Here are the steps to solve this problem: a) Given: Refrigerant enters condenser at 1.2 MPa and 50°C, leaves at same pressure subcooled by 5°C = 45°C Use energy balance in condenser: m_r * (h_in - h_out) = m_w * Cp_w * (T_out - T_in) Solve for m_r b) Refrigeration load = Heat rejected in condenser = m_r * (h_in - h_out) c) COP = Refrigeration load / Power input = [m_r * (h_in - h_out)]

1. By: Natnael Mesfin(Msc.) ASTU
CHAPTER 5

2. INTRODUCTION TO THE SECOND LAW
So far, we applied the first law of thermodynamics, or the
conservation of energy principle, to processes involving closed and
open systems.
As pointed out repeatedly energy is a conserved property, and no
process is known to have taken place in violation of the first law of
thermodynamics.
Therefore, it is reasonable to conclude that a process must satisfy
the first law to occur. However, as explained here, satisfying the
first law alone does not ensure that the process will actually take
place.
 It is common experience that a cup of hot coffee left in a cooler
room eventually cools off .This process satisfies the first law of
thermodynamics, since the amount of energy lost by the coffee is
equal to the amount gained by the surrounding air.

3. The second law of thermodynamics asserts that processes occur in a certain
direction, not in any direction.
Physical processes in nature can proceed toward equilibrium
spontaneously:
Water flows down a waterfall.
Gases expand from a high pressure to a low pressure.
Heat flows from a high temperature to a law temperature.
fig. Heat transfer from a hot container to the cold surroundings is possible

4. Examples
These processes cannot occur
even though they are not in
violation of the first law.

5. Cont.
 A process can occur only when it satisfies both the first
and the second laws of thermodynamics.
 The second law also asserts that energy has a quality.
 Preserving the quality of energy is a major concern of
engineers.
 The second law is also used in determining the
theoretical limits for the performance of commonly used
engineering systems, such as heat engines and
refrigerators etc.

6.  The second law may be used to identify the direction of
processes.

7. THERMAL ENERGY RESERVOIRS
 A hypothetical body with a relatively large thermal energy
capacity (mass x specific heat) that can supply or absorb finite
amounts of heat without undergoing any change in temperature
is called a thermal energy reservoir, or just a reservoir.
 In practice, large bodies of water such as oceans, lakes, and
rivers as well as the atmospheric air can be modeled accurately
as thermal energy reservoirs because of their large thermal
energy storage capabilities or thermal masses.

8. Cont.
Thermal reservoir can be of two types depending upon nature of
heat interaction that heat rejection or absorptions from it.
 Source: A reservoir that supplies energy in the form of heat
 Sink: A reservoir that absorbs energy in the form of heat

9.  HEAT
ENGINES
Heat engines and other cyclic devices usually involve a
fluid to and from which heat is transferred while
undergoing a cycle.
This fluid is called the working fluid.
HEAT ENGINES: The devices that convert
heat to work.
There are several types of heat engines, but
they are characterized by the following:
1. They receive heat from a high-temperature source
(solar energy, oil furnace, nuclear reactor, etc.).
2. They convert part of this heat to work (usually in
the form of a rotating shaft.)
3. They reject the remaining waste heat to a low-
temperature sink (the atmosphere, rivers, etc.).
4. They operate on a cycle.

10. A steam power plant
Fig: Simple Steam power plant.

11. Thermal efficiency
It is a measure of the performance of a heat engine
Or

13. Even the most efficient heat engines reject most heat as waste heat
Thermal efficiency can not reach 100%

14. Example
 Heat is transferred to a heat engine from a furnace at a rate of 80
MW. If the rate of waste heat rejection to a nearby river is 50 MW,
determine the net power output and the thermal efficiency for this
heat engine.
solution

15. Net Power Production of a Heat Engine
Solution

16. Can we save Qout?
 In a steam power plant, the condenser is the device where large
quantities of waste heat is rejected to rivers, lakes, or the atmosphere.
 Can we just take the condenser out of the plant and save all that
waste energy?
 The answer is, unfortunately, a firm no for the simple reason that
without a heat rejection process in a condenser, the cycle cannot be
completed.
Every heat engine must waste some energy by transferring it to a
low-temperature reservoir in order to complete the cycle, even
under idealized conditions.

17.  REFRIGERATORS AND HEAT PUMPS
• The transfer of heat from a
low-temperature medium to
a high-temperature one
requires special devices
called refrigerators.
• Refrigerators, like heat engines,
are cyclic devices.
• The working fluid used in the
refrigeration cycle is called a
refrigerant.
• The most frequently used
refrigeration cycle is the vapor-
compression refrigeration cycle.
In nature, heat flows from high‐temperature regions to low‐temperature
ones. the reverse process, however, can’t occur by itself.

