Isı pompası uygulamaları için helis tipi toprak ısı değiştiricilerinin optımum tasarımı

Efficient use of energy allows decreasing energy consumption as well as less environmental pollution and leads to sustainable development. For an efficient way of heating and cooling of spaces, ground can be used as an energy storage medium for heat exchange processes of heat pump systems to reach higher efficiency. These types of systems are called ground source heat pump (GSHP) systems, which are well established in Western and European countries for space heating and cooling applications.The installation cost and performance of GSHP systems can be greatly affected by optimal design of ground heat exchangers (GHEs). One of the most important GHEs that is used these days is helix GHEs. These GHEs are cheaper than others and also have some advantages. Three parameters affecting the helix GHE's performance are GHEs spacing, GHE major diameter and helical configuration of the pipe. In large scale GSHP applications, more than one GHE is needed, therefore determining the distance between GHEs becomes as an important issue. In this thesis, the effects of distance between vertical helix GHEs on the heat transfer ratio (HTR) are studied. Performance of GHE is determined and optimal distance is examined. Furthermore, the influence of the pitch between the turns of the helix on HTR and GHE major radius are numerically studied in COMSOL environment. The available experimental data are used to validate the numerical results. It is seen that they are in a good agreement. Computational model in this study may provide useful guidance for designing the helical shaped GHE for GSHP systems. The heat transfer from/to the pipe wall of the heat exchanger to/from the ground depends on the turns number, location, material and configuration of the pipes. In this study, computer modeling of 1, 2, 3 and 5 helix GHEs are described. For modeling the heat transfer of a GHE, the ground soil/rock is usually approximated as an infinite homogeneous medium and heat transfer is assumed mainly to take place by conduction. The wall temperature of GHE is assumed as constant and average value of inlet and outlet water temperature. Performance losses due to thermal interactions for different GHE spacings are determined for different number of helix GHEs. Distances between GHEs vary from 1 m to 11 m. Also different characteristics of soil in the ground and its temperature play an important role in GHE performance. The temperature of the ground around GHE is a function of the soil thermal properties like thermal diffusivity (α), thermal conductivity (k) and the heat capacity (Cp) as well as temperature of fluid, time, position. The soil thermal diffusivity is a defined property and is the ratio of the thermal conductivity and the volumetric heat capacity. Therefore, three soil properties (k, α and Cp) should be known or estimated to predict the thermal behavior of GHEs. Obtaining accurate values of the thermal properties of the soil requires a detailed site survey. Soil composition varies widely not only with locations but also from wet clay to sandy soil. In order to better understand the behavior of the system, the performance of the GHE is experimentally analyzed for 5 day non-stop operation. Evolution of inlet and outlet temperatures for vertical helix circuits are measured. It is seen that inlet temperature is almost constant as 40 oC. The outlet temperature reaches the maximum value of 39.2 oC. It is also observed that the outlet temperature gets higher values for longer operation times, due to increament of soil temperature. Based on the observation of temperature change, we consider that the temperature of the pipe's wall is almost constant and is equal to the average value of inlet and outlet temperature amounts. Furthermore, a single helix GHE 3D model is made using COMSOL multi-physics program based on the geometrical parameter and thermal properties of solid domains (ground, grout, and polyethylene inlet and outlet pipes). The model conditions is the same as the experimental conditions The accuracy of the model is proved by experimental data. After validating single helix GHE with experimental results, the same model is extended for 2, 3 and 5 GHEs in order to determine their HTR values and performance losses due to their thermal interactions. Based on the single GHE results, the same study is repeated for 2, 3 and 5 GHEs. The amount of HTR in critical GHE is determined computationally and performance losses are calculated. Critical GHE is defined as central GHE in all part of this thesis. As a result of the study, when the number of GHEs is increased in a finite size domain, increament of the system performance becomes smaller for addition of each GHE. It is found that when the distance between GHEs is d=3m, performance losses of critical GHE (cGHE) for the cases of 2, 3 and 5 GHEs are around 6%, 14% and 22% respectively. Similarly when the distance is 10 m, performance loss is 1%, 2% and 4% for 2, 3 and 5 number of GHEs. Furthermore, the effects of pitch distance (Lp) and major radius (D) on performance of GHE are investigated. The results of the simulations prove that Lp and D important parameters for the design of a GSHP. Although increament of these parameters also improve the performance of GSHP, a higher investment is needed for installation stage. Therefore an optimum size should be found, which minimizes the total cost over the system lifetime.