All conventional power sources—from coal and gas plants to high-tech nuclear fission and potential future nuclear fusion reactors—boil down to the same basic thing: heating water into steam to spin a turbine. The varying efficiency of these different types depends on how much water they can heat up and how much fuel it costs to do so.
Steam turbines are simple but far from the most efficient way to generate electricity. It just takes a lot of energy to turn room-temperature water into steam. That's energy that isn't being used to spin a turbine which means it's basically wasted. There's no way to fix this problem short of swapping out steam for something else.
So that's exactly what some groups are trying to do. Their best replacement candidate? Carbon dioxide, according to a new study in Science.
CO2 is most commonly depicted as a greenhouse gas and the primary driver of climate change, but in this case it might actually reduce the amount of carbon pollution in the atmosphere. Researchers are looking at using CO2 in a "supercritical" state, which has many ideal properties.
Supercritical CO2 is produced when normal CO2 gas is raised to very high temperatures and pressures. At that level, it has the same density of a liquid but expands to fill its container like a gas. The increased density means that CO2 turbines can be built much smaller than steam turbines, and the lack of boiling means that less energy is wasted.
But building a working CO2 turbine is far from easy. Pressurizing supercritical CO2 is much harder than liquid water and takes far more energy. The temperature and pressure of the supercritical CO2 have to be maintained exactly or else the CO2 will condense into a liquid, which can cause problems for the machinery. The turbine also has to work at the high pressures and temperatures required for supercritical CO2.
Even though the idea has been around since the 1940s the possible benefits of CO2 turbines have never been worth the challenges. It was always cheaper just to make the steam turbine a little more efficient than invent a whole new kind of turbine. But the steam system is now about as efficient as it can get, and engineers are running out of ways to make improvements. That makes this the perfect time for supercritical CO2, and a few plants are trying it out.
One of the first large-scale CO2 turbines, managed by NET Power, is being installed right now at a demonstration plant in Texas. When the turbine is completed later this year it will become the first functional supercritical CO2 turbine in the world—assuming it works.
The NET Power plant is innovative in other ways, too. It burns natural gas in pure oxygen instead of air to eliminate all byproducts other than CO2, and it buries the excess CO2 in the ground to make the entire plant emissions-free. The most important innovation is the supercritical CO2 turbines, which almost double the plant's efficiency singlehandedly.
If the experimental NET Power plant in Texas is as successful as its designers hope, the efficient CO2 turbine design could be adopted by hundreds of plants around the country, drastically reducing greenhouse gas emissions and increasing energy efficiency. NET Power is already planning to build a large utility-scale plant with the supercritical CO2 technology if the results are promising. That proposed second plant could power 200,000 homes and be operational by 2021.