Advice on boreholes and ground array alternatives on space-restricted sites

Installing a ground source heat pump on a site with limited land area brings its own particular challenges. Guy Cashmore offers advice on boreholes and ground array alternatives on space-restricted sites as well as guidance on such installation methods.
Ground arrays for ground source heat pumps can take many different forms, but the two most commonly used in the UK are vertical drilled boreholes (with a U tube in them) and horizontal loops (usually referred to as ‘slinkies’).
Both have advantages and disadvantages. Drilled systems are usually the most expensive (especially on smaller jobs) and getting physical access for the drilling rig can sometimes be a challenge, but boreholes can usually be installed in any type of ground, and the smallest of gardens can usually meet the heat load of the property – the depth of the hole(s) is simply varied to suit the heat requirement.
Boreholes can also be sited under car parks and even under the buildings themselves as unlike horizontal arrays they are far less affected by what’s directly above them (see Case Study panel).
Horizontal systems, on the other hand, are usually cheap and easy to install because they don’t need a specialist drilling contractor – often this work can be done by a digger that is already on site doing other work – but they must have sufficient exposed land area to collect and deliver the energy requirements of the building.
Adding more loops in a fixed area of land doesn’t increase the amount of energy available because the limiting factor is the land not the pipes within it.
Critical calculation
What becomes obvious is that an accurate heat loss calculation for the building is absolutely essential when designing any ground array – get this part wrong and everything about the system is likely to be wrong.
Other factors also affect the ground array sizing – for example, the water temperature the heating distribution system runs at changes the operating efficiency (CoP) of the heat pump.
This means that a system using underfloor heating will normally need a slightly larger ground array than one using radiators in an identical building. The efficiency of the heat pump itself also has an effect.
One of the ironies of using a highly efficient heat pump (such as the new Kensa Evo) is that the ground array size needs to increase slightly. According to the laws of physics, if less energy is coming from electricity then more has to come from the ground array in order to attain a given heat output.
Think differently
Surprisingly often, calculations for a horizontal array will show that the available garden area isn’t quite big enough to meet the load. The usual solution for this situation is to go down the borehole route, but this will typically add several thousand pounds to the overall cost. An alternative worth investigating is spending some or all of this money on insulation and other measures to reduce the load.
By reducing the load, the available garden area may well become viable as the heat source, and, at the same time, it may also be possible to specify a smaller heat pump, further offsetting the insulation costs. By going this route, the installer can also be certain that the annual running costs will be significantly lower for the occupier, which has to be good for everyone in the long run.
Common ground
Ground arrays don’t have to follow the usual ‘one heat pump, one array’ route – although this is probably only applicable to blocks and social housing type situations.
Ground arrays can be shared between multiple properties in a similar fashion to district heating but because the flow is cold, the system doesn’t suffer any heat losses. Equally, because each property has its own heat pump, the system doesn’t need any metering or billing arrangements.
Also, the boreholes or slinkies can be sited where practical rather than individually in gardens. Another advantage of sharing ground arrays is that as the system grows in size, a degree of diversification can be applied. This means that the total amount of ground array can be reduced a little compared to individual systems.
Less commonly, other sources of free energy can sometimes be found – existing water wells, old mines, natural springs, river water and even sea water all feature regularly in installation enquiries.
While these will never be mainstream, they do crop up and the best ones can make highly efficient, low-cost energy sources.
When using water as the source, a secondary heat exchanger is used so that the heat pump itself remains on a closed loop containing glycol.
This secondary exchanger can take several forms – commonly loops of pipe or metal panels immersed in the source water or a gasketed plate heat exchanger (to allow for cleaning) with the water piped to it. Choice of pipework material needs to take into account the water being used, although plastic pipes and stainless steel plates are good for most.