“We’ve Got the Power”: Sandia Technology Test Delivers Electricity to the Grid

“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,” Fleming said. “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.”

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,” said Logan Rapp, a Sandia mechanical engineer who was involved in the test. “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.”

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,” Fleming said. “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.”