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[en] Hydrogen is considered to become a main energy vector in sustainable energy systems to store large amounts of intermittent wind and solar power. In this work, exergy efficiency and cost analyses are conducted to compare pathways of hydrogen generation (PEM, alkaline or solid oxide electrolysis), storage (compression, liquefaction or methanation), transportation (trailer or pipeline) and utilization (PEMFC, SOFC or combined cycle gas turbine). All processes are simulated with respect to their full and part-load efficiencies and resulting costs. Furthermore, load profiles are estimated to simulate a whole year of operation at varying loads. The results show power-to-power exergy efficiencies varying between about 17.5 and 43 %. The main losses occur at utilization and generation. Methanation features both lower efficiency and higher costs than compressed hydrogen pathways. While gas turbines show very high efficiency at full load, their efficiency drops significantly during load-following operation , while fuel cells (especially solid oxide) can maintain their efficiency and exceed the combined cycle gas turbine full-load efficiency. Overall specific costs between 245 €/MWh and 646 €/MWh are resulting from the simulation. Lower costs are commonly reached in chains with higher overall efficiencies. Installation costs are identified as predominant because of the low amount of full-load hours. To decrease the energy storage overall costs of the process chains, the options to use revenue generated by by-products such as oxygen and heat as well as changing the system application scenario are investigated. While the effect of the oxygen sale is negligible, the revenue generated by heat can significantly decrease overall costs. An increase of full-load by accounting for an electrolysis base-load to provide hydrogen for vehicles also shows a significant decreases in costs per stored energy down to 151 €/MWh at 2337 h/a full-load hours. The optimization of the exergy efficiency is performed by analysing physical and heat exergy recovery options such as expansion machines in the gas grid, the use of additional thermodynamic cycles (both Joule and Clausius-Rankine), as well as providing heat for steam electrolysis from compression inter-cooling, methanation or stored heat from a solid oxide fuel cell. The analysis shows that at full-load, process chains using solid oxide electrolysis, compressed hydrogen and a combined cycle gas turbine or a solid oxide fuel cells with a heat exergy recovery cycle can reach exergy efficiencies of 47 % and 45.5 %, respectively. A reversible solid oxide cell systems with metal-hydride heat and hydrogen storage can also reach 46.5 % exergy efficiency. The energy storage costs for these processes can be as low as 35 to 40 €/MWh at full-load. At load-following operation the efficiency of the fuel cell systems is expected to increase.
[en] In this paper, a thermodynamic analysis of a Sulfur-Iodine thermochemical hydrogen production cycle was performed. At first, a new heat exchanger network configuration was designed by means a heuristic method. Then, an exergy and anergy analysis was carried out in order to analyze the thermal efficiency of the proposed heat exchanger network compared to the reference case. With this study, a reduction of the energy inputs of the process was achieved; being 58.59% for cooling and 52.31% for heating, both lower than the reference case. Regarding the exergy y and anergy calculations for the new heat exchanger configuration, the calculated exergy was 365.202 MJ/K mol-H2 with an anergy of 187.66 MJ/ K mol-H2, being the latest lower than that for the reference case 338.97 MJ/K mol-H2. This means that less energy it is being wasted improving the thermal efficiency of the cycle and reducing the plant operational cost. (author)
[en] The residential building sector is one of the major areas to reduce energy demand. In this study, the single-sided top vent-based natural ventilation parameters are optimized using Taguchi technique. The computational fluid dynamics tool is used to simulate the room model. The grid sensitivity analysis is executed to predict more precise results. The operating parameters for top vent and window-based ventilation investigated are top vent width, top vent height, the location of the top vent, the number of top vents and window type with different levels. The orthogonal array, L8, is selected for the conduct of experiments. The signal-to-noise ratio and analysis of variance technique are used to optimize the results. The optimized results showed that the maximum top vent width, top vent height and more number of vents with higher window opening area reduce the maximum indoor air temperature and increase the mass flow rate of a single-sided residential room. Of all, around 60.64% numbers of top vents contributed to indoor temperature reduction. In addition, thermal exergy is also investigated on various vent sectional planes.
