Above a certain temperature, it becomes possible to replace the steam with supercritical carbon dioxide. This works more efficiently, potentially providing a boost of more than 20 percent, but it requires temperatures in excess of 1,000K. That makes things a bit more challenging, given that many metals will melt at such temperatures; others will react with carbon dioxide under these conditions. Finding a material that could work involves balancing a lot of factors, including heat and chemical resistance, ea

Source: New material could up efficiency of concentrated solar power | Ars Technica