The investigation of heat transfer in ground heat exchangers under building foundation

The need for energy is compulsory in order to provide comfort conditions in living spaces. In general, electricity, fossil fuels or biomass are used to obtain the desirable air temperatures in living spaces for heating and cooling. While such energy sources need long distance transportation, ground energy exists in abundance in settlement areas. As the heat in the ground gets transferred slowly and the ground has greater heat storage capacity, the temperature change in the ground takes months and even years depending on the depth. Low thermal conductivity of the ground enables the transfer of the some heat from the cooling season to heating season, i.e. the opportunity to use the heat obtained in summer in winter. This relationship between the air and ground represents a thermal energy potential as an aid to heat and cool a building. In the heating season, the ground has a higher temperature than the air whereas in the cooling season it is vice versa. In order to transfer this energy to living spaces by using ground energy, Ground Source Heat Pumps (GSHPs) are developed. In the design of GSHPs, Ground Heat Exchangers (GHEs) as the most critical component consist of heat exchanger pipes buried in soil. By using these heat exchanger pipes, the energy of the ground is transferred into fluid in the heating season and it is vice versa in the cooling season. When previous research on the design of GHEs is studied, there are two analytical solution methods in principal, namely Kelvin Line-Source Method and Cylindrical Source Method. There are numerical methods apart from the analytical solution methods as well as studies that include both methods. However, many heat fluxes on the soil surface (convection, evaporation, longwave and solar radiation), that is, dynamic boundary conditions are not included in the solution with analytical methods, and numerical methods are employed to a certain extent. A numerical model is developed with realistic boundary conditions using meteorological data to solve transient-three dimensional temperature distribution in soil. Since the temperature gradient along the pipe axis is quite small, heat transfer inside the soil along the pipe axis is neglected. Thus, the heat transfer equations in the soil are solved by considering two dimensional space (in a plane perpendicular to pipe axis) and dynamic boundary conditions. . The heat transfer equations are associated with one another by employing the law of conservation of energy between two dimensional solution regions along the pipe. Finite difference methods among the numerical solutions methods are preferred due to the dynamic boundary conditions and complexity of the equations. Alternating-direction implicit (ADI) formulation method among the finite difference methods is employed as this method displays a consistent structure for each time step and grid size, and allows for less need of computer memory and processor power for greater time steps. The matrix system obtained by using the ADI formulation method is tridiagonal and the solution can be easily found using the Thomas algorithm. At the first stage of the thesis, a model is designed for GHEs buried in the ground, the surface of which is open to the atmosphere in or around the garden of the buildings to be heated or cooled. The hourly heat need of this building during the heating and cooling seasons is determined with HAP software, and thanks to the code written in MATLAB, the fluid inlet temperature corresponding to the hourly heat need of the living space in heating and cooling seasons is simulated for a ten-year period. Several parameters which have an impact on the design of the GHE, such as the burial depth, pipe spacing, number of parallel tubes (NPT), pipe length, pipe diameter and surface effects, are studied in the simulation studies. The hourly fluid inlet and outlet temperatures in the pipe and soil temperatures around the pipe in accordance with the various heat loads transferred from unit pipe lengths are analyzed. Horizontal and vertical temperature distribution in the soil at the beginning, middle and end of the first, fifth and tenth year of the heating and cooling seasons during the ten-year simulation study are obtained. Fluid inlet and outlet temperatures and the soil temperatures as input parameters are provided, and then the energy analysis in the heat pump cycle is carried out with a new code developed in MATLAB. As a result, hourly COP values for a ten-year period are obtained for various simulation studies. Afterwards, an economic analysis is performed and the reference function, which is an optimization parameter, is defined. Reference functions for all simulation studies are acquired. In the implementations of GSHPs in Turkey, it is seen that horizontal GHEs are generally buried in the gardens or surroundings of buildings. The increase in initial investment costs due to the excavation works makes the users avoid employing these systems in comparison with the conventional systems. In order to diminish or avoid the excavation costs, two methods are adopted. Firstly, the excavation cost is reduced by placing heat exchanger pipes in the ground while the building is constructed. Secondly, heat exchanger pipes are laid in the foundation of the building during the construction to eliminate excavation costs. At the second stage of the thesis, a model is designed for the horizontal heat exchanger pipes laid in the ground under the foundations and in the foundations of the buildings reconstructed in Turkey. An optimization study is carried out for the heat exchanger pipes laid in the ground under the foundation and in the foundation of a shopping mall in Istanbul thanks to a code written in MATLAB. The effect of the installation place of the horizontal heat exchanger pipes, the burial depth and the NPTs on GSHP performance is studied. Reference functions for all simulation studies are obtained. As part of the experimental studies, horizontal heat exchanger pipes are placed in the ground under the foundation and in the foundation of the newly built 2400 square meter Central Laboratory building at Yıldız Technical University, Davutpaşa Campus and a section of the building is heated and cooled. These are Concrete lines buried in the foundation of the building and Ground lines buried in the ground under the foundation of the building. Ground and Concrete lines are let into the building by connecting them with the collectors outside the building and then these lines are attached to the heat pumps. The heat pumps of the Ground and Concrete lines feed two locations (each location consists of two rooms) with equivalent area values. The fluid inlet and outlet temperatures of these lines are measured by using RTDs. At the outlet of the lines, turbine flow meters are used to measure the flow rates. RTDs and turbine flow meters are integrated with the PLC system to import the data. During the excavation work of the building, several thermocouples are emplaced to determine the horizontal and vertical temperature changes in the Ground and Concrete lines buried in the foundation and in the ground under the foundation. Thermocouples for each line are linked with the data logger units so that data can be acquired. As a result of the experimental studies, while maximum differences are obtained as 8.36% and 7.14% between the daily average experimental and theoretical fluid inlet temperatures for the Ground and Concrete lines, they are 5.58% and 6.38% for the daily average experimental and theoretical fluid outlet temperatures, respectively. The experimental and theoretical temperature distributions of the Ground and Concrete lines on vertical and horizontal axes are gathered 10,100, 250, 500 and 864 hours following the starting time and are seen to be in good agreement with each other. The energy drawn from the daily average Ground and Concrete lines is 5.51 and 4.63 kWh/ day, respectively, and the cumulative total energy of these lines is also obtained. The cumulative total energy drawn from the Ground has proven to provide approximately 15.98 % better results than the cumulative total energy drawn from the Concrete. As a result of the technical and economical analyzes carried out within the scope of numerical analysis, higher COP at all time and higher Fref values at the low NPT are obtained when the heat exchanger pipes are placed in the ground under the building foundation. However, it is seen that the placement of the heat exchanger tubes inside the building foundation (concrete) is more suitable for high NPT (NPT≥50).