Infinite borehole field model-a new approach to estimate the shallow geothermal potential of urban areas applied to central Budapest, Hungary

Deploying shallow geothermal solutions is critical for meeting energy demands while supporting decarbonisation targets. In densely populated areas, drilling large numbers of boreholes may lead to thermal interactions between closely located borehole heat exchangers. This paper presents a novel method termed the infinite borehole field model to estimate the technical shallow geothermal potential, especially in urban regions. The thermal in-teractions between boreholes are considered using finite element models simulating the operation of a single borehole in a larger field. Mathematical optimisation is used to find the amount of thermal energy that can be annually extracted while keeping the borehole wall temperature above freezing point of water. The method considers thermogeological details of geological formations including downward-increasing ground temperature, geothermal heat flux, thermal conductivity, heat capacity, porosity, density, and advective heat transfer. Results of our case study indicate that 100 m deep thermally independent boreholes can produce 14.20 MWh/a for 50 years on average. However, boreholes in an infinite borehole field spaced 20 m apart produce 7.80 MWh/a. A further investigation including advective heat transfer indicated that high velocity groundwater flow can significantly enhance borehole yield. Our method provides a generalised approach which can be beneficial prior to detailed site investigations.