Heat pump energy sources: ground, water or air

How to choose a heat pump by heat source

The external circuit of a heat pump collects heat from the environment. In most cases, it is a polyethylene pipe loop installed in the ground or in water, with an antifreeze liquid — brine — circulating inside.

The main energy sources for a heat pump are ground, water and outdoor air. Each option has its own advantages, limitations, installation cost and level of seasonal stability.

Ground source heat pump

A ground source heat pump uses heat stored in the earth. This is one of the most stable solutions, because the ground temperature at a certain depth changes very little throughout the year.

The ground accumulates solar energy and is also warmed by heat from deeper layers of the earth. It is always available under the site and can provide heat regardless of current weather conditions.

When the ground is used as a heat source, the external heat pump circuit is installed either in boreholes 50–100 m deep as vertical geothermal probes, or horizontally in the ground at a depth of about 1.2–2.0 m.

Ground temperature becomes much more stable below approximately 5 m, and at depths greater than 15 m it remains almost independent of outdoor air temperature. This is why vertical geothermal probes can provide a high and stable heat pump efficiency coefficient.

The main advantage of a ground source system is stable performance throughout the year. The main drawback is the higher initial cost caused by drilling or excavation work.

Water-to-water heat pump

A water-to-water heat pump uses heat from groundwater or open water sources. This option can be highly efficient if there is a suitable water source nearby and the technical conditions allow it to be used.

Water, like the ground, stores solar energy well. The heat source can be groundwater, a lake, a river or another open body of water.

To use groundwater, an extraction well and a discharge well are usually required. In many cases, this also requires approval from the relevant authorities.

If a nearby lake or river is used as the heat source, the external circuit is placed on the bottom of the water body. Under suitable conditions, this solution can be more efficient than a ground loop, because water temperature can be higher than ground temperature in summer and usually remains above freezing in winter.

A relatively high source temperature improves heat exchange with the external heat pump circuit. This can reduce the size of the circuit, lower material costs and increase the overall efficiency of the system.

The advantage of a water-to-water system is potentially lower installation cost and high efficiency. The disadvantages are dependence on water quality, permits, source stability and the risk of equipment damage.

Air-to-water heat pump

An air-to-water heat pump uses outdoor air as the heat source. It is the simplest option to install, but its efficiency depends strongly on outdoor temperature.

Outdoor air is easy to use as a heat source because it is available in unlimited quantity. Modern air source heat pumps can operate during most of the year.

However, at low outdoor temperatures the efficiency coefficient and heat output of such a pump decrease noticeably. For this reason, air-to-water heat pumps are often used in bivalent heating systems together with an electric boiler, solid fuel boiler or another backup heat source.

Compared with ground and water source systems, air source heat pumps require lower initial investment. However, in Ukrainian climate conditions they are not always suitable as the only heat source without a backup heating system.

Another option is to use warm air from exhaust ventilation or industrial processes. Such systems can be useful for facilities where excess heat is constantly generated, for example bakeries, ceramic production plants and other manufacturing processes.

Which heat source is best for a heat pump?

The most stable heat source is the ground, the easiest to install is outdoor air, and the most efficient under favorable conditions can be water. The final choice depends on the site, budget, heat loss of the house, water availability, drilling options and reliability requirements.

For a private house with year-round living, ground source and air-to-water heat pumps are usually considered first. A ground source system is more expensive at the start but more stable in winter. An air-to-water heat pump is cheaper to install, but it requires careful sizing and a backup heat source.

Read more articles about heat pumps.