PARAMETRIC STUDY OF THE THERMAL-HYDRAULIC CHARACTERISTICS OF DEEP BOREHOLE CO-AXIAL HEAT EXCHANGER IN A GEOTHERMAL WELL

Geothermal energy constitutes an additional option for energy planners and can have significant advantages when compared to other renewable sources. The work presented here relates to geothermal wells located in areas of average and high geothermal gradients, i.e. 30 and 80 K/km thermal diffusivity of 9.27 m2/s and a soil temperature of 12 oC, as an average value. A closed-loop 3 and 5 km deep borehole co-axial heat exchanger was modelled using in-house developed software (WellTH). Water flows into the annulus space of the heat exchanger and upwards in the inner pipe. Studies were conducted for two different types of commercially available inner upward flow pipes, namely a High-Density Polyethylene (HDPE) and a vacuum-insulated tubing (VIT). The mass flow rate was varied from 1 to 15 kg/s. The water inlet temperature ranged from 10 to 80 oC to allow for the varying return temperature from possible different heating or power plants using the geothermal energy at ground level. Results are presented in the form of outlet temperature and thermal output as a function of the inner pipe insulation, mass flow rate, and water inlet temperature for operational periods up to 30 years and demonstrate the importance of these parameters. The pressure drop in the heat exchanger and the power required at the pump are included in the analysis. Finally, the possible output in terms of electricity production and heating using a heat pump are calculated