Sandia team puts power into local grid with supercritical CO2 closed-loop Brayton-cycle turbine
22 August 2022
Sandia National Laboratories researchers recently delivered electricity produced by a new power-generating system to the Sandia-Kirtland Air Force Base electrical grid. The system uses heated supercritical carbon dioxide instead of steam to generate electricity and is based on a closed-loop Brayton cycle. The Brayton cycle is named after 19th century engineer George Brayton, who developed this method of using hot, pressurized fluid to spin a turbine, much like a jet engine.
Supercritical carbon dioxide is a non-toxic, stable material that is under so much pressure it acts like both a liquid and a gas. This carbon dioxide, which stays within the system and is not released as a greenhouse gas, can get much hotter than steam—1,290 degrees Fahrenheit or 700 Celsius. Partially because of this heat, the Brayton cycle has the potential to be much more efficient at turning heat from power plants—nuclear, natural gas or even concentrated solar—into energy than the traditional steam-based Rankine cycle.
Because so much energy is lost turning steam back into water in the Rankine cycle, at most a third of the power in the steam can be converted into electricity. In comparison, the Brayton cycle has a theoretical conversion efficiency upwards of 50%.
We’ve been striving to get here for a number of years, and to be able to demonstrate that we can connect our system through a commercial device to the grid is the first bridge to more efficient electricity generation. Maybe it’s just a pontoon bridge, but it’s definitely a bridge. It may not sound super significant, but it was quite a path to get here. Now that we can get across the river, we can get a lot more going.—Rodney Keith, manager for the advanced concepts group working on the Brayton cycle technology
On 12 April, the Sandia engineering team heated up their supercritical CO2 system to 600 degrees Fahrenheit and provided power to the grid for almost one hour, at times producing up to 10 kilowatts. Ten kilowatts isn’t much electricity, an average home uses 30 kilowatt hours per day, but it is a significant step. For years, the team would dump electricity produced by their tests into a toaster-like resistive load bank, said Darryn Fleming, the lead researcher on the project.
A diagram of Sandia National Laboratories’ simple closed-loop Brayton cycle test loop. The working fluid being compressed, heated and expanded to produce power is supercritical carbon dioxide. Supercritical carbon dioxide is a non-toxic, stable material that is under so much pressure it acts like both a liquid and a gas. (Graphic courtesy Sandia National Laboratories)
We successfully started our turbine-alternator-compressor in a simple supercritical CO2 Brayton cycle three times and had three controlled shutdowns, and we injected power into the Sandia-Kirtland grid steadily for 50 minutes. The most important thing about this test is that we got Sandia to agree to take the power. It took us a long time to get the data needed to let us connect to the grid. Any person who controls an electrical grid is very cautious about what you sync to their grid, because you could disrupt the grid. You can operate these systems all day long and dump the power into load banks, but putting even a little power on the grid is an important step.—Darryn Fleming
In a simple closed-loop Brayton cycle, the supercritical CO2 is heated by a heat exchanger. Then the energy is extracted from the CO2 in a turbine. After the CO2 exits the turbine, it is cooled in a recuperator before entering a compressor. The compressor gets the supercritical CO2 up to the necessary pressure before it meets up with waste heat in the recuperator and returns to the heater to continue the cycle. The recuperator improves the overall efficiency of the system.
For this test, the engineers heated up the CO2 using an electrical heater, fairly similar to a home water heater. In the future, this heat could come from nuclear fuel, burning fossil fuels or even highly concentrated sunlight.
Importance of advanced power electronics. In fall 2019, Fleming began exploring how Sandia’s closed-loop supercritical CO2 Brayton cycle test loop could be connected to the grid. Specifically, he was looking for advanced power electronic control systems that could regulate supplying electricity into the grid. The team then found KEB America who produces advanced power electronics for elevators that could be adapted for this application.
Elevators use electricity to lift the elevator car up to the top floor of the building, and some elevators convert the potential energy stored in the lifted car back into electricity for the grid as the car is lowered to another floor. These elevators use equipment very similar to that used in the Brayton cycle test loop, called a permanent magnet rotor, to convert this energy, Fleming said. This similarity allowed the Sandia team to adapt commercial-off-the-shelf power electronics from an elevator parts company to control feeding power from their test loop into the grid.
The achievement here was coupling the system with the advanced power electronics and syncing it to the grid. We have never done that before; we’d always gone to the load banks. You can draw a pretty clear line from the work we’re doing at 10 kilowatts to about one megawatt. One megawatt is pretty useful; it can power 500-1,000 homes or replace diesel generators for remote applications. Our industry partners are targeting 1- to 5-megawatt systems.—Logan Rapp, a Sandia mechanical engineer who was involved in the test
Rapp primarily works on refining other supercritical CO2 Brayton cycle equipment, but during the test he was in control of heating the supercritical CO2 before it reached the turbine and operating the recuperator. Fleming focused on controlling and monitoring the turbine and generator.
Having successfully completed this test, the team will work on modifying the system so that it can operate at higher temperatures, 1,000 degrees Fahrenheit and above, and thus produce power with greater efficiencies, said Fleming and Rapp.
