of Biochar from Switchgrass Carbonization
Energies 2014, 7(2), 548-567; doi:10.3390/en7020548
Received: 17 November 2013 / Revised: 24 December 2013 / Accepted: 14 January 2014 / Published: 24 January 2014
Cited by 21 | PDF Full-text (683 KB) | HTML Full-text | XML Full-text
Abstract
Switchgrass is a high yielding, low-input intensive, native perennial grass that has been promoted as a major second-generation bioenergy crop. Raw switchgrass is not a readily acceptable feedstock in existing power plants that were built to accommodate coal and peat. The objective of
[...] Read more.
Switchgrass is a high yielding, low-input intensive, native perennial grass that has been promoted as a major second-generation bioenergy crop. Raw switchgrass is not a readily acceptable feedstock in existing power plants that were built to accommodate coal and peat. The objective of this research was to elucidate some of the characteristics of switchgrass biochar produced via carbonization and to explore its potential use as a solid fuel. Samples were carbonized in a batch reactor under reactor temperatures of 300, 350 and 400 °C for 1, 2 and 3 h residence times. Biochar mass yield and volatile solids decreased from 82.6% to 35.2% and from 72.1% to 43.9%, respectively, by increasing carbonization temperatures from 300 °C to 400 °C and residence times from 1 h to 3 h. Conversely, biochar heating value (HV) and fixed carbon content increased from 17.6 MJ kg−1 to 21.9 MJ kg−1 and from 22.5% to 44.9%, respectively, under the same conditions. A biomass discoloration index (BDI) was created to quantify changes in biochar colors as affected by the two tested parameters. The maximum BDI of 77% was achieved at a carbonization temperature of 400 °C and a residence time of 3 h. The use of this index could be expanded to quantify biochar characteristics as affected by thermochemical treatments. Carbonized biochar could be considered a high quality solid fuel based on its energy content. Full article
(This article belongs to the Special Issue Renewable Energy for Agriculture)
Open AccessArticle Factors Affecting Alkaline Sodium Silicate Gelation for In-Depth Reservoir Profile Modification
Energies 2014, 7(2), 568-590; doi:10.3390/en7020568
Received: 28 October 2013 / Revised: 7 January 2014 / Accepted: 16 January 2014 / Published: 27 January 2014
Cited by 7 | PDF Full-text (1963 KB) | HTML Full-text | XML Full-text
Abstract
Alkaline sodium silicate (Na-silicate) is environment-friendly and possesses water-like viscosity during the injection stage for in-depth reservoir treatment to enhance sweep efficiency. Gel setting time (tg) and gel strength are interrelated. Factors that accelerate tg are Na-silicate content (wt%),
[...] Read more.
Alkaline sodium silicate (Na-silicate) is environment-friendly and possesses water-like viscosity during the injection stage for in-depth reservoir treatment to enhance sweep efficiency. Gel setting time (tg) and gel strength are interrelated. Factors that accelerate tg are Na-silicate content (wt%), low pH, presence of divalent ions and temperature. Pressure drop across the gel accelerates syneresis; however, the gel appeared to remain intact. Presence of Ca2+ and Mg2+ ions is shown to increase gel strength. With a Na-silicate content of 4.5 wt%, for example, at a pH of 10.3 and a temperature of 20 °C, gel strength almost tripled and was reached about eight times faster at the combined tested concentration of 0.009 M, based on the average effect from the coexistence of both ions. Low-salinity water (LSW) has an ion composition of 25-fold diluted seawater, did not show precipitation, and could accordingly be a candidate for a pre-flush before the injection of a Na-silicate solution in the event of a field application. This is important since LSW for enhancing oil recovery is a popular method in oil industry. A suggested predictive tool (simple graphical method) to estimate the effect of different factors on gelation time and gel strength is presented. Full article
Open AccessArticle Linearization and Input-Output Decoupling for Nonlinear Control of Proton Exchange Membrane Fuel Cells
Energies 2014, 7(2), 591-606; doi:10.3390/en7020591
Received: 14 October 2013 / Revised: 4 January 2014 / Accepted: 23 January 2014 / Published: 27 January 2014
Cited by 2 | PDF Full-text (334 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a nonlinear control strategy utilizing the linearization and input-output decoupling approach for a nonlinear dynamic model of proton exchange membrane fuel cells (PEMFCs). The multiple-input single-output (MISO) nonlinear model of the PEMFC is derived first. The dynamic model is then
[...] Read more.
This paper presents a nonlinear control strategy utilizing the linearization and input-output decoupling approach for a nonlinear dynamic model of proton exchange membrane fuel cells (PEMFCs). The multiple-input single-output (MISO) nonlinear model of the PEMFC is derived first. The dynamic model is then transformed into a multiple-input multiple-output (MIMO) square system by adding additional states and outputs so that the linearization and input-output decoupling approach can be directly applied. A PI tracking control is also introduced to the state feedback control law in order to reduce the steady-state errors due to parameter uncertainty. This paper also proposes an adaptive genetic algorithm (AGA) for the multi-objective optimization design of the tracking controller. The comprehensive results of simulation demonstrate that the PEMFC with nonlinear control has better transient and steady-state performance compared to conventional linear techniques. Full article
(This article belongs to the Special Issue Polymer Electrolyte Membrane Fuel Cells)
Open AccessArticle Nanofibrillated Cellulose (NFC): A High-Value Co-Product that Improves the Economics of Cellulosic Ethanol Production
Energies 2014, 7(2), 607-618; doi:10.3390/en7020607
Received: 19 November 2013 / Revised: 22 January 2014 / Accepted: 27 January 2014 / Published: 7 February 2014
Cited by 10 | PDF Full-text (984 KB) | HTML Full-text | XML Full-text
Abstract
Cellulosic ethanol is a sustainable alternative to petroleum as a transportation fuel, which could be made biologically from agricultural and forestry residues, municipal waste, or herbaceous and woody crops. Instead of putting efforts on steps overcoming the natural resistance of plants to biological
[...] Read more.
