Answer: All power plants or work producing devices which work on thermodynamic principles must have the following 4 processes to produce continuous:
According to the second law of thermodynamics, there must be a heat rejection in the surroundings for a device to work in a cycle and produce continuous work output.
One may say let's remove the heat rejection device and save waste heat and in turn, increase the thermodynamic efficiency to 100%. But that can not be done because it may violate the second law of thermodynamics. Every heat engine must reject some heat energy by transferring it to a low-temperature reservoir in order to complete the cycle even under idealized conditions.
In the specific case of the Rankine cycle, the condenser is the device in the cycle where the waste heat is rejected to the surroundings. This is done by condensing the steam in the vapor phase into the liquid phase i.e. water by rejecting heat to the cooling water reservoir.
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