What is renewable energy?
The United States currently relies heavily
on coal, oil, and natural gas for its energy.
Fossil fuels are non-renewable, that is,
they draw on finite resources that will
eventually dwindle, becoming too expensive
or too environmentally damaging to retrieve.
In contrast, renewable energy resources-such
as wind and solar energy-are constantly
replenished and will never run out.
Most renewable energy comes either directly
or indirectly from the sun. Sunlight, or
solar energy, can be used directly for heating
and lighting homes and other buildings,
for generating electricity, and for hot
water heating, solar cooling, and a variety
of commercial and industrial uses.
The sun's heat also drives the winds, whose
energy is captured with wind turbines. Then,
the winds and the sun's heat cause water
to evaporate. When this water vapor turns
into rain or snow and flows downhill into
rivers or streams, its energy can be captured
using hydroelectric power.
Along with the rain and snow, sunlight causes
plants to grow. The organic matter that
makes up those plants is known as biomass.
Biomass can be used to produce electricity,
transportation fuels, or chemicals. The
use of biomass for any of these purposes
is called biomass energy.
Hydrogen also can be found in many organic
compounds, as well as water. It's the most
abundant element on the Earth. But it doesn't
occur naturally as a gas. It's always combined
with other elements, such as with oxygen
to make water. Once separated from another
element, hydrogen can be burned as a fuel
or converted into electricity.
Not all renewable energy resources come
from the sun. Geothermal energy taps the
Earth's internal heat for a variety of uses,
including electric power production, and
the heating and cooling of buildings. And
the energy of the ocean's tides comes from
the gravitational pull of the moon and the
sun upon the Earth.
In fact, ocean energy comes from a number
of sources. In addition to tidal energy,
there's the energy of the ocean's waves,
which are driven by both the tides and the
winds. The sun also warms the surface of
the ocean more than the ocean depths, creating
a temperature difference that can be used
as an energy source. All these forms of
ocean energy can be used to produce electricity.
re layers of different materials with different
band gaps. The higher band gap material
is on the surface, absorbing high-energy
photons while allowing lower-energy photons
to be absorbed by the lower band gap material
beneath. This technique can result in much
higher efficiencies. Such cells, called
multi-junction cells, can have more than
one electric field.
Why is renewable energy important?
Renewable energy is important because of the benefits it provides. The key benefits are:
Environmental benefits - Renewable energy technologies are clean sources of energy that have a much lower environmental impact than conventional energy technologies.
Energy for our children's children's children - Renewable energy will not run out. Ever. Other sources of energy are finite and will some day be depleted.
Jobs and the economy - Most renewable energy investments are spent on materials and workmanship to build and maintain the facilities, rather than on costly energy imports. Renewable energy investments are usually spent within the United States, frequently in the same state, and often in the same town. This means your energy dollars stay home to create jobs and fuel local economies, rather than going overseas. Meanwhile, renewable energy technologies developed and built in the United States are being sold overseas, providing a boost to the U.S. trade deficit.
Energy security - After the oil supply disruptions of the early 1970s, our nation has increased its dependence on foreign oil supplies instead of decreasing it. This increased dependence impacts more than just our national energy policy.
how solar energy is captured for lighting products
You've probably seen calculators that have solar cells -- calculators that never need batteries, and in some cases don't even have an off button. As long as you have enough light, they seem to work forever. You have probably also been hearing about the "solar revolution" for the last 20 years -- the idea that one day we will all use free electricity from the sun. This is a seductive promise: On a bright, sunny day, the sun shines approximately 1,000 watts of energy per square meter of the planet's surface, and if we could collect all of that energy we could easily power our homes and offices for free. But, to start with, solar lighting for around your home and garden makes great sense as it is so fast and easy to install with no wires and no electricity costs. Its safe, simple and we have many designs to choose from. For those technically minded or you simply want to know more about solar energy and how it is created then read on...
Converting Photons to Electrons
The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power your house. How do these devices work?
