The Second Law of Thermodynamics

The Second Law of Thermodynamics defines the property entropy, which can be used as a measure of the disorder in a system. A process that does not generate entropy is called a reversible process if it can be performed and then returned to its initial state (reversed) without leaving any traces on the surroundings. Therefore, in a reversible process, by the First Law, no net exchange of heat or work occurs in either the system or surroundings: both return to their original states. An irreversible process, on the other hand, does generate entropy because of, for example, uncontrolled expansion, heat loss from friction, or heat transfer through a finite temperature difference. A process involving heat transfer can be made reversible if the finite temperature difference, or temperature gradient, is minimized to an infinitesimal difference (at the expense of the rate of heat transfer). Entropy is based on this reversible heat transfer, and as a property, is expressed as an exact differential in Eq. (3.6).

electrolyte system boundary fuel oxidant electrolyte system boundary fuel oxidant

FIGURE 3.2 A fuel cell represented as a control volume. E stands for electrical potential, measured in volts. (With permission from Chen, E.L. and Chen, P.I., Proceedings of the ASME 2001 IMECE, vol. 3, Nov. 11-16, 2001.)

FIGURE 3.2 A fuel cell represented as a control volume. E stands for electrical potential, measured in volts. (With permission from Chen, E.L. and Chen, P.I., Proceedings of the ASME 2001 IMECE, vol. 3, Nov. 11-16, 2001.)

The change in entropy is dependent only on the initial and final states of the system, as seen in the integrated form of this equation in Eq. (3.7).

For a process that undergoes reversible heat transfer, Qrev, at a constant temperature, To, entropy is expressed as in Eq. (3.8).

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

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