I've read (1) that the efficiency (COP) of a heat pump does not change much with size. If that is true, why don't power companies employ them to produce yet more power from fossil fuels or renewables? I'm aware that air source heat pump efficiency drops in cold wet climates, but aren't there other more stable sources that could be tapped by an industrial scale plant? Ocean currents, for example. (1) Are smaller heat-pumps more efficient?