Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 38

Reflection radiation on tilted, aligned surfaces. A certain proportion of the
global radiation incident in the neighbourhood of a defined surface is reflected
onto the receiving tilted, aligned surface G r,t,a . This reflected radiation can be cal-
culated using the albedo A G (i.e. the ratio of reflected to the incident global radia-
tion), the global radiation incident on the horizontal receiving surface G g , and the
inclination angle towards the horizontal α according to Equation (2.13).

sin 2

G = A G ( α 2 )

(2.13)

r , t , a G g

The albedo depends on site-specific conditions. Values can, for example, range
between 0.7 and 0.9 with snow, between 0.25 and 0.35 with sand, and between
0.1 and 0.2 in forest and farmland areas.

Global radiation on tilted, aligned surfaces. The global radiation incident on an
tilted and aligned surface, for example the surface of a photovoltaic module, con-
sists of the incident direct ( G b,t,a , Equation (2.10)) and diffuse radiation ( G d,t,a ,
Equation (2.11) and (2.12)) as well as the radiation reflected by the neighbour-
hood on this receiving surface ( G r,t,a , Equation (2.13)). The total global radiation
incident G g,t,a on an tilted and aligned surface is thus calculated according to
Equation (2.14).

G = G + G + G

(2.14) g , t , a b , t , a d , t , a r , t , a

2.2.2 Supply characteristics

Measuring radiation. In order to be able to measure short and long-wave radia-
tion fluxes through the atmosphere, a number of different measuring instruments
are available. Relative and absolute instruments are generally differentiated /2-3/,
/2-10/.
If radiation energy has to be measured in absolute terms, the incident solar en-
ergy has to be converted into a measurable parameter first. Most of these radiation
measurement instruments thus absorb the radiation energy on a blackened surface
which converts it into heat. Due to this process the temperature of the surface
increases. Correspondingly, an amount of heat per time unit is released by heat
transfer in the instrument or into the air by thermal radiation. Based on this bal-
anced conditions are established. In this case, the resulting temperature increase is
a measure for the radiation energy. Types of such absolute instruments are for
example, the Moll-Gorcynsky pyranometer, and the compensation-pyrheliometer
named after Angström. A relative instrument can be used to calibrate such an
absolute instrument. One example of a relative instrument is the Michelson-
Marten Actinometer.