For a few hours on April 12, engineers at Sandia National Laboratories in New Mexico were able to send electrons back into the Sandia-Kirtland Air Force Base electrical grid using a new type of power generating system. This small proof-of-concept, announced earlier this month, showed the team that their system—which they have been tinkering with since 2007—-was stable, controllable, and able to sync up well to the electrical grid. 

The new system, which the team purports could be more efficient than existing power-generating systems, is based off of a thermodynamics process described through a closed-loop Brayton cycle, and works similarly to a jet engine. Most jet engines are powered by an open Brayton cycle. Ambient air is brought in and compressed. Then, it's heated with fuel and expanded through a turbine. That turbine can then mechanically operate a generator to produce electricity (if you're ground-based), or thrust (if you're on an aircraft).

"Previous years, we just made electricity, and put it into resistive load banks. That's like a huge toaster oven, it turns electricity into heat," says Darryn Fleming, a mechanical engineer at Sandia National Laboratories who worked on this project. "Being able to put power back on the grid was the main outcome for this program." 

So what engine operations do Brayton cycles describe? "Imagine you take a pipe and you connect the [gas] discharge of the turbine back with the compressor, and you keep the working fluid just in a loop. That's a closed Brayton cycle," says Logan Rapp, a Sandia mechanical engineer that was part of the test. "It has the same compressor, turbine, and heat exchange, but the working fluid always stays inside the pipes." 

This system can work with a wide variety of gasses, like air, helium, nitrogen. The Sandia team used carbon dioxide in the supercritical state, which is kind of like a gas, but also kind of like a liquid. "It has a really high density, but it can also be expanded through a turbine like a gas," Rapp says. "So, we reduced the amount of work needed for compression because it's so dense, and that allows us to get to higher thermal-to-electric [conversion] efficiencies."