Why is the combined cycle power generation system so much more efficient that the straight steam cycle?

  Because it uses the residual energy of the fluid used by the first engine.

Explanation:

  A combined cycle power generation counts with two heat engines that work in tandem from the same source of heat. The engines turn the energy into mechanical energy.

  The cycle is much more efficient than the other, almost 60% more.

  I hope this answer helps you.