Long Term Performance Prediction of a Borehole Ground Heat Exchanger by Green's Function Method

Abstract

The ground source heat pump systems have received considerable attention in last decades as an energy efficient heating/cooling system for residential and commercial spaces. In these systems, long term performance prediction of a borehole is a critical issue since the heat exchange performance of a borehole decreases through the life time of the system. Therefore it is important to find a reliable method determining the long term performance of a ground heat exchanger (GHE). In this study, an analytical expression for heat transfer ratio per unit length (HTR value) of a borehole GHE is derived for 1D and 2D models at constant borehole wall temperature condition by using Greens function method (GFM). A 100m-depth borehole consisting of U-shaped polyethylene pipes with 32mm outside diameter is considered. Experimental measurements are done by pumping warm water of 40 oC into the pipe system for 120 hours. Variation of HTR value in time is calculated from the experimental data. Theoretical results based on GFM are compared with those of computational model in COMSOL environment and validated by experimental measurements. Thermal conductivity and diffusion constant of the ground is determined by fitting the theoretical model to the experimental data and then long term HTR values are predicted. Also a comparison of the long predictions of theoretical and computational models is given. Effectiveness of GFM is shown. Long term HTR predictions based on GFM can be used for a realistic engineering design of ground source heat pump systems.