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Top 20 Most Cited Articles
The 20 most cited articles over time based on CrossRef data.
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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) | Cited 15 times 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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Nanofluid-based direct absorption solar collector J. Renewable Sustainable Energy 2, 033102 (2010); http://dx.doi.org/10.1063/1.3429737 (13 pages) | Cited 11 times Online Publication Date: 26 May 2010
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Solar energy is one of the best sources of renewable energy with minimal environmental impact. Direct absorption solar collectors have been proposed for a variety of applications such as water heating; however the efficiency of these collectors is limited by the absorption properties of the working fluid, which is very poor for typical fluids used in solar collectors. It has been shown that mixing nanoparticles in a liquid (nanofluid) has a dramatic effect on the liquid thermophysical properties such as thermal conductivity. Nanoparticles also offer the potential of improving the radiative properties of liquids, leading to an increase in the efficiency of direct absorption solar collectors. Here we report on the experimental results on solar collectors based on nanofluids made from a variety of nanoparticles (carbon nanotubes, graphite, and silver). We demonstrate efficiency improvements of up to 5% in solar thermal collectors by utilizing nanofluids as the absorption mechanism. In addition the experimental data were compared with a numerical model of a solar collector with direct absorption nanofluids. The experimental and numerical results demonstrate an initial rapid increase in efficiency with volume fraction, followed by a leveling off in efficiency as volume fraction continues to increase.
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Wearable electronics self-powered by using human body heat: The state of the art and the perspective J. Renewable Sustainable Energy 1, 062701 (2009); http://dx.doi.org/10.1063/1.3255465 (14 pages) | Cited 9 times Online Publication Date: 6 November 2009
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In this paper, we present our vision of what kind of wearable devices and how they can be powered by the heat of human beings and by using ambient light. The basic principles of designing body-powered devices and ways of their hybridizing with photovoltaic cells are discussed. The mechanisms of thermoregulation in humans and the laws of thermodynamics enable placing a distinct boarder between realistic targets and the science fiction. These allow prediction of application areas for wearable energy harvesters accounting for competitive batteries with long service life. The existing family of body-powered wearable devices and new technologies for thermopiles are discussed. The theory and practice point at the necessity of using microelectronic and microelectromechanical system technologies for the target application area. These technologies for thermopiles offer the possibility of reduced production cost. Therefore, autonomous systems powered thermoelectrically could be successfully marketed. The related aspects of design and fabrication are discussed.
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J. Renewable Sustainable Energy 1, 013102 (2009); http://dx.doi.org/10.1063/1.3005376 (8 pages) | Cited 7 times Online Publication Date: 6 November 2008
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Flexible Cu(In,Ga)Se2 (CIGS) solar cells were fabricated using alkali-silicate glass thin layers (ASTL) as an alkali source material deposited on various flexible substrates prior to the Mo backcontact layer deposition. Control of univalent alkali metal incorporation into the CIGS layer with the use of ASTL was demonstrated regardless of the presence of other multivalent metal impurities in ASTL. Dramatic enhancement in cell efficiencies using ASTL was observed though the use of excessively thick ASTL led to a degradation in cell performance. The quantum efficiency curves of CIGS solar cells fabricated using ASTL showed enhanced absorption in the long wavelength region. The photovoltaic performance dependence on a variety of substrate materials is also discussed.
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J. Renewable Sustainable Energy 1, 063107 (2009); http://dx.doi.org/10.1063/1.3278518 (12 pages) | Cited 6 times Online Publication Date: 31 December 2009
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We have successfully obtained a new figure of merit for qualifying fluorine-doped tin oxide (FTO) glass in dye-sensitized solar cells (DSCs) through two equivalent testing methods. These methods are demonstrated and applied to change the equivalent transmittance and sheet resistance of FTO glass even after the glass is assembled in DSCs. By recording the I-V characteristic of a DSC with changed equivalent transmittance and sheet resistance of FTO glass, the dependent relations between the DSC performance (short circuit current density, open circuit voltage, and fill factor) and FTO properties (transmittance and sheet resistance) are found. With these relations, the new figure of merit MTC for FTO glass is successfully defined to be in linearly increasing dependence on the efficiency of DSCs. A series of DSCs with different FTO glasses is prepared to test the effectiveness of MTC and Haacke’s figure of merit ΦTC, which has been widely used for more than 30 years. The result shows that MTC is proportional to the efficiency of DSCs, while ΦTC is not. MTC could be very useful as a guideline to greatly simplify the process of optimizing the FTO glass to improve the efficiency of DSCs.
