Infrastructure Engineering - Theses
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ItemDevelopment of design tools for direct geothermal systems( 2015)Direct geothermal energy systems use the earth as a heat source or sink to provide heating and cooling for buildings. In these systems, a heat pump is employed to transfer heat from a building to the ground and vice versa by circulating water (sometimes with antifreeze) through Ground Heat Exchanger (GHE) pipes buried in the ground. By reducing the electricity demand for heating/cooling applications, these systems have the potential to significantly reduce both electricity consumption and greenhouse gas emissions. GHEs come in two basic configurations: closed loop and open loop systems. The focus of this thesis is on horizontal and vertical closed loop GHEs. The total length of a GHE has a significant influence on the efficiency of heat transfer between the circulating water and the surrounding ground and hence the efficiency and cost of a direct geothermal system. Despite the importance of the GHE to system performance, there are no Australian (and few international) field experiments on fully operational systems that study GHE heat transfer in the ground in detail. Therefore, there is little data available which allows the assessment of available design tools and how well these predict performance. Based on a critical review of current design methods, this research aims to improve current design techniques by performing field experiments using instrumented pilot geothermal systems. The ultimate goal of this research is to increase the performance and reduce the cost of direct geothermal systems, by improving the understanding of GHE heat transfer. To pursue this goal, in this research, a full scale horizontal geothermal system with more than 200 temperature sensors was installed at Main Ridge (80km from Melbourne) and a full scale vertical system with more than 100 sensors was installed at the Walter Boas Building at the University of Melbourne. Ground and GHE temperatures were measured at various depths within and around the GHEs to observe the effect of thermal loading on the ground. Heat flux has been applied to the GHEs using either heat pump(s) capable of extracting and rejecting heat to the ground or electrical heating elements. Two new robust numerical models have been developed on the basis of the experimental results. The first is the Horizontal Conductive Flux and Energy System (HCFES) that has been developed specifically for horizontal GHE modelling using ExcelTM. Although this model is useful, it has a few drawbacks. Therefore, the second model, a more Generalized GHE model referred to as the GGHE model has been developed to cover a much broader range of GHE geometries to include both horizontal and vertical systems as well as energy piles. These models have been validated against the experimental results. The results of this research suggest that the models developed may form the basis for better design practice through the adoption of a less empirical approach. There is a potential to significantly improve GHE efficiency, especially with regard to the steps provided within the GGHE model. The models developed create a potential to lessen the degree of conservatism in design and reduce the installation cost of the GHEs. This thesis also presents details of the equipment used in the experimental work and provides a framework for future experimental studies of direct geothermal energy systems.