Second Law and Entropy A Broader View

We saw in Section 5.3.3 that any process that brings about a total increase of entropy AS results in the loss of an amount of useful energy equal to TqAS, where Tq is the temperature of the atmosphere. Notice that this energy, while it is still around us, is lost for ever with respect to its ability to provide useful work such as, for example, to produce mechanical work. 

But we also know that all real life processes are, by their very nature, irreversible causing an increase in total entropy and, consequently, leading continuously to loss of useful energy. Thus, according to Table 5.2, we only use between 10% and 50% of the useful energy of the fuels, our main source of energy. 

While this percentage will be increasing with technological advances, we should keep in mind that it can never reach 100%, for a degree of irreversibility is a practical requirement for all real life processes. A reversible heat exchanger, for example, must operate with a vanishingly small temperature difference, hence requiring an infinite area for heat transfer. 

---