Boreholes or horizontal arrays?

The answer to whether the development should use boreholes or horizontal ground arrays depends on a number of factors, mainly the amount of ground available and the load required from the building.

For small developments, where there is sufficient land, digging horizontal trenches generally is the lower cost option and is generally done at the same time as the building’s ground works (because the ground excavators are on-site). Horizontal ground arrays are generally dug in trenches 30, 40 or 50m long and separated by 5m between centres. As a rule of thumb every 10m of slinky will provide 1kW of heat. If there is insufficient land available then the only option would be to use boreholes.

Boreholes require specialist equipment and operators and hence tend to be more expensive to install, however due to economies of scale on larger developments (above 100kW) the cost becomes comparable to digging trenches. The amount of energy that can be obtained from a borehole depends on the material that the hole is drilled into. As a rough rule of thumb 3-5 kW of energy can be obtained from a 70-100m deep borehole. Each borehole needs to be separated by 5-6m between centres. The borehole has a single (or double) loop of pipe inserted and is then backfilled with a thermal grout to provide good contact with the surrounding ground.

Ideally in any development above 100kW a Thermal Response test should be conducted, which involves drilling a test borehole and measuring how much energy can be obtained from it.

Kensa Engineering are happy to advise which system would be suitable for the application. Please send your site and building drawings to us for an appraisal.