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J. Renewable Sustainable Energy 1, 033101 (2009); http://dx.doi.org/10.1063/1.3138922 (14 pages) | Cited 5 times Online Publication Date: 21 May 2009
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Sulfonated poly(ether ether ketone) (S-PEEK) with 40% of degree of sulfonation was studied using full atomistic molecular dynamics simulation in order to investigate the nanophase-segregated structures, focusing on the sulfonate group and water phase at various water contents such as 10, 13, and 20 wt %. By analyzing the pair correlation function, it is found that as the water solvation of sulfonate groups proceeds more with increasing water content, the distance between sulfonate groups is increased from 4.4 Å (10 wt %) to 4.8 Å (13 wt %) to 5.4 Å (20 wt %), and the hydronium ions (H3O+) become farther apart from the sulfonate groups. The water coordination number for water and the water diffusion are enhanced with increasing water content because the internal structure of the water phase in S-PEEK approaches that of bulk water. Compared to the Nafion and Dendrion membranes, the S-PEEK membrane shows less internal structure in the water phase and smaller water diffusion, indicating that the S-PEEK has less nanophase segregation than the Nafion and Dendrion membranes.
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J. Renewable Sustainable Energy 1, 033109 (2009); http://dx.doi.org/10.1063/1.3156004 (12 pages) | Cited 5 times Online Publication Date: 24 June 2009
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Solid-state dye sensitized solar cells (DSSCs) could become alternative to liquid electrolyte-based cells which suffer from several problems such as leakage, encapsulation, and cell interconnections within the module. In this paper we focus on ZnO solid-state DSSCs employing thiophene-based polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) as hole conducting agent. The effect of the addition of multiwalled carbon nanotubes (MWCNTs) in P3HT on the solar cell parameters was investigated. Current-voltage (I-V) characteristics of the different cells were obtained having different wt % of MWCNTs. Using the I-V characteristics and single diode model for the solar cell, various parameters such as the photocurrent, the saturation current of the diode, the series resistance, the shunt resistance, and the ideality factor were determined. The use of MWCNTs with polymer P3HT increased the photovoltaic performance of the solar cells. Highest device performance was achieved with 0.8 wt % of MWCNTs in P3HT. The role of MWCNTs for efficient hole transfer was discussed in terms of series and shunt resistances.
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J. Renewable Sustainable Energy 1, 013106 (2009); http://dx.doi.org/10.1063/1.3081510 (6 pages) | Cited 4 times Online Publication Date: 24 February 2009
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A low-cost dichroic mirror can be used successfully for solar spectrum splitting to enhance solar to electrical energy conversion. The mirror is optimized for use with a polycrystalline silicon photovoltaic cell (pc-Si). With the dichroic mirror simultaneous excitation of a medium-efficient (11.1%) commercial pc-Si and a custom-made high band gap GaInP cell (12.3%), yields 16.8% efficiency, with both cells operating at maximum power. Our results clearly show that what is missing for this simple low-cost enhancement of Si solar cell efficiency are low-cost high band gap cells.
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J. Renewable Sustainable Energy 2, 013106 (2010); http://dx.doi.org/10.1063/1.3289735 (25 pages) | Cited 4 times Online Publication Date: 27 January 2010
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When wind turbines are deployed in large arrays, their ability to extract kinetic energy from the flow decreases due to complex interactions among them, the terrain topography and the atmospheric boundary layer. In order to improve the understanding of the vertical transport of momentum and kinetic energy across a boundary layer flow with wind turbines, a wind-tunnel experiment is performed. The boundary layer flow includes a 3×3 array of model wind turbines. Particle-image-velocity measurements in a volume surrounding a target wind turbine are used to compute mean velocity and turbulence properties averaged on horizontal planes. Results are compared with simple momentum theory and with expressions for effective roughness length scales used to parametrize wind-turbine arrays in large-scale computer models. The impact of vertical transport of kinetic energy due to turbulence and mean flow correlations is quantified. It is found that the fluxes of kinetic energy associated with the Reynolds shear stresses are of the same order of magnitude as the power extracted by the wind turbines, highlighting the importance of vertical transport in the boundary layer.
