Informing the practice of ground heat exchanger design through numerical simulations
Haslam, Simon R.
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Closed-loop ground source heat pumps (GSHPs) are used to transfer thermal energy between the subsurface and conditioned spaces for heating and cooling applications. A basic GSHP is composed of a ground heat exchanger (GHX), which is a closed loop of pipe buried in the shallow subsurface circulating a heat exchange ﬂuid, connected to a heat pump. These systems oﬀer an energy eﬃcient alternative to conventional heating and cooling systems; however, installation costs are higher due to the additional cost associated with the GHX. By further developing our understanding of how these ground loops interact with the subsurface, it may possible to design them more intelligently, eﬃciently, and economically. To gain insight into the physical processes occurring between the GHX and the subsurface and to identify eﬃciencies and ineﬃciencies in GSHP design and operation, two main research goals were deﬁned: comprehensive monitoring of a fully functioning GSHP and intensive simulation of these systems using computer models. A 6-ton GSHP was installed at a residence in Elora, ON. An array of 64 temperature sensors was installed on and surrounding the GHX and power consumption and temperature sensors were installed on the system inside the residence. The data collected were used to help characterize and understand the function of the system, provide motivation for further investigations, and assess the impact of the time of use billing scheme on GSHP operation costs. To simulate GSHPs, two computer models were utilized. A 3D ﬁnite element model was employed to analyse the eﬀects of pipe conﬁguration and pipe spacing on system performance. A unique, transient 1D ﬁnite diﬀerence heat conduction model was developed to simulate a single pipe in a U-tube shape with inter-pipe interactions and was benchmarked against a tested analytical solution. The model was used to compare quasi-steady state and transient simulation of GSHPs, identify system performance eﬃciencies through pump schedule optimization, and investigate the eﬀect of pipe length on system performance. A comprehensive comparison of steady state and pulsed simulation concludes that it is possible to simulate transient operation using a steady state assumption for some cases. Optimal pipe conﬁgurations are identiﬁed for a range of soil thermal properties. Optimized pump schedules are identiﬁed and analysed for a speciﬁc heat pump and ﬂuid circulation pump. Finally, the eﬀect of pipe spacing and length on system performance is characterized. It was found that there are few design ineﬃciencies that could be easily addressed to improve general design practice.