Abstract:
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 fullydeveloped
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 firstand
second-law efficiencies.
