Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 36

( )

cos ψ = cos α ϕ ϕ β α δ

ϕβα αϕδ ω
βαδ ω

sin - cos cos sin sin +

( )

sin cos sin + cos cos cos cos + (2.6)

h

sin sin cos sin h

Reception surface

ψ

α
90°

G b

α

Reception surface

ψ

90°

G b

ϕ

ϕ = α

Equator

Fig. 2.10 Geometric interrelationships of radiation incident on tilted surfaces (see /2-8/)

ω h can be calculated using True Solar Time ( TST ), resulting from the local time
LT (in h) according to international conventions (winter time) and the equation of
time E , which takes into account the perturbations in the earth's rate of rotation
which affect the time the sun crosses the observer's meridian, see Equation (2.7)
(according to /2-8/).

( )

( )

⋅ − with ω = LT ⋅ +⋅ 60 4 λ λ −+ E / 60 15 180

h 0

E = 229.2 (0.000075 + 0.001868 cos ⋅ ⋅ B - 0.032077 sin ⋅ B -

(2.7)

( ) ( )

− 0.014615 cos ⋅ 2 B - 0.04089 sin ⋅ 2 B )

) 360 (

Bn1 365 =−

n is the observed day of the year (1 ... 365), λ O the reference meridian (-15° at
Greenwich Mean Time (GMT), -30° at Central European Time (CET)) and λ the
longitude of the site. The declination of the sun δ , describing the angular distance
of the sun at its highest point from the equator of the sky, is calculated according
to Equation (2.8). It assumes values between -23.45° on December, 22 nd , and
+23.45° on June, 22 nd .