Energy storage

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 or dust storms, which makes energy storage necessary. Energy storage facilities may be local or distributed and are part of the overall energy distribution system of the settlement.

It is important to note that before energy storage is considered, it should always be priority to reduce energy needs in the first place and best match the time of peak energy consumption to the time of peak energy production. Enclosed spaces can use heavier insulation and active day cooling to better regulate temperatures, and agricultural and industrial processes may be actively altered to match the current energy production and biologic drives to work during the day.

Nuclear power is often considered the preferred energy source for most plans for medium-to-long-term human expeditions to Mars. However, it may be a difficult option for an autonomous colony due to the vast effort of the nuclear enrichment process.

Mechanical storage

Compressed-air storage

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.

Gravity storage

Gravity storage can take the form of pumped storage or 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 then flow when energy demand exceeds supply, generating electricity. Efficiencies are good, reaching up to 70-80% in large systems.

Various systems lifting weight have been considered but no significant installations are in use today. Proposals include moving masses up a rail on a slope or moving a large massive cylinder. Since Mars has lower gravity than Earth, less energy can be stored with the same height displacement.

Kinetic storage

Flywheels can be used for short term high intensity energy storage. Requirements for sophisticated materials and controls make these unlikely as short or mid-term solutions.

Chemical storage

Flow battery

Electrical energy can be stored in a flow battery. The capacity depends on the size of the tanks and can be easily extended. Most batteries can provide power for hours or days. Large flow batteries can provide power for days, weeks, or months (taking a proportional time to charge).

Batteries

Batteries can be used for daily power demand but are rather heavy for longer term storage. High tech batteries would (at first) have to be shipped from Earth. Low tech batteries such as lead acid batteries could be created on Mars. Many flow batteries (see above) are easy to construct, tho the ideal chemistry suited for early Martian development needs to be decided. Modern batteries have a system efficiency of around 95%.

Nickel Iron

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 complex or poisonous components. It might be a good candidate for local production.

Lithium ion

Lithium ion batteries should be quite mature technologies by the time the Martian settlement is built. Combined with solar power they can provide off peak power and help stabilize a power network.

Hydrogen storage

Hydrogen storage uses electrolysis to convert water into hydrogen and oxygen for storage, then recombining them in fuel cells to produce electrical energy. Overall system efficiency is about 40%.[1]

Hydrogen has a very low volumetric energy density, and hydrogen easily escapes from storage. If it is created, and promptly used, this loss is trivial, it hydrogen is to be stored for a long time, turning it into a compound (such as methane) with a higher volumetric energy density and better storage properties would be wise.

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 hydrocarbon 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 dynamo or fuel cells, in which 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.

Alternatively, hydrocarbons can be used to operate an internal combustion engine if an oxidizer is provided. This would allow for convenient vehicular energy production or quick deployment for other mechanical devices, such as dynamos, pumps, or excavators.

Some companies have already developed the technology to store excess electricity in methane, such as the German company SolarFuel.

Using methane and oxygen storage, for example, would store about 55 MJ/kg of methane. This would be burned in electrical generators very similar to today's gensets, although the absence of a nitrogen component would probably change the combustion behavior significantly. A large 3,5MW electrical generator masses about 10 000 kg (ref. Caterpillar) including an external cooling array. A solar array providing the same power on Mars would mass about 100 W/kg, or 30 000 kg.

If you wish to transport Hydrogen from Earth to Mars (say for building fuel for the return trip), some hydrogen will boil off during the trip. However, Benzene is a space storable way of transporting hydrogen.

Thermal storage

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 use 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.

Management of production processes (Negawatts)

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.

Other

Most conventional schemes (e.g. batteries and flywheels) are practical on a small scale, but do not scale up very easily.