Modeling vertical U-tube ground heat exchangers for ground-coupled heat pump applications

Buildings were responsible for 41% of the primary energy use in the United States in 2011. If the energy consumed in the building section was broken down by end-users, it were about 21% and 13% of primary energy used for space heating and space cooling respectively. Ground-couple heat pump (GCHP) systems use stored energy in the ground at a relatively constant temperature as a heat source (in the winter) and a heat sink (in the summer) so that they offer higher energy efficiency than the conventional heating and cooling system. However, drilling of deep boreholes comes with a high initial cost, which hinders the application of the GCHP systems. To effectively use a GCHP, it is necessary in the design phase to accurately predict the thermal performance of GCHP systems throughout their entire life spans. This research aims to develop the heat conduction model and the hybrid moisture and groundwater model for vertical U-tube ground heat exchangers (GHEs) to evaluate its performance within their life spans. Based on these models, the sensitivity analysis will be conducted to study the influence of parameters on the thermal performance of vertical U-tube GHEs. In order to achieve this goal, it can be broken down to three sub-objectives: • Objective 1: Develop the two-dimensional heat conduction model for constant volume and variable volume vertical U-tube GHEs in cylindrical coordinate system. • Objective 2: Develop hybrid moisture and groundwater model in the three-dimensional unstructured grid to consider the influence of the moisture transfer and groundwater movement on the thermal performance. • Objective 3: Conduct the sensitivity analysis of the parameters on the performance of vertical U-tube GHEs. The screening design will select the relatively important parameters and the sensitivity analysis will rank these screened parameters. The result of sensitivity analysis will provide the guidance for further optimization of the model of vertical U-tube GHEs. Based on the local geological surveys or the data from testing boreholes, the models developed in this research will significantly help designers to predict the thermal performance of GCHP systems. The appropriate design for the GCHP systems will be achieved to trade off the initial cost and the operating cost in their life spans. In the future research, the models of vertical U-tube GHEs could be integrated with the models of cooling towers or solar collectors to simulate the hybrid GCHP systems, which provide much higher efficiency for the thermally imbalanced buildings