The Moon is a tough place to survive, and not just for humans. The wild temperature extremes between day and night make it extremely difficult to build reliable machinery that will continue to operate. But an engineering team from Nagoya University in Japan have developed an energy-efficient new way to control Loop Heat Pipes (LHP) to safely cool lunar rovers. This will extend their lifespan, keeping them running for extended lunar exploration missions.
How do you keep a rover insulated well enough to survive the frozen lunar nights, without cooking it during the day? A team of engineers led by Dr Masahito Nishikawara of Nagoya University may have found an answer. By combining a loop heat pipe (LHP) with an electrohydrodynamic pump (EHP), they have created a mechanism to cool machinery efficiently in the vacuum of space, but in a form which can also be turned off at night. Crucially, it is so efficient that it uses practically no power at all.
The Moon is an extraordinarily harsh environment for machinery. Aside from the highly abrasive regolith, which sticks to everything and is found everywhere, the Moon has no atmosphere and a very slow rotational period. This means that days and nights on the moon last 14 Earth days each, and reach extreme temperatures. With no atmosphere to insulate and transport heat around the Moon, night-time temperatures can drop all the way down to -173º Celsius, while the unfiltered heat from the Sun causes daytime temperatures to climb as high as 127º Celsius.
It is very difficult to design complex machinery to work reliably under such conditions. The long nights mean that the energy harvested from solar panels needs to be stored in very large batteries, but batteries do not cope well with low temperatures. They can be electrically warmed, but heaters need a constant flow of electricity, draining the batteries. Alternatively, a machine can be heavily insulated to keep it functional when idle, but this leads to overheating when it is active, and when the Sun rises.
Overheating can damage batteries, but it’s equally bad for electronic components. Active cooling systems are the traditional answer. They work similarly to the radiator in a car by pumping coolant through a large radiator, but these require power to run. This is a problem when you need your batteries to last 14 days before the next recharge. Passive systems, such as LHPs, are effective and don’t require power, but they run continuously, even when you would prefer heating.
“Heat-switch technology that can switch between daytime heat dissipation and nighttime insulation is essential for long-term lunar exploration,” said lead researcher Masahito Nishikawara. “During the day, the lunar rover is active, and the electronic equipment generates heat. Since there is no air in space, the heat generated by the electronics must be actively cooled and dissipated. On the other hand, during extremely cold nights, electronics must be insulated from the outside environment so that they don’t get too cold.”
LHPs can be thought of as a cross between the machinery of a refrigerator or air conditioner, and the heat pipes in modern laptop computers. Like a refrigerator, a liquid refrigerant is allowed to absorb heat which causes it to vaporise. The vapour then passes through a radiator, which cools it back to ambient temperatures. This turns it back into a liquid, and the cycle repeats. The phase changes, from liquid to gas and back, allow the refrigerant to transfer heat very efficiently. Heat pipes, by contrast, use capillary action to move a liquid between a heat source (such as your computer’s CPU or graphics accelerator) and a radiator. LHPs combine the capillary transport action of a heat pipe with the phase changes of a refrigeration unit.
LHPs have been used in space before, where they have been equipped with valves to block the flow of refrigerant when cooling is not needed. However, these valves significantly reduce the system’s cooling efficiency. Nishikawara’s innovation is to replace the valves with an Electrohydrodynamic pump. EHPs are low-powered pumps which work by inducing electric currents in a fluid, and then using the resulting magnetic field to apply force to the fluid. This has the advantage of not intruding into the plumbing of the system, which means there is no interference with flow when it isn’t active.
Nishikawara’s team have added low-powered EHPs to an LHP to act as a very efficient valve: When they need to turn cooling off, the EHP is activated to create a small opposing force that stops the flow of refrigerant, while sipping only a tiny amount of power.
“This groundbreaking approach not only ensures the rover’s survival in extreme temperatures but also minimizes energy expenditure, a critical consideration in the resource-constrained lunar environment,” Nishikawara said. “It lays the foundation for potential integration into future lunar missions, contributing to the realization of sustained lunar exploration efforts.”