A new 1D analytical model for investigating the long term heat transfer rate of a borehole ground heat exchanger by Green's function method

In ground source heat pump (GSHP) applications, long term performance prediction of a borehole is an important issue since performance of a borehole ground heat exchanger decreases by the life time of the system. Therefore, it is important to find a reliable method determining the long term performance of a borehole. In this study, a new 1D analytical expression for heat transfer rate (HTR) per unit length of a borehole is derived at constant borehole wall temperature condition by using Green's function method (GFM). A 50 m-depth borehole containing U-tube polyethylene pipes with 32 mm outside and 26.6 inside diameter is considered. Experimental measurements are done by pumping warm water of 40 °C into the pipe system for 120 h. Variation of HTR value per length of borehole in time is determined from the experimental data. Analytical results based on GFM are verified by experimental measurements. Based on the validated analytical model, long term HTR values (thermal performance) per length of borehole are predicted. Verification of numeric and experiment results illustrates the compromise of them. Furthermore, temperature distribution around borehole is investigated by using new given analytical expression. Engineers can use analytical models that are given in this study in designing of GSHP systems.