Tip:
Highlight text to annotate it
X
Every second our sun produces more energy
than humanity has ever used.
The light of this brilliant star has bathed
our planet since it was formed 4,5 billion years ago.
And it will illuminate our days for another 4 billion years.
The enormous importance of the Sun
has been recognized since prehistoric times.
Many cultures regarded our G-type main-sequence star as a God.
Today we know that sunlight is not a deities magic
but a constant stream of photons, the force-carrier
of the electromagnetic force.
Solar energy technologies include
solar heating, solar thermal electricity and solar photovoltaics.
In this video I will shed light on the photovoltaic cell.
If we zoom in on a light-ray,
we find that it consists of individual particles.
Proposed by Max Planck and Albert Einstein as the quanta of light
those particles have since been named photons.
The photons of a light beam have a characteristic energy.
For visible light we can see this energy as colour.
A photon of sufficient energy can free an electron
from binding with its individual atom,
thereby creating a free electron and a positively charged electron hole.
The charge carriers are separated by the built-in field of the semiconductor.
Thus holes move toward the anode, and electrons toward the cathode,
and a current is produced.
For Silicon, the most common material for solar cells,
the energy needed to free an electron is 1,107 eV,
about what you get from infra-red light.
Light with less energy won´t bother the electrons at all
and the excess energy of more powerful light is lost.
Hence the possible efficiency of a silicon-based solar cell
is limited to about 30%.
A combination of different semiconductors would achieve better efficiencies,
but these technologies are still in pre-commercial development.
Now we need make a slight detour round the solar system,
to determine the energy we get from the sun.
The Sun radiates light equally in all directions.
A hollow sphere centred at the middle of the Sun
would have its entire interior surface illuminated.
As the radius increases, the surface area will also increase.
Because the constant luminosity is spread over a greater surface area
the flux density or power per unit area decreases.
With 4*pi*r² for the surface area of a sphere
we get the function for the flux density.
This relation is also called an Inverse-square law.
The Earth is a long way from the Sun.
It takes light about 8 minutes and 19 seconds
to cover the 149,6 billion meters between Sun and Earth.
The Suns luminosity is 3,846 × 10^26 W.
Put this into the equation and we get a flux density of 1367 W/m².
Called the solar-constant, this is the amount of energy per second
that hits the edge of earth´s atmosphere.
As it passes through the atmosphere
the solar radiation is significantly attenuated.
Depending on latitude the flux density at earth´s surface
amounts to between 0 and 1000 W/m².
The figure varies with the Sun´s angle at different times of year,
according to the distance the sunlight travels through the air
and depending on the extent of atmospheric haze and cloud cover.
Taking into account the lower radiation intensity
in early morning and evening, and its absence at night
gives us a world map of the average solar irradiance.
A modern solar cell can convert up to 30%
of the radiant flux displayed here into electricity.
But let´s be modest and only use a 20% efficient solar cell
to power the world.
To achieve this we have to produce 472,89 exajoule,
neglecting demand peaks and lows.
The relatively sparse energy density of solar radiation means,
that covered land area will be the crucial measure.
Built near the equator where we get a maximum of solar irradiance,
the solar cells would cover about 300 000 km².
Closer to home in Central Europe, North America or Central Asia
an area of 500 000 km² is required.
To put this into perspective the global urban built up area is 660 000 km²,
enough to power the earth from our roofs.
In the previous video of my energy series I talked about
the carbon batteries, charged millions of years ago by sunlight.
Today we no longer need to rely on burning fossils.
Solar power has long been insignificant,
but since it´s humble beginnings the photovoltaic cell has been perfected.
Form powering towns to dreaming about large power-stations in space,
we can harness the force of light directly and unlock
this vast potential to appease our growing hunger for energy