Unknown Advanced Thermodynamics for Engineers Chapter 2 Availability and Exergy英文电子书.pdf

Availability and Exergy Many of the analyses performed by engineers are based on the First Law of Thermo- dynamics, which is a law of energy conservation. Most mechanical engineers use the Second Law of Thermodynamics simply through its derived property - entropy ( S ) . However, it is possible to introduce other ‘Second Law’ properties to define the maximum amounts of work achievable from certain systems. Previously, the properties Helmholtz energy ( F ) and Gibbs energy ( G )were derived as a means of assessing the equilibrium of various systems. This section considers how the maximum amount of work available from a system, when interacting with surroundings, can be estimated. This shows, as expected, that all the energy in a system cannot be converted to work: the Second Law stated that it is impossible to construct a heat engine that does not reject energy to the surroundings. 2.1 Displacementwork The work done by a system can be considered to be made up of two parts: that done against a resisting force and that done against the environment. This can be seen in Fig 2.1. The pressure inside the system, p, is resisted by a force, F , and the pressure of the environment. Hence, for System A, which is in equilibrium with the surroundings p A = F + p d (2.1) where A is the area of cross-section of the piston. Svstem A Fig. 2.1 Forces acting on a piston If the piston moves a distance dx, then the work done by the various components shown in Fig 2.1 is pA d x = F d x + p J dx (2.2) where PA dx =p dV = SW,,, = work done by the fluid in the system, F dx = SW,,