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Mind The Gap: How To Build A Power Plant Fueled By The Sun And CO2

September 06, 2016
In March this year, Doug Hofer, a steam turbine specialist at GE Global Research, designed a prototype of a supercritical CO2 turbine small enough to fit on his desk but powerful enough to generate electricity for 10,000 homes.
Six months later, he’s ready to raise that number fiftyfold. Hofer and his colleagues are using a grant from the U.S. Department of Energy to design key new parts for the turbine and boost its output to 500 megawatts (MW), enough to supply a large city with electricity.

Today, you need a locomotive and two rail cars to move even the most advanced turbine in this class. Hofer says his design will fit on a semi-truck.

GRC Miniturbine-05 Above: The CO2 turbine prototype in Doug Hofer's hands could one day power 10,000 homes. Image credit: GE Global Research Top: Concentrated solar power plants like this one, which GE Renewable Energy is building in Ashalim, Israel, could use the technology. Image credit: GE Renewable Energy

Unlike traditional steam or gas turbines, the medium turning the blades inside Hofer’s machine is supercritical CO2. The gas, a major climate change culprit, is squeezed to 250 bar pressures—the equivalent of being more 1.5 miles beneath the surface of the ocean— and heated to 700 degrees Celsius—so high that parts glow like charcoal during operation—so that it forms a supercritical fluid and a acquires marvelous new properties. The difference between gas and liquid essentially disappears and makes the turbine “superefficient,” Hofer says.

The titanic pressures and hellish temperatures hint at the decidedly unsexy ingredients Hofer needs to advance his quest: better seals between the rotor and the stator of the turbine. Just like washers or gaskets that stop your bathroom faucet from dripping, these rotor-stator seals stop the supercritical CO2 turning the blades from leaking out. The high-speed rotation of the sealing elements means they have to be much more technologically sophisticated than the laundry room variety.

greenville5_extra_large It takes a locomotive to pull a gas turbine like this one from GE Power's factory in Greenville, South Carolina. Image credit: GE Power

The seals Hofer used on the original prototype had 5-micron gaps between the high-speed rotor and the stator, one-tenth the width of a human hair. He and his team now must figure out how to make a dry gas seal 10 times larger without increasing that distance. Anything much wider would lead to too much leakage and make the turbine inefficient.

Fortunately for Hofer, Global Research has a dedicated Seals Lab onsite that specializes in this niche area. One of the lab’s top experts, Rahul Bidkar, is leading the project to develop these new seals with help from Southwest Research Institute in San Antonio, Texas, using “film-riding” gas seals that remain stable at critical high temperatures and pressures and advanced manufacturing techniques. He says the new larger seal design also could be used in aircraft engines and conventional gas and steam turbines.

Supercritical CO2 technology holds promise for a wide range of applications. including waste heat recovery in industrial and thermal solar plants. “Further in the future, this could also replace steam as the working fluid in large multi-hundred-megawatt coal-fired power plants, and then even beyond that is the next generation of nuclear reactors,” Hofer says. The new seals being developed are an enabling technology for these future power plants.

Those are some not-so-tiny changes from a much smaller turbine.