The U.S. Department of Energy hopes to create a more efficient turbine that uses CO2 to make electricity
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. […]”<