Development of a Lunar Regolith Thermal Energy Storage Model for a Lunar Outpost

Abstract: The Moon has always been an important milestone in space exploration. After the Apollo landings, it is logical to think that the next step should be a permanent habitation module, which would serve as a testing ground for more ambitious projects to Mars and beyond. For a lunar base to come into realization, it is necessary to assess a number of technological challenges which are due to the harsh conditions that can be found on the Earth's satellite. One of these tasks revolves around energy storage: During the day it is possible to use photovoltaic cells and convert the solar irradiance into electrical energy to power an outpost, however during the lunar night this source is not available. Current investigations establish that the optimal landing site for a permanent mission would be on the rim of the Shackleton crater, near the South Pole. This would reduce the night duration from 14 days to 52 hours of the lunar cycle, which is 29.5 days. While this significantly decreases the exposure to the cold temperatures of the Moon when there is no sunlight, there is still a need for a system to provide energy to the lunar base over this period. Therefore, this study pretends to serve as a possible solution for the aforementioned problem, by developing a system storing energy as thermal energy and then harvesting it as electricity using thermoelectrics. First, a theoretical introduction is presented, where the problem statement is exposed, along with background information regarding the solar illumination and the lunar soil. At the same time, an insight on regolith sintering techniques is given. These techniques are important as a means to providing thermal energy storage during the night cycle. After this, the core of the study is developed: The ideal system for energy storage is broken down into segments, and each of them is explained attending to the possible requirements of a lunar base, while providing supporting simulations when deemed appropriate. These are the solar concentrator, thermal mass, thermoelectric array, cold sink and, if necessary, a pipe network. Following this chapter, a device is proposed. Based on the previously mentioned guidelines, an ideal thermal energy system is simulated and evaluated. Although it is not optimized for efficient energy harvesting, it serves as insight on the design and simulation constraints that appear when one wants to collect electrical energy from thermoelectrics with relatively low efficiency. It was estimated that the prototype would output a mean power of 3.6 Watts over the whole duration of the lunar night. Although in its current state this technology would not present significant benefits over existing energy storage methods such as nickel-hydrogen batteries, this study also proposed several optimization methods which could vastly increase the performance of the device. These include adding more efficient thermoelectric patterns, or modifying the properties of the semiconductors by doping or using nanostructures, and present follow-on opportunities for further research.