Closed loop Brayton cycle (IMAGE) DOE/Sandia National Laboratories Caption 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. Credit Graphic courtesy Sandia National Laboratories Usage Restrictions With credit License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.