Thermal impact of underground car parks on urban groundwater

Built-up areas are known to heavily impact the thermal regime of the shallow subsurface. In many cities, the answer to densification is to increase the height and depth of buildings, which leads to a steady growth in the number of underground car parks. These underground car parks are heated by waste heat from car engines and are typically several degrees warmer than the surrounding subsurface, which makes them a heat source for ambient subsurface and groundwater. Thus, the objective of this study is to investigate the thermal impact of 31 underground car parks in six cities and to upscale the thermal impact that underground car parks have on the subsurface in Berlin, Germany. Underground car parks have daily, weekly, and seasonal temperature patterns that respond to air circulation and traffic frequency, resulting in net heat fluxes of 0.3 to 15.5 W/m² at the measured sites. For the studied underground car parks in Berlin, the emitted annual thermal energy is about 0.65 PJ. Recycling this waste heat with geothermal heat pumps would provide a sustainable alternative for green energy and counteract the urban heat island by cooling of the shallow subsurface.