An extended two-dimensional borehole heat exchanger model for simulation of short and medium timescale thermal response

Common approaches to the simulation of Borehole Heat Exchangers (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 the fluid around the U-tube loop. Such effects have been shown to have an impact on peak temperatures and hence operation of heat pumps systems when short time scales are considered. A model has been developed that combines a two-dimensional numerical model and models of the pipe loop components. A novel heat exchanger analogy is employed to calculate the heat exchanger outlet temperatures such that iterative procedures can be avoided and numerical stability is unconditional. These approaches result in a model that is computationally efficient and captures much of the short timescale dynamic effects represented in fully three-dimensional models. This is demonstrated by comparison with experimental data and by comparing two and three-dimensional model behaviour in the frequency domain. Predicted monthly outlet temperatures and heat transfer rates are furthermore shown to be in close agreement with experimental values and in good agreement with existing borehole heat exchanger models. The model is computationally efficient enough to allow use in routine analysis and design tasks.