In Switzerland, energy company FlexBase Group is currently building the largest battery ever - at least that is the claim. But how do you build such a battery and what is its significance? The Engineer answers five questions.

1) What is FlexBase Group doing?

The company makes an energy storage system good for 2.1 gigawatt hours. That's as much as 210,000 households use for a day, or a million households for an hour. If it succeeds, it will be, as far as we can tell, the largest battery in the world in terms of storage capacity.

This is a gigantic 'redox flow' battery, to be built near the municipality of Laufenburg, roughly 35 kilometres east of Basel. It will be part of the Laufenburg Technology Centre, which consists of offices, laboratories, and an AI data centre.

The aim is to cope with fluctuations on the electricity grid, storing energy when there is a surplus and using it when there is a shortage. The system responds in a fraction of a second, according to FlexBase, and has a maximum capacity of 1.2 gigawatts.

2) What is a redox flow battery?

In a flow battery, energy is stored in liquid electrolytes: substances within which electrons can travel. In this process, two different electrolyte liquids are located in large tanks. During charging and discharging, the liquids are pumped from those tanks into cells, where they are separated only by a membrane. In that stack (stack) of cells, electrons move back and forth between the two substances, through the membranes. That is where the electrochemical reactions take place. In FlexBase, the electrolytes are two varieties of vanadium (see question 4).

Image: Depositphotos

3) Why is FlexBase going to use this technique?

Flow batteries are safer than, for example, lithium-ion batteries, which are known for their high energy density. Because the liquid electrolytes are 75 per cent water, there is no risk of fire or explosions. And the materials last a long time without sacrificing quality. On top of this, the redox flow battery is easily scalable, with power and storage capacity sizes being independently adjustable. Storing a lot of energy requires large tanks, while creating a lot of power requires large stacks.

4) So why don't we see redox flow batteries everywhere?

Large-scale energy storage has only become important relatively recently, due to the advance and capriciousness of solar and wind energy. As a result, industrial development and hence the reduction of the cost of membranes, pumps, tanks and stacks also came late in the game. For less large-scale storage, lithium-ion batteries are well suited, so the focus was on that.

Actually, it is thanks to space agency NASA that redox flow batteries broke through after all. NASA saw this form of storage in the 1960s and 1970s as ideal for lunar missions, so it did a lot of research on it. FlexBase will use vanadium as an electrolyte, a relatively new electrolyte. Vanadium occurs in different oxidation states, and can therefore be used as both a positively charged electrolyte and a negatively charged electrolyte at the same time. This prevents contamination of the membrane.

5) When is the battery ready?

Scheduled for 2029.

Opening image: FlexBase