Photovoltaic (PV) cells are made of special
materials called semiconductors such as
silicon, which is currently the most commonly
used. Basically, when light strikes the
cell, a certain portion of it is absorbed
within the semiconductor material. This
means that the energy of the absorbed light
is transferred to the semiconductor. The
energy knocks electrons loose, allowing
them to flow freely. PV cells also all have
one or more electric fields that act to
force electrons freed by light absorption
to flow in a certain direction. This flow
of electrons is a current, and by placing
metal contacts on the top and bottom of
the PV cell, we can draw that current off
to use externally. For example, the current
can power a calculator. This current, together
with the cell's voltage (which is a result
of its built-in electric field or fields),
defines the power (or wattage) that the
solar cell can produce.
That's the basic process, but there's really
much more to it. Let's take a deeper look
into one example of a PV cell: the single
crystal silicon cell.
Silicon in Solar Cells
Silicon has some special chemical properties,
especially in its crystalline form. An atom
of silicon has 14 electrons, arranged in
three different shells. The first two shells,
those closest to the center, are completely
full. The outer shell, however, is only
half full, having only four electrons. A
silicon atom will always look for ways to
fill up its last shell (which would like
to have eight electrons). To do this, it
will share electrons with four of its neighbor
silicon atoms. It's like every atom holds
hands with its neighbors, except that in
this case, each atom has four hands joined
to four neighbors. That's what forms the
crystalline structure, and that structure
turns out to be important to this type of
PV cell.
We've now described pure, crystalline silicon.
Pure silicon is a poor conductor of electricity
because none of its electrons are free to
move about, as electrons are in good conductors
such as copper. Instead, the electrons are
all locked in the crystalline structure.
The silicon in a solar cell is modified
slightly so that it will work as a solar
cell.
Our cell has silicon with impurities --
other atoms mixed in with the silicon atoms,
changing the way things work a bit. We usually
think of impurities as something undesirable,
but in our case, our cell wouldn't work
without them. These impurities are actually
put there on purpose. Consider silicon with
an atom of phosphorous here and there, maybe
one for every million silicon atoms. Phosphorous
has five electrons in its outer shell, not
four. It still bonds with its silicon neighbor
atoms, but in a sense, the phosphorous has
one electron that doesn't have anyone to
hold hands with. It doesn't form part of
a bond, but there is a positive proton in
the phosphorous nucleus holding it in place.
When energy is added to pure silicon, for
example in the form of heat, it can cause
a few electrons to break free of their bonds
and leave their atoms. A hole is left behind
in each case. These electrons then wander
randomly around the crystalline lattice
looking for another hole to fall into. These
electrons are called free carriers, and
can carry electrical current. There are
so few of them in pure silicon, however,
that they aren't very useful. Our impure
silicon with phosphorous atoms mixed in
is a different story. It turns out that
it takes a lot less energy to knock loose
one of our "extra" phosphorous
electrons because they aren't tied up in
a bond -- their neighbors aren't holding
them back. As a result, most of these electrons
do break free, and we have a lot more free
carriers than we would have in pure silicon.
The process of adding impurities on purpose
is called doping, and when doped with phosphorous,
the resulting silicon is called N-type ("n"
for negative) because of the prevalence
of free electrons. N-type doped silicon
is a much better conductor than pure silicon
is.
Actually, only part of our cell is N-type.
The other part is doped with boron, which
has only three electrons in its outer shell
instead of four, to become P-type silicon.
Instead of having free electrons, P-type
silicon ("p" for positive) has
free holes. Holes really are just the absence
of electrons, so they carry the opposite
(positive) charge. They move around just
like electrons do.
So where has all this gotten us? The interesting
part starts when you put N-type silicon
together with P-type silicon. Remember that
every PV cell has at least one electric
field. Without an electric field, the cell
wouldn't work, and this field forms when
the N-type and P-type silicon are in contact.
Suddenly, the free electrons in the N side,
which have been looking all over for holes
to fall into, see all the free holes on
the P side, and there's a mad rush to fill
them in.
Before now, our silicon was all electrically
neutral. Our extra electrons were balanced
out by the extra protons in the phosphorous.
Our missing electrons (holes) were balanced
out by the missing protons in the boron.
