A 2-kW small-scale organic Rankine cycle power plant system targeted for medium-low temperature heat sources was established in this study. The experimental research was aimed at investigating how expansion parameters affect the system performance. The performance indicators include isentropic efficiency

filling factor

cycle thermal efficiency

and output electricity. The results indicate that the expander pressure ratio is an important parameter that influences the cycle performance. The output electricity increases with the increase of pressure ratio

while there exists an optimum point for both isentropic efficiency and cycle thermal efficiency. Considering the effect of internal leakage and mechanical friction loss

the optimum operating pressure ratio should be slightly higher than the built-in volume ratio of the expander. It turns out that increasing the expander rotational speed can effectively decrease the internal leakage.