[00:08] The Earth intercepts a lot of solar power:
[00:11] 173 thousand terawatts.
[00:14] That's ten thousand times more power
than the planet's population uses.
[00:19] So is it possible that one day
[00:20] the world could be completely 
reliant on solar energy?
[00:24] To answer that question,
[00:25] we first need to examine how solar panels
convert solar energy to electrical energy.
[00:31] Solar panels are made up of smaller units
called solar cells.
[00:36] The most common solar cells
are made from silicon,
[00:39] a semiconductor that is the second
most abundant element on Earth.
[00:43] In a solar cell,
[00:44] crystalline silicon is sandwiched
between conductive layers.
[00:48] Each silicon atom is connected
to its neighbors by four strong bonds,
[00:53] which keep the electrons in place
so no current can flow.
[00:58] Here's the key:
[00:59] a silicon solar cell uses
two different layers of silicon.
[01:03] An n-type silicon has extra electrons,
[01:07] and p-type silicon has extra spaces
for electrons, called holes.
[01:12] Where the two types of silicon meet,
[01:14] electrons can wander across 
the p/n junction,
[01:17] leaving a positive charge on one side
[01:19] and creating negative charge on the other.
[01:23] You can think of light 
as the flow of tiny particles
[01:26] called photons,
[01:28] shooting out from the Sun.
[01:30] When one of these photons strikes
the silicon cell with enough energy,
[01:34] it can knock an electron from its bond,
leaving a hole.
[01:38] The negatively charged electron and
location of the positively charged hole
[01:43] are now free to move around.
[01:45] But because of the electric field
at the p/n junction,
[01:48] they'll only go one way.
[01:51] The electron is drawn to the n-side,
[01:53] while the hole is drawn to the p-side.
[01:56] The mobile electrons are collected by
thin metal fingers at the top of the cell.
[02:01] From there, they flow through 
an external circuit,
[02:04] doing electrical work,
[02:06] like powering a lightbulb,
[02:07] before returning through the conductive
aluminum sheet on the back.
[02:11] Each silicon cell only puts out
half a volt,
[02:15] but you can string them 
together in modules to get more power.
[02:18] Twelve photovoltaic cells are enough
to charge a cellphone,
[02:22] while it takes many modules 
to power an entire house.
[02:26] Electrons are the only moving parts
in a solar cell,
[02:29] and they all go back where they came from.
[02:31] There's nothing to get worn out
or used up,
[02:33] so solar cells can last for decades.
[02:37] So what's stopping us from being
completely reliant on solar power?
[02:42] There are political factors at play,
[02:44] not to mention businesses that lobby
to maintain the status quo.
[02:48] But for now, let's focus on the physical
and logistical challenges,
[02:53] and the most obvious of those
[02:54] is that solar energy 
is unevenly distributed across the planet.
[02:58] Some areas are sunnier than others.
[03:01] It's also inconsistent.
[03:02] Less solar energy is available 
on cloudy days or at night.
[03:07] So a total reliance would require
[03:09] efficient ways to get electricity 
from sunny spots to cloudy ones,
[03:14] and effective storage of energy.
[03:17] The efficiency of the cell itself
is a challenge, too.
[03:20] If sunlight is reflected 
instead of absorbed,
[03:23] or if dislodged electrons fall back into
a hole before going through the circuit,
[03:28] that photon's energy is lost.
[03:30] The most efficient solar cell yet
[03:33] still only converts 46% of 
the available sunlight to electricity,
[03:38] and most commercial systems are currently
15-20% efficient.
[03:43] In spite of these limitations,
[03:45] it actually would be possible
[03:47] to power the entire world 
with today's solar technology.
[03:50] We'd need the funding 
to build the infrastructure
[03:52] and a good deal of space.
[03:54] Estimates range from tens 
to hundreds of thousands of square miles,
[03:59] which seems like a lot,
[04:00] but the Sahara Desert alone is over
3 million square miles in area.
[04:06] Meanwhile, solar cells are getting
better, cheaper,
[04:09] and are competing 
with electricity from the grid.
[04:11] And innovations, like floating solar farms,
may change the landscape entirely.
[04:16] Thought experiments aside,
[04:18] there's the fact 
that over a billion people
[04:21] don't have access 
to a reliable electric grid,
[04:24] especially in developing countries,
[04:26] many of which are sunny.
[04:28] So in places like that,
[04:30] solar energy is already much cheaper
and safer than available alternatives,
[04:34] like kerosene.
[04:36] For say, Finland or Seattle, though,
[04:38] effective solar energy 
may still be a little way off.