Experimental and numerical analysis of variable volume ratio as additional optimization parameter in organic Rankine cycle expanders

Abstract

• Variable volume ratio piston expander has been experimentally characterized. • Semi-empirical ORC model quantifies the effects of a variable volume ratio expander. • Good selection of built-in volume ratio can eliminate need for variable valve timing. • Variable valve timing extends the working conditions of a volumetric expander. In this paper, the use of a variable volume ratio expander for a 10 kWe organic Rankine cycle (ORC) to be integrated in a Carnot battery has been studied. The reciprocating piston expander used in this work has a built-in mechanism which allows changing the inlet valve timing of the expander and hence the expander internal volume - and pressure ratio. The experimental results of a 10 kWe ORC prototype are presented. A total of 58 steady state points has been collected by varying the heat source and cold sink temperature, the refrigerant mass flow rate, the expander speed and the expander inlet valve timing. The experimental results for the ORC are presented in terms of electrical power output and cycle efficiency and for the expander the isentropic and volumetric efficiencies are determined for different valve timings. Although the experimental results clearly show the effect of changing the internal volume ratio on the expander performance we here present a combined experimental-numerical approach which is necessary to evaluate the performance under varying external conditions. Therefore, an empirical quasi-steady state model of the ORC has been calibrated based on the experimental results. Subsequently, in a numerical analysis, the use of various valve timings has been evaluated under time-varying boundary conditions. A maximal difference of 14.2 % in the net power production has been observed between the most optimal and least optimal valve timing. However, for the presented numerical study the use of variable valve timing only increased the net power production maximally 1 % compared to the optimal fixed timing. Altogether, this study demonstrates the benefit of variable volume ratio reciprocating piston expanders for ORCs under varying external conditions.