| Abstract |
A typical downhole heat exchanger consists of a single loop of tubing placed in a geothermal well. Energy is extracted from the geothermal fluid in the well by transfer of heat through the tube wall to clean water, the working fluid, circulating in the loop. The performance of a DHE can be improved by placing a "promoter", a single length of tubing which is open or slotted at both ends, in the well to promote circulation of the geothermal fluid within the wellbore itself. In this study, two configurations involving such a "promoter" are considered: in the first, the DHE tubing loop carrying the clean water is placed inside the promoter tube, while in the second configuration the loop is outside, beside, the promoter. In the present context, the first configuration can include the case where a slotted liner hangs loosely in the and fluidcan move between it and the well face. Here, modelling of such systems is canied out by using mass and heat balances, resulting in a set of linear ordinary differential equations which describe the steady-state flow. With the assumption that the circulation of the clean water is maintained at a fixed constant and that the geothermal fluid is circulating in the wellbore by natural convection (under a balance of buoyancy and frictional forces), the equations are solved by a combination of analyticaland numerical methods. Material parameters and dimensions are varied to investigate their effect on the power output of the DHE. One major theoretical result is that a much higher power output is obtained if the loop is placed beside the promoter tube, rather than inside it. |