A crater in Sirenum Fossae, in the southern highlands, whose gullies were formed by water (NASA/JPL/University of Arizona)

Martian air and water might be all that’s needed to store energy on the Red Planet, according to a recent discussion on the NewMars forums.  Solar power, perhaps in the form of a Solar Power Tower, is one of the strongest contenders for a system to generate energy for a pioneering Martian settlement. However, solar power systems only generate energy when the Sun is shining.  This creates a need to store energy to keep the lights on at night.

Users on the Forum believe they may have a solution: Freeze Carbon Dioxide out of the atmosphere, then use the heat from lukewarm water to boil it off and drive an engine.

Here’s how the system would work:

1. Freeze Carbon Dioxide out of the atmosphere:  Carbon Dioxide freezes at a temperature of 195 K (-78°C, -103°F).  Mars during the day is generally around 250 K (-23°C, -10°F) but this can be variable.  Basically what this means is that it’s a lot easier to freeze CO2 on Mars than Earth. Mars’s atmosphere is over 95% carbon dioxide.
2. Melt ice to store heat: Melting ice requires energy, which is released when it freezes. Specifically, it requires 334 kJ/kg of heat.  This can be done with a simple system to absorb and transfer solar heat.
3. Melt and pressurize the dry ice.  The energy to do this is free: It comes from the “hot” end of the freezer you used to freeze the dry ice out of the atmosphere.  At an elevated pressure of 5 to 10 times Earth’s atmosphere, CO2 will be liquid at normal martian temperatures, meaning that there’s no temperature problems with storage.
4. Heat the liquid Carbon Dioxide into a gas around -20°C (-5°F) using energy from the water and run it through a piston or turbine to generate energy.  Repeat until there’s no usable pressure left.

This system has the potential to be both incredibly efficient and incredibly simple.  A high-performing battery will return 70-90% of the energy input. This system could be expected to return 60-110%: In certain cases it will release more energy than it stores.  Meanwhile, the technology involved is very simple. Refrigeration and piston engines fall into the category of what Dr. Robert Zubrin likes to call “Gaslight Era Technology”, meaning that it would be familiar to people in an advanced country at the turn of the previous century.

A system of pumps, pipes, tanks, and engines similar to what you might expect to see if this system were implemented on Mars (Siemens)

Some challenges remain: A system to freeze cold, low pressure Carbon Dioxide out of the atmosphere presents design challenges in maximizing the rate of heat transfer while also maximizing efficiency.  The efficiency of the system depends strongly on the efficiency of this process. Any system to provide power at night needs to be extremely reliable and will require extensive testing before deployment.

The settlement of Mars will necessarily be based on the use of resources available on Mars; the development of a system like this one would bring us that much closer to permanent human settlement of the Red Planet.