Study on heat transfer surrounding pile foundation ground heat exchangers with groundwater advection

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Study on heat transfer surrounding pile foundation ground heat exchangers with groundwater advection

 

Author: Zhang, Wenke
Title: Study on heat transfer surrounding pile foundation ground heat exchangers with groundwater advection
Degree: Ph.D.
Year: 2015
Subject: Heat pumps.
Ground source heat pump systems.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxxv, 212 leaves : illustrations (some color) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2816416
URI: http://theses.lib.polyu.edu.hk/handle/200/8168
Abstract: Currently, the ground-coupled heat pump (GCHP) technology has gained more and more attentions due to its outstanding high performance characteristics in energy saving, environmental protection and associated benefits. The development of investigations have led to a new type of ground heat exchanger (GHE) entitled "energy pile". In brief, it is proposed for spiral heat exchange tubs to be enclosed in the pile foundation of buildings, thus integrating the bearing structure of buildings with a heat transfer component. A certain proportion of the heating load or cooling load can be met in this way; thereby reducing the initial cost of the whole cooling and heating system.A range of research studies have been conducted on the pure conduction of pile foundation GHE, however, little references or documentation exists in the literature concerning the influence exerted by groundwater flow. It should be recognized that varying degrees of groundwater seepage phenomena exist below ground. As the depth of the pile is usually more than ten meters or even many more meters, the underground hydraulic gradient inevitably leads to the groundwater flow. Accordingly the heat transfer involving conduction and groundwater advection should be taken into account when considering the role of seepage. The study presented in this thesis includes corresponding studies on simulation models for an energy pile with groundwater advection, and each type of model consists of both infinite and finite length sources. Heat transfer experiments were then conducted to indirectly verify the seepage models. In addition, a reasonable methodology to obtain the groundwater velocity is also proposed. The models were investigated and then further developed to be more advanced from some classical models such as line and hollow cylindrical heat sources. Firstly, a new model, referred to as the "solid" cylindrical source model, is initially proposed. In this model, the pile diameter is much thicker and the depth is usually shorter than in the case of the borehole, and the spiral coils are disposed in the vicinity of the pile circumference. The cylinder is filled with medium identical to that out of it; thus, the interior heat capacity of the pile cannot be ignored. Analytical solutions for the solid cylindrical models while groundwater flows through it were obtained. Secondly, the ring-coil model is put forward to represent the configuration of spiral coils set inside the pile by a more appropriate method. The solid cylindrical model only regards the pile foundation GHE as a uniform surface heat source and thus the coil intervals are not considered. Compared to the solid cylindrical model, the ring-coil model is relatively advanced because it takes into consideration the discontinuity of the heat source along the depth. This heat source is deemed as a series of separated coils arranged along the z-direction so that focus is on the impacts of the coil pitch. If groundwater goes through the energy pile with spiral coils, the temperature response induced by conduction and groundwater advection at any point except the heat source can be achieved.
Thirdly, focus is on comparing the solid cylindrical and ring-coil models with an improved model. An attempt is made to reduce the deficiencies of the former two by introducing a spiral line, hence proposing the spiral heat source model. The improvement is the fact that a spiral line replaces the separated coils and thus a succession of connected coils with a certain diameter are distributed down the depth of pile. Accordingly, not only the coil pitch but also the helix angles are dealt with. Thereby, the spiral heat source seepage model efficiently shows the thermal transfusions with greater accuracy and precision at the time of the combined contributions of both conduction and groundwater advection.However and regrettably the on-site heat exchange experiments of energy pile have not yet been conducted to validate the theoretical models. Next, the significant experiments based on the actual engineering projects were conducted to check the heat transfer ability and verify the conduction model of the pile foundation GHE. The experiments conducted in this study focused on the pure conduction of energy piles, and two groups of trials were conducted and the heat transfer superiority of the energy pile is shown by comparing pile foundation GHE with borehole GHE. The pure conduction model can then be validated by the experimental results. The combined heat transfer models including pure conduction and groundwater advection are indirectly proven because it derives from the pure conduction case. Thus, the combined model could be certificated indirectly in the event of the pure conduction model being confirmed.Lastly, the groundwater velocity is a vital parameter embodying the influence degrees of seepage, the comprehension of both value and orientation of velocity is becomingly increasingly important. To avoid the huge difficulties of directly measuring the velocity by means of the test equipments, a significant methodology named "back calculation" is suggested. The conditional extreme values of the sum of the variance and the corresponding derivatives combined with the theoretical models were put to use when the target functions had been established. By this means, the groundwater velocity can be obtained based on the temperature responses at different points around a borehole GHE.In summary, all models involving both energy pile and groundwater advection developed and presented in the thesis are introduced one by one on the simple to complex principle. The spiral heat source model is the optimum case from the perspective of academic study as it most approximately simulates the energy pile with groundwater advection. However, other models can still be employed to provide theoretical basis for some situations such as engineering projects and so on, because in these cases the requirements for calculation accuracy are not too high. The application of energy piles can reduce the initial cost of meeting a building's need for heating and cooling. Groundwater advection does improve the heat transfer performance of energy piles as it enables heat accumulation around the pile to be alleviated, therefore the temperature difference between GHE and the surrounding underground medium is maintained or even increased. Thermal transmission is further promoted and accordingly heat exchange quantity is enlarged. The flow velocity including its value and orientation can be deduced when the back calculation method is employed. The pure conduction experiments have produced satisfactory effects, and an aim of the future work is the implementation of the heat transfer experiments with groundwater advection to directly validate the groundwater seepage models. Full understanding of groundwater advection is helpful to emphasize not only the significance but also the popularization of energy pile technology.

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