20 September 2011

FLUID DYNAMICS AND HEAT TRANSPORT IN CO2 GEOTHERMAL HEAT PROBES


In this research project the fluid dynamics and thermodynamics of a CO2
geothermal heat probe have been investigated. The probe, which works like a thermosyphon, was analyzed by means of a simulation model in Matlab. The model couples the flow behaviour and heat transport inside the heat probe with the unsteady heat conduction in the soil. The study revealed that the self-circulation within the probe leads to more uniform heat
removal. This “energy shifting effect” due to the evaporation/condensation behaviour of CO2 works even during the regeneration phase. The heat transfer resistance of the liquid film is
very small compared to the soil conduction resistance. Therefore, no performance differences have been found between a conventional heat pipe and a configuration with multiple injection heights. It is estimated that the seasonal performance factor of heat pumps can be improved by 15 to 25%. The main advantage is the heat transfer to the heat pump evaporator which is much more efficient than in a conventional brine probe. Finally, practical guidelines for the design of CO2 geothermal heat probes are presented in this contribution.