Power conversion breakthrough could increase plant efficiency

Today's power plants - nuclear, gas and coal - use a steam-based Rankine cycle to convert the heat they produce into electricity, but these steam-based systems lose some two-thirds of the energy they could potentially produce because steam must be converted back into water to repeat the cycle. The Sandia team instead developed a simple recuperated closed-loop Brayton cycle that uses supercritical carbon dioxide as the working fluid.

Supercritical carbon dioxide, which stays within the system and is not released as a greenhouse gas, can get much hotter than steam - up to 700°C. The Brayton cycle has the potential to be much more efficient at turning heat from power plants into energy than the traditional steam-based Rankine cycle, with a theoretical conversion efficiency upwards of 50%.

In the system developed by the Sandia team, the carbon dioxide gas runs through a continuous loop of being pressurised, heated, and expanded through a turbine to generate electricity. It is then cooled in a recuperator before returning to a compressor to complete the cycle.
 

A diagram of the closed-loop Brayton cycle test loop (Image: Sandia)
The gas runs through a continuous loop of being pressurised, heated, and expanded through a turbine to generate electricity. After the fluid exits the turbine, it is then cooled in a recuperator before returning to a compressor to complete the cycle.

For the test, researchers used an electric heater to heat up the supercritical carbon dioxide up to 600°F (316°C) and then returned power into the Sandia-Kirtland Air Force Base electrical grid. The test delivered continuous power to the grid for 50 minutes and, at times, produced up to 10 kilowatts of electricity. This is the first time that grid operators have agreed to take the power and is a significant step, the researchers said.

"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," said Rodney Keith, manager for the advanced concepts group working on the Brayton cycle technology.

The team will work on modifying the system so that it can operate at higher temperatures and aim to demonstrate a 1 MW supercritical CO2 Brayton cycle system by the autumn of 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," lead researcher Darryn 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."

The project has been supported through the Department of Energy's Supercritical Transformational Electric Power (STEP) programme, which is co-sponsored by the offices of Nuclear Energy, Fossil Energy and Carbon Management, and Energy Efficiency and Renewable Energy.