Chapter 12 - Ultra-high temperature space power applications

Abstract

This chapter discusses the application of ultrahigh temperature thermal energy storage (TES) and conversion to spacecraft systems. The use of silicon and boron as phase change materials (PCMs) is of primary interest for spacecraft in the context of a thermal rocket. The history of this concept is discussed as applied to solar thermal propulsion and dual-use radioisotope propulsion and power systems. In both the cases the use of high-temperature PCMs as a primary thermal capacitor has been frequently cited as beneficial, but investigations seldom go beyond the conceptual phase. For solar thermal propulsion, the inclusion of PCMs can drastically decrease the volume and mass of a propulsion system, provided that effective convective coupling to the PCM is achieved in the heat exchanger. However, the benefits of solar thermal propulsion have historically been relegated to niche mission scenarios. The more compelling application of ultrahigh temperature PCMs is in a pulsed radioisotope propulsion and power system where it is a truly enabling technology offering both high specific energy and power. It is suggested that future development efforts would focus on characterizing convective coupling to a latent heat storage medium since all applications require some form of heat exchange with a working fluid. A meaningful mission design cannot just look at material properties and must include effective energy transfer for an optimized heat exchanger to determine the net system benefit. Ultimately, this chapter highlights benefits that can be obtained by latent heat TES but cautions that the ultimate benefit to a spacecraft system is mission-dependent and will rely on a distinct mission need for a high specific power and energy system to justify technological investment.