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J. Renewable Sustainable Energy 3, 013102 (2011); http://dx.doi.org/10.1063/1.3529427 (17 pages) | Cited 4 times Online Publication Date: 6 January 2011
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Lithium zirconates have attracted researchers’ interests because they can also be used as solid sorbents for CO2 capture. The structural, electronic, and phonon properties of Li2ZrO3, Li6Zr2O7, and monoclinic phase ZrO2 are investigated by the density-functional theory and phonon dynamics. Their electrochemical properties and their thermodynamics of CO2 absorption/desorption are analyzed. The calculated results show that their optimized structures and calculated bulk moduli as well as cohesive energies are in good agreement with experimental measurements. The calculated band gaps are 3.90 eV (indirect), 3.98 eV (direct), and 3.76 eV (direct) for Li2ZrO3, Li6Zr2O7, and ZrO2, respectively. The calculated Li intercalation voltage and energy densities of Li2ZrO3 are higher than that of Li6Zr2O7, which indicates that as a cathode material Li2ZrO3 is better than Li6Zr2O7. The calculated phonon dispersions and density of states show that there is one soft mode in Li2ZrO3 and two soft modes in Li6Zr2O7. From the calculated thermodynamic properties of these two lithium zirconates reacting with CO2, we found that the performance of Li2ZrO3 as a CO2 sorbent is better than that of Li6Zr2O7. In the first half cycle, sorbents absorbing CO2 to form lithium carbonate, Li6Zr2O7 performs better than Li2ZrO3 because the former releases more heat of reaction and has a lower Gibbs free energy and a higher CO2 capture capacity. However, during the second half cycle, regenerating sorbent from carbonate and zirconia to release CO2, the main product is the thermodynamically favorable Li2ZrO3 rather than forming Li6Zr2O7.
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Visible light photocatalytic properties of polymorphic brookite titania J. Renewable Sustainable Energy 1, 023101 (2009); http://dx.doi.org/10.1063/1.3096606 (7 pages) | Cited 4 times Online Publication Date: 20 March 2009
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Visible light (VL) active titania was prepared via ambient condition sol process. The particles precipitated from titanium tetrachloride solution followed by lattice dehydroxylation using hot N-methylpyrrolidone (NMP) yielded a mixed crystal phase of brookite titania with anatase as the minor phase. The mixed phase titania exhibited superior photocatalytic activities for an organic dye and hydrogen evolution from an aqueous solution under 14 W VL as compared with several other available reference titanias. The superior VL photocatalytic activity may be explained as the effective charge separation by the intercrystalline electron transport from brookite to anatase grains complemented by the strong VL absorption by the nitrogen species in NMP. The probable electron transport mechanism is illustrated.
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Effects of global warming on wind energy availability J. Renewable Sustainable Energy 2, 052301 (2010); http://dx.doi.org/10.1063/1.3486072 (5 pages) | Cited 4 times Online Publication Date: 1 September 2010
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The use of wind energy reduces our greenhouse gas emissions into the atmosphere. In this study, we proposed a generic power-law relationship between global warming and the usable wind energy (Betz’s law). The power law index ( ∼ 4, region dependent) is then determined using simulated atmospheric parameters from eight global coupled ocean-atmosphere climate models (CGCMs). It is found that the power-law relationship holds across all eight climate models and also is time scale independent. Reduction of wind power scales with the degree of warming according to a generic power-law relationship. Thus, the earlier we switch to clean energy, and thereby decrease the global climate warming trend, the more cost-effective will be the harnessing of wind energy. This relationship is an area-averaged consequence of the reduced poleward temperature gradient as the climate warms during the 21st Century; it does not imply spatial uniformity over a region of interest.