Cellulosic ethanol is a sustainable alternative to petroleum as a transportation fuel, which could be made biologically from agricultural and forestry residues, municipal waste, or herbaceous and woody crops. Instead of putting efforts on steps overcoming the natural resistance of plants to biological breakdown, our study proposes a unique pathway to improve the outcome of the process by co-producing high-value nanofibrillated cellulose (NFC), offering a new economic leverage for cellulosic ethanol to compete with fossil fuels in the near future. In this study, glucose has been produced by commercial enzymes while the residual solids are converted into NFC via sonification. Here, we report the morphology of fibers changed through the process and yield of glucose in the enzymatic hydrolysis step. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2013)
Open AccessArticle An FMEA-Based Risk Assessment Approach for Wind Turbine Systems: A Comparative Study of Onshore and Offshore
Energies 2014, 7(2), 619-642; doi:10.3390/en7020619
Received: 13 November 2013 / Revised: 14 January 2014 / Accepted: 24 January 2014 / Published: 10 February 2014
Cited by 12 | PDF Full-text (1510 KB) | HTML Full-text | XML Full-text
Abstract
Failure mode and effects analysis (FMEA) has been extensively used by wind turbine assembly manufacturers for analyzing, evaluating and prioritizing potential/known failure modes. However, several limitations are associated with its practical implementation in wind farms. First, the Risk-Priority-Number (RPN) of a wind turbine
[...] Read more.
Failure mode and effects analysis (FMEA) has been extensively used by wind turbine assembly manufacturers for analyzing, evaluating and prioritizing potential/known failure modes. However, several limitations are associated with its practical implementation in wind farms. First, the Risk-Priority-Number (RPN) of a wind turbine system is not informative enough for wind farm managers from the perspective of criticality; second, there are variety of wind turbines with different structures and hence, it is not correct to compare the RPN values of different wind turbines with each other for prioritization purposes; and lastly, some important economical aspects such as power production losses, and the costs of logistics and transportation are not taken into account in the RPN value. In order to overcome these drawbacks, we develop a mathematical tool for risk and failure mode analysis of wind turbine systems (both onshore and offshore) by integrating the aspects of traditional FMEA and some economic considerations. Then, a quantitative comparative study is carried out using the traditional and the proposed FMEA methodologies on two same type of onshore and offshore wind turbine systems. The results show that the both systems face many of the same risks, however there are some main differences worth considering. Full article
(This article belongs to the Special Issue Wind Turbines 2014)
Open AccessArticle Development and Field Test of Voltage VAR Optimization in the Korean Smart Distribution Management System
Energies 2014, 7(2), 643-669; doi:10.3390/en7020643
Received: 11 November 2013 / Revised: 9 January 2014 / Accepted: 24 January 2014 / Published: 10 February 2014
Cited by 2 | PDF Full-text (2304 KB) | HTML Full-text | XML Full-text
Abstract
This paper is a summary of the development and demonstration of an optimization program, voltage VAR optimization (VVO), in the Korean Smart Distribution Management System (KSDMS). KSDMS was developed to address the lack of receptivity of distributed generators (DGs), standardization and compatibility, and
[...] Read more.
This paper is a summary of the development and demonstration of an optimization program, voltage VAR optimization (VVO), in the Korean Smart Distribution Management System (KSDMS). KSDMS was developed to address the lack of receptivity of distributed generators (DGs), standardization and compatibility, and manual failure recovery in the existing Korean automated distribution system. Focusing on the lack of receptivity of DGs, we developed a real-time system analysis and control program. The KSDMS VVO enhances manual system operation of the existing distribution system and provides a solution with all control equipment operated at a system level. The developed VVO is an optimal power flow (OPF) method that resolves violations, minimizes switching costs, and minimizes loss, and its function can vary depending on the operator’s command. The sequential mixed integer linear programming (SMILP) method was adopted to find the solution of the OPF. We tested the precision of the proposed VVO on selected simulated systems and its applicability to actual systems at two substations on the Jeju Island. Running the KSDMS VVO on a regular basis improved system stability, and it also raised no issues regarding its applicability to actual systems. Full article
(This article belongs to the Special Issue Smart Grids: The Electrical Power Network and Communication System)
Open AccessArticle Modelling of PEM Fuel Cell Performance: Steady-State and Dynamic Experimental Validation
Energies 2014, 7(2), 670-700; doi:10.3390/en7020670
Received: 10 December 2013 / Revised: 29 January 2014 / Accepted: 6 February 2014 / Published: 10 February 2014
Cited by 2 | PDF Full-text (1022 KB) | HTML Full-text | XML Full-text
Abstract
This paper reports on the modelling of a commercial 1.2 kW proton exchange membrane fuel cell (PEMFC), based on interrelated electrical and thermal models. The electrical model proposed is based on the integration of the thermodynamic and electrochemical phenomena taking place in the
[...] Read more.
