Rock: Boreholes

Boreholes are the collectors used to extract heat energy from rock for ground source heat pumps. They are a very discreet and compact solution when space saving and minimal ground disruption is a priority, and ideal for heat network or district heating schemes.

Hot rocks

Solar energy stored in surface soil dissipates through the many rock layers beneath our feet to form a very stable heat source. Depending on the make-up of the geology, this can provide an excellent heat source for ground source heat pumps.

Boreholes are the collectors used to extract heat energy from rock for ground source heat pumps. They are a very discreet and compact solution when space saving and minimal ground disruption is a priority.

Typically just one borehole is required per dwelling depending on the underlying ground conditions. Boreholes are the most expensive collector but become especially economically viable on large commercial projects, clusters of properties connected via a micro heat network, and sites with heat loads above 100kW due to the costs involved of assembling the drilling rig and its speed when on site.

Typically a 75-100m deep borehole will provide 3-5kW of extractable heat, based on 1800 running hours a year.

Geology

“I will liken him to a wise man, he who built his house on rock.”

As in the Parable of the Wise and Foolish Builder, a house built on rock is far more beneficial for a borehole than a house built on sand. Geology plays a significant role in the depth and efficiency of rock boreholes due to their conductivity and thermal mass.

Kensa can recommend companies that can assess your geology and specifications for a borehole.

How it works

A drilling contractor is employed to drill a borehole approximately 90m deep x 150mm wide, depending on ground conditions, located within approx 20m of the property.  Straight pipe is inserted into the borehole, and connected via a trench to the ground source heat pump.

Typically one borehole is required per property, with multiple boreholes being used for larger installations with greater heat demands or heat network schemes.

Borehole process

Borehole drilling

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.

Borehole pipework

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.

Testing

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.

Connection of boreholes

If more than one borehole is required, the pipes should be connected in such a way that equal distribution of flow in  the different channels is secured. Manifolds can be at the  building, or the pipes can be connected in trenches in the field.

To avoid any joints, and to eliminate the need for any electro-fusion welding (which requires specialist equipment and attracts a significant cost), it is sensible to use a specially extended borehole probes. This eliminates the need for any electrofusion joints, which are generally beyond the scope of most heating contractors. This pipe is simply laid in a trench, which is required between the top of the borehole, and the manifold on the side of the building. The groundworking crew can provide the trenching since their prices will generally be more competitive than any drilling contractor.

The design of boreholes is a complicated issue and the above is simply meant as an overview. Expert advice should be taken regarding sizing and placement and consideration of the buildings heating profile should be taken into account. Any figures quoted within this document are for an initial guide only and Kensa cannot be held responsible for any sizing based on this data.

 

Facts at a glance:

Borehole performance

This is very much dependant on the geology, heating distribution system, building heating profile, location, etc.
As a guide a 75—100m bore-hole will provide 3—5 kW of extractable heat.

Borehole design

The design of boreholes for small, individual applications can be done with tables, empirical values and guidelines.
For larger commercial projects (>100kW) then a thermal response test is required to obtain accurate sizing.

Borehole make up

Boreholes generally consist of a 60 to 100m deep hole, with a loop of PE100 pipe encased in a bentonite thermal grout.

Borehole spacing

Boreholes should be spaced at 5 to 6 metres between centres to avoid interference between boreholes.

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