Vapour compression heat pumps can upgrade ambient heat sources to match the desired heating load
temperature.
They can offer considerable increase in operational energy efficiency compared to
current water heating systems. Solar heat pumps collect energy not only from solar radiation but also
from the ambient air.
They can operate even at night or in totally overcast conditions. Since the
evaporator/collector operates at temperatures lower than ambient air temperature it does not need
glazing or a selective coating to prevent losses.
Currently, however, they are not used much at all in
domestic or commercial water heating systems.