Geothermal Heat Pump - Ground Heat Exchanger

Ground Heat Exchanger

Heat pumps provide winter heating by extracting heat from a source and transferring it into a building. Heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground source heat pump uses the top layer of the earth's crust as a source of heat, thus taking advantage of its seasonally moderated temperature.

In the summer, the process can be reversed so the heat pump extracts heat from the building and transfers it to the ground. Transferring heat to a cooler space takes less energy, so the cooling efficiency of the heat pump gains benefits from the lower ground temperature.

Ground source heat pumps employ a heat exchanger in contact with the ground or groundwater to extract or dissipate heat. This component accounts for a fifth up to half of the total system cost, and would be the most cumbersome part to repair or replace. Currently they are often designed to have minimal installation cost, while just not freezing up the ground. Even marginally under sizing this component leads to reduced energy efficiency and increased electricity bills. In the longer term this can lead to freezing up of the ground, resulting in spiralling electricity costs and the heat pump automatically stopping to prevent frost damage to the condenser. Correct sizing this component is highly beneficial in the long term: the energy efficiency of the system improves with roughly 4% for every degree Celsius that is won through correct sizing. Spending extra money on correct sizing of the ground source heat exchanger is more economical than spending additional extra money on a better heat pump.

Shallow 3–8 feet (1 to 3 metres) horizontal heat exchangers experience seasonal temperature cycles due to solar gains and transmission losses to ambient air at ground level. These temperature cycles lag behind the seasons because of thermal inertia, so the heat exchanger will harvest heat deposited by the sun several months earlier, while being weighed down in late winter and spring, due to accumulated winter cold. Deep vertical systems 100–500 feet (33 to 160 metres) rely on migration of heat from surrounding geology, unless they are recharged annually by solar recharge of the ground or exhaust heat from air conditioning systems.

Several major design options are available for these, which are classified by fluid and layout. Direct exchange systems circulate refrigerant underground, closed loop systems use a mixture of anti-freeze and water, and open loop systems use natural groundwater.