Sürdürülebilir tasarım için jeotermal enerji kazıklarının kullanımı

There is a developing trend around the world to explore alternative energy sources. The main driving forces are growing global energy demand, depleting natural resources and the adverse effects of greenhouse gas emissions from fossil fuel consumption. Geothermal energy is one of the promising renewable sources that can be utilized to offset such trends. Ground temperature remains constant after a depth of about 6 to 10 m as the near surface soils act as a thermal insulator as the ambient temperatures vary seasonally. In most regions this constant temperature is about 10 to 24°C. The relatively constant temperature and the heat storage capacity of near-surface soils in any region represent a significant potential for stored thermal energy which in turn can be harvested with ground-source heat pump (GSHP) systems. These systems exemplify a highly efficient renewable energy technology for space heating and cooling. Over the past 20 years, the ground coupling concept has been expanded from geothermal borehole systems to the use of building foundation elements as heat exchangers. Heat exchanger pile (also mentioned as energy pile) in particular is an innovative technology that combines geothermal heat exchange and structural foundation support. In this hybrid system, geothermal circulation loops are integrated into the deep foundation elements, such as piles, piers, or drilled shafts that are already designed to provide structural support. The heat energy is fed into the ground for cooling in the summer and withdrawn from the ground for heating in the winter. The temperature differential between the ground and the outside temperature acts as an energy pathway to harvest stored ground energy for bringing the building temperature to comfort zone levels. As an added benefit, the additional cost of geothermal borehole drilling for loop placement is offset by this combined use and installation costs of geothermal heat exchangers are significantly reduced. Thermal operation of a heat exchanger pile induces temperature changes in the piles. Such temperature changes create thermal stresses and displacements along the pile that, if ignored in the design process, can result on overstressing the piles and/or unacceptable building deformations. Numerous numerical and analytical approaches were developed to approximate the thermally induced stresses and displacements in heat exchanger piles. All the techniques for analyzing the thermal effects on the mechanical behavior of the piles rely on accurate estimates of the temperature changes in the pile. Sustainable operation of heat exchanger piles or any other thermo-active foundation element relies on maintaining the constant temperature of the ground. Ground temperature changes can directly affect the heat exchange capacity and efficiency of operations. It is critical to evaluate how the heat exchanger pile operation over the life cycle of the structure will change the temperature gradients around the pile. This is particularly important at regions where the respective heating and cooling energy demands are not balanced. The objectives of this dissertation can be listed as follows: (1) estimating the thermal properties of the ground accurately, (2) assessing the thermo-mechanical behavior of heat exchanger piles, and (3) predicting the long-term performance of heat exchanger piles and pile groups. The pile subjected to heating and cooling cycles will eventually expand and/or contract. It is thought that the volume changes and the constraints of the pile, i.e., the degree of fixity at the pile head and the pile toe will have an impact on the thermo-mechanical behavior. The amount of slip displacements at the pile-soil interface and the magnitudes of changes in pile axial stress, shear resistance and radial pressures on the pile soil-interface are the main points of concern. The efficiency and sustainability of the system will be investigated for long-term seasonal operation using actual energy demands for locations with extreme climatic conditions, i.e., hot, moderate and cold. Single pile and pile groups with numerous grid sizes will be investigated. Long-term thermo-mechanical behavior of single piles in each location will be assessed. In order to meet the above mentioned research objectives, in the scope of this study, robust and competent finite element analysis tools will be developed for modeling geothermal heat exchangers. The numerical tools with different levels of complexity will be utilized to analyze various problems related to the operation of GHEs. These include modeling of field thermal conductivity testing, full scale thermo-mechanical testing of heat exchanger piles and long-term seasonal operation of heat exchanger piles and pile groups. The models are planned to be built extensible in nature so that modifications for considering various aspects such as the geothermal gradient or ambient temperature changes could be easily incorporated. Validation and calibration of the proposed numerical models are to be performed using available analytical methods and field testing data. The dissertation includes literature review on the theory of heat transfer with the academic background needed to study the thermal processes occurring in the geothermal heat exchangers. The concept of ground-source heating/cooling, types of commonly used geothermal heat exchangers, detailed discussion on field thermal conductivity testing and currently available analytical and numerical analysis methods are presented. Three numerical modeling techniques are developed for simulating the thermal processes in GHEs by utilizing finite elements. The validation of the models is performed via an analytical method, and a detailed comparison of the analysis results is presented. Several methods to improve the modeling and analysis of the field TCT are also discussed. Case studies of field TCTs performed in two different sites are analyzed. The concept of thermo-mechanical behavior of heat exchanger piles subjected to temperature changes is explained. Thermo-mechanical numerical models are developed and utilized to assess the change in the stresses in the pile, contact pressures and the behavior of the pile-soil interface by performing parametric analyses with respect to soil strength and pile fixity. The proposed thermo-mechanical numerical model is utilized to study the long-term performance of the heat exchanger piles. This study includes analyses in three different cities, which have hot, moderate and cold climates. Finally, the conclusions of the research is presented along with the discussion of recommendations for future research.