Environmental Engineering Reference
In-Depth Information
Area
10
10
m
2
, or 360 km or 223 miles, on a side. This is
comparable to the area of the state of Iowa, which is equivalent to 237miles on a side!
The positive aspect is that the turbines do not necessarily preclude the normal use of
the land, for example, to grow wheat or corn. But there is no escape from the reality
that both wind energy and solar energy are diffuse sources.
Or we may ask how many wind turbines at 1.56MW per turbine? That number is
¼
500
10
9
/3.86
¼
12.95
10
9
/1.56
10
6
N
0.394 miles
per turbine or 2.53 turbines per mile, we could imagine turbines along 126 684 miles
of highway. The total mileage in the Interstate Highway system is 46 751miles, while
the total U.S. highways extend 162 156 miles. The cost of the turbines at $1 per Watt
is $500 billion. The cost of the U.S. Interstate Highway system is said to be $425
billion in 2006 dollars (http://en.wikipedia.org/wiki/Interstate_Highway_System.).
$500 billion is approximately equal to 0.07 of the U.S. military budget for a period of
10 years.
The same kinetic energy extraction analysis applies to water flow in a river, which
bene
ts immediately from the factor 1000/1.2
¼
500
¼
320 513 turbines. At a spacing of 635m
¼
833 increase in density. A recent
measurement of Mississippi water
flow (http://blog.gul
ive.com/mississippi-press-
news/2011/05/mississippi_river_
ooding_vic.html.) recorded 11 mph velocity and
16 million gallons per second
¼
flow under a bridge near Vicksburg, MS. With
10
3
m
3
, we have
conversions 1mph
¼
0.447m/s and 1 U.S. gallon
¼
4.404
10
4
m
3
is for water
4.92m/s and dV/dt
¼
7.06
flow at this location. The power is
then (dM/dt)(v
2
/2)
1000 kg/m
3
(7.06
10
4
m
3
/s)(4.92m/s)
2
/2
94.8MW.
The ef
ciency can be at most 0.59, corresponding to loss of speed by 2/3, and the
resulting disruption of the river
flow if the full cross section were
filled with rotor
blades would be prohibitive. Still it seems that tens of MW could be extracted from
such a
flow if it were continuous and if the installations could be sited to avoid
blocking of commerce.
The size of a 1MW river-
ow or tidal-
ow turbine is much smaller than a 1MW
wind turbine because of the 1000-fold increase in water density. Probably, this means
the water turbine would be cheaper. Water turbines, highly developed for hydro-
electric installations, in smaller forms for river-
ow applications are not as well
established commercially as are wind turbines.
¼
¼
1.1.2.2 Hydroelectric Power
Water running through turbines is used to generate electricity, with a typical
efficiency of 90%. It is evident from Figure 1.2 that hydroelectric power is at present
by far the largest renewable energy source, amounting to about 1.07 TWworldwide in
2008, or about 7.3%. These are extremely large projects typically, and the easiest sites
are already utilized (see Figure 1.10). The situation, often, for a large installation is
that it is close to a copper mine or an aluminum smelting facility, which has
supported the capital investment. The availability of ef
cient DC power transmission
lines may make the bene
t of these large installations more widely available.
Similar large facilities are at Niagara Falls in the United States and the Akosombo
Dam in Ghana, Africa. The Three Gorges dam in China at completion has a capacity
of 22.5GW. The planned Grand Inga Dam in Congo is projected as 39GW. The Belo
