Forms of energy Energy transfer by heat Energy transfer by work Mechanical forms of work The first law of thermodynamics Energy balance Energy change of a system Mechanisms of energy transfer (heat, work, mass flow) Energy conversion efficiencies Efficiencies of mechanical and electrical devices (turbines, pumps, etc...)

1. ENERGY, ENERGY
TRANSFER AND ENERGY
ANALYSIS
Lecture 5
Keith Vaugh BEng (AERO) MEng
Reference text: Chapter 3 - Fundamentals of Thermal-Fluid Sciences, 3rd Edition
Yunus A. Cengel, Robert H. Turner, John M. Cimbala
McGraw-Hill, 2008
KEITH VAUGH

2. OBJECTIVES
}
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3. OBJECTIVES
Introduce the concept of energy and define its
various forms.
}
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4. OBJECTIVES
Introduce the concept of energy and define its
various forms.
Discuss the nature of internal energy.
}
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5. OBJECTIVES
Introduce the concept of energy and define its
various forms.
Discuss the nature of internal energy.
}
Define the concept of heat and the terminology
associated with energy transfer by heat.
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6. OBJECTIVES
Introduce the concept of energy and define its
various forms.
Discuss the nature of internal energy.
}
Define the concept of heat and the terminology
associated with energy transfer by heat.
Discuss the three mechanisms of heat transfer:
conduction, convection, and radiation.
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7. OBJECTIVES
Introduce the concept of energy and define its
various forms.
Discuss the nature of internal energy.
}
Define the concept of heat and the terminology
associated with energy transfer by heat.
Discuss the three mechanisms of heat transfer:
conduction, convection, and radiation.
Define the concept of work, including electrical
work and several forms of mechanical work.
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8. }
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9. Introduce the first law of thermodynamics, energy
balances, and mechanisms of energy transfer to or
from a system.
}
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10. Introduce the first law of thermodynamics, energy
balances, and mechanisms of energy transfer to or
from a system.
}
Determine that a fluid flowing across a control
surface of a control volume carries energy across
the control surface in addition to any energy transfer
across the control surface that may be in the form of
heat and/or work.
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11. Introduce the first law of thermodynamics, energy
balances, and mechanisms of energy transfer to or
from a system.
}
Determine that a fluid flowing across a control
surface of a control volume carries energy across
the control surface in addition to any energy transfer
across the control surface that may be in the form of
heat and/or work.
Define energy conversion efficiencies.
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12. Introduce the first law of thermodynamics, energy
balances, and mechanisms of energy transfer to or
from a system.
}
Determine that a fluid flowing across a control
surface of a control volume carries energy across
the control surface in addition to any energy transfer
across the control surface that may be in the form of
heat and/or work.
Define energy conversion efficiencies.
Discuss the implications of energy conversion on
the environment.
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13. FORMS OF ENERGY
Energy can exist in numerous forms such as thermal, mechanical,
kinetic, potential, electric, magnetic, chemical, and nuclear, and
their sum constitutes the total energy, E of a system.
}
Thermodynamics deals only with the change of the total energy.
Macroscopic forms of energy
Those a system possesses as a whole with respect to some outside
reference frame, such as kinetic and potential energies.
Microscopic forms of energy
Those related to the molecular structure of a system and the
degree of the molecular activity.
Internal energy, U
The sum of all the microscopic forms of energy.
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14. }
Kinetic energy, KE
The energy that a system
possesses as a result of its motion
relative to some reference frame.
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15. }
Kinetic energy, KE 1 2
KE = mV ( kJ )
The energy that a system 2
possesses as a result of its motion
relative to some reference frame.
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16. }
Kinetic energy, KE 1 2
KE = mV ( kJ )
The energy that a system 2
possesses as a result of its motion
relative to some reference frame. V2
ke =
2 ( kg)
kJ
Per unit mass
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17. }
Kinetic energy, KE 1 2
KE = mV ( kJ )
The energy that a system 2
possesses as a result of its motion
relative to some reference frame. V2
ke =
2 ( kg)
kJ
Per unit mass
}
Potential energy, PE
The energy that a system
possesses as a result of its
elevation in a gravitational field.
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18. }
Kinetic energy, KE 1 2
KE = mV ( kJ )
The energy that a system 2
possesses as a result of its motion
relative to some reference frame. V2
ke =
2 ( kg)
kJ
Per unit mass
}
Potential energy, PE PE = mgz ( kJ )
The energy that a system
possesses as a result of its
elevation in a gravitational field.
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19. }
Kinetic energy, KE 1 2
KE = mV ( kJ )
The energy that a system 2
possesses as a result of its motion
relative to some reference frame. V2
ke =
2 ( kg)
kJ
Per unit mass
}
Potential energy, PE PE = mgz ( kJ )
The energy that a system
possesses as a result of its
elevation in a gravitational field. pe = gz ( kg)
kJ
Per unit mass
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20. KEITH VAUGH

