{ 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

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What is the Urban Heat Island Effect? Photo:

An aquifer is permeable rock (made of clay, gravel, sand, chalk, limestone, sandstone etc) underground that can contain or transmit groundwater

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 Change  Sewage Leakage  Land Use Change  Groundwater flow

Effect of raised temperature on aquifers Positive: 1. Aquifers become attractive thermal reservoirs for space heating and cooling 2. Higher temperatures mean higher amount of energy stored, and therefore more geothermal potential 3. Aquifers can improve the sustainability of geothermal systems 4. Energy extraction is more efficient Negative: 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.

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 aquifer V = aquifer volume N = porosity C w = Volumetric heat capacity of water C s = Volumetric heat capacity of solid Q w = heat content stored in groundwater Q s = heat content stored in solid Δ T = 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 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 The geothermal potential of urban heat islands. Environmental Research Letters 5: uifer1.jpg Sources