A redox flow battery is a battery where two tanks of chemicals are held as a liquid which is pumped thru a reaction chamber with a membrane separating two electrodes. Chemically they are identical to a normal battery. However, a limited amount of chemical can fit into a sealed battery cell. A flow battery allows huge tanks to hold the 'charged' fluid. With a sufficient sized tank, the flow battery can provide power for weeks or months (which is long enough to outlast a huge Martian Dust storm).
This page will discuss why flow batteries are useful on Mars, and which chemistries are suitable for Mars. For general information on flow batteries see...
Advantages of Flow Batteries on Mars
-- The very long duration that a flow battery can provide power is the main attraction. If the Settlement is dependent on Solar panels, then they can lose 95% of their power during massive dust storms – which can last several weeks. A battery which can provide power for this long, is thus more attractive than short term batteries.
-- The reaction chamber (electrodes & membrane) can be shipped from Earth, while the tanks and pumps can be built with rudimentary Martian industry. This is a huge savings as most of the mass of the installation is from local materials.
-- A different number of reaction chambers can be used to charge and discharge them. Thus a single (low voltage) current can charge the cell, but several reaction chambers wired in series can discharge them more quickly, at a HIGHER VOLTAGE. Thus they serve double purpose, acting as both a battery and transformer, reducing the amount of mass shipped from Earth.
Disadvantages of Flow Batteries on Mars
-- These batteries depend on pumping two liquids from their tanks past the electrodes and membrane. They stop working if these liquids freeze. Mars is very cold, so they must be insulated and kept warm. On the surface, this power loss lowers their efficiency. (How much of the power that they store is lost to heating?) If they are deeply buried and well insulated, then the heat loss would be slow, but the cost of building them is higher.
If the base is built on permafrost then care must be taken to prevent the ice supporting the battery from melting (tho this is a concern to ALL heated volumes built on permafrost not just batteries).
Chemistries Suitable for Mars
This section is a STUB, someone with a better background with such batteries please feel free to expand it. The following chemistries seem likely as they provide a reasonable voltage and use chemicals common on Mars:
-- Lead-Acid (methanesulfonate). 1.82 Volts
-- Sulfur-Oxygen-Salt. ???
-- Zinc-Bromine. 1.85 Volts (and this battery chemistry enjoys a large number of charge / recharge cycles).
If settlement is near a vanadium ore, then some attractive chemistries open up.