Environmental Engineering Reference
In-Depth Information
TABLE 3.2
Summary of Global Values for Renewable Sources
Extractable
Power, W
Power, W
Energy, quads/year
Solar
1.8*10
17
Wind
3.6*10
15
1.3*10
14
3,900
Hydro
9.0*10
12
2.9*10
12
86
Geothermal
2.7*10
13
1.3*10
11
4
Tides
3.0*10
12
6.0*10
11
1.9
3.3 POWER IN THE WIND
The moving molecules of air have kinetic energy, so locally the amount of air molecules moving
across some area during some time period determines the power (Figure 3.3). This area is not the
surface area of the earth, which was referred to in the estimation of extractable power and energy,
but the area perpendicular to the wind flow. The mass,
m
, in the volume of the cylinder that will pass
across the area,
A
, in time,
t
, can be determined from the density of the air,
l
, and the volume of the
cylinder,
V
. The power is the kinetic energy (KE) of the air molecules divided by the time:
P
KE
/
t
0.5 m v
2
/t
l
m
/
V
(3.1)
V
area * length
A
*
L
m
l
*
V
l
*
A
*
L
Substitute this value of mass into Equation 3.1. Only those molecules with a velocity,
v
L
/
t
, will
cross the area in time,
t
, and those further to the left will not, so the power is given by
P
0.5
l ALv
2
/
t
0.5
l AL
/
tv
2
0.5
l Avv
2
0.5
l Av
3
The power/area, referred to as wind power potential or wind power density, is
P
/
A
0.5
l v
3
(3.2)
L
A
V
FIGURE 3.3
Flow of wind through a cylinder of area
A
.
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