Numerical study on a multi-unit heat-driven double-acting free-piston Stirling heat pump system

•A multi-unit Stirling heat pump system with one moving part per unit is proposed.•Larger loss exists in the gap between piston and cylinder with less unit number.•The overall coefficient of performance reaches 1.92 with heat block at 650 °C. Heat pump technology has gained widespread attention in recent years. Stirling heat pumps show great promise due to their high efficiency, environmental benignity and ability to operate across a wide range of temperatures. In conventional heat-driven Stirling heat pumps, to obtain an ideal acoustic field, which is critical in determining Stirling machines’ performance, three moving parts’ parameters should be adjusted simultaneously, making the design very complicated. This paper presents a novel heat-driven double-acting free-piston Stirling heat pump system with multiple units. In this configuration, the adjustment of the acoustic field is very simple, which is achieved by changing the number of units. The Stirling engine and the Stirling heat pump are coupled by a small-diameter piston rod with minimal damping losses, greatly improving the efficiency of acoustic power delivery and simplifying the design. In addition, there is only one moving part per unit, significantly increasing system reliability. A detailed analysis is carried out to explore the system’s performance. The optimal acoustic field varies for different operating temperatures. The number of units corresponding to the highest engine efficiency and the optimal heat pump subsystem performance differs. With a unit number of 5, the engine efficiency reaches a maximum value of 47.3 %. The heat pump subsystem’s coefficient of performance remains at around 3.4 with a unit number of more than 6. When there are six units in the system, the highest overall coefficient of performance of 1.92 is achieved with a heat block temperature of 650 °C, an ambient temperature of 0 °C and a heat-pumping temperature of 60 °C.