Turbines can use CO2 to cut CO2

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Science  26 May 2017:
Vol. 356, Issue 6340, pp. 805-806
DOI: 10.1126/science.aam8281

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Two-thirds of the electricity in the United States is generated from fossil fuel via combustion-powered steam turbines. To get to the high temperatures needed for high efficiency, steam must first be vaporized from liquid water. The steam is further heated, expanded through the turbine, and condensed to water on the other side. In this process, called the Rankine cycle, the vaporization step is a phase change that requires a large heat input but delivers no increase in temperature (or efficiency). Advanced steam turbines try to avoid the phase change by going to supercritical conditions, but attempts to exhaust heat at low temperatures push parts of this cycle to operate just above water's critical point (374°C and 218 atm). Near this point, steam's heat capacity increases sharply, so up to 36% of total heat input still goes to a low-temperature, vaporizer-like process (see the figure). By switching from steam to supercritical CO2 (scCO2) and running a Brayton cycle (the same cycle run by natural gas turbines), the “vaporizer” step can be avoided, providing an opportunity to replace subcritical steam plants with a cycle that could be up to 30% more efficient. These gains are expected to persist in the smaller turbine sizes suited for harvesting solar thermal energy.