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  Influence of system parameters on thermal performance of energy slab installed in underground structures

  The energy slab is a ground coupled heat exchanger equipped in building slab structures, which has a similar layout to the horizontal ground heat exchanger (GHEX). As the energy slab is generally installed at the shallow depth, its thermal performance is notably inferior to the conventional vertical GHEXs. This is because the energy slab cannot use the high thermal capacity of a deep underground formation, and is significantly affected by ambient air temperature. However, if the energy slab can be installed as a floor slab or side wall of underground structure, it will be a strategic way to utilize stable geothermal energy for cooling/heating underground space. In the energy slab, the effect of ambient temperature, the presence of thermal insulation layer, the thermal conductivity of each component, and the hydraulic characteristics of ground formation become considerably important comparing to the conventional GHEX. However, there is no reliable method to evaluate the system parameters in the energy slab. In this paper, a series of experimental studies and numerical analyses was conducted to assess the effect of the system parameters (i.e., the pipe material, the thermal conductivity of surrounding ground formation, concrete slab and thermal insulation material, and the flowrate of working fluid) on the thermal performance of energy slabs. The Thermal Performance Test (TPT) was performed in the floor-type energy slab constructed in a test bed to evaluate the effect of pipe material. In addition, a CFD model was developed by calibrating with the TPT result to estimate the effect of the other system parameters. Note that an optimum flowrate of working fluid should be determined to maximize the thermal performance of energy slabs with consideration of the configuration of energy slab and thermal properties of surrounding medium.

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