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The Geothermal Potential of Urban Heat Islands

By: Ke Zhu, Philipp Blum, Grant Ferguson, Klaus-Dieter Balke and Peter Bayer

The statements presented in this presentation are excerpts from the above paper, or additionally listed sources, not my own original findings

To estimate the potential and sustainable use of shallow

geothermal energy on the large scale (in urban environments).

Geothermal Energy: heat energy produced by the inner heating of the Earth due to the kinetic energy of its atoms/molecules

Objective

Source: http://www.conserve-energy-future.com/Advantages_GeothermalEnergy.php

What is the Urban Heat Island Effect?

Photo: http://www.weatherquestions.com/urban_heat_island.jpg

An aquifer is permeable rock (made of clay, gravel, sand, chalk, limestone, sandstone etc)

underground that can contain or transmit groundwater

What is

an

aquifer?

http://shelledy.mesa.k12.co.us/staff/computerlab/images/COLifeZones_Plains_Aquifer1.jpg

The urban heat island effect not only effects the surface temperature of the Earth, but also the subsurface temperature. This in turn raises the temperature of aquifers, which serve as thermal (heat) energy

reservoirs.

Suspected causes for the increase in subsurface temperature include:

Climate ChangeSewage LeakageLand Use ChangeGroundwater flow

Effect of raised temperature on aquifers

Positive:

1. Aquifers become attractive thermal reservoirs for

space heating and cooling2. Higher temperatures mean

higher amount of energy stored, and therefore more

geothermal potential3. Aquifers can improve the

sustainability of geothermal systems

4. Energy extraction is more efficient

Negative:

1. Considered underground thermal

pollution

Data was collected from several cities through kriging, a geostatistical technique used to

estimate the value of an unknown based on linear least squares.

The data was then compiled into two tables, one with raw data comparing Cologne and Winnipeg, the other comparing all 7 cities.

Method:

Data:

Findings:

The subsurface beneath green spaces in the cities has lower temperatures than business districts in the city centers.

Q (KJ) = Total theoretical potential heat content of the aquiferV = aquifer volumeN = porosityCw = Volumetric heat capacity of waterCs = Volumetric heat capacity of solidQw = heat content stored in groundwaterQs = heat content stored in solidΔT = temperature reduction of the whole aquifer

Balke K D 1997

Findings:The natural geothermal flux substantially

decreases the amount of natural heat supply

Due to urbanization and the natural geothermal flux, only 3% of this energy is available

Example: Approximately 10% of the annual heating demand in Cologne, Germany could potentially be met with the Earth’s natural heat supply.

Conclusion: Large amounts of the Earth’s stored subsurface

energy is capable of fulfilling some of the Earth’s space heating demand

Megacities such as Shanghai, China have an existing potential heat content in the urban aquifer that is at least 22 times the city’s annual heating demand.

The energy of the subsurface is slowly, but continuously replenished

Uniform extraction is virtually impossible, each instance is case specific based on a ratio of producible and stored thermal energy (recovery factor, R)

Relation to NYC:

Geothermal energy use of shallow aquifers is on the rise

More important for highly urbanized cities with higher heating demand

Population of New York City: Over 8 million

A dual heating/cooling system or aquifer thermal energy storage (ATES) system would be more environmentally and economically more efficient

The Bottom Line

Further Research to be done:

Specific hydrological/geological and urbanized conditions to improve our understanding of energy fluxes in urban heat islands.

Zhu, K., P. Blum, G. Ferguson, K.-D. Balke, P. Bayer. 2010. The geothermal potential of urban heat islands. Environmental Research Letters 5: 044002.

http://shelledy.mesa.k12.co.us/staff/computerlab/images/COLifeZones_Plains_Aquifer1.jpg

http://www.weatherquestions.com/urban_heat_island.jpg

http://www.conserve-energy-future.com/Advantages_GeothermalEnergy.php

Sources