Experimental studies of the expansion cycle in a double-acting piston expander using intermittent supply of humid air for micro-CAES applications

Although compressed air expanders have been investigated, a clear research gap remains regarding experimentally validated control strategies that ensure both efficient and stable operation of piston expanders, contributing to their low round-trip efficiency. This study presents an experimental investigation of a piston expander for a micro-CAES, demonstrating PWM-based control to actively shape the thermodynamic cycle, including piston velocity and residual pressure. During the experimental investigations, the residual chamber pressure was reduced to 0.69 bar at a supply pressure of 2.1 bar using a PWM duty cycle of 0.5. Moreover, near-isothermal expansion conditions were observed for supply pressures up to 3.8 bar, with maximum temperature variations limited to 7.6 °C. Computational analyses showed that nearly complete air expansion yields the highest cycle efficiency of 0.65 achieved at a PWM value of 0.2 and a mechanical power output of 1.07 kW for a supply pressure of 4 bar. In contrast, the highest mechanical power output of 2.72 kW was obtained at PWM = 0.7 with a significantly lower efficiency of 0.38. The results demonstrate that PWM control allows the operating regime to be adjusted toward either higher efficiency or higher mechanical power output. To maintain a power output of 2.72 kW at a low PWM value of 0.2 while preserving high efficiency, the actuator can be appropriately oversized by increasing its diameter by a factor of 1.37. These findings confirm the potential of PWM-based control to enhance the efficiency and controllability of piston expanders for micro-CAES systems.