Kelvin–Planck Statement

The Kelvin–Plank statement of the second law of thermodynamics refers to a thermal reservoir. A thermal reservoir is a system of infinite heat capacity that remains at a constant temperature even though energy is added or removed by heat transfer. A reservoir is an idealization, of course, but such a system can be approximated in a number of ways—by the Earth’s atmosphere, large bodies of water (oceans), and so on.

The Kelvin–Planck statement of the second law can be given as: It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir.

In Figure 1.11, it is shown that there are two reservoirs from which heat is interacted to do a work W_net. Heat, Q_H is taken from higher temperature reservoir and work is done and rest amount of heat is rejected to lower temperature reservoir. Thus, the total conversion of heat to work is impossible; there will always be rejection of some part of the heat supplied by the heat engine.

Figure 1.11 Heat Engine