Abstract
This paper investigates the feasibility of Coaxial Deep Borehole Heat Exchanger (CDBHE) applications to the campus at the University of California San Diego (UCSD). By collecting different geophysical source data for various formations and well logs around the UCSD campus, a multilayered thermophysical model for the ground on the site is established. Circulation of water within a closed coaxial loop system considers the geothermal energy extraction under uncertainity consideration of the unknown deeper layers heat flow gradient as coupled with the variation of pipe insulation properties, flow rates, outer pipe diameter, grout and depths between 1 km and 4 km. A finite-element framework is constituted to model the Navier-Stokes fluid flow and heat transfer in the CDBHE system, and validated with a field test on CDBHE from the literature. It was found that a 4 km CDBHE could produce a thermal power of 600 kW under the optimum geological conditions at the UCSD site. Thermal power shares from different layers indicate that deeper formation layers contribute more to the thermal power than the shallower layers. An inner pipe with an insulated depth of 2 km produces only 1–6% less power than a fully insulated inner pipe for the 4 km CDBHE, and thus a partially insulated VIT-plastic inner pipe is suggested. Furthermore, the CDBHE thermal power increases by 5% when the grout thermal conductivity increases from 1 to 3.65 W/(K∙m), close to the formation thermal conductivity, and then maintains almost the same, and the 4 km CDBHE with flow rates of 2.78–6.94 L/s at the UCSD site can directly supply a low-temperature heating radiator system for room heating. The effects of the investigated factors provide guidelines for future geothermal resource exploitation in southern California.