The principles of closed loop boreholes
The benefits of using a closed loop borehole in a ground source heat pump installation include reduced maintenance and no need for filtration units.
As with all closed loop systems, the principle of a closed loop borehole is that at no point does the fluid in the borehole leave the system, it is continually cycled through the closed system.
The benefit of using a closed loop borehole over an open loop borehole includes a reduced risk of freezing within the heat pump and reduced maintenance, as there is no need for any filtration units.Read more
Designing the borehole
A geological survey by the driller should provide an indication of the type of material that the borehole is to be drilled into. This material can determine the design of the borefield particularly in large commercial projects. For example a borehole in loose stones has an extraction rate of approx. 20 W/m and granite of 55-70 W/m. The survey should also indicate whether there are any mine workings or aquifers present.Read more
Sizing of boreholes
The design of boreholes for small, individual applications can be done with tables, empirical values and guidelines. A popular parameter to calculate the required length of borehole heat ex-changers is the specific heat extraction, expressed in Watt per meter borehole length. Typical values range between 40-70 W/m, dependent upon geology (thermal conductivity), annual hours of heat pump operation, number of neighbouring boreholes, location, etc. Typically a 75-100m deep borehole will provide 3-5kW of extractable heat, based on 1800 running hours a year.Read more
This element of the project is quite distinct from the installation of the ground source heat pump, underfloor heating and DHW cylinder. It should be considered as contractually equivalent to the installation of the gas main, which is handled separately from the fitting of any boiler. Care must be taken to ensure the borehole ‘tails’ are left in a suitable configuration for the M & E contractor.Read more