Air, ground or water: which heat source is most economical for a heat pump?

Ground-source heat pumps are often described as more efficient, and air-source heat pumps as simpler. But the client is not just choosing a refrigeration machine. They are choosing a heat source together with its installation, pumps and maintenance.

A favourable source temperature is useful only if that source is available in sufficient quantity and at an acceptable cost.

Air: no boreholes, but variable conditions

An air source does not require a ground loop. However, its temperature changes with the weather, and frosting and defrosting must be accounted for in outdoor unit operation. Capacity and efficiency must be checked at the relevant air and water temperatures. (Panasonic — Aquarea T-CAP M Series: technical specifications) (Energies — field study of heat pump defrosting algorithms)

At the concept stage, we would check the installation location, service access and design winter operating conditions. Equipment should not be undersized simply because its most favourable rated operating point appears sufficient.

Ground: a more stable source, but not an unlimited reserve

Ground-source systems use an underground loop as a heat source in winter and, with a suitable configuration, as a heat sink in summer. Ground temperature is more stable than outdoor air temperature, but the loop must be sized for the building’s load. (US DOE — ground-source heat pumps)

Borehole depth or horizontal loop area should not be determined solely by the unit’s capacity. Ground properties and the duration of heat extraction matter. DOE specifically highlights thermal conductivity, source temperature stability and accurate ground loop sizing. (US DOE FEMP — source selection for ground- and water-source heat pumps)

We would also check the annual balance: whether heat is rejected in summer, which loads operate continuously and how the source will change over long-term operation.

Water: more than just a borehole is needed

For an open-loop water-source system, temperature, sustainable yield and water chemistry matter. These are explicit inputs for water-source selection. (US DOE FEMP — source selection for ground- and water-source heat pumps)

Having a domestic water borehole does not establish that it is suitable for continuous abstraction for a heat pump. Where to return the water and how to ensure permitted use of the source must also be resolved.

Let us calculate an illustrative operating condition. The heat pump delivers 20 kW of heat, while the compressor consumes 5 kW. In a simplified steady-state balance with no other heat flows, the heat required from the source is:

Q₍source₎=20-5=15 kW

If the permitted temperature drop in the source water is 3 K, the required flow rate is approximately:

V̇=(15) / (1,163×3)≈4,3 m³/h

The coefficient 1,163 is used here for water under normal conditions. This is a substantial continuous flow, not a brief domestic draw-off.

Auxiliary pumps consume energy too

If supplying water in this example requires an additional 0,8 kW, the ratio falls when the system boundary is expanded:

COP₍unit₎=20/5=4;

COP₍including water supply₎=20/(5+0,8)≈3,45

This is not an argument against a water source. It is an argument for an honest energy balance. All auxiliary energy consumption must be included, rather than comparing one unit’s rated figures with another system’s total consumption.

At the first stage, we would compare several concepts for the same building demand: capital investment, seasonal energy use, maintenance and site constraints.

The best source is not the one with the highest temperature figure. It is the one that delivers a reliable and economically justified result for this particular building.