Comparative study on operating strategies of the organic Rankine cycle under transient heat source

Abstract

An effective operating strategy is crucial to the adaptive operation of the ORC unit driven by the transient heat source. The dynamic model of a small-scale ORC prototype is established and validated by the tested data in this study. Three operating strategies are proposed and compared: the constant vapor superheat operation, constant evaporation temperature operation, and optimal evaporation temperature operation. It is found that PI controllers can satisfy the control of the ORC system with evaporation temperature controlled at ±1 °C and vapor superheat at ± 3 °C. The expander output power varies more significantly than the system thermal and exergy efficiencies under heat source fluctuation. The constant evaporation operation is sensitive to the transient heat source and is only recommended for the expander with a strict constraint on vapor temperature and pressure. The optimal evaporation temperature operation is recommended as it has the highest system performance and enables the ORC unit to be adaptive to the transient heat source. The constant vapor superheat mode can achieve close-to-optimal performance by carefully setting the rotating speed of the expander. Thus, it can also be preferred considering its performance, simplicity, and the unique advantage of power supply with a stable frequency.