Environmental Engineering Reference
In-Depth Information
Table 5.2
Important parameters for photovoltaic modules.
Parameter
Symbol
Unit
Description
Open-circuit
voltage
V
OC
Volt, V
Voltage of PV module in open-circuit
operation without connected load
Short-circuit
current
I
SC
Ampere, A
Current of PV module in short-circuit
operation with short-circuited module
MPP voltage
V
MPP
Volt, V
Voltage when a photovoltaic module
produces maximum power
MPP current
I
MPP
Ampere, A
Current belonging to MPP voltage
MPP power
P
MPP
Watt, W
Maximum power a PV module can produce
Standard test conditions (STC) for the comparison of photovoltaic modules have
been agreed internationally. Accordingly, the MPP power of solar cells and modules
is determined on the basis of solar radiation intensity of 1000 watts per square metre
and a module temperature of 25 °C. As radiation intensity is usually lower in practice
and photovoltaic modules can warm up to 60 °C in the summer, the MPP power
determined on the STC basis represents a maximum value. This value is achieved
in very few cases and seldom exceeded. Therefore, this power also has the unit watt
peak, abbreviated W
p
.
5.2 Production of Solar Cells - from Sand to Cell
5.2.1 Silicon Solar Cells - Electricity from Sand
Silicon, the raw material for computer chips and solar cells, is the second most
common element in the earth's crust after oxygen. However, in nature silicon occurs
almost exclusively as an inclusion in quartz, sand and silicate rock or as silicic acid
in the world's oceans. Even the human body contains around 20 milligrams of
silicon per kilogram of body weight.
Pure silicon is usually extracted from quartz sand. Chemically, quartz sand is pure
silicon dioxide (SiO
2
). For silicon to be produced from it, high temperatures are
used to sequester the oxygen atoms (O
2
). This process is called reduction and is
carried out in arc furnaces at temperatures of around 2000 °C. The result is industrial
raw silicon with a purity of 98 to 99%.
Raw silicon has to be purifi ed further before it can be used to produce solar
cells. The Siemens method is the normal procedure followed. Hydrogen chloride is
used to convert the raw silicon into trichlorosilane, which is then distilled. At high
temperatures of 1000 to 1200 °C the silicon is then separated again into long
bars. The polycrystalline solar silicon produced in this way has 99.99% purity
(Figure 5.4 ).
