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Top 20 Most Read Articles
April 2013
The 20 articles with the most full-text downloads during the month, in descending order.
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Effect of temperature and gas flow on the efficiency of an air bottoming cycle J. Renewable Sustainable Energy 5, 021409 (2013); http://dx.doi.org/10.1063/1.4798486 (10 pages) Online Publication Date: 27 March 2013
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The objective of this research is to analyze the performance of an Air Bottoming Cycle (ABC) by considering the effect of temperature and gas flow. Based on the energetic analysis, a thermodynamic calculation has been performed to investigate improving the thermal efficiency of the ABC. In this research, we consider the full cycle should consist of two sub-cycles: Braysson and Joule. The flue gas from the top cycle is used in Braysson cycle turbine to generate power and the rest of the energy of this flow is exchanged to the pressurized Joule cycle air flow. The results indicate that the higher inlet temperatures of the Braysson turbine and lower Braysson turbine outlet temperatures lead to higher thermal efficiencies. Moreover, the lower overall pressure ratios in Joule cycle are required to achieve the higher overall thermal efficiencies. The maximum thermal efficiency and also the optimal design point of this ABC have been found. The results of this ABC analysis show an increase in net output work and the thermal efficiency up to more than 55%, which in turn leads to have less specific fuel consumption. The proposed cycle has been found to have more energy recovery potential compared to some other gas turbine based ABCs. Besides, in contrast to common steam bottom cycles, some water can be generated using the flue gas stream which is important because the lack of water resources is going to be a major problem in many regions worldwide.
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J. Renewable Sustainable Energy 2, 042701 (2010); http://dx.doi.org/10.1063/1.3467511 (37 pages) Online Publication Date: 26 July 2010
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Photocatalysis process, as an environmental application, is a relatively novel subject with tremendous potential in the near future. A number of studies determine the economic viability of applying various ultraviolet advanced oxidation processes for the purpose of degradation and destroying of various pollutant solutions. The present paper reviews the treatment of hazardous wastewater bearing organic compounds and the effects of various parameters such as pH, initial concentration, mass of catalyst, wavelength, light intensity, electron acceptor, temperature, etc., with the treatment cost for different reactor type. This study investigates the effect of various types of waste-water treatment parameters and the treatment cost for various designs of photocatalytic reactors, using both solar and artificial light, have been proposed for their different types of pollutants.
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Effects of using vortex tubes on events during cold start of a direct injection diesel engine J. Renewable Sustainable Energy 5, 021411 (2013); http://dx.doi.org/10.1063/1.4798489 (8 pages) Online Publication Date: 27 March 2013
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Vortex tubes use a compressed gas flow and separate it into low and high temperature regions without using any moving mechanical parts. Cold and hot exhaust flows of vortex tubes can be used for different purposes. In this study, a vortex tube was used to improve the cold start performance of a six cylinders, four stroke and direct injection diesel engine of a truck. To increase the engine intake air temperature, hot exhaust of vortex tube was introduced to the engine intake manifold. Pressured air was supplied to the vortex tube from the air tank of the compressed-air brake system of the truck. Variation in the cold starting events was observed by both in cylinder pressure data and revolution of engine crank shaft. Experiments were started when the ambient air and engine cooling fluid temperatures were at 0 °C, which is a critical temperature for cold-starting diesel engines. Experimental results showed that the cold starting performance of the engine can be successfully improved by using vortex tubes. Durations of engine starting-cranking, cranking-idling, and idling-stabling were decreased due to the increase in the intake air temperature.
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J. Renewable Sustainable Energy 5, 013121 (2013); http://dx.doi.org/10.1063/1.4792847 (10 pages) Online Publication Date: 19 February 2013
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China's wind power capacity is increasing so rapidly that some areas are not able to consume all the produced wind power. Therefore, it has already become the inevitable trend in wind power integration to achieve the controlled operation of wind turbine. This paper first offers a method to predict the maximum generation capacity of every type of wind turbine, including both under-controlled wind turbine and uncontrolled wind turbine. And then a strategy for active power control of wind farm based on the predicting results is proposed. Considering thoroughly the active power fluctuation of wind turbine and the error of the prediction, the strategy adopts both long and short controlling periods to carry out coordinated optimization control. The simulation results show that the strategy can keep the active power output of wind farm in step with the control order, and consequently the generating efficiency is improved. In addition, the strategy is also applicable to the wind farm with some uncontrollable wind turbines, and the controlling precision can be satisfied.