[en] The main focus of this paper is to analyze the exergetic characteristics of a frost-free air source heat pump (ASHP) integrated with solid dehumidification and phase change heat storage. The exergy loss, exergy loss ratio and exergy efficiency of the system have been calculated to investigate the influence of the operation modes on the system characteristics. In addition, a comprehensive exergy loss ratio is defined to evaluate the system performance. It is concluded that the comprehensive highest exergy loss ratio is obtained as 51.4 % for compressor, followed by the energy storage device (ESD) with 15.8 %, the extra heat exchanger coated with a solid desiccant (EHECSD) with 10.7 %. Also the variation of the exergy loss, exergy loss ratio and exergy efficiency of the system and the key components with the ambient temperature changes from -20 to 5 °C are discussed. This study can provide guidance for optimizing the structure and improving the frost-free ASHP system in the engineering applications.
[en] This present work contributes to the improvement in thermal energy storage capacity of an all-glass evacuated tube solar water heater by integrating it with a phase change material (PCM) and with a nanocomposite phase change material (NCPCM). Paraffin wax as PCM and a nanocomposite of paraffin wax with 1.0 mass% SiO2 nanoparticles as NCPCM had been used during the experiments. The results were acquired through the real-time experimental measurements on the all-glass evacuated tube solar water heater integrated with built-in thermal energy storage, functioning under thermosyphonic flow. Three different cases, namely, without PCM, with PCM, and with NCPCM, were considered. The testing procedure involved the observation of total temperature variation in the tank water from 6.00 a.m. to 6.00 a.m. of next morning. Meanwhile, the water was completely renewed for every 12 h. The system performance was studied using energy efficiency, exergy efficiency, and temperature of hot water supply during the next morning, for all the three cases. The investigation exemplifies that the tank water temperature at 6.00 a.m. after one complete day of operation was notably improved to 37 °C and 39.6 °C, respectively, with PCM and NCPCM, whereas it was 33.1 °C for the case without PCM. The energy efficiencies for the three cases were found to be 58.74%, 69.62%, and 74.79%, respectively, and exergy efficiencies of the system were determined as 19.6%, 22.0%, and 24.6%, respectively, for without PCM, with PCM, and with NCPCM. Also, it was evidenced that the thermal conductivity of paraffin wax was considerably increased to 22.78% through the diffusion of SiO2 nanoparticles. Put together, this indicates that the incorporation of PCM and explicitly the dispersion of SiO2 nanoparticles in NCPCM had been significantly improved the thermal performance of the system.
[en] In this paper, the thermodynamic (energy and exergy) analysis and water analysis of a modified solar still augmented with copper tube heat exchanger in coarse aggregate have been carried out and compared with conventional still performance under the same climatic conditions. Basin water temperature, solar intensity, wind velocity, cumulative yield, water conductivity, total hardness, pH value and fluoride concentration are obtained from experimental results for saline, basin and distilled water. Energy efficiency, evaporation and convective heat transfer coefficient, exergy evaporation rate and exergy efficiency are determined from energy and exergy analysis. The results show that the modified still has an efficiency of 28% and 17% greater than the conventional still. The productivity of modified and conventional still is 6.23 kg m−2 and 2.41 kg m−2, respectively. The exergy efficiency depends on the time of the test day and reaches a maximum value of 5.5% and 1.1%, respectively, for the modified and conventional still. From the water analysis, it is observed that the maximum distilled water pH, water conductivity, hardness and fluoride content are 7.5, 0.8 × 10−4 S m−1 (0.8 µS cm−1), 0.5 × 10−3 kg m−1 (0.5 mg L−1) and 0.7 × 10−3 kg m−3 (0.7 mg L−1), respectively, with the still salinity removal efficiency of 99%. The results indicate that the modified still has higher energy and exergy efficiencies and better water quality with cumulative yield.
