Environmental Engineering Reference
In-Depth Information
7.5.3
Photovoltaic Cells
Photovoltaic cells are solid-state devices that generate electric power when irradiated by solar light.
They provide an alternative method of electric power production to that of solar thermal systems
generating electric power by a heat engine supplied with heat from a solar collector. Like solar
thermal systems, only a fraction of the solar irradiance incident upon a solar cell can be converted to
electric power. Nevertheless, their mechanical and electrical simplicity, negligible operating cost,
and ability to produce power in any quantity makes the use of photovoltaic power systems very
attractive. A major obstacle to widespread use of photovoltaic cells is their current high cost per
unit of power output.
The process whereby the energy flux in solar radiation is converted to electrical power is
quantum mechanical in origin. The energy of sunlight is incorporated in packets of electromagnetic
radiation called photons, each of which possesses an energy of amount
hc
/λ
, where
h
is Planck's
constant,
c
is the speed of light, and
is the wavelength of light. Sunlight contains photons of
all wavelengths and energies, but most of the solar energy content lies in the wavelength range of
0.3-2.5
λ
m (see Figure 7.5). If a photon can deliver all its energy to an electron in a semiconductor,
for example, it can move the electron to a higher energy state of electric potential magnitude equal
to
hc
µ
, where
e
is the electron charge. For solar photons, the corresponding electric potential
range is 0.5-2.5 V. If the energetic electron can flow through an electric circuit while experiencing
this potential change, it can deliver electrical energy to an external load.
The components of a photovoltaic cell are sketched in Figure 7.13. The cell consists of a
base
layer of a
p
-type semiconductor, about 250
/
e
λ
µ
m in thickness, joined to an extremely thin
emitter
layer of an
n
-type semiconductor about 0.5
m thick that is exposed to the solar irradiance. Solar
radiation is absorbed mostly in the thin region close to the junction of the two materials. Current
generated by the cell is collected at positive and negative electrodes attached to the exterior of
µ
Emitter
Base
+
I
-
jA
V
Load
Figure 7.13
The elements of a photovoltaic cell of area
A
exposed to a solar irradiance
I
and supplying a
total current
jA
to an external load at a potential difference
V
. The current density in the cell is
j
A/m
2
.
