Dynamic Response Simulations of Circulating Fluid and a Borehole Heat Exchanger

Abstract

A three-dimensional numerical model has been developed to simulate transient fluid transport and heat transfer in and around Borehole Heat Exchangers (BHEs). Common approaches to the simulation of BHEs assume heat transfer in circulating fluid and grout to be in a quasi-steady state and ignore fluctuations in fluid temperature due to transport of fluid through the loop. However, the dynamic response of the circulating fluid has important implications for system design and operation under some circumstances. By applying the numerical model, this paper examines the dynamics of fluid transport and the transient response of a BHE at short timescales in detail by considering the variations in outlet temperature in response to the variations of inlet temperature. The response at short timescales has been characterised in terms of response to periodic variation in inlet temperature at a range of frequencies. This frequency response is compared to that of related two-dimensional models and the transient behaviours of the BHE which can only be captured by the three-dimensional model are highlighted. The model is being used to develop improved simplified models of BHEs.