Solar cells are also called “photovoltaic cells” or simple PVs; they get this
name from the fact that photons (light) interact in the cell to caused electrons to
flow to metal plates, so they produce a voltage. We’ll discuss how they do this in
the next chapter, on quantum physics. Traditional solar cells were based on
crystals of silicon, and they typically converted about 10% of the sunlight to
electricity. For a 1 Gigawatt plant, when the sun is directly overhead, that would
require 10 square kilometers of these cells. Traditional solar cells cost about $3
per installed watt, and so are not really considered competitive with fossil fuels.
These are the kinds being installed by homeowners, sometimes because they think
they are saving money, but more often because they want to reduce the CO2
that they are personally responsible for. But the field is advancing rapidly.
One of the truly hopeful developments in recent years has been the
development of highly efficient solar cells. These are complex devices because
extracting as much energy as possible out of sunlight requires having separate
layers to convert different colors. These sophisticated solar cells are now being
built, and one major producer, Boeing (yes, the airplane company; it started
producing solar cells when they were needed for space) is selling solar cells that
convert 41% of the incident sun power to electric power. They say that the
efficiency should rise to 45% in the near future. Wow!
There is a catch, of course. Even when purchased in large quantities, these
special cells cost about $10 per square centimeter; that’s equal to $70 per square
inch, or about $10,000 per square foot. A foot-sized cell would yield 41 watts—
not much for the $10,000 investment. Why do I call this hopeful? The reason is
that sunlight (if there are no clouds) can be focused using a lens or mirror. You can
make a plastic lens that is 1 foot square for less than $1 and use it to focus the
sunlight onto a cell 0.4 inch on a side. A cell of that size costs $10. Your total cost
for the 41 watts is now reduced to $10, plus $1 for the lens, plus whatever you
spend to build the module. That sounds very attractive. The tricky part is that you
have to keep the cell pointed at the sun, and that requires a mechanical system. If
our goal is to spend no more than $1 per installed watt, then the total cost for the
square-foot device must be less than $41. Can that be done? It is not obviously
impossible, and several companies in California are already building such systems
to see if they can be cost-effective. Even if it costs three times that, this system still
becomes the cheapest form of solar power.
This approach is called solar concentrator technology. Its greatest drawback
is that it works only on sunny days, when the sun is visible and its rays can be
focused. Imagine now an array of foot-sized concentrator solar cells covering a
square mile of sunny Nevada. Since there are 5280 feet in a mile, there would be
5280 5280 = 27,878,400 modules. Each module would be only a foot high,
making the system quite robust against wind. Driven by tiny electric motors, the
modules would all point in the same direction: toward the sun. With 41 watts from
each, the total electric power output at midday would be over a gigawatt. Of
course, there may be other expenses, such as keeping the reflectors clean. In a
recent trip to Nevada I found that much of the region I visited had over a foot of
“bug dust” that whirled around every time the wind blew.
Another hopeful developing are cheap solar cells made without growing
crystals; these are called “amorphous” (non-crystalline) cells. There is much
excitement over a particular kind called CIGS. (The letters stand for the elements
that go into the material: Copper Indium Gallium and Selenium.) CIGS are
manufactured using a technique similar to that of an ink-jet printer: they are
basically sprayed on a piece of plastic. CIGS have already achieved an efficiency
of 19%, and enough people are convinced of their future that factories costing
hundreds of million of dollars are under construction to build CIGS cells. For
business reasons, many of the details have not yet been released; as of April 2008
(when I am writing this) these companies have not released a public number for
the price at which they will sell their cells. The price may be determined by
competition, since sales of the cells will have to pay back the huge investment
being put into these plants. And people worry that with huge numbers of solar
cells being built, that the world will not be able to supply enough gallium for the
cells! But optimism in the solar energy business is rampant. Many investors are
jumping in. They believe the future of solar is sunny.
