Wednesday, March 4, 2015

Can Carbon Dioxide Replace Steam to Generate Power?

The U.S. Department of Energy hopes to create a more efficient turbine that uses CO2 to make electricity
Engineers are looking into replacing steam with supercritical carbon dioxide, a technique that could unlock up to 50 percent greater thermal efficiency using a smaller, cheaper turbine. 
Credit: Michael Spiller/Flickr

Much has changed in the modern electric power plant since Thomas Edison's era, but the parts that actually turn heat into electrons haven't changed since his eureka moments.
Whether burning coal, concentrating sunlight or splitting atoms, most thermal power plants use the energy for the same thing: heating water into steam to drive a turbine. Steam-based generation produces 80 percent of the world's electricity.
After more than a century of incremental improvements in the steam cycle, engineers have plucked most of the low-hanging fruit and are chasing diminishing returns, spending millions of dollars for every percentage point of efficiency improvement. These upgrades propagate to other steps in electricity production, allowing power plants to extract more work for a given unit of fuel.
In a fossil fuel-fired generator, this means less carbon dioxide emissions for the same unit of electricity produced. For a solar thermal plant, this results in higher capacity at lower operating costs.
Now engineers are looking into replacing steam with supercritical carbon dioxide, a technique that could unlock up to 50 percent greater thermal efficiency using a smaller, cheaper turbine.
Last month, in a budget briefing and in two different hearings before Congress, Energy Secretary Ernest Moniz specifically mentioned the Department of Energy's supercritical carbon dioxide initiatives. The department's 2016 budget request allocates $44 million for research and development on this front, including a 10-megawatt supercritical turbine demonstration system.
A simpler, smaller, cleaner machine
The term "supercritical" describes the state of carbon dioxide above its critical temperature and pressure, 31 degrees Celsius and 73 atmospheres. Under these conditions, carbon dioxide has a density similar to its liquid state and fills containers the way it would as a gas.
Coffee producers are already using supercritical carbon dioxide to extract caffeine from beans. Materials companies are also using it to make plastics and ceramics.
"From a thermodynamic perspective, it's a very good process fluid," said Klaus Brun, machinery director at the Southwest Research Institute, a nonprofit research and development group. "You get a fairly efficient cycle and a reasonable firing temperature."
In its supercritical state, carbon dioxide is nearly twice as dense as steam, resulting in a very high power density. Supercritical carbon dioxide is easier to compress than steam and allows a generator to extract power from a turbine at higher temperatures.
The net result is a simpler turbine that can be 10 times smaller than its steam equivalent. A steam turbine usually has between 10 and 15 rotor stages. A supercritical turbine equivalent would have four.
"We're looking at a turbine rotor shaft with four stages on it that's 4 inches in diameter, 4 feet long and could power 1,000 homes," said Richard Dennis, turbine technology manager at the National Energy Technology Laboratory.
He noted that the idea of a supercritical carbon dioxide power cycle dates back to the 1940s, but steam cycles were already very efficient, well-understood and cheap, creating an uphill slog for a new power block to catch on. In addition, engineers were still finding ways to improve the combustion side of power production, so the need to improve the generation side of the plant wasn't as acute until recently.
Regulations could create an expanding market
Now regulations and climate concerns are forcing power producers to consider ideas like supercritical carbon dioxide.
"In just the closed, indirect cycle, numbers suggest that the thermodynamic efficiency, at similar operating temperatures to a steam cycle, you would see a 3- to 4-percentage-point improvement," Dennis said. With further optimization and better materials, engineers could push performance even higher.
The indirectly heated power block would essentially be a drop-in replacement for a steam power block. Such a device would be a boon for nuclear power stations, concentrating solar farms, geothermal installations, combined heat and power systems, and fossil fuel-fired power plants.
Supercritical turbines would also be an attractive upgrade from steam systems aboard ships and submarines, producing the same power while occupying less space. Because they use carbon dioxide instead of water as their process fluid, these turbines would also work well in drought-stricken areas.
In addition, a supercritical turbine could fit into a directly heated cycle, where a fuel like natural gas burns in the presence of pure oxygen inside the turbine, creating only water and carbon dioxide as waste.
Operators could then remove water and sequester the excess carbon dioxide. "With modest success in the technology management program, these cycles could compete with combined-cycle natural gas turbines and carbon capture and storage," Dennis said. The system could also route waste heat back into the front end of the system, further increasing overall efficiency.
However, the turbine is not the only component of the power block; a supercritical carbon dioxide system still needs heat exchangers, cooling systems and piping, which add to cost and complexity. And carbon dioxide can corrode materials, so engineers will have to redesign much of the plumbing and support hardware from steam systems to account for these problems.
Another concern is that most of the supercritical carbon dioxide systems demonstrated to date were built at kilowatt scales, too small to offer useful lessons for a full-sized version. DOE's pilot proposal would go a long way toward demonstrating the feasibility of supercritical carbon dioxide. "Building a 10-megawatt plant would be a huge, huge step forward," Dennis said.

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