Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 84

posed by microorganisms. The net storage of biomass per hectare and year is ap-
proximately 5.7 t above ground and approximately 2.4 t as roots and humus un-
derground

Influence of various growth factors. The formation of biomass is mainly influ-
enced by irradiation, water, temperature, soil, nutrients, and plant cultivation
measures. These parameters are discussed below /2-28/.

Irradiation. Net photosynthesis increases with an increased level of irradiation
intensity until it reaches a saturation point. If irradiation is very low, respiration of
carbon dioxide exceeds its assimilation. The level of irradiation intensity, at which
the respired amount of CO 2 equals the amount assimilated, is called photo com-
pensation point and for most plants it varies between 4 and 12 W/m 2 .
Only part of the irradiation incident on a plant is absorbed. The rest is reflected
by the plant or penetrates it. The absorption of irradiation in the plant tissue is
selective, i.e. it depends on the wavelength. Especially in the infrared range be-
tween 0.7 and 1.1 µm a major share of energy penetrates the plant body without
being absorbed (Fig. 2.49).
The net incident irradiation is a result of the non-reflected total irradiation and
the long-wave reflection. The reflection coefficient represents the ratio of re-
flected to incident energy. This coefficient mainly depends on the incident angle,
the surface texture, and the colour of the plant. For a green plant body, the reflec-
tion coefficient is between 0.1 and 0.4.
CO 2 -assimilation of individual leaves of different plants increases proportion-
ally to the incident irradiation and the type of photosynthesis. At the same level of
irradiation, the assimilation of C 4 -plants is higher than that of C 3 -plants (see e.g.
/2-29/).

uv Visible Infrared

100

80

60

Light absorption

Reflection

40

20

0

0.4 0.5 0.6 0.7 0.8 1.0 1.5 2 4 10

Wavelength in µm

Fig. 2.49 Absorption and reflection spectrum of poplar leaves (see /2-26/, /2-37/)