You're forgetting something on the Carnot cycle efficiency, which is that the efficiency of the storage cycle can be GREATER than unity. Heat pumps (of which th

You're forgetting something on the Carnot cycle efficiency, which is that the efficiency of the storage cycle can be GREATER than unity. Heat pumps (of which this is an example) can move more than one joule of heat per joule of input power. I agree that the output efficiency (Joules of power : Joule of heat) is going to be low, given the relatively small temperature differential between the reservoirs, but the round-trip efficiency might not be that bad.