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322 CHAPTER 9 Design Principles of Photovoltaic Irrigation Systems

Diffuse

Diffuse Direct

Direct

Reflected β
Diffuse

FIGURE 9.24
Solar radiation components.

• Reflected irradiance: Reflected by the earth’s surface. The amount of reflected
radiation depends on the reflection coefficient of the surface, also termed
albedo.
However, these solar radiation components are not usually known when
designing a PV project; therefore it is necessary to estimate them using commonly
available data, namely, global daily surface radiation, H, which is the incoming
radiant energy per square meter throughout a whole day. Fig. 9.25 shows the evolu-
tion of H in the city of Cordoba (Spain) throughout the year.
The Clarity Index (K t ) is the ratio between the measured global daily radiation
(H) at a specific location and the maximum or extraterrestrial radiation (H o ) that

Solar irradiation
30000000
25000000
Irradiation
Irradiation (J/m2) 15000000
20000000

10000000
5000000

0
1 2 3 4 5 6 7 8 9 10 11 12
Month
FIGURE 9.25
Monthly Mean irradiation measured in Cordoba (Spain).