How compressed-air storage works
Hydrostor Willow Rock, Kern County CA: 500 MW · 4,000 MWh · 8 hours · 60–65% round trip
Squeeze air into a water-filled cavern deep underground, keep the heat of compression, and let a column of water hold the pressure steady. Run it back through a turbine at the evening peak. Every number comes from one model.
Sources: ESS News, “Hydrostor signs offtake agreement for CAES project in southern California” (Aug 2026: 500 MW / 4,000 MWh, under construction since July 2026, state permit Dec 2025, 60–65% round trip, 60 MW with Clean Energy Alliance); ESS News, California Community Power offtake (Feb 2026, 50 MW); Energy-Storage.news, DOE $1.76B conditional loan. Cavern depth and layout, the charging window and the heat-store temperature are illustrative.