Technical feasibility of a gravity-type pumpless ORC system with one evaporator and two condensers

Based on the problems encountered in the earlier-established pumpless ORC (Organic Rankine Cycle) system, i.e. that the shaft power of the system output is unstable and intermittent, a novel gravity-type pumpless ORC system is proposed in this paper, which consists of an evaporator, two condensers, a scroll expander and some valves. The theoretical model of the system is established and the model is also validated by the earlier-established pumpless ORC system. Simulation results show that the system can output the shaft power continuously and the instantaneous shaft power of the scroll expander fluctuates in a relatively small range. The instantaneous shaft power at the hot water temperature of 90 °C ranges from 201.6 W to 349.9 W. The energy efficiencies at the hot water temperatures of 80–95 °C are 2.4–3.1%, and the exergy efficiencies are almost constant and about 18%. The novel gravity-type pumpless ORC is very appropriate for the small-scale applications, which is due to the characteristics of the low cost and relatively high net power efficiency.