Experimental and numerical analysis of a reciprocating piston expander with variable valve timing for small-scale organic Rankine cycle power systems

This paper presents a reciprocating expander concept for organic Rankine cycle applications using a novel rotating variable timing admission valve system, enabling the adjustment of the expansion ratio in real time while the expander is running. An organic Rankine cycle experimental test rig with n-pentane as the working fluid and a single-cylinder reciprocating piston expander was developed. Experiments were conducted for evaporation temperatures ranging from 125 °C to 150 °C and condensation temperatures ranging from 20 °C to 40 °C. The performance of the reciprocating piston expander was investigated in terms of the torque of the expander, pressure inside the cylinder, isentropic efficiency of the expander, and net power produced by the expander. Based on the experimental data, a dynamic model of the system was formulated in the object-oriented language, Modelica. The model was validated using the experimental results and then used to predict the performance of the expander. Special attention was paid to the robust modelling of the valve actuation to avoid computational inefficiencies caused by singularities of state variables or their derivatives. The results indicate that the expander produces up to 2.5 kW of electricity from a low-temperature heat source while operating at pressure ratios ranging from 10 to 16.5 with an isentropic efficiency of approximately 70%. The relative differences between the model and the measurements of the isentropic efficiency and power output of the expander per revolution were ±10% and ±30%, respectively.