Wind is Sun's heat transformed into kinetic energy
through the
greatest solar collector currently available, Earth's atmosphere. Wind total
power is estimated between 1 700 and 3 500 TeraWatt; by comparison, the
whole mankind primary energy needs are estimated at approx. 14 TW. Wind
energy is not uniformely distributed, but mainly found in the higher layers of
the troposphere at the mid latitudes of both North and South hemispheres. The
most favourable altitude in term of wind power is, quite difficult to exploit,
at approx. 10 000 m (32 800 ft), where average wind speeds can exceed 45
m/s (equal to 162 km/h or 100 mph). The graphic below shows ten years of data
at 250 hectoPascal (measure of atmospheric pressure, equal to millibar)
equivalent to approx. 10 400 m of altitude From these heights, wind speed
and energy decrease moving towards the ground, the layers of air giving back
heat through friction between them and with the orography of the Earth's
surface. In the below graphic at 500 hPa (approx. 5 600 m of altitude) average
wind speeds are already significantly reduced.
At 80 m above the ground, the hub height representative
of
the latest generation of wind turbines, the global average wind speed is
estimated at 4.6 m/s, not enough for economical wind power generation; at
10 m above the ground is even lower, estimated at 3.3 m/s (see Evaluation
of global wind power, Cristina L. Archer - Mark Z. Jacobson, Journal of
Geophysical Research – Atmospheres, 2005).
But to break free from the ground and to reach
altitude winds is
already very beneficial at 800 m, the height representative of a Kite Gen
power plant, where the average wind speed is estimated by Kite Gen
Research at 7.2 m/s. At this altitude wind power, as shown by the following
table, is almost 4 times the amount available to wind turbines.
Altitude Wind
speed Wind power
m m/s W/m²
800 7.2 205
80 4.6 58
10 3.3 22
Every point on the Earth surface, on average, 800 m above
it, has
enough wind power to be exploited