When the holes and electrons mix at the
junction between N-type and P-type silicon,
however, that neutrality is disrupted. Do
all the free electrons fill all the free
holes? No. If they did, then the whole arrangement
wouldn't be very useful. Right at the junction,
however, they do mix and form a barrier,
making it harder and harder for electrons
on the N side to cross to the P side. Eventually,
equilibrium is reached, and we have an electric
field separating the two sides.
This electric field acts as a diode, allowing
(and even pushing) electrons to flow from
the P side to the N side, but not the other
way around. It's like a hill -- electrons
can easily go down the hill (to the N side),
but can't climb it (to the P side).
So we've got an electric field acting as
a diode in which electrons can only move
in one direction. Let's see what happens
when light hits the cell.
When Light Hits the Cell
When light, in the form of photons, hits
our solar cell, its energy frees electron-hole
pairs.
Each photon with enough energy will normally
free exactly one electron, and result in
a free hole as well. If this happens close
enough to the electric field, or if free
electron and free hole happen to wander
into its range of influence, the field will
send the electron to the N side and the
hole to the P side. This causes further
disruption of electrical neutrality, and
if we provide an external current path,
electrons will flow through the path to
their original side (the P side) to unite
with holes that the electric field sent
there, doing work for us along the way.
The electron flow provides the current,
and the cell's electric field causes a voltage.
With both current and voltage, we have power,
which is the product of the two.
How much sunlight energy does our PV cell
absorb? Unfortunately, the most that our
simple cell could absorb is around 25 percent,
and more likely is 15 percent or less. Why
so little?
Energy Loss
Why does our solar cell absorb only about
15 percents of the sunlight's energy? Visible
light is only part of the electromagnetic
spectrum. Electromagnetic radiation is not
monochromatic -- it is made up of a range
of different wavelengths, and therefore
energy levels.
Light can be separated into different wavelengths,
and we can see them in the form of a rainbow.
Since the light that hits our cell has photons
of a wide range of energies, it turns out
that some of them won't have enough energy
to form an electron-hole pair. They'll simply
pass through the cell as if it were transparent.
Still other photons have too much energy.
Only a certain amount of energy, measured
in electron volts (eV) and defined by our
cell material (about 1.1 eV for crystalline
silicon), is required to knock an electron
loose. We call this the band gap energy
of a material. If a photon has more energy
than the required amount, then the extra
energy is lost (unless a photon has twice
the required energy, and can create more
than one electron-hole pair, but this effect
is not significant). These two effects alone
account for the loss of around 70 percent
of the radiation energy incident on our
cell.
Why can't we choose a material with a really
low band gap, so we can use more of the
photons? Unfortunately, our band gap also
determines the strength (voltage) of our
electric field, and if it's too low, then
what we make up in extra current (by absorbing
more photons), we lose by having a small
voltage. Remember that power is voltage
times current. The optimal band gap, balancing
these two effects, is around 1.4 eV for
a cell made from a single material.
We have other losses as well. Our electrons
have to flow from one side of the cell to
the other through an external circuit. We
can cover the bottom with a metal, allowing
for good conduction, but if we completely
cover the top, then photons can't get through
the opaque conductor and we lose all of
our current (in some cells, transparent
conductors are used on the top surface,
but not in all). If we put our contacts
only at the sides of our cell, then the
electrons have to travel an extremely long
distance (for an electron) to reach the
contacts. Remember, silicon is a semiconductor
-- it's not nearly as good as a metal for
transporting current. Its internal resistance
(called series resistance) is fairly high,
and high resistance means high losses. To
minimize these losses, our cell is covered
by a metallic contact grid that shortens
the distance that electrons have to travel
while covering only a small part of the
cell surface. Even so, some photons are
blocked by the grid, which can't be too
small or else its own resistance will be
too high.
Finishing the Cell
There are a few more steps left before we
can really use our cell. Silicon happens
to be a very shiny material, which means
that it is very reflective. Photons that
are reflected can't be used by the cell.
For that reason, an antireflective coating
is applied to the top of the cell to reduce
reflection losses to less than 5 percent.
The final step is the glass or plastic cover
plate that protects the cell from the elements.
PV modules are made by connecting several
cells (usually 36) in series and parallel
to achieve useful levels of voltage and
current, and putting them in a sturdy frame
complete with a glass cover and positive
and negative terminals on the back.
