Events / Science communication

The heat beneath our feet

We were expecting chilly weather here in northern Scotland, but so far the sun has been shining on the British Science Festival in Aberdeen! We don’t think of Scotland -or the UK for that matter -as a particularly warm place, but underground it’s a different story.

Yesterday, scientists from the British Geological Survey explained the untapped resource which they’ve already established has the potential to provide up to 40% of Glasgow’s heating.

It’s all thanks to the Earth’s ability to retain heat – not from the core, but from the atmosphere. Most rocks can do this, although some are better than others – sandstone has a much higher thermal conductivity than granite, for example. Even at very shallow depths, the ground retains its heat far better than the air does – which means that even as far north as Aberdeen, we could still use ground source heat pumps to keep our houses warm. Athough the temperatures involved are often as low as 10%, that’s enough, say BGS, to keep our homes warm in winter and cool in summer.

There are lots of different ways to get hold of this heat. BGS are currently focusing on water held in abandoned and flooded mines – of which there are plenty to go around in the UK. The picture above is a 3D model showing the extent of mine workings underneath Glasgow – and they think there are plenty more such sites across the UK.

“The mine workings are hugely extensive. Perhaps 50% of the Glasgow conurbation sits above them,” said Diarmad Campbell of the BGS.

To get the heat out, a refridgeration fluid is injected into the ground, and heat from the rocks is transferred. When it’s pumped back up to the surface, an evaporator, a compressor and a condenser extract the heat from the fluid and transfer it to domestic heating systems.

What kind of impact could this have on our energy supply? We’re already way behind many other countries, particularly Sweden and Germany. In the nineties Sweden was already getting 75% of domestic heat from the ground. Britain remains dependent on gas, and so far the high cost of installation has put us off.

Even if we do get underway, the technology isn’t likely to be implemented in individual homes –  the cost of converting heating systems can be as high as £15,000 per home. It would take large scale investment to start using the Earth’s heat resources in a way which is affordable, and still carbon friendly.

Whilst we are still lagging behind countries like Germany and Sweden, interest in research aimed at tapping into the Earth’s thermal store is growing. There are now around 10,000 heat pumps installed in the UK compared with only nine in 1999.

Combined with research in wave and wind power – particularly prevalent in northern Scotland -investment in ground source heat pump technology could become an important  part of the UK’s renewable energy future.

Edit: We’ve had quite a few questions now about whether this system has anything in common with ‘fracking’, and comes with similar risks of causing small earthquakes. Jon Busby, of the British Geological Survey, has given us the following explanation of why this isn’t the case:

There is no risk of seismic activity or creating new flow pathways in the sub-surface from ground source heat. For a start, a closed loop ground source heat pump is a passive system that is drawing heat slowly from the ground due to the temperature difference between the water in the pipes and the ground. During the summer when heating is not required, heat will flow back to the areas from where it has been abstracted and equilibrium will be restored. For an open loop system where ground water is used the impact of the ground source heat pump is no different to water abstraction for drinking or agriculture.

Another way of looking at it for shallow horizontal loops (“slinkies”) is that the top 1 m of the ground goes through a temperature cycle due to the seasons that is no different to the temperature cycle imposed by a ground source heat pump.

There are a few occurrences of problems with vertical closed loop ground source heat pumps where ground heave has occurred. This is because they were not designed properly; specifically the boreholes were to close together leading to the ground freezing. However, these sort of problems are very different to fraking and should become a thing of the past now that the MCS (Microgeneration Certification Scheme) for ground source heat has been introduced which defines the professional standard required for installations.

4 thoughts on “The heat beneath our feet

  1. Sarah, this sounds like a wonderful idea, except for the fluid injection. Is this injected under pressure, like traditional petroleum drilling? Would this be similar to the effects of hydrofracking? Or can be this be done with non-toxic fluids and minimal potential seismic effects?

  2. Caryn If its the same system I am familar with in the states its a closed loop system under low pressure and no danger of seismic effects its just a fluid that takes and releases heat efficently in a loop of tubing planted deep underground

  3. Caryn, this is a really good point, and something which did come up in the session, so I’ll add in an edit on it. Basically, the depths involved are much shallower, and as Gary says, the pressure much lower, so it’s not something to be too concerned about – although obviously potential risks need to be considered in the design. The fluids are basically what we have in our fridges at home. I’ll track down a more detailed answer to your question and add it into the post!

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