Optimization of heat exchanger size of a 10 kW organic Rankine cycle system

Most of the heat exchangers design in a conventional Rankine cycle power generation system is based on the assumption of a fixed temperature difference between the heat source and the inlet temperature of the expander. Since the temperature difference between the evaporator and the condenser is high, the effect of this estimated temperature difference on the system performance is not important. However, for an ORC system, since the temperature of heat source is low, a minor difference on the temperature estimation will affect the system thermal efficiency significantly. This study provides an experimental measurement of a practical 10 kW organic Rankine cycle system using HFC-245fa as working fluid subject to the influence of heat exchangers with different NTU. The effect of NTU of evaporator on heat transfer rate, network output, system thermal efficiency and investment payback years were studied. The results show that with increasing NTU of evaporator leads to the increase of total heat transfer rate, work output and system thermal efficiency. However, since the system installation cost increases with increasing heat exchangers NTU, in combining with the increase of power output, an optimal heat exchanger NTU for the shortest investment payback years is found.