Thermal analysis models of deep borehole heat exchangers

With the advantages of much less land demand and higher temperature available, the deep borehole heat exchanger (DBHE) gets down to a depth of 1000-3000 m below the ground surface, and provides a new variance of ground-coupled heat pump systems especially for applications in cold-climate regions. Coaxial tubes, instead of U-tubes, are usually used in DBHEs. Two models are presented for DBHE thermal analysis in this paper. One follows the traditional approach based on analytical solutions and the concept of the effective borehole thermal resistance for the boreholes with coaxial tubes. The other is a numerical simulation scheme based on the FDM which takes the geothermal gradient into account. The latter features much higher efficiency in computation than most commercially-available software toolkits based on FEM. The performance of DBHEs is then assessed with parameter analyses