This paper reports on the modelling of a commercial 1.2 kW proton exchange membrane fuel cell (PEMFC), based on interrelated electrical and thermal models. The electrical model proposed is based on the integration of the thermodynamic and electrochemical phenomena taking place in the FC whilst the thermal model is established from the FC thermal energy balance. The combination of both models makes it possible to predict the FC voltage, based on the current demanded and the ambient temperature. Furthermore, an experimental characterization is conducted and the parameters for the models associated with the FC electrical and thermal performance are obtained. The models are implemented in Matlab Simulink and validated in a number of operating environments, for steady-state and dynamic modes alike. In turn, the FC models are validated in an actual microgrid operating environment, through the series connection of 4 PEMFC. The simulations of the models precisely and accurately reproduce the FC electrical and thermal performance. Full article
(This article belongs to the Special Issue Polymer Electrolyte Membrane Fuel Cells)
Open AccessArticle Wave Basin Experiments with Large Wave Energy Converter Arrays to Study Interactions between the Converters and Effects on Other Users in the Sea and the Coastal Area
Energies 2014, 7(2), 701-734; doi:10.3390/en7020701
Received: 2 December 2013 / Revised: 18 January 2014 / Accepted: 6 February 2014 / Published: 12 February 2014
Cited by 9 | PDF Full-text (2098 KB) | HTML Full-text | XML Full-text
Abstract
Experiments have been performed in the Shallow Water Wave Basin of DHI (Hørsholm, Denmark), on large arrays of up to 25 heaving point absorber type Wave Energy Converters (WECs), for a range of geometric layout configurations and wave conditions. WEC response and modifications
[...] Read more.
Experiments have been performed in the Shallow Water Wave Basin of DHI (Hørsholm, Denmark), on large arrays of up to 25 heaving point absorber type Wave Energy Converters (WECs), for a range of geometric layout configurations and wave conditions. WEC response and modifications of the wave field are measured to provide data for understanding WEC array interactions and to evaluate array interaction numerical models. Each WEC consists of a buoy with a diameter of 0.315 m and power take-off (PTO) is modeled by realizing friction based energy dissipation through damping of the WEC’s motion. Wave gauges are located within and around the WEC array. Wave conditions studied include regular, polychromatic, long- and short-crested irregular waves. A rectilinear arrangement of WEC support structures is employed such that several array configurations can be studied. In this paper, the experimental arrangement and the obtained database are presented. Also, results for wave height attenuation downwave a rectilinear array of 25 heaving WECs are presented, for the case of irregular waves. Up to 16.3% and 18.1% (long-crested) and 11.2% and 18.1% (short-crested waves) reduction in significant wave height is observed downwave the WEC array, for the radiated wave field only and for the combination of incident-diffracted-radiated (perturbed) wave field, respectively. Using spectra at different locations within and around the array, the wave field modifications are presented and discussed. Full article
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Open AccessArticle Effects of Organic Loading Rate on the Performance of a Pressurized Anaerobic Filter in Two-Phase Anaerobic Digestion
Energies 2014, 7(2), 736-750; doi:10.3390/en7020736
Received: 16 December 2013 / Revised: 30 January 2014 / Accepted: 7 February 2014 / Published: 13 February 2014
Cited by 6 | PDF Full-text (434 KB) | HTML Full-text | XML Full-text
Abstract
The effect of organic loading rate (OLR) on a pressurized anaerobic filter was studied in a laboratory two-phase anaerobic digestion system. The anaerobic filter was operated successively at two working pressures (9 bar and 1.5 bar). The OLR(COD) for each pressure was
[...] Read more.
The effect of organic loading rate (OLR) on a pressurized anaerobic filter was studied in a laboratory two-phase anaerobic digestion system. The anaerobic filter was operated successively at two working pressures (9 bar and 1.5 bar). The OLR(COD) for each pressure was increased from 5 to 17.5 kg·m−3·day−1. The best performance of the reactor at 9 bar was observed at OLR(COD) of 12.5 kg·m−3·day−1 and hydraulic retention time (HRT) of 1.8 day, with specific biogas productivity (SBP) of 5.3 L·L−1·day−1 and COD degradation grade of 90.6%. At higher OLRs and shorter HRTs, the process became unstable. In contrast, there was no indication of digester failure during the experiments at 1.5 bar. The SBP peaked at OLR(COD) of 17.5 kg·m−3·day−1 with 8.2 L·L−1·day−1, where COD degradation grade was 90.4%. The biogas collected from the reactor at 9 bar and 1.5 bar contained approximately 74.5% CH4 and 66.2% CH4, respectively, regardless of OLR variation. At OLR(COD) of 5–12.5 kg·m−3·day−1, the reactor at 9 bar had the same specific methane yield as at 1.5 bar, which was in the range of 0.31–0.32 LN·g−1COD. Raising the working pressure in the reactor resulted in an increase of methane content of the produced biogas. However, the low pH value (approximately 6.5) inside the reactor, induced by high CO2 partial pressure seemed to limit the reactor performance at high OLRs and short HRT. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2013)
Open AccessArticle Energy and GHG Analysis of Rural Household Biogas Systems in China
Energies 2014, 7(2), 767-784; doi:10.3390/en7020767
Received: 20 November 2013 / Revised: 24 January 2014 / Accepted: 29 January 2014 / Published: 17 February 2014
Cited by 3 | PDF Full-text (473 KB) | HTML Full-text | XML Full-text
Abstract
The Chinese government has taken great efforts to popularize rural household scale biogas digesters, since they are regarded as an effective approach to address energy shortage issues in rural areas and as a potential way of reducing greenhouse gas (GHG) emissions. Focusing on
[...] Read more.
