The document discusses zenith angles related to solar irradiance, particularly at London's latitude of 51.5° during different solstices and equinoxes. It explains how zenith angles affect the direct normal irradiance (DNI) received on surfaces angled perpendicularly to sun rays. Key calculations are provided to illustrate variations in solar energy capture due to changes in sun position relative to the Earth.

1. Examples of zenith angle:
London latitude f=51.5º
Ecliptic
plane
N
S
N
S
Winter solstice Summer solstice
23.5º
23.5º
51.5º
Zenith angle q =51.5º + 23.5º = 75º
Cos 75º = 0.26, Ib =351 W/m2
51.5º
q=28º
q=75º
Zenith angle q =51.5º - 23.5º=28º
Cos 29º = 0.88, Ib =1188 W/m2
sun
Vernal Equinox
Autumnal Equinox

2. Irradiance
Irradiance is the rate that radiant
energy arrives on a surface of specific
area at a particular time (W/m2)
Direct Normal Irradiance (DNI) is the
solar radiation received per unit area
on a surface that is perpendicular
(i.e. normal) to incoming solar rays
By maintaining a surface such that is
always normal to incoming radiation
we can maximise the energy captured
DNI (W/m2)
90º

3. Sun-earth geometry
N
δ
Sun rays
(DNI)
Equator
normal
ω
θ
The sun’s position relative to the earth is first described by the
earth’s rotation about its polar axis.
Latitude, ϕ
Hour angle, ω
declination, δ
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