CO2 heat pump water heating systems have been commercialized and widespread. It has become more and more important to enhance the system performance. In this paper, the performance of the systems is investigated from the exergy viewpoint. Exergy destructions results from the overall temperature drop due to heat loss from the tank surface and the temperature mixture due to heat conduction. A conventional system as well as a system revised by extracting water with middle temperatures from the middle of the storage tank are investigated. First, a numerical simulation is conducted to determine the hourly changes in the flow rates and states of the systems. Then, the exergy analysis is conducted based on the results obtained by the numerical simulation. The exergy destructions and efficiencies of the components are clarified quantitatively for both the systems.
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