Revised 24 May 2019

For a heat engine to be 100% thermally efficient, the engine is re­quired to reject no heat to a reservoir. This require­ment has proved to be a physical impossibility. The energy of heat com­prises the inter­nal energy of random mol­ecular motion as opposed to the ordered kin­etic motion of mecha­nica­l work. It is not feasible to com­pletely convert the random molecular motion of heat to ordered molecular motion of mechanical work because the motions of each indi­vidual mol­ecule would need to be fully con­trolled. Furthermore, part of the portion of heat that can be con­verted into mech­anical work dissi­pates to the heat form of energy due to the unavoidable fric­tion present in all machines. The degree of convertibility of energy - stored work - into applied work is often called availability. The Second Law of Thermodynamics can be stated as: 

All physical processes proceed in such a way that the availability of the energy involved decreases.

The Second Law of Thermodynamics is not deduced from the First Law of Thermodynamics, but stands as a separate law of nature. The First Law of Thermodynamics denies the pos­sibility of creating or des­troying energy, but does not preclude the pos­sibility of running a power station that extracts heat from the at­mosphere. The Second Law of Thermodynamics denies any possibility of per­petual motion of the second kind whereby a machine utilizes the inter­nal energy of one, and only one, heat reser­voir.

The Second Law of Thermodynamics may appear equally simple to understand as the First Law of Thermodynamics, but the Second Law of Thermodynamics is very deceptive (Andrews, 1980) and there are many far reaching implications which connect back to this law. An outcome of the Second Law of Thermodynamics is that all forms of energy ultimately degrade into dispersed heat energy. There is no process whose sole result is the complete transformation of energy into another form of energy which is of higher grade than heat. This is why it is impossible to drive a steamship across the ocean by extracting heat from the ocean or why the perpetual motion machine does not exist. In other words, there are some fixed limits to technological innovation, placed there by fundamental laws of nature.