The siren call of the flow battery has lured many a renewable energy researcher with its promise of large scale, long duration energy storage. In contrast to conventional battery arrays that pour out the juice for four hours or so, flow batteries can store a day’s worth of wind or solar power. That kind of long duration output is something that the US Department of Energy has been dreaming about, and with that in mind let’s take a look at the flow battery specialist ESS Inc.

For Germany, 2020 was a banner year in the production of renewable energy. Clean energy sources—wind farms and solar arrays as well as hydroelectric and biogas plants—ratcheted their share of power consumption up to 46 percent, nearly equaling that of coal, gas, oil, and nuclear power combined. And after a period of stagnation in the 2010s, the greenhouse emissions of the world’s fourth-largest economy have been dropping again, last year by around 80 million tons of carbon di