Thermodynamic assessment of downhole heat exchangers for geothermal power generation

Abstract

Downhole heat exchanger is a device to extract heat from geothermal fluid. While it is widely used for heating purposes, its use for power generation has not been reported. The aim of this study is to examine the feasibility of power generation from a 2500 m deep existing geothermal well with high temperature gradient and insufficient flowrate by using a downhole heat exchanger. For this purpose, a thermodynamic and an economic evaluation model are developed by the use of Engineering Equation Solver software. Additionally, the parametric studies have been carried out to identify the effects of insulation, geothermal well conditions, geometry of downhole heat exchanger, mass flowrate and type of working fluids on the performance of downhole heat exchanger system. Consequently, work output of the best alternative is computed as 2511 kWe with 64 kg/s mass flowrate of R-134a for 2500 m-deep downhole heat exchanger having inner pipe diameter of 0.127 m. Electricity generation cost and simple payback time are calculated as 46 $/MWh and 2.25 years, respectively. The obtained results showed that the downhole heat exchanger system can be a feasible alternative for wells with very low geothermal flowrate to generate power.