  Geothermal heat, as renewable energy, shows great advantage with respect to
its environmental impact due to its significantly lower CO2 emissions than
conventional fossil fuel. Open and closed-loop geothermal heat pumps, which
utilize shallow geothermal systems, are an efficient technology for cooling and
heating buildings, especially in urban areas. Integrated use of geothermal
energy technologies for district heating, cooling, and thermal energy storage
can be applied to optimize the subsurface for communities to provide them with
multiple sustainable energy and community resilience benefits. The utilization
of the subsurface resources may lead to a variation in the underground
environment, which might further impact local environmental conditions.
However, very few simulators can handle such a highly complex set of coupled
computations on a regional or city scale. We have developed high-performance
computing (HPC) based hydrothermal finite element (FE) simulator that can
simulate the subsurface and its hydrothermal conditions at a scale of tens of
km. The HPC simulator enables us to investigate the subsurface thermal and
hydrologic response to the built underground environment (such as basements and
subways) at the community scale. In this study, a coupled hydrothermal
simulator is developed based on the open-source finite element library deal.II.
The HPC simulator was validated by comparing the results of a benchmark case
study against COMSOL Multiphysics, in which Aquifer Thermal Energy Storage
(ATES) is modeled and a process of heat injection into ATES is simulated. The
use of an energy pile system at the Treasure Island redevelopment site (San
Francisco, CA, USA) was selected as a case study to demonstrate the HPC
capability of the developed simulator. The simulator is capable of modeling
multiple city-scale geothermal scenarios in a reasonable amount of time.
