Solar Panel Physics
Before we compare and contrast different panel types it is necessary to briefly look at
the solar energy spectrum, and how this interacts with solar glass.
What is the Solar Spectrum?
Sunlight, in the broad sense, is the total frequency spectrum
of electromagnetic radiation given off by the Sun. On Earth, sunlight has been filtered
through the Earth's atmosphere, and solar radiation is obvious as daylight
when the Sun is above the horizon. Although we experience daylight as white light,
it is infact made up of a band of colours, some visible and some invisible.
Solar Radiation on the Earths Surface
The athmosphere absorbs some of this radiation. But it doesn't do it evenly.
As can be seen by the graph below, there are areas within the spectrum where water vapour,
CO2 and even O2 absorb/reflect a particular wavelenght. This is known as Rayleigh scattering. In addition
other mechanisms absorb/reflect energy, namely;
- Reflection back into space by the atmosphere
- Absorption of energy by various molecules in the atmosphere (see the marked dips in the graph below)
- Mie scattering by dust and pollutants in the air
- Rayleigh scattering by interaction with the air molecules
The spectrum outside the atmosphere is often termed AM0 air mass 0.
At the equator with the sun directly overhead the reduction in light intensity
due to passing this shortest distance through the atmosphere is termed AM1.
Over the course of our year the air mass in Ireland would vary from a value of about 1.2 in mid-June
to about 4 in mid-December. Thus the reduction or attenuation factor due to the Sun'
radiation travelling through the atmosphere and being absorbed and scattered,
would give have a value of between 20-40% in June and about 50-85% in December.
The Mie scattering component is particularly susceptible to pollution in industrial
areas and is primarily responsible for the variations in these total attenuation values.
The clean air that sweeps in over Ireland and Britain usually keeps Mie
attenuation at the low end of the values given, except around major cities
or when air masses stagnate over the country for apreciable times.
The graph above gives an impression of the effect of these factors across the
wavelengths of radiation in the Sun's spectrum.
One very interesting thing to note on this graph.
Our eyes have evolved to take advantage of the most energy rich (i.e. visible)
part of the sprectrum, and secondly, the significant amount of energy in the
near infrared part of the graph. Good solar panel glazing takes advantage of as much
of this energy as possible by letting through Ultraviolet and the Near Infrared parts
of the sprectrum. Specifically low iron content in the glass helps the performance.
Historically ordinary window glass was not manufactured to be transparent
to the non-visible parts of the spectrum hence has worse performance than solar glass.
As eluded to in the history page,
Clear glass has a peculiar property: it easily allows sunshine to pass through,
but inhibits thermal radiation from doing the same. Basically the visible part of the spectrum
passes through the plate glass. It heats up the solar panel and because even a very hot solar panel is much cooler than
the surface of the sun, it tries to emit radiation with a (huge) wavelenght above 3,000nm. This wavelenght is
so big that it cannot pass between the atomic spaces of the glass and so is trapped inside.