Simulation of Ground Coupled Vertical U-Tube Heat Exchangers

Abstract

Ground coupled heat pumps are an efficient alternative to conventional methods of conditioning homes, because instead of using the ambient air they utilize the ground as an energy source or sink. However, ground coupled heat pumps have high installation costs that makes it critical to design the system to maximize performance. Vertical u-tube heat exchangers are commonly used as the ground coupled heat exchanger, but estimating their performance is difficult because of the unique heat transfer conditions of this configuration. This thesis focuses on modeling the vertical u-tube heat exchanger. Several initial attempts to model the heat exchanger were made, and finally an explicit euler finite difference numerical technique was employed. The ground storage volume is divided axially into sections, and each section is a two dimensional cylindrical mesh representing the fluid, tubes, grout, and soil at a specific depth. The tubes are approximated by non circular sections of the mesh, and is accurate to within 8%. A local coupling factor can increase this accuracy to 3% for most systems, and comparisons with an existing model showed good agreement. The finite difference model has provided an approach that is fundamental and readily extended to more realistic conditions. It is accurate and fast enough to be useful as both a comparison to existing models and as a design tool.