Design of an Air-Cooled Condenser for CO2-Based Mixtures: Model Development, Validation and Heat Exchange Gain with Internal Microfins

CO2 blends provide tremendous advantages when used as a working fluid in transcritical power cycles with respect to pure CO2. The benefits become especially apparent if coupled with concentrated solar power since increasing the critical temperature of the blend with respect to pure CO2 allows dry condensing at high ambient temperatures in locations of high solar radiation. One key cycle component is the cooler, which in this work is designed as an air-cooled condenser with a MATLAB in-house code. The internal, condensation heat transfer model used in this paper relies on a correlation developed by Cavallini (2006). The model itself is validated against experimental data from a test rig for heat transfer measurements on a CO2 + R1234ze(E) mixture. The resulting design of the condenser is compared with the commercial software HTRI for a specific case study which is representative of the condenser of a recuperated cycle working with a CO2 + C6F6 blend. The authors also present an upgraded heat exchanger design with microfinned tubes, the DIESTA tubes, and groovy fins on the air side. The design of the heat exchanger adopting the mixture is compared to a case with pure CO2 as the working fluid.