Study on the efficiency of deep borehole heat exchangers

Abstract

The following paper shows results of several studies, carried out with the Earth Energy Designer program (EED3.21). Having used the EED, analyses on efficiency of the heat extraction were conducted, and measurements of productivity were taken in dependence of variable parameters of a theoretical deep coaxial borehole heat exchanger. The depth of 1000 m and the constant heating load 80 MWh per year were assumed for the borehole exchanger. The variables analyzed were: type of the heat carrier fluid, flow rate of the heat carrier, diameter of the borehole, wall thickness of the inner tube (insulation), diameter of the inner tube, thermal conductivity of the inner tube material, wall thickness of the outer tube, diameter of the outer tube and thermal conductivity of the outer tube material. Due to mathematical basis of the EED program, results of the calculations can be considered as diminished. To operate correctly, the program can use parameters from a borehole not deeper than 300 meters. At greater depths of borehole heat exchangers, results are understated. Therefore the results of analyses presented below can be seen as a pessimistic scenario of the calculations.