The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 142

Gross Hydroelectric Potential: the hydroelectric potential of a country if all
its water flows were turbined until sea level or to the country borders (if the
flow continues into other countries) under 100% system e ciency. It has been
estimated at 4,200 GW.
Technically Useful Hydroelectric Potential: estimated at 1,800 GW, it repre-
sents the hydroelectric energy obtained from all the exploitable or exploited
places under existing technological limits, without taking into account environ-
mental, economic or other restrictions (Table 6.8).
Economically Exploitable Hydroelectric Potential: estimated as around 1,200
GW, it is the part of the technically feasible potential that can be or that has
been developed under the local economic conditions, in a competitive way with
other energy supply sources. Some of the places that can be exploited econom-
ically can have restrictions from an environmental point of view. Nonetheless,
this limitation is not taken into account when determining this potential.
At the end of 2010, worldwide hydropower consumption was 775.6 Mtoe, ac-
counting for about 6% of the total energy consumption (BP, 2011).
6.6.3 Wind energy
Around 350 TW of solar energy is used for driving winds and ocean waves (Skinner,
1986). Wind is defined as horizontal air movement arising from differences in air
pressure created by the uneven heating of the atmosphere. It always flows from a
place of high pressure to one of low. Wind speed and direction are also affected by
the Coriolis effect 9 and the friction occurring between the wind and solid objects of
any kind such as the ground, trees, etc. Most places around the world have wind
speeds that average between 10 and 30 km/h (Skinner et al., 1999). The average
wind speed at 50 m is 6.6 m/s (23.7 km/h) (NASA, 2004) with an exergy content
of about 336 W=m 2 perpendicular to the wind direction (Hermann, 2006).
Estimates of the total global wind power are very large (in the order of 10 15 W).
Most of that power is in high altitude winds and is not recoverable by devices on
the land surface (Skinner, 1986). Global wind power generated at locations with
mean annual wind speeds of 6.9 m/s at 80 m is found to be 72 TW (Archer
and Jacobson, 2005). A technical potential of 72 TW installed global capacity at
a 20% average capacity factor would generate 126,000 TWh/yr (around 14.5 TW
— Table 6.8). In 2005, the existing exergy power capacity worldwide was 59 GW
(Martinot, 2006). This figure has notably increased, reaching near 200 GW in 2010
and covering almost 3% of the global electricity demand, according to the Global
Wind Energy Council (Fig. 6.2).
9 The Coriolis effect is the deviation from a straight line in the path of a moving body due to the
Earth's rotation.