I WAS delighted to see your report of the work being done at Glasgow Caledonian University on the possibility of using ground-source heat pumps to extract heat from flooded mine workings ("Abandoned coal mines could hold key to heat up Glasgow", The Herald, February 13.

Having spent much of the last two decades working on mine water management, and investigating its use as a heat source, I would flag up two points:

First, ground-source heat pumps only work effectively when delivering heat to buildings at about 50C to 55C at most – at temperatures above that they effectively become extremely expensive immersion heaters. This means that they are not suitable for feeding directly into existing central heating systems, but need to be deployed in well-insulated buildings with either underfloor heating or blown-air delivery (unpopular in the UK). For this reason they are best suited to new-build houses – though it is to be hoped the Clyde Gateway development will in time include many.

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Secondly, to get large amounts of heat from this source it is necessary to actively pump the groundwater and re-inject it, after processing in the heat pump. Apart from the added expense of pumping, the big worry with doing this in shallow mine workings is the possibility of induced flows through old workings eroding pillars and/or lowering water levels sufficiently that buoyant support of void roofs is lost: both processes have been known to trigger formation of subsidence craters at the surface. In densely-populated urban areas, this is a risk demanding very careful forethought. The existing system at Glenalmond Street is sufficiently small-scale that it does not run this risk: the challenge comes in scaling up to the level of ambition stated in your report; that is, 40% of Glasgow's heating.

I hope no-one in Iceland or the United States sees the claim that Glasgow would be the third city in the world to de-ice streets using warm groundwater: Reykjavik and Klamath Falls (Oregon) have prior claims over Hamburg and Stockholm.

None of these points is a show-stopper and I offer the GCU team any help I can give to help bring this to fruition.

Professor Paul L Younger, FREng,

Rankine Chair of Engineering, and Professor of Energy Engineering,

School of Engineering,

University of Glasgow.