Environmental Engineering Reference
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generate the photovoltaic effect are based on silicon cells. Advanced designs have
achieved conversion efficiencies (defined as ratio between power of solar radiation
to maximum power supplied by the device) of almost 30% in laboratory condi-
tions, while efficiencies of 10-15% are available on current commercial devices
[ 21 ]. Research in this field is being also conducted in order to produce competitive
organic photovoltaic cells with significantly lower costs than silicon cells. As the
voltage obtainable from a single photovoltaic cell is too low for the most common
applications (about 0.5 V), the connection in series of several cells is necessary,
realizing arrays of cells usually installed in metallic frames and covered by anti-
reflection glass (photovoltaic modules for different applications).
Wind energy is currently proposed in the contest of new renewable sources for
the direct production of electricity by wind generators, with power capability
ranging from few Watts to Megawatts. The possibility to exploit the wind energy in
a specific geographic place depends on three main parameters: intensity, direction
and velocity of the wind in that particular site. Turbines are generally spaced
approximately three blade diameters apart, in all directions, leaving significant area
of land still available to be exploited for other purposes. Wind generators with
blades of 50 cm diameter are used as battery rechargers, while those with blades of
about 50 m can be used for electric energy production up 2 MW. The most diffuses
types of wind generators adopt two or three blades of about 30 m, and are capable to
generate an electric power of about 1500 kW. The wind generators are connected
each other to realize the so-called wind farm. Wind energy is classified according to
the range 1-5, corresponding to the average wind velocity in a site, where 5 is
the strongest. Typical output power densities for class 5 sites range from 3.2 to
5.7 W/m 2 [ 22 ]. As wind change and variability is caused by uneven heating of the
ground by the sun, the power of the wind can be considered as a form of solar
power. Assuming an average solar power density of 200 W/m 2 , the output power
density of 5.7 W/m 2 before considered corresponds to an efficiency of 2.85% for a
wind generator. This efficiency, although not directly dependent on solar radiation
as for photovoltaic, permits a comparison between the two renewable resources.
Wind power efficiencies are lower than photovoltaic of about one order of
magnitude, but whereas a solar photovoltaic array requires 100% of the land on
which the modules are placed, a wind farm occupies only a small fraction of the
land for the base of the turbine and access roads.
Biomass can be defined as any organic material available on a recurring base,
which can be transformed in a variety of products, such as energy, chemicals and
biomaterials, a possibility which has led in recent years to the concept of biore-
finery. The most obvious types of biomass are wood and crops, which are con-
sidered to be renewable as they can be continually regenerated by taking up carbon
dioxide from atmosphere during growth (through photosynthesis). In addition also
many types of organic wastes (forestry residues, straw, food waste, manure, etc.)
can be considered as biomass. At the end of their utilization biomass resources
return to the air, so creating a closed loop cycle. This evidences that also biomass,
like wind, can be regarded as a form of solar energy, which is transformed by
living organisms through their normal functions of growth and metabolism.
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