21. Mass flow rate
& &
m = ρV = ρ AcVavg ( s)
kg
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22. Mass flow rate
& &
m = ρV = ρ AcVavg ( s)
kg
Energy flow rate
& &
E = me ( kJ s or kW )
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23. Total energy of a system
1 2
E = U + KE + PE = U + mV + mgz ( kJ )
2
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24. Total energy of a system
1 2
E = U + KE + PE = U + mV + mgz ( kJ )
2
Energy of a system per unit mass
V2
e = u + ke + pe = u +
2
+ gz ( kg)
kJ
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25. Total energy of a system
1 2
E = U + KE + PE = U + mV + mgz ( kJ )
2
Energy of a system per unit mass
V2
e = u + ke + pe = u +
2
+ gz ( kg)
kJ
Total energy per unit mass
E
e=
m ( kg)
kJ
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26. MECHANICAL ENERGY
The form of energy that can be converted to mechanical work
completely and directly by an ideal mechanical device such
as an ideal turbine
}
Kinetic and Potential energies are the familiar forms of
mechanical energy
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27. MECHANICAL ENERGY
The form of energy that can be converted to mechanical work
completely and directly by an ideal mechanical device such
as an ideal turbine
}
Kinetic and Potential energies are the familiar forms of
mechanical energy
P V2 Mechanical energy of a
emech = + + gz
ρ 2 flowing fluid per unit mass
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28. Rate of mechanical energy of a flowing fluid
& ⎛ P V 2 ⎞
&
Emech = memech &
= m ⎜ + + gz ⎟
⎝ ρ 2 ⎠
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29. Rate of mechanical energy of a flowing fluid
& ⎛ P V 2 ⎞
&
Emech = memech &
= m ⎜ + + gz ⎟
⎝ ρ 2 ⎠
Mechanical energy change of a fluid during incompressible flow per unit mass
P2 − P1 V22 − V12
Δemech =
ρ
+
2
+ g ( z2 − z1 ) ( kg)
kJ
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30. Rate of mechanical energy of a flowing fluid
& ⎛ P V 2 ⎞
&
Emech = memech &
= m ⎜ + + gz ⎟
⎝ ρ 2 ⎠
Mechanical energy change of a fluid during incompressible flow per unit mass
P2 − P1 V22 − V12
Δemech =
ρ
+
2
+ g ( z2 − z1 ) ( kg)
kJ
Rate of mechanical energy change of a fluid during incompressible flow
& ⎛ P2 − P1 V22 − V12 ⎞
&
ΔEmech = mΔemech &
= m ⎜ + + g ( z2 − z1 )⎟ ( kW )
⎝ ρ 2 ⎠
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31. ENERGY TRANSFER BY HEAT
Heat
The form of energy that is transferred between
two systems (or a system and its surroundings)
by virtue of a temperature differential
}
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32. ENERGY TRANSFER BY HEAT
Heat
The form of energy that is transferred between
two systems (or a system and its surroundings)
by virtue of a temperature differential
}
Heat transfer per unit mass
Q
q=
m ( kg)
kJ
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33. ENERGY TRANSFER BY HEAT
Heat
The form of energy that is transferred between
two systems (or a system and its surroundings)
by virtue of a temperature differential
}
Heat transfer per unit mass
Q
q=
m ( kg)
kJ
Amount of heat transfer when heat transfer rate is
constant
&
Q = QΔt ( kJ )
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34. ENERGY TRANSFER BY HEAT
Heat
The form of energy that is transferred between
two systems (or a system and its surroundings)
by virtue of a temperature differential
}
Heat transfer per unit mass
Q
q=
m ( kg)
kJ
Amount of heat transfer when heat transfer rate is
constant
&
Q = QΔt ( kJ )
Amount of heat transfer when heat transfer rate
changes with time
&
t2
Q = ∫ Qdt
t1
( kJ )
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35. During an adiabatic process, a system
exchanges no heat with its surroundings
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36. ENERGY TRANSFER BY WORK
Work
The energy transfer associated with a force
acting through a distance.
}
Heat transfer to a system and work done by a
system are positive; heat transfer from a system
and work done on a system are negative
Specifying the directions of
Alternative is to use the subscripts in and out to
heat and work indicate the direction
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37. ENERGY TRANSFER BY WORK
Work
The energy transfer associated with a force
acting through a distance.
}
Heat transfer to a system and work done by a
system are positive; heat transfer from a system
and work done on a system are negative
Specifying the directions of
Alternative is to use the subscripts in and out to
heat and work indicate the direction
Work done per unit mass
W
w=
m ( kg)
kJ
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38. KEITH VAUGH

