Experimental study of an organic Rankine cycle with a variable-rotational-speed scroll expander at various heat source temperatures

In this study, an organic Rankine cycle with R134a as the working fluid was developed, and a variable-rotational-speed dual expander was used. To improve the cycle performance at various heat source temperatures, the expander was operated in single-expander (SE) and dual-expander (DE) modes at different rotational speeds. Although the electrical output became unstable when the superheat degree was insufficient, it improved when the heat source temperature increased and the expander rotational speed reduced. At a low heat transfer rate, reducing the expander rotational speed effectively decreased the refrigerant mass flow rate and increased the evaporation pressure. In DE mode, the isentropic efficiency ranged from 55.8% to 65.8%. A performance map was created to divide the operating region into three subregions depending on heat transfer rate: 10–17 kW in SE mode at 900 rpm, 18–26 kW in SE mode at 1350 rpm, and ≥27 kW in DE mode at 900 rpm. During a dynamic test conducted at a low heat transfer rate, when the operating mode was switched to SE mode at 900 rpm, the cycle performance was 66% and 108% higher than that in SE mode at 1800 rpm and that in DE mode at 900 rpm, respectively.