The theoretically achievable maximum efficiency of thermal power generation is limited by a simple relation based only on the lowest and the highest temperature of the cycle.This equation is well known as Carnot’s Law.

The higher steam temperatures of Super-Critical (540 to 600 C)& Ultra-Super Critical (650 to 720 C) boilers allows these boilers to obtain higher efficiencies than conventional boilers.  Material suitable for the steam tubes at temperatures of super-critical are now widely available and these boilers are now being built.  Research into suitable materials for ultra-super-critical boilers is currently been done in Europe, Japan and USA, and a few demonstration units have been built.