Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 24

radiation is approximately equivalent to that of a black body. The radiant flux
density of the sun M S can be derived from the temperature within the photo-
sphere (approximately 5,785 K), the degree of emission, and the Stefan-
Boltzmann-constant; it is approximately 63.5 10 6 W/m 2 .

Corona
approx. 1 Mio. K
several Mio. km thick

Parameters

Chromosphere
approx. 5,000 K and above
several 1,000 km thick

Diameter 1,390,000 km (approx. 109 times

the diameter of the earth)

Energy transfer
by gamma- and
X-ray radiation

1.4 g/cm 3

Mean density

Core density 80 bis 100 g/cm 3

r

0.23 r

33

Mass 2 10 g (approx. 330,000 times

the mass of the earth)

0.7 r

Gravitational
acceleration

approx. 275 m/s (approx. 28
times earth gravity)

Energy transfer
by convection

Components 75 % hydrogen (H 2 )

23 % helium (He)
2 % other components

Photosphere
approx. 5,785 K
approx. 300 km thick
(emits the sunlight)

Core
90 % of total generated energy
40 % of total mass
10 bar
8 to 40 Mio. K

Condition gaseous

11

Fig. 2.1 Schematic structure and main parameters of the sun (see 2-25/)

The radiant flux density of the sun decreases - if losses are not considered - with
the square of the distance travelled. Thus the radiant flux density at the outer rim
of the earth atmosphere E SC can be calculated according to Equation (2.2).

2

M π d

E =

S S

(2.2) SC

( ) 2

π 2 L

ES

If the diameter of the sun d S is assumed up to the photosphere (approximately
1.39 10 9 m) and a mean distance between the sun and the earth ( L ES ) of approxi-
mately 1.5 10 11 m is taken into consideration, a radiant flux density of approxi-
mately 1,370 W/m 2 can be calculated at the top rim of the earth atmosphere (see
/2-3/). This mean value is called the solar constant. Over several years it varies
less than 0.1 % due to a fluctuation in solar activity.
The solar radiation incident on the atmospheric rim throughout the course of
the year is nevertheless characterised by seasonal variations. They are caused by
the elliptical orbit, where the earth moves around the sun during the course of one
year (Fig. 2.2). This changes the distance between the two celestial bodies. And
this distance variation leads to a fluctuation in the radiation incident on the atmos-
pheric rim; this results in the course of the solar constants shown in Fig. 2.3.