A geothermal heat pump, also called a ground-source heat pump, is a machine that heats and cools a building by moving heat between the building and the ground. A few meters below the surface, the ground stays at a more stable temperature than the air above it. This makes the earth a useful heat source in winter and a useful heat sink in summer.
The system uses electricity to move heat rather than to create heat directly, so it can be very efficient.
Key Facts
- A geothermal heat pump moves heat between a building and the ground using a refrigerant cycle and underground fluid loops.
- In heating mode, heat flows from the ground loop into the refrigerant and then into the building.
- In cooling mode, heat flows from the building into the refrigerant and then into the ground.
- Coefficient of performance: COP = useful heat moved / electrical energy input.
- Heat transfer rate can be estimated by Q/t = kA(ΔT)/d for conduction through a material.
- A heat pump does not create most of its delivered heat, it transports thermal energy using work from a compressor.
Vocabulary
- Geothermal heat pump
- A heating and cooling system that transfers heat between a building and the stable-temperature ground.
- Ground loop
- A buried pipe system that circulates fluid to exchange thermal energy with the earth.
- Refrigerant
- A working fluid that absorbs and releases heat as it changes pressure and phase inside a heat pump.
- Compressor
- A device that raises the pressure and temperature of the refrigerant so heat can be delivered to a warmer location.
- Coefficient of performance
- A measure of heat pump efficiency equal to useful heating or cooling energy moved divided by electrical energy used.
Common Mistakes to Avoid
- Thinking geothermal heat pumps make heat from the ground like a furnace, which is wrong because they mainly move existing thermal energy using a refrigeration cycle.
- Confusing geothermal heat pumps with geothermal power plants, which is wrong because heat pumps condition buildings while power plants generate electricity from high-temperature underground heat.
- Assuming the ground loop must be very hot to work, which is wrong because a heat pump can extract useful heat even from cool ground by using compression and phase changes.
- Ignoring cooling mode, which is wrong because the same system can reverse heat flow and dump indoor heat into the ground during warm weather.
Practice Questions
- 1 A geothermal heat pump delivers 18,000 J of heat to a house while using 4,500 J of electrical energy. What is its coefficient of performance?
- 2 A system removes 36,000 J of heat from a room in 12 s. What is the average heat transfer rate in watts?
- 3 Explain why a geothermal heat pump can be more efficient than an electric resistance heater even though both use electricity.