ASSESSING THE ROLE OF PCM AND WELL DESIGN IN ENHANCING THE PERFORMANCE OF COAXIAL GEOTHERMAL WELLS

Abstract

Geothermal energy is becoming a promising source of clean energy as the world transitions away from fossil fuels. It can be exploited through a closed loop system where a fluid circulates from the surface to underground formations in a closed well and exits at higher temperatures. Major problems in closed loop geothermal wells include the early thermal depletion of the formation around the well, and thermal short circuiting due to the long distance travelled by the fluid back to the surface. Conventional ways to reduce the impact of such problems is to install an insulation around the tubing and regulate the operation conditions of the well to optimize heat extraction and formation thermal depletion.

In this work, a two-dimensional axisymmetric coaxial geothermal well model of depth 1000 m is simulated by coupling fluid flow and heat transfer using the finite element method. The simulated well model is benchmarked and validated against a steady state model. The thermal energy production is studied for different well design by varying the inner pipe thickness, the properties of the grout, and including an air gap or phase change material to the inner pipe.

The results displayed improvement in the well performance after increasing the inner pipe thickness from 1 cm to 2 cm and using thermally enhanced grout by 65% and 3.6%, respectively. Moreover, geothermal well performance improved after the deployment of an air gap inside the inner pipe with an optimal performance improvement of 5.54% achieved for an air gap height of 400 m. Furthermore, adding PCM in the inner pipe shows enhancement in the overall well performance. Finally, the analyses were complemented by an economic evaluation by calculating the levelized cost of energy for each model. Models with an inner pipe gap have the lowest LCOE among the studied models.