In this blog, first published in Installer magazine July/Aug issue 2017, Guy Cashmore, Technical Director at Kensa Heat Pumps, explains how best to install a ground source heat pump when you have a plot with a limited land area
Ground source heat pumps can extract renewable heat energy from a variety of heat sources, including surface soil, rock and water courses, and can do this via a number of different collectors.
A few key things to remember about these heat collectors are that ground arrays are sized to meet the heat needs of the building, not the heat pump; all collectors have broadly the same performance and a building’s thermal efficiency affects heat pump performance more than the type of collector.
Thinking differently about ground arrays
First and foremost, it is important to note that an accurate heat loss calculation for the building is absolutely essential when designing any ground array for a ground source heat pump system. If you get this part wrong, then everything else about the system is likely to be wrong as well!
Other factors also affect the ground array sizing. For example, the water temperature the heating distribution system runs at changes the efficiency (CoP) at which the heat pump operates; hence 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 an ultra-efficient heat pump like the new Kensa Evo model is that the ground array size needs to increase slightly. If less energy for a given heat output is coming from electricity, then more has to come from the ground array.
Surprisingly often calculations for a horizontal array will show that the available garden area isn’t quite big enough to meet the heat 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 installation cost. An alternative worth investigating is spending some or all of this money on insulation and other measures to reduce the heat load. By reducing the heat load, the available garden area may well become viable as the heat source. It may also be possible to specify a smaller heat pump, further offsetting the insulation costs. By adopting this solution, 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.
Ground arrays don’t always have to follow the usual ‘one heat pump – one ground array’ route either. Although probably only applicable to clusters of dwellings and social housing-type situations, ground arrays can be shared between multiple properties in a similar fashion to district heating. However, because the flow is cold, the system doesn’t suffer any heat losses, and because each property has its own heat pump, the system doesn’t need any metering or billing arrangements either. 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, so 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 in existing water wells, old mines, natural springs, river water and even sea water. Whilst these type projects will never be mainstream, the best ones can make highly efficient, low cost energy sources and they do crop up from time to time in the Kensa special projects department. When using water as the heat source, a secondary heat exchanger is used, so the heat pump itself remains on a closed loop containing glycol. This secondary exchanger can take several forms; 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 materials needs to take into account the water being used, although plastic pipes and stainless steel plates are good for most installations.
Ground collectors can take many different forms, but the two most commonly used in the UK are horizontal loops, usually called slinkies, and drilled vertical boreholes with a U tube in them. Both have advantages and disadvantages.
Boreholes versus slinkies – pros and cons
Horizontal slinky systems are usually cheaper and easier to install as they don’t need a specialist drilling contractor and often the installation work can be undertaken by a digger that is already onsite doing other work. However slinkies 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 – the limiting factor is the land, not the pipes in it.
Drilled borehole systems on the other hand are typically a more expensive option (especially on smaller jobs), and getting physical access for the drilling rig can sometimes be a challenge. However boreholes can usually be installed in any type of ground and the smallest of gardens can often meet the heat load of the property as the depth of the hole(s) is 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.
Case Study: Northumberland Fire and Rescue Service – Prudhoe Fire Station
This summer Northumberland County Council is upgrading the night storage heaters in three of its fire stations with Kensa ground source heat pumps. One of the three fire stations, Prudhoe, does not have any available grass land and so it is necessary to drill boreholes through the reinforced concrete parking area to extract heat energy from the ground.
A detailed assessment of the anticipated geological conditions was conducted. The site is above the Pennine Middle Coal Measures Formation which is made up of sandstone with frequent coal bands. A thermal conductivity of 2.1 W/mK, a diffusivity of 0.085 m2/day and an undisturbed ground temperature of 9°C were used for the design along with published data available for the location. These characteristics are considered to be in line with UK average ground conditions, which do vary from location to location. Permission was granted from the Coal Authority to drill in this location.
The fire station has a peak heat load of around 23kW with an estimated heating and hot water requirement of approximately 62,500kWh per year. Using computer modelling the ground array was designed to feature four boreholes, each to a depth of 174 metres with a spacing of 8 metres between boreholes. The spacing was kept relatively close as this is a tight site requiring access for the fire engine and ambulances at all times, and so only a small amount of space could be given up for the boreholes. With larger spacing, borehole depth could have been reduced. All boreholes were positioned in the car park underneath a concrete layer that is about 0.5 metres thick.
Kensa Heat Pumps is publishing regular blog updates on the Northumberland Fire Station Projects that are available to view here: www.kensaheatpumps.com/ncc