Influence of Convection on Production from Borehole Heat Exchangers

Abstract

Convection cells in a porous medium form when the medium is subjected to sufficient heating from below (or equivalently, cooling from above) or when cooled or heated from the side. In the context of geothermal energy extraction from a sealed system, we are interested in how the convection cells transport heat when the porous medium is subjected to a cool, sealed borehole, also known as a borehole heat exchanger. Using pseudospectral methods together with domain decomposition we consider two scenarios; one system initialized with constant temperature and one system initialized with a vertical temperature gradient in the rock surrounding the borehole heat exchanger. We find the convection cells to have a positive effect on the heat extraction for a constant initial rock temperature, but a negative effect for some of the systems with an initial temperature gradient in the rock: Convection gives a negative effect when the borehole temperature is close to the rock temperature in the borehole, but gradually provides a positive effect when the borehole temperature gets colder and the convection stronger.