The Chinese government has taken great efforts to popularize rural household scale biogas digesters, since they are regarded as an effective approach to address energy shortage issues in rural areas and as a potential way of reducing greenhouse gas (GHG) emissions. Focusing on a typical rural household biogas system, the aim of this study is to systematically quantify its total direct and indirect energy, concentrating on non-renewable energy and the associated GHG emission cost over the entire life cycle to understand its net dynamic benefits. The results show that the total energetic cost for biogas output is 2.19 J/J, of which 0.56 J is from non-renewable energy sources and the GHG emission cost is 4.54 × 10−5 g CO2-equivalent (CO2-eq), with respect to its design life cycle of 20 years. Correspondingly, a net non-renewable energy saving of 9.89 × 1010 J and GHG emission reduction of 50.45 t CO2-eq can be obtained considering the coal substitution and manure disposal. However, it must be run for at least 10 and 3 years, to obtain positive net non-renewable energy savings and GHG emission reduction benefits, respectively. These results have policy implications for development orientation, follow-up services, program management and even national financial subsidy methods. Full article
Open AccessArticle The Influence of Loading Rate and Variable Temperatures on Microbial Communities in Anaerobic Digesters
Energies 2014, 7(2), 785-803; doi:10.3390/en7020785
Received: 26 November 2013 / Revised: 22 January 2014 / Accepted: 28 January 2014 / Published: 18 February 2014
Cited by 3 | PDF Full-text (835 KB) | HTML Full-text | XML Full-text
Abstract
The relationship between seasonal temperatures, organic loading rate (OLR) and the structure of archaeal communities in anaerobic digesters was investigated. Previous studies have often assessed archaeal community structure at fixed temperatures and constant OLRs, or at variable temperatures not characteristic of temperate climates.
[...] Read more.
The relationship between seasonal temperatures, organic loading rate (OLR) and the structure of archaeal communities in anaerobic digesters was investigated. Previous studies have often assessed archaeal community structure at fixed temperatures and constant OLRs, or at variable temperatures not characteristic of temperate climates. The goal of this study was to determine the maximum OLR that would maintain a balanced microbial ecosystem during operation in a variable temperature range expected in a temperate climate (27–10 °C). Four-liter laboratory digesters were operated in a semi-continuous mode using dairy cow manure as the feedstock. At OLRs of 1.8 and 0.8 kg VS/m3·day the digesters soured (pH < 6.5) as a result of a decrease in temperature. The structure of the archaeal community in the sour digesters became increasingly similar to the manure feedstock with gains in the relative abundance of hydrogenotrophic methanogens. At an OLR of 0.3 kg VS/m3·day the digesters did not sour, but the archaeal community was primarily hydrogenotrophic methanogens. Recommendations for operating an ambient temperature digester year round in a temperate climate are to reduce the OLR to at least 0.3 kg VS/m3·day in colder temperatures to prevent a shift to the microbial community associated with the sour digesters. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2013)
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Open AccessArticle Availability of Biomass Residues for Co-Firing in Peninsular Malaysia: Implications for Cost and GHG Emissions in the Electricity Sector
Energies 2014, 7(2), 804-823; doi:10.3390/en7020804
Received: 14 December 2013 / Revised: 14 January 2014 / Accepted: 8 February 2014 / Published: 18 February 2014
Cited by 5 | PDF Full-text (2446 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Fossil fuels comprise 93% of Malaysia’s electricity generation and account for 36% of the country’s 2010 Greenhouse Gas (GHG) emissions. The government has targeted the installation of 330 MW of biomass electricity generation capacity by 2015 to avoid 1.3 Mt of CO2
[...] Read more.
Fossil fuels comprise 93% of Malaysia’s electricity generation and account for 36% of the country’s 2010 Greenhouse Gas (GHG) emissions. The government has targeted the installation of 330 MW of biomass electricity generation capacity by 2015 to avoid 1.3 Mt of CO2 emissions annually and offset some emissions due to increased coal use. One biomass option is to co-fire with coal, which can result in reduced GHG emissions, coal use, and costs of electricity. A linear optimization cost model was developed using seven types of biomass residues for Peninsular Malaysia. Results suggest that about 12 Mt/year of residues are available annually, of which oil-palm residues contribute 77%, and rice and logging residues comprise 17%. While minimizing the cost of biomass and biomass residue transport, co-firing at four existing coal plants in Peninsular Malaysia could meet the 330 MW biomass electricity target and reduce costs by about $24 million per year compared to coal use alone and reduces GHG emissions by 1.9 Mt of CO2. Maximizing emissions reduction for biomass co-firing results in 17 Mt of CO2 reductions at a cost of $23/t of CO2 reduced. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2013)
Open AccessArticle Fractionation for Biodiesel Purification Using Supercritical Carbon Dioxide
Energies 2014, 7(2), 824-833; doi:10.3390/en7020824
Received: 30 October 2013 / Revised: 16 February 2014 / Accepted: 17 February 2014 / Published: 19 February 2014
Cited by 1 | PDF Full-text (341 KB) | HTML Full-text | XML Full-text
Abstract
In recent years, biodegradable and alternative biodiesel has attracted increased attention worldwide. Producing biodiesel from biomass involves critical separation and purification technology. Conventional technologies such as gravitational settling, decantation, filtration, water washing, acid washing, organic solvent washing and absorbent applications are inefficient, less
[...] Read more.
