Borehole (Array) Dimensioning
The importance of correct GSHP and associated borehole dimensioning cannot be overstated. It is not unheard of for salesmen to "underestimate" the dimensions of borehole requirements in order to make a quick heat pump sale. Invariably, it is the end user that pays the cost for this in the long term, simply because the energy savings that were predicted, fail to materialise.
Get the procedures/documentation wrong → the installation will not qualify for government support
Get the design/dimensions wrong → the reason for getting the GSHP i.e. energy/cost economy will simply disappear.
The comparison with a conventional oil/gas boiler can be used to illustrate this point in the following scenarios.
Q What happens if the oil/gas boiler you install in your house is too small?
A The house does not get warm enough, The boiler runs continuously. In the extreme, up to 365 days a year, using gas from the mains or delivered oil, an expensive, but practically inexhaustible supply
Q What happens if the Ground Source Heat Pump/Borehole system you install in your house is too small?
A The house does not get warm enough, the heat pump runs continuously, if it has a back up electric immersion heater, this will kick in (very expensively). The system will try to extract more and more heat from the ground around the boreholes. This will lower the ground temperature. The heat energy in the ground close to the borehole loops becomes exhausted. This in turn makes it harder (more expensive) for the pump to extract more heat. A vicious circle is formed. The heat pump starts to operate outside its comfort zone, it loses efficiency, the savings you expected, or were promised, do not materialise. Ultimately the system may fail.
Q What happens if the GSHP is correctly dimensioned, but the associated boreholes are underdimensioned, i.e. too few and/or too shallow?
A Similarly to Scenario 2, the ground will become overcooled as the GSHP overworks the inadequate borehole length, a similar vicious circle forms.
Q What happens if the GSHP is correctly dimensioned, but the associated boreholes are overdimensioned?
A The system will perform perfectly at full capacity, for a longer period. (perhaps longer than you require) There may therefore be an element of wasted money in the borehole installation.
Q What happens if the GSHP is oversized, and the associated boreholes are correspondingly overdimensioned?
A The system will stop and start too often. Like a large car that is used for urban shopping trips, the "m.p.g." or efficiency will not be optimal. There will therefore be an element of wasted money in the installation.
Simply put, an oil or gas boiler can draw upon a practically inexhaustable energy supply via delivered oil or mains gas.
A GSHP can only extract the heat available in the ground which is only very slowly replenished. It is therefore essential not to "overdo it" by trying to extract heat over too long a period or from too small an area/volume of earth.
If longer run times are required, more boreholes are needed.
In different parts of the country the heat extraction/replenishment rates are affected by the differing geological strata. Thermal Conductivity, the scientific name for this property, can vary by a ratio of up to 3:1
This is where the correct, geology based, dimensioning of boreholes and borehole arrays has a critical role to play.
From the above it can be seen that two parameters and one characteristic are of great importance to the long term efficiency of a GSHP/borehole system.
The Parameters are
TEMPERATURES, more accurately temperature differences
TIME, i.e. heating periods, running times/durations
The Characteristic is: GEOLOGY, more accurately, thermal conductivity properties of the geology
These essential elements are reflected in the new MCS guidelines.
The government RHI subsidies will probably (and rightly!) only be awarded to clients who can demonstrate the following:
That they have used an MCS accredited installer to put in the GSHP system, and
That the installer has made use of the correct design procedures for both the sizing of the heat pump and also the dimensioning of the borehole(s) serving the heat pump.
For larger installations, i.e. > 20 kW, a geological survey and professional borehole array design are strongly recommended