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Energy Payback and Harvest Factors
The PV manufacturing process requires energy. The highest energy requirement
for making cells is for crystalline cells and the glass. This is because of the high
temperatures needed in wafer and silicon and glass fabrication. The crystalliza-
tion of solar silicon currently requires about 10,000 kWh per kilowatt peak
(kWp). Kilowatt peak stands for peak power. This value specifies the output
power achieved by a solar module under full solar radiation under standard set
conditions (STC). Designers can reduce energy consumption when manufac-
turers produce silicon separately for PV systems.
Manufacturing thin-film modules requires only about half the amount of
energy that’s needed for making other types of modules. It takes about 1,000 kWh
per kWp to fabricate the aluminum frame for monocrystalline modules. It takes
about 2,000 kWh per kWp for amorphous silicon modules, which are thin-film
silicon cells manufactured by depositing layers of doped silicon on a substrate.
You can view the energy used to fabricate PV systems as a credit paid back
with the generation of solar power. An explanation of this repayment follows:
Presume a real system output for a particular system
Compare it with the energy produced by a traditional power station
Calculate the annual energy savings
Calculate amortization periods based on each installation site
The harvest factor indicates how much more energy the PV system produces
than was needed to generate it. This would consider the predicted service life of
the system. Given a service life of 30 years there is 5.3 to 11 times as much energy
produced as needed to fabricate the system.
Pollutants in the Production Process
Environmentally harmful substances occur in the production of silicon solar cells.
These substances are comparable to the harmful substances in the semiconductor
field. The substances occurring in solar cell production stay within closed cycles
due to strict regulations in some countries, though this is less true elsewhere.
Unlike the semiconductor field, which took years to develop and provide stan-
dards, the PV industry is moving actively to adopt standards to avoid pollu-
The completed silicon solar module does not have any damaging compo-
nents with the exception of the soldered joints and the use of some products like
176 INTRODUCTION TO PHOTOVOLTAICS