Indeed. The Carnot Cycle has nothing to do with the efficiency, since it can be reversed in what is typically referred to as a "Coefficient of Performance", whi

Indeed. The Carnot Cycle has nothing to do with the efficiency, since it can be reversed in what is typically referred to as a "Coefficient of Performance", which is always bigger than 1 (assuming you aren't using straight resistive/friction heating). I seem to recall from Thermos lecturers that a CoP for consumer fridges is often in the 4 - 6 range, and it increases the smaller the difference in temperature between sink and source is, inversely to the Carnot efficiency. That said, due to heat capacity of pump components and conduction losses, I would be surprised if the total cycle efficiency was above 80%, putting this well below gravity and battery storage schemes. Maybe it is just incredibly cheap? Water does have a high heat capacity...