Ground heat storage: Thermal analyses of duct storage systems. I. Theory
Abstract (summary)
The thesis presents a theory for the thermal processes in the ground. The thermal process is divided into a local process around each ground heat exchanger (duct, borehole, plastic tube, etc.) and a macroscale or global process in the storage volume and the surrounding ground. There is also a thermal process between the heat carrier fluid and the ground immediately outside the heat exchanger, which is represented by a fluid-to-ground thermal resistance.
The fluid-to-ground resistance and the local process determine the heat transfer capacity of the ground heat exchanger. It depends on the specific arrangements of the flow channels in the ground heat exchanger, the thermal properties of the materials involved in the thermal process, and the distances between adjacent heat exchanger channels. The heat transfer in the flow channels is subject to a survey of convective heat transfer literature. The local thermal process in the ground is studied for steady-flux, step-change, and periodic variations. The new handy concept of a steady-flux regime is exploited extensively. Explicit analytical expressions for the heat transfer capacity of several types of ground heat exchangers are derived. Relatively simple formulas for an effective fluid-to-ground thermal resistance, which includes the effect of varying temperature of the heat carrier fluid along the flow channels and the heat exchange between these channels, are obtained for cases with simplified boundary conditions.
The global heat flows through the storage boundaries determine the heat losses from the store. During the initial years there is a transient thermal build-up of the temperature field around the store. The annual heat losses will gradually approach a steady-state value. These heat losses have been calculated numerically with the genuinely three-dimensional character taken into account. During the storage cycle there is a superimposed periodic variation, which is treated by an analytical method. (Abstract shortened by UMI.)