Geothermal energy is a renewable energy source that can be used to heat and cool buildings. In HVAC systems, geothermal energy is used to transfer heat from the ground into the building. This is done by using a series of pipes that are buried in the ground and filled with a liquid. The liquid absorbs heat from the ground and is then pumped into the building, where it is used to heat the air. The same process is used in reverse to cool the building, with the liquid absorbing heat from the building and transferring it back into the ground. This process is highly efficient and can significantly reduce energy costs.