Tidal Current Energy

228 D. Infield
Spectral irradiance (W m �2 �m �1 )
2000
1500
1000
500
Figure 13.4 . The AM1.5G solar spectrum.
Incident on earth’s surface
Outside atmosphere
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Wavelength/�m
AM1 represents the shortest path through the atmosphere, i.e. normal to the
earth’s surface, and AM x refers to x times this shortest path. Formally this is calculated
as 1/cos ( θ ), with θ the angle between the path in question and the normal
to the surface. Also by convention, the European standard air mass value
used for cell and module testing is AM1.5. After adding the diffused light intensity
this becomes AM1.5G (where G stands for global). Spectra corresponding to
this and AM0 are shown in Figure 13.4 .
A comprehensive account of the solar resource can be found in Ref. [1] ,
together with the formulae for undertaking calculations of the radiation falling
on differently oriented surfaces at different times of day and year, and at different
locations.
3 . Outline of the Conversion Process
All photovoltaic devices convert incident photons to free charge carriers through
the process of absorption. The heart of a photovoltaic device is a p–n junction
made from the doping of semiconductors, explained later in the section. In semiconductors,
which are by far the most common PV materials, the process starts
with an incoming photon promoting a valence electron to the conduction band of
the semiconductor in question, thus creating electron–hole pairs. These electrons
and holes are then separated by the existing electric field at the p–n junction
and get collected at the end contacts to deliver open circuit voltage. This process
is called photovoltaics. In dye cells and organic polymer devices the process
is more complicated. The solar cell action contrasts to free carrier transport in
a conventional semiconductor p–n junction in that the charge transfer is mediated
via excitons or polarons, which are a kind of bound form of electron–hole
pair as a neutral entity or particle. The so-called exciton that results from photon
absorption in a conjugated polymer or in a dye system then dissociates into free