Chap 4 251_2nd_Law_MAR-2.pdfhhhhhhhhhhhh

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1. 1
Introduction To
The Second Law of
Thermodynamics
Understanding of
Thermal Efficiency
Prepared by
PM Muhammad Abd Razak
FKM UiTMPP

2. 2
The Second Law of Thermodynamics The 2
nd
law of thermodynamics is a natural law that
states that
‰processes can occur in a certain direction, not in just
any direction
Gases expand from a high pressure to a low pressure.
Heat flows from a high temp. to a low temperature.
‰No heat engine is able to convert completely all the
heat supplied into work output and there must be
some heat rejection at a lower temperature than the
source.

3. 3
Heat Engine
An energy conversion system which:
•operates in a thermodynamic cycle
•operates between two heat reservoirs
where
¾net heat is transferred
¾net work is delivered
.
Heat (Thermal) Reservoir
•a sufficiently large system in stable equilibrium
•has finite amounts of heat that can be transferred in/out
without any change in its temperature.
¾high temperature heat reservoir : a heat source
.
¾low temperature heat reservoir : a heat sink
.

4. 4
Example of a heat engine : Steam Plant

5. 5
Thermal Efficiency, η
th
•the index of performance
of a heat engine
•defined by the ratio of the net work output to the heat input
η
th
=
DesiredResult
Required Input η
th
net out
in
W
Q
=
,
WWW
QQ
net out out in
in net
,
=
−
≠
where
thermal efficiency is 0 < η< 100 %

6. 6
Applying the 1
st
Law of Thermodynamics
QW U
WQ
WQQ
net in net out
net out net in
net out in out
,,
,,
,
−
= =
=−
Δ
0 for cyclic
process
η
th
net out
in
in out
in
out
in
W
Q
QQ
Q
Q
Q
=
=
−
=−
,
1
Then
η
th
L
H
Q
Q
=−1
or

7. 7
Refrigerator & Heat Pump
‰operates in a thermodynamic cycle
‰absorbs heat
from a low temperature body
and
delivers heat
to a high temperature body
‰must receives external energy (work or heat) from the surroundings.
refrigerator: extracts
heat from low-temperature media.
heat pump: rejects
heat to the high-temperature media.

8. 8
Coefficient of Performance, COP
•index of performance of a refrigerator & heat pump is in
terms of the coefficient of performance, COP,
•the ratio of desired result to input larger than 1 and the
COP to be as large as possible.
For a refrigerator or an air conditioner
Heat is transfered
from
the low temperature reservoir.
Then

9. 9
COP
Q
QQ
R
L
HL
=
−
From the 1
st
Law Equation
Then
For a “heat pump”
heatis transfered
to
the high temperature system, then
COP
Q
W
Q
QQ
HP
H
net in
H
HL
==
−
,
We can also show that
COP COP
HPR
=
+
1

10. 10
The Carnot Cycle
•Sadi Carnot (1769-1832) was among the first to study the
principles of the 2
nd
law of t/dynamics on cyclic operations
•devised
a reversible cycle composed of four reversible
processes:
•two isothermal and
•two adiabatic.
A vapour cycle
Process 1 –2
: Reversible
adiabatic expansion
(in turbine).
•System produces work, W
out
•The working fluid temperature
decreases from T
H
to T
L
.
Process 2-3
: reversible
isothermal heat rejection
Q
L
(in a
condenser)
T
T
H
T
L

11. 11
Process 3-4
: reversible adiabatic
compression
(in a compressor)
•system receives work input, W
in
•working fluid temperature
increases from T
L
to T
H
Process 4-1
: reversible isothermal
heat addition
, Q
H
(in a boiler)
Note that
•the Carnotpower cycle
operates in the
clockwise direction when plotted on a process diagram.(T-v, P-v, T-s)
•for a refrigerator & heat pump
, the
Carnot cycle is reversed, the cycle operates in the counter clockwise
direction.
A gas cycle
Q
H
Q
L
W
in
W
out

12. 12
Again, the thermal efficiency is For a reversible heat engine
, the energy transfer ratio Q
L
/Q
H
can
be replaced by ratio of absolute temp T
L
/T
H
η
th rev
L
H
T
T
,
=−1
This is the maximum possible efficiency
of a heat engine
operating between two heat reservoirs at constant temperatures T
H
and T
L
.

13. 13
The Kelvin scale, relates the heat transfers in a reversible device
between the high and low-temperature heat reservoirs at
constant temperature as Summarising all ‘heat in’ and ‘heat out’ For a cyclic process

14. 14
The term δQ/T depends only on the initial & final states
,
not on the process. Thus it isa point function or property
defined as entropy, s Then
1
2
δQ = T ds
[kJ/kg]
Then
Q
12
= T(s
2
–s
1
)
[kJ/kgK]