3 - Energy Conservation

Abstract

The first law of thermodynamics states that energy may be transformed from one type to another, but is neither created nor destroyed. In nature, two processes can be distinguished: the spontaneous processes, which occur naturally without an input of energy from outside, and the nonspontaneous processes, which require an input of energy from outside. These facts are included in the second law of thermodynamics, which states that processes involving energy transformations will not occur spontaneously unless there is a degradation of energy from a nonrandom to a random form or from a concentrated into a dispersed form. In other words, all energy transformations will involve energy of high quality being degraded to energy of lower quality (for example, potential energy to heat energy). Exergy is defined as the amount of work a system can perform when it is brought into thermodynamic equilibrium with its environment. The loss of exergy and production of entropy are two different descriptions of the same reality—namely, that all processes are irreversible. Therefore, the formulation of the second law of thermodynamics in terms of exergy is that all real processes are irreversible, which implies that exergy is lost. Energy is, of course, conserved by all processes according to the first law of thermodynamics.