Investigation on improvement potential of ORC system off-design performance by expander speed regulation based on theoretical and experimental exergy-energy analyses

Abstract

In this paper, the potential of expander speed regulation for optimization of ORC system adaptability to off-design working conditions has been explored through experiments. According to the obtained results, it has been found that there existed two different optimal speeds: optimal power speed (rop), optimal efficiency speed (roee/rote) which could endow the ORC system with maximum output power or highest thermoelectric/exergy efficiency. For working conditions of tw = 70, 73, 80 °C, power of expander at rop has been increased by 199.1%, 137.4%, 6.5% compared with the power output at design speed (rds). The thermoelectric efficiency at rote was 11.1%, 27.7% higher than that at rop and rds. To further investigate the influences of expander speed on the performance, energy and exergy analyses were conducted based on theoretical calculation and experiment results within the range of 400–1600 r/min. The results showed that actual exergy destruction of expander and working fluid pump was closely related to expander speed. Gaps between theoretical and actual exergy destruction rates proved that optimization of expander and working fluid pump would be more effective for the ORC system performance.