| dc.description.abstract | The focus of this thesis is to model a ground loop cooling system to substitute for typical cooling systems of a thermal power plant. Steam power plants generate heat from fuel (e.g., coal, natural gas, and nuclear) which is used to convert water to steam and which, in turn, expands through turbines to turn generators which produce electricity. After passing through the turbine, the mixture of low pressure steam and water must be cooled to become all liquid water in order to be reused. Cooling towers play a crucial role in the removal of waste heat for thermal power generation. This waste heat is commonly rejected to lakes, rivers or air with the help of cooling towers and condensers. Although these methods are efficient, they are no longer considered the best choices due to consumption of large amounts water, water scarcity in certain geographical locations, and environmental effects where power plants are needed. For the ground loop cooling technique, the warm cooling water coming out of the condenser is sent into the earth through a number of closed loop tubes in vertical bore holes. The heat is then transferred from the warm cooling water to the earth, and the cooling water’s exit temperature [from the closed loop tubes] is reduced from that of the inlet. This ‘cooled’ cooling water is then used to condense the mixture of low pressure steam and water in the power plant’s condenser. Therefore, in modeling a ground loop cooling system to substitute for a wet or dry cooling system, so as to maintain the cycle efficiency of the plant, it is necessary to determine the number of bore holes required and the spacing that has to be provided between the bore holes in order to reach the needed bore hole exit water temperature. To model the ground loop cooling system, ANSYS-CFX, a computational fluid dynamics (CFD) software tool, was used to evaluate different results (e.g., bore hole exit water temperature and temperature distribution in the surrounding earth). This software can simulate a wide range of fluid flow problems with good accuracy and allows defining a conjugate heat transfer problem with different input parameters. The results are determined for different values of various input parameters such as thermal conductivity of the earth, water mass flow rate, and depth of the bore holes, operational time, and inner diameter of the tubes in the bore holes. All of the results, including the number of bore holes required, spacing that has to be provided between the bore holes, and the cost to install a ground loop cooling system, are estimated for a 1000 MW thermal power plant operating at full load with a cycle thermal efficiency of 40%. It was determined that, in order to maintain the cycle thermal efficiency of the power plant for an earth thermal conductivity of 5 W/m-K and bore hole depth of 150 m, 84230 bore holes would be needed for a total installation cost of 5,206.24 million USD. In addition, it was determined that, when reducing the depth of the bore holes from 150 m to 100 m (while keeping earth thermal conductivity at 5 W/m-K), the installation cost of the ground loop cooling systems decreased by 81.43 million USD. Comparable wet cooling tower installation costs are currently 39 million USD. Considering these huge ground source cooling costs, future studies must be done. Since the total cost of the project is hugely dependent on the number of bore holes required and their diameter, future studies should be aimed at decreasing the number and diameter of bore holes required for the project. It costs less to drill a bore hole of smaller diameter. For instance, it costs approximately 1/3 to drill a 0.16 m diameter bore hole as compared to a 0.6 m diameter bore hole. Therefore, the installation cost can be reduced by changing the inner diameter of the tubes and decreasing the spacing between the U-tubes. Although, this would increase the number of bore holes required, since the mass flow rate through each bore hole decreases, the effect of decreasing the bore hole diameter on the installation cost can be examined. Since the cost of the project increases by 81.43 million USD when the depth of the bore hole is increased from 100 m to 150 m (at an earth thermal conductivity of 5 W/m-K), studies can also be done to determine the cost sensitivity of the project to bore hole depth. All results of the study and future recommendations are presented in an easy-to-use systematic form so that estimates for individual cases may be made. | |
