Heat Pumps
A vapor-compression heat pump moves heat from a low-temperature source to a higher-temperature sink. In heating mode, the outdoor coil acts as the evaporator, absorbing Q_L; the compressor raises the refrigerant pressure and temperature; the indoor coil acts as the condenser, releasing Q_H; and the expansion valve reduces refrigerant pressure before it returns outdoors. A reversing valve can interchange the evaporator and condenser roles for cooling. The steady-flow energy balance is Q_H = Q_L + W, where W is compressor work. Consequently, heating COP can exceed 1 because both absorbed heat and input work are delivered indoors. Actual COP decreases as the temperature lift between source and sink increases. Frost accumulation on an air-source outdoor coil also degrades heating performance and requires periodic defrosting.
Knowledge Check
1.What is the normal refrigerant flow sequence in a simple vapor-compression heat pump?
2.A heat pump absorbs 8 kW from the outdoor source while the compressor uses 2 kW. How much heat is delivered indoors?
3.If a heat pump delivers 10 kW of heat using 2 kW of electrical input, what is its heating COP?
4.A reversible heat pump has a cooling COP of 4. What is its heating COP for the same operating temperatures and ideal energy balance?
5.Why can a heat pump have a heating COP greater than 1 without violating thermodynamics?
6.What is the primary function of the expansion valve in a heat pump?
7.What does the reversing valve do in a reversible heat pump?
8.What generally happens to heat-pump COP when the temperature lift between source and sink increases?
9.For an ideal heat pump operating between T_H = 320 K and T_L = 280 K, what is the maximum theoretical heating COP?
10.Why does frost formation reduce the heating capacity of an air-source heat pump?