39. Heat Vs. Work
Both are recognised at the boundaries of a
system as they cross the boundaries. That is,
both heat and work are boundary phenomena.
Systems possess energy, but not heat or work.
Both are associated with a process, not a state.
Unlike properties, heat or work has no meaning
at a state.
Both are path functions (i.e., their magnitudes
depend on the path followed during a process as
well as the end states).
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40. KEITH VAUGH

41. Properties are point functions; but heat and work
are path functions (their magnitudes depend on the
path followed
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42. Properties are point functions
and have exact differentials (d).
2
∫ dV = V2 − V1 = ΔV
1
Properties are point functions; but heat and work
are path functions (their magnitudes depend on the
path followed
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43. Properties are point functions
and have exact differentials (d).
2
∫ dV = V2 − V1 = ΔV
1
Path functions have inexact
differentials (δ)
2
∫ δ W = W12
1
Properties are point functions; but heat and work
are path functions (their magnitudes depend on the
path followed
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44. ELECTRICAL WORK
}
Electrical power in terms of resistance
R, current I, and potential difference V.
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45. ELECTRICAL WORK
Electrical Work
We = VN
}
Electrical power in terms of resistance
R, current I, and potential difference V.
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46. ELECTRICAL WORK
Electrical Work
We = VN
}
Electrical Power (rate form)
&
We = VI (W )
Electrical power in terms of resistance
R, current I, and potential difference V.
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47. ELECTRICAL WORK
Electrical Work
We = VN
}
Electrical Power (rate form)
&
We = VI (W )
When potential difference and
current change with time
&
2
We = ∫ VI dt
1
( kJ )
Electrical power in terms of resistance
R, current I, and potential difference V.
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48. ELECTRICAL WORK
Electrical Work
We = VN
}
Electrical Power (rate form)
&
We = VI (W )
When potential difference and
current change with time
&
2
We = ∫ VI dt
1
( kJ )
Electrical power in terms of resistance
R, current I, and potential difference V. When potential difference and
current remain constant
We = VI Δt ( kJ )
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49. MECHANICAL FORMS OF WORK
There are two requirements for a
work interaction between a system
and its surroundings to exist
}
✓ there must be a force acting on
the boundary
✓ the boundary must move
The work done is
proportional to the force
(F) and the distance
traveled (s)
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50. MECHANICAL FORMS OF WORK
There are two requirements for a
work interaction between a system
and its surroundings to exist
Work = Force × Distance
We = VI Δt ( kJ )
}
✓ there must be a force acting on
the boundary
✓ the boundary must move
The work done is
proportional to the force
(F) and the distance
traveled (s)
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51. MECHANICAL FORMS OF WORK
There are two requirements for a
work interaction between a system
and its surroundings to exist
Work = Force × Distance
We = VI Δt ( kJ )
}
✓ there must be a force acting on
When force is not constant
the boundary
✓ the boundary must move &
2
We = ∫ VI dt ( kJ )
1
The work done is
proportional to the force
(F) and the distance
traveled (s)
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52. KEITH VAUGH

