Maximum Obtainable Useful Energy from a System

1 Maximum Obtainable Useful Energy from a System 

If the qualitative picture presented in the previous Section is to be used for practical applications, some quantitative measure of useful energy must be established. 

To this purpose, let us consider the following general question If we have a system at a temperature T and a pressure P, what is the maximum amount of useful energy that can be extracted from it If, for example, we have one kg of steam at 60 bar and 550°C, what is the maximum useful energy that can be obtained from it  

The extraction of useful energy from a given system will involve, of course, a change to some final state characterized by the temperature Tq and the pressure Pq. We must, therefore, specify this final state, before we can determine the obtainable useful energy. 

It is apparent that if the production of work involves expansion - of the steam for example - the lower the exhaust pressure Po, the larger the amount of useful energy produced. Or, if work is to be produced by setting up a Carnot cycle - that absorbs heat from the system and rejects part of it to a heat sink that is at a temperature Tq - the lower the value of Tq, the more the amount of useful energy produced. 

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