Last week we discussed Estonia’s under-development underground pumped hydro project, with the fundamental point of interest being its price tag of just $800 million for 500 MW / 6 GWh. This week there’s another type of long-duration facility to discuss, also built into excavated caverns – coming from China’s compressed-air energy storage (CAES) fleet, which accounts for a couple of percentage points of national energy storage capacity, on par with the development of sodium-ion or vanadium-flow battery technologies, or concentrated solar power (CSP). One such is a 300 MW / 1.2 GWh compressed-air facility in Xinyang, China, being developed by China Energy Storage National Engineering Research Center. The crucial capex figure in this case is $300 million, which in a…