Researchers at Sandia National Laboratories in Albuquerque, N.M., have moved into the demonstration phase of a novel gas turbine system for power generation, with the promise that thermal-to-electric conversion efficiency will be increased to as much as 50 percent – an improvement of 50 percent for nuclear power stations equipped with steam turbines, or a 40-percent improvement for simple gas turbines. The system is very compact, meaning that capital costs would be relatively low, according to Sandia.
Research focuses on supercritical carbon dioxide
Brayton-cycle turbines, which typically would be used for bulk thermal and
nuclear generation of electricity, including next-generation power reactors.
The goal is eventually to replace steam-driven Rankine cycle turbines, which
have lower efficiency, are corrosive at high temperature and occupy 30 times as
much space because of the need for very large turbines and condensers to
dispose of excess steam. The Brayton cycle could yield 20 MW of electricity
from a package with a volume as small as four cubic meters.
The Brayton cycle originally functioned by heating air in a
confined space and then releasing it in a particular direction. The same
principle is used to power jet engines today.
“This machine is basically a jet engine running on a hot
liquid,” says principal investigator Steve Wright of Sandia’s Advanced Nuclear
Concepts group. “There is a tremendous amount of industrial and scientific
interest in supercritical CO2systems for power
generation using all potential heat sources, including solar, geothermal,
fossil fuel, biofuel and nuclear.”
Sandia currently has two supercritical CO
2test loops. The term “loop” derives from the shape taken by the working fluid
as it completes each circuit. A power production loop is located at the Arvada,
Colo., site of contractor Barber Nichols Inc., where it has been running and
producing approximately 240 kW of electricity during the developmental phase
that began last year. It is now being upgraded and is expected to be shipped to
Sandia this summer.
A second loop, located at Sandia in Albuquerque, is used to
research the unusual issues of compression, bearings, seals, and friction that
exist near the critical point, where the carbon dioxide has the density of
liquid but otherwise has many of the properties of a gas.
Immediate plans call for Sandia to continue to develop and
operate the small test loops to identify key features and technologies. Test
results will illustrate the capability of the concept, particularly its
compactness, efficiency and scalability to larger systems. Future plans call
for commercialization of the technology and development of an industrial
demonstration plant at 10 MW of electricity.
A competing system, also at Sandia and using Brayton cycles
with helium as the working fluid, is designed to operate at about 1,697°F
(925°C) and is expected to produce electrical power at 43 to 46 percent
efficiency. By contrast, the supercritical CO2Brayton
cycle provides the same efficiency as helium- Brayton systems but at
considerably lower temperatures 482 to 572°F (250 to 300°C). The S-CO2equipment is also more compact than that of the helium cycle, which in turn is
more compact than the conventional steam cycle.
Under normal conditions, materials behave in a predictable,
classical, “ideal” way as those conditions cause them to change phase, as when
water turns to steam. But this model tends not to work at lower temperatures or
higher pressures than those that exist at these critical points. In the case of
carbon dioxide, it becomes an unusually dense supercritical liquid at the point
where it is held between the gas phase and liquid phase. The supercritical
properties of carbon dioxide at temperatures above 932°F (500°C) and pressures
above 7.6 megapascals enable the system to operate with very high thermal
efficiency, exceeding even those of a large coal-generated power plant and
nearly twice as efficient as that of a gasoline engine (about 25 percent).
In other words, as compared with other gas turbines the S-CO2Brayton system could increase the electrical power produced per unit of fuel by
40 percent or more. The combination of low temperatures, high efficiency and
high power density allows for the development of very compact, transportable
systems that are more affordable because only standard engineering materials
(stainless steel) are required, less material is needed, and the small size
allows for advanced-modular manufacturing processes.
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Giant Leap Nears in Thermal-to-Electric Conversion Efficiency
April 6, 2011