Solar tower CSP plants with transcritical cycles based on CO2 mixtures: A sensitivity on storage and power block layouts

In this work three CO2-based binary mixtures, CO2 + C6F6, CO2 + C2H3N and CO2 + C4F8, are compared as innovative working fluids for closed power cycles in CSP plants. Adopted in transcritical cycles, they lead to cycle efficiencies higher than sCO2 cycles at minimum temperatures above 50 degrees C, a typical condition for arid regions with high solar radiation. The analysis considers four plant configurations: the first with direct storage, solar salts as HTF and cycle maximum temperatures of 550 degrees C, while the three other plants adopt sodium as HTF and an indirect storage system, designed for cycle maximum temperatures of 550 degrees C, 625 degrees C and 700 degrees C. Detailed models are used to characterize the solar fields optical performance, the receiver thermal efficiency and the HTF pump consumption, both at design and off-design conditions, for large scale plants located in Las Vegas. Different power block layouts are considered, spanning from the more efficient ones to cycles with a high heat recovery capacity. In addition, the impact of the mixtures on the design of heat exchangers is evidenced, with convincing results with respect to the heat transfer characteristics of CO2. Considering the resulting yearly performances and LCOE of each configuration, the adoption of indirect storage systems is considered a viable solution for high temperature solar plants. The three innovative mixtures allow for a reduction in LCOE with respect to sCO2 cycles (up to 10 $/MWh, depending on the configuration), capacity factors above 70% for the specific location, optimal solar multiples around 2.8 and 12 equivalent hours of TES.