Albedo irradiance

The reflectivity of most types of ground is rather low. Consequently, the contribution of the albedo irradiance falling on a receiver is generally small. (An exception occurs in the case of snow.) There is therefore no point in developing very sophisticated models for albedo. It is usual to assume that the ground is horizontal and of infinite extent and that it reflects isotropically. On this basis, the albedo irradiance on an inclined surface is given by

1 — cos в

R(fi. a) = pG(0)——– ^ (22.38)

where p is the reflectivity of the ground and depends on the composition of the ground. When the value of p is unknown, it is common to take p = 0.2.

It is now opportune to go forward with the example of the previous section, by calculating the irradiance components over a surface tilted to the latitude, for 15 April in Portoalegre, Brazil. Using Equations (22.34-22.37) to deal with diffuse radiation and p = 0.2, the results, expressed in W/m2, are as follows:

ki

оФ)

Вс(ф)

В(ф)

Я(ф)

С(ф)

Ms

0

0

0

0

0

0

±60°

0.2205

73.40

31.80

147.56

2.73

255.49

±30°

0.3082

124.14

76.94

357.14

6.26

564.48

0.3403

138.97

98.09

455.31

7.80

700.18

It becomes clear that the albedo can be generally neglected in PV calculations.

Updated: August 23, 2015 — 1:40 am