SOLAR RADIATION ON AN INCLINED SURFACE

2.2.1 Hourly Variation

The total solar radiation incident on a surface consists of (i) beam solar radiation (ii) diffuse solar radiation and (iii) solar radiation reflected from the ground and the surroundings. After

Figure 2.7a Hourly variation of beam and diffuse radiation (June 22, 2010).

Figure 2.7b Hourly variation of beam and diffuse radiation (December 21, 2010).

5:00 6:00 7:00 800 9:00 10:00 11:00 1200 13:00 1400 15:00 16:00 1700 18:00 19:00 Time (h)

Figure 2.8a Hourly variation of beam and diffuse radiation (June, 2010) (Source, IMD).

Figure 2.8b Hourly variation of ambient air temperature (June, 2010) (Source, IMD).

500 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 Time (h)

Figure 2.8c Hourly variation of beam and diffuse radiation and ambient air temperature (December 21, 2010) (Source, IMD).

Figure 2.8d

Hourly variation of ambient air temperature (December 21, 2010) (Source, IMD).

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month in Year

Figure 2.8e

Monthly data of beam and diffuse radiation.

и ■SJ i­ s

a.

i- <

la В <

Figure 2.8f Monthly data of ambient air temperature.

determining the beam and diffuse radiation on horizontal surface, Liu and Jordan gave a formula to evaluate total radiation on a surface of arbitrary orientation.3

I = Ib Rb + Id Rd +pRr{Ib + Id) (2.8)

where Rb, Rd and Rr are known as conversion factors for beam, diffuse and reflected components respectively, and p is the reflection coefficient of the ground (= 0.2 and 0.6 for ordinary and snow covered ground, respectively). The expressions for Rb, Rd and Rr are given below:

i) Rb: This is defined as the ratio of flux of beam radiation incident on an inclined surface (Eq. (2.4)) to that on a horizontal surface (Eq. (2.5)). Now, Rb, for beam radiation can be obtained as,

I’b cos 0i

I b cos 0z

Depending on the orientation of the inclined surface, the expression for cos 0; and cos 0z can be obtained from Eqs. (2.4) and (2.5).

ii) Rd: This is defined as the ratio of the flux of diffuse radiation falling on the tilted surface to that on the horizontal surface.

This conversion factor depends on the distribution of diffuse radiation over the sky and on the portion of sky seen by the surface. But a satisfactory method of estimating the distribution of diffuse radiation over the sky is yet to be found. It is, however, widely accepted that sky is an isotropic source of diffuse radiation. If (1+cos b)/2 is the radiation shape factor for a tilted surface with respect to sky, then

iii)

Rr: The reflected component comes mainly from the ground and other surrounding objects. If the considered reflected radiation is diffuse and isotropic, then the situation is opposite to that in the above case.

It may be mentioned here that both the beam and diffuse components of radiation undergo reflection from the ground and the surroundings.