Heat source

Ashley Patton measuring soil properties in Cardiff.

Ashley Patton, BGS engineering geologist, measuring soil properties in Cardiff.

The untapped reservoir of hot water beneath UK cities could provide green energy for millions of homes.

A pilot scheme in Cardiff shows that it is possible to heat buildings using groundwater beneath the Earth’s surface. If this technology was rolled out across the UK it could provide green energy for millions of homes and small businesses.

The UK has set an ambitious target of reducing its net greenhouse gas emissions to zero by 2050. The solution may be lying right under our feet; there is potentially enough geothermal energy under the Earth’s crust to provide for all our power needs for billions of years, all we need do is harness it.

If you were to dig a big hole straight down into the Earth, you would notice the temperature increasing the deeper you go. That's because the inside of the Earth is full of heat – emanating from the planet’s core, and the radioactive decay of uranium, thorium and potassium in certain types of rocks.

Unfortunately, most of this heat is inaccessible; but tapping just a fraction of it would make a substantial contribution to reducing greenhouse gas emissions. So much so that a recent report from the British Geological Survey (BGS) warns that the only way the UK can meet its 2050 zero emissions goal is through harnessing geothermal energy.

One option is to drill deep down into rocks to access hot waters directly. In most regions of the UK this would be madness, as the drilling costs would be astronomical. However in the south west of England, where the crust contains large amounts of heat producing granites, it may be feasible, and a geothermal power plant is currently under development at United Downs.

One study even predicted that water extracted from Cornwall’s granite crust could provide a fifth of the UK’s power needs (see (www. theguardian.com/environment/2012/may/30/geothermal-energy-uk-power). However the risks from drilling deep boreholes into the Earth’s crust are not yet fully understood, with some fearing the impacts on seismic activity.

BGS scientists in Cardiff believe that a more promising route lies in accessing shallow groundwater just meters beneath the ground. 

“In the UK a lot of urban areas tend to be built on low lying flat land near rivers, where the groundwater tends to be shallow,” says Gareth Farr, a BGS researcher who leads the Cardiff Urban Geo Observatory, one of three such observatories in the UK (see www.ukgeos.ac.uk/observatories/cardiff).

“When you have cities plonked on top of rocks and water like this, you can get an anthropogenic warming effect where heat from the surface is lost underground.”

This is known as the 'Urban Heat Island' (UHI) effect – a phenomenon where cities like London are one to three degrees Celsius hotter than the surrounding countryside. While the effects of the UHI on air temperatures are well known, the warming effect can be felt below ground too.

“In cities you can lose heat underground through heated basements, underground car parks, or through underground tube systems,” says Farr.

“Underground infrastructure such as sewers also generate heat, as do old landfill sites. All these players go into a big mixing pot and contribute to that subsurface urban heat island.”

To find out more about this effect, Farr and his team fitted temperature sensors to a pre-existing network of 61 boreholes spread throughout Cardiff. They took readings every 30 minutes for a period of over four years, resulting in more than 3.5 million measurements.

“We went out and measured borehole temperature profiles all the way to the bottom using in effect a glorified thermometer on a large measuring tape,” says Farr. “Although this was fairly low tech stuff, it enabled us to create a city wide map of temperatures which showed much warmer temperatures below ground than we expected”.

The map showed that between the months of September and December, groundwater temperatures can reach 16.1°C – much hotter than surface waters which can approach zero degrees during these months.

According to Farr, this temperature gradient could be exploited to provide energy efficient heating for homes and businesses around the UK using ground source heat pumps (GSHPs). 

GSHPs work by physically pumping groundwater out of a borehole, passing it through a heat exchanger, and then returning the cooled water to the ground. The heat exchanger effectively passes the heat to the building, where it can be used to heat hot water, radiators and underfloor heating systems.

Unlike deep geothermal, ground-source heat pumps can be installed almost anywhere. To prove this, Farr and his colleagues installed boreholes and heat pumps at a nursery school  in Cardiff. 

“We chose a small nursery school in Grangetown whose gas boiler was nearing the end of its life,” says Farr. “They very kindly agreed to let us dig up their playground. However we replaced the swings and all you can see now on the surface is two manhole covers and a small green shed where the heat pumps live.”

The installation enabled the school to almost entirely turn off their gas supply. While they do have to use some electricity to run the pump, in theory as the council moves towards totally renewable technology, in a few years CO2 emissions could diminish to almost zero. If the scheme was a standard installation, the government’s renewable heat incentive would even leave the school a few thousand pounds a year in pocket.

“In theory the school is not beholden to any fluxes in the gas market or gas supply, so that provides them with an element of security,” says Farr. “They need electricity to operate their site, but in theory they could generate on their roof with solar panels and be totally off grid”.

As well as helping to keep children warm, the pilot scheme shows that the technology works, and can be scaled up. A study by BGS showed that existing boreholes operated by Cardiff Harbour Authority could supply enough energy to heat about 74 homes, and the groundwater beneath Cardiff could generate the equivalent of 26% of the city's 2020 heating demand.

The technology could really be applied anywhere where there is substantial water availability underground. However there are still a number of hurdles that need to be overcome.

“One of the reasons we did this was to show regulators what the technology looks like and how it works, as I think one of the main barriers is people  being unfamiliar with this technology”, says Farr.
“Another issue is that we don’t really understand who owns heat in the subsurface yet. It is a bit of a wild west setup, where in theory you could have as many GSHPs as you wanted down there.

However when you have multiple users in a city all next door to each other, you really want to avoid stealing your next door neighbours’ heat supply.”