MODELING PERFORMANCE OF HYBRID THERMIONIC-THERMOELECTRIC POWER SYSTEMS FOR SPACE APPLICATIONS

Current radioisotope power system technologies for deep space applications rely on thermoelectric energy conversion to convert heat into electrical power. However, these thermoelectric materials are susceptible to damage in high temperature applications and are limited in their conversion efficiency. Thermionic energy conversion is an alternative form of direct energy conversion that operates most effectively at high temperatures. Due to advances in manufacturing techniques, thermionic technology is being revisited as a practical energy conversion technology with comparable performance to modern thermoelectric technology. A cascaded system incorporating both thermionic and thermoelectric energy conversion technologies can reduce the strain on the thermoelectric materials while increasing the overall power conversion efficiency for space power systems. The purpose of this investigation is to explore, through modeling, how a hybrid conversion system of this type might be optimized using modern technologies for each of the two types of subsystems. To construct the system-level model we developed component-level, physics-based models and integrated them appropriately into a cascaded system. The thermionic model used in this study includes a more accurate accounting of space charge effects than used previously. This model includes the effects of back-emission and both emitter and collector electron reflection and the subsequent effect on output current density. This modification allows for improved performance prediction across a broad spectrum of emitter and collector temperatures. The hybrid model was used to explore strategies for maximizing hybrid performance within the temperature limits of the expected components. Thermionic and thermoelectric systems alone have typically achieved a conversion efficiency in the 5-10% range for typical heat input and rejection temperatures. Our modeling indicates that a hybrid system combining thermionic technology with the currently available thermoelectric energy conversion technologies could increase the beginning-of-life power output from a pure system efficiency of 5% to a hybrid system efficiency above 30% for input temperatures between 900 K - 2500 K and heat rejection at 300 K. Keywords: Energy Conversion, Thermionic, Thermoelectric, Space Charge, Heat Transfer