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We have some good reasons to completely

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switch over to solar power.

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It's cheaper in many cases,

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and definitely more sustainable

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than our dependance on traditional power plants

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that use resources like coal,

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which will eventually run out.

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So why don't we replace these

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traditional plants with solar energy?

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Because there's one factor that makes

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solar power very unpredictable:

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cloud cover.

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As the sun's rays move towards Earth,

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some get absorbed by the Earth's atmosphere,

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some are reflected back into outer space,

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but the rest make it to the Earth's surface.

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The ones that aren't deviated

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are called direct irradiance.

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The ones that are deflected by clouds

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are called diffuse irradiance.

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And those rays that first get reflected by a surface,

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like a nearby building,

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before reaching the solar energy system

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are called reflected irradiance.

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But before we can examine how clouds

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affect the sun's rays and electricity production,

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let's see how these solar energy systems work.

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First up, we have solar towers.

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These are made up of a central tower

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surrounded by a huge field of mirrors

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that track the sun's path and focus

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only the direct rays onto a single point on the tower,

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kind of like an eager beachgoer.

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The heat generated by these rays

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is so immense that it can be used to boil water

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producing steam that drives a traditional turbine,

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which makes electricity.

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But when we say solar energy systems,

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we're usually talking about photovoltaics,

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or solar panels,

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which are the systems most commonly used

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to generate solar power.

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In solar panels,

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photons from the sun's rays hit the surface of a panel,

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and electrons are released

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to get an electric current going.

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Solar panels can use all types of irradiance,

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while solar towers can only use direct irradiance,

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and this is where clouds become important

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because depending on their type and location

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relative to the sun,

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they can either increase or decrease

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the amount of electricity produced.

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For instance, even a few cumulus clouds

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in front of the sun can reduce

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the electricity production in solar towers

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to almost zero because of this dependence on direct rays.

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In solar panels, those clouds would decrease

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energy output as well,

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though not as much

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because solar panels can use all types of irradiance.

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However, all this depends on the clouds exact positioning.

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Due to reflection, or a particular phenomeon

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called Mie scattering,

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the sun's rays can actually be focused forward

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by clouds to create a more than 50% increase

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in the solar irradiance reaching a solar panel.

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If this potential increase isn't accounted for,

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it could damage the solar panel.

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Why does this matter?

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Well, you wouldn't want this lesson to stop

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just because a cloud passed over the panel on your roof.

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In solar towers, huge tanks of molten salt or oil

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can be used to store any excess heat

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and use it when needed,

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so that's how they manage the problem

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of fluctuating solar irradiance to smooth out

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electricity production.

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But in the case of solar panels,

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there currently isn't any way to affordably

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store extra energy.

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That's where traditional power plants come in

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because to correct for any fluctuations

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in these solar powered plants,

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extra electricity from traditional sources

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always needs to be available.

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But then why aren't these tradtional power plants

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just used as a backup,

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instead of us humans depending on them

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as our main sources of energy?

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Because it's impossible for an employee

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at a coal fired or a nuclear plant

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to turn a knob to produce

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more or less electricity depending

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on how many clouds there are in the sky.

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The response time would simply be too slow.

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Instead, to accommodate these fluctuations,

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some extra electricity from traditional power plants

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is always being produced.

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On clear sky days,

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that extra electricity might be wasted,

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but when cloudy skies prevail,

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it's what fills the gap.

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This is what we currently depend on

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for a constant supply of energy.

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For this reason, a lot of researchers

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are interested in forcasting the motion

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and formation of clouds through satellite images

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or cameras that look up at the sky

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to maximize the energy from solar power plants

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and minimize energy waste.

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If we could accomplish that,

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you'd be able to enjoy this video

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powered solely by the sun's rays,

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no matter what the weather,

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although if the sun is shining,

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you may be tempted to venture outside

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to go and do a different kind of cloud gazing.