Ground source heat pumps continue to grow in popularity but Vaillant’s Shaun Edwards warns system designers and installers to do their homework before choosing this option.
The renewables market is a bright spot in a generally depressed economy and there is a healthy interest in ground source heat pumps (GSHPs) in particular. However, we must remain constantly on guard against mis-selling and poor application of all carbon reducing technologies.
GSHPs are proving their worth in many projects and will produce coefficients of performance (COPs) of up to 4.5 depending on the flow temperatures achieved.
However, like many renewables, they are not the right solution for every application and may not deliver these COPs if they are not properly designed. Designers and installers must make sure the building has the right level of heating demand, can accommodate a GSHP and has the right heating system to extract maximum efficiency.
You cannot just attach a heat pump to a standard wet central heating system. The heat produced by a GSHP is 'low grade' - up to 60˚C - so should be used with underfloor heating or, at a pinch, a conventional system with radiators oversized by 40-60%.
A full geological survey is essential before starting. Hidden faults and undetected changes in rock formations can prove extremely expensive. Depending on the plot size and ground conditions, slinky loops can be used but these should be buried at least 1.2 metres below the surface in horizontal trenches.
Alternatively, the pipework can be inserted into vertical boreholes drilled down to around 100 metres. The higher the temperature of the heat source, the more efficient the heat pump but the key is having a constant temperature. The earth around UK buildings is on average a constant 15˚C at 100m.
The heat is collected by polyethylene pipes filled with a water and anti-freeze mixture that extracts about 65W of heating energy per metre from a borehole or one kilowatt per 40m2 if you use horizontal coils.
Ground loop system
For a ground loop system an area of between three and 10 times the floor area of the property is needed. Boreholes can be drilled in more restricted areas, but you need access for the drilling rigs. As a rule, a 100 metre borehole will provide about 6kW of heating.
The presence of water or wet conditions in the ground usually offers excellent thermal conductivity. However, the ground must also be stable so that no movement can interfere with the integrity of the ground loop.
The heat pump units can weigh in excess of 150kg so the installation location must be properly designed to accommodate them.
Open loop system
Open loop water source heat pumps are another option and these deliver the highest efficiencies,. In this case, the water from the source itself is pumped directly through the heat pump. However, there must be a convenient standing body of water and care must be taken to ensure components are corrosion proof.
A buffer store is essential with most ground source heating systems. It performs two functions. It ensures maximum circulation around the heating system/heat pump and allows the heat pump to operate for longer periods without repeat cycling, so maximising COP and minimising the load on the compressor. For a domestic system of up to 10kW a 300 litre buffer store would be sufficient.
At Vaillant, we believe the key to creating a sustainable market for GSHPs is harnessing as much technical expertise as possible and providing support to the installers. This is a fledgling market and building services firms are going through a steep learning curve.
However, it is worth the time and investment because there is clearly a large potential market.
Installers have expertise in the heating aspects of ground source systems but are, obviously, less familiar with other elements of these projects. Therefore, Vaillant provides training and has developed partnerships with geological specialists and ground drilling companies to deliver a complete package of technical support.
//Shaun Edwards is Vaillant's UK and Ireland commercial director//