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Energy and the Environment
Published in Marc J. Assael, Geoffrey C. Maitland, Thomas Maskow, Urs von Stockar, William A. Wakeham, Stefan Will, Commonly Asked Questions in Thermodynamics, 2022
Marc J. Assael, Geoffrey C. Maitland, Thomas Maskow, Urs von Stockar, William A. Wakeham, Stefan Will
So in energy terms, thermal storage can operate very efficiently, subject to minimizing losses and inefficiencies of process equipment such as pumps and heat exchangers, particularly where, as in this example, the charging and discharging phases only involve heat. The “quality” of the heat does reduce steadily through the process, as measured by the exergy, and this needs to be taken into account in the use of the stored heat. In simple terms, the energy efficiency tells us how effectively the available energy has been transferred to its end application, electricity generation, whereas the exergy efficiency tells us whether this is a sensible use of the energy content of that heat stream. For instance, storing solar-derived heat in a thermal storage device where the storage fluid operated between 100°C and 200°C would not deliver heat of high enough quality to superheat steam significantly, resulting in a significant reduction in power cycle efficiency (cf. Question 3.8).
Geothermal Heat Pumps
Published in Vasile Minea, Heating and Cooling with Ground-Source Heat Pumps in Cold and Moderate Climates, 2022
The exergy efficiency is defined as the ratio of exergy output rate (or useful exergy being produced by the system) to exergy input rate: ηex=(Usefulexergyoutput)/(Totalexergyinput)
Energy and Exergy Analysis
Published in Neha Gupta, Gopal Nath Tiwari, Photovoltaic Thermal Passive House System, 2022
Performance of the thermodynamic system can be evaluated using exergy efficiency. As discussed, the thermal efficiency of the system is based on the first law of thermodynamics, which is comprised of energy balance of the system to account energy input, desired energy output, and energy losses. The exergy efficiency of the system is based on the second law of thermodynamics and accounts for total exergy inflow, exergy outflow and exergy destruction for the process. Exergy efficiency is defined as the ratio of energy output to the exergy input.
Experimental study on photovoltaic/thermal system performance based on microencapsulated phase change material slurry
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Liting Tian, Jianzhen Liu, Zhuanzhuan Wu, Jiří Jaromír Klemeš, Jin Wang
The exergy efficiency indicates the ability of the system to capture available energy. Figure 15 shows variations of exergy efficiency at different MPCMS mass concentrations, and the variation trend is consistent with that of the thermal exergy efficiency. When the mass concentration is 0, the thermal exergy efficiency is lower than the electrical exergy efficiency. As the concentration increases, the thermal exergy efficiency is greater than the electrical exergy efficiency due to the addition of MPCM particles, which absorbs more latent heat. The exergy efficiency is improved with the increment of concentration. Under the concentration of 0, the electrical, thermal, and exergy efficiencies are 15.41%, 9.44%, and 24.85% separately. Under the mass concentration of 5%, the electrical, thermal, and exergy efficiencies are 15.62%, 21.35%, and 36.97%.
Comparative analysis of solid and perforated fins for thermal enhancement of a latent heat storage unit positioned at various inclinations
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2021
Lokesh Kalapala, Jaya Krishna Devanuri
Similar to the energy efficiency, exergy efficiency is defined as the ratio of exergy stored to exergy input. As mentioned in Equation (8) exergy stored depends upon ambient temperature and the average temperature of PCM. As ambient temperature is same for all the cases, exergy stored will be a function of average temperature. Exergy input (Equation (9)) depends upon inlet and outlet temperatures of HTF and the ambient temperature. Figure 12b shows the comparison of exergy efficiency among solid fins and perforated fins at various orientations. From the figure, it can be noticed that LHSU with solid fins has relatively higher exergy efficiencies for majority of the charging process at all the orientations. During the early stages of melting, perforated fins showed slightly higher exergy efficiency, but it reduced significantly in the later stages of melting. Exergy stored will be higher if average temperature of the PCM is more. It is observed that in the case of perforated fins the PCM’s average temperature is lesser (when compared with solid finned LHSU) in the latter stages of melting. This could be the reason for the reduction in exergy efficiency when perforations are provided on the fins. As can be observed from Figure 12b, perforated fins exhibited lesser exergy efficiency in comparison with solid fins for most of the time during charging. Hence it can be said that solid fins are exergetically efficient than perforate fins.
Thermoeconomic analysis and optimization of a solar micro CCHP by using TLBO algorithm for domestic application
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Behzad Azizimehr, Ehsanolah Assareh, Rahim Moltames
Exergy is the maximal amount of work that a system can generate. In other words, the energy quality and the performance of a system can be evaluated using exergy efficiency. Exergy efficiency is defined as the ratio of the output exergy to the input exergy of the system. The exergy input is considered as the energy variation in the heat source, and the exergy output is taken as the total output exergy of the generator and the refrigeration cycle (Wang, Dai, and Sun 2009). Equations of the exergy considering the first and second laws of thermodynamics are written as follows: