UC San Diego

ABSTRACT: This paper presents an investigation into the feasibility of low thermal-gradient deep Borehole Heat Exchanger (BHE) applications on campus at the University of California San Diego (UCSD). With a review of primary source data for various formations and well logs around the UCSD campus, the stratigraphy and thermo-physical properties of the formation were evaluated. Based on the collected information, a numerical model for the BHE application was developed. Circulation of water through a closed coaxial loop system considers the variation of several parameters in order to develop the feasibility study and obtain guidelines for the future UCSD geothermal resource. For example, multiple layers of the formation and thus the variance of ground thermal conductivities along the vertical direction were considered, and fluid velocity and borehole depth were varied. To do that, a numerical scheme that employs finite element schemes for modeling Navier-Stokes fluid flow and heat flow and transport in the BHE system constructed the numerical model. The numerical model verification was first performed via comparisons with a field test on BHE from the literature. Then the effects of the thermal properties, volumetric flow rate, and other relevant parameters were systematically analyzed by using the developed numerical modeling. The proposed model can be used to evaluate the suitability of BHE applications with low thermal-gradient in southern California. INTRODUCTION: The utilization of fossil fuels has surged significantly during the last half-century, resulting in enormous carbon dioxide (CO2) emissions and thus significant climate change. To tackle that, the United Nations, the G7 economies and many countries around the world have started or are going to promote the shift to renewable energy (UN DESA 2017; Gielen et al. 2019; G7 2021). Among different renewable energy, geothermal energy has been used for centuries and it is expected to meet 3%-5% of global demand by 2050 (Craig and Gavin 2018). Based on the depth of geothermal energy reservoirs, there are two major types of geothermal systems: shallow geothermal systems (≤400 m) and deep geothermal systems (> 400 m) according to White (1966).