Optimized Combination of a Cooling Pond and Cooling Tower System for Condenser Cooling at a Steam Cycle Power Plant

Abstract

The performance of the condenser cooling cycle of a steam cycle power plant is examined for the Columbia Generating Station of the Wisconsin Power & Light Company located near Portage, Wisconsin. The Columbia station is a coal fired power plant with a capacity of 1000 MW. A closed circuit cooling system is used for heat rejection that consists of a cooling pond and two cooling towers. Pond and towers are connected in a parallel arrangement. To study the behavior of the cooling system, computer models of the components are developed. The cooling tower model is based on the analogy approach of a cooling tower and an heat exchanger. An effectiveness-Ntu method is employed to predict the cooling tower performance with respect to ambient and load conditions. For the calculation of cooling capacity of the cooling pond surface, a semi-empirical approach is selected that is based on the combination of free and forced convection. The water flow in the pond is approximated by a plug flow model. From the energy transfer and flow model a simulation program is developed that is capable of predicting cooling pond temperatures dependent on meteorological conditions and heat load on the pond. The possibility of adding different cooling devices to the system is examined. A simple model of an atmospheric cooling spray is employed. A comparison between cooling tower and cooling spray performance is performed to study if the addition of a cooling spray can be an advantage for the system performance. The impact of dredging the pond to a greater depth is evaluated. To link the cooling system to the steam cycle performance a condenser model is introduced that predicts the condenser back pressure for a given circulating water temperature and plant iv load. From an empirical relation the influence of condenser back pressure on the steam cycle heat rate is determined. The performance of the cooling system is then evaluated economically. The modular system simulation program TRNSYS is used to analyze the behavior of the cooling system in combination with the power generation unit. Transient simulations are performed in hourly time steps to examine the impact of weather conditions on the whole system and to study diurnal fluctuations. A simulation program usable by the plant operators is a product of this work. Using the simulation program the cooling system is examined under different operation modes utilizing the available equipment. Dependent on weather conditions the best plant operation mode is found. Recommendations are given for the most cost effective alteration of the cooling towers and pond.