5 Exergy

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5 Exergy
5.1 Introduction
Quantity
Evaluation of heat
Quality
Exergy and Anergy
Full convertible energy mechanical Partial
convertible energy heat Unconvertible energy
environmental
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Exergy useful work potential available energy
The maximum useful work a system can delivered
from a specified state to the state of its
environment in theory. Anergy unavailable
energy, unconvertible energy
Conditions for definition of exergy
(1)based on environment exergy of
environmental energy is zero
(2)reversible process
(3)there is no other heat resource in the process.
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5.2 Calculation of exergy
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5.2.1 Work resource Electricity, mechanical
energy, pneumatic energy, hydraulic energy, can
be converted to work entirely. Exergy of work
resource its total energy
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5.2.2 Heat exergy Potential work of heat
W0
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For constant temperature heat resource
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T0
For finite heat resource
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• Influential factors
• Heat quantity
• Heat resource temperature

• Environmental temperature T0

If the system absorbs heat, it absorbs exergy If
the system discharges heat, it discharges exergy
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Example 1kg air with temperature of 200? was
cooled to 40?. Please calculate the heat exergy.
The specific heat of air is cp1.004kJ/(kg.K).
the environmental temperature is T025??
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Solution
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5.2.3 Low temperature heat exergy Potential work
of heat at a temperature below the environmental
temperature
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For constant temperature
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For finite heat resource
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• Influential factors
• Heat quantity
• Heat resource temperature

• Environmental temperature T0

If the system absorbs heat, it discharges
exergy If the system discharges heat, it absorbs
exergy
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5.2.4 Inner energy exergy
For closed system
From initial state
to final state
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For A?B?O
The dead work to resist environment
The maximum work
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For m kg substance
The maximum available work
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Example
Please calculate the inner energy exergy of air
in the state of 1MPa and 50?. The environmental
pressure is p00.1MPa,the temperature is T025?
and the specific heat is cv0.716kJ/(kgK)?
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5.2.5 Enthalpy exergy
For steady flow system
From initial state
to final state
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For A?B?O
For m kg substance
The maximum available work
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???????
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Example
Please compare the enthalpy exergy value of
saturated steam of 0.5MPa with 5MPa. The
environmental state is p00.1MPa and T020??
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Solution
Look up properties in the table of water or steam
h084 kJ/kg s00.2963 kJ/(kgK) h12747.5
kJ/kg s16.8192 kJ/(kgK) h22794.2
kJ/kg s25.9735 kJ/(kgK)
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ex1 (h1-h0)-T0(s1-s0)
(2747.5-84)-293(6.8192-0.2963)
752.3kJ/kg
ex2(h2-h0)-T0(s2-s0)
(2791.2-84)-293(5.9735-0.2963)
1046.8kJ/kg?
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5.3 Exergy loss
Exergy loss caused by temperature difference
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TA
Q
TB
Q
H.E
T0
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Exergy loss
Entropy production
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T
H.E
T0
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??????????
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Exergy loss caused by temperature difference
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If discharging temperature is
The whole exergy loss
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5.4 exergy equation
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For many streams
Technical work
Heat exergy
Enthalpy exergy
Exergy loss
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5.5 Exergy efficiency
1
2
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Example The high temperature is TH1800K
and the low temperature is the environmental
temperature that is T0290K. A heat engine
absorbs heat at T1900K and discharges heat at
T2320K. The engine efficiency is 70 of that of
corresponding carnot cycle?If each kilogram
substance absorbs heat 100kJ,please calculate
(1)the practical work of heat engine (2)the
heat exergy at given temperature (3)the entropy
production and exergy loss of each process
(4)the entropy increase of isolated system and
the whole exergy loss.
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TH 1800 K
T1 900 K
R
W
T2 320 K
T0 290 K
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Solution (1) The carnot cycle works between T1
and T2
Practical work of engine
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Discharged heat
(2) Heat exergy
Heat exergy at 1800K
Heat exergy at 900K
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Heat exergy at 320K
(3) Entropy production and exergy loss
Entropy production caused by temperature
difference in absorbing heat process
Exergy loss
or
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Friction work loss
Entropy production
Exergy loss
0r
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Entropy production caused by temperature
difference in discharging heat process
Exergy loss
0r
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(4)Entropy increase of isolated system
0r
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The whole exergy loss
0r
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