Various technologies pertaining to tuning composition of a fluid mixture in a supercritical Brayton cycle power generation system are described herein. Compounds, such as Alkanes, are selectively added or removed from an operating fluid of the supercritical Brayton cycle power generation system to cause the critical temperature of the fluid to move up or down, depending upon environmental conditions. As efficiency of the supercritical Brayton cycle power generation system is substantially optimized when heat is rejected near the critical temperature of the fluid, dynamically modifying the critical temperature of the fluid based upon sensed environmental conditions improves efficiency of such a system.

Here is described a process to transform energy in chemical form in fuels into electric power through a thermal process. It combines advantages of the traditional internal combustion engine and the steam engine by producing supercritical combustion to allow direct mixture of combustion gases with additional working fluid to cool the mixture to operational conditions. The process allows the control of the inlet temperature of the turbine or expander and makes direct heat exchange by mixing working fluids. The combustion gases are completely used as working fluid in contrast to steam generator. The process improves the efficiency compared to combined cycle or traditional supercritical plants.