Page 54 - Solar Power in Building Design The Engineer's Complete Design Resource

24 SOLAR POWER TECHNOLOGIES

The chemical vaporization and crystallization process described here is energy
intensive and requires a considerable amount of electric power. To produce purified
silicon ingots at a reasonable price, in general, silicon ingot production plants are
located within the vicinity of major hydroelectric power plants, which produce an
abundance of low-cost hydroelectric power. Ingots produced from this process are in
either circular or square form and are cleaned, polished, and distributed to various
semiconductor manufacturing organizations.

Solar photovoltaic cell production The first manufacturing step in the production
of photovoltaic modules involves incoming ingot inspection, wafer cleaning, and quality
control. Upon completion of the incoming process, in a clean-room environment the
ingots are sliced into millimeter-thick wafers and both surfaces are polished, etched, and
diffused to form a PN junction. After being coated with antireflective film, the cells are
printed with a metal-filled paste and fired at high temperature. Each individual cell is
then tested for 100 percent functionality and is made ready for module assembly.

Photovoltaic module production The photovoltaic module production process
involves robotics and automatic controls where a series of robots assemble the solar
cells step by step: laying the modules, soldering the cells in a predetermined pattern,
and then laminating and framing the assembly as a finished product. Upon completion
of framing, each PV module is tested under artificial insolation conditions and the
results are permanently logged and serialized. The last step of production involves
a secondary module test, cleaning, packaging, and crating. In general, the efficiency
of the PV modules produced by this technique range from 15 to 18 percent. Figure 2.3
depicts silicon ingot inspection process.

Photovoltaic module life span and recycling To extend the life span of solar
power photovoltaic modules, PV cell assemblies are laminated between two layers of
protective covering. In general the top protective cover is constructed from 1/4- to
5/8-inch (in) tempered glass and the lower protective cover either from a tempered
glass or a hard plastic material. A polyurethane membrane is used as a gluing mem-
brane, which holds the sandwiched PV assembly together. In addition to acting as the
adhesive agent, the membrane hermetically seals the upper and lower covers prevent-
ing water penetration or oxidation. As a result of hermetical sealing, silicon-based PV
modules are able to withstand exposure to harsh atmospheric and climatic conditions.
Figure 2.4 depicts ingot production chamber.
Even though the life span of silicon-based PV modules is guaranteed for a period
of at least 20 years, in practice it is expected that the natural life span of the modules
will exceed 45 years without significant degradation. Figure 2.5 depicts fabricated
monosilicon solar cell inspection.
In order to minimize environmental pollution, SolarWorld has adopted a material
recovery process whereby obsolete, damaged, or old PV modules (including the alu-
minum framing, tempered glass, and silicon wafers) are fully recycled and reused to
produce new solar photovoltaic modules.