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J. Renewable Sustainable Energy 1, 043102 (2009); http://dx.doi.org/10.1063/1.3167284 (11 pages) | Cited 3 times Online Publication Date: 1 July 2009
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The use of auxiliary power and the cost of using main power decrease when the reactive gas is applied at low humidity for the operation of polymer electrolyte membrane fuel cells. The electrolyte membrane and the three-phase boundary of the electrode need to be improved to maintain this low humidity. We carried out a study of the electrode catalyst at low humidity in order to improve the cathode catalyst. Carbon support with a catalyst and fluorocarbon resin were mixed and then were treated at the melting temperature of fluorocarbon resin to fabricate the cathode-electrode catalyst layer. The transmission electron microscopy images of this electrode catalyst revealed that the surfaces of the catalyst particle and the carbon support were partly coated with a thin film of melted fluorocarbon resin. Ionomer electrolyte material was added to this electrode catalyst, and an electrode catalyst with a four-layer structure was fabricated. The durability of this four-layer electrode catalyst being operated at low humidity (42% relative humidity on the anode and the cathode) was evaluated by using a cell that contained the catalyst. The results demonstrated that the rate of deterioration was smaller than that of a conventional three-layer electrode catalyst. In addition, the load change test to assess the durability of the cell (current range between 75 and 600 mA cm−2) produced good results.
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Determining optimum tilt angles of photovoltaic panels at typical north-tropical latitudes J. Renewable Sustainable Energy 1, 033104 (2009); http://dx.doi.org/10.1063/1.3148272 (6 pages) | Cited 3 times Online Publication Date: 9 June 2009
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An application of renewable energy technology is the installation of photovoltaic systems that generate power without emitting pollutants and requiring no fuel. Solar radiation varies with geographic latitude, season, and time of day due to the various sun positions in the sky. Hence, the problem of designing the optimal tilt angle and the orientation of a solar panel arises for maximizing solar radiation collection at a fixed latitude. An approach of employing sky radiance models for determining optimal tilt angle values of solar collectors with respect to a set of geographic latitudes was conducted by using a data archive of the daily global solar radiation collected by the Italian Institute of ENEA. The results provided a set of tilt angles for winter months that enables a solar panel to absorb the maximum amount of global solar radiation and another set of smaller tilt angle values for summer months, as well. The great difference between these two sets suggested to us to plan semifixed solar panels whose tilt angle can be changed twice a year. The relative gain in solar radiation collectable should not be negligible, as shown in Sec. 3.
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Critical evaluation of wind speed frequency distribution functions J. Renewable Sustainable Energy 2, 013102 (2010); http://dx.doi.org/10.1063/1.3294127 (16 pages) | Cited 3 times Online Publication Date: 13 January 2010
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Over the past few years a number of new mathematical functions have been proposed for wind speed probability density distributions. The most commonly used function that has been cited in literature has been the two-parameter Weibull function. However, in recent years studies have shown that the two-parameter Weibull function might be inadequate in modeling the wind speed probability density distributions or independent of whether the distribution is of unimodal or bimodal nature. For the unimodal distributions, the inadequacy may be due to the intricate behavior of the distribution, which prevents it to be satisfyingly modeled by a two-parameter model. For the bimodal behavior, the two-parameter Weibull function, which produces only a unimodal distribution, is simply inadequate to model it appropriately. Therefore, in recent years, alternative functions have been suggested for both unimodal and bimodal distributions, seeking more involved functions to better model these distributions. This article involves the modeling of observed wind speed probability density distributions using the main body of models found in the literature, namely, Rayleigh, Lognormal, two-parameter Weibull, three-parameter Weibull, and bimodal Weibull probability distribution functions. One of the important steps in the evaluation of different functions is the interpretation of the statistical parameters, namely, slope, R2, mean bias error, and root mean squared error, as are presently used in this article. A novel statistical tool is developed in the present article using these four statistical parameters. The novel tool can be used to evaluate the relative performance of models when more than one model is involved or to determine the overall accuracy of a particular model for a specific site. The calculations are made based on the long term wind speed data collected at 4-s interval at the experimental site at Edinburgh Napier University.
