Environmental Engineering Reference
In-Depth Information
Wind has been used since the most remote antiquity for navigation based on the
use of sails, but it was also used to grind grains in the 7th century. In the 17th
century windmills became very popular in Europe and by around 1750, there were
6,000-8,000 mills in operation in Holland, with typical powers of 7.5 kW. In the
beginning of the 20th century these mills were used very extensively for water lift-
ing and irrigation and the generation of electricity. In isolated systems, batteries are
necessary to store electricity for when the wind is not blowing.
The amount of power obtained from a windmill increases rapidly with the ve-
locity of the wind. Typical wind velocities are in the range of 3-10 m/sec, that is,
10-36 km/hour. Contrary to common belief, the average wind velocity is rather
regular and fluctuations do not deviate more than 10-15% from the average wind
velocity, over a year. If the wind velocity is 6 m/sec, one can generate approxim-
ately 140 W/m
2
.
Modern wind machines are very large, with blades more than 80 m in length and
generating approximately 5 MW. The world's installed capacity of wind machines
for the production of electricity at the end of 2010 was 198 GW.
Those wanting to install wind machines sometimes meet with community res-
istance due to environmental problems such as noise or degradation of the scenery.
To solve such problems, many wind machines are installed offshore, far from pop-
ulated places.
Photovoltaic (PV) cells, discovered in 1954 by Bell Laboratories researchers, con-
vert solar energy directly into electricity. The incident solar radiation (or photons)
displaces free electrons from the semiconductor material; when the electrons leave
their positions, the imbalance of the electric charges at the front and at the back of
the cells generates a difference in potential, in turn generating an electric current.
This is what happens in conventional lead-acid batteries. One can visualize the pro-
cess by thinking of a garage that has two levels and is so completely full of cars
that they cannot move. If one removes one of the cars to an upper level, the remain-
ing cars in the lower level can move. The movement of the cars is the analogy to
an electric current. A photovoltaic module is composed of cell panels, each hav-
ing a width of 1-10 cm and producing 1-2 W. The current generated is continuous,
