Thermofluid Design and Performance Evaluation of a Natural Draft Air-Cooled Condenser Towards Annual Performance Modeling of Concentrated Solar Power Plants

Abstract

This paper presents the sizing and performance evaluation of a natural draft air-cooled condenser, with a nominal heat rejection rate of 75 MWth, for implementation at a concentrated solar power plant in the Northern Cape province of South Africa. Initial sizing and optimization of the tower geometry is done with the aid of a one-dimensional thermofluid model at design point conditions. A high-density Latin hypercube sampling-based parametric sweep was conducted that covers the geometric design envelope, which is defined via the tower and heat exchanger heights, and the tower base and outlet diameters. Following this, the performance of the best-performing tower geometry is verified via detailed three-dimensional computational fluid dynamics (CFD), and the geometry adjusted slightly to achieve the desired heat rejection rate. This process includes refinement and validation of the CFD model compared to previous work, with the heat rejection rate matching the previous results within 0.1%, as well as performing grid convergence studies to ensure mesh independence. The refinements include a more direct coupling with the solver continuity equation, improving the accuracy of the heat exchanger integration via porous media, and a decrease in computational overhead to reduce the time required for parametric studies. The best-performing geometry implemented in the CFD model features a tower height of 80 m, base diameter of 58 m, outlet diameter of 40.15 m, heat exchanger height of 11.25 m and heat exchanger width of 3.551 m, with the model predicting a conservative heat rejection rate of 76 MWth at the design point. Finally, a methodology is presented to evaluate the performance of the system over the full range of ambient conditions encountered during an annual operating cycle. The methodology will be applied in further work to develop a reduced-order surrogate model for application in annual performance studies.