Stirling Thermal Motors, Inc. (STM) of Ann Arbor, Michigan, has been developing and testing a general purpose, 40 kW Stirling engine designated the STM4-120. This engine configuration features a heat pipe heater head designed to receive heat from the condensing metal vapor (sodium, Na) of a heat pipe using any one of a variety of heat sources.
In support of the engine development program, STM has designed a general purpose Heat Pipe Gas Combustion (HPGC) system under a GRI contract. Use of the HPGC system with the STM4-120 will result in a Gas-Fired Stirling Engine (GFSE) that may be advantageous when used for cogeneration, heat-driven heat pumps and other similar applications.
The principal component in the HPGC is the heat pipe evaporator which collects the condensed liquid sodium and distributes it equally over the heat transfer surfaces. The liquid sodium evaporates and flows to the heat pipe condenser where it delivers its latent heat. When used in combination with the STM4-120, the heat pipe condensers also function as the Stirling engine heaters.
Several quarter-scale models of this evaporator design have been fabricated and successfully demonstrated, first, on a test bed with a calorimeter for measuring performance and, later, in combination with the STM4-120 Stirling engine.
The STM4-120, operating on heat supplied from the aforementioned evaporators, has been tested extensively on a dynamometer facility. This facility has been supported, in part, by the Department of Energy (DOE) Automotive Stirling Engine Development Program. The maximum power achieved to date was 23 kW at 1800 rpm, a heater temperature of 775°C and a mean cycle pressure of 10.8 MPa. The engine conversion efficiency based on the heat input to the heater head was approximately 40%.
This paper describes the test hardware, the performance and related problems of the engine and heat pipe systems. Plans for future development and testing are outlined and include a heat pipe solar receiver supported by the DOE Solar Thermal Program.