00:00The Earth intercepts a lot of solar power, 173,000 terawatts.
00:08That's 10,000 times more power than the planet's population uses.
00:13So is it possible that one day the world could be completely reliant on solar energy?
00:18To answer that question, we first need to examine
00:21how solar panels convert solar energy to electrical energy.
00:26Solar panels are made up of smaller units called solar cells.
00:30The most common solar cells are made from silicon,
00:33a semiconductor that is the second most abundant element on Earth.
00:37In a solar cell, crystalline silicon is sandwiched between conductive layers.
00:42Each silicon atom is connected to its neighbors by four strong bonds,
00:47which keep the electrons in place so no current can flow.
00:52Here's the key.
00:53A silicon solar cell uses two different layers of silicon.
00:58An N-type silicon has extra electrons,
01:01and P-type silicon has extra spaces for electrons, called holes.
01:06Where the two types of silicon meet, electrons can wander across the P-N junction,
01:12leaving a positive charge on one side and creating negative charge on the other.
01:17You can think of light as the flow of tiny particles, called photons, shooting out from the sun.
01:24When one of these photons strikes the silicon cell with enough energy,
01:28it can knock an electron from its bond, leaving a hole.
01:32The negatively charged electron and location of the positively charged hole are now free to move around.
01:39But because of the electric field at the P-N junction, they'll only go one way.
01:44The electron is drawn to the N-side, while the hole is drawn to the P-side.
01:50The mobile electrons are collected by thin metal fingers at the top of the cell.
01:55From there, they flow through an external circuit, doing electrical work like powering a light bulb,
02:01before returning through the conductive aluminum sheet on the back.
02:05Each silicon cell only puts out half a volt, but you can string them together in modules to get more power.
02:12Twelve photovoltaic cells are enough to charge a cell phone,
02:16while it takes many modules to power an entire house.
02:20Electrons are the only moving parts in a solar cell, and they all go back where they came from.
02:25There's nothing to get worn out or used up, so solar cells can last for decades.
02:30So what's stopping us from being completely reliant on solar power?
02:36There are political factors at play, not to mention businesses that lobby to maintain the status quo.
02:42But for now, let's focus on the physical and logistical challenges.
02:46And the most obvious of those is that solar energy is unevenly distributed across the planet.
02:52Some areas are sunnier than others.
02:55It's also inconsistent.
02:57Less solar energy is available on cloudy days or at night.
03:01So a total reliance would require efficient ways to get electricity from sunny spots to cloudy ones,
03:08and effective storage of energy.
03:11The efficiency of the cell itself is a challenge, too.
03:14If sunlight is reflected instead of absorbed,
03:17or if dislodged electrons fall back into a hole before going through the circuit,
03:22that photon's energy is lost.
03:25The most efficient solar cell yet still only converts 46% of the available sunlight to electricity,
03:32and most commercial systems are currently 15 to 20% efficient.
03:37In spite of these limitations, it actually would be possible to power the entire world with today's solar technology.
03:44We'd need the funding to build the infrastructure and a good deal of space.
03:48Estimates range from tens to hundreds of thousands of square miles.
03:53Which seems like a lot, but the Sahara Desert alone is over 3 million square miles in area.
04:00Meanwhile, solar cells are getting better, cheaper, and are competing with electricity from the grid.
04:05And innovations like floating solar farms may change the landscape entirely.
04:11Thought experiments aside, there's the fact that over a billion people don't have access to a reliable electric grid,
04:18especially in developing countries, many of which are sunny.
04:22So in places like that, solar energy is already much cheaper and safer than available alternatives like kerosene.
04:30For, say, Finland or Seattle, though, effective solar energy may still be a little way off.
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