In 2023, they plan to work on getting two turbine-alternator generators operating in a recompression configuration on the same system, which is even more efficient. The team’s goal is to demonstrate a 1-megawatt supercritical CO2 Brayton cycle system by fall 2024. Throughout this process, they hope to occasionally test the system by supplying electricity to the grid, provided they get approval from the grid operators to do so.
For actual commercial applications we know that we need bigger turbo machinery, power electronics, larger bearings and seals that work for supercritical CO2, closed Brayton cycles. There’s all these different things that need to be done to de-risk the system, and we’re working on those now. In 2023 we’ll be putting it all together into a recompression loop and then we’ll take it to even higher power output, and that’s when the commercial industry can take it from there.—Darryn Fleming
This work is supported by the Department of Energy’s Supercritical Transformational Electric Power program. Collaborators at Barber-Nichols helped with getting the specifications for the advanced power electronics.
We've pretty much go the whole technological toolbox needed to create a demi-paradise, if only we are not too dumb to implement it.
Nuscale and other SNR's can provide power, right where it is needed, and that is an excellent source power source, especially using highly efficient supercritical CO2 as this article outlines, for growing much agricultural produce in cities in vertical farms, with hours from growing it, to eating it:
All inputs save energy are drastically reduced, land use, water, pesticides, so cheap low to zero power is the key, and SNRs can provide that.
So that is the salads etc taken care of.
And for the staples, wheat and potatoes etc, agrivoltaics means that we can both generate energy and provide needed shade and far less evaporation:
So the issue of conflicts between using land for solar and crop growth is potentially resolved.
That leaves meat, and artificial meat will surely become practical.
There is no reason why we can't essentially live in a landscape primarily given over to being a wildlife park, with the cities far greener with extensive greenroofs, and utilising water as 'sponge cities' instead of fighting it with concrete:
contradicting Erlich and the doomsters, we have the technical means to do really well, if only we seize the day.
Posted by: Davemart | 22 August 2022 at 03:47 AM
I don't know whether this is identical to the supercritical CO2 turbine that Energy Dome are to use, as discussed extensively on this thread:
Posted by: Davemart | 22 August 2022 at 04:47 AM
Here is a fairly large agrivoltaic installation approved for Colorado:
Shade for the sheep, the grass gets cut for free, and reduced evaporation on top of the power output.
What's not to like?
Posted by: Davemart | 23 August 2022 at 01:34 AM
re Energy Dome vs Sandia Supercritical C02 turbine:
Energy Dome uses Carbon Dioxide's phase change between gas and liquid to store energy. When the energy is cheap they compress the stored carbon dioxide gas to a liquid at 5.1 bar and 31° C. When energy is needed they expand the liquid carbon dioxide to a gas to drive a turbine. The expanded gas output from the turbine is captured for reuse. Carbon dioxide has an expansion ratio of 535 meaning the that it occupies roughly 535 more space in gas form than in liquid. The Energy Dome also stores the generated latent heat of compression for use in expansion.
The Sandia Supercritical Turbine, on the other hand, keeps the Carbon Dioxide within the closed loop above the critical temperature of 31.1° C and critical pressure of 73.8 bar. They add energy to the compressed high pressure supercritical carbon dioxide before using it to drive a turbine. From the turbine they extract the output energy and use part of the turbine output to recompress the cooler lower pressured carbon dioxide to a higher pressure.
Of note is that the Sandia graphic is poorly drawn as the turbine inputs and outputs are not the normal compression/expansion locations.
Posted by: Dkwolfe4 | 23 August 2022 at 06:08 PM
I first read about the Sandia work with CO2-based Brayton cycle reseach and development more than 15 years ago, if memory serves me right.
Rather slow development.
High pressure difference between the two sides leading to a ventilation flow that causes drag on the generator windings in the center - not least due to high rpm.
@Dkwolfe4: the parallelograms used to symbolize compressor and turbine have been mirrored. A mistake that a turbomachinery engineer would never make.
Posted by: Thomas Pedersen | 24 August 2022 at 05:52 AM
Thanks for your informed commentary, guys.
I'm guessing from that that the Energy Dome implementation sounds a lot more readily do-able, have I got that right?
Posted by: Davemart | 24 August 2022 at 09:37 AM
@ Thomas Pedersen... good point on the mirrored parallelograms... mirrors my point in that the inputs and outputs of the graphic were at the wrong ends of the parallelograms for the function of that side of the turbine.
@Davemart - Energy dome actually has signed contracts already and is quickly moving out of development into commercialization... none of the equipment is unique and is basically off the shelf stuff. For more info, see Mat Ferrells Undecided Youtube video on it... he does a good job of explaining it. https://www.youtube.com/watch?v=GSzh8D8Of0k
Posted by: Dkwolfe4 | 24 August 2022 at 10:48 PM
Thanks. Yep, I have had a good look through the Energy Dome stuff and am well impressed.
But I have to be aware continually of my lack of technical chops, so can easily miss gotchas, and am always up for picking other's brains! ;-)
The reason I know so much is because I steal so many ideas from other people!
Posted by: Davemart | 25 August 2022 at 04:08 AM