53. Shaft work is proportional to the torque
applied and the number of revolutions
of the shaft
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54. A force (F) acting through a moment
arm (r) generates a torque (T)
T
T = Fr → F =
r
Shaft work is proportional to the torque
applied and the number of revolutions
of the shaft
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55. A force (F) acting through a moment
arm (r) generates a torque (T)
T
T = Fr → F =
r
This force acts through a distance (s)
s = ( 2π r ) n
Shaft work is proportional to the torque
applied and the number of revolutions
of the shaft
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56. A force (F) acting through a moment
arm (r) generates a torque (T)
T
T = Fr → F =
r
This force acts through a distance (s)
s = ( 2π r ) n
Shaft work Shaft work is proportional to the torque
applied and the number of revolutions
⎛ T ⎞ of the shaft
Wsh = Fs = ⎜ ⎟ ( 2π rn ) = 2π nT ( kJ )
⎝ r ⎠
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57. A force (F) acting through a moment
arm (r) generates a torque (T)
T
T = Fr → F =
r
This force acts through a distance (s)
s = ( 2π r ) n
Shaft work Shaft work is proportional to the torque
applied and the number of revolutions
⎛ T ⎞ of the shaft
Wsh = Fs = ⎜ ⎟ ( 2π rn ) = 2π nT ( kJ )
⎝ r ⎠
The power transmitted through the shaft
is the shaft work done per unit time
& &
Wsh = 2π nT ( kW )
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58. KEITH VAUGH

59. Spring Work
When the length of the spring changes by a
differential amount dx under the influence of a
force (F), the work done is
δ Wspring = Fdx
Elongation of a spring under the
influence of a force
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60. Spring Work
When the length of the spring changes by a
differential amount dx under the influence of a
force (F), the work done is
δ Wspring = Fdx
For linear elastic springs, the displacement (x)
is proportional to the force applied
F = kx ( kN )
k: spring constant (kN/m)
Elongation of a spring under the
influence of a force
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61. Spring Work
When the length of the spring changes by a
differential amount dx under the influence of a
force (F), the work done is
δ Wspring = Fdx
For linear elastic springs, the displacement (x)
is proportional to the force applied
F = kx ( kN )
k: spring constant (kN/m)
Substituting and integrating yields
1
Wspring
2
(
= k x2 − x12
2
) ( kJ ) Elongation of a spring under the
influence of a force
x1 and x2: the initial and the final displacements
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62. KEITH VAUGH

63. Work done on Elastic Solid Bars
2 2
Welastic = ∫ F dx = ∫ σ n A dx
1 1
( kJ )
Solid bars behave as springs
under the influence of a force
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64. Work done on Elastic Solid Bars
2 2
Welastic = ∫ F dx = ∫ σ n A dx
1 1
( kJ )
Solid bars behave as springs
under the influence of a force
Work associated with the stretching of a
liquid film
2
Wsurface = ∫ σ s dA
1
( kJ )
Stretching a liquid film with a
movable wire
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65. KEITH VAUGH

