G : How Deep?

How many, how deep?

The GSHP (ground source heat pump) specified by the installer or consultant will be defined by its power rating (kW) and COP (Coefficient Of Performance), these two parameters, together with the predicted borehole heat harvesting capability will determine the total effective length of borehole(s) required. This will be expressed in metres, e.g. 1000m.

So, one borehole 1000m deep?, 10 boreholes at 100m deep?, or 20 boreholes at 50m deep?, which is "best"?

This decision depends upon many factors, these are listed below

1) Geology

The UK has many, many different types of geology. The thermal efficiency of boreholes will depend upon

a) the substance drilled through, i.e. sand, gravel, chalk, hard rock, clay, etc, etc.

b) The presence or absence of moisture/water and its movement

c) The drilling method used in terms of its utilisation of the geology encountered.

2) Site Area Limitations

Area restrictions may require fewer deeper boreholes

3) Equipment capacity

Deeper boreholes require more capable drilling rigs

Therefore some GSHP suppliers may specify more, shallower boreholes to allow borehole drillers with less capable, i.e. cheaper  rigs, to quote.

4) System Design Parameters

Deeper boreholes require more higher quality, more resistant probes because the pressure loading on the collector loop increases with depth if the probe is unsupported.

Also, fewer, deeper boreholes increase the circulation energy losses due to friction compared to fewer shallower boreholes

5) Overriding factors

Some sites may exhibit overriding factors, such as 

  1. Mine workings, possible dangers from methane, or other gases
  2. Protected or vulnerable aquifers
  3. Artesian groundwater

Any of the above can limit the dimensions/depth of borehole drilling

Q. How to get the optimimum borehole (array) design?

A. Commission a qualified borehole array designer who will take all of the above factors into consideration.

Synergy will propose this as a matter of course.

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