MUSE and the underground urban heat island effect

Cornelia Steiner, 21 October 2020

The project MUSE deals with managing urban shallow geothermal energy in urban areas. It investigates resources of shallow geothermal energy in 14 pilot area cities in Europe. Knowledge about the temperature regime of the underground is essential to calculate the energy content, that is available for heating and cooling with for example groundwater heat exchangers. These systems mainly use shallow groundwater bodies.

Figure 1: Underground temperatures throughout the year. The temperatures on the surface only affect the first 20 meters, deeper areas are influenced by heat fluxes from the Earth’s interior.

The temperature in shallow depths down to around 20 m strongly depends on the surface temperatures (see figure 1). In urban areas they are generally higher than in rural areas due to human activities. Surface sealing, little vegetation and waste heat cause this widely known urban heat island (UHI) effect, which causes also increasing underground temperatures. Underground installations such as sewage systems, district heating pipes, basements, tunnels and geothermal energy systems are additional heat sources (see figure 2), that amplify this underground UHI effect. A quantification of these heat fluxes into the underground is unfortunately rather complex and until now afflicted with many uncertainties. But it is possible to measure the resulting groundwater temperatures, which are higher in urban areas compared to the sourrounding rural areas. Mean annual groundwater temperatures of above more than 20 °C are not uncommon anymore in European cities.

Figure 2: Heat source below urban areas due to human activities that lead to an increase of underground temperatures (underground urban heat island effect). Source: Menberg et al, 2013, Subsurface urban heat islands in German cities, Science of The Total Environment 442, 123-133.

Groundwater heat exchangers are a mixed blessing in areas with already strongly increased groundwater temperatures. If used predominatley for cooling they further heat up the groundwater. However, if cooling dominates, the system withdraws heat from the groundwater and therefore helps to reduce the underground UHI. A dense monitoring network for groundwater temperatures in cities is important to detect underground UHIs and to take counteractive measures in advance.

The underground UHI is an integral part of MUSE. By elaborating groundwater temperature maps based on field measurements, we identify areas with elevated groundwater temperatures. Furthermore, we suggest management strategies for a sustainable and efficient use of shallow geothermal energy in urban areas.

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