A heat pump is a renewable energy machine that heats or cools a building by moving thermal energy instead of making heat by burning fuel. In heating mode, it takes heat from outside air, ground, or water and delivers it indoors, even when the outside feels cold. This matters because moving heat usually requires much less electrical energy than producing the same amount of heat with resistance heaters.
Heat pumps can reduce energy use, operating cost, and carbon emissions when powered by clean electricity.
A heat pump uses a refrigerant that changes pressure and temperature as it circulates through a closed loop. The refrigerant absorbs heat in an evaporator, is compressed to a higher temperature by a compressor, releases heat in a condenser, and then expands through a valve to cool down again. The same machine can often reverse the cycle to provide air conditioning in summer.
Its performance is measured by coefficient of performance, which compares useful heat moved to electrical work supplied.
Key Facts
- A heat pump transfers heat from a colder region to a warmer region using electrical work.
- Coefficient of performance for heating: COP_heating = Q_hot / W_in.
- Energy balance for a heat pump: Q_hot = Q_cold + W_in.
- A COP of 3 means the system delivers 3 J of heat indoors for every 1 J of electrical energy used.
- Main parts of a vapor compression heat pump: evaporator, compressor, condenser, and expansion valve.
- Heat pumps are most efficient when the temperature difference between indoors and outdoors is small.
Vocabulary
- Heat pump
- A device that uses work, usually electrical energy, to move heat from one place to another.
- Refrigerant
- A fluid that circulates through a heat pump and absorbs or releases heat as it changes pressure and phase.
- Compressor
- The component that raises the pressure and temperature of the refrigerant vapor.
- Condenser
- The heat exchanger where hot refrigerant releases heat and usually changes from a gas to a liquid.
- Coefficient of performance
- A measure of heat pump efficiency equal to useful heat transferred divided by work input.
Common Mistakes to Avoid
- Thinking a heat pump creates heat directly is wrong because it mainly moves existing thermal energy from one region to another.
- Assuming cold outdoor air contains no heat is wrong because air above absolute zero still has thermal energy that can be extracted.
- Confusing COP with percent efficiency is wrong because COP can be greater than 1 since it counts heat moved plus electrical work input.
- Ignoring temperature difference is wrong because a heat pump must work harder and loses performance when it moves heat across a larger temperature gap.
Practice Questions
- 1 A heat pump delivers 9000 J of heat to a room while using 3000 J of electrical energy. What is its heating COP?
- 2 A heat pump has COP_heating = 4.0 and uses 2.5 kWh of electrical energy. How much heat energy does it deliver indoors in kWh?
- 3 Explain why a heat pump can have a COP greater than 1 without violating conservation of energy.