Extracting heat from the ground

renewable energy, RHI, Renewable Heat Incentive, Rehau, borehole, Slinky
Exploiting ground-source energy — Steve Richmond.

The deeper you can go the more useful is the ground as a source of energy. Steve Richmond of Rehau burrows deeper.

Ground-source heat pumps have the lowest operating costs of any mainstream renewable heating system, which obviously goes some way to explaining their popularity around the world. Added to this, the revised non-domestic RHI tariffs that have recently been announced make them particularly appealing to clients who can calculate a healthy and viable payback.

However, whilst there is an understandable focus on the performance and efficiency of the heat pump, there is generally much less focus on the means of extracting the heat from the ground, but this can have a significant impact on the performance and efficiency of the overall installation.

There are three main material options for the pipes used in ground source energy extraction: PE-Xa; PE 100-RC; and PE 100. As a polymer specialist, Rehau manufactures pipes in all three materials and can give impartial advice on which is the most appropriate solution for each application.

In general terms, PE 100 (high-density polyethylene) is the standard material used across the UK market. The newest generation of PE 100 is PE 100-RC (resistant to crack) and is a modified polyethylene with improved mechanical resistance. It has a much higher resistance to point loads and can be used typically without a sand bed for backfilling — offering potential savings on installation costs. Many central-European markets have already made the switch from PE100 to PE 100-RC.

It is these pipes which are generally used for horizontal ground source energy collectors. They are typically installed around 1.2 to 1.5 m below ground at a pipe spacing of around 0.75 m.

The other option for horizontal installation is ‘slinkies’ which are often used because they offer lower installation costs. However, they require careful overall system design to ensure that they do not extract too much heat from the ground and exhaust the resource.

This helical pipework will be incorporated into the piles of the Central Institute for Mental Health in Mannheim, Germany.

The Ground Source Heat Pump Association (second link below) is currently finalising a new ‘Shallow Ground Source Standard’, which will cover these installation methods.

In contrast, PE-Xa cross-linked polyethylene is the highest-quality option and is the preferred choice for underground thermal energy storage — such as Rehau installed in conjunction with Icax at Suffolk One and for boreholes beneath buildings like those which were installed recently at Derby Hospital. Rehau uses PE-Xa for its Raugeo probes because it does not require a joint at the probe tip and uses an Everloc compression sleeve joint rather than an electro-fusion joint.

In terms of application, PE-Xa is typically used for vertical boreholes which, in the UK, are typically 70 to 150 m deep. PE 100 and PE 100-RC are also used for probes but they do not have some of the properties of PE-Xa.

There are other alternatives as well for heat extraction, such as thermal piles and Helix probes. Helix probes are spirally wound probes generally installed to a depth of around 5 m; they offer an alternative to boreholes and collectors.

There is a huge amount of innovation taking place in this area — driven largely by markets in Europe where ever-more ambitious ground-source heat-projects are being delivered.

Helix XXL, for example, is a spirally wound pipe for thermal piles (pipes inserted into foundation piles) which can be installed without the need to drill additional boreholes. The Helix XXL’s pipe design has been proven independently to extract up to 12% more than heat loops or meanders in thermal piles.

One of the first such installations was carried out at the Central Institute for Mental Health in Mannheim, Germany. Thermal piles were installed as part of an extension at the Institute and the 82 durable helix-shaped pipework probes were factory fitted into the reinforcing cages being lowered into the piles. This saved time-consuming threading and bending, and the helix-shaped flow on the outside and the vertical return on the inside meant that there were no potentially weakening connections required for the probe base or the probe head.

With a depth of 800 m, this installation of Rehau’s Raugeo ground-source energy probe and HPR pipe taps into geothermal energy.

Thermal piles are becoming increasingly common, so to assist system designers the GSHPA has published a ‘Thermal pile standard’ to cover the structural and installation issues which arise.

There is also a new high-pressure-reinforced (HPR) pipe available for closed-loop applications up to 800 m deep. Normal probes typically go to a maximum of 200 m depth, so 800 m is truly exceptional. At this depth the system is extracting true geothermal energy rather than ground-source energy. Typical temperatures are closer to 40ºC than the 10ºC we see with ground-source systems, so they deliver increased GSHP efficiency. However, they also place greater demands on the pipework, which is a steel reinforced PE-Xa probe with a coaxial or double U-loop configuration.

For an installation in Germany, at Landau in der Pfalz, Rehau installed HPR pipe at 800 m to provide direct geothermal heat for the underfloor heating system in a new car dealership. With output estimated at 80 kW and the return temperature at 40ºC, the annual heating costs for the dealership are expected to be less than €500 per year, without any requirement for an intermediate heat pump.

Our experience in this market shows that innovation in mainland Europe very quickly finds its way to the UK, so I predict that the kind of applications which now seem fairly cutting edge will soon become commonplace.

Steve Richmond is business team manager for renewable energy at Rehau.

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