The direct solar energy received at a certain time interval on an oriented surface situated on the Earth is given by the expression:

rt0 +At

Eb = J Ibn(t )cos idt (36)

Jt0

where At (time interval) may vary (an hour, a day, a month, etc.) and Ibn(0 stands for the normal direct radiation.

As a rule, it is not possible to use this equation to calculate Eb since Ibn(t) depends on local atmospheric conditions which cannot be known in advance. It is possible, instead, to calculate that quantity per surface on soil during clear sky days using one of the models described in par. 5.

It is often useful to calculate using eqn (36) the radiation received during the day on a horizontal surface placed outside the atmosphere, which is equal to:

Г h

Hex = Г Icse(t)sen a dt (37)

J ha

where ha and ht are, respectively, the hour angles at dawn and sunset. Through a few passages we get:

Hex = 24/nIcs[1 + 0.0033cos(2n«/365)]

• (cos L cos d sen ha + hasen L send) (38)

where ha, in the last term, should be expressed in radians. The daily extraterrestrial radiation calculated using eqn (38) is expressed in watt-hour per square metres (W-h/m2) [1].