In recent years, biodegradable and alternative biodiesel has attracted increased attention worldwide. Producing biodiesel from biomass involves critical separation and purification technology. Conventional technologies such as gravitational settling, decantation, filtration, water washing, acid washing, organic solvent washing and absorbent applications are inefficient, less cost effective and environmentally less friendly. In this study supercritical carbon dioxide (SC-CO2) with few steps and a low environmental impact, was used for biodiesel fractionation from impure fatty acid methyl ester (FAME) solution mixes. The method is suitable for application in a variety of biodiesel production processes requiring subsequent stages of purification. The fractionation and purification was carried out using continuous SC-CO2 fractionation equipment, consisting of three columns filled with stainless steel fragments. A 41.85% FAME content solution mix was used as the raw material in this study. Variables were a temperature range of 40–70 °C, pressure range of 10–30 MPa, SC-CO2 flow rate range of 7–21 mL/min and a retention time range of 30–90 min. The Taguchi method was used to identify optimal operating conditions. The results show that a separated FAME content of 99.94% was verified by GC-FID under optimal fractionation conditions, which are a temperature of 40 °C of, a pressure level of 30MPa and a flow rate of 7 mL/min of SC-CO2 for a retention time of 90 min. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2013)
Open AccessArticle Convex Optimization for the Energy Management of Hybrid Electric Vehicles Considering Engine Start and Gearshift Costs
Energies 2014, 7(2), 834-856; doi:10.3390/en7020834
Received: 28 November 2013 / Revised: 30 January 2014 / Accepted: 5 February 2014 / Published: 19 February 2014
Cited by 28 | PDF Full-text (549 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a novel method to solve the energy management problem for hybrid electric vehicles (HEVs) with engine start and gearshift costs. The method is based on a combination of deterministic dynamic programming (DP) and convex optimization. As demonstrated in a case
[...] Read more.
This paper presents a novel method to solve the energy management problem for hybrid electric vehicles (HEVs) with engine start and gearshift costs. The method is based on a combination of deterministic dynamic programming (DP) and convex optimization. As demonstrated in a case study, the method yields globally optimal results while returning the solution in much less time than the conventional DP method. In addition, the proposed method handles state constraints, which allows for the application to scenarios where the battery state of charge (SOC) reaches its boundaries. Full article
(This article belongs to the Special Issue Advances in Hybrid Vehicles)
Open AccessArticle Impact of Hot-Water Extraction on Acetone-Water Oxygen Delignification of Paulownia Spp. and Lignin Recovery
Energies 2014, 7(2), 857-873; doi:10.3390/en7020857
Received: 9 December 2013 / Revised: 7 February 2014 / Accepted: 17 February 2014 / Published: 19 February 2014
Cited by 2 | PDF Full-text (606 KB) | HTML Full-text | XML Full-text
Abstract
A hardwood-based biorefinery process starting with hot-water extraction (HWE) is recommended in order to remove most of the hemicelluloses/xylans before further processing. HWE may be followed by delignification in acetone/water in the presence of oxygen (AWO) for the production of cellulose and lignin.
[...] Read more.
A hardwood-based biorefinery process starting with hot-water extraction (HWE) is recommended in order to remove most of the hemicelluloses/xylans before further processing. HWE may be followed by delignification in acetone/water in the presence of oxygen (AWO) for the production of cellulose and lignin. In this study, the HWE-AWO sequence was evaluated for its effectiveness at removing lignin from the fast-growing species Paulownia tomentosa (PT) and Paulownia elongata (PE), in comparison with the reference species, sugar maple (Acer saccharum, SM). HWE might lead to a remarkable increase in lignin accessibility, and as a result, a greater AWO delignification degree was observed for extracted PT, PE, and SM than for unextracted ones. Organosolv lignin was recovered from the spent liquor of AWO delignification of PT with/without prior HWE and characterized to evaluate the benefits of HWE on the lignin structure and purity. The lignin recovered from the spent liquor of HWE-AWO sequence is of higher purity and lighter color than that recovered from the AWO spent liquor. These properties along with low sulfur content are desirable for lignin high-value applications. Full article
(This article belongs to the Special Issue Biomass and Biofuels 2013)
Open AccessArticle Modelling and Internal Fuzzy Model Power Control of a Francis Water Turbine
Energies 2014, 7(2), 874-889; doi:10.3390/en7020874
Received: 17 November 2013 / Revised: 6 February 2014 / Accepted: 8 February 2014 / Published: 19 February 2014
Cited by 8 | PDF Full-text (1082 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents dynamic modelling of a Francis turbine with a surge tank and the control of a hydro power plant (HPP). Non-linear and linear models include technical parameters and show high similarity to measurement data. Turbine power control with an internal model
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This paper presents dynamic modelling of a Francis turbine with a surge tank and the control of a hydro power plant (HPP). Non-linear and linear models include technical parameters and show high similarity to measurement data. Turbine power control with an internal model control (IMC) is proposed, based on a turbine fuzzy model. Considering appropriate control responses in the entire area of turbine power, the model parameters of the process are determined from a fuzzy model, which are further included in the internal model controller. The results are compared to a proportional-integral (PI) controller tuned with an integral absolute error (IAE) objective function, and show an improved response of internal model control. Full article
Open AccessArticle Large Eddy Simulation of Vertical Axis Wind Turbine Wakes
Energies 2014, 7(2), 890-912; doi:10.3390/en7020890
Received: 11 December 2013 / Revised: 5 February 2014 / Accepted: 11 February 2014 / Published: 20 February 2014
Cited by 16 | PDF Full-text (4867 KB) | HTML Full-text | XML Full-text
Abstract
In this study, large eddy simulation (LES) is combined with a turbine model to investigate the wake behind a vertical-axis wind turbine (VAWT) in a three-dimensional turbulent flow. Two methods are used to model the subgrid-scale (SGS) stresses: (a) the Smagorinsky model; and
