What is an example in Kelvin-Plank statement in which heat energy is transferred into mechanical, & then mechanical energy into electrical energy

 

We represent a heat engine and a heat pump cycle in a minimalist abstract format as in the following diagrams. In both cases there are two temperature reservoirs T_H and T_L, with T_H > T_L.

 

In the case of a heat engine heat Q_H is extracted from the high temperature source T_H, part of that heat is converted to work W done on the surroundings, and the rest is rejected to the low temperature sink T_L. The opposite occurs for a heat pump, in which work W is done on the system in order to extract heat Q_L from the low temperature source T_L and "pump" it to the high temperature sink T_H. Notice that the thickness of the line represents the amount of heat or work energy transferred.

We now present two statements of the Second Law of Thermodynamics, the first regarding a heat engine, and the second regarding a heat pump. Neither of these statements can be proved, however have never been observed to be violated.

The Kelvin-Planck Statement: It is impossible to construct a device which operates on a cycle and produces no other effect than the transfer of heat from a single body in order to produce work.

We prefer a less formal description of this statement in terms of a boat extracting heat from the ocean in order to produce its required propulsion work: