Everyone has got very excited by the claim this week from Siemens Gamesa, that is has a new form of storage – one well known, but mostly seen as impractical – based on heating rock – a form of electrothermal energy storage system or ETES. It has opened a pilot plant in Hamburg-Altenwerder which it says can store 130 MWh of energy for up to one week, and says it will gradually raise this above 1 GWh. Before we get too excited, recent white papers from Siemens Gamesa show that this technology is fine if you want to go from heat to heat, and is able to retain 98% of heat. But the round trip from electricity to heat back…