Some Final Comments Regarding Exergy: Mech 330: Applied Thermodynamics Ii [PDF]

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MECH 330: APPLIED THERMODYNAMICS II



LECTURE 07



Some final comments regarding Exergy Uses for exergetic efficiency: - For distinguishing means for utilizing energy resources that are thermodynamically effective from those that are less so. - To evaluate the effectiveness of engineering measures taken to improve the performance of a thermal system. - To gauge the potential for improvement in the performance of a thermal system by comparing the efficiency of the system with the efficiency of like systems. Methods of improving energy resources use by using some of the exergy that would otherwise be vented to the environment. Waste Heat Recovery



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MECH 330: APPLIED THERMODYNAMICS II



Cogeneration Steam Extraction for Thermal Load:



Combined Power Cycles:



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MECH 330: APPLIED THERMODYNAMICS II



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Advanced Vapour Power Systems – Rankine Cycle



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MECH 330: APPLIED THERMODYNAMICS II



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Analysis Our analysis for each component is based upon the steady-state energy balance equation with ∆KE , ∆PE = 0 :



0 = Q − W + m(∆h) Turbine No Q with surroundings, thus:



Wt = (h1 − h2 ) m Condenser No W done, thus:



Qout = (h2 − h3 ) m



Pump No Q with surroundings, thus:



WP = (h4 − h3 ) or m



WP ≈ υ3 ( P4 − P3 ) m



Boiler No W is done, thus:



Qin = (h1 − h4 ) m 72



where υ3 = υ f ( P3 )



MECH 330: APPLIED THERMODYNAMICS II



LECTURE 07



The thermal efficiency, η , is:



η= η=



net work heat input



(Turbine work ouput ) − ( Pump work input ) ( Heat input to Boiler )



η=



η=



Wt / m − WP / m Qin / m



(h1 − h2 ) − (h4 − h3 ) (h1 − h4 )



The back work ratio, bwr, is:



bwr =



WP / m Wt / m



bwr =



(h4 − h3 ) (h1 − h2 )



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MECH 330: APPLIED THERMODYNAMICS II



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Example Saturated vapour steam enters the turbine at 8 MPa and saturated liquid leaves the condenser at 0.008 MPa. Find: η , bwr, m , Qin (for boiler), Qout (for condenser) if Wnet = 100 MW .



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MECH 330: APPLIED THERMODYNAMICS II



Solution



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MECH 330: APPLIED THERMODYNAMICS II



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MECH 330: APPLIED THERMODYNAMICS II



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MECH 330: APPLIED THERMODYNAMICS II



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Extra question:



Calculate the m for the cooling water through the condenser if it enters at 15oC and leaves at 35oC. 2 6 Cooling water (e.g., Lake water)



mcw = ?



5 Condensate



3



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