Effect of cut-off and compression ratio on the isentropic efficiency during off-design and part-load operations of a Wankel rotary steam expander used for small scale cogeneration

•A new Wankel rotary expander has been developed for use with saturated steam.•Peak isentropic efficiency is 88% and in the range 60 to 88% for off-design.•Analysis reveals clear impact of cut-off on isentropic efficiency.•Any cut-off and pressure ratio combination can provide peak isentropic efficiency.•Dynamic admission-volume control can maintain high efficiency for all mass flow. Expanding steam from boiler pressure to process pressure using an expansion device instead of a pressure reducing valve is recommended for improving energy efficiency and reducing operational costs. However, there are challenges in realizing this potential with existing technologies (such as turbines) due to wetness of process steam and mass flow rate variations caused by load changes. Positive displacement expanders, such as screw and reciprocating engines, can handle wet steam but, they are designed to handle variable mass flow only by throttling leading to exergy destruction and poor economic performance. In this paper we present an improved Wankel steam expander by analysing the impact of mass flow variations and translating it into key design elements: cut-off and compression ratio of the expander. Theoretical analysis in this study resolves conflicting definitions found in literature on compression ratio and cut-off used for designing a positive displacement expander (such as Wankel) and its impact on isentropic efficiency prediction. Experimental analysis of the Wankel expander reveals that a peak isentropic efficiency of over 85% can be achieved for high cut-off. The range of measured isentropic efficiencies were from 60 to 88% for different pressure ratios and at 40% and 62% cut-off. As the pressure ratio increased, the peak efficiency shifted to a lower cut-off condition. When compared to the Wankel, the peak efficiency of a reciprocating expander was only in the range of 50–55% . Thus, the Wankel expander developed in this study presents an opportunity to improve the exergetic efficiency of process steam.