[en] This paper reports the energy and exergy performance of a photovoltaic/thermal solar-assisted heat pump system (PV/T-SAHPS) with different solar radiation levels. From the heat pump, the solar evaporator/collector extracts the thermal energy required, while the cooling effect of the refrigerant reduces the working temperature of the PV cells. Therefore, this integrated PV/T-SAHPS exhibits a relatively high thermal performance with improved coefficient of performance (COP) and PV efficiency. The Engineering Equation Solver tool embedded with the Hottel–Whillier equation was used to stimulate the system to quantify the enhancement of heat transfer due to the use of a refrigerant (R134a). Simulation results indicated that the maximum efficiency of the hourly electricity generation of the PV system could reach 11.56% at 1000 W/m2, the average electrical efficiency was 11.88%, and the maximum efficiency of the hourly thermal generation was 88.68% when the solar irradiance variation ranges from 300 to 1000 W/m2. The maximum PV panel temperature was 35.68 °C at 14:00. The average values of COP and COPex were 6.14 and 1.49, respectively. These result indicated that the heat pump system performance is relatively high.
[en] Domestic refrigerator is one of the major energy-consuming appliances, and the enhancement of its energy efficiency plays a vital role in implementing the energy conservation policies and green building concepts in residential sector. The major strategies used to improve the performance of domestic refrigerators are the replacement of existing components, use of alternative refrigerants and the reduction in condensing pressure. Therefore, in this work, the conventional air-cooled condenser has been replaced with water-cooled condensers such as shell-and-coil and brazed-plate heat exchangers to maintain a low compression ratio and condensing pressure. The performance of a domestic refrigeration system retrofitted with water-cooled condensers has been studied using experimental methods. The result showed that the system with water-cooled condensers reduces the pull-down time and the per day energy consumption by 70% and 3.5%, respectively. Moreover, the proposed system can improve the COP and exergy efficiency by 6.4% and 4.9%, respectively. Compared to shell-and-coil heat exchanger, the system with brazed-plate heat exchanger can reduce irreversibility and TEWI by 3.9% and 3.7%, respectively. In this study, the system with brazed-plate heat exchanger showed better performance than the shell-and-coil heat exchanger for all operating conditions.
[en] In this work, the thermal performance of single-pass double-duct jet plate solar air heater with various designs of artificial roughness is analyzed by developing a mathematical model. The influence of artificial roughness designs on the performance of the solar air heater is analyzed based on energy and exergy basis, and the results are compared with single-pass single-duct jet plate solar air heater. Further, year-around enviro-economic analysis is carried out for artificial roughness design providing the better performance. The results show that single-pass double-duct jet plate solar air heater with arc-shaped rib roughness produces highest energetic and exergetic efficiency. The results are compared with single-pass single-duct jet plate solar air heater in the form of energetic and exergetic efficiency enhancement ratios, annual overall energy and exergy gain and amount of carbon credit earned. The mathematical model has been validated with analytical and experimental results available in the literature with acceptable deviation. On the basis of numerical calculations, it has been concluded that single-pass double-duct jet plate solar air heater improves annual usual energy and exergy gain by 111.7% and 185.6%, respectively. Further, it reduces 2.11 times of production of CO2 mitigation per year and enhances the revenue of annual carbon credit by 2.85 times.
[en] This paper had carried out the optimization screening experiment for the nucleating agent and thickening agent of sodium acetate trihydrate (SAT) targeting its problems of supercooling and phase separation. Our results verified that the addition of 2 mass% disodium hydrogen phosphate dodecahydrate in SAT could effectively suppress the SAT supercooling and control the degree of supercooling within 2 K. Besides, the addition of 1–1.5 mass% xanthan gum in SAT could effectively suppress its phase separation during melting/freezing cycle and could evenly disperse the nucleating agent in SAT. Moreover, comparative test analysis on the modified SAT suggested that the thickening agent in the additive had great influence on the SAT physical property parameters. Additionally, to improve the comprehensive utilization quality of the modified SAT, the modified SAT thermodynamic analysis was performed, which discovered that the initial charging temperature had little influence on the exergy efficiency of the modified SAT, while the final charging temperature had great influence on the exergy efficiency. Typically, the optimal final charging temperature of the modified SAT was 72 °C.