Influence of short-term dynamic effects on geothermal borefield size

A borefield is often oversized in practice, leading to unnecessary investment costs. This is the result of using a steady-state borefield model to describe the heat transfer inside the borehole, thereby neglecting the dynamic behavior of the fluid, the pipe, and especially the grout. When using a dynamic model (which incorporates transients), the thermal mass of the materials between the fluid and the ground introduces inertia, reducing the heat transfer peak to the ground. Consequently, the borefield size can be reduced while still meeting the predetermined temperature limitations, leading to a significant decrease in investment cost. The aim of this study is to investigate the influence of short-term dynamic effects on the sizing of geothermal borefields. This study starts from the Finite Line Source (FLS) model as baseline. This model is corrected to account for the cylindrical geometry of the boreholes and coupled to an explicit 1-D numerical model of an equivalent borehole, thereby extending the scope of the response factors to short time scales. Validation was based on the dynamic Modelica borefield model of the IDEAS Library and experimental borefield data. Results show that the relevance of using a sizing method based on a dynamic model is highly dependent on the thermal load profile. In case a building has a high variation in peak demand and the sizing is not dominated by imbalance a size reduction up to 34.7% is obtained for the case investigated.