GE Global Research is testing a desk-size turbine that could produce 10 megawatts — enough to power 10,000 homes — and, more importantly, spin up in minutes:
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 watts.
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.
How does this compare to natural gas fueled turbines? I bet the cost per kWh is higher.
Depending on how I’m interpreting your statement, Bob, that might not be a sensible question. In a gas turbine power-plant the turbine is the power source itself: NatGas is burned, turning a turbine which is coupled to a generator. The system is a Brayton cycle.
The system above isn’t a power source like the turbine in the above-mentioned power-plant, but as a component in a Rankine cycle system where an external heat source is used to move a working fluid (in this case super-critical CO2). Examples are coal or nuclear power-plants where the energy source merely provides heat. Usually that heat generates steam that runs a turbine. The turbine spins a generator just like in the NatGas plant.
In modern power-plants, Rankine and Brayton cycles are roughly similar in efficiency, and have interlocking strengths and weaknesses.
Thanks. I should have read the link.
Apparent typo, now fixed at the link: “scale it to 33 megawatts”, not just “watts”.
(Though I could see someone trying to make a system requiring 700C CO2 that only dumps out 33 watts, just for the geek cred.)
“Steam-based systems are typically in the mid-40 percent (efficiency) range.”
This is true, but plants that aim for maximum efficiency aren’t pure steam; they are combined-cycle, with a gas turbine and a steam turbine, which is fed by waste heat from the gas turbine. GE’s own “H” series combined-cycle turbines are quoted at efficiencies of 61%.
So presumably the CO2 turbine is intended for situations where a first-stage gas turbine is infeasible, or the physical size of a combined-cycle system would be excessive. Maybe faster start-up time as well.
I think responsiveness is the big plus here, combined maybe with the simplicity of a single-loop system versus multiple. Less plumbing is usually a good thing, even if it’s more challenging plumbing to get right the first time.
A GE H series gas turbine is a component of a combined-cycle application, with a gas turbine and a separate steam turbine. GE makes separate Steam turbine families like ST-D600, D400, D200 etc. The Supercritical Carbon Dioxide Turbine in this article would replace the steam turbine. A combined-cycle plant with a GE H series gas turbine driving a Supercritical Carbon Dioxide Turbine would likely be over 61% efficient.
https://powergen.gepower.com/products/steam-turbines.html