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[en] Highlights: → Chemical energy values are 2.16-5.20 times as the physical energy values. → Chemical exergy values are 4.50-13.45 times as the physical exergy values. → Efficiencies mainly increase first and then decline when ER/temperature increases. → Higher carbon and hydrogen content generates higher energy and exergy values. → Higher ash content results in lower energy and exergy values/efficiencies. - Abstract: Biomass gasification with air in autothermal gasifiers is studied and compared with another fuel from thermodynamic aspect. The results indicate that the chemical energy values of product gases from biomass are 2.16-5.20 times as the corresponding physical energy values, while the chemical exergy values are 4.50-13.45 times as the corresponding physical exergy values. The energy and exergy efficiencies of biomass gasification are respectively in ranges of 52.38-77.41% and 36.5-50.19%, and mainly increase first and then decline when ER or gasification temperature increases. Higher carbon and hydrogen content in the ultimate analysis generates higher gaseous energy and exergy values, while results in lower energy and exergy efficiencies. Higher ash content makes biomass produce lower energy and exergy values/efficiencies.
[en] In this work, the thermodynamic performance of a single slope solar still with cotton cloth energy storage medium was compared with a simple solar still without energy storage. Two solar stills with similar dimensions (one with cotton cloth energy storage and another without energy storage) were fabricated and investigated its performance under the hot humid climatic conditions of Chennai in India during the summer months of 2017. The performance was evaluated in terms of energy and exergy analysis based on first and second law of thermodynamics, respectively, for 2 mm, 4 mm, 6 mm and 8 mm cotton cloth thickness. The results showed that the maximum energy and exergy efficiency of a solar still was observed to be 23.8% and 2.6%, respectively, for 6 mm cotton cloth thickness. The results confirmed that the cotton cloth regenerative medium has enhanced the still productivity by about 24.1% when compared to the solar still without heat storage.
[en] Thermal energy recovery of pyrometallurgy slags is a worldwide problem that is widely concerned for decades. As chemical recovery method, molten slag cascade recovery method (MS-WHCR) is proposed in this work. As typical endothermic chemical reactions, pyrolysis, gasification, calcination and reforming reactions are applied in this method. Gasification–pyrolysis system, calcination–pyrolysis system, enhanced pyrolysis system (R-SEP) and fixed carbon gasification and sorption-enhanced pyrolysis system (CG–SEP) systems of MS-WHCR method are designed. Based on the first law of thermodynamics and second law of thermodynamics, enthalpy–exergy compass analysis method is applied to analyze the exergy efficiency, consumption of reactants and products of designed MS-WHCR method, compared with traditional water quenched (WQ) method and gravity bed waste heat recovery (GWHR) method. As calculation example, 1000kg copper slag is used in this paper. The results showed that the exergy efficiency and exergy loss of WQ method are 20.7% and −947 MJ respectively. By WQ method, energy quality of molten copper slag is discounted. Copper slag particles should be fast cooled during granulation process. Thus, lots of air is blown in to make enough heat transfer with copper slag particles, which generate some exergy loss. And exergy efficiency of GWHR method is 76.9%. Using chemical endothermic reactions, MS-WHCR method improves the exergy efficiency of molten slag waste heat recovery. There is a slight fluctuation of exergy efficiency by MS-WHCR method for four kinds of systems from 66.6 to 70.1%. Fixed carbon and combustible syngas are acquired by MS-WHCR. And enhanced pyrolysis process in proposed R-SEP and CG–SEP systems improves hydrogen contents in syngas.
[en] The manner is investigated in which exergy-related parameters can be used to minimize the cost of a copper-chlorine (Cu-Cl) thermochemical cycle for hydrogen production. The iterative optimization technique presented requires a minimum of available data and provides effective assistance in optimizing thermal systems, particularly in dealing with complex systems and/or cases where conventional optimization techniques cannot be applied. The principles of thermoeconomics, as embodied in the specific exergy cost (SPECO) method, are used here to determine changes in the design parameters of the cycle that improve the cost effectiveness of the overall system. The methodology provides a reasonable approach for improving the cost effectiveness of the Cu-Cl cycle, despite the fact that it is still in development. It is found that the cost rate of exergy destruction varies between $1 and $15 per kilogram of hydrogen and the exergoeconomic factor between 0.5 and 0.02 as the cost of hydrogen rises from $20 to $140 per GJ of hydrogen energy. The hydrogen cost is inversely related to the exergoeconomic factor, plant capacity and exergy efficiency. The results are expected to assist ongoing efforts to increase the economic viability and to reduce product costs of potential commercial versions of this process. The impact of the results are anticipated to be significant since thermochemical water splitting with a copper-chlorine cycle is a promising process that could be linked with nuclear reactors to produce hydrogen with no greenhouse gases emissions, and thereby help mitigate numerous energy and environment concerns.
[en] In this study, thermo-economic optimization of single-pass SAHs (solar air heaters) with obstacles of arcuate shape has been carried out. The research is conducted in order to compare the exergy efficiency of three different types of flat plate SAHs. Also, using NSGA-II (non-dominated sorting genetic algorithm) influencing factors were optimized. The SAHs were two different arcuate obstacles (type II and type III), and the other one had no obstacles (type I). All heater types with single and double glass cover were evaluated. The results showed that the heater with double glass cover and the obstacles (type III) had the highest function in both economic and exergetic aspects. Based on the NSGA-II results, collector with flow rate, area and the outlet temperature of 0.017 kg s−1, 2.6 m2 and 78 °C, respectively, had the best performance.
