A borehole basically consists of a hole drilled between 60 and 100m deep vertically down. Generally the borehole diameter is around 110 – 145mm, but this diameter depends on the type of machine being used to drill the borehole and the diameter of the borehole pipe (usually 32-40mm). The drilling rigs used by drillers come in many shapes and sizes some small enough to operate and gain access to small gardens others designed for larger commercial projects.
The first few meters of a borehole is generally sleeved with a casing to avoid the sides collapsing. The depth of this casing is dependent on the material that the borehole is sunk into and the depth of soil.
Boreholes are generally placed at 5-6m centres however for large commercial projects the interference from one borehole to another may well need to be calculated to ensure that adequate spacing or sufficient depth is provided.
A loop of pipe (usually PE100 HDPE or Pex pipe) is inserted within the hole. Normally in the UK a single loop is used. It is possible to use a twin loop or duplex system, to try and extract more energy. However for a twin loop system a larger diameter hole is required and the energy yield from the borehole only increases by an approximate factor of 1.25. (This is dependent on the hole and pipe diameter, distance from the next borehole, how the pipe is inserted, grouting, etc). The pipe is generally either filled with water or weighted at the end to aid with the insertion.
Along with the borehole pipe a small tremie pipe (25– 40mm) is also inserted attached to the borehole pipe. The tremie pipe is used to fill the borehole completely with thermal grout and is withdrawn as the grout is injected. The thermal grout provides an enhanced thermal path to allow the energy within the ground to be absorbed by the fluid circulating around the borehole pipe. The driller will take responsibility for grouting the hole using specialist pumping equipment.
The drilling contractor will perform a pressure test, cap the plastic ground array pipe and issue a certificate before leaving site.
For larger commercial projects, (nominally over 100kW) guidelines tend to overestimate the number of required boreholes and it is advisable that a thermal response test or TRT is carried out on a test and representative borehole. A thermal geologist can combine the results from a TRT, with the heating and cooling profile of the building, to calculate the type, depth, number and spacing of boreholes. The cost of completing a TRT is generally recovered in the reduction in the number of boreholes required.