Solar Energy Perspectives - IEA

Chapter 2: The solar resource and its possible uses
All places on earth have the same 4 380 hours of daylight hours per (non-leap) year, i.e. half
the total duration of a year. However, they receive varying yearly average amounts of energy
from the sun. The earth’s axis of rotation is tilted 23.45° with respect to the ecliptic – the plane
containing the orbit of the earth around the sun. The tilting is the driver of seasons. It results
in longer days, and the sun being higher in the sky, from the March equinox to the September
equinox in the northern hemisphere, and from the September equinox to the March equinox
in the southern hemisphere.
When the sun is lower in the sky, its energy is spread over a larger area, and is therefore
weaker per surface area. This is called the “cosine effect”. More specifically, supposing no
atmosphere, in any place on a horizontal surface the direction of the sun at its zenith forms
an angle with the vertical. The irradiance received on that surface is equal to the irradiance
on a surface perpendicular to the direction of the sun, multiplied by the cosine of this angle
(Figure 2.3).
Figure 2.3 The cosine effect
Surface normal
Surface A.
parallel to earth
? x
? x
l
?
Hypothetical surface B.
normal to sun’s rays
l
Limits of earth’s
atmosphere
lcosè
Note: As a plate exposed to the sun tilts, the energy it receives varies according to the cosine of the tilt angle.
Source: Stine and Geyer, 2011 (left).
Solar_Figure_02.03
Key point
Solar irradiance is maximal when the sun is directly overhead.
Tilting also leads to definition of two imaginary lines that delineate all the areas on the
earth where the sun reaches a point directly overhead at least once during the solar year.
These are the tropics, situated at 23.45° latitude on either side of the equator. Tropical
zones thus receive more radiation per surface area on yearly average than the places that
are north of the Tropic of Cancer or south of the Tropic of Capricorn. Independent of
atmospheric absorption, the amount of available irradiance thus declines, especially in
winter, as latitudes increase. The average extraterrestrial irradiance on a horizontal plane
depends on the latitude (Figure 2.4). Irradiance varies over the year at diverse latitudes –
very much at high latitudes, especially beyond the polar circles, and very little in the
tropics (Figure 2.5).
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© OECD/IEA, 2011