Alkali Metal Loop Heat Pipe Development for Solar Dynamic Energy Conversion

Future space exploration missions and outposts on the Moon and Mars would benefit from compact, efficient, high temperature energy conversion devices such as nuclear and solar reactors. In the case of solar dynamic systems, extending operation beyond the hours of solar exposure can be achieved by incorporating Thermal Energy Storage (TES) if the concentrated radiation can be decoupled from the power conversion unit and stored. Conventional heat pipes have been previously developed for this purpose in terrestrial solar receivers and dish-Stirling systems, but the low pumping capacity provided by the capillary structure limits their operational range to horizontal configurations and short distances. Here a high temperature Loop Heat Pipe (LHP) is investigated to transport the concentrated solar radiation from a parabolic dish´s focal point to alternative locations. LHP can perform well at high power, transporting heat over large distances and in different orientations. However, the large power transport (> 10 kW) and high temperature (> 600ºC) requirements of dish-Stirling systems are above current state-of-the-art LHP technology, making this a challenge in terms of materials, fluids, and design. A trade-off study is performed taking into account system requirements, performance and cost, developing a numerical model to determine the most promising solution for a future prototype. The working fluids options at these temperatures are limited to liquid metals: mainly sodium, potassium, and cesium. Although sodium might seem like the most promising working fluid candidate, potassium is anticipated to work better within the system requirements. This paper will show through analysis that, in contrast to conventional LHP where working fluids have negligible thermal conductivity, when using a highly conductive liquid metal the parasitic heat fluxes might be extremely important. This is a novel problem, indicating that design parameter optimization has to be performed differently to ensure proper operation.