18. Cont.
 Heat pumps transfer heat from a
low‐temperature medium to a
high‐temperature one.
 Refrigerators and heat pumps are
essentially the same devices; they
differ in their objectives only.
 Refrigerator is to maintain the
refrigerated space at a low
temperature.
 Air conditioners are basically refrigerators whose refrigerated space is a
room or a building instead of the food compartment.

19.  Coefficient of Performance (COP).
The efficiency of a refrigerator is expressed
in terms of the coefficient of performance
(COP).
The objective of a refrigerator is to remove
heat (QL) from the refrigerated space.

20.  Heat Pumps
The efficiency of a heat pump is
expressed in term of the coefficient
of performance (COPHP).

21. Cont.
 Relationship between Coefficient of Performance of a Refrigerator
(COPR) and a Heat Pump (COPHP).
,
,
net in L
H H
HP
net in H L H L
W Q
Q Q
COP
W Q Q Q Q

  
 
,
1
net in L
HP R
H L H L
W Q
COP COP
Q Q Q Q
   
 
1
HP R
COP COP
 

22. The Second Law of Thermodynamics
It is impossible for any device that
operates on a cycle to receive heat from a
single reservoir and produce a net amount
of work.
No heat engine can have a thermal
efficiency of 100%, or as for a
power plant to operate, the
working fluid must exchange heat
with the environment as well as the
furnace.
The two most common statements:
 Kelvin‐Planck and
 Clausius statement
 Kelvin–Planck Statement

23.  Clausius Statement
It is impossible to construct a device
that operates in a cycle and produces
no effect other than the transfer of
heat from a lower-temperature body
to a higher-temperature body.
It states that a refrigerator cannot
operate unless its compressor is driven
by an external power source, such as
an electric motor.
This way, the net effect on the
surroundings involves the consumption
of some energy in the form of work, in
addition to the transfer of heat from a
colder body to a warmer one.

24. PERPETUAL-MOTION MACHINES
Perpetual-motion machine: Any device that violates the first or
the second law.
 A device that violates the first law (by creating energy) is called a
PMM1.

25.  A device that violates the second law is called a PMM2.

26.  The second law of thermodynamics state that no heat
engine can have an efficiency of 100%.
 Then one may ask, what is the highest efficiency that a
heat engine can possibly have.
 Before we answer this question, we need to define an
idealized process first, which is called the reversible
process.

27. REVERSIBLE AND IRREVERSIBLE PROCESSES
 A reversible process is defined as a process that can be reversed without leaving
any trace on the surroundings. It means both system and surroundings are
returned to their initial states at the end of the reverse process. Processes that are
not reversible are called irreversible.
 Reversible processes do not occur and they are only idealizations of actual
processes.
 All the processes occurring in nature are irreversible

28.  Why are we interested in reversible processes?
(1) They are easy to analyze and
(2) They serve as idealized models (theoretical limits) to which
actual processes can be compared.
 Irreversibilities
 The factors that cause a process to be irreversible are called
Irreversibilities.

29. Some factors that cause a process to become irreversible:
o Friction
o Unrestrained expansion
o mixing
o Heat transfer (finite ΔT)
o Inelastic deformation
o Chemical reactions
The presence of any of these effects renders a process irreversible.

30.  Friction renders a
process
irreversible.
 Irreversible
compression and
expansion processes.
(a) Heat transfer through a temperature difference is
irreversible, and
(b) the reverse process is impossible.

31. • The efficiency of a heat‐engine cycle greatly depends on how
the individual processes that make up the cycle are executed.
• The net work (efficiency) can be maximized by using reversible
processes.
• Note that the reversible cycles cannot be achieved in practice
because of irreversibilities associated with real processes.
• But, the reversible cycles provide upper limits on the
performance of real cycles.
THE CARNOT CYCLE
• The best known reversible cycle is the Carnot cycle. The
theoretical heat engine that operates on the Carnot cycle is
called the Carnot heat engine.
• The Carnot cycle is composed of four reversible processes-two
isothermal and two adiabatic, and it can be executed either in a
closed or a steady-flow system.

32. Process 1-2: A reversible isothermal
expansion
 The gas is allowed to expand
isothermally by receiving heat
(QH) from a hot reservoir.
Execution of the Carnot cycle in a closed system.

33. Process 2-3: A reversible adiabatic expansion
 The cylinder now is insulated and
the gas is allowed to expand
adiabatically and thus doing work
on the surrounding.
The gas temperature decreases from
TH to TL.

34. Process 3-4: A reversible isothermal compression
 The insulation is removed
and the gas is compressed
isothermally by rejecting
heat (QL) to a cold
reservoir.

35. Process 4-1: A reversible adiabatic compression
 The cylinder is insulated
again and the gas is
compressed adiabatically to
state 1, raising its
temperature from TL to TH

36.  Net work done by Carnot cycle is the area enclosed by all
process
 The Carnot cycle is the most efficient cycle operation between
two specified temperatures limits.
• The area under the curve 1-2-3 is
the work done by the gas during the
expansion part of the cycle and
• The area under the curve 3-4-1 is
the work done on the gas during the
compression part of the cycle.
• The difference b/n these two gives
the net work done by the gas
during the cycle.
• The efficiency of an irreversible
(real) cycle is always less than the
efficiency of the Carnot cycle
operating between the same two
reservoirs.

