A ground-source heat pump uses the earth or ground water or other water source (lake, river or well) as the source of heat in the winter, and as the "sink" for heat removed from the home in the summer. For this reason, ground-source heat pump systems are known as earth-energy systems (EESs). Heat is removed from the earth through a liquid, such as ground water or an antifreeze solution, upgraded by the heat pump, and transferred to indoor air. The process is reversed during the summer: heat is extracted from indoor air and transferred to the earth through the ground water or antifreeze solution. A direct-expansion (DX) earth-energy system uses refrigerant in the ground-heat exchanger instead of an antifreeze solution.
Earth-energy systems are available for use with either forced-air or hydronic heating systems. They can also be designed and installed to provide heating only, heating with passive cooling, or heating with active cooling. Heating-only systems do not provide cooling. Passive-cooling systems provide cooling by pumping cool water or antifreeze through the system without using the heat pump to assist the process.
The following description is for active cooling systems.
All EESs have two parts:
- a circuit of underground piping outside the house unless it is an open system
- a heat pump unit inside the house
Unlike air-source heat pumps, which have one heat exchanger (and frequently the compressor) located outside, the entire ground-source heat pump unit is inside the house.
The outdoor piping system can be either an open system or closed loop. An open system takes advantage of the heat retained in an underground body of water or a lake or river. The water is drawn up through a well directly to the heat exchanger, where its heat is extracted. The water is discharged either to an above-ground body of water, such as a stream or pond, or back to the underground water body through a separate well.
Closed-loop systems collect heat from the ground by means of a continuous loop of piping buried underground. An antifreeze solution (or, in a direct-expansion earth-energy system, a refrigerant) is chilled by the heat pump's refrigeration system to several degrees colder than the outside soil. It then circulates through the piping, absorbing heat from the surrounding soil.
In the heating cycle, the ground water, the antifreeze mixture, or refrigerant is brought back to the heat pump unit inside the house. It then passes through the refrigerant-filled primary heat exchanger for ground water or antifreeze mixture systems. In direct-expansion systems the refrigerant enters the compressor directly, with no intermediate heat exchanger.
The heat is transferred to the refrigerant, which boils to become a low-temperature vapour. In an open system, the ground water is then pumped back out and discharged into a pond or down a well. In a closed-loop system, the anti-freeze mixture or refrigerant is pumped back out to the underground piping system to be heated again.
The reversing valve sends the refrigerant vapour to the compressor. The vapour is then compressed, reducing its volume and heating it up.
Finally, the reversing valve sends the now-hot gas to the condenser coil, where it gives up its heat. Air is blown across the coil, heated, and then forced through the ducting system to heat the home. Having given up its heat, the refrigerant passes through the expansion device, where its temperature and pressure are dropped further before it returns to the first heat exchanger, or to the ground in a direct-expansion system, to begin the cycle again.
In some EESs, a heat exchanger, sometimes called a desuperheater, takes heat from the hot refrigerant after it leaves the compressor. Water from the home's water heater is pumped through a coil ahead of the condenser coil so that some of the heat that would have been dissipated at the condenser is used to heat water. Excess heat is always available in the cooling mode; it is also available in the heating mode during mild weather when the heat pump is above the balance point and not working to full capacity. Other EESs heat domestic hot water when it is required.
Hot water heating is practical with EESs because the compressor is located inside. Because EESs have relatively constant heating capacity, they generally have many hours’ more heating capacity than is required for space heating.
The cooling cycle is basically the reverse of the heating cycle. The direction of the refrigerant flow is changed by the reversing valve. The refrigerant picks up heat from the house air and transfers it directly in direct-expansion systems or to the ground water or antifreeze mixture. The heat is then pumped outside, into a water body or return well (in the case of an open system), or into the underground piping (in the case of a closed-loop system). Once again, some of this excess heat can be used to preheat domestic hot water.
Unlike air-source heat pumps, EESs do not require a defrost cycle. Temperatures underground are much more stable than air temperatures, and the heat pump unit itself is located inside; therefore, the same problems with frost do not arise.
Earth-energy systems have three main components:
- the heat pump unit
- the liquid heat exchange medium (open system or closed loop)
- the air delivery system (ductwork)
Ground-source heat pumps are designed in different ways. Self-contained units combine the blower, compressor, heat exchanger, and condenser coil in a single cabinet. Split systems allow the coil to be added to a forced-air furnace and use the existing blower and furnace.
Supplementary Heating Systems
Most heat pump installations require a supplementary heating system. Supplementary heat can be supplied by any type of heating system, as long as it can be activated by the thermostat controlling the heat pump. Many new ground-source heat pumps use duct heaters to supply auxiliary heat beyond the rated capacity of the EES.