A geothermal ground loop is a buried pipe system that helps a heat pump move heat between a building and the Earth. A few meters below the surface, ground temperature stays much steadier than air temperature through the year. This makes the ground a useful heat source in winter and a heat sink in summer.
Ground loops matter because they can provide heating and cooling with much less electricity than electric resistance heaters or many air conditioners.
A fluid, usually water mixed with antifreeze, circulates through pipes buried in soil, rock, or groundwater. In heating mode, the fluid absorbs heat from the ground and carries it to a geothermal heat pump, which raises the temperature for use indoors. In cooling mode, the process reverses, and unwanted indoor heat is carried into the cooler ground.
The system does not create energy from nothing, it uses work from a compressor to move thermal energy efficiently.
Key Facts
- Heat naturally flows from higher temperature to lower temperature unless a heat pump uses work to move it the other way.
- Heating mode: the ground loop absorbs heat from Earth and the heat pump delivers it indoors.
- Cooling mode: the heat pump removes heat from the house and the ground loop releases it into Earth.
- Coefficient of performance for heating: COP = Qhot / W, where Qhot is useful heat delivered and W is electrical work input.
- Heat transfer rate can be estimated by Q / t = kAΔT / L for conduction through soil or pipe walls.
- Typical shallow ground temperatures are often near the local annual average air temperature and change slowly with season.
Vocabulary
- Ground loop
- A buried network of pipes that circulates fluid to exchange heat with the ground.
- Geothermal heat pump
- A machine that uses a refrigeration cycle to move heat between a building and the ground loop.
- Heat exchanger
- A device or surface that allows thermal energy to move between two materials without mixing them.
- Coefficient of performance
- A measure of heat pump efficiency equal to useful heating or cooling output divided by electrical work input.
- Thermal conductivity
- A material property that describes how easily heat conducts through a substance.
Common Mistakes to Avoid
- Thinking a geothermal heat pump burns fuel underground is wrong because the ground loop only exchanges heat and the compressor is usually powered by electricity.
- Assuming the ground is always hot is wrong because shallow ground loops use stable moderate temperatures, not volcanic heat.
- Confusing heating mode and cooling mode is wrong because the direction of heat transfer reverses between winter and summer operation.
- Ignoring soil moisture is wrong because wet soil usually transfers heat better than dry loose soil, which can affect loop size and performance.
Practice Questions
- 1 A geothermal heat pump delivers 12,000 W of heat to a house while using 3,000 W of electrical power. What is its heating COP?
- 2 In cooling mode, a system removes 8,000 W of heat from a house and uses 2,000 W of electrical power. How much total heat is released into the ground?
- 3 Explain why a ground loop can heat a house in winter even when the outdoor air is below freezing.