Consider a complete cycle, such as a power cycle which starts at state 1, undergoes several steps, and returns to state 1. What is the net change in energy?
Since, the cycle is complete and the working fluid reaches the same state after the process where it was before the process so net change in internal energy and Enthalpy is zero since these two are point functions and does not depend upon the path taken to reach the state point. In power producing cycle the net interaction is equal to net work interaction that is heat is converted to work.
