CO2 mixtures as innovative working fluid in power cycles applied to solar plants. Techno-economic assessment

This paper discusses the adoption of CO2 mixtures for improving the thermal-to-power efficiency conversion in solar tower plants and reducing the Levelized Cost of Electricity. Two different fluids are considered for blending the CO2: N2O4 and TiCl4. The main advantage of the innovative mixtures relies in a higher critical temperature with respect to pure CO2, which allows condensing cycles even at relatively high ambient temperatures typical of solar plants locations. Thermodynamic results show that the innovative cycles can achieve conversion efficiencies as high as 43% and 50% at 550 °C and 700 °C maximum temperature respectively, outperforming the reference CO2 cycle by 2 points percent. In addition, the simpler lay-out and the liquid compression reduce the power block capital costs below 700 $/kW. Detailed solar plant annual simulation is performed to assess the overall solar to electricity efficiency which can be around 21% for the innovative fluid, corresponding to 10% increase with respect to state-of-the-art solar plant. The higher performance and lower costs lead to a Levelized Cost of Electricity reduction of 10% with respect to conventional steam cycle power block.