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.
Understanding Renewable Energy Machines: Geothermal Heat Pumps
The working fluid inside the heat pump follows a repeating path. In one part of the path, it is a cold liquid at low pressure. It passes through a heat exchanger where it absorbs energy from water or antifreeze flowing through buried pipes.
The fluid then evaporates into a gas. A compressor squeezes this gas, raising its pressure and temperature. The hot gas releases heat through another heat exchanger connected to the building air system or a water heating system.
After releasing energy, the refrigerant passes through an expansion device. Its pressure falls, it becomes cold again, and the cycle repeats. This is the same basic idea used by refrigerators, but the useful heat is delivered to rooms instead of discarded outdoors.
The underground pipe system must be matched to the site. Horizontal loops are placed in long trenches and need plenty of open land. Vertical loops go deep into boreholes, so they suit smaller plots but drilling can cost more.
Some systems use a nearby pond, while others pump groundwater through a heat exchanger before returning it safely underground. Pipe length matters because the ground must be able to supply or absorb heat steadily.
If a loop is too small, the nearby soil temperature can change too much over a season. Soil type, moisture, rock layers, groundwater movement, and local climate all affect the design.
A useful way to judge performance is the coefficient of performance. It compares the heat delivered to the electricity used by the equipment. If a system moves four units of heat while using one unit of electrical energy, its heating coefficient of performance is four.
This does not break conservation of energy. The electrical input runs the compressor, pumps, and controls, while most of the delivered energy was already present as low temperature thermal energy.
Performance usually improves when the temperature difference is smaller. A mild ground temperature is easier to use than very cold winter air, so ground systems can avoid some of the efficiency drop seen in outdoor air heat pumps.
Students can connect this topic to everyday heating bills, home insulation, and the temperature of surfaces. A heat pump works best when a building loses heat slowly. Good insulation, sealed gaps, and low temperature heating systems reduce the demand placed on the machine.
Underfloor heating and large radiators can be helpful because they provide warmth without requiring extremely hot water. During cooling, the same ideas matter because shade, ventilation, and insulation reduce unwanted heat entering the building. When studying heat pumps, keep separate the ideas of heat, temperature, and energy.
Temperature tells how hot something is. Heat is energy transferred because of a temperature difference. The compressor uses electrical work to direct that transfer in a useful direction.
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.