Ground-source Heat Pumps In Ireland: Heat Transfer Experimentation For The Design And Modelling Of A Sub-slab Installation.
Authors:   ,   Cork Institute of Technology, O'Connell Sarah 
Published: June 2024

"Ground-source heat pump technology is currently undergoing rapid growth in Ireland. Three aspects of ground-source heat pumps have been studied in this thesis. The market expansion taking place in Ireland, heat transfer from the soil to the pipe of a ground- source heat pump collector, and the design of an innovative sub-slab ground-source heat pump system. Initial investigations into the use of ground-source heat pumps in Ireland revealed surprising growth in the use of the technology. There have been approximately 1500 systems installed mainly since 2000. The ground-source heat pump market in Ireland has an estimated annual growth rate of 45%. Ground-source heat pumps are being targeted as a means by which CO2 emissions can be reduced and help Ireland meet the terms of the Kyoto protocol. It is estimated that heat pumps have the potential to reduce CO2 emissions by 617000 tonnes C02/year. In order for continued successful growth of the ground-source heat pump market in Ireland, policy decisions, incentives and regulation of the quality of the installed systems are necessary. The study of the heat transfer mechanisms by which heat is transferred from the soil to the ground-source heat pump collector is vital for greater understanding of collector operation. The effect of theimal contact resistance between the soil and the collector pipe had not previously been measured. An experimental test rig was designed and constructed to measure thermal contact resistance between soil, sand, gravel, broken stone samples and a polyethylene surface. The test rig was based on the guarded hot plate apparatus specified in ASTM Standard Cl77-85. The design criteria for the test rig were established based on the operation of a sub-slab ground-source heat pump collector. The results of the thermal contact resistance testing revealed that thermal contact resistance for all samples was less than 5% of the total thermal resistance between the soil and the pipe. For wet sand thermal contact resistance was less than 1% of the total thermal resistance. Wet soil exhibited no thermal contact resistance. It was concluded that thermal contact resistance does not obstruct conduction from the soil to the collector pipe for a sub-slab ground-source heat pump installation. The third section of the project developed a design for a sub-slab ground-source heat pump collector. The design of a horizontal collector pipe array for a ground-source heat pump installed in the footprint of a building was evaluated using a finite element model. Structural and geotechnical analysis established the conditions for which the design of a sub-slab system is best suited. The thermal finite element analysis found that the successful operation of a sub-slab system is achievable if half the heating demands of the building are met by the sub-slab collector; an area of exposed ground greater than half the footprint area is available; and the correct insulation and backfill materials are used. Full soil temperature recovery occurred by the end of August, 4 months after the heating season had ended."