Energy storage selection and operation for night-time survival of small lunar surface systems

Highlights

• Night survival is the greatest challenge for small lunar surface systems.

• Temperature dependent capacity for a selection of energy storage devices.

• Temperature dependent capacity determines the optimum operation temperature.

• Optimal operational temperatures for batteries depend on insulation.

• Majority of energy storage devices survive cryogenic temperatures.

Abstract

In the coming decade, a multitude of small, carry-on payloads in the form of miniature rovers, scientific instruments and surface deployed stand-alone systems will provide many new and exciting possibilities for lunar exploration. For these system, the survival of and operation in the extreme cold of the lunar night will be the biggest challenge, as most will be purely battery driven and unable to rely on radioisotope heaters. Maximizing battery lifetime in these conditions requires a trade-off between energy density, low-temperature performance and insulation, but comparable temperature and current dependent discharge capacities of batteries and other energy storage devices are often not reported in datasheets or literature. For this reason, a selection of energy storage devices was chosen and tested to determine their low-temperature performance in a representative environment. Based on this data, the theoretical lifetime per battery weight and optimal operational temperature for varying degrees of insulation was determined. It was shown that the selection of the optimal energy storage device and operational temperature depends on the insulation, emphasizing the need for multi-disciplinary design optimization.

Introduction

After decades of very limited activity, a renaissance of lunar exploration has started in recent years. Perhaps kick-started by the Lunar X-Prize and by the desire of space agencies to explore the resource potential of the Moon, a large number of surface missions have recently been announced by agencies and private enterprises. This new wave of lunar surface exploration, spearheaded by the landing of Chang'e 3 in 2013, and possibly culminating in the announced crewed NASA Artemis landings in the mid-2020s, targets more ambitious science objectives and will be more sustainable than Apollo era exploration.

For any lunar surface mission, one of the most demanding challenges is the extreme environment of dust, radiation, vacuum and especially its thermal environment. In the absence of a tempering atmosphere, surface temperatures range from about 50 K to 390 K, depending on daytime, latitude and surrounding topography [1]. Maximum daytime temperatures are more moderate near the poles, but low solar elevation causes significant shadowing by local topography that severely impact surface temperatures. In addition, the relatively long duration of a lunar month (average synodic lunar day) of 29.531 [2] lead to a nighttime of 354 h. In the absence of solar illumination and faced with the extremely low temperatures of the surface and space, there will be no external energy source available. Ensuring reliable operation during, or survival of this period is “probably the most demanding energy storage challenge that will be faced in the exploration of the solar system” [3].

Some of the previous lunar missions have faced the same problem and have coped with varying degrees of success. Prior to the Apollo landings, the USA successfully landed a total of five solar powered Surveyor landers. Surveyor 3 died during its first lunar night, the other four managed to survive up to six nights with varying degrees of damage. Surveyor 6 and 7 even achieved a combined 120 h of night-time operation [4,5]. A set of long term experiment packages was also deployed during the manned Apollo missions: The Early Apollo Scientific Experiments Package (EASEP) of Apollo 11 and the Apollo Lunar Surface Experiments Packages (ALSEP) of Apollo 12 to 17. EASEP was solar powered but with Radio-Isotopic Heating Units (RHUs) and survived its first lunar night but died during noon on the second day. The following ALSEP packages operated flawlessly for up to 98 lunar day/night cycles [5], thanks to a Radioisotope Thermo-Electric Generator (RTG). On the soviet side, the two solar powered but RHU assisted Lunokhod rovers managed to survive 4 and 10 months. Similarily, the Chinese Chang'e 3 and recently landed Chang'e 4 lander and rover use solar power and RHU assistance to cope with the lunar conditions. So far, both landers and the second Yutu rover are still fully operational while the first Yutu rover survived at least 31 months but lost mobility in the first lunar night [6].

While these past achievements must not be understated, the challenge is significantly greater for some of the currently proposed systems. Building on the advances in spacecraft miniaturization demonstrated by CubeSat technology in recent years, many of the proposed or planned missions involve surface systems with a mass of less than 25 kg. Examples include the Sorato (4 kg) and Polar Ice Explorer (12 kg) rovers of ispace inc. [7], Cuberover (4 kg) from Astrobotics [8], Mission One (5 kg) of Pulispace [9] or the stationary Robex Remote Unit concept (3 kg and 10 kg) [10]. Together with further similar examples, these standalone units make a new class of lunar surface systems that are considerably smaller than what has flown previously. Considering the significant time, effort and cost involved in the preparation and execution of any lunar mission, it should be a priority to maximize the operational lifetime of deployed systems. Despite this, it is expected that the majority of early commercial lunar systems are not planned to survive the first lunar day [3].

Keeping systems operational during the lunar night requires thermal control to keep internal temperatures within component envelopes. Insulation, such as multi-layer insulation (MLI), low emissivity coatings or aerogel reduce heat loss at night, but heating is required to keep temperatures constant. For this reason, efficient energy storage is crucial to enable nighttime operation. Radio-isotope based energy sources would be ideal because of their extremely high energy density, but access to such systems is restricted, procurement is expensive and their usage can be politically challenging. Smaller systems, especially those of non-institutional origin, will therefore typically rely on battery storage [3].

Various battery technologies have been used in space systems to date, but in recent years rechargeable lithium ion cells have become the “preferred choice because of their high energy & power density, reliability, robustness and long cycle life” [11]. This includes application on planetary missions with highly challenging environmental conditions. For example, both Mars Exploration Rovers (Spirit and Opportunity), the Mars Science Laboratory (Curiosity) and Mars Insight use lithium ion batteries for energy storage [12]. Multiple different chemistries of lithium ion batteries exist, some of which are specifically tailored for low temperature operation [13], though these specialty products usually don't exhibit the energy densities of currently available cells. High energy density chemistries and architectures for future batteries are a topic of intense investigation, with promising developments into the direction of lithium metal and lithium sulphur batteries, but early prototypes are still constrained by limited temperature ranges, limited cycle life and safety concerns. Primary lithium batteries have also been used for exploration missions, where recharging was not necessary, e.g. for the Philae [14] or Huygens landers [15]. For the application of sustaining operation during the long and cold lunar night, a compromise is needed between high energy density, capacity at low temperature and high temperature tolerance.

However, nominal capacity and temperature ranges are only of limited use for the comparison and selection of energy storage devices [16], as battery capacity generally significantly decreases towards lower temperatures, but increases for lower discharge currents [17]. The extent of these effects is not always reported in datasheets and presented values may not always be comparable. For example, self-heating of batteries is seldom documented, which can lead to higher apparent capacities at higher discharge currents [e.g.18]. In addition, the presented problem uses very low discharge currents, which are also not usually covered for rechargeable batteries.

Passive hibernation could be another strategy to extend mission operations beyond the first lunar day. In this case, the surface system would be switched off during the night and reawaken once the sun rises again. This would require non-operational survival of cryogenic temperatures. This has already been demonstrated by some lithium batteries [19,20], but it is unclear if this also applies to other cells.

For these reasons, the present study investigates the suitability of a broad selection of batteries and other energy storage devices. A selection of batteries are subjected to low current discharge testing over a wide temperature range. Temperature dependent capacity data is then used to calculate theoretical self-heated survival times to compare their performance. In addition, the selected batteries are tested for their ability to survive passive exposure to cryogenic temperatures.