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Borehole quality / thermal efficiency (Or Not!)

Almost all renewable energy experts and heat pump installers know that the performance of closed loop borehole arrays is very much dependent upon the geology of the site.

The characteristics of the strata drilled through, the presence and possible movement of water, have a very significant influence upon the transfer speed of heat into the borehole and collector loop.

Much less well known is that the drilling speed and methodology employed can also determine the thermal performance of the borehole to a great extent.

A fast and forced drilling method which smears and compacts the drilling shavings into the borehole walls, rather than expelling them to the surface, will effectively manufacture an insulating sheath around the column of grout. This impervious layer will significantly obstruct the transfer of heat from the surrounding strata into the borehole. This effect is particularly prevalent in substances which lend themselves to plastic forming, good examples are common varieties of clay and chalk

Some drillers have evolved such drilling practices which greatly enhance drilling speed, reduce spoil (returns to the surface) and thus improve initial drilling cost effectiveness by producing more borehole metres per day and reducing spoil handling costs. They are therefore able to offer drilling quotations which are very, very competitive.

The downside is significant. Unless the borehole array designer has allowed for this effect, i.e. increased the specified borehole meterage, the boreholes drilled will not provide the heat transfer which is indicated by the site geology.

This is not a theory. Thermal Response Testing is able to measure and quantify this. It can be reliably shown that the effect of forced drilling can reduce thermal borehole performance by up to 50%

Therefore, it is easily possible that a borehole quote which is initially 15% cheaper, can lead to a borehole array which is 30% or more less productive than one drilled using best practice. This will, at the very least impair the efficiency of the GSHP installation (reducing the COP) and in the worst case can lead to medium term deterioration and failure of the GSHP system performance.

Boreholes drilled by Synergy have been frequently TRT tested and consisitently demonstrated to fully utilise the thermal potential of the site geology.

How can an ultra cheap quote still be very profitable to the salesman/contractor?

  1. If too few borehole metres are specified in depth and/or number of boreholes, either through ignorance of design procedures, or cost pressures.
  2. If the borehole array is incorrectly dimensioned in terms of borehole spacing and or connections
  3. If the holes are drilled without regard to thermal efficiency, as described above
  4. If inferior quality loops are used, even home made ones
  5. If inferior quality grout is used, even builders yard materials/home made mixes
  6. If the holes are drilled short, i.e. not to specified depth
  7. If the loops are not inserted to the design depth