Iron-redox-flow batteries

One of the core problems of the energy transition is the storage of green electricity on a large scale. This is what needs to be solved. Together with ESS Tech Inc (ESS), a leading global manufacturer of long-term energy storage systems, we are focusing on iron redox flow technology. At the Boxberg site, we want to realise a corresponding 50 MW/500 MWh, which is to be commissioned in 2027 - our first energy center.

Redox flow batteries - also called liquid batteries, flow batteries or wet cells - are based on a uniform principle, it is already contained in the name:

Redox stands for reduction/oxidation and symbolises the circuit as a storage principle. Here, the electrolytic fluid flows through the battery cells in two circuits. When electrical energy is stored, the electrolyte is reduced in the negative half-cell and oxidised in the positive half-cell. When discharging, this process is reversed again. 

Flow stands for the pumping system that guides the electrolytic fluid through the battery cell. The advantage is that the electrolytic liquid can be stored outside the actual battery in tanks with different oxidation levels.

The electricity is produced in a cell, similar to the fuel cell. The size of the cell determines the power (kW), the energy (kWh) depends on the tank size - the amount of liquid used.

What is special about the iron redox flow battery is that it is based on materials that are cheap, abundant and non-toxic: Iron, salt and water. The ESS iron redox flow battery uses the same electrolyte on both sides of the battery, positive and negative. It is iron chloride dissolved in water.