Influence of a non-condensable gas on the performance of a piston expander for use in carbon dioxide trans-critical heat pumps

Non-condensable gas is proved to accelerate the phase change process and improve the expander performance by experimental researches on the carbon dioxide trans-critical heat pump. Using expander instead of throttle valve in carbon dioxide trans-critical heat pump can recover energy and improve system efficiency. Accelerating phase change process is a main way to improve expander efficiency. The theoretical analysis indicates that a non-condensable gas can reduce the required molecule number to form critical cluster, and convert homogeneous nucleation process into heterogenic nucleation process, thus increase the number of gas nucleus. In order to investigate the influence of a non-condensable gas to expander performance, experiments on expander efficiency adding different mass ratios of nitrogen gas into system are carried out. The results indicate that when the inlet temperature of expander is 35 °C and the inlet pressure keeps at 8 MPa, the efficiency of expander used in the system with non-condensable gas is 25% higher on average. The results also show that the more the addition quantity of non-condensable gas is used in the system, the more the expander efficiency increases. Moreover, the non-condensable gas can improve the expander efficiency more when the inlet temperature of expander is higher.