66. Work done to raise or to accelerate a body
The work transfer needed to raise a body is
equal to the change in the potential energy of
the body.
The work transfer needed to accelerate a body
is equal to the change in the kinetic energy of
the body.
The energy transferred to a body
while being raised is equal to the
change in its potential energy
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67. Non-mechanical forms of work
Electrical work
The generalised force is the voltage (the electrical
potential) and the generalised displacement is the
electrical charge.
Magnetic work
The generalised force is the magnetic field strength
and the generalised displacement is the total
magnetic dipole moment.
Electrical polarisation work
The generalised force is the electric field strength
and the generalised displacement is the polarisation
of the medium.
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68. FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics (the conservation of
energy principle) provides a sound basis for studying the
relationships among the various forms of energy and
energy interactions.
}
The first law states that energy can be neither created nor
destroyed during a process; it can only change forms.
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69. FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics (the conservation of
energy principle) provides a sound basis for studying the
relationships among the various forms of energy and
energy interactions.
}
The first law states that energy can be neither created nor
destroyed during a process; it can only change forms.
Energy cannot be created or
destroyed; it can only change
forms
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70. The First Law
For all adiabatic processes between two specified
states of a closed system, the net work done is the
same regardless of the nature of the closed system
and the details of the process.
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71. KEITH VAUGH

72. In the absence of any work
interactions, the energy change of
a system is equal to the net heat
transfer
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73. In the absence of any work The work (electrical) done on an
interactions, the energy change of adiabatic system is equal to the
a system is equal to the net heat increase in the energy of the
transfer system
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74. ENERGY BALANCE
}
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75. ENERGY BALANCE
The net change (increase or decrease) in the total energy of the
system during a process is equal to the difference between the
total energy entering and the total energy leaving the system
}
during that process.
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76. ENERGY BALANCE
The net change (increase or decrease) in the total energy of the
system during a process is equal to the difference between the
total energy entering and the total energy leaving the system
}
during that process.
⎛ Total energy ⎞ ⎛ Total energy ⎞ ⎛ Change in the total ⎞
⎜ entering the system ⎟ − ⎜ leaving the system ⎟ = ⎜ energy of the system ⎟
⎝ ⎠ ⎝ ⎠ ⎝ ⎠
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77. ENERGY BALANCE
The net change (increase or decrease) in the total energy of the
system during a process is equal to the difference between the
total energy entering and the total energy leaving the system
}
during that process.
⎛ Total energy ⎞ ⎛ Total energy ⎞ ⎛ Change in the total ⎞
⎜ entering the system ⎟ − ⎜ leaving the system ⎟ = ⎜ energy of the system ⎟
⎝ ⎠ ⎝ ⎠ ⎝ ⎠
Ein − Eout = ΔEsystem
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78. KEITH VAUGH

79. The work (boundary) done on an
adiabatic system is equal to the
increase in the energy of the
system
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80. The work (boundary) done on an The energy change of a system
adiabatic system is equal to the during a process is equal to the net
increase in the energy of the work and heat transfer between
system the system and its surroundings
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81. Energy change of a system, ΔEsystem
Energy change = Energy at final state - Energy at initial state
KEITH VAUGH

82. Energy change of a system, ΔEsystem
Energy change = Energy at final state - Energy at initial state
ΔEsystem = E final = Einitial = E2 − E1
KEITH VAUGH

83. Energy change of a system, ΔEsystem
Energy change = Energy at final state - Energy at initial state
ΔEsystem = E final = Einitial = E2 − E1
ΔE = ΔU + ΔKE + ΔPE
KEITH VAUGH

84. Energy change of a system, ΔEsystem
Energy change = Energy at final state - Energy at initial state
ΔEsystem = E final = Einitial = E2 − E1
ΔE = ΔU + ΔKE + ΔPE
Internal, kinetic and potential energy changes
ΔU = m ( u2 − u1 )
1
(
ΔKE = m V22 − V12
2
)
ΔPE = mg ( z2 − z1 )
KEITH VAUGH

85. Mechanisms of Energy Transfer, Ein and Eout
Energy can be transferred to or from a system in three forms
✓ Heat transfer
✓ Work transfer
✓ Mass flow
KEITH VAUGH

