Environmental Engineering Reference
As more and more mills were built on rivers and streams, the activity was strictly
regulated and mill operators were instructed on how long mills could be used and
how large they could be. This may have been an annoyance for them, but it was a
good thing because it promoted technical development and ensured that optimal use
was made of existing mills. This constant drive to improve gave rise to the modern,
highly effi cient turbines used in hydropower plants today.
The introduction of the steam engine slowly displaced water-powered systems. But,
in contrast to wind power, the use of hydropower did not vanish from the scene
with the exploitation of fossil energies. When widespread electrifi cation began at
the end of the nineteenth century, hydropower was still very much part of the scene.
At the beginning small turbines were used to power electric generators, but the size
of the systems grew rapidly.
9.1 Tapping into the Water Cycle
The colour of our planet is blue when seen from space. The reason is that 71% of
the earth's surface consists of water. However, without the sun our blue planet
would not be blue. Water, which gives the earth its characteristic appearance, would
be completely solidifi ed into ice. Because of the heat of the sun, 98% of water is
A Water-Powered System in the Rain Gutter?
The roof of a house collects many cubic metres of water each year. A rain
gutter diverts the water without making any use of its energy. A water-
powered system for each rain gutter could actually be a good idea.
The annual precipitation in Berlin amounts to around 600 litres per square metre, so that
a 100 m 2 house roof would collect 60000 litres or 6000 10-litre buckets of water.
Compared with a similar bucket on the ground, the content of a 10-litre bucket on a
10 m-high roof has a potential energy of 0.000273 kilowatt hours. Six thousand buckets
together create around 1.6 kilowatt hours, just enough to boil water for 80 cups of coffee
but unfortunately too little to make it worthwhile to harness. Thus considerably larger
quantities of water than are available in a rain gutter are necessary to produce viable
quantities of energy.
Altogether there are around 1.4 billion cubic kilometres of water on earth. Of this
amount, 97.5% is salt water in the oceans and only 2.6% is fresh water. Almost
three-quarters of the fresh water is bound in polar ice, ocean ice and glaciers; the
rest is mainly in the groundwater and the soil moisture. Only 0.02% of the water
on earth is in rivers and lakes.
Due to the infl uence of the sun, on average 980 litres of water evaporate from each
square metre of the earth's surface and come down again somewhere else in the