[en] Magnetic refrigeration has the potential to replace conventional refrigeration-with often problematic refrigerants-in several niche markets or even some main markets of the refrigeration domain. Based on this insight, for the Swiss Federal Office of Energy a list of almost all existing refrigeration technologies was worked out. Then an evaluation how good magnetic refrigeration applies to each of these technologies was performed. For this purpose a calculation tool to determine the coefficient of performance (COP) and the exergy efficiency as a function of the magnetic field strength and the rotation frequency of a rotary-type magnetic refrigerator was developed. The evaluation clearly shows that some application domains are more ideal for a replacement of conventional refrigerators by their magnetic counterparts than others. In the pre-study, four good examples were chosen for a more comprehensive investigation and working out of more detailed results. In this article, the calculation method is briefly described. COP values and exergy efficiencies of one very suitable technology, namely the magnetic household refrigerator, are presented for different operation conditions. Summarizing, it is stated that magnetic refrigeration is a serious environmentally benign alternative to some conventional cooling, refrigeration and air-conditioning technologies
[en] In this paper we thermodynamically assess the performance of an ammonia-water Rankine cycle that uses no boiler, but rather the saturated liquid is flashed by a positive displacement expander (e.g., reciprocating, centrifugal, rotating vane, screw or scroll type expander) for power generation. This cycle has no pinch point and thus the exergy of the heat source can be better used by matching the temperature profiles of the hot and the working fluids in the benefit of performance improvement. The second feature comes from the use of the ammonia-water mixture that offers further opportunity to better match the temperature profiles at the sink level. The influence of the expander efficiency, ammonia concentration and the coolant flow rate is investigated and reported for a case study. The optimized cycle is then compared to four organic Rankine cycles and a Kalina-type cycle and shows the best performance. It is also shown that, in order to determine the best cycle configuration and parameters, energy efficiency must be used only in conjunction with the amount of the heat recovered from the source. The efficiency of the cycle running with ammonia-water is 0.30 in contrast to steam-only case showing 0.23 exergy efficiency, which means an increment of 7.0% is obtained for the same operating conditions. If cogeneration is used the cycle effectiveness may even be over 70%. The cycle can be applied for low power/low temperature heat recovery from geothermal sources, ocean thermal energy conversion, solar energy or process waste heat, etc
[en] Highlights: • An irreversible Brayton cycle operating quantum gasses is considered. • Exergetic sustainability index is derived for nano-scale cycles. • Nano-scale effects are considered. • Calculation are conducted for irreversible cycles. • Numerical results are presented and discussed. - Abstract: In this study, a nano-scale irreversible Brayton cycle operating with quantum gasses including Bose and Fermi gasses is researched. Developments in the nano-technology cause searching the nano-scale machines including thermal systems to be unavoidable. Thermodynamic analysis of a nano-scale irreversible Brayton cycle operating with Bose and Fermi gasses was performed (especially using exergetic sustainability index). In addition, thermodynamic analysis involving classical evaluation parameters such as work output, exergy output, entropy generation, energy and exergy efficiencies were conducted. Results are submitted numerically and finally some useful recommendations were conducted. Some important results are: entropy generation and exergetic sustainability index are affected mostly for Bose gas and power output and exergy output are affected mostly for the Fermi gas by x. At the high temperature conditions, work output and entropy generation have high values comparing with other degeneracy conditions
[en] This paper experimentally investigated exergetic performance analysis of vapor compression refrigeration system using R450a as a replacement for R134a at different evaporator and condenser temperatures within controlled environmental conditions. The exergetic performance analysis of the vapor compression refrigeration system with test parameters including efficiency defects in the components, total irreversibility, and exergy efficiency of the refrigeration system was performed. Findings showed that the total irreversibility and exergy efficiency of the vapor compression refrigeration system using R450A refrigerant were lower and higher than R134a by about 15.25–27.32% and 10.07–130.93%, respectively. However, the efficiency defect in the condenser, compressor, and evaporator of the R450A refrigeration system was lower than R134a by about 16.99–26.08%, 5.03–20.11%, and 1.85–15.85%, respectively. Conversely, efficiency defect in the capillary tube of the R450A refrigeration system was higher than R134a by about 14.66–78.97% under similar operating conditions. Overall, it was found that the most efficient component was the evaporator, and the least efficient component was the compressor for both refrigerants.
[en] Turbulent flow of nanofluid inside a solar collector has been presented in the current research. Turbulence mixing has been increased with insertion of new turbolentor. Heat transfer and exergy behavior within the test tube equipped with multi-channel twisted tape elements. Al2O3–water nanofluid flows inside the pipe. Absorber plates are made of aluminum. The scrutinized parameters in current paper are number of revolution (N), diameter ratio (D*), Reynolds number (Re), number of channel (n). Better convection heat transfer can be observed for greater values of diameter ratio. Exergy drop declines with increasing scrutinized parameters due to giving strongly swirling flows with rise of them.