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In this study, large eddy simulation (LES) is combined with a turbine model to investigate the wake behind a vertical-axis wind turbine (VAWT) in a three-dimensional turbulent flow. Two methods are used to model the subgrid-scale (SGS) stresses: (a) the Smagorinsky model; and (b) the modulated gradient model. To parameterize the effects of the VAWT on the flow, two VAWT models are developed: (a) the actuator swept-surface model (ASSM), in which the time-averaged turbine-induced forces are distributed on a surface swept by the turbine blades, i.e., the actuator swept surface; and (b) the actuator line model (ALM), in which the instantaneous blade forces are only spatially distributed on lines representing the blades, i.e., the actuator lines. This is the first time that LES has been applied and validated for the simulation of VAWT wakes by using either the ASSM or the ALM techniques. In both models, blade-element theory is used to calculate the lift and drag forces on the blades. The results are compared with flow measurements in the wake of a model straight-bladed VAWT, carried out in the Institute de Méchanique et Statistique de la Turbulence (IMST) water channel. Different combinations of SGS models with VAWT models are studied, and a fairly good overall agreement between simulation results and measurement data is observed. In general, the ALM is found to better capture the unsteady-periodic nature of the wake and shows a better agreement with the experimental data compared with the ASSM. The modulated gradient model is also found to be a more reliable SGS stress modeling technique, compared with the Smagorinsky model, and it yields reasonable predictions of the mean flow and turbulence characteristics of a VAWT wake using its theoretically-determined model coefficient. Full article
(This article belongs to the Special Issue Wind Turbines 2014)
Figures

Open AccessArticle Investigation of the Wind Resource Assessment over 2D Continuous Rolling Hills Due to Tropical Cyclones in the Coastal Region of Southeastern China
Energies 2014, 7(2), 913-933; doi:10.3390/en7020913
Received: 9 December 2013 / Revised: 15 February 2014 / Accepted: 18 February 2014 / Published: 20 February 2014
Cited by 3 | PDF Full-text (2929 KB) | HTML Full-text | XML Full-text
Abstract
The effect of tropical cyclones on the turbulent flow over 2D continuous rolling hills was numerically investigated based on a field test analysis of the coastal region of Southeast China. A computational fluid dynamics (CFD) method was first developed and verified using previously
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The effect of tropical cyclones on the turbulent flow over 2D continuous rolling hills was numerically investigated based on a field test analysis of the coastal region of Southeast China. A computational fluid dynamics (CFD) method was first developed and verified using previously published experimental results. Then two typical beneficial and destructive cyclone cases were studied above different locations of the hills. Results showed that the continuous hilly flow was much more drastic and variable than previously reported normal wind; the mean and turbulent magnitudes became the strongest around the hill top, with the maximum speed-up ratio, turbulence intensity and gust-speed ratio of 1.1, 0.32 and 1.6; the flow over lower hill was greatly affected by the nearby higher hills; the mean and fluctuating quantities were mostly smaller than the corresponding single hill case. These phenomena were considered to be related with the rather strong detachment and attachment of the cyclone flow around the two hills. In addition, the mean and fluctuating wind velocities were found to be underestimated by at least 20% if the widely accepted IEC standard equations were utilized, suggesting the necessity to supplement the field test analysis in the standard for more reasonable wind resource evaluation within the Southeast China coastal area. Full article
(This article belongs to the Special Issue Wind Turbines 2014)
Open AccessArticle Research on the Power Management Strategy of Hybrid Electric Vehicles Based on Electric Variable Transmissions
Energies 2014, 7(2), 934-960; doi:10.3390/en7020934
Received: 12 December 2013 / Revised: 26 January 2014 / Accepted: 14 February 2014 / Published: 21 February 2014
Cited by 5 | PDF Full-text (1208 KB) | HTML Full-text | XML Full-text
Abstract
Electric variable transmission is a new electromechanical energy conversion device structure, which is especially suitable as the driving force distribution device for hybrid electric vehicles. This paper focuses on the power management strategy of hybrid electric vehicles based on an electric variable transmission,
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Electric variable transmission is a new electromechanical energy conversion device structure, which is especially suitable as the driving force distribution device for hybrid electric vehicles. This paper focuses on the power management strategy of hybrid electric vehicles based on an electric variable transmission, and a kind of hierarchical control ideology is proposed. The control strategy is composed of four control levels, namely analysis of force requirement, operation mode switching, force distribution and coordinate control, which are designed respectively in this paper. Then a simulation model is built based on the notion of energetic macroscopic representation, and an experimental test bench is built. The simulation and experiment results demonstrate the feasibility of the proposed strategy, and it can be taken as a new theory and method for the study of hybrid electric vehicle based on electric variable transmission. Full article
(This article belongs to the Special Issue Advances in Hybrid Vehicles)
Open AccessArticle Performance Evaluation of an In-Wheel Motor Cooling System in an Electric Vehicle/Hybrid Electric Vehicle
Energies 2014, 7(2), 961-971; doi:10.3390/en7020961
Received: 6 November 2013 / Revised: 22 January 2014 / Accepted: 14 February 2014 / Published: 21 February 2014
Cited by 3 | PDF Full-text (526 KB) | HTML Full-text | XML Full-text
Abstract
High power and miniaturization of motors in an in-wheel drive system, which is installed inside the wheels of a vehicle, are required for directly driving the wheels. In addition, an efficient cooling system is required to ensure high driving performance and durability. This
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High power and miniaturization of motors in an in-wheel drive system, which is installed inside the wheels of a vehicle, are required for directly driving the wheels. In addition, an efficient cooling system is required to ensure high driving performance and durability. This study experimentally evaluated the heat dissipation performance of a 35-kW-class large-capacity in-wheel motor equipped with an internal-circulation-type oil-cooling system that exhibits high cooling performance and can be easily miniaturized to this motor. Temperatures of the coil and stator core of cooling systems with and without a radiator were measured in real time under in-wheel motor driving conditions. It was found that operating the cooling system at a continuous-rating maximum speed without the radiator was difficult. We confirmed that under continuous-rating base speed and continuous-rating maximum speed driving conditions, the cooling system with the radiator showed thermally stable operation. Furthermore, under maximum-rating base speed and maximum-rating maximum speed driving conditions, the cooling system with the radiator provided additional driving times of approximately 22 s and 2 s, respectively. Full article
