Performance Analysis of an Updraft Tower System for Dry Cooling in Large-Scale Power Plants
An updraft tower cooling system is proposed to eliminate water use and utilize heat rejection in large-scale power plants. The updraft tower has an annular heat exchanger distributed around an inlet at the base; ambient air is drawn up through the tower chimney due to the density difference between the heated air at the base and the cooler air at the top. A secondary loop, configured as a vapor-compression cooling cycle, is used to transfer the heat from the power plant condenser to the tower base. Recuperative turbines in the tower harness the energy available in the waste heat air stream to assist in powering the secondary cooling cycle and increase the cooling system efficiency. The system feasibility is evaluated from thermodynamic perspective by comparing the net power needed to operate the system versus alternative dry cooling schemes. A detailed thermodynamic model coupling the power plant, secondary loop, and updraft tower systems is developed. Parametric studies involving changes in updraft tower geometry, heat exchanger size, and operating conditions are conducted to develop system design methodologies for optimized performance; water, R134a, R600a, and R290 are evaluated as alternative secondary cooling cycle refrigerants. The size of the updraft tower is critical to the system performance. Increases in the tower diameter and height (up to reasonable construction limits) increase the system work output. The cooling system efficiency can be improved by lowering the secondary loop condensing temperature to reduce the compression work, but this requires a larger size tower to maintain the same heat rejection rate. Similarly, increasing the secondary loop evaporating temperature can reduce the compression work, but requires a larger heat exchange surface area. An optimum depth of the heat exchanger at the tower base (i.e., the secondary loop condenser) is identified that maximizes the work output. For an optimized system design, the required work input to the updraft tower cooling system is shown to be less than for a typical air-cooled condenser at the same heat rejection rate. By allowing the power plant condensing temperature to increase, a simple pumped loop can be used to replace the secondary vapor-compression cooling cycle as the secondary loop. With the same updraft tower parameters and heat exchangers design, the simulation model predicts the pumped loop provides better performance and consumes less power as compared to the vapor compression loop. With a tower height similar to existing water-cooled cooling towers, the updraft tower system with a pumped secondary loop can allow for dry cooling with significantly less efficiency penalty compared to air-cooled condensers. This work provides a framework for judgement of the updraft tower dry cooling system feasibility from a thermodynamic perspective.
Weibel, Purdue University.
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