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dioxide don’t melt but rather sublimate into a gas; solid CO2 is known as “dry ice.” Indeed, CO2 won’t liquefy at all unless a pressure greater than five atmospheres is applied. But at a somewhat greater pressure—around 73 atmospheres—and roughly room temperature, CO2 makes a strange transition from a gas to a state known as a supercritical fluid. Supercriticality is a hybrid state. A supercritical fluid is dense, like a liquid, but it expands to fill a volume the way a gas does. Small changes in temperature near the critical point—31 °C—will cause large changes in density, similar to boiling where the liquid changes to a vapor. The density change, however, is only a factor of three or four, not a thou- sand as when water becomes steam at atmospheric pressure. Similarly, it takes a lot of energy to increase the temperature a small amount when the fluid is near the critical point, much the way the heat of vaporization requires energy to convert a liquid to a vapor. Consequently, a large spike in heat capacity occurs near the critical point of CO2 . There are also viscosity changes that mimic the viscosity difference caused by transitioning from a very dense liquid- like fluid to a vapor-like fluid. And there are no drops and no bubbles because there can be no free surface. These properties make supercritical carbon dioxide an incredibly tantalizing working fluid for Brayton cycle gas turbines. For the past several years, I have been part of a team of researchers at Sandia National Laboratory that has investi- gated these sorts of turbines for power generation, and we are now moving into the demonstration phase. Such gas turbine systems promise an increased thermal-to-electric conversion efficiency of 50 percent over conventional gas turbines. The system is also very small and simple, meaning that capi- tal costs should be relatively low. The plant uses standard materials like chrome-based steel alloys, stainless steels, or nickel-based alloys at high temperatures (up to 800 °C). It can also be used with all heat sources, opening up a wide array of previously unavailable markets for power production. For these reasons the technology is quite promising. It could represent a breakthrough power system for the 2A1st century. s an illustration of the broad interest these power sys- tems are generating, Sandia and Barber Nichols, a con- tractor from Arvada, Colo., organized and hosted a sympo- sium dedicated to supercritical carbon dioxide power cycles at the University of Colorado at Boulder in 2011. The confer- ence presented 58 papers, and had 140 registered participants from 34 companies and 13 countries. While recent interest is strong, the idea of using supercriti- calCO2 inapowersystemisnotanewone.SulzerBros. submitted a patent for a partial condensation Brayton cycle as early as 1948, but nothing ever came of that patent. The idea was rediscovered two decades later and was thoroughly described by Gianfranco Angelino in 1968. System designs were developed and some fabrication was started in the early 1970s, but then the momentum of the research dropped off. It is likely that the early attempts to fabricate these systems faltered because the high power density made small-scale systems impractical or made the costs of systems that could be fabricated unaffordable for a first of a kind. And without such benchtop systems to prove the concept, the idea was left to the realm of myth. After another three decades of neglect, the cycle began re- ceiving more interest at the turn of this century. Vaclav Dos- tal, now an assistant professor at the Czech Technical Univer- sity in Prague, studied the use of supercritical CO2 in Brayton cycle turbines for his Ph.D. thesis, and this work has led to the development of multiple research and power cycles. At Sandia, we began studying these turbines more than five years ago as part of the lab’s work on advanced nuclear reac- tors. We have selected supercritical CO2 as the working fluid operating at approximately 73 bar and 33 °C at the compres- sor inlet. Under those conditions, the CO2 gas has the density of 0.6-0.7 kg per liter—nearly the density of water. Even at the turbine inlet (the hot side of the loop) the CO2 density is high, about 0.1 kg/liter. The high density of the fluid makes the power density very high because the turbomachinery is very small. The machine is basically a jet engine running on a hot liquid, though there is no combustion because the heat is added and removed us- ing heat exchangers. A 300 MWe S-CO2 power plant has a turbine diameter of approximately 1 meter and only needs 3 stages of turbomachinery, while a similarly sized steam sys- tem has a diameter of around 5 meters and may take 22 to 30 blade rows of turbomachinery. Eventually, this compactness will be a design advantage, but as we develop prototypes to study the concept, it presents a distinct challenge. Early proof-of-concept demonstrations are often performed at the 1-to-20 kWe power level because many research labs have sufficient financial resources and support equipment to fabricate and operate power systems on this scale. It is quite easy to estimate the physical size of turbomachinery if one uses the similarity principle, which guarantees that the velocity vectors of the fluid at the inlet and outlet of the compressor or turbine are the same as in well-behaved efficient turbomachines. Using these relationships, one finds that a 20 kWe power engine with a pressure ratio of 3.1, would ideally use a turbine that is 0.25 inch in diameter and spins at 1.5 million rpm! Its power cycle efficiency would be around 49 percent. This would be a wonderful machine indeed. But at such small scales, parasitic losses due to friction, thermal heat flow losses due to the small size, and large by- pass flow passages caused by manufacturing tolerances will dominate the system. Fabrication would have been impos- sible until the mid-1990s when the use of five-axis computer numerically controlled machine tools became widespread. The alternative is to pick a turbine and compressor of a size that can be fabricated. A machine with a 6-inch (outside Steven Wright recently retired from Sandia National Laboratories in Albuquerque, where he was the principal investigator for S-CO2 systems. He has started his own consulting company, Critical Energy LLC. January 2012 | mechAnIcAl engIneerIng 41 BarBer Nichols iNc.PDF Image | A turbine that uses supercritical carbon dioxide can deliver great power from a small package.
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