Heat Transfer Analyses Of Ground Heat Exchangers And Their Engineering Applications

In this dissertation the heat transfer in vertical GHEs is investigated on basis of a series of analytical solutions of the heat transfer processes and the principle of superimposition. In this approach the temperature response and, then, the resistance of a single borehole experiencing a constant heating load is cited repeatedly to obtain the GHE transient performance. Thus, better understanding of every thermal resistances of the single-borehole GHE is crucial, and their analytical solutions are especially preferred to facilitate the computations. Three important analytical solutions have been derived for both the heat transfer processes inside and outside the boreholes, which can be easily incorporated into computer programs for thermal analysis of the ground heat exchangers. First, an explicit solution of the transient two-dimensional temperature response in a semi-infinite medium with a line-source of finite length has been derived to take the place of the one-dimensional infinite line-source model. It is a more appropriate model for boreholes in GHEs, especially for their long duration of operations. Meanwhile, the steady-state temperature distribution has also been obtained as a limit of this solution. A mistake on this problem that appears in heat transfer handbooks and textbooks has been pointed out. Secondly, the thermal resistance inside the borehole bears strong impact on GHE performance, defined by the thermal properties of the construction materials and the arrangement of flow channels of the borehole. Taking the fluid axial convective heat transfer and thermal "short-circuiting"among U-tube legs into account, a quasi-three-dimensional model for boreholes in GHEs is established, which provides a clearer insight into the heat transfer processes in the GHE boreholes. Analytical solutions of the fluid temperature profiles along the borehole depth have been obtained. On this basis analytical expressions of the borehole resistance have been derived for different configurations of single and double U-tube boreholes. Then, different borehole configurations and flow circuit arrangements are assessed in regard to their borehole resistance. Calculations show that the double U-tubes boreholes are superior to those of the single U-tube ones in regard of their borehole resistance. Finally, in order to investigate the impact of groundwater advection on the performance of GHEs, a governing equation of conduction-advection is established for heat transfer in saturated porous media, and an explicit solution is also obtained for a line heat source in an infinite medium by means of the Green function analysis. Being clear and concise, the solution of this transient two-dimensional conduction-advection problem has never been found in literature. Dimensionless criteria that dictate the process are then summarized, and influence of the groundwater flow on the heat transfer in GHEs is discussed accordingly. Computations show that water advection in the porous medium may alter significantly the conductive temperature distribution, results in much lower temperature rises and leads to a steady-state regime eventually. These models have more solid theoretical basis, and are helpful to improve design procedures recommended by available literature, and are yet suitable for engineering applications. Engineering applications of GHE heat transfer are also included in this dissertation. On basis of the theoretical achievements a complete set of algorisms have been presented for practical design of GHEs made up of multiple boreholes with variable and/or intermittent loads. According to this improved approach computer software has also worked out to facilitate GHE design and performance simulations. The software has been crosschecked with an American computing program for the similar purpose. Finally, a GCHP air-conditioning system has been designed and constructed successfully by means of the software; observations and measurements on the ground heat exchanger of this system have been providing valuable understanding on GHE heat transfer. An experimental device of a single U-tube GHE has also been established to investigate and verify the theory of GHE heat transfer. Tests were carried out on U-tube heat transfer with and without water advection in the porous medium, respectively. Temperature responses in the medium and the circulating fluid were recorded and compared with the results of theoretical solutions. Their reasonable agreements have verified the models and their solutions on the respective processes of GHE heat transfer.