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Session: System design and optimization IV
Session starts: Tuesday 08 October, 11:20
Presentation starts: 11:40
Room: Ruys & Rijckenvorsel Zaal

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Hsiao-Wei D. Chiang (Department of Power Mechanical Engineering, National Tsing Hua University)
Sung Wei Hsu (Green Energy Research Laboratories, Industrial Technology Research Institute)
Chih-Yung Huang (Department of Power Mechanical Engineering, National Tsing Hua University)

Abstract:
Among low-medium heat energy converting to power technologies, transcritical ORC systems have demonstrated to have better thermal efficiency and higher heat recovery rate than subcritical ORC’s. This paper starts with working fluids of R125, R218, R32, R134a, R227ea, R152a, R236fa, R236ea, isobutane, and R245fa, for heating fluid temperature levels of 125C, 150C, 175C, 200C, and 225C. Using the maximum power output of a subcritical cycle as baseline, the effects of different expander inlet temperatures on the transcritical system thermal efficiency, heat recovery rate, and net power output were investigated. Using our performance analysis, transcritical cycle performance behaviors can be studied. As demonstrated, for those working fluids with low critical temperatures, the heat recovery rate is relatively high, and the variation of expander inlet temperature has little effects on the heat recovery. However, for working fluids with high critical temperatures, the heat recovery rate is relatively low, and decreases sharply with elevating expander inlet temperature. It was demonstrated that the temperature difference between the inlet temperature of the heat source stream and the temperature of the working fluid at expander inlet needs to be designed in the range of 30~35C for maximum power output. Consequently, the high critical temperature working fluid would generate higher power output for a heat source with higher temperature level. The results demonstrate that the candidate working fluids for a transcritical cycle can be R227ea, R134a, R236fa, and R245fa for inlet temperatures of the heat source stream of 125C, 150C, 175C, and 200C, respectively. For the 150C inlet temperature of the heat source stream, sensitivity analyses on the isentropic efficiencies of the pump and expander were performed for subcritical and transcritical cycles. The results demonstrate that the pump efficiency has larger effects on the transcritical cycle than the subcritical one, since the transcritical pumping power accounts for a major portion of the power consumption than that of a subcritical cycle. For the increasing expander efficiency effects, both the subcritical and transcritical power outputs are increased similarly. As a result, when the pump and expander isentropic efficiencies are over 80%, the transcritical ORC cycles are demonstrating 20% and more power output than the subcritical cycles.