The principle of the conservation of energy states that energy can neither be
created nor destroyed. If a system undergoes a process by heat and work
transfer, then the net heat supplied, Q, plus the net work
input, W, is equal to the change of intrinsic energy of the
working fluid, i.e.
where U1 and U2 are intrinsic energy of the system at initial and final states, respectively. The special case of the equation applied to a steady-flow system is known as steady-flow energy equation. Applying this general principle to a thermodynamic cycle, when the system undergoes a complete cycle, i.e. U1 = U2, results in:
Q= The algebraic sum of the heat supplied to (+) or rejected from (-) the system.
W= The algebraic sum of the work done by surroundings on the system (+) or by the system on surroundings (-).
Applying the rule to the power plant shown in figure below,
Q = Qin - Qout
W = Win - Wout
Qin + Win - Qout - Wout = 0
Qin = Heat supplied to the system through boiler,
Win = Feed-pump work,
Qout = Heat rejected from the system by condenser,
Wout = Turbine work.