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Preface to Special Topic: Low-Carbon Society for a Green Economy J. Renewable Sustainable Energy 4, 041301 (2012); http://dx.doi.org/10.1063/1.4747721 (3 pages) Online Publication Date: 23 August 2012
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Dewatering of microalgal cultures: A major bottleneck to algae-based fuels J. Renewable Sustainable Energy 2, 012701 (2010); http://dx.doi.org/10.1063/1.3294480 (15 pages) Online Publication Date: 12 January 2010
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Microalgae dewatering is a major obstruction to industrial-scale processing of microalgae for biofuel production. The dilute nature of harvested microalgal cultures creates a huge operational cost during dewatering, thereby, rendering algae-based fuels less economically attractive. Currently there is no superior method of dewatering microalgae. A technique that may result in a greater algal biomass may have drawbacks such as a high capital cost or high energy consumption. The choice of which harvesting technique to apply will depend on the species of microalgae and the final product desired. Algal properties such as a large cell size and the capability of the microalgae to autoflocculate can simplify the dewatering process. This article reviews and addresses the various technologies currently used for dewatering microalgal cultures along with a comparative study of the performances of the different technologies.
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Correlation analysis for wind speed and failure rate of wind turbines using time series approach J. Renewable Sustainable Energy 4, 032301 (2012); http://dx.doi.org/10.1063/1.4730597 (13 pages) Online Publication Date: 21 June 2012
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The correlation between wind speed and failure rate (FR) of wind turbines is analyzed with time series approach. The time series of power index (PI) and FR of wind turbines are established based on historical data, which are pretreated by singularity processing, stationarity processing, and wavelet de-noising. The trend variations of the time series are analyzed from both time domain and frequency domain by extracting the indicator functions, including auto-correlation function, cross-correlation function, and spectral density function. A case study is given out to verify the validity of the model and the method, which is based on the wind speed and failure data from January 1995 to December of 2002 in Nordjylland, Denmark. Auto-correlation function and spectral density function show that time series of PI and FR have strong seasonal characteristics and quite similar periodicity, while the cross-correlation function shows they keep high consistency and strong correlation. The results indicate that by calculating and monitoring PI, the failure rule of wind turbines can be forecast, which provides theoretical basis for preventive maintenance of wind turbines.
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High performance FeFx/C composites as cathode materials for lithium-ion batteries J. Renewable Sustainable Energy 5, 021402 (2013); http://dx.doi.org/10.1063/1.4798423 (9 pages) Online Publication Date: 27 March 2013
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FeFx precursors were synthesized by a simplified one-step hydrothermal synthesis route with commercial Fe(OH)3 and HF as raw materials; then the as-prepared precursor was calcined in different temperature and obtained FeF3, FeF2, and amorphous mixture FeF3-FeF2. These materials were characterized by X-ray diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy and used as cathode materials for lithium-ion batteries. The electrochemical tests show that the initial discharge capacities of FeF3, FeF2, and amorphous mixture FeF3-FeF2 are as high as 204.6 mAh/g, 162 mAh/g, and 208.6 mAh/g, respectively, at the rate of 0.1 C in the range 2.0-4.5 V at 25 °C, and display very excellent discharge capacity retention rate after the first discharge process. Furthermore, the cyclic voltammogram test was used to illustrate the reaction mechanisms of the nanocomposites.
