Environmental Engineering Reference
In-Depth Information
However, not every light particle ensures that an electron is separated from the hole.
If the energy of the photon is too low, the electron will fall back into the hole. On
the other hand, if the energy of the photon is too high, only part of it is used to
separate the electron from the hole. Some photons also move through the solar cell
unused; others are refl ected by the front contacts.
5.1.2 Effi ciency, Characteristics and MPP
The effi ciency of a solar cell specifi es which part of the solar radiant power is con-
verting the cell into electric power.
Solar Cell Effi ciency
P
electrical power output
incoming solar radiant
⎛
⎜
⎞
⎟
el
Φ
η =
efficiency
=
power
The higher the effi ciency, the more electric power the solar cell can generate per
square metre. In addition to the type of materials used, the quality of the manufac-
turing also plays a major role. Today silicon cells in mass production reach a
maximum effi ciency of over 20%. Close to 25% effi ciency has already been reached
in laboratories (Table 5.1 ).
Table 5.1
Effi ciency of different solar cells.
Cell material
Max. cell
effi ciency
(lab)
Max. cell
effi ciency
(mass prod.)
Typical
module
effi ciency
Surface
req. for
1 kW
p
6.7 m
2
Monocrystalline silicon
24.7%
22.0%
15%
7.2 m
2
Polycrystalline silicon
20.3%
17.4%
14%
16.7 m
2
Amorphous silicon
12.1%
6.8%
6%
CIS/CIGS
20.0%
11.6%
10%
10.0 m
2
CdTe
16.5%
12.0%
7%
14.3 m
2
Concentrator cells
41.1%
36.5%
28%
3.6 m
2
Incidentally, conventional petrol engines do not reach a higher level of effi ciency
than silicon cells. Compared to the 5% effi ciency of the fi rst solar cells in 1954,
technology has advanced considerably. If individual solar cells are packaged into
photovoltaic modules, the effi ciency drops somewhat due to the space necessary
between the cells and the module frames. It is hoped that other materials can be
used in future to produce further cost savings. In comparison with silicon cells, more
