Analytical and Experimental Study on Coaxial Borehole Heat Exchangers

This research focuses on methods of direct-use geothermal energy considering a coaxial borehole heat exchanger (BHE) as a major component in a ground-source heat pump (GSHP) system. A GSHP system is a sustainable energy system that transfers thermal energy between the surrounding ground and the conditioned space of a building. Various methods exist to accomplish the ground-side heat exchange for a GSHP, where the focus of this thesis remains on closed-loop systems which utilize loops of fused high-density polyethylene (HDPE) pipes buried vertically in boreholes ranging between 80 and 200 meters deep. This thesis provides an overview of the critical design considerations used in sizing a BHE where a comparison is made between a typical U-tube BHE and a thermally improved coaxial BHE where various benefits may be realized by the latter. The motivation for this research is to provide a tool to accurately compare various coaxial systems, where a semi-analytical model for heat transfer is proposed. The proposed model, referred to as the composite coaxial (CCx) model, is semi-analytical in nature being that it relies on a curve-fitted cylindrical response function, or g-function. The CCx model is made to produce accurate simulations for the fluid temperature measured at the outlet of a coaxial BHE over the course of a typical thermal response test (TRT). The model considers coaxial configurations where the inner and outer pipes may have differing thermal properties, diameters, and thicknesses. The model is validated using known input parameters and physical measured temperature data for three different TRTs showing root mean square errors (RMSE) as low as 0.09 °C, which is well within the uncertainty of the measurement for the given test. The general development of the model is largely empirical in nature, where various aspects were introduced keeping logical constraints in mind to produce an acceptable fit to each of the three physical tests. Further experimental analysis is performed using a lab-scale coaxial heat exchanger to verify the trends produced by the CCx model during short term operation considering laminar annular flow. The measured outlet fluid temperature is again compared to the temperature simulated by the CCx model showing an RMSE of 0.16 °C, which is again found to be within the uncertainty of the measurement. In summary, the primary contribution of this research is the CCx model itself, where this model has been developed as a tool for future use in the case-by-case optimization of coaxial systems. This model is capable of capturing the effect of various pipe materials and sizes as shown through the validation presented in this thesis