Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 80

Forggensee, for example, serves as a head reservoir for the run-of-river power
stations at the Lech. One of the oldest high-pressure power stations in Germany
for example operates between the Lake Kochel and Lake Walchen. Most of the
reservoirs, have several functions and energy generation is not always their main
function.

Maximum value
Average high water
Mean value
Average low water
Minimum value

2,700 4,500

2,400 4,000

2,100 3,500

1,800 3,000

2,500

1,500

1,200 2,000

900 1,500

600 1,000

300 500

0 0

123456789 0 1 2 123456789 0 1 2

Time in mont hs Time in months

Fig. 2.46 Mean values, average high and low water as well as minimum and maximum
fluctuations of the monthly mean runoff at the river levels Rockenau/Neckar (left) and
Maxau/Rhine (right) (data see /2-25/)

In the low mountain ranges there might exist reservoir lakes which sometimes
mainly generate energy, but sometimes have the main purpose of ensuring the
supply of drinking and domestic water. This is mainly the case in those parts
where - without a reservoir - supply shortages would mainly occur during the
summer due to the low water supply. Often both aspects are interconnected.
Typical examples of water reservoirs for energy generation are the annual res-
ervoirs in the Alps. They are usually empty by the end of winter and are refilled
by melting snow in the catchment area and possibly by water transfers from other
valleys during the summer months. During this time, they only release little water.
At the end of the summer, the reservoirs should be entirely refilled. During the
winter months, when energy is high in demand, and there is practically no inflow
into the reservoir, the stored water is used. These reservoirs also increase the flow
of the downstream rivers.

2.5 Photosynthetically fixed energy

Biomass in the broader sense is all phytomass and zoomass. An estimated total of
1.84 10 12 t of dry mass exists on the continents. The major part of phytomass or
plant mass is generated by autotrophic organisms that can generate the energy