Comparison of the thermal and hydraulic performance of single U-tube, double U-tube and coaxial medium-to-deep borehole heat exchangers

Brown, C. S. , Kolo, I. , Banks, D. and Falcone, G. (2024) Comparison of the thermal and hydraulic performance of single U-tube, double U-tube and coaxial medium-to-deep borehole heat exchangers. Geothermics, 117, 102888. (doi: 10.1016/j.geothermics.2023.102888)

Abstract

Decarbonisation of heat is essential in curbing carbon dioxide emissions and can be achieved through the use of geothermal systems. Recently, single-well, closed-loop, deep borehole heat exchangers, using a coaxial design, have become the focus of attention, partly due to the potential to repurpose existing infrastructure (such as oil and gas wells); however, few have investigated the potential for other types of heat exchanger for middle-deep geothermal systems. Therefore, in this study, a comprehensive numerical analysis was undertaken using OpenGeoSys software to investigate the thermal and hydraulic performance of coaxial, single U-tube and double U-tube middle-deep borehole heat exchangers (MDBHEs). The purpose of this paper is to test the maximum operational depth for each type of pipe configuration as few wells have been completed to depths exceeding 500 m using single/double U-tube configurations. The best performing MDBHEs should minimise parasitic and hydraulic losses, whilst maximising thermal output. Furthermore, ground sourced heat pumps require electricity; therefore, at times where electricity prices are high (and drilling costs can be minimised) it may be more beneficial to utilise MDBHEs to encounter greater temperatures. Results indicate that coaxial MDBHEs provide the best performance in terms of specific heat extraction and lowest pressure/parasitic losses. Double U-tube MDBHEs can provide a similar thermal performance to the coaxial design, but have significantly greater hydraulic pressure losses across all simulations, which translates to greater parasitic pumping power costs. Single U-tube MDBHEs demonstrate the poorest performance in terms of heat extraction and pressure losses. At the end of the 25-year base case scenario for a 800 m MDBHE, coaxial, U-tube and double U-tube configurations, all with a fluid circulation rate of 5 L/s, provided specific heat extraction rates of 39.1 W/m, 32.8 W/m, and 36.0 W/m, respectively, with the fluid inlet temperature set as a constant of 5 °C. For these simulations, pressure losses were estimated as 85 kPa (coaxial), 1.46 MPa (single U-tube) and 423 kPa (double U-tube)—the single U-tube value being close to the nominal 16 bar (1.6 Mpa) pressure rating of SDR11 high density polyethylene pipe. Further parametric analysis was also undertaken, investigating depth, flow rate, rock thermal conductivity, pipe diameter and shank spacing.

• Item Type: Articles
• Additional Information: This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) grant 871 EP/T022825/1. The funding source is for the NetZero GeoRDIE (Net Zero Geothermal Research for 872 District Infrastructure Engineering).
• Keywords: Single U-tube, double U-tube, coaxial, borehole heat exchanger, OpenGeoSys, middle-deep, deep, DBHE.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Brown, Dr Christopher and Falcone, Professor Gioia and Banks, Mr David and Kolo, Dr Isa

Creator Roles

Brown, C.: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review and editing
Kolo, I.: Formal analysis, Methodology, Software, Writing – original draft, Writing – review and editing
Banks, D.: Formal analysis, Supervision, Writing – original draft, Writing – review and editing
Falcone, G.: Funding acquisition, Project administration, Resources, Supervision, Writing – review and editing

• Authors: Brown, C. S., Kolo, I., Banks, D., and Falcone, G.
• College/School: College of Science and Engineering > School of Engineering > Systems Power and Energy
• Journal Name: Geothermics
• Publisher: Elsevier
• ISSN: 0375-6505
• ISSN (Online): 1879-3576
• Published Online: 07 December 2023
• Copyright Holders: Copyright © 2023 The Authors
• First Published: First published in Geothermics 117: 102888
• Publisher Policy: Reproduced under a Creative Commons License