Green Car Congress recently posted a story about a DOE program to fund the development of a demonstration plant for supercritical CO2 (sCO2) Brayton cycle electrical generator at the 10MW scale. Supercritical CO2 is carbon dioxide held above the critical temperature and pressure at which there is no phase transition between the liquid and gaseous states. The sCO2 Brayton cycle is an external combustion engine like the Rankine cycle steam engines that are currently used in a variety of electrical generation system (e.g. nuclear power plants, coal fired power plants, geothermal power plants, concentrating solar thermal power plants, and the secondary power cycle in a combined cycle natural gas fired power plant.). The energy community has long been interested in sCO2 power generators because they are potentially much more efficient at converting thermal energy to electrical energy than Rankine cycle generators, and because they should be inherently much more compact (The turbine for an sCO2 generator can be 30 times smaller than the turbine for a Rankine cycle plant of the same power rating.) thus leading to lower capital costs. Another advantage sCO2 Brayton cycle is that the possibility of maintaining a reasonably high thermal to electrical conversion efficiency using air cooling rather than water cooling. This feature is regarded as of special importance for solar thermal plants in desert regions where water supply issues may limit the use of water cooled generators. However, air cooling in other contexts would also help to reduce the environmental impact of thermal pollution of natural water sources by all types of generators which currently rely on water cooled cycles.
Sandia National Laboratory has already developed laboratory scale (125Kw) Brayton cycle sCO2 generators. It is hoped that the recent DOE proposal will be an intermediate step on the road to commercialization of the sCO2 Brayton cycle technology.