37. Carnot cycle can be executed in many different ways

38. The Reversed Carnot Cycle
The Carnot heat-engine cycle is a totally reversible cycle.
Therefore, all the processes that comprise it can be reversed, in
which case it becomes the Carnot refrigeration cycle.
Process 1-2: The gas expands
adiabatically (throttling valve)
reducing its temp from TH to TL.
Process 2-3: The gas expands
isothermally at TL while
receiving QL from the cold
reservoir.
Process 3-4: The gas is
compressed adiabatically
raising its temperature to TH.
Process 4-1: The gas is compressed isothermally
by rejecting QH to the hot reservoir.

39. Reversed Carnot Cycle

40. THE CARNOT PRINCIPLES
1. The efficiency of an
irreversible heat engine is
always less than the
efficiency of a reversible
one operating between the
same two reservoirs.
2. The efficiencies of all
reversible heat engines
operating between the
same two reservoirs are the
same.

41.  For a reversible cycle the amount of heat transferred is proportional to
the temperature of the reservoir.
Cont.
NB The thermal efficiency of a
Carnot Cycle depends only on the
temperatures of the thermal reservoirs
with which it interacts.

42. Efficiency of a Carnot Engine
H
L
rev
Q
Q

 1

H
L
T
T

 1
 COP of a Reversible Heat Pump and a Reversible Refrigerator
H
L
rev
HP
Q
Q
COP


1
1
,
H
L T
T


1
1
1
1
,


L
H
rev
R
Q
Q
COP
1
1


L
H T
T
The equation that defines this relationship is :

43. THE CARNOT HEAT ENGINE
Any heat engine
Carnot heat engine
 Reversible Carnot Heat Engine
compare with real engines?

44. THE CARNOT REFRIGERATOR AND HEAT PUMP
Carnot refrigerator or
heat pump
Any refrigerator or
heat pump

45. The COP of a reversible refrigerator or heat pump is the maximum
theoretical value for the specified temperature limits.
Actual refrigerators or heat pumps may approach these values as
their designs are improved, but they can never reach them.

46. 1.A Carnot heat engine, shown in Fig. below, receives 500 kJ of heat per cycle
from a high-temperature source at 652oC and rejects heat to a low-temperature sink
at 30oC. Determine
a)The thermal efficiency of this Carnot engine and
b)The amount of heat rejected to the sink per cycle.
Example and Exercise

47. SOLUTION
Given
 The heat supplied to a Carnot heat engine(QH)
 This Carnot heat engine
converts 67.2 % of the heat it
receives to work
(a) The efficiency can be determined as
b) The amount of heat rejected QL by this reversible heat engine is easily
determined from
 Note that this Carnot heat engine rejects to a low-temperature
sink 164 kJ of the 500 kJ of heat it receives during each cycle

48. 2.An air conditioner removes heat steadily from a house at a rate of
750 kJ/min while drawing electric power at a rate of 6 kw. Determine
a) the COP of this air conditioner and
b)the rate of heat transfer to the outside air.

49. solution
Analysis
(a) The coefficient of performance of the air-conditioner (or
refrigerator) is determined from its definition
b)The rate of heat discharge to the out side air is determined from the
energy balance

50. 3.The food compartment of a refrigerator, shown in Fig. below, is maintained at
48o
C by removing heat from it at a rate of 360 kJ/min. If the required power
input to the refrigerator is 2 kW, determine
(a) the coefficient of performance of the refrigerator
(b) the rate of heat rejection to the kitchen
SOLUTION
(a) The coefficient of performance of the
refrigerator is
(b) the rate of heat rejection to the kitchen
from the conservation of energy relation for
cyclic devices

51. 4.A heat pump is to be used to heat a building during the winter. The building is to
be maintained at 21o
C at all times. The building is estimated to be losing heat at a
rate of 135,000 kJ/h when the outside temperature drops to -5o
C. Determine the
minimum power required to drive the heat pump unit for this outside temperature

52. Solution:

53. 5. A commercial refrigerator with refrigerant-134a as the working fluid is used
to keep the refrigerated space at 35°C by rejecting waste heat to cooling
water that enters the condenser at 18°C at a rate of 0.25 kg/s and leaves at
26°C. The refrigerant enters the condenser at 1.2MPa and 50°C and leaves at
the same pressure sub cooled by 5°C. If the compressor consumes 3.3 kW of
power, determine
a) The mass flow rate of the refrigerant,
b) The refrigeration load,
c) The COP, and
d) The minimum power input to the
compressor.

54. solution

55. Cont.

56. THANK YOU