COP=Qh/W=Qn/(Qh-Qc=Th/Th-Te) The CoP will diminish if the outside temperature drops So electrically driven air-to-air heat pumps are most useful in moderate climate(>-10oC)In addition to the air-to-air heat pumps, it is common to use the ground at several feet of depth, or surface water such as a river for the cold reservoirCOP = Qh /W = Qh /(Qh – Qc ) = Th /(Th – Tc ) The COP will diminish if the outside temperature drops. So electrically driven air-to-air heat pumps are most useful in moderate climate (> -10oC). In addition to the air-to-air heat pumps, it is common to use the ground at several feet of depth, or surface water such as a river for the cold reservoir