Design and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS)

• Published in: Renewable energy Vol. 55; pp. 128 - 137
• Main Authors: Yekoladio, P.J., Bello-Ochende, T., Meyer, J.P.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.07.2013; Elsevier
• Subjects: Applied sciences; Binary cycle; Computational fluid dynamics; cooling; Devices using thermal energy; Downhole coaxial heat exchanger; Earth; Energy; Energy. Thermal use of fuels; Enhanced geothermal system; entropy; Entropy generation minimization analysis; Exact sciences and technology; Exergy analysis; Fluid flow; friction; Geothermal; Heat exchangers; Heat exchangers (included heat transformers, condensers, cooling towers); heat transfer; mass flow; Natural energy; Optimization; pipes; renewable energy sources; temperature; Turbulence; Turbulent flow; Binary cycle; Downhole coaxial heat exchanger; Exergy analysis; Enhanced geothermal system; Entropy generation minimization analysis; Energy analysis; Renewable energy; Heat exchanger; Optimization
• ISSN: 0960-1481; 1879-0682
• DOI: 10.1016/j.renene.2012.11.035

The present study considers the design, performance analysis and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS). The optimum mass flow rate of the geothermal fluid for minimum pumping power and maximum extracted heat energy was determined. In addition, the coaxial pipes of the downhole heat exchanger were sized based on the optimum geothermal mass flow rate and steady-state operation. Transient effect or time-dependent cooling of the Earth underground, and the optimum amount and size of perforations at the inner pipe entrance region to regulate the flow of the geothermal fluid were disregarded to simplify the analysis. The paper consists of an analytical and numerical thermodynamic optimization of a downhole coaxial heat exchanger used to extract the maximum possible energy from the Earth's deep underground (2 km and deeper below the surface) for direct usage, and subject to a nearly linear increase in geothermal gradient with depth. The thermodynamic optimization process and entropy generation minimization (EGM) analysis were performed to minimize heat transfer and fluid friction irreversibilities. An optimum diameter ratio of the coaxial pipes for minimum pressure drop in both limits of the fully turbulent and laminar fully-developed flow regime was determined and observed to be nearly the same irrespective of the flow regime. Furthermore, an optimum geothermal mass flow rate and an optimum geometry of the downhole coaxial heat exchanger were determined for maximum net power output. Conducting an energetic and exergetic analysis to evaluate the performance of binary power cycle, higher Earth's temperature gradient and lower geofluid rejection temperatures were observed to yield maximum first- and second-law efficiencies. ► Optimum diameter ratio of coaxial pipes in an EGS application to minimize pressure drop does not dependent on the flow regime. ► Optimum geofluid mass flow rate for maximum extracted heat energy from the Earth's underground to increase exponentially with the flow Reynolds number. ► First- and second-law efficiencies are maximized by higher Earth's temperature gradient and lower geofluid rejection temperatures.