Why aren't we only using solar power?

We have some good reasons to completely

switch over to solar power.

It's cheaper in many cases,

and definitely more sustainable

than our dependance on traditional power plants

that use resources like coal,

which will eventually run out.

So why don't we replace these

traditional plants with solar energy?

Because there's one factor that makes

solar power very unpredictable:

cloud cover.

As the sun's rays move towards Earth,

some get absorbed by the Earth's atmosphere,

some are reflected back into outer space,

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

The ones that aren't deviated

are called direct irradiance.

The ones that are deflected by clouds

are called diffuse irradiance.

And those rays that first get reflected by a surface,

like a nearby building,

before reaching the solar energy system

are called reflected irradiance.

But before we can examine how clouds

affect the sun's rays and electricity production,

let's see how these solar energy systems work.

First up, we have solar towers.

These are made up of a central tower

surrounded by a huge field of mirrors

that track the sun's path and focus

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

kind of like an eager beachgoer.

The heat generated by these rays

is so immense that it can be used to boil water

producing steam that drives a traditional turbine,

which makes electricity.

But when we say solar energy systems,

we're usually talking about photovoltaics,

or solar panels,

which are the systems most commonly used

to generate solar power.

In solar panels,

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

and electrons are released

to get an electric current going.

Solar panels can use all types of irradiance,

while solar towers can only use direct irradiance,

and this is where clouds become important

because depending on their type and location

relative to the sun,

they can either increase or decrease

the amount of electricity produced.

For instance, even a few cumulus clouds

in front of the sun can reduce

the electricity production in solar towers

to almost zero because of this dependence on direct rays.

In solar panels, those clouds would decrease

energy output as well,

though not as much

because solar panels can use all types of irradiance.

However, all this depends on the clouds exact positioning.

Due to reflection, or a particular phenomeon

called Mie scattering,

the sun's rays can actually be focused forward

by clouds to create a more than 50% increase

in the solar irradiance reaching a solar panel.

If this potential increase isn't accounted for,

it could damage the solar panel.

Why does this matter?

Well, you wouldn't want this lesson to stop

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

In solar towers, huge tanks of molten salt or oil

can be used to store any excess heat

and use it when needed,

so that's how they manage the problem

of fluctuating solar irradiance to smooth out

electricity production.

But in the case of solar panels,

there currently isn't any way to affordably

store extra energy.

That's where traditional power plants come in

because to correct for any fluctuations

in these solar powered plants,

extra electricity from traditional sources

always needs to be available.

But then why aren't these tradtional power plants

just used as a backup,

instead of us humans depending on them

as our main sources of energy?

Because it's impossible for an employee

at a coal fired or a nuclear plant

to turn a knob to produce

more or less electricity depending

on how many clouds there are in the sky.

The response time would simply be too slow.

Instead, to accommodate these fluctuations,

some extra electricity from traditional power plants

is always being produced.

On clear sky days,

that extra electricity might be wasted,

but when cloudy skies prevail,

it's what fills the gap.

This is what we currently depend on

for a constant supply of energy.

For this reason, a lot of researchers

are interested in forcasting the motion

and formation of clouds through satellite images

or cameras that look up at the sky

to maximize the energy from solar power plants

and minimize energy waste.

If we could accomplish that,

you'd be able to enjoy this video

powered solely by the sun's rays,

no matter what the weather,

although if the sun is shining,

you may be tempted to venture outside

to go and do a different kind of cloud gazing.