Rejection of waste heat from oxygen liquefaction operations at a lunar oxygen production plant

Oxygen liquefaction is a key processing step of proposed lunar oxygen production plants. A baseline oxygen liquefaction process has been developed. The process operates continuously with a production rate of 200 tonnes per year. Oxygen is liquefied using a cascade refrigeration scheme in which methane and ethane refrigeration loops are used to increase the rejection temperature to ≈ 270 K. Heat is transferred from three process coolers to a utility cooling fluid and subsequently rejected by conduction to the lunar subsurface. An initial subsurface regolith exchanger consists of smooth tubes buried horizontally at a depth of ≈ 2 m. A heat transfer analysis is performed to determine approximations for the spacing required between parallel tubes and the required heat exchange area. The analysis demonstrates that rejection of waste heat by conduction to the lunar subsurface does not appear to be a viable option. This is due to the large heat transfer area required.