Propellant storage facilities are required to store propellant for return vehicles and for Mars transit vehicles. Large scale propellant storage can also serve as backup energy storage for a settlement in case of emergencies.
Cryogenic oxygen storage requires cooling down to -183°C, or about 90°K. Specific heat of liquid Oxygen, 0.92 kJ/kg. Density of 1100 kg/m3.
Methane storage on Mars can use the technologies developed for natural gas (which is mostly methane) storage on Earth.
Cryogenic methane storage
Liquefied methane is cooled down to liquid form for ease and safety of non-pressurized storage. It takes up about 1/600th the volume of methane in the gaseous state (at standard conditions for temperature and pressure). It is odorless, colorless, non-toxic and non-corrosive. Hazards include flammability after vaporization into a gaseous state, freezing and asphyxia. The liquefaction process involves removal of certain components, such as dust, acid gases, helium, water, and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is condensed into a liquid at close to atmospheric pressure by cooling it to approximately −162 °C (−260 °F); maximum pressure is set at around 25 kPa (4 psi). See Liquified Natural Gas.
Pressurised methane storage
Natural gas storage on Earth is usually used for network load variations. Geological formations are often used for this purpose. Although this is likely possible on Mars, one possible solution for natural gas storage on Mars might be the Gasholder.
Hydrogen storage is more difficult than methane storage. If the transportation system uses methane for propulsion, then the hydrogen storage is limited to the time required to convert it to methane, from the hydrogen liberated by electrolysis of water. Liquid hydrogen is cooled down to -252°C, or 20°K.
If the ships are going to stay an entire synod on Mars, then their tanks can serve for propellant storage, reducing the infrastructure required. Care must be taken to avoid condensation of CO2 on the exterior of ship stored propellant. At Martian atmospheric pressure, CO2 becomes a solid at about -125°C.
Propellant can be stored in surface or underground tanks. This allows for ships from Earth to stay just a few days on the Mars surface and then return to Earth for the next synod, reducing ship inventory.