Study on the power optimization of transcritical and subcritical rankine power cycles for a low-grade heat source

Abstract

In this dissertation, in order to compare the power output of the transcritical cycle and subcritical Rankine cycle for a low-grade heat source of about $100^\circ C$, both cycles were optimized for the power output. In contrast to conventional approaches, each working fluid`s heat transfer and pressure drop characteristics within the heat exchangers were taken into account. To fairly compare the power output of the cycles by using different working fluids, the inlet temperatures and the flow rates of both the heat source and the heat sink were fixed. The optimization results show that the R125 transcritical cycle has a potential of power enhancement compared to subcritical cycles. When an enough heat exchange surface is available, the R125 transcritical cycle yields 13% more power than the best subcritical cycle. In addition to this, the R125-based binary HFC mixtures were found to increase the power of the transcritical cycle. However, the impact was small in the present study. The results also show that the condition of a large value of the thermal capacitance-rate ratio is proper to the R125 transcritical cycle rather than subcritical cycles. In summary, the R125 transcritical cycle is recommended for the utilization of low-grade heat source of about $100^\circ C$