Computer optimization of dry and wet/dry cooling tower systems for large fossil and nuclear power plants

Abstract

There is a projected shortage of water supply for evaporative cooling in electric power industry by the end of this century, Thus, dry and wet/dry cooling tower systems are going to be the solution for this problem. This study has determined the cost of dry cooling compared to the conventional cooling methods. Also, the savings by using wet/dry instead of all-dry cooling has been determined. A total optimization has been performed for power plants with dry cooling tower systems using metal-finned-tube heat exchangers and surface condensers. The optimization minimizes the power production cost. The program does not use pre-designed heat exchanger modules. Rather, it optimizes the heat exchanger and its air and water flow rates. In the base case study, the-method of replacing lost capacity assumes the use of gas turbines. As a result of using dry cooling towers in an 800 MWe fossil plant, the incremental costs with the use of high back pressure turbine and conventional turbine over all-wet cooling are 11% and 15%, respectively. For a 1200 MWe nuclear plant, these are 22% and 25%, respectively. Since the method of making up lost capacity depends on the situation of a utility, considerable effort has been placed on testing the effects of using different methods of replacing lost capacity at high ambient temperatures by purchased energy. The results indicate that the optimization is very sensitive to the method of making up lost capacity. It is, there- fore, important to do an accurate representation of all possible methods of making up capacity loss when optimizating power plants with dry cooling towers. A solution for the problem of losing generation capability by a power plant due to the use of a dry cooling tower is to supplement the dry tower during the hours of peak ambient temperatures by a wet tower. A separate wet/dry cooling tower system with series tower arrangement has been considered in this study. In this cooling system, the physical separation of the dry and wet towers protects the dry tower airside heat transfer surface from the corrosion problem. It also allows complete freedom of design and operation of the dry and wet towers. A wet/dry cooling system can be tailored to meet any amount of water available for cooling. The results of the optimization show that wet/dry cooling towers have significant savings over all-dry cooling, For example, in either fossil or nuclear plant, the dry tower heat transfer surface of 30% makeup water wet/dry cooling system is only about fifty percent of that in all-dry cooling using high back pressure turbines. This results in a reduction of 27% and 37% of the incremental cost in the fossil and nuclear plant, respectively, over all-wet cooling. Even the availability of a small percentage of makeup water reduces the incremental cost significantly. Thus, wet/dry cooling is an economic choice over all-dry cooling where some water is available but supplies are insufficient for a totally evaporative cooling towers. On the other hand, the advantage of wet/dry cooling over evaporative towers is conservation of water consumption.