Numerical simulation and sensitivity study of double-layer Slinky-coil horizontal ground heat exchangers

Abstract

In ground-source heat pump (GSHP) systems, the application of Slinky-coil horizontal ground heat exchangers (HGHEs) greatly reduces the initial costs for the system since the HGHEs can be constructed using common excavation machines. Though HGHEs have been commonly used in the United States and Canada, where abundant land space is available for installing HGHEs, the reduction of the land area requirement is important for the wider application of the system to other regions of the world. For this purpose, the introduction of a double-layer Slinky-coil HGHE is considered an effective choice if the heat exchange rates are much more than those for single-layer HGHEs.In this study, long-term cooling and heating tests, using single-layer and double-layer Slinky-coil HGHEs as the heat source, were conducted in Fukuoka, Japan to compare their heat exchange capacities. The tests showed that the heat exchange capacity of HGHEs per unit land area is remarkably enhanced by the introduction of double-layer HGHEs. Numerical simulation models were then developed for the HGHEs on the basis of the procedures of Fujii et al. (2012) after modifications of surface boundary conditions. The models could successfully reproduce the temperature behaviors of the heat medium (heat carrier fluid) and ground temperatures in the cooling and heating tests, demonstrating the reliability of the numerical model for double-layer Slinky-coil HGHEs.Using the model, sensitivity studies were performed to optimize the design of the double-layer HGHEs. The results of the sensitivity study on installation depth showed that the optimum depth of the upper layer was 1.5m in case that the depth of the lower layer was fixed at 2.0m. The preferable direction of heat-medium circulation was then investigated and it was concluded that circulation from the upper layer to the lower layer is the most suitable direction. Finally, the influence of the reflectance of the land surface was investigated by changing the albedo of the land surface as 0.1, 0.3 and 0.6. The numerical simulations showed that lower albedo is preferable in heating operations, while higher albedo is favorable in cooling operations.