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Concentrated illumination
Increasing the illumination intensity gives a higher short circuit
current density which is not surprising. However, it is also an
important option for higher efficiency. Let us look again at the
equation for the open circuit voltage:
Concentrated illumination will actually increase the open circuit
voltage Voc. Simply by illumination with concentrated
light we can get higher efficiency out of any solar cell!
Levels of concentration
If we could concentrate enough to mach the short circuit current jsc
with the current prefactor j00 we could get an open
circuit voltage equivalent to the bandgap. This would require
concentration levels in the range of millions or even billions.
Simple geometrical considerations give an upper limit for concentration which
is much lower; the sun is not a point source but it appears as a disk with
an angular width just below half a degree. Maximum concentration is achieved
if we enlarge the disk so much that
it fills the full solid angle. If we go through the maths it will work out to
something around 46600. In this scenario we need a bifacial solar cell.
Flat cells see a hemisphere at best, resulting in a maximum concentration of
23300 times.
With high levels of concentration we do have a problem with getting the current
out of the cell. Concentration of 10000 or 1000 suns will create current densities of
around 400 or 40 A/cm2, respectively, from a Silicon solar cell. In order
to carry such currents with minimum loss we need quite big metal contacts. The
problem of shadowing was neatly circumvented by back surface contacted cells
where both, p- and n-contacts, are located on the back of the cell.
A still lower limit than the contacing is set by Auger recombination; due
to the photogeneration we get a lot of electron-hole pairs. These free
carriers are subject to Auger recombination and make the Fill Factor
trail off at concentration levels around 200 to 500 in most practical
concentrator cells.
Concentrator modules
Concentration is also an option for lower module prices because the
solar irradiation is a rather dilute form of energy. Conventional solar cells
made from bulk semiconductors require a lot of material which makes
them quite expensive. Concentration of light with Fresnel lenses or external
reflectors is easy and after concentration only small area solar cells
are required. The cost of the actual solar cell may even become secodary.
However, there are a few more things to consider:
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Concentrators work only for ideal alignment with respect to the sun
and it becomes necessary to follow the illumination. In case of
concentration by lenses this is done by mounting the module on a solar
tracker, in case of mirror fields the solar module remains stationary
but the mirrors have to follow the sun. In any case mechanical parts
are involved, so the amount of maintenance of the system will be
higher due to wear and tear.
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Concentration also means a higher thermal load. Remember, even if high
quality cells are used, about 70% or more of the incident power will heat
the cell. High concentration levels require active cooling!
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Concentration only works for direct sunlight. It will not be
advantageous in climates where clouds or overcast skies prevail. Ideal
conditions are typically found in deserts where the conditons may be
particularly harsh on the mechanics of trackers.
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