Carbon dioxide is one hell of a molecule. Perhaps you only know it as the stuff humans exhale and plants inhale, or the primary culprit for climate change. But CO₂ is capable of so much more. For instance, some engineers think it could help make the power industry a little greener.

Now, you're probably thinking this is a twist on carbon capture and storage. Nope. It's about turbine generators---the enormous machines that convert heat into electricity. Most power plants use steam turbines. But turning water into a gas (steam) requires a lot of energy. Carbon dioxide exists as a gas at room temperature, saving you that trouble. Plus, it compresses far more easily, meaning you can cram a lot more of it through a turbine. A paper published in Science says extremely hot and extremely compressed—a state called supercritical—CO₂ could generate more power with smaller turbines.

More than two thirds of all the electricity in the US is generated using steam generators operating on what engineers call the Rankine cycle. You start with water, pressurized using a pump. Then apply heat—by burning coal, letting radioactive material decay, or focusing sunlight reflected off thousands of mirrors onto a single point. This boils the water, creating steam. Add more heat. And still more heat. You want that steam as hot as possible before sending it through the turbine: More heat means more energy means more electricity. The turbine blades spin, and the generator attached to them creates electricity. Then steam goes through a condenser, becomes water, and returns to the pump. The cycle begins anew.

The Rankine cycle has worked quite nicely for well over a century. No one had any reason to change things because, until recently, generating electricity was pretty cheap and the consequences of using coal (read: climate change) to do it weren't so readily apparent. But the Rankine cycle is inefficient, mostly because it uses water. "It's an interesting accident of physics, that in order to get anything to change phase, like, from ice to water, or water to steam, you need to add lots of energy," says Avi Shultz, a program manager at the DOE’s SunShot Initiative. In other words, a steam generator going through the Rankine cycle wastes a whole lot of energy boiling water.

This is especially rankling when you remember that the hotter the steam going through a turbine, the more electricity that turbine generates. All that heat energy wasted boiling the water could have been used to generate more power.

A CO₂-driven turbine like those described in the Science paper skips the liquid phase entirely with what's called the Brayton cycle. "It uses a gas phase throughout, so you really end up with a better use of energy," says Levi Irwin, a DOE contractor, and author of the paper. Carbon dioxide also compresses more easily than water. That means you can pack more energized (heated) CO₂ in a smaller volume. Irwin's paper proposes heating and compressing CO₂ until it enters a supercritical state in which it is a bit like a liquid and a bit like a gas. "This lets you push energy through a turbine at 10 times the rate of steam," Irwin says. Vrooooom!

This makes a supercritical CO₂ generator 30 percent more efficient at converting energy into electricity, Irwin writes in his paper. Those generators are smaller and simpler---because they deal with only a single phase (gas) and therefore have fewer parts. The only thing that might make them better is if they somehow harvested CO₂ from the atmosphere. Instead, they rely upon industrial grade carbon dioxide that remains in a closed system.

So what's the hangup? Well, the intense heat can play hell on turbines. "When you talk about high energy, you have a lot of large temperature gradients that are going to put mechanical stresses on the turbine," says Irwin. That means building CO~2 ~turbines with metals that won't crack, distend, or deform, and making them big enough to take the abuse. Also, there are a few engineering problems to work out. Like, the turbine blades, which need to be designed to work efficiently with the not quite liquid, not quite vapor consistency of supercritical CO₂.

The DOE announced in October that it is building a prototype power plant that uses supercritical CO₂ turbines. When the $80 million project goes online in about six years, it will generate 10 megawatts of energy—about enough to operate a few thousand homes. That explains why Shultz doesn't expect supercritical CO₂ turbines to start replacing traditional steam turbines en masse for at least a decade. And if coal happens to be obsolete by then, no problem. This technology works with any power plant that converts heat into electricity, including solar thermal and nuclear energy. That's one hell of a machine.