The hourly values of global radiation received on a horizontal surface Hh can be divided into diffuse components Dh and direct components Bh by Liu and Jordan’s method using the following expressions:

Dh/Hh = 1 — 0.09k if k < 0.22

Dh/Hh = 0.9511 — 0.1604k + 4.388k2

— 16.638k3 + 12.336k4 if 0.22 < k < 0.8 (46)

Dh/Hh= 0.165 ifk > 0.8

where k is the hourly clearness index defined by the ratio between the hourly global energy Hh received on a horizontal plane and the hourly energy received on a horizontal plane Hh, ex situated outside the atmosphere.

k=Hh/Hh>ex (47)

Hh, ex can be calculated using the equation:

Hh, ex = /J1 + 0.033cos(2rc«/365)]

• (cos L cos d cos h + senL send) (48)

The hour angle h is calculated at the centre of the considered hour or using the exact equation:

Hhex = 12/n/cs[1 + 0.033cos(2n«/365)]

• {cos L cos d(sen h1 — sen h2) + (h1 — h2)sen L sen d} (49)

h1 and h2 are the hour angles at the extremities of the said hour; in eqns (48) and (49) Hh, ex is expressed in W-h/m2.

Once we have obtained the value of the hourly global radiation Hh received on a horizontal plane, we can calculate the hourly diffuse radiation Dh. The hourly direct radiation on a horizontal plane is calculated by difference:

Bh=Hh-Dh (50)

The hourly global radiation received on an inclined surface turns out to be:

Eh=Rb Bh+Rd Dh+Rr(Bh+Dh) (51)

In this case, Rb is calculated at the centre of the said hour [1].