General ElectricRendering of a combined cycle gas plant that can adjust quickly to compensate for the less reliable flow of renewable energy to the grid.
Making electricity from natural gas is cleaner than making it from coal or oil, but as gas gains a bigger market share in the business in the United States and elsewhere, the idea of what constitutes a good generator is shifting. General Electric announced a line of new models on Wednesday, with two interesting environmental twists.
One is efficiency. New natural gas plants burn the fuel in something that resembles a jet engine that is chained to the ground and turning a generator. Over the years, as the technology of such models has improved, they have been converted a given quantity of gas to larger and larger amounts of electricity.
Modern plants are often “combined cycle” generators, meaning that the exhaust of one engine is used as the heat source for another. The hot exhaust from the jet is used to boil water into steam, and the steam is used to turn a second turbine; if the jet engine extracts 40 percent of the energy in the gas and the steam turbine extracts 40 percent of the remainder, the combination is about 60 percent efficient.
In 2000, G.E. introduced a new model that reached that 60 percent level. At the time, the Energy Department likened that milestone to running a four-minute mile. The new models it is introducing now are a little over 61 percent efficient.
Boosting efficiency in a jet engine is tricky. The amount of work that the machine can get out of a fossil fuel depends on the temperature of combustion, but if the designer pushes the temperature too high, the metal components could melt. The solution is to hollow out the blades that extract energy from the passing hot gas and to circulate cool air inside them. “Cool” is relative here; G.E. is using air at 1,000 degrees to cool blades whose exterior is heated to 2,800 degrees. The metal blades are coated with a ceramic that acts as an insulator, helping keep the metal temperature down.
A key trick in raising efficiency, said Paul F. Browning, the president of the company’s thermal products division, is using technology borrowed from G.E.’s aircraft engine business to prevent the 1,000-degree air from leaking into the 2,800-degree gas path, which would cool and it thus reduce efficiency.
But in the years since G.E. introduced the 2000 model, new players have entered the grid: wind and solar plants that produce energy with a much smaller environmental footprint yet are highly variable in their output. A system that can integrate a large amount of renewable energy can far more significantly reduce the average amount of carbon dioxide produced per kilowatt-hour than one that relies on natural gas alone.
Still, relying on renewable energy requires other generators on the grid to rapidly adjust their production, up and down, to compensate. Some grid operators will therefore pay the owners of generators not only for how much energy they produce but for how quickly they can change their output to help balance the supply as demand shifts.
So G.E. has introduced a product line called FlexEfficiency that allows operators to adjust quickly as renewable energy comes on on and off the grid, including a 750-megawatt combined-cycle plant that can vary its output by 100 megawatts in one minute. The model introduced in 2000 could only change its output by 50 megawatts in one minute.
The company says it has $1.2 billion advance orders, including one from the Japanese utility Chubu Electric Power, which, as Mr. Browning delicately put it, would help the company “respond to events of March 11.’’ He was referring to last year’s quake and tsunami in Japan, which caused a meltdown at the Fukushima Daiichi nuclear plant and led to the shutdown of all of the country’s nuclear reactors.
The combined-cycle gas plants are likely to replace the output of some G.E.-designed nuclear reactors.
A Saudi electric company also placed a big order to replace generators that run on oil so it could use the oil for export, G.E. said. And Xcel Energy has ordered one to replace a coal-burning power plant in Denver, the company added.
