The coefficient of performance, also called as the COP, of the heat pump is defined as the ratio of the desired effect from the heat pump to the power consumed by the heat pump. The desired effect from the heat pump is producing the heat inside the room and the power consumed by the heat pump is the power consumed by the compressor, hence COP is also defined as the ratio of heat produced inside the room to the power consumed by the compressor.
Let us suppose:
H1 – enthalpy of the refrigerant leaving the evaporator and entering the compressor
H2- Enthalpy of refrigerant leaving the compressor and entering the condenser
H3- Enthalpy of refrigerant leaving the condenser and entering the expansion valve
H4 – Enthalpy of the refrigerant leaving the expansion valve and entering the evaporator
During the expansion of the refrigerant in the expansion valve, its enthalpy remains constant, hence H3 = H4.
Coefficient of Performance or COP = Heat produced inside the room / Power consumed by the compressor
Heat produced inside the room is the heat absorbed from the condenser = H2 – H3, since the enthalpy of the refrigerant entering the condenser is more than the enthalpy of refrigerant leaving the condenser.
The power consumed by the compressor = H2 – H1
Hence COP of the Heat Pump = H2 – H3 / H2 – H1