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J. Renewable Sustainable Energy 1, 033110 (2009); http://dx.doi.org/10.1063/1.3152431 (13 pages) | Cited 3 times Online Publication Date: 29 June 2009
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In this paper, a computational fluid dynamics analysis using the FLUENT package 6.2 was carried out to predict the performance characteristics such as power coefficient (Cp), torque coefficient (Ct), and tip speed ratio of a combined three-bucket Savonius and three-bladed Darrieus rotor for various overlap conditions, namely, 16.2%, 20%, 25%, 30%, and 35%. In the upper part of the rotor model, there was a three-bucket Savonius rotor of bucket diameter of 8 cm and height of 10 cm, whereas, in the lower part, there was a three-bladed Darrieus rotor of blade diameter of 8 cm and height of 10 cm. A two-dimensional unstructured computational grid was developed for the combined Savonius–Darrieus rotor model. A k-ε turbulence closure model with enhanced wall treatment function was chosen. A first-order upwind discretization scheme was adopted for pressure-velocity coupling of the flow. The values of Cp and Ct obtained computationally were then compared with those of the values of Cp and Ct obtained experimentally for all the overlap conditions. The experimental values of Cp and Ct for different overlap conditions were obtained from the tests conducted previously in an open-circuit subsonic wind tunnel available in the department. The comparison of experimental and computational studies is quite encouraging.
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Fabrication of organic solar array for applications in microelectromechanical systems J. Renewable Sustainable Energy 1, 013101 (2009); http://dx.doi.org/10.1063/1.2998825 (8 pages) | Cited 3 times Online Publication Date: 6 November 2008
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We have developed an innovative way to fabricate organic solar arrays for application in dc power supplies for electrostatic microelectromechanical systems devices. A solar array with 20 miniature cells interconnected in series was fabricated and characterized. Photolithography was used to isolate the individual cells and output contacts of the array, whereas the thermal-vacuum deposition is employed to make the series connections of the array. With 1 mm2 for single cell and a total device area of 2.2 cm2, the organic solar array based on bulk heterojunction structure of π-conjugated polymers and C60 derivative [6,6]-phenyl C61 butyric acid methyl ester produced an open-circuit voltage of 7.8 V and a short-circuit current of 55 μA under simulated air mass (AM) 1.5 illumination with an intensity of 132 mW/cm2. The procedure described here has the full potential for use in future fabrication of microarray with the size as small as 0.01 mm2.
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Energy use and environmental impacts: A general review J. Renewable Sustainable Energy 1, 053101 (2009); http://dx.doi.org/10.1063/1.3220701 (29 pages) | Cited 3 times Online Publication Date: 18 September 2009
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Globally, buildings are responsible for approximately 40% of the total world annual energy consumption. Most of this energy is for the provision of lighting, heating, cooling, and air conditioning. Increasing awareness of the environmental impact of CO2 and NOx emissions and chlorofluorocarbons triggered a renewed interest in environmentally friendly cooling and heating technologies. Under the 1997 Montreal Protocol, governments agreed to phase out chemicals used as refrigerants that have the potential to destroy stratospheric ozone. It was therefore considered desirable to reduce energy consumption and decrease the rate of depletion of world energy reserves and pollution of the environment. One way of reducing building energy consumption is to design buildings that are more economical in their use of energy for heating, lighting, cooling, ventilation, and hot water supply. Passive measures, particularly natural or hybrid ventilation rather than air conditioning, can dramatically reduce primary energy consumption. However, exploitation of renewable energy in buildings and agricultural greenhouses can also significantly contribute toward reducing dependency on fossil fuels. Therefore, promoting innovative renewable applications and reinforcing the renewable energy market will contribute to preservation of the ecosystem by reducing emissions at local and global levels. This will also contribute to the amelioration of environmental conditions by replacing conventional fuels with renewable energies that produce no air pollution or greenhouse gases. The provision of good indoor environmental quality (IEQ) while achieving energy and cost efficient operation of the heating, ventilating, and air-conditioning plants in buildings represents a multivariant problem. The comfort of building occupants is dependent on many environmental parameters including air speed, temperature, relative humidity, and