21
Chapter 2
ENERGY, ENERGY TRANSFER, AND GENERAL
ENERGY ANALYSIS
Forms of Energy
21C
In electric heaters, electrical energy is converted to sensible internal energy.
22C
The forms of energy involved are electrical energy and sensible internal energy.
Electrical energy is
converted to sensible internal energy, which is transferred to the water as heat.
23C
The
macroscopic
forms of energy are those a system possesses as a whole with respect to some
outside reference frame.
The
microscopic
forms of energy, on the other hand, are those related to the
molecular structure of a system and the degree of the molecular activity, and are independent of outside
reference frames.
24C
The sum of all forms of the energy a system possesses is called
total energy
.
In the absence of
magnetic, electrical and surface tension effects, the total energy of a system consists of the kinetic,
potential, and internal energies.
25C
The internal energy of a system is made up of sensible, latent, chemical and nuclear energies.
The
sensible internal energy is due to translational, rotational, and vibrational effects.
26C
Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily
life.
27C
The
mechanical energy
is the form of energy that can be converted to mechanical work completely
and directly by a mechanical device such as a propeller
.
It differs from thermal energy in that thermal
energy cannot be converted to work directly and completely. The forms of mechanical energy of a fluid
stream are kinetic, potential, and flow energies.
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28
A river is flowing at a specified velocity, flow rate, and elevation. The total mechanical energy of the
river water per unit mass, and the power generation potential of the entire river are to be determined.
Assumptions
1
The elevation given is the elevation of the free surface of the river.
2
The velocity given is
the average velocity.
3
The mechanical energy of water at the turbine exit is negligible.
Properties
We take the density of water to be
ρ
=
1000 kg/m
3
.
Analysis
Noting that the sum of the flow energy and
the potential energy is constant for a given fluid
body, we can take the elevation of the entire river
water to be the elevation of the free surface, and
ignore the flow energy. Then the total mechanical
energy of the river water per unit mass becomes
kJ/kg
887
.
0
/s
m
1000
kJ/kg
1
2
)
m/s
3
(
m)
90
)(
m/s
(9.81
2
2
2
2
2
2
mech
=
+
=
+
=
+
=
V
gh
ke
pe
e
90 m
River
3 m/s
The power generation potential of the river water is obtained by multiplying the total mechanical energy by
the mass flow rate,
kg/s
500,000
/s)
m
00
)(5
kg/m
1000
(
3
3
=
=
=
V
&
&
ρ
m
MW
444
=
=
=
=
=
kW
000
,
444
kJ/kg)
7
kg/s)(0.88
000
,
500
(
mech
mech
max
e
m
E
W
&
&
&
Therefore, 444 MW of power can be generated from this river as it discharges into the lake if its power
potential can be recovered completely.
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 Spring '08
 Hassan
 Energy, electrical energy

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