Unveiling the impact of shallow geothermal heat recycling and underground heat sources on urban heat fluxes and climate resilience

Temperatures in the shallow urban subsurface are typically elevated. Locally, e.g. near underground car parks or other underground infrastructures, they may be ≥5 K warmer than background temperatures. Often these temperature anomalies are discussed as a heat source for shallow geothermal heat recycling. This could significantly lower local temperatures. Here we investigate the short-term impacts of underground temperature anomalies in the soil on atmospheric energy fluxes in Berlin after two days using the large eddy simulation (LES) microclimate model PALM-4U. Two scenarios are compared: a reference case and one where temperatures at 3 m depth are increased by 5 K.

We find pronounced changes in sensible heat flux (SHF), ground heat flux (GHF), surface temperature and 2 m potential temperature. Even after the very short time period of two days a maximum increase of 0.64 K in 2 m potential temperature is simulated. With increasing height, the influencing effect diminishes. These findings demonstrate that soil temperature anomalies significantly alter the temperature distribution and energy budget within urban systems.

Why are these findings important? In urban environments like Berlin, changes in soil heat fluxes can exacerbate urban warming, emphasizing their importance for urban heat island (UHI) dynamics and related societal challenges. Hence, the understanding of the soil-land-atmosphere coupling is of utmost importance not only regarding energy flux dynamics but even more for shallow geothermal energy applications and its effects on urban microclimates, particularly for urban heat mitigation. Subsurface heat recycling and alleviating underground thermal pollution have been disregarded so far. However, subsurface heat recycling can provide a green, renewable, and carbon free energy solution for heating or cooling demands, when the technical feasibility of the geothermal potential is given. We demonstrate that soil temperature anomalies significantly influence the temperature distribution and energy budget of a system. Thus, in a reverse sense the potential of cooling the subsurface through shallow geothermal systems can be a sustainable method for creating climate resilient cities.