The availability of energy is one of the vital requirements for a settlement on Mars. Solar power and wind turbines are subject to changing weather conditions, especially during the Martian night, which makes energy storage necessary. Energy storage facilities may be local or distributed. They are part of the overall energy distribution system of the settlement.
However, having production follow availability can reduce the need for energy storage significantly. Insulation can reduce heat loss during the night with active day cooling while agricultural and industrial production can be stopped or significantly reduced at night. This follows the natural cycles than humans and plants that have evolved for exactly these circumstances.
Nuclear power has been considered as the preferred energy source for most plans for medium- to long-term human expeditions to Mars. However, this may be a difficult option for an autonomous colony due to the vast effort of the nuclear enrichment process.
Compressed-air storage has been used since the nineteenth century to store large amounts of energy. Natural Martian caves can be used as a pressure accumulator or artificial pressure vessels can be viable alternatives. Standard compressed air storage systems have 42% overall efficiency, increasing to 55% if there is heat recovery, as would be likely on Mars.
Adiabatic compression could raise the overall efficiency to 70%, making this a very interesting alternative for colony wide, utility scale energy storage systems.
Electrical energy can be stored in a flow battery. The capacity depends on the size of the tanks and can be easily extended.
While the nickel–iron battery is rather heavy, it is very robust and durable, making it a good candidate for a stationary energy storage in a Martian settlement. It does not require poisonous substances.
The storage of energy as hydrogen in the natural gas distribution system of Europe has been studied(ref). If part of the energy is used as heat, then the efficiency can be increased significantly.
Methane and other hydrocarbons storage
Hydrocarbons have a significant amount of inherent energy, which can be used as stored energy. Excess electric energy can be used to produce hydrocarbons out of carbon dioxide and water. The hydrocarbons can be stored in large tanks. In periods of energy deficiency the hydrocarbons can be oxidized to produce heat or electricity . The heat can be produced by a boiler or furnace, and the electricity using a electrical generator or fuel cells. carbon dioxide and water are the reaction products. Since propellant production is a large part of the activities on a Mars settlement, there should always be large quantities of hydrocarbons, mainly methane, available. Overall efficiencies should be about 70% for heating and 30% for electricity.
Gravity storage can take the form of pumped storage or of lifted storage. In pumped storage a reversible pump/generator is used to pump water form a lower point to a higher point when energy is available. Water can latter flow down through the system to generate electricity.
Various systems lifting weight have been projected but no significant installations are in use today. Moving masses up a rail on a slope, or moving a large massive cylinder are both been proposed. Since Mars has lower gravity than on Earth, the energy stored will be lower than for equivalent systems on Earth.
A substantial amount of energy is required for heating the settlement buildings and greenhouses. Materials with a high heat capacity inside of buildings, combined with an excellent insulation on the outside, help to keep the inside warm during the night. It may be advantageous to use artificially lit greenhouses, rather than try to compensate for the night time heat loss of greenhouses. Alternatively mobile insulation systems, already in house in Earth greenhouses, might be developed for Mars.
Heat can be stored in a big block of concrete, which is used to turn water to steam. The steam powers a turbine that produces electricity using turbo-alternators. The concrete block has a high heat capacity and can store the heat for many hours. However, the conversion of heat to electrical energy for such a system may be low.
A more refined form for this type of storage is available as Pumped Heat Electrical Storage . These installation can used steel tanks and an inert gas such as argon as working medium, storing energy with an efficiency of 75 to 80%. A reversible compressor is used to both compress the working fluid and to extract the energy from the expanding gas. The use of materials readily available on Mars make this an attractive storage solution.
Flywheels are used for short term high intensity energy storage. Requirements for sophisticated materials and controls make these unlikely as short or mid term solutions.
Capacitors and superconducting magnets storage
Management of production processes
Another way to reduce energy storage requirements is a sophisticated management of production processes. Energy consuming production processes can be carried out during periods of available energy (e.g. daylight), such as the production of liquid hydrocarbons out of atmospheric carbon dioxide or food production. At night the exothermic production processes are carried out, such as some recycling processes.
Most conventional schemes (e.g. batteries and flywheels) are practical on a small scale, but do not scale up very easily.
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