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1. AvailabilityandIrreversibility MECH3400

2. Availability or exergy • is total energy useful? • maximum work potential = availability or exergy • work is generally a function of the: • initial state • process path • final state given work maximised when process reversible ?

3. Dead state and maximum work • dead state = equilibrium with the surroundings • thermal • mechanical (potential and kinetic) • chemical, • magnetic...... “a system must go to the dead state at the end of the process in order to maximise the work output”

4. Reversible work and availability • reversible work is the maximum amount of useful work that can be obtained as a system undergoes a process between the specified initial and final states • when final state=dead state • then availability=reversible work examples: furnace and windmill

5. Irreversibility reversible work useful work irreversibility = - • for a work producing device irreversibility =lost opportunity to do work • for a work consuming device irreversibility =wasted work examples: pie chart, engine, iron block

6. 2nd law efficiency • 1st law efficiency (COP) is conversion efficiency • no reference to best possible performance • that is where 2nd law efficiency helps

7. 2nd law efficiency(example) Sources 1000 K 600 K A B Heat engines =30% =30% 300 K Sink example

8. 2nd law efficiency(definition) for work producing devices (engines): for work consuming devices : generally:

9. 2nd law analysisfor a control mass 1 W surroundings To , Po 2 gas Qo 1st law 2nd law total actual work done in the process:

10. Po 2 Po 1 V2 V1 Surroundings work surroundings work: surroundings air which is displaced

11. Useful work total actual work useful work surroundings work - =

12. Reversible work and availability reversible work: 0 availability or exergy:

13. Irreversibility example

14. Additional heat transfer to other systems 2 W 1 QR reservoir at TR Qsurr surroundings To , Po

15. Additional heat transfer to other systems (cont.) 1st law 2nd law 0 subtract the surroundings work from W:

16. Additional heat transfer to other systems (final)

17. 2nd law analysis for a control volume CV 1st law 0 steady state flow

18. 2nd law analysis for a CV 1st law

19. Actual and useful work for a CV 1st law 2nd law actual work: what about useful work?

20. Stream availabilityor stream exergy (CV) actual/useful work for a single inlet/outlet : reversible work for a single inlet/outlet : let us define stream exergy (availability):

21. reservoir at TR surroundings To , Po Additional heat transfer to other systems (CV) CV QR Qsurr

22. 2nd law efficiencyfor a CV In the case of no heat transported between device and surrounding: for work producing devices (engines) for work consuming devices