Page 208 - Materials Chemistry, Second Edition

196 E. Martínez Cámara et al.

was connected to a 12 kW dynamo. It charged the batteries at the house for 20
years. His turbines had 144 blades, whereas modern turbines have just 3
(Rivkin et al. 2012).
Renewable energy sources, and particularly wind power, have undergone
considerable development in recent years. This new boom in wind power dates
back to the early 1990s and is due mainly to the need to find viable alternatives to
fossil fuels, reserves of which are finite and will in the long term be incapable of
sustaining current levels and trends in consumption around the world.
The wind turbines now in use are models developed in recent years. There are
numerous types of turbine capable of generating electricity, but most of those
currently installed use horizontal-axis technology. Their rated power outputs range
from 500 kW to 5 MW. They are used basically in direct connections to the
electricity grid and are grouped into wind farms to take advantage of economies of
scale and facilitate monitoring and maintenance tasks.
Figure 1 shows the world-wide trend over time in total generating capacity
from wind power. There has been a clear, sustained year-by-year increase in
capacity in recent years, to almost 200 GW in 2010. However, a breakdown of
these data by regions (Fig. 2) clearly shows the influence of the worldwide
recession, with less capacity being installed in 2010 than in 2009, especially in
Europe and North America.
The basic idea underlying the generating of electricity via modern, horizontal-
axis wind turbines of the kind that can be found in most wind farms around the
world is to convert wind energy into mechanical energy, by using the force of the
aerodynamic thrust of the blades to generate torque on the main shaft. That
mechanical energy is then turned into electricity by means of a generator. The
disadvantage of this system compared with other conventional generators is that it
can only generate electricity when there is enough wind. Moreover, it is not
currently possible, from the economical point of view, to store the electricity
generated in order to use or to transfer it to the grid at a later time, because this
type of energy is noncontrollable and is subject to fluctuations depending on
changes in wind speed. From the viewpoint of electricity grids as a whole, the

Global cumulative installed wind capacity and forecasting
500
Annual Capacity [GW]
400
Cumulative capacity [GW]
300
GW
200
100
0
2006 2007 2008 2009 2010 2011 2012 2013 2014 2015
Year
Fig. 1 Global cumulative installed wind capacity and forecasting