The five types of heat-pump system

An explanation of each of the five types of heat-pump system, how each works and the factors to consider when choosing a system to install.

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There are five types of heat-pump system, which differ according to the heat source.

Vertical closed loop boreholes

This approach is very effective and requires minimal land. Boreholes (usually around 150 mm in diameter) are drilled vertically, typically up to 150 metres deep. Collector pipes filled with glycol liquid are inserted as a ‘closed loop’ into the borehole, which is backfilled with a thermal grout. The loops are connected via flow and return to a manifold chamber and the heat pump.

Drilling boreholes can be costly, so this type of heat-pump installation is used when there is no water source available or the amount of land available is insufficient or unsuitable.

Pros	Cons
Reliable. Boreholes sized to meet system requirements No planning, EA approval or licences required High efficiency (though less than water-source heat pump) Minimal maintenance Can also provide cooling as the chilled water returning to the boreholes can be intercepted and used, which also replenishes the boreholes so that fewer are required	Drilling can be expensive, typically £35/m Higher capital cost Most horticultural applications will require many vertical boreholes. At 150 m deep, 13 boreholes in 550 m³ would be needed to yield 100 kW, approximately costing around £70K

Pros

Cons

Reliable. Boreholes sized to meet system requirements

No planning, EA approval or licences required

High efficiency (though less than water-source heat pump)

Minimal maintenance

Can also provide cooling as the chilled water returning to the boreholes can be intercepted and used, which also replenishes the boreholes so that fewer are required

Drilling can be expensive, typically £35/m

Higher capital cost

Most horticultural applications will require many vertical boreholes. At 150 m deep, 13 boreholes in 550 m³ would be needed to yield 100 kW, approximately costing around £70K

Open loop boreholes

Open loop boreholes can be utilised when there is a reliable, consistent subterranean supply of ground water. Establishing the suitability of a water supply accurately requires the drilling of a test borehole, but an initial desktop study can be undertaken by a hydrogeologist, usually at low cost.

In an open loop system, water taken from a borehole provides the heat. The water can then be used for other purposes or returned to the ground in a second borehole that is some distance from the first.

Water abstraction

Abstracting water for use in an open loop heat pump requires consent and a licence from the Environment Agency. Water is being returned to the ground after use. Obtaining approvals and licences can be time-consuming – sometimes taking up to 12 months – although non-consumptive licences tend to be easier and quicker. Temporary licences may be obtained while the application is being progressed. However, obtaining a temporary licence is no guarantee of full approval and drilling costs will be significant, so proceeding without approval is risky.

Desk surveys are not a guarantee that water will be available in sufficient quantity. A desk survey will provide a strong indication of the available water supply and your consultant hydrogeologist should advise you of any potential issues, as well as the degree of certainty associated with your land. It is advisable to make a test drilling to measure the actual water flow before committing to the full heat-pump installation.

Nurseries or holdings with an existing borehole for irrigation water can consider using them as a heat source, thereby saving additional drilling costs, albeit the amount of heat may be limited unless the water quantity is sufficiently large or more water can be made available.

Pros

Cons

Cheaper than closed loop boreholes

Very efficient

Water temperature stable year round

Minimal disruption to site

Drilling can be expensive, typically £35/m

Water extraction licences required

Filtration set required, specified by the Environment Agency and potentially expensive

Test borehole required with no guarantee of performance or grant of licence

Example of a lake/river water source for a closed loop heat pump.

Image courtesy of NFU Energy.

Horizontal loop ground source

This approach is more cost-effective than a vertical loop system. A horizontal loop system can be a good solution when sufficient land is available. Trenches are excavated to a depth of 1.2 m and collector pipes laid. As an approximate guide, for every acre of glasshouse requiring a frost-free environment (heating to 6˚C), approximately one acre of land will be needed for the collectors.

Pros	Cons
Reliable Groundworks likely to be considerably cheaper than a vertical closed loop borehole system for equivalent heat output	Requires a relatively large land footprint. Typically, 1,000 m²to provide 12˚C temperature lift for each 1,000 m²glasshouse Potential damage to buried collectors by agricultural machinery unless sufficiently deep Restriction of future groundworks on the land

Pros

Cons

Reliable

Groundworks likely to be considerably cheaper than a vertical closed loop borehole system for equivalent heat output

Requires a relatively large land footprint. Typically, 1,000 m²to provide 12˚C temperature lift for each 1,000 m²glasshouse

Potential damage to buried collectors by agricultural machinery unless sufficiently deep

Restriction of future groundworks on the land

Water-source closed loop heat pump – lake/river

Reservoirs, lakes or even rivers can be used as closed loop water sources and, if these are available, are cost-effective solutions. The collectors are placed in the water in stacked coils and tied down. The collectors will function even when the surface water has frozen.

The labour costs associated with this are less than both the ground-source options and there is less disturbance during the installation.

Pros	Cons
Cheaper than closed loop boreholes No licences required for lakes High efficiencies Very reliable Little maintenance	Lake or body of water needed nearby Large lake required, at least 1.5 m deep Waterway approval may be needed for rivers Boats/debris could damage pipework

Pros

Cons

Cheaper than closed loop boreholes

No licences required for lakes

High efficiencies

Very reliable

Little maintenance

Lake or body of water needed nearby

Large lake required, at least 1.5 m deep

Waterway approval may be needed for rivers

Boats/debris could damage pipework

Example of a lake/river water source for a closed loop heat pump.

Image courtesy of NFU Energy.

Air source heat pump (ASHP)

As the name suggests, air-source heat pumps use neither ground or water as the heat source but the ambient air around them. However, the technology is identical after the heat has been collected. The RHI rate for ASHP is considerably less than for ground-source and water-source heat pumps and while they could be a practical alternative to fossil-fuel heating for packing sheds, offices and other relatively small spaces, the size and low return on investment for an ASHP of sufficient capacity for heating greenhouses will be prohibitive.

Pros	Cons
Lowest capital cost Reliable Can be used for heating and cooling	Available capacities generally insufficient for heating greenhouses Can sometimes be noisy Considerably lower RHI tariff

Pros

Cons

Lowest capital cost

Reliable

Can be used for heating and cooling

Available capacities generally insufficient for heating greenhouses

Can sometimes be noisy

Considerably lower RHI tariff