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Τ Τ Decrease in exergy loss Heat source Pure working fluid Zeotropic mixture Pure working fluid Q 100% ΔΤpinch Q 100% Zeotropic mixture 0% (b) Fig. 7. Q-T diagrams of the heat exchange in the a) evaporator and b) condenser for pure working fluids and zeotropic mixtures. The grey area is equal to the decrease in exergy loss achieved due to the phase change taking place under variable temperature This effect is illustrated in Fig. 7. The area enclosed by the Q-T lines of the heat source and the working fluid (evaporator) and cooling medium and working fluid (evaporator) is equal to the ΔΤpinch (α) 0% Decrease in exergy loss irreversibility I 2 2222 (Th Tc)dQThdQTcdQdEh dEc Eh Ec I 1 1111 In the above equation, 1 stands for one side and 2 for the other side of the heat exchanger. It is evident that the exergy losses are minimized when the temperature profiles of the hot and cold stream are perfectly matched. For a fixed pinch point value, this happens when they are almost parallel to each other, minimizing the area between them. The absolute difference |ΔΤh-ΔΤc| can be used as an indicator of the matching between the temperatures of the hot and cold stream during the phase change of either. As |ΔΤh-ΔΤc| approaches zero, the Q-T lines tend to have the same slope and the exergy destruction decreases. On the other hand, the higher the |ΔΤh-ΔΤc| value, the higher the irreversibility of the heat exchange. It has been previously stated [23, 24] that because the temperature glide in the condenser is usually higher than the glide in the evaporator, the overall exergy efficiency can be mostly improved when the exergy destruction losses in the condenser are minimized. An emphasis has been consequently given on the matching of the temperature profiles in the condenser (indicated by |ΔΤcw-ΔΤglide|), as a criterion for optimizing the cycle. The second parameter that determines the exergetic efficiency of zeotropic fluid mixtures is their critical temperature (and thus evaporation temperature range) compared to their parent components, which greatly influences the heat source utilization efficiency as well as the thermal efficiency of the WHR-ORC (Section 3.1.1.) especially in lower heat source temperatures.. Similarly to the case of pure working fluids, for the heat source temperature of 150 oC the impact of the critical temperature is dominant in determining the exergetic efficiency of the cycle. It is therefore no surprise that the mixture with the smallest critical temperature (Butane/Propane) has also the biggest second law efficiency, despite its energetic efficiency being among the lowest ones. It is also notable that higher second law efficiencies are attainable by using mixtures of Butane/Cyclopentane, Butane/Hexane, Butane/Pentane, and Hexane/Pentane than by using their of the heat exchange: 15 (19) Cooling mediumPDF Image | Thermodynamic investigation of waste heat recovery
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