Urban underground holds sustainable energy

October 28, 2013
Where does the heat in the underground of large cities come from? Researchers analyzed various factors. Credit: AGW/KIT

Vast energy sources are slumbering below big cities. Sustaina-ble energies for heating in winter and cooling in summer may be extracted from heated groundwater aquifers. Researchers from KIT and ETH Zurich developed an analytical heat flux model and found that increasing heat in the underground is mainly caused by an increase in surface temperatures and heat release from buildings. Work of the scientists is now reported in the renowned journal Environmental Science and Technology.

Temperatures in big cities are far above those in the surrounding rural area: Dense settlement, surface sealing, industry, traffic, and lacking vegetation cause an urban microclimate with increased temperatures in the atmosphere. Temperature anomalies also develop in the . They spread laterally and vertically. In the past decades, groundwater in conurbations heated up considerably. "In Karlsruhe, the average heat flux density into subsurface aquifers was 759 milliwatts per square meter in 1977. In 2011, a heat flux density of 828 milliwatts per square meter was reached," says Junior Professor Philipp Blum, Head of the Engineering Geology Division of the KIT Institute of Applied Geosciences (AGW). "This amount of heat corresponds to 1 petajoule per year and would suffice to supply at least 18,000 households in Karlsruhe with heat."

In cooperation with Kathrin Menberg and Axel Schaffitel from KIT and Dr. Peter Bayer from ETH Zurich, Philipp Blum also studied the origin of heat in the underground of conurbations. The scientists developed an analytical heat flux model to examine factors, such as the increase in temperatures of sealed surfaces, heat release from buildings, sewage ducts, and underground district heating networks as well as discharge of thermal waste water. By modeling anthropogenic heat fluxes into the underground of the city of Karlsruhe, the researchers determined long-term trends of processes. They found that the increased surface temperature and heat release from buildings in particular are responsible for the increase in temperatures in the subsurface area.

The energy from close-to-surface groundwater aquifers might be used for heating in winter and cooling in summer via geothermal heat pumps and groundwater pumps. Use of this geothermal potential would not only cover part of the growing demand for energy, but also reduce the emission of greenhouse gases, which would counteract heating of the cities.

Explore further: Mining for heat

More information: Kathrin Menberg, Philipp Blum, Axel Schaffitel, and Peter Bayer: Long-Term Evolution of Anthropogenic Heat Fluxes into a Subsur-face Urban Heat Island. Environmental Science and Technology, 2013, 47 (17), pp. 9747–9755. DOI: 10.1021/es401546u

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1.5 / 5 (8) Oct 28, 2013
"This amount of heat corresponds to 1 petajoule per year

278 GWh in more familiar units.

On further examination, the average power comes out to 31 MW which gives you 1.76 kW per household if you divide it to 18,000 parts, which is clearly too little. Something in the claim is off.

An average urban household in Germany consumes 81 GJ per year in heating, which is 2.57 kW on average. The estimate for number of households heated is therefore 45% exaggerated, unless Karlsruhe is an exceptionally energy-efficient city.

(ref: Household direct energy consumption and CO2 emissions in European countries. Koen Meirmans EES 2013-171 T page 23)
1 / 5 (8) Oct 28, 2013
Also note that the energy required to pump this heat out of the ground with a heat pump is not free. The vast majority (~78%) of urban German households are heated with natural gas and heating oil and the rest are heating by distric heat. Very few are heated directly by electricity, so there's actually no direct energy savings in converting households to geothermal pumps.

The problem is thus: if we take some of that natural gas and oil so as to not increase the total energy consumption, and turn it into electricity at a loss of 3:1 and then use the electricity to pump heat out of the ground at a gain of 1:3 which is a typical average CoP of heat pumps, we arrive at exactly where we started. Nothing gained.

So the sustainability of the heat source is really dependent on the sustainability of your actual energy source.
1 / 5 (10) Oct 28, 2013
I wonder if centuries of elevated heat in the ground under urban environments would leave a detectable change in the future geology of a region. Might alien geologists be able to tell intelligent beings were here (if we went extinct in future aeons and our cities crumbled) by the previously warmed mineral patches showing a non-natural pattern of city placement?
1 / 5 (10) Oct 28, 2013
@Sean_W, what cause the trace elements not produced in nature wouldn't be enough of a clue? The vast quantities of minerals displaced from naturally occuring sources, the hard fired ceramics, etc?

Not to mention the various isotopes with half lifes well into the millions of years...

1 / 5 (1) Oct 29, 2013
"Urban underground holds sustainable energy"
Sure, if reliable and cost effective methods can be devised to get at it. In fact, why not take the heat now pumped out of the urban environment and pump it deeper into the earth, to be harvested when needed.

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