Vast geothermal energy sources ‘slumbering’ below cities

Heat generated by cities from pollution, energy waste and lack of green spaces could be put to good use 

(Pic: KIT)

The city of Karlsruhe. (Pic: KIT)

By Nilima Choudhury

Cities are missing a huge opportunity to use untapped resources of geothermal energy to heat and cool households, say researchers.

Scientists from the Karlsruhe Institute of Technology (KIT) and ETH Zurich say underground temperatures generated by buildings, sewage ducts and the sun could be used to power urban areas.

“In Karlsruhe, the average heat flux density into subsurface aquifers [underground layers of rock] was 759 milliwatts per square meter in 1977. In 2011, a heat flux density of 828 milliwatts per square meter was reached,” said Professor Philipp Blum from KIT.

“This amount of heat corresponds to 1 petajoule per year and would suffice to supply at least 18,000 households in Karlsruhe with heat.”

Ground temperatures in big cities are far above those in the surrounding rural area. Dense settlements, industry, traffic and a lack of vegetation causes an urban microclimate which increases ground temperatures compared to levels in the countryside.

Ground source heat pumps, where pipes full of water are run underground, soaking up rising heat, are increasingly common.

The water increases the temperature of the ground heat captured in the pipe and can then heat water for the heating and hot water circuits of the house.

Where does the heat in the underground of large cities come from? Researchers analyzed various factors. (Graphics: AGW/KIT)

Where does the heat in the underground of large cities come from? Researchers analyzed various factors. (Graphics: AGW/KIT)

In 2011 the IPCC said that by 2015 it envisages geothermal energy to generate 122 terrawatt hours per year (TWh/yr) of electricity and by 2050 historical growth trends suggest that geothermal could produce 1,180 TWh/yr for electricity. One TWh is essentially enough energy to power a city of 200,000 people for an entire year.

This would meet more than 3% of global electricity demand by 2015 and about 5% of the global demand for heat by 2050.

Policy is likely to be one of the main driving factors for future geothermal development.

In a previous study, KIT analysed from more than 1100 individual geothermal heating systems and found the average capital cost is about €23,500 to €6800, but several countries like the US, the UK, Australia and Japan offer tax reductions, subsidies or other benefits to facilitate the use of heat pumps.

In the UK, this time last year, the British Geological Survey (BGS) mapped the mine workings below Glasgow which could provide up to 40% of Glasgow’s heating requirements, and similar levels could be reached in other UK locations that overlie mine workings.

There are now around 10,000 heat pumps installed in the UK compared with only nine in 1999.

Despite the price tag, the sector is growing rapidly and is expected to increase by 33% this year compared to 31% last year.

Globally, in 2000 electricity generated by geothermal energy represented only 0.3% of the world’s total electrical energy use. This figure is expected to rise over the coming years.

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