86. Mechanisms of Energy Transfer, Ein and Eout
Energy can be transferred to or from a system in three forms
✓ Heat transfer
✓ Work transfer
✓ Mass flow
} The energy content of a control volume can be changed
by mass flow as well as heat and work interactions
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87. Mechanisms of Energy Transfer, Ein and Eout
Energy can be transferred to or from a system in three forms
✓ Heat transfer
✓ Work transfer
✓ Mass flow
}
The energy content of a control volume can be changed
by mass flow as well as heat and work interactions
Ein − Eout = (Qin − Qout ) + (Win − Wout ) + ( Emass, in − Emass, out ) = ΔEsystem ( kJ )
14 2 4 3 123
Net energy transfer Change in internal, kinetic,
by heat, work and mass potential, etc... energies
KEITH VAUGH

88. Mechanisms of Energy Transfer, Ein and Eout
Energy can be transferred to or from a system in three forms
✓ Heat transfer
✓ Work transfer
✓ Mass flow
}
The energy content of a control volume can be changed
by mass flow as well as heat and work interactions
Ein − Eout = (Qin − Qout ) + (Win − Wout ) + ( Emass, in − Emass, out ) = ΔEsystem ( kJ )
14 2 4 3 123
Net energy transfer Change in internal, kinetic,
by heat, work and mass potential, etc... energies
Expressed in the Rate form
dEsystem &
Q = QΔt
& &
Ein − Eout = ( kW )
14 2 4 3 dt &
W = W Δt
Rate of net energy transfer 123
by heat, work and mass Rate of change in internal,
kinetic, potential, etc... energies ⎛ dE ⎞
ΔE = ⎜ ⎟ Δt
⎝ dt ⎠
KEITH VAUGH

89. The energy balance can be expressed in a per unit mass basis
ein − eout = Δesystem ( kg)
kJ
KEITH VAUGH

90. The energy balance can be expressed in a per unit mass basis
ein − eout = Δesystem ( kg)
kJ
The energy balance can also be expressed in a differential form
KEITH VAUGH

91. The energy balance can be expressed in a per unit mass basis
ein − eout = Δesystem ( kg)
kJ
The energy balance can also be expressed in a differential form
δ Ein − δ Eout = dEsystem
or
δ ein − δ eout = desystem
KEITH VAUGH

92. The energy balance can be expressed in a per unit mass basis
ein − eout = Δesystem ( kg)
kJ
The energy balance can also be expressed in a differential form
δ Ein − δ Eout = dEsystem
or
δ ein − δ eout = desystem
For a closed system undergoing a cycle, the initial
and final states are identical, then the energy balance
for a cycle simplifies to Ein = Eout
The energy balance for a cycle can be expressed in
terms of heat and work interactions
KEITH VAUGH

93. The energy balance can be expressed in a per unit mass basis
ein − eout = Δesystem ( kg)
kJ
The energy balance can also be expressed in a differential form
δ Ein − δ Eout = dEsystem
or
δ ein − δ eout = desystem
For a closed system undergoing a cycle, the initial
For a cycle ΔE = 0
and final states are identical, then the energy balance
thus Q = W
for a cycle simplifies to Ein = Eout
The energy balance for a cycle can be expressed in
terms of heat and work interactions
& &
Wnet , out = Qnet , in or Wnet , out = Qnet , in for a cycle
KEITH VAUGH

94. ENERGY CONVERSION
EFFICIENCIES
Efficiency
is one of the most frequently used terms in thermodynamics,
and it indicates how well an energy conversion or transfer
process is accomplished
}
Desired output
Performance =
Required output
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95. ENERGY CONVERSION
EFFICIENCIES
Efficiency
is one of the most frequently used terms in thermodynamics,
and it indicates how well an energy conversion or transfer
process is accomplished
}
Desired output
Performance =
Required output
Efficiency of a water heater
The ratio of the energy delivered to the house by hot water to
the energy supplied to the water heater.
KEITH VAUGH

96. Q Amount of heat released during combustion
ηcombustion = =
HV Heating value of the fuel burned
Heating value of the fuel
The amount of heat released when a unit amount of fuel at
room temperature is completely burned and the
combustion products are cooled to the room temperature.
Lower heating value (LHV)
When the water leaves as a vapour.
Higher heating value (HHV)
When the water in the combustion gases is completely
condensed and thus the heat of vaporisation is also
recovered.
KEITH VAUGH

