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Abstract: This review paper summarizes recent developments regarding geothermal exploitation using coaxial deep borehole heat exchangers (DBHE). Specifically, this study focuses on field tests, analytical and semi-analytical approaches, and numerical simulations. First, field tests and applications of coaxial DBHE are summarized and future work for the field tests is suggested. Then, the ongoing analytical and numerical modeling approaches on coaxial DBHE are evaluated regarding the capability and incapability of describing physical behaviors. Lastly, key factors for the design of coaxial DBHE are summarized and discussed based on collected results. Regarding field tests, future work should focus more on (1) long-term performance; (2) effect of groundwater flow within formation and fractures; (3) technology for larger diameter boreholes; (4) new and cheap materials for insulated inner pipe; (5) treatment of fluid, pipe wall, and different working fluid; (6) economic analysis of coaxial DBHE-based geothermal power plant. As for the analytical methods and numerical simulations, it is important to consider the dependence of fluid and formation properties on pressure and temperature. Besides, verification and calibration of empirical models for working fluids other than water such as CO2 should be performed based on laboratory and field tests. Different borehole properties and pump parameters should be optimized to obtain the maximum thermal power of a coaxial DBHE, and an insulated inner pipe is recommended by many researchers. An intermittent working pattern of the DBHE could be more realistic when modeling a DBHE. To further improve the performance of coaxial DBHE, continuous research to enhance heat transfer and working fluid performance is still important.

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).