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On the velocity distribution for hydro-kinetic energy conversion from tidal currents and rivers J. Renewable Sustainable Energy 5, 023115 (2013); http://dx.doi.org/10.1063/1.4795398 (19 pages) Online Publication Date: 21 March 2013
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Tidal currents and rivers are promising sources of renewable energy given that suitable turbines for kinetic energy conversion are developed. To be economically and technically feasible, a velocity distribution that can give a high degree of utilization (or capacity factor) while the ratio of maximum to rated velocity is low would be preferable. The rated velocity is defined as the velocity at which rated power is achieved. Despite many attempts to estimate the resource, however, reports on the possible degree of utilisation from tidal currents and rivers are scarce. In this paper, the velocity distribution from a number of regulated rivers, unregulated rivers, and tidal currents have been analysed regarding the degree of utilisation, the fraction of converted energy, and the ratio of maximum to rated velocity. Two methods have been used for choosing the rated velocity; one aiming at a high fraction of converted energy and one aiming at a high degree of utilisation. Using the first method, with a rated velocity close to the maximum velocity, it is unlikely that the turbine will reach the cut-out velocity. This results in, on average, a degree of utilisation of 23% for regulated rivers, 19% for unregulated rivers, and 17% for tidal currents while converting roughly 30%–40% of the kinetic energy. Choosing a rated velocity closer to the mean velocity resulted in, on average, a degree of utilisation of 57% for regulated rivers, 52% for unregulated rivers, and 45% for tidal currents. The ratio of maximum to rated velocity would still be no higher than 2.0 for regulated rivers, 1.2 for unregulated rivers, and 1.6 for tidal currents. This implies that the velocity distribution of both rivers and tidal currents is promising for kinetic energy conversion. These results, however, do not include weather related effects or extreme velocities such as the 50-year velocity. A velocity factor is introduced to describe what degree of utilisation can be expected at a site. The velocity factor is defined as the ratio Umax/Urate at the desired degree of utilisation, and serves as an early indicator of the suitability of a site.
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J. Renewable Sustainable Energy 5, 022701 (2013); http://dx.doi.org/10.1063/1.4798483 (27 pages) Online Publication Date: 27 March 2013
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This article reviews the use of Density Functional Theory (DFT) to study the electronic and optical properties of solar-active materials and dyes used in solar energy conversion applications (dye-sensitized solar cells and water splitting). We first give a brief overview of the DFT, its development, advantages over ab-initio methods, and the most commonly used functionals and the differences between them. We then discuss the use of DFT to design optimized dyes for dye-sensitized solar cells and compare between the accuracy of different functionals in determining the excitation energy of the dyes. Finally, we examine the application of DFT in understanding the performance of different photoanodes and how it could be used to screen different candidate materials for use in photocatalysis in general and water splitting in particular.
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J. Renewable Sustainable Energy 5, 021401 (2013); http://dx.doi.org/10.1063/1.4798422 (7 pages) Online Publication Date: 27 March 2013
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In recent years, some metal borides, especially cobalt boride, have been regarded as anode candidates of high energy alkaline secondary batteries for their potential of carrying out multi-electron reactions. So far, there are some reports on the synthesis of metal borides, such as ball milling method, arc melting method, high-temperature solid-phase method, and so on. Herein, a vacuum freeze-drying method is adopted to synthesize cobalt boride with small size, uniform particle size distribution, high specific surface area, as well as excellent electrochemical activity. The crystal structures, particle size, and specific surface area of the as-prepared Co–B alloys were characterized and analyzed via X-ray diffraction, scanning electron microscopy, and nitrogen adsorption–desorption test. The electrochemical activities of the Co–B samples were examined by cyclic voltammetry (CV) and charge–discharge test. It is found that the Co-B alloy synthesized via the vacuum freeze-drying method exhibits good reversibility and cycle stability, and achieves a maximum discharge capacity of 437 mAh/g at the current density of 300 mA/g, which is much superior to that of the sample prepared via traditional chemical reduction method.
