Abstracts Category : Other

Analytical and Experimental Study on Coaxial Borehole Heat Exchangers

by David Tyler Gordon

This research focuses on methods of direct-use geothermal energy considering a coaxialborehole 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 thesurrounding ground and the conditioned space of a building. Various methods exist to accomplishthe ground-side heat exchange for a GSHP, where the focus of this thesis remains on closed-loopsystems which utilize loops of fused high-density polyethylene (HDPE) pipes buried vertically inboreholes ranging between 80 and 200 meters deep. This thesis provides an overview of thecritical design considerations used in sizing a BHE where a comparison is made between a typicalU-tube BHE and a thermally improved coaxial BHE where various benefits may be realized by thelatter. The motivation for this research is to provide a tool to accurately compare various coaxialsystems, 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 relieson a curve-fitted cylindrical response function, or g-function. The CCx model is made to produceaccurate simulations for the fluid temperature measured at the outlet of a coaxial BHE over thecourse of a typical thermal response test (TRT). The model considers coaxial configurations wherethe inner and outer pipes may have differing thermal properties, diameters, and thicknesses. Themodel is validated using known input parameters and physical measured temperature data forthree different TRTs showing root mean square errors (RMSE) as low as 0.09 C, which is wellwithin the uncertainty of the measurement for the given test. The general development of themodel is largely empirical in nature, where various aspects were introduced keeping logicalconstraints in mind to produce an acceptable fit to each of the three physical tests. Furtherexperimental analysis is performed using a lab-scale coaxial heat exchanger to verify the trendsproduced by the CCx model during short term operation considering laminar annular flow. Themeasured outlet fluid temperature is again compared to the temperature simulated by the CCxmodel showing an RMSE of 0.16 C, which is again found to be within the uncertainty of themeasurement. 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 ofcoaxial systems. This model is capable of capturing the effect of various pipe materials and sizesas shown through the validation presented in this thesis.Advisors/Committee Members: Ting, David, Bolisetti, Tirupati.