Regulation strategies and optimizations of the expander and pump in organic Rankine cycle under off-design conditions

Abstract

For the present investigations of the off-design performance of organic Rankine cycle systems, which are mainly based on indirect parameters such as temperature and pressure, it is difficult to provide direct theoretical guidance for the actual operation of the system. Therefore, this paper introduces a novel, directly regulated coupling model based on a quasi-two-stage single screw expander and a multistage centrifugal pump. In addition, to enhance the matching between the expander and the pump during the regulation, the coupled model is optimized using the particle swarm optimization algorithm. The results reveal that the net efficiency initially increases and then decreases when exploring regulation strategies for the expander and the pump, suggesting the existence of optimal operating points. Specifically, after applying the particle swarm algorithm, the net efficiency reached 13.23 % at an expander speed of 5076 RPM and a pump frequency of 46.8 Hz, reflecting a better match between the pump and expander regulation with the heat source conditions. For instance, at a heat source temperature of 200 °C, the net efficiency increased by up to 1.45 % compared to the original results.