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J. Renewable Sustainable Energy 4, 011501 (2012); http://dx.doi.org/10.1063/1.3687647 (2 pages) Online Publication Date: 17 February 2012
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Photovoltaic properties of thin film heterojunctions with cupric oxide absorber J. Renewable Sustainable Energy 5, 011205 (2013); http://dx.doi.org/10.1063/1.4791779 (11 pages) Online Publication Date: 27 February 2013
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In this work, we report on the fabrication, characterization, and photovoltaic properties of sputter-deposited, thin film heterojunctions combining p-type cupric oxide (CuO) absorber with n-type ZnO. The structural investigation reveals highly crystalline, columnar growth of the layers and confirms that the absorber's phase is purely CuO, with only negligible traces of Cu2O. The optical characterization yields for CuO an indirect bandgap of 1.2 eV and a direct optical transition at approximately 3 eV. The short circuit current, open circuit voltage, fill factor, and power conversion efficiency of the heterojunction solar cells were extracted as a function of the CuO thickness under AM1.5 G (1 kW/m2) illumination. From the observed dependencies, we conclude that the photovoltaic performance is compromised by a restricted carrier collection efficiency, caused by the small carrier lifetime in CuO. Indeed, the carrier population is found to decay with time constants of 40 and 460 ps. A maximum power conversion efficiency of 0.08% was obtained for the solar cell with CuO thickness of 500 nm.
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J. Renewable Sustainable Energy 2, 033106 (2010); http://dx.doi.org/10.1063/1.3449299 (7 pages) Online Publication Date: 22 June 2010
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The impact of escalating human activities on greenhouse gas emission, global warming, and changes in global climate patterns is almost certainly the most discussed issue in the first decade of the 21st century. Two-thirds of Africa’s energy consumption consists of various forms of highly inefficient traditional biomass. In rural areas, low energy consumption is both a cause and consequence of poor development and also of the degradation of the natural environment. The burning of biomass or vegetation as a conventional land preparation method has a net negative impact on the soil organic carbon as well as on the environment through the oxidation of carbon into carbon dioxide, an anthropogenic greenhouse gas. This paper reports the findings of an experiment to investigate the impact of the heat due to burning on soil organic carbon at soil depths of 0–5 and 5–10 cm. It was observed that 21% decline in soil organic carbon resulted in the release of 1446 CO2 kg ha−1 into the atmosphere. This underscores the fact that burning of biomass as a land preparation method may not be sustainable when viewed against the backdrop that the African continent encompasses the most vulnerable regions and populations for current climate variability. This study is significant in that it is the first to shed light on the effects of burning on soil carbon in the coastal savanna ecosystem of Ghana and would potentially provide the impetus for further research.
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J. Renewable Sustainable Energy 4, 063133 (2012); http://dx.doi.org/10.1063/1.4768812 (12 pages) Online Publication Date: 5 December 2012
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This research article deals with the employment of organic Rankine cycle (ORC) to generate electricity from a set of low-temperature abandoned gas wells in Iran. At first, a thermodynamic analysis was performed to select an appropriate power cycle; consequently organic Rankine cycle was chosen. Then, a comprehensive investigation was carried out to find a typical low-temperature abandoned gas reservoir, so an abandoned gas reservoir in the central part of Iran was considered. The next step was selecting the working fluid; in this regard, a vast range of common organic fluids were studied and R125 was chosen. Finally, the gas well and the power plant were simulated, and then a parametric optimization of the ORC plant was performed in order to achieve optimum power generation, and also to compute generated power at different operational parameters of gas wells and power cycle.
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A critical review on state of charge of batteries J. Renewable Sustainable Energy 5, 021403 (2013); http://dx.doi.org/10.1063/1.4798430 (10 pages) Online Publication Date: 27 March 2013
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The accurate and reliable SOC (state of charge) estimation is difficult, although it is a core factor especially for battery energy storage system. Current studies of SOC estimation theories and models are mainly focused on voltage model, current model, resistance model, intelligent algorithm model, and hybrid model. Specific attentions were paid to estimate the model and their impact factors on SOC. Then, a critical review of available literature on major SOC issues was presented. Influencing factors on SOC were studied and analyzed, such as dynamic hysteresis characteristics, temperature, aging, self-discharge, and rate of charge/discharge. The economy and practicability of the SOC estimation are analyzed and compared objectively.
