The Cyclic Devices
P M V SubbaraoProfessor
Mechanical Engineering Department
Create Resources Parallel to Nature…..
Cyclic Device: A single system or a group of
thermodynamic systems, which are responsible for
execution of thermodynamic cycle by a pure substance.
Greatness of Heat Engines
• Balance Sheet for A Heat Engine:– All heat inputs consume resources : Total Input Heat :
Qin (Cost incurred).– All heat outputs are just mere a loss.– Net work Out put is positive and this is the final
benefit : Wnet (Benefit Achieved)– Performance : Benefit to cost ratio (in energy units).
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iiin
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Thermodynamic Model for a General Heat Engine
Wnet
First law:Wnet = Qin - Qout
Qin
High Temperature Reservoir (Source)
Qout
Low Temperature Reservoir (Sink)
HE
in
net
Q
WEfficiency ,
in
i
i
in Q
Q
Q
W
1
If Qin = Wnet, then the efficiency of A Heat Engine will be 100%
Cost Benefit Analysis
Cost Total
BenefitNet Ratiocost Benefit to
Greatness of A Heat Pump• Heat Pump or Refrigerator:
– Input : Win (Cost incurred)
– Out put : Qin or Qout (Benefit Achieved)
– Performance : Benefit to cost ratio.
in
outin
W
QQ or (COP) ePerformanc oft Coefficien
outin
in
in
in
Q
W
Q
ref
outin
out
in
out
Q
W
Q
hp
If Win = 0, then COP = .
While using as a Refrigerator :
While using as a heat pump :
Thermodynamic Model for a General Heat Pump
Qout
Qin
Win
First law:Win = Qin - Qout
HTR (Sink)
LTR (Source)
HP
Kelvin Planks postulate
“It is impossible to construct a heat engine which produces no effect other than the extraction of
heat from a single source and the production of an equivalent amount of work”
On the other side
It is possible to construct a heat engine which produces no effect other than the extraction of
work from a single source and the production of an equivalent amount of heat
Clausius postulate
“The Clausius statement: It is impossible to construct a heat pump produces no effect other than the transfer of heat from a cooler body to a hotter body.”
However,
It is possible to construct a heat pump produces no effect other than the transfer of heat from a hotter body to a colder body.
The Impossible Heat PumpPerpetual Motion Machine II : Heat Pump Model
QH
QL
First law|QH| = QL
HTR (Sink)
LTR (Source)
Discussion of Second Law
• It is impossible to construct a heat engine with 100% efficiency.
• It is impossible to construct a heat pump or refrigerator with infinite COP.
• Both are negative statements.
• They cannot be proved.
• They will remain correct till they are disproved.
• Violation of Kelvin Planks statement leads to violation of Clasius statement and vice versa.
• Can a heat engine be reversed to work as heat pump or refrigerator?
• If yes, what will be the COP of this reversed engine?
• Can a reversed engine perform same as forward engine between same reservoirs?
• What can really affect the performance of a reversible engine?
Perpetual Heat Pump Model
QH
QL
First law|QH | = QL
HTR (Source)
LTR (Sink)
Let us assume this is possible
Perpetual Motion Machine II : Compound Engine
QHP
QLP
First lawQHP = QLP
QHE
E Wnet
QHE = QLP
QLE = QLP-Wnet
QLP - QLE=Wnet
LTR (Source)
HTR (Sink)