Sandia National
Laboratories researchers are moving into the demonstration phase of a gas
turbine system for power generation, with the promise that thermal-to-electric
conversion efficiency will be increased to as much as 50%—an improvement of 50%
for nuclear power stations equipped with steam turbines, or a 40% improvement
for simple gas turbines.
Research focuses on
supercritical carbon dioxide (S-CO2) 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
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,
named after George Brayton, 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,” said principal investigator
Steve Wright of Sandia’s Advanced Nuclear Concepts group. “There is a
tremendous amount of industrial and scientific interest in supercritical CO2
systems for power generation using all potential heat sources including solar,
geothermal, fossil fuel, biofuel, and nuclear.”
Sandia currently has
two supercritical CO2 test loops. 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 in March 2010. 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 925?C and is expected to
produce electrical power at 43% to 46% efficiency. By contrast, the
supercritical CO2 Brayton cycle provides the same efficiency as
helium Brayton systems but at a considerably lower temperature (250 to 300?C).The S-CO2 equipment 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
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 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%).
In other words, as
compared with other gas turbines the S-CO2 Brayton system could
increase the electrical power produced per unit of fuel by 40% 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.
“Sandia is not alone
in this field, but we are in the lead,” Wright said. “We’re past the point of
wondering if these power systems are going to be developed; the question
remains of who will be first to market. Sandia and DOE have a wonderful
opportunity in the commercialization effort.”
Sandia’s
S-CO2 Brayton cycle program is supported by DOE with funding from
the Labs’ Laboratory Directed Research & Development (LDRD) program.
