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Availability and Irreversibility

Availability and Irreversibility. MECH3400. 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. ?.

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Availability and Irreversibility

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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

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