How can old mines heat our cities?
Ben - British Geological Survey Scientists have come up with a strategy using heat pumps to extract warmth from the water deep within flooded mineshafts to supply homes, returning about 4 kilowatts of heating for every 1 kilowatt of energy consumed just to run the system. From Edinburgh, Diarmid Campbell.
Diarmid - The idea is that we use heat pump technology to exploit the resource which lies under a city and we've been using Glasgow as a test case. We've been using Glasgow mainly because we've been doing a lot of work there in the last few years, developing high-resolution 3D models of the subsurface of Glasgow. We've acquired a very good understanding of the extent of the abandoned mine workings which underlie very large parts of the city of Glasgow and the wider Glasgow conurbation.
Chris - How much of Glasgow is sitting on a mineshaft?
Diarmid - Well, looking at the Glasgow conurbation as a whole, it's approximately 50%.
Chris - It's a lot.
Diarmid - In fact, most of the eastern conurbation of Glasgow is sitting on top of mine workings.
Chris - How deep?
Diarmid - They vary up to a maximum depth of about 610 metres, but on average, they're generally in the range of 250 to 300 metres deep as a maximum, but some of the mine workings are very shallow. Some of the oldest mine workings come to surface.
Chris - Are they still intact or have they all collapsed?
Diarmid - Various mining techniques were used and there have been various collapses of some of the very near surface workings which are the oldest workings. But by and large, as the mines developed, they started to use a technique called longwall mining which actually allowed the workings to collapse immediately after the coal had been extracted. So, most of the collapse in subsidence took place a long time ago when the mines were actually active and working.
Chris - So, what we've got underground will be layers of rock which is intact and therefore, very strong and impervious, and it'll be punctuated by this sort of lateral shafts where even if collapses happen, there's lots of debris, things can flow through there.
Diarmid - Yes and the idea of flowing is the key principle in all of this because once the mine had stopped operating, pumps were turned off, then the actual groundwater system was allowed to reinvade the rocks. So essentially, the abandoned mine workings are now flooded and the important thing is, that it's an important aquifer - contains a lot of water and that water flows very readily through the mine workings and through the tunnels which interconnect the mine workings to the shafts. That's the important factor that we can exploit in using ground-source heat pump technology.
Chris - So, you could put a tube or a borehole down to some of these lateral shafts that are full of water at ambient temperature.
Diarmid - Yes.
Chris - Will this therefore exploit the fact that some bits of the mine are going to be at a different temperature than others and you're using that temperature gradient?
Diarmid - Very much so. Now the idea here is not entirely new. There are working systems usually on a small scale and in fact, there's an example in Glasgow which has been operating now for 13 years, providing space heating for some 16, 17 houses in a housing association project. It was a very innovative piece of thinking by the architect who developed the project. And the deeper essentially you drill into the workings, the warmer the water is likely to get. And you can then pump that water to surface, extract some of the heat using heat exchange - it's essentially the technology we have in our fridges - and then return slightly cooler water to a shallower level in the mine workings.
Chris - So, you pull out some of the warmer water and extract the heat from it using the heat pump technology and then return the colder water to a different part of the mine.
Diarmid - Yes and a shallower part of the mine typically and this is what is being done in the largest current working example of such a system in the southern Netherlands. It's a demonstrative project which came on stream in 2008. It had some support from the European Union to raise funds to get the project up and running, but developers buy the heat technology from a company that was setup by the local authority there.
Chris - Is there any risk though, with doing this because you're putting water, albeit a tiny amount cooler in one part of the mine, having taken the energy out? This could potentially affect the pressure, could it not? Or the effect on the rock, it could make the rock contract or expand differentially across the mine workings. Is that not a risk?
Diarmid - I don't think pressure is the real issue at stake because the sorts of volumes of water that we're moving around the systems would be tens of litres a second potentially. So, we're not moving colossal amounts of water around the system, but certainly, the change in temperature of the rock mass is something that we have to take account of, especially in relation to some of the oldest workings which left the pillars of rock in place, holding the roof of the workings up. And colleagues of ours at Edinburgh University are looking at this aspect now and looking at the potential stresses that this might create. But we think on the sort of scale that we were likely to operate the process, it wouldn't be a major issue.
Chris - Based on the maps you've made, how much energy do you think is down there in terms of the capacity of that system to supply how much of Glasgow for example?
Diarmid - Well, we've used various ways of looking at this, but a flow rate method which considers the sort of borehole yields that you might get from mine workings and the temperature of the waters that you will be exploiting, we've come up with various scenarios. One of which suggests that using 4 boreholes per square kilometre in areas where there are mine workings extracting up to 10 degrees centigrade which is more than we would normally anticipate of doing. You could potentially provide up to 40% of Glasgow's space heating requirements and that's on a fairly sustainable basis, potentially up to 100 years.
Chris - How many people in Glasgow?
Diarmid - 1.2 million people live in the Glasgow conurbation, 600,000 within the city of Glasgow itself, but about half of those live above mine workings.
Chris - Presumably, Glasgow is not unique in having mine workings under it either.
Diarmid - It certainly isn't, no. In fact, a large proportion of the midland valley of Scotland for example lives above or close to mine workings and there are major coal field areas in northeast of England and northwest of England, the Yorkshire coalfield, the midlands, South Wales, and so on so, quite a significant part of the British population live close to mine workings.
Chris - So if you get this working here, then you could extrapolate this across the country.
Diarmid - Very much so and that would be something that we would really like to see if it could be achieved, yes.