Environmental Engineering Reference
In-Depth Information
form of precipitation. Altogether about 500 000 cubic kilometres of water collect in
this way each year. This gigantic water cycle converts around 22% of all the solar
energy radiated onto earth (Figure 9.2).
Figure 9.2
Earth's water cycle.
If the evaporation were concentrated onto a single square kilometre on earth, the
water would come pouring out at a speed of over 50km/h. The column of water
would reach the moon in less than one year. Fortunately, however, the water remains
spread on the surface of the earth, because otherwise we would not have any liquid
water left in 3000 years.
The energy that is converted by the sun in the earth's water cycle is around 3000
times the primary energy demand on earth. If we were to stretch a tarpaulin at a
height of 100 m around the earth, collect all the rainwater into it and use it to produce
energy, we would be able to satisfy all the energy needs on earth.
Altogether 80% of precipitation comes down over the ocean. Of the 20% that falls
on land, a large amount evaporates. Around 44000 cubic kilometres of water
reach the ocean again as return fl ow in groundwater or in rivers. This is still
more than one billion litres per second. We could be making use of the energy of
the water from this return fl ow without stretching large tarpaulins over the earth.
This water could be delivering some of its energy on its way back to the ocean -
energy that it fi rst received from the sun. Yet the useable amount of energy that
rivers carry with them comprises only a small fraction of the energy of the water
cycle.
To convert water into power, it is not just the amount of water that is important
but the height at which water is found. Water in a small mountain stream with
