An investigation of unsteady thermal exchange processes in geothermal heat exchangers

“Geothermal heat exchangers, has become one of the highly recognized non conventional heat exchangers as they utilize renewable energy source earth to initiate the thermal energy transfer. However, these heat exchangers suffer from a expensive installation cost due to their conservative design model, thereby facilitating the need to identify and improve the design. In this dissertation, three gaps, which identifies improvement over conservative design were investigated, to assess the heat exchanger performance. In the first approach, borehole thermal and geometrical properties were investigated keeping the outlet leg of the heat exchanger as adiabatic, which neglects the thermal short-circuiting between the pipes. The results indicated that increasing the thermal mass of the borehole 5 times to that of the conventional properties and placing the pipes as close to the centre of the borehole improved the heat exchanger performance by 42%. In the second approach, these modified properties were compared with the conventional properties by allowing the thermal short-circuiting between the pipes. The results indicated that energy losses were up to 35% for conventional properties and 45% for the modified properties. Hence, thermal solutions such as placing an inverse L-shaped barrier close to the outlet pipe were offered which not only improved the thermal performance by 50% but also reduced the thermal losses by 20%. In the third approach, the thermal exchange between the borehole and the ground were investigated treating the ground as a porous medium. The results indicated that, absent ground water flow, the impact of adiabatic ground surface, where the borehole first meets the ground, was significant when compared against the similarity solutions. The presence of groundwater flow caused by hydraulic gradients produces mixed convection and the dimensionless group, Lb/D √PeD/RaL is shown to capture the mixed convection domain between the natural and forced convection limits. The values of the dimensionless group are in the order one range, corresponding to the transition between the limiting behaviors”--Abstract, page iv