Civil Engineering - Theses
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ItemTowards improvement of the design of borehole ground heat exchangers( 2017)Ground source heat pump (GSHP) systems utilise ground thermal energy stored at shallow, up to 500 m, depths for heating and cooling applications. The underground elements of these systems, ground heat exchangers (GHEs), provide essential thermal interactions of the systems with the ground. In urban areas, GHEs are typically installed vertically to efficiently use the limited available land. Commonly, such GHEs comprise U-loops of plastic pipes installed in boreholes grouted with cement-based grouts after the installation of the pipes. The installation costs of borehole GHEs are usually the largest component of the capital costs of GSHP systems, so accurate sizing of borehole GHEs is crucial to provide a balance between adequate long-term performance and their financial feasibility. This research aims to contribute to the improvement of the design of borehole GHEs by advancing several aspects of current GHE design practices. Firstly, this work verifies some common borehole GHE models and modelling assumptions against experimental data. To do so, a 120kW commercial GSHP system installed in the Elizabeth Blackburn School of Sciences in Melbourne, Australia, was instrumented to monitor thermal performance of its borehole GHEs and ground-GHE thermal interactions. The system uses twenty-eight 50m deep borehole GHEs to extract and inject ground thermal energy. The GHEs and the adjacent ground were fitted with temperature probes and other instruments. The predictions made by common GHE analytical models are assessed against the 2-year monitoring data collected. This includes predictions of ground temperatures around the GHEs, estimations of borehole thermal resistances and modelling of GHE fluid temperatures. The study suggests that, in general, analytical models of borehole GHEs can be successfully used for simulations of their thermal performance, but the models have to be carefully selected for particular conditions based on their limitations. In addition, this work proposes a method for the estimation of the uncertainty of the GHE design length by considering the uncertainties involved with the selection of design parameters when a particular set of design recommendations is followed. Using the proposed method, a case study is presented where borehole GHEs are sized following a commonly applied design process. The uncertainty of the resulting length of GHEs is estimated and the sensitivity of this uncertainty to the uncertainties of individual design parameters is discussed. Also, possible measures to reduce the length uncertainty, including an in-situ thermal response test, are considered. Furthermore, this work examines the design of borehole GHEs for district hybrid GSHP (district HGSHP) systems. In such systems, two or more buildings share GHEs. This study discusses the benefits of district over individual HGSHP systems and presents a method for the optimisation of borehole GHEs for district HGSHP systems that considers thermal demand regimes of individual buildings. Such optimisation can reduce the total lifetime costs of heating and cooling, capital investments and payback periods of HGSHP systems. The importance of considering the demand regimes in the optimisation is demonstrated through a case study. The case study shows that an optimised district HGSHP system can have significant financial benefits over individual HGSHP systems.