97. The efficiency of space heating systems of
residential and commercial buildings is
usually expressed in terms of the annual
fuel utilisation efficiency (AFUE).
This accounts for the combustion
efficiency as well as other losses such as
heat losses to unheated areas and start-up
and cool down losses.
The definition of the heating value of gasoline
KEITH VAUGH

98. Generator
A device that converts mechanical energy to electrical energy.
Generator efficiency
The ratio of the electrical power output to the mechanical
power input.
Thermal efficiency of a power plant
The ratio of the net electrical power output to the rate of fuel
energy input.
KEITH VAUGH

99. Generator
A device that converts mechanical energy to electrical energy.
Generator efficiency
The ratio of the electrical power output to the mechanical
power input.
Thermal efficiency of a power plant
The ratio of the net electrical power output to the rate of fuel
energy input.
&
Wnet , electric Overall efficiency
ηoverall = ηcombustionηthermalηgenerator =
HHV × mnet & of a power plant
KEITH VAUGH

100. Generator
A device that converts mechanical energy to electrical energy.
Generator efficiency
The ratio of the electrical power output to the mechanical
power input.
Thermal efficiency of a power plant
The ratio of the net electrical power output to the rate of fuel
energy input.
&
Wnet , electric Overall efficiency
ηoverall = ηcombustionηthermalηgenerator =
HHV × mnet & of a power plant
Lighting efficiency
The amount of light output in lumens per W of electricity
consumed. (refer to text page 88)
KEITH VAUGH

101. Using energy-efficient appliances conserve energy.
It helps the environment by reducing the amount of
pollutants emitted to the atmosphere during the combustion
of fuel
The combustion of fuel produces
✓ carbon dioxide, causes global warming
✓ nitrogen oxides and hydrocarbons, cause smog
✓ carbon monoxide, toxic
✓ sulphur dioxide, causes acid rain
The efficiency of a cooking appliance represents the
fraction of the energy supplied to the appliance that
is transferred to the food
KEITH VAUGH

102. Efficiencies of mechanical and electrical devices
Mechanical energy output Emech, out Emech, loss
ηmech = = = 1−
Mechanical energy input Emech, in Emech, in
KEITH VAUGH

103. Efficiencies of mechanical and electrical devices
Mechanical energy output Emech, out Emech, loss
ηmech = = = 1−
Mechanical energy input Emech, in Emech, in
The effectiveness of the conversion process between the mechanical
work supplied or extracted and the mechanical energy of the fluid is
expressed by the pump efficiency and turbine efficiency,
KEITH VAUGH

104. Efficiencies of mechanical and electrical devices
Mechanical energy output Emech, out Emech, loss
ηmech = = = 1−
Mechanical energy input Emech, in Emech, in
The effectiveness of the conversion process between the mechanical
work supplied or extracted and the mechanical energy of the fluid is
expressed by the pump efficiency and turbine efficiency,
& &
Mechanical energy increase of the fluid ΔEmech, fluid W pump, u
η pump = = =
Mechanical energy input & , in
Wshaft &
W pump
KEITH VAUGH

105. Efficiencies of mechanical and electrical devices
Mechanical energy output Emech, out Emech, loss
ηmech = = = 1−
Mechanical energy input Emech, in Emech, in
The effectiveness of the conversion process between the mechanical
work supplied or extracted and the mechanical energy of the fluid is
expressed by the pump efficiency and turbine efficiency,
& &
Mechanical energy increase of the fluid ΔEmech, fluid W pump, u
η pump = = =
Mechanical energy input & , in
Wshaft &
W pump
& & &
ΔEmech, fluid = Emech, out − Emech, in
KEITH VAUGH

