Page 105 - Applied Photovoltaics

12. Bypass diode failure—Bypass diodes used to overcome cell mismatching
problems can themselves fail, usually due to overheating and often due to
undersizing (Durand, 1994). The problem is minimised if junction
temperatures are kept below 128°C.

13. Encapsulant failure—UV absorbers and other encapsulant stabilisers ensure
a long life for module encapsulating materials. However, slow depletion, by
leaching and diffusion, does occur and, once concentrations fall below a
critical level, rapid degradation of the encapsulant materials occurs. In
particular, browning of the EVA layer, accompanied by a build-up of acetic
acid, has caused gradual reductions in the output of some arrays, especially
those in concentrating systems (Wenger et al., 1991; Czanderna & Pern,
1996; King et al., 2000), although recent improvements in EVA
photostability have reduced this problem.

5.12 EMBODIED ENERGY AND LIFE CYCLE ISSUES
Early solar cells required significant levels of material use and processing energy.
This raised the issue of net energy production by photovoltaics. Modern commercial
products are higher in efficiency than the early cells, production techniques minimise
material use and wastage, while energy use has become more efficient, so that there is
no longer any question that photovoltaic cells pay back their manufacturing energy in
the early years of their operation. Effort is now being directed to developing
manufacturing methods that facilitate module recycling (Wambach, 2004; Arai et al.,
2004).
A number of methods, including process analyses or input-output techniques, are
used to calculate and report on the energy used to manufacture components and
modules, which is referred to as the embodied energy of manufacture (E man ). This is
the primary energy requirement during module manufacture, including materials
mining.
Life cycle analysis (LCA) techniques (sometimes known as cradle-to-grave analyses)
are also used to follow both the materials and the energy flow through manufacturing,
operation and end-of-life. Methodologies used are covered under the International
Standards Organisation Life Cycle Assessment standard ISO 14040. For energy use,
E input can be defined as the sum of primary energy requirements for manufacturing
(E man ), transport (E trans ), installation (E inst ), operation (E use ) and decommissioning
(E decomm ) (Kato, 2000)

E E E E E E (5.4)
input man trans inst use decomm
For each year the PV module is in operation, it will displace a certain amount of
energy that would otherwise have been used for electricity generation. This is referred
to as E gen and will vary with site and fuel displaced. The term energy payback time
(EPT or EPBT) refers to the time taken for the input energy to be repaid by PV
generation. Hence

E input
EPT (5.5)
E
gen

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