Oh, man.
Not again.
My battery died.
Where am I going
to get power now?
There's no way I can
get power around here.
But it's such a sunny day.
Maybe I can get
power from the sun.
Indeed, the sun is a huge and
reliable source of energy.
But how can we convey this
energy into electricity?
You must have heard
about solar cells.
But do you know how they work?
And how many of them would we
need to power Mike's laptop
so that he can keep
playing this awesome game?
First, let's take a
look at a solar cell.
Wow.
That's really thin.
And, in fact, there are even
two super-thin layers in there.
When the sunlight
hits the cell, it
gives enough energy
for electrons
to break free and travel through
an external circuit, generating
electricity.
But let's have a
closer look at the cell
to understand how
it really works.
Most of the cells
are made of silicon.
By themselves, the
atoms of silicon
would be organized in a
very stable structure.
However, when phosphorus
atoms are added to the silicon
structure, they disrupt
the nice arrangement,
because they have one extra
electron, compared to silicon.
This electron cannot bond
to the neighboring atoms,
and it becomes a free electron.
If we add a lot of
phosphorus into silicon,
we end up with a lot of free
electrons in the structure.
This arrangement
is called N-type,
because it's going to
be on the negative side.
We can also add boron instead
of phosphorus to the silicon.
This time, the boron
atoms have one less
electron than would be
necessary for a nice day out.
You can think of
the lack of electron
as a hole that would be free to
travel through the structure.
And therefore, adding a lot
of boron atoms to the silicon
will result in a lot of
free holes in the structure.
This arrangement
is called P-type,
as it's going to be
on the positive side.
To form a solar cell, you first
need to bring these two types
together.
At the region of contact, the
extra electrons of the N-type
would love to combine with
the extra holes of the P-type.
However, since the electrons
are negative charges,
the atoms they left now
become positive ions.
Conversely, the holes that
combined with the electrons
left negative ions
on the P side.
This difference
of charges results
into an electrical field
between the two sides, which
prevents more electrons
and holes from combining
with each other.
In fact, new electrons
trying to reach the junction
are pushed back by
negative charges of P-type,
while new holes are repulsed
by the positive charges
of the N-type.
Overall, the field
acts as some kind
of walls that keeps the
electrons to the N side
and the holes to the P side.
So what happens with the sun?
Well, when the light
reaches the junction,
it may have enough
energy to break
a hole-electron combination.
However, because of the field
that has been created before,
they electron is sent
away towards the N side,
while the hole is sent
towards the P side.
They can only recombine
if this extra electron
is collected by an external
circuit connected to P side.
In that case, the
electron will run
through whatever electrical
device that would be on the way
before going back to the
cell and finally recombining
with the hole.
Now, one cell like
this only provides
1.5 watt of electrical power,
which is fairly limited.
So if we need more
power, well, we simply
have to use more cells.
Hey, Mike.
Here's a solar setup for you.
It has 36 cells.
That's enough to
charge your laptop.
Awesome.
I can play again.
And all I needed was
energy from the sun.
Isn't that great?
Great indeed.
And, in fact, we
can even power more
than a laptop with solar
panels, if we use a lot of them.
For example, this solar farm
can power 600 average US homes.
Wow, that's pretty
big, isn't it?
Awesome.
So couldn't we just power the
whole country with solar cells?
How big would that panel be?
Let's do simple estimation.
Over one year, this one
provides about 8 gigawatt hours
of electrical energy.
For the year of 2010, the
total electricity production
in the US summed up to four
million gigawatt hours--
the equivalent of 500,000
solar farms like this one.
The area you would
need for these farms
would be about 1/9 the
size of California.
Hey, if that's all
the area it requires,
why don't we have solar panels
powering everything yet?
Well, as of today, there are
two main limiting factors
to solar panels-- cost
and electricity storage.
Wait, wait, wait.
You said cost?
Don't you just get energy
from the sun for free?
That's correct but the
construction of solar cells
is so expensive that even if
we used them during 30 years,
it would be cheaper to get
the electricity from coal.
However, improvements in
technology and materials
will probably make solar cells
more affordable in the future.
Also, the electricity
provided by solar cells
is not available all the time.
We could theoretically store
the surplus of electricity
during the day and
use it at night,
but storing these huge
amounts of electricity
is still complicated
and expensive.
These two reasons mainly
explain why today only 0.04%
of US electricity
comes from solar cells.
However, solar energy
is very valuable,
because it is sustainable.
The sun will still be shining
and sending its energy
to the Earth for
billions of years.
As new technologies
appear, we will most likely
be able to convert this
energy into electricity
for a cost that will compete
with other resources.
Anyway, if anything, the
solar cell technology
enabled Mike to keep
playing his game.
