When carbon dioxide is held above its critical pressure (73.773 bar) and temperature (30.9780 °C), it can behave both as a gas and as a liquid, that is it expands to fill entirely its container like a gas but has a density similar to that of a liquid.
Supercritical CO is chemically stable, very cheap, and non-flammable, making it suitable as a working fluid for transcritical cycles. For example, it is employed in domestic water heat pumps, which can reach high efficiencies.
Moreover, when used in power generation plants that employ Brayton and Rankine cycles, it can improve efficiency and power output. Its high density enables a strong reduction in turbomachines dimensions, still ensuring the high efficiency of these components. Simpler designs can therefore be adopted, while steam turbines require multiple turbine stages, which necessarily yield larger dimensions and costs.
By contrast, mechanical components within sCO Brayton cycles, especially turbomachinery and heat exchangers, suffer from corrosion.