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

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 timedependent 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 first-and second-law efficiencies.

Second law analysis of a fossil‐geothermal hybrid power plant with thermodynamic optimization of geothermal preheater

“…Once the stream (indicated by arrows) reaches the bottom of the well, it is heated and then returned to the surface through the inner pipe that is well insulated to minimize heat loss to the annulus. 23,24 The fundamental design question concerns the selection of the diameter of the inner pipe D i . The flow is at risk of being impeded if the inner diameter D i is much smaller than the outer diameter D o or if D i is nearly the same as D o .…”

“…The development of the entropy generation minimization method for DCHE is presented below. The ratio of local pressure drop to distributed pressure drop was developed by Yekoladio 24 in the following form:…”

“…The analysis of the pressure drop ratio proved that, for a downhole coaxial pipe with length that is 20 times greater than its diameter, the local pressure drop at the lower extremity of the well can be neglected for both laminar and turbulent flow. 24 From the first and second laws of thermodynamics, for a pipe of arbitrary cross section "A" and wetted perimeter "p," the entropy generation rate is derived as follows 24 :…”

“…Following further evaluation and applying turbulent flow conditions, the following equations were obtained 24 :…”

Thermodynamic analysis and performance optimization of organic rankine cycles for the conversion of low-to-moderate grade geothermal heat

“…The geothermal fluid flows down the annular space of a downhole coaxial heat exchanger of an enhanced geothermal system (EGS) [2,16]. The coaxial pipes dimensions and fluid circulation flow rate were optimized by Yekoladio et al [17] for minimum pumping power and maximum extracted heat energy from the Earth"s deep underground. The warmer geothermal fluid is used to completely vaporize a secondary or binary fluid with a lower boiling point and higher vapour pressure through a closed pipe system heat exchanger.…”