The supercritical carbon dioxide power cycle operates in a manner similar to other turbine cycles, but it uses CO2 as the working fluid in the turbomachinery. The cycle is operated above the critical point of CO2 so that it does not change phases (from liquid to gas), but rather undergoes drastic density changes over small ranges of temperature and pressure. This allows a large amount of energy to be extracted at high temperature from equipment that is relatively small in size. Supercritical CO2 turbines will have a nominal gas path diameter an order of magnitude smaller than utility scale combustion turbines or steam turbines.
The Advanced Turbines Program at NETL conducts R&D for directly and indirectly heated supercritical carbon dioxide based power cycles for fossil fuel applications. The focus is on components for indirectly heated fossil fuel power cycles with turbine inlet temperature in the range of 1300 - 1400ºF (700 - 760ºC) and oxy-fuel combustion for directly heated supercritical CO2 based power cycles.
The turbomachinery R&D focuses on advancing technologies and designs of turbomachinery to be used in the supercritical CO2 power cycle. Operating power cycles, either directly or indirectly, with supercritical CO2 offers potential for further improvements in power cycle efficiencies and lower costs. However, the utilization of supercritical CO2 as the working fluid must be considered when designing the turbines. Extremely compact turbine sizes are possible for use in the supercritical CO2-based power cycles. These turbines will have high power density, lower peripheral speeds, high blade loading, and high shaft speeds, all of which will factor into the final turbine designs. The high pressure, relatively high temperature, uncertainty of the CO2 state near the critical point, and high power density create design challenges for the supercritical CO2 turbomachinery. The R&D will consider all aspects of the turbomachinery, including the turbo-expander, compressors, pumps, airfoils, turbine coupling with the motor/generator, seals, casings, bearings, shafts, and valves.