quality in addition to lighting and noise. The overall objective is to provide a high level of building performance, which can be defined as IEQ, energy efficiency (EE), and cost efficiency (CE). IEQ is the perceived condition of comfort that building occupants experience due to the physical and psychological conditions to which they are exposed by their surroundings. The main physical parameters affecting IEQ are air speed, temperature, relative humidity, and quality. EE is related to the provision of the desired environmental conditions while consuming the minimal quantity of energy. CE is the financial expenditure on energy relative to the level of environmental comfort and productivity that the building occupants attained. The overall CE can be improved by improving the IEQ and the EE of a building. The increased availability of reliable and efficient energy services stimulates new development alternatives. Anticipated patterns of future energy use and consequent environmental impacts (acid precipitation, ozone depletion, and greenhouse effect or global warming) are comprehensively discussed in this paper. Throughout the theme several issues relating to renewable energies, environment, and sustainable development are exam-ined from both current and future perspectives. It is concluded that renewable environmentally friendly energy must be encouraged, promoted, implemented, and demonstrated by full-scale plant especially for use in remote rural areas.
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J. Renewable Sustainable Energy 2, 053104 (2010); http://dx.doi.org/10.1063/1.3501336 (11 pages) | Cited 3 times Online Publication Date: 13 October 2010
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Hydropower is the largest renewable energy resource in the United States and the world. However, hydropower dams have adverse ecological impacts because migrating fish may be injured or killed when they pass through hydroturbines. In the Columbia and Snake River basins, dam operators and engineers are required to make those hydroelectric facilities more fish-friendly through changes in hydroturbine design and operation after fish population declines and the subsequent listing of several species of Pacific salmon under the Endangered Species Act of 1973. Public Utility District No. 2 of Grant County, Washington, requested authorization from the Federal Energy Regulatory Commission to replace the ten turbines at Wanapum Dam with advanced hydropower turbines designed to improve survival for fish passing through the turbines while improving operation efficiency and increasing power generation. As an additional measure to the primary metric of direct injury and mortality rates of juvenile Chinook salmon using balloon tag-recapture methodology, this study used an autonomous sensor device—the Sensor Fish—to provide insight into the specific hydraulic conditions and physical stresses experienced by the fish as well as the specific causes of fish biological response. We found that the new hydroturbine blade shape and the corresponding reduction of turbulence in the advanced hydropower turbine were effective in meeting the objectives of improving fish survival while enhancing operational efficiency of the dam. The frequency of severe events based on Sensor Fish pressure and acceleration measurements showed trends similar to those of fish survival determined by the balloon tag-recapture methodology. In addition, the new turbine provided a better pressure and rate of pressure change environment for fish passage. Overall, the Sensor Fish data indicated that the advanced hydroturbine design improved passage of juvenile salmon at Wanapum Dam.
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Determination and comparison of kinetic parameters of low density biomass fuels J. Renewable Sustainable Energy 1, 023109 (2009); http://dx.doi.org/10.1063/1.3126936 (12 pages) | Cited 3 times Online Publication Date: 30 April 2009
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Biomass has great potential as a clean, renewable feedstock for producing modern energy carriers. Low density biomass, such as rice husk, sawdust, bamboo dust, etc., can provide a continuous supply of liquid and gaseous fuels through thermochemical conversion processes. In the present study, kinetics of the thermal decomposition of three biomasses such as rice husk, sawdust, and bamboo dust were evaluated under air atmosphere from ambient temperature to 1000 °C at a heating rate of 10 °C min−1. Two distinct reaction zones were observed for all the three biomasses. From the thermogravimetric (TG) and differential TG curves, the activation energies, pre-exponential factors, and order of reaction were determined for both the reaction zones. Experimental results were validated numerically and these results are found to be very close to the numerical results. As observed, thermal decomposition rates in the first reaction zone were found to be significantly higher than those in the second reaction zone.
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