'Secret' underground energy could heat every home in capital city

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There could be an untapped resource under everyone's feet in Wales' capital that could lead to cheaper heating bills

ByDani Thomas

BBC Wales

• Published

  5 November 2025

• 270 Comments

Updated 5 hours ago

On an unassuming street in Cardiff, engineering geologist Ashley Patton is lifting the lid on what looks like an ordinary drain cover.

https://www.bbc.co.uk/news/articles/cy5q9e1e4zpo

I expect @Ken Fabian will be interested in this.

Edited November 6Nov 6 by studiot

@studiot Thanks for the heads up. Yes, this is something I think has a lot of potential for cities and towns in climates that are less suited to air source heat pumps aka reverse cycle A/C. Wider application than that but warmer climates have other options with less up front costs. And there are air source types now that handle below freezing temperatures.

A site that looks at the emerging possibilities, that I recommend is Volts Substack - https://www.volts.wtf/archive?sort=new - that does interviews with people involved in them, rather than the more usual press release with some (inexpert) commentary. David Roberts usually asks the questions that I want answered but often don't get asked.

The boreholes in this case appear to tap into an aquifer and those have been a preferred option for borehole ground-source geothermal. Very good heat transfer, even potential for using the existing water as the working fluid, may not need very deep boreholes and potentially less of them than in dry ground and rock. In this case the boreholes are pre-existing, hopefully of a diameter suitable.

It does sound like the aquifer is being warmed by the city above but not sure by what means. Groundwater inflows? An aquifer can be isolated or there could be continuous underground flows; if it has been gaining heat then it may be an isolated one or partially so. Which may in turn allow 'reversible' use, where summer cooling transfers heat to the underground water, which gives inter-seasonal energy storage. Where the water flows they don't provide storage per se, but have advantages like more constant temperature range.

Some other variations I've encountered include the conversion of a city gas supply pipeline infrastructure, enhanced by industrial waste heat and added boreholes (without aquifer). https://www.volts.wtf/p/thermal-energy-networks-are-the-next The pipelines are not buried deep but are more like the pipes buried in trenches style, yet do appear to have reversibility, shedding heat into the ground when used for cooling as well as using the heat for homes and businesses.

There is at least one US company doing retrofitted systems for larger city buildings with small drill rigs and continuous feed of one piece pipe lining. Some of their innovation is in building databases of underground conditions including mapping the geology as well as existing infrastructure to avoid; the advantage of that seems cumulative, ultimately allowing more accurate quoting of costs in place of the surcharges to cover unexpected conditions. https://www.volts.wtf/p/making-geothermal-heat-pumps-work

Some improved drilling technology is a big part of that last example. For that other kind of geothermal - the sort intended for electricity production - we may see "millimetre microwave" deep drilling that overcomes some of the cost limitations of accessing heat enough to produce electricity at scale, potentially almost anywhere. Not a technology that gives advantage over conventional drilling at shallow depths - too much variability of ground geology there - but seems suitable for hard rock and at depths where the temperatures are a problem for conventional drilling. https://www.volts.wtf/p/super-deep-geothermal-drilling-with

Note, I haven't fully re-read each of these interviews; I think I've got the correct ones.

Edited November 7Nov 7 by Ken Fabian

Thank you for replying, Ken.

+1 not only for that but also for the manner in which you have replied.

This really is a good example of how discussion should work.
I have not heard of any of your references, but rest assured I will investigate them as potentially very interesting.

A most informative reply.

@studiot The links are to interviews, so there is some reading or listening to find and get to the significant bits. That format may not be to everyone's taste but I found it informative. Unusual to get answers about emerging technologies from people doing them.

51 minutes ago, Ken Fabian said:

@studiot The links are to interviews, so there is some reading or listening to find and get to the significant bits. That format may not be to everyone's taste but I found it informative. Unusual to get answers about emerging technologies from people doing them.

Noted. Hopefully these interviews are in English.