Materials For A Stirling Engine Heater Head

The achievable performance of Stirling engines is limited by the strength properties of the construction materials. For high efficiency and power density, it is desirable to maximize the hot end temperature and the pressure of the working gas. For systems using liquid metal heat transport systems, the materials must be compatible with the liquid metal. This paper describes work done on the 25 kW Advanced Stirling Conversion System (ASCS) Terrestrial Solar program in establishing criteria and selecting materials for the engine heater head and heater tubes. Various mechanisms contributing to incompatibility between materials are identified and briefly discussed. Large thermal gradients, coupled with requirements for long life (60,000 hours at temperature) and a large number of heatup and cooldown cycles (20,000) drive the design from a structural standpoint. The pressurized cylinder is checked for creep rupture, localized yielding, reverse plasticity, creep and fatigue damage, and creep ratcheting, in addition to the basic requirements for burst and proof pressure. In general, creep rupture and creep and fatigue interaction are the dominant factors in the design. A wide range of materials for the heater head and tubes was evaluated. Factors involved in the assessment were strength and effect on engine efficiency, reliability and cost. A preliminary selection of Inconel 713LC for the heater head is based on acceptable structural properties but mainly driven by low cost. The criteria for failure, the structural analysis, and the material characteristics with basis for selection are discussed.