Most
commercially available photovoltaic solar modules are comprised
of individual cells made from silicon, connected in series and
laminated behind glass or plastic. Single-crystal silicon cells
are the most efficient. Polycrystalline (or multi-crystalline)
cells are slightly less efficient than single crystal cells. Thin-film
is 30% less efficient that single crystal. Efficiency is also
effected by cell coverage in a PV module.
Square
cells on these solar panels can be packed very closely, allowing
most of the module surface to generate power. Modules made with
round cells will have a lower cost, but the space between these
cells in a module is effectively wasted space, and causes the
module to have less power output for any given area. Some cells
are semi-round and will have an efficiency between round and square
cells. A solar panel made with square polycrystalline cells will
be slightly more efficient than one made with round single crystal
cells.
Module
efficiency determines the surface area that is required for any
given power output. The increased cost of higher efficiency PV
modules becomes beneficial if there are space constraints. Less
efficient modules will require more mounting structures, increasing
the balance-of-systems cost. This is most important when the modules
are mounted on tracking mounts.
Solar
panels come in various output voltages. The standard is 12VDC
nominal, but other single output voltages (6, 24, 51VDC) and dual
output voltage panels (6 and 12VDC, 12 and 24VDC) are also available.
When working with higher voltage, such as water pumping or utility-intertie
systems, higher voltage panels are convenient. Or if you need
to charge a 6 volt battery, the lower voltage panels are required.
We
sell PV modules in sizes that range from ½ watt to 300
watts. Most large power systems are built from arrays of modules
in the 75 to 165 watt range. This wattage is high enough to be
a significant part of the system and that the physical size is
not too large to lift and position. 40 to 55 watt modules are
a good choice for small systems such as small water pumping systems,
or in any system where less than 75 watts is required.
It
is important to use UL listed panels for NEC compliance in inspected
systems. However, UL listed panels generally do not differ from
unlisted panels. UL listing is expensive and may add to the panel
price. Class 1, Division 2 FM approval for explosion resistance
is required near gas and oil pipelines. A class A fire rating
may be required in some commercial rooftop applications.
The
mechanical specifications of PV modules are also important. Junction
boxes differ by their size, type of connectors and whether they
will accommodate conduit. Presently, many commercially available
panels have well-designed junction boxes. Some module manufacturers
offer interconnection cables that can simplify and expedite installation.
Some small modules (usually 30 watts or less) come with wire leads
and have no junction boxes.
Most
PV modules are framed with aluminum and have bolt holes that allow
them to be attached to mounting structures. There are some unique
frame options available that simplify mounting without racks for
RV, roof and flush mounting. (Some modules are also available
unframed for special applications.) Most manufacturers use tempered,
high light transmission glass that allows more of the sunlight
to the cells and breaks without shards.
The
encapsulant (the material sandwiched between the face glass and
PV cells) has been greatly improved since PV production began,
and discoloration and breakdown is no longer an issue in modern
modules. |
