Dinitrogen tetroxide and carbon dioxide mixtures as working fluids in solar tower plants

The competitiveness of concentrated solar power technology in the near-future electricity generation scenario, requires a substantial reduction of the Levelized Cost of Energy which can be achieved with an increase of the energy conversion efficiencies while maintaining or reducing the investment costs. This paper discusses the use of pure Dinitrogen tetroxide N2O4, and N2O4/CO2 mixture, as working fluids in supercritical Brayton cycles applied to solar tower power plants. When N2O4 is combined with CO2, the resulting mixture has a comparatively higher critical temperature than pure CO2, allowing a condensing cycle even at the fairly high ambient temperatures of desert areas, where solar power plants are typically installed. This allows the adoption of simpler cycle configurations than the one used in sCO2 cycles (cost reduction) while achieving very high thermodynamic efficiency (47% at 700 °C). The N2O4/CO2 mixture with optimized composition, integrated in a solar tower unit, increases the solar-electric efficiency by 1% with respect to commercial plants based on steam cycle with 550 °C maximum temperature (22.3% vs. 21.3%). At 700 °C, the overall solar-electric efficiency can reach 24.5% which is slightly higher than supercritical CO2 cycles, yet with a foreseeable reduction of the investment costs as consequence of the simpler plant lay-out.