Solar cells
Last updated: December 10, 2009.
Why do we waste time drilling for oil
and shovelling coal when
there's a gigantic power station in the sky up above us, sending out
clean, non-stop energy for free? The Sun, a seething ball of
nuclear power, has enough fuel onboard
to drive the Universe for another five billion years—and solar panels
can turn this energy into an endless, convenient
supply of electricity.
Solar power might seem strange or futuristic, but it's already quite
commonplace. You might have a solar-powered quartz watch on your wrist or a
solar-powered pocket calculator. Many people have solar-powered lights
in their garden. Spaceships and satellites
usually have solar panels on them too. The American space agency NASA has even developed a solar-powered
plane! As global warming
continues to threaten our environment, there seems little doubt that solar power
will become an even more important form of renewable energy in future.
But how exactly does it work?
Photo: NASA's solar-powered Pathfinder airplane.
The upper wing surface is covered with lightweight solar panels that
power the plane's propellers.
Picture courtesy of Great
Images in NASA.
What is solar power? How much energy can we get from the Sun?
Solar power is amazing. On average, every square meter of Earth's
surface receives 164 watts of solar energy. In other words, you could
stand a really powerful (150 watt) table lamp on every square meter of
Earth's
surface and light up the whole planet with the Sun's energy! Or, to put
it another
way, if we covered just one percent of the Sahara desert with solar
panels, we could generate enough electricity
to power the whole world. That's the good thing about solar power:
there's an awful lot of it—much more than we could ever use.
But there's a downside too. The energy the Sun sends out arrives on
Earth as a mixture of light and heat. Both of these are incredibly
important—the light makes plants grow, providing us with food, while
the heat keeps us warm enough to survive—but we can't use either the
Sun's light or heat directly to run a television or a car. We have to
find some way of converting solar energy into other forms of energy we
can use more easily, such as electricity. And that's exactly what solar
panels do.
What are solar panels?
Photo: This typical solar panel is made of eight smaller panels, each of which
contains about three dozen separate solar cells. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
A solar panel is a large flat rectangle, typically somewhere between
the size of a radiator and the size of a door, made up of many
individual solar energy collectors called solar
cells covered with a
protective sheet of glass. The cells, each of which is about the size
of an
adult's palm, are usually octagonal and coloured bluish black. Just
like the cells in a battery, the cells in
a
solar panel are designed to generate electricity; but where a battery's
cells make electricity from chemicals, a solar panel's cells generate
power by capturing sunlight instead. They are sometimes called photovoltaic cells because they use sunlight
("photo" comes from the Greek word for light) to make electricity (the
word "voltaic" is a reference to electricity pioneer Alessandro Volta).
Photo: A single solar cell.
Picture by Rick Mitchell, courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Each cell generates a few volts of electricity and the panel
combines the energy they produce to make a bigger electric current and
voltage. The cells are made from silicon, a
very common chemical
element found in sand. When sunlight shines on a solar cell, the energy
it carries blasts electrons out of the silicon. These can be forced to
flow around an electric circuit and power anything that runs on
electricity. That's a pretty simplified explanation! Now let's take a
closer look...
A more detailed look at solar cells
Silicon is the stuff from which the transistors
(tiny switches) in microchips are made—and solar cells work in a
similar way.
Silicon is a type of material called a semiconductor.
Some materials, notably metals, allow electricity to flow through them
very easily; they are called conductors. Other materials, such as
plastics and wood, don't really let electricity flow through them at
all; they are called insulators. Semiconductors like silicon are
neither conductors nor insulators: they don't normally conduct
electricity, but under certain circumstances we can make them do so.
Photo: A colorful collection of solar cells.
Picture courtesy of NASA Glenn Research Center
(NASA-GRC).
A solar cell is a sandwich of two different layers of silicon that
have been specially treated or doped so they
will let electricity flow them in a particular way. The lower layer is
doped so it has slightly too few electrons. It's called p-type or positive-type silicon (because electrons
are negatively charged and this layer has too few of them). The upper
layer is
doped the opposite way to give it slightly too many electrons. It's
called n-type or negative-type silicon. (You
can read more about semiconductors and doping in our articles on transistors and
integrated circuits.)
When we place a layer of n-type silicon on a layer of p-type
silicon, a barrier is created at the junction of the two materials. No
electrons can cross the barrier so, even if we connect this silicon
sandwich to a flashlight, no current will flow: the bulb will not light
up. But if we shine light onto the sandwich, something remarkable
happens. We can think of the light as a stream of energetic "light
particles" called photons. As photons enter
our sandwich, they give up their energy to the atoms in the silicon.
The incoming energy knocks electrons out of the lower, p-type layer so
they
jump across the barrier to the n-type layer above and flow out around
the circuit. The
more light that shines, the more electrons jump up and the more current
flows.
This is what we mean by photovoltaic—light making voltage—and it's one kind of what
scientists call the photoelectric effect.
What is solar thermal power?
Photovoltaic cells, which make electricity from light, are not the
only way of making solar power. Many homes use solar-thermal
panels to make hot water instead of electricity. These work in a completely
different way that doesn't involve electricity at all. Although they
look similar to photovoltaic panels, solar thermal panels use a large
sheet of black glass to soak up the sun's heat. Water trickles through
the panels, warming up as it goes, before flowing into the home water
system and out through people's faucets (taps).
Many homes are also designed to heat themselves through what's known
as passive solar gain. They are built with
large areas of glass facing the Sun so they soak up as much sunlight as
possible. Solar gain is a useful way of reducing heating bills in
relatively cold parts of Europe and North America. In hot countries,
the real challenge is keeping the Sun out and keeping buildings cool.
So buildings tend to have thick walls and small windows to reduce the
solar gain (the amount of energy a building absorbs from the Sun).
Photo: Just right for hot weather! The electric
refrigerator on the right
is powered by the solar panels on the left.
Picture courtesy of NASA Glenn Research Center
(NASA-GRC).
Further reading
Books you can read
- Eyewitness Energy by Jack Challoner.
New York/London, England: Dorling Kindersley, 2000: Explains the basic concepts of energy and the history of how people have harnessed it.
- Energy by Chris Woodford.
New York/London, England: Dorling Kindersley, 2007: My colorful book about energy in the modern world.
- Power and Energy by Chris Woodford.
New York: Facts on File, 2004. Another of my books, this one covers humankind's efforts to harness energy.
Websites