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J. Renewable Sustainable Energy 5, 021301 (2013); http://dx.doi.org/10.1063/1.4802938 (1 page) Online Publication Date: 25 April 2013
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An integrated energy storage scheme for a dispatchable solar and wind powered energy system J. Renewable Sustainable Energy 3, 043101 (2011); http://dx.doi.org/10.1063/1.3599839 (12 pages) Online Publication Date: 5 July 2011
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This research analyzed an integrated energy system that includes a novel configuration of wind and solar coupled with two storage methods to make both wind and solar sources dispatchable during peak demand, thereby enabling their broader use. Named DSWiSS for Dispatchable Solar and Wind Storage System, the proposed system utilizes compressed air energy storage (CAES) that is driven from wind energy and thermal storage supplied by concentrating solar thermal power (CSP) in order to achieve firm power from intermittent, renewable sources. Although DSWiSS mimics the operation of a typical CAES facility, the replacement of energy derived from fossil fuels with energy generated from renewable resources makes this system unique. West Texas is a useful geographical testbed for this system because it has abundant co-located wind and solar resources; it has competitive electricity markets, which give producers an economic incentive to store night-time wind energy in order to be sold during peak price times; and it has a significant number of locations with geological formations suitable for CAES. Through a thermodynamic and a levelized lifetime cost analysis, the power system performance and the cost of energy are estimated for this integrated wind-solar-storage system. We calculate that the combination of these components yields an energy efficiency of 46% for the CAES main power block, and the overall system cost is only slightly more expensive per unit of electricity generated than the current technologies employed today.
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Validation of a FAST semi-submersible floating wind turbine numerical model with DeepCwind test data J. Renewable Sustainable Energy 5, 023116 (2013); http://dx.doi.org/10.1063/1.4796197 (29 pages) Online Publication Date: 26 March 2013
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There are global efforts in the offshore wind community to develop reliable floating wind turbine technologies that are capable of exploiting the abundant deepwater wind resource. These efforts require validated numerical simulation tools to predict the coupled aero-hydro-servo-elastic behavior of such systems. To date, little has been done in the public domain to validate floating wind turbine simulation tools. This work begins to address this problem by presenting the validation of a model constructed in the National Renewable Energy Laboratory (NREL) floating wind turbine simulator FAST with 1/50th-scale model test data for a semi-submersible floating wind turbine system. The test was conducted by the University of Maine DeepCwind program at Maritime Research Institute Netherlands' offshore wind/wave basin, located in the Netherlands. The floating wind turbine used in the tests was a 1/50th-scale model of the NREL 5-MW horizontal-axis reference wind turbine with a 126 m rotor diameter. This turbine was mounted to the DeepCwind semi-submersible floating platform. This paper first outlines the details of the floating system studied, including the wind turbine, tower, platform, and mooring components. Subsequently, the calibration procedures used for tuning the FAST floating wind turbine model are discussed. Following this calibration, comparisons of FAST predictions and test data are presented that focus on system global and structural response resulting from aerodynamic and hydrodynamic loads. The results indicate that FAST captures many of the pertinent physics in the coupled floating wind turbine dynamics problem. In addition, the results highlight potential areas of improvement for both FAST and experimentation procedures to ensure accurate numerical modeling of floating wind turbine systems.
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Electrochemical performance of lithium ion capacitors using aqueous electrolyte at high temperature J. Renewable Sustainable Energy 5, 021404 (2013); http://dx.doi.org/10.1063/1.4798432 (10 pages) Online Publication Date: 27 March 2013
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Aqueous lithium ion capacitors (LICs) with LiFePO4 positive electrodes and activated carbon (AC) negative electrodes were developed and their electrochemical performances were investigated by cyclic voltammetry, galvanostatic charge/discharge, and ac impedance spectroscopy at various temperatures. It is found that the stable voltage window for the aqueous LICs is 0–1.7 V and the optimized mass ratio between the positive electrode and negative electrode is 1:1. The rate capability of the LIC is much higher than that of the LiFePO4 electrodes. The Ohmic resistance and electrochemical impedance of the LiFePO4 and AC electrodes decrease with increasing the temperature, leading to high rate capability of the LIC. The performance of the LIC is deteriorated when it is cycled at 70 °C which is caused by the reduction of Fe2+ in LiFePO4.
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