Solar Energy Perspectives - IEA

Solar Energy Perspectives: The solar resource and its possible uses
may influence cloud cover and reduce clarity, and the potential of the solar energy resource
in different regions of the globe. Current models suggest, however, that monthly average
changes in solar fluxes will remain very low – though this is not necessarily true with respect
to direct normal irradiance.
Two basic ways to capture the sun’s energy
Solar rays can be distinguished according to their wavelengths, which determine visible light,
infrared and ultraviolet radiation. Visible light constitutes about 40% of the radiated energy,
infrared 50% and ultraviolet the remaining 10%. Most of the infrareds are “near infrared” or
“short-wave infrared” rays, with wavelengths shorter than 3 000 nanometres, so they are not
considered “thermal radiation”.
The sun’s primary benefit for most people is light, the use of which can be improved in
buildings to reduce energy consumption. This area of development, called day lighting, is
one of the avenues to reducing energy consumption in buildings.
Solar irradiative energy is easily transformed into heat through absorption by gaseous, liquid
or solid materials. Heat can then be used for comfort, in sanitary water heating or pool water
heating, for evaporating water and drying things (notably crops and food), and in space
heating, which is a major driver of energy consumption. Heat can also be transformed into
mechanical work or electricity, and it can run or facilitate chemical or physical transformations
and thus industrial processes or the manufacture of various energy vectors or fuels, notably
hydrogen.
However, solar radiation can also be viewed as a flux of electromagnetic particles or photons.
Photons from the sun are highly energetic, and can promote photoreactions such as in
photosynthesis and generate conduction of electrons in semiconductors, enabling the
photovoltaic conversion of sunlight into electricity. Other photoreactions are also being used,
for example photocatalytic water detoxification.
Note that the two fundamental methods to capture energy from the sun – heat and
photoreaction – can also be combined in several ways to deliver combined energy vectors –
e.g. heat and electricity.
Thus, from the two basic ways of capturing the sun’s energy, apart from day lighting, i.e. heat
and photoreaction, we distinguish four main domains of applications: photovoltaic electricity,
heating (and cooling), solar thermal electricity, and solar fuel manufacture. The relevant
technologies are detailed in Chapters 6 to 9 of this publication.
How this resource varies
Although considerable, the solar resource is not constant. It varies throughout the day and
year, and by location. For a large part, these variations result directly from the earth’s
geography and its astronomical movements (its rotation towards the East, and its orbiting the
sun). But these variations are accentuated and made somewhat less foreseeable from day to
day by the interplay between geography, ocean and land masses, and the ever-changing
composition of the atmosphere, starting with cloud formation.
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