GE Global Research is testing a desk-size turbine that could power a small town of about 10,000 homes. The unit is driven by “supercritical carbon dioxide,” which is in a state that at very high pressure and up to 700 °C exists as neither a liquid nor a gas. After the carbon dioxide passes through the turbine, it's cooled and then repressurized before returning for another pass.
The unit’s compact size and ability to turn on and off rapidly could make it useful in grid storage. It’s about one-tenth the size of a steam turbine of comparable output, and has the potential to be 50 percent efficient at turning heat into electricity. Steam-based systems are typically in the mid-40 percent range; the improvement is achieved because of the better heat-transfer properties and reduced need for compression in a system that uses supercritical carbon dioxide compared to one that uses steam. The GE prototype is 10 megawatts, but the company hopes to scale it to 33 megawatts.
In addition to being more efficient, the technology could be more nimble—in a grid-storage scenario, heat from solar energy, nuclear power, or combustion could first be stored as molten salt and the heat later used to drive the process.
While such a heat reservoir could also be used to boil water to power a steam turbine, a steam system could take 30 minutes to get cranked up, while a carbon dioxide turbine might take only a minute or two—making it well-suited for on-the-spot power generation needed during peak demand periods.
GE's system might also be better than huge arrays of batteries. Adding more hours of operation just means having a larger or hotter reservoir of the molten salt, rather than adding additional arrays of giant batteries. “The key thing will come down to economics,” says Doug Hofer, the GE engineer in charge of the project. While there’s work ahead, he says, “at this point we think our economic story is favorable compared to batteries.”