(This article belongs to the Special Issue Advances in Hybrid Vehicles)
Open AccessArticle Temperatures and Heat Flows in a Soil Enclosing a Slinky Horizontal Heat Exchanger
Energies 2014, 7(2), 972-987; doi:10.3390/en7020972
Received: 20 November 2013 / Revised: 22 January 2014 / Accepted: 12 February 2014 / Published: 24 February 2014
Cited by 8 | PDF Full-text (558 KB) | HTML Full-text | XML Full-text
Abstract
Temperature changes and heat flows in soils that host “slinky”-type horizontal heat exchangers are complex, but need to be understood if robust quantification of the thermal energy available to a ground-source heat pump is to be achieved. Of particular interest is the capacity
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Temperature changes and heat flows in soils that host “slinky”-type horizontal heat exchangers are complex, but need to be understood if robust quantification of the thermal energy available to a ground-source heat pump is to be achieved. Of particular interest is the capacity of the thermal energy content of the soil to regenerate when the heat exchangers are not operating. Analysis of specific heat flows and the specific thermal energy regime within the soil, including that captured by the heat-exchangers, has been characterised by meticulous measurements. These reveal that high concentrations of antifreeze mix in the heat-transfer fluid of the heat exchanger have an adverse impact on heat flows discharged into the soil. Full article
Open AccessArticle Multi-Objective Structural Optimization Design of Horizontal-Axis Wind Turbine Blades Using the Non-Dominated Sorting Genetic Algorithm II and Finite Element Method
Energies 2014, 7(2), 988-1002; doi:10.3390/en7020988
Received: 26 December 2013 / Revised: 30 January 2014 / Accepted: 10 February 2014 / Published: 24 February 2014
Cited by 6 | PDF Full-text (564 KB) | HTML Full-text | XML Full-text
Abstract
A multi-objective optimization method for the structural design of horizontal-axis wind turbine (HAWT) blades is presented. The main goal is to minimize the weight and cost of the blade which uses glass fiber reinforced plastic (GFRP) coupled with carbon fiber reinforced plastic (CFRP)
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A multi-objective optimization method for the structural design of horizontal-axis wind turbine (HAWT) blades is presented. The main goal is to minimize the weight and cost of the blade which uses glass fiber reinforced plastic (GFRP) coupled with carbon fiber reinforced plastic (CFRP) materials. The number and the location of layers in the spar cap, the width of the spar cap and the position of the shear webs are employed as the design variables, while the strain limit, blade/tower clearance limit and vibration limit are taken into account as the constraint conditions. The optimization of the design of a commercial 1.5 MW HAWT blade is carried out by combining FEM analysis and a multi-objective evolutionary algorithm under ultimate (extreme) flap-wise load and edge-wise load conditions. The best solutions are described and the comparison of the obtained results with the original design is performed to prove the efficiency and applicability of the method. Full article
(This article belongs to the Special Issue Wind Turbines 2014)
Open AccessArticle Voltage Support Provided by STATCOM in Unbalanced Power Systems
Energies 2014, 7(2), 1003-1026; doi:10.3390/en7021003
Received: 5 December 2013 / Revised: 6 February 2014 / Accepted: 19 February 2014 / Published: 24 February 2014
Cited by 5 | PDF Full-text (647 KB) | HTML Full-text | XML Full-text
Abstract
The presence of an unbalanced voltage at the point of common coupling (PCC) results in the appearance of a negative sequence current component that deteriorates the control performance. Static synchronous compensators (STATCOMs) are well-known to be a power application capable of carrying out
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The presence of an unbalanced voltage at the point of common coupling (PCC) results in the appearance of a negative sequence current component that deteriorates the control performance. Static synchronous compensators (STATCOMs) are well-known to be a power application capable of carrying out the regulation of the PCC voltage in distribution lines that can suffer from grid disturbances. This article proposes a novel PCC voltage controller in synchronous reference frame to compensate an unbalanced PCC voltage by means of a STATCOM, allowing an independent control of both positive and negative voltage sequences. Several works have been proposed in this line but they were not able to compensate an unbalance in the PCC voltage. Furthermore, this controller includes aspects as antiwindup and droop control to improve the control system performance. Full article
(This article belongs to the Special Issue Smart Grids: The Electrical Power Network and Communication System)
Open AccessArticle Anticipating and Coordinating Voltage Control for Interconnected Power Systems
Energies 2014, 7(2), 1027-1047; doi:10.3390/en7021027
Received: 19 November 2013 / Revised: 27 January 2014 / Accepted: 20 February 2014 / Published: 24 February 2014
Cited by 5 | PDF Full-text (544 KB) | HTML Full-text | XML Full-text
Abstract
This paper deals with the application of an anticipating and coordinating feedback control scheme in order to mitigate the long-term voltage instability of multi-area power systems. Each local area is uniquely controlled by a control agent (CA) selecting control values based on model
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This paper deals with the application of an anticipating and coordinating feedback control scheme in order to mitigate the long-term voltage instability of multi-area power systems. Each local area is uniquely controlled by a control agent (CA) selecting control values based on model predictive control (MPC) and is possibly operated by an independent transmission system operator (TSO). Each MPC-based CA only knows a detailed local hybrid system model of its own area, employing reduced-order quasi steady-state (QSS) hybrid models of its neighboring areas and even simpler PV models for remote areas, to anticipate (and then optimize) the future behavior of its own area. Moreover, the neighboring CAs agree on communicating their planned future control input sequence in order to coordinate their own control actions. The feasibility of the proposed method for real-time applications is explained, and some practical implementation issues are also discussed. The performance of the method, using time-domain simulation of the Nordic32 test system, is compared with the uncoordinated decentralized MPC (no information exchange among CAs), demonstrating the improved behavior achieved by combining anticipation and coordination. The robustness of the control scheme against modeling uncertainties is also illustrated. Full article