106. Efficiencies of mechanical and electrical devices
Mechanical energy output Emech, out Emech, loss
ηmech = = = 1−
Mechanical energy input Emech, in Emech, in
The effectiveness of the conversion process between the mechanical
work supplied or extracted and the mechanical energy of the fluid is
expressed by the pump efficiency and turbine efficiency,
& &
Mechanical energy increase of the fluid ΔEmech, fluid W pump, u
η pump = = =
Mechanical energy input & , in
Wshaft &
W pump
& & &
ΔEmech, fluid = Emech, out − Emech, in
Mechanical energy output &
Wshaft , out &
Wturbine
ηturbine = = =
& &
Mechanical energy decrease of the fluid ΔEmech, fluid Wturbine, e
KEITH VAUGH

107. Efficiencies of mechanical and electrical devices
Mechanical energy output Emech, out Emech, loss
ηmech = = = 1−
Mechanical energy input Emech, in Emech, in
The effectiveness of the conversion process between the mechanical
work supplied or extracted and the mechanical energy of the fluid is
expressed by the pump efficiency and turbine efficiency,
& &
Mechanical energy increase of the fluid ΔEmech, fluid W pump, u
η pump = = =
Mechanical energy input & , in
Wshaft &
W pump
& & &
ΔEmech, fluid = Emech, out − Emech, in
Mechanical energy output &
Wshaft , out &
Wturbine
ηturbine = = =
& &
Mechanical energy decrease of the fluid ΔEmech, fluid Wturbine, e
& & &
ΔEmech, fluid = Emech, in − Emech, out
KEITH VAUGH

108. The mechanical efficiency of a fan is the ratio
of the kinetic energy of air at the fan exit to
the mechanical power input
KEITH VAUGH

109. &
⎛ mV22 ⎞
&
ΔEmech, fluid ⎜
⎝ 2 ⎟ ⎠
ηmech, fan = =
&
Wshaft , in &
Wshaft , in
The mechanical efficiency of a fan is the ratio
of the kinetic energy of air at the fan exit to
the mechanical power input
KEITH VAUGH

110. &
⎛ mV22 ⎞
&
ΔEmech, fluid ⎜
⎝ 2 ⎟ ⎠
ηmech, fan = =
&
Wshaft , in &
Wshaft , in
⎝(
⎛ 0.50 kg
)(
2
m ⎞
s 12 s ⎠ )
ηmech, fan = 2
50W
The mechanical efficiency of a fan is the ratio
of the kinetic energy of air at the fan exit to
the mechanical power input
KEITH VAUGH

111. &
⎛ mV22 ⎞
&
ΔEmech, fluid ⎜
⎝ 2 ⎟ ⎠
ηmech, fan = =
&
Wshaft , in &
Wshaft , in
⎝(
⎛ 0.50 kg
)(
2
m ⎞
s 12 s ⎠ )
ηmech, fan = 2
50W
The mechanical efficiency of a fan is the ratio
of the kinetic energy of air at the fan exit to ηmech, fan = 0.72 or 72%
the mechanical power input
KEITH VAUGH

112. &
Mechanical power output Wshaft , out
ηmotor = = Pump efficiency
Electric power input &
Welect , in
&
Electrical power output Welect , out
ηgenerator = = Generator efficiency
&
Mechanical power input Wshaft , in
& &
W pump, u ΔEmech, fluid Pump-motor
η pump−motor = η pumpηmotor = =
&
Welect , in &
Welect , in overall efficiency
&
Welect , out &
Welect , out Turbine-generator
ηturbine−gen = ηturbineηgenerator = =
& &
Wturbine, e ΔEmech, fluid overall efficiency
KEITH VAUGH

113. Forms of energy
Energy transfer by heat
Energy transfer by work
Mechanical forms of work
The first law of thermodynamics
 Energy balance
 Energy change of a system
 Mechanisms of energy transfer (heat, work, mass flow)
Energy conversion efficiencies
 Efficiencies of mechanical and electrical devices (turbines, pumps, etc...)
Energy and environment
 Ozone and smog
 Acid rain
 The Greenhouse effect - Global warming
KEITH VAUGH