On the Design of Recuperator for Transcritical Cycle Adopting CO2-Based Mixture as Working Fluid: A Focus on Transport Properties Prediction

Transcritical cycles working with CO2-based mixtures gain considerable attention due to thermodynamic efficiency gain compared to pure sCO(2) in hot environments. Previous literature works prove that the adoption of CO2 mixtures provides a reduction of the levelized cost of electricity in concentrated solar power applications and medium-high temperature heat recovery. However, for techno-economic analysis and heat exchanger design, proper evaluation of transport properties of the CO2-based mixtures in power cycle conditions is necessary. Herein, it deals with the analysis of the proper transport properties models for CO2 mixtures to assess their actual thermal behavior. A literature review on transport properties models, and their validation with available experimental data, proves that the friction theory is suitable for CO2 blended with dopants having high molecular complexity. The impact of the different model selection on the recuperator sizing, considering optimized power cycle conditions, is assessed on the CO2 mixtures with hexafluorobenzene and decane: The TRAPP and Chung-Lee-Starling models are imported from Aspen Plus, while the friction theory model is implemented and calibrated in an in-house MATLAB code. The optimal design of the recuperator for the CO2 + C6F6 mixture in a 100 MWel power block coupled with a solar power plant located in Sevilla is carried out.