Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 89

with a low level of water-holding capacity and a low level of precipitation, like
for example in Brandenburg (i.e. the mid eastern part of the country), the biomass
production potential is low. Areas with a higher level of biomass productivity for
winter wheat cultivation are mostly characterised by high-quality soils which oc-
cur in areas with loessic soil, or in regions with marshy soil. The far northern part
of Germany, Schleswig-Holstein, is for example particularly favoured by soil
quality and balanced precipitation. Centres of high soil quality and biomass pro-
ductivity can also be found for example in the centre of Germany, at the far west-
ern side (i.e. Cologne Bay), and in large areas of Central Bavaria (i.e. the south-
eastern part of Germany).

Temporal supply characteristics. The increase in biomass is characterised by a
daily and annual cycle.
As the process of photosynthesis is dependent on insulated solar energy, the
diurnal rhythm of the photosynthesis is determined by the daily course of solar
irradiation (Fig. 2.52). The photosynthetic activity increases with increasing inci-
dent irradiation, and reaches its peak when the sun is at its highest level at mid-
day and decreases again towards the evening. Reduced irradiation caused by
cloudiness reduces photosynthetic activity.

Global radiation Global radiation

C 4 -plant (e.g. Miscanthus) C 4 -plant (e.g. Miscanthus)

C 3 -plant (e.g. rape) C 3 -plant (e.g. rape)

6 6 8 8 10 10 12 12 14 14 16 16 18 18

Fig. 2.52 Schematic daily cycle of CO 2 -gas exchange (bottom) depending of the insulated
solar irradiation (top) (see /2-31/)

Time withinthecourseof thedayin h Time withinthecourseof thedayin h

The annual cycle of biomass production is determined by the course of the am-
bient temperature and the length of the day. The lower temperature limit of an
increase in biomass by most crops in Central Europe, for instance, is at a daily
mean temperature of 5 °C; this might be different in other parts of the world (e.g.
tropical areas). In general, the photosynthetic activity and biomass accumulation
increase with increasingly long days and higher temperatures, and in Central
Europe reach peak levels during the months from May to August (Fig. 2.53).