Experimental characterization of a free-piston Stirling engine intended for integration with high-temperature thermal energy storage and combined heat and power applications

A high-temperature thermal energy storage (TES) system coupled with a free-piston Stirling engine offers a potential route for converting stored thermal energy into electricity while simultaneously recovering useful heat for domestic applications. This study experimentally characterises a commercially available 1 kW free-piston Stirling engine under operating conditions representative of its future integration with a high-temperature TES system using sodium heat pipes and domestic combined heat and power (CHP) heat recovery. A dedicated experimental test rig was developed to independently investigate the effects of hot-end heat acceptor temperature, cooling-water temperature and cooling-water flow rate. Experiments were conducted for hot-end heat acceptor temperatures between 160 and 350 °C, cooling-water temperatures between 10 and 60 °C, and flow rates of 8 and 14 L/min. The results demonstrate that cooling-water temperature is the dominant operating parameter affecting electrical performance. Increasing the cooling-water temperature from 10 to 60 °C reduced the electrical power output by approximately 150–170 W over the investigated hot-end temperature range, whereas increasing the cooling-water flow rate from 8 to 14 L/min produced only a limited improvement in electrical output. Heat-to-electricity conversion efficiency reached up to approximately 21% at the highest investigated hot-end temperature and lowest cooling-water temperature. The cooling circuit simultaneously provided up to approximately 2.3 kW of recoverable thermal power, demonstrating the potential for domestic hot water production and space heating. The experimental results establish quantitative relationships between hot-end temperature, cooling-water conditions, electrical output and recoverable thermal power, providing design and control information for future TES-based Stirling CHP systems. Although the absolute performance values are specific to the investigated MEC engine, the identified thermodynamic trends and operating trade-offs may provide useful guidance for evaluating other liquid-cooled Stirling engine configurations.