ESS Tech, a US producer of iron-based flow batteries, has agreed to build a 500 MW h storage battery facility for the German energy provider LEAG. The facility will be at the site of LEAG’s lignite-fired power station in Boxberg, in eastern Germany. It is scheduled to start up in 2027. It will feature ESS Tech’s proprietary electrolyte, composed of ferrous chloride in an aqueous solution. In optimal conditions the facility could store energy at a cost of about 3 cents per kW, the company claims.