howzit work? | answered here with pictures
concentrating photovoltaics By SETh MASIA
It’s all done with mirrors and super-efficient solar cells.
primary mirror secondary mirror
iLLustrAtion By kurt struve
to dissipate heat
One way to reduce the cost of solar power is to reduce the mass of expensive photovoltaic (PV) material needed in a module. This can involve using lenses or mirrors to focus sunlight onto a small
PV cell. The technology is called concentrating photovoltaics, or CPV.
Shown here is the CPV system developed by SolFocus in Mountain
View, Calif. ( solfocus.com). SF-1100 modules consist of small indium
gallium arsenide phosphide (InGaAsP) triple-junction PV cells, each
measuring one square centimeter, placed at the focal point of what
amounts to a reflecting telescope. The parabolic mirror system concentrates sunlight by a factor of 650. InGaAsP cells, developed for powering
space satellites, are more efficient than monocrystalline silicon. They
convert sunlight at roughly 39 percent efficiency, compared to about
19 percent for the best silicon. Equally important, InGaAsP efficiency
doesn’t degrade with high temperatures as it does with silicon PV cells.
For example, at 40°C (104°F) ambient, a polysilicon PV panel will only
perform at 79 percent of its rated power, whereas InGaAsP cells will
perform at 96 percent of their rated power.
SolFocus systems engineer Dr. Steffen Jensen won’t reveal how much
heat is involved. Total heat absorbed can’t exceed the same 1 kilowatt per
square meter absorbed by all solar modules, so total heat in the system is
not the issue, only the transfer of concentrated heat out of the InGaAsP
cells. For example, Jensen says that in the SolFocus design, a 37 percent-efficient cell will produce about 15 watts; the math suggests that each cell
may absorb 50 to 55 watts of solar energy. That’s equivalent to an automobile headlight. A modern 55-watt halogen headlamp bulb operates
at about 250°C (482°F) to 500°C (932°F), well within the engineering
specifications of a glass lens with coated aluminum reflectors. In SolFocus
modules, this heat is dissipated into the aluminum back-plate, shaped like
a muffin tin. Cooling is therefore passive, with no fans, pumps or motors,
improving reliability and cost.
Externally, each telescope looks like a high-performance headlight.
The inside is more complex, with primary and secondary mirrors and an
optical rod to conduct light to the InGaAsP cell. Power units are ganged
20 to a module. Each 32-kilogram (70-pound) module has a nominal
capacity of 300 watts at 50 volts DC. Because focus depends on aiming
at the sun with an accuracy within about 1 degree of arc, modules are
mounted on two-axis trackers. In utility-scale installations, SolFocus
has been mounting 28 modules on a tracker — roughly a metric ton,
producing 8. 4 kilowatts at peak power. In September, SolFocus signed
an agreement to supply 8. 5 megawatts for a utility in Portugal. ST