Ground arrays are a critical part of any heat pump system and it is vitally important that these are sized and installed correctly. Incorrectly sized or installed ground arrays can result in higher heating bills and ultimately the ground can run out of energy during the heating season leaving the building and occupants cold.
There are a number of ways of installing ground arrays and for small developments Kensa would generally recommend installing ground arrays using the slinky method. The invention of the slinky® in 1985 is credited to Oklahoma University in the USA, which could be considered the heart of ground source heat pump technology.
Slinkies are considered by many ground array design experts to be the best method of “horizontal ground coupling” for heat pumps. Every one metre of trench is roughly equivalent to 5m of slinky pipe.
They are particularly well suited to the UK, and require less digging per kW of heat delivered than any other method and therefore offer the most economic installation costs.
If installed and sized correctly they offer the same performance as straight pipe and will not overcool the ground any more than straight pipe.
The amount of ground required for the ground arrays depends on the heat demand for the building, which in turn depends on the insulation levels, size of the building and temperatures required. The lower the heat demand the less ground required. As a rough rule of thumb, every 10m of slinky will provide 1kW of heat. Kensa slinkies are available for 30m, 40m or 50m trenches, the 30m uses 200m of MDPE pipe (32mm OD), the 40m uses 250m of pipe and the 50m uses 300m of pipe. Each slinky has 25m headers to enable the flow and return to be connected to the slinky manifold at the building. It is important to keep all slinkies the same length, this equalises the pressure drops within each slinky and means equal flow around each slinky.
Slinkies can be installed in two ways and the easiest way will depend on the site. To reduce the amount of digging, placing the slinkies on their edge within a 2m deep trench is generally preferred. The trenches need to be 2m deep (so the top of the slinky is 1m below the surface of the ground) but only 300-400mm wide, i.e. the narrowest digger bucket. If more than one slinky trench is required then each trench needs to be separated by 5m between centres. (This is for the coiled section of the slinky only).
The bottom of the trench should be covered with a layer of sand and if the ground is heavy clay or contains sharp stones we would recommend that the slinky is completely covered in sand before the excavated ground is placed on top.
If ground conditions mean that a trench 2m deep cannot be dug then it is possible to place the slinkies flat within a trench that is 1.2m deep by 1.2m wide. This would also reduce the amount of sand required to cover the slinky if ground conditions require this. Trenches would still need to be separated by 5m between centres and so there is no overall increase or decrease in required area.
It is possible to change between a narrow or wide trench at anytime.
Straight pipe ground arrays are also possible and Kensa can supply these if required. Please contact Kensa for further information.
For further information on installing Slinky ground arrays please see the Kensa Slinky Guide.
Boreholes
A borehole is basically a 150-200mm diameter hole, which is drilled vertically downwards for 60-100m. Each borehole is approximately 5 to 6 meters apart. Generally boreholes are closed loop systems and a single loop of pipe is inserted to the bottom of the borehole. (Double pipe systems are also available). The hole is then backfilled with a thermal grout providing close contact between the pipe and the surrounding ground. Water/antifreeze is then pumped around the borehole and this absorbs energy from the surrounding ground.
For small developments with sufficient land, the cost of installing boreholes is far greater than the cost of installing slinkies, due to the specialist teams and equipment required. However, if there is insufficient land a borehole solution might be the only means of extracting the energy from the ground.
For larger developments the cost of drilling boreholes reduces due to the economies of scale and as a guide at around 100kW the cost of installing a borehole becomes roughly equivalent to the cost of installing a horizontal ground array.
The amount of energy that can be obtained from a borehole depends on the ground that the borehole is being drilled into and its depth. As a rough rule of thumb 3-5kW can be extracted from a 60-100m borehole. While estimating outputs using rules of thumb can be acceptable for small projects, for projects larger than 100kW Kensa would always recommend that a thermal response test (TRT) is carried out. A TRT calculates the conductivity of an installed borehole and hence the amount of energy that can be obtained. It involves drilling a test borehole that is representative of the rest of the proposed field and injecting a constant heat while monitoring the return temperatures. Plotting the results can provide the conductivity of the borehole. Once the conductivity is known, a borehole specialist can be employed to calculate the correct number of boreholes and propose a bore field layout.
Kensa are one of the few companies who are able to offer a Thermal Response Test within the UK and are able to recommend borehole specialists and drillers to help design the bore field.
Some clients have expressed an initial desire to have all the work handled by a single contractor, primarily because of some ‘nervousness’ linked to the performance of the technology. The groundwork contractor is merely providing a connection to the ‘heat source’ in the same way that the utility companies are responsible for providing connections to the gas, electricity or water grids. In a typical project featuring a gas boiler, the contractor would only be responsible for installing the boiler and would not handle the provision of the gas supply to the property. Kensa is merely suggesting a mirroring of this approach.
Kensa can also fully advise on using other heat sources such as lakes, open loop boreholes or rivers. Please contact Kensa for further information and guidance.
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