Open AccessArticle Influence of Prewhirl Regulation by Inlet Guide Vanes on Cavitation Performance of a Centrifugal Pump
Energies 2014, 7(2), 1050-1065; doi:10.3390/en7021050
Received: 30 November 2013 / Revised: 17 January 2014 / Accepted: 10 February 2014 / Published: 24 February 2014
Cited by 14 | PDF Full-text (1969 KB) | HTML Full-text | XML Full-text
Abstract
The influence of prewhirl regulation by inlet guide vanes (IGVs) on a centrifugal pump performance is investigated experimentally and numerically. The experimental results show that IGVs can obviously change the head and increase the efficiency of the tested centrifugal pump over a wide
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The influence of prewhirl regulation by inlet guide vanes (IGVs) on a centrifugal pump performance is investigated experimentally and numerically. The experimental results show that IGVs can obviously change the head and increase the efficiency of the tested centrifugal pump over a wide range of flow rates. Although the cavitation performance is degraded, the variation of the cavitation critical point is less than 0.5 m. Movement of the computed three-dimensional streamlines in suction pipe and impeller are analyzed in order to reveal the mechanism how the IGVs realize the prewhirl regulation. The calculated results show that the influence of IGVs on the cavitation performance of centrifugal pump is limited by a maximum total pressure drop of 1777 Pa, about 7.6% of the total pressure at the suction pipe inlet for a prewhirl angle of 24°. Full article
Open AccessArticle Development and Improvement of an Intelligent Cable Monitoring System for Underground Distribution Networks Using Distributed Temperature Sensing
Energies 2014, 7(2), 1076-1094; doi:10.3390/en7021076
Received: 15 November 2013 / Revised: 4 February 2014 / Accepted: 11 February 2014 / Published: 24 February 2014
Cited by 3 | PDF Full-text (2063 KB) | HTML Full-text | XML Full-text
Abstract
With power systems switching to smart grids, real-time and on-line monitoring technologies for underground distribution power cables have become a priority. Most distribution components have been developed with self-diagnostic sensors to realize self-healing, one of the smart grid functions in a distribution network.
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With power systems switching to smart grids, real-time and on-line monitoring technologies for underground distribution power cables have become a priority. Most distribution components have been developed with self-diagnostic sensors to realize self-healing, one of the smart grid functions in a distribution network. Nonetheless, implementing a real-time and on-line monitoring system for underground distribution cables has been difficult because of high cost and low sensitivity. Nowadays, optical fiber composite power cables (OFCPCs) are being considered for communication and power delivery to cope with the increasing communication load in a distribution network. Therefore, the application of distributed temperature sensing (DTS) technology on OFCPCs used as underground distribution lines is studied for the real-time and on-line monitoring of the underground distribution power cables. Faults can be reduced and operating ampacity of the underground distribution system can be increased. This paper presents the development and improvement of an intelligent cable monitoring system for the underground distribution power system, using DTS technology and OFCPCs as the underground distribution lines in the field. Full article
(This article belongs to the Special Issue Smart Grids: The Electrical Power Network and Communication System)

Review

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Open AccessReview Performance of Different Experimental Absorber Designs in Absorption Heat Pump Cycle Technologies: A Review
Energies 2014, 7(2), 751-766; doi:10.3390/en7020751
Received: 12 November 2013 / Revised: 16 January 2014 / Accepted: 10 February 2014 / Published: 13 February 2014
Cited by 4 | PDF Full-text (391 KB) | HTML Full-text | XML Full-text
Abstract
The absorber is a major component of absorption cycle systems, and its performance directly impacts the overall size and energy supplies of these devices. Absorption cooling and heating cycles have different absorber design requirements: in absorption cooling systems, the absorber works close to
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The absorber is a major component of absorption cycle systems, and its performance directly impacts the overall size and energy supplies of these devices. Absorption cooling and heating cycles have different absorber design requirements: in absorption cooling systems, the absorber works close to ambient temperature, therefore, the mass transfer is the most important phenomenon in order to reduce the generator size; on the other hand, in heat transformer absorption systems, is important to recover the heat delivered by exothermic reactions produced in the absorber. In this paper a review of the main experimental results of different absorber designs reported in absorption heat pump cycles is presented. Full article

Other

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Open AccessCorrection Palazzi, E.; Currò, F.; Fabiano, B. Accidental Continuous Releases from Coal Processing in Semi-Confined Environment. Energies 2013, 6, 5003–5022
Energies 2014, 7(2), 735; doi:10.3390/en7020735
Received: 21 January 2014 / Accepted: 22 January 2014 / Published: 12 February 2014
PDF Full-text (113 KB) | HTML Full-text | XML Full-text
Abstract The authors wish to make the following corrections, due to typographical errors, to this paper [1]. [...] Full article
(This article belongs to the Special Issue Coal Combustion and Gasification)
Open AccessCorrection Li, R.; Ma, H.; Wang, F.; Wang, Y.; Liu, Y.; Li, Z. Game Optimization Theory and Application in Distribution System Expansion Planning, Including Distributed Generation. Energies 2013, 6, 1101–1124
Energies 2014, 7(2), 1048-1049; doi:10.3390/en7021048
Received: 5 February 2014 / Accepted: 11 February 2014 / Published: 24 February 2014
PDF Full-text (131 KB) | HTML Full-text | XML Full-text
Abstract The authors wish to make the following corrections to this paper [1]. [...] Full article

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