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Some Final Comments Regarding Exergy: Mech 330: Applied Thermodynamics Ii

Exergy, or available energy, can be used to evaluate the effectiveness of energy resource usage and engineering systems. Exergy that would otherwise be lost can be recovered through methods like waste heat recovery, cogeneration, steam extraction, and combined power cycles. The Rankine cycle is commonly used in advanced vapor power systems. Exergetic efficiency calculations can distinguish more thermodynamically effective energy utilization methods and evaluate improvements to system performance.

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137 views10 pages

Some Final Comments Regarding Exergy: Mech 330: Applied Thermodynamics Ii

Exergy, or available energy, can be used to evaluate the effectiveness of energy resource usage and engineering systems. Exergy that would otherwise be lost can be recovered through methods like waste heat recovery, cogeneration, steam extraction, and combined power cycles. The Rankine cycle is commonly used in advanced vapor power systems. Exergetic efficiency calculations can distinguish more thermodynamically effective energy utilization methods and evaluate improvements to system performance.

Uploaded by

Yosua Wijaya
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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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 LECTURE 07

Cogeneration

Steam Extraction for Thermal Load:

Combined Power Cycles:

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MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

Advanced Vapour Power Systems – Rankine Cycle

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MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

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 WP
= (h4 − h3 ) or ≈ υ3 ( P4 − P3 ) where υ3 = υ f ( P3 )
m m

Boiler

No W is done, thus:

Qin
= (h1 − h4 )
m

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MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

The thermal efficiency, η , is:

net work
η=
heat input

(Turbine work ouput ) − ( Pump work input )


η=
( Heat input to Boiler )

Wt / m − WP / m
η=
Qin / m

(h1 − h2 ) − (h4 − h3 )
η=
(h1 − h4 )

The back work ratio, bwr, is:

WP / m
bwr =
Wt / m

(h4 − h3 )
bwr =
(h1 − h2 )

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MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

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 .

74
MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

Solution

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MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

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MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

77
MECH 330: APPLIED THERMODYNAMICS II LECTURE 07

Extra question:

Calculate the m for the cooling water through the condenser if it


enters at 15oC and leaves at 35oC.

6
Cooling water (e.g., Lake water)
mcw = ?
5
3
Condensate

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