Hydrostor’s 500 MW, 8-hour duration Willow Rock Energy Storage Center – which is an advanced compressed-air energy storage system (A-CAES) in California – now has government backing, thanks to the Department of Energy’s Loan Programs Office, which has made a conditional commitment to a $1.76 billion loan guarantee. Compressed-air is a long-duration energy storage technology – eight hours is on the shorter side – and in future may win out over the few alternatives in that segment. The biggest long-duration energy storage option, pumped hydro, faces geographical and environmental constraints – a ‘flat-land underground version doesn’t exist yet. Pumped hydro also represents a major engineering challenge – China constantly builds pumped hydro plants at below $1 billion per GW, but…