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.

從各方面來看它比較便宜，並且毫無疑問的，比起其它我們所依賴的傳統發電廠所使用的原料如煤炭，更能永續發展，因為煤炭終究會耗盡。

So why don't we replace these traditional plants with solar energy?

那麼為何我們不以太陽能取代這些傳統的發電廠呢?

Because there's one factor that makes solar power very unpredictable: cloud cover.

因為其中一個原因使得太陽能非常不好預測，那就是：雲的遮擋。

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.

當太陽光朝著照射地球時，有些會被地球的大氣層所吸收，有些會反射回到外太空，而剩下的才會照射到地球表面。

The ones that aren't deviated are called direct irradiance.

那些沒有被反射回的稱做直射。

The ones that are deflected by clouds are called diffuse irradiance.

而那些因為雲層而偏射的則稱作散射。

And those rays that first get reflected by a surface, like a nearby building, before reaching the solar energy system are called reflected irradiance.

那些直接被表面反射的太陽光，例如旁邊的高樓大廈，沒能到達太陽能系統的稱做反射。

But before we can examine how clouds affect the sun's rays and electricity production, let's see how these solar energy systems work.

然而在我們能夠觀測雲層如何影響太陽光及電力的產生之前，我們來看看太陽能系統的工作原理。

First up, we have solar towers.

首先，我們需要有太陽能塔。

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 beach-goer.

這個高塔是由一個很大的，豎滿了很多鏡子的空地所圍繞用來追蹤太陽的軌跡及焦點，並且直接照射到塔上固定的一點，就有點像是一個非常熱切的海灘迷一般。

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.

這些光線會產生熱，而產生的熱是非常巨大足以使水沸騰並且產生水蒸氣來推動傳統的渦輪機，而發電。

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.

當我們說太陽能系統時，我們通常是指光伏電池，或是太陽能面板，這些也是非常普遍的太陽能系統發電系統。

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.

在太陽能板裡，從太陽光而來的光子打到面板表面，釋放出電子並且產生電流流動。

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.

太陽能板可以接受全部的輻射來源，而太陽能塔只能接受直射，這就是為什麼雲層變得如此重要，因為會非常依賴它們的種類及地點與太陽之間的關係，它們可以直接增加或減少電量的產生。

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.

例如，只要有少量的積雲擋在太陽前方，就會減少太陽塔中電量的產生甚至幾乎為零，因為它非常倚賴直接照射的光線。

In solar panels, those clouds would decrease energy output as well, though not as much because solar panels can use all types of irradiance.

太陽能板方面，這些雲層也會減少能量的輸出沒錯，只不過沒有這麼多，因為太陽能板可以接收各類型的輻射。

However, all this depends on the clouds exact positioning.

然而，全部都需看雲層實際的位置而定。

Due to reflection, or a particular phenomenon 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.

由於反射，或是稱做米氏散射的特殊現象，太陽光其實可以在雲層前端聚焦產生多於50%的太陽輻射增加量到太陽能板上。

If this potential increase isn't accounted for, it could damage the solar panel.

如果沒有考慮這些潛在的增加，可能會損壞太陽能板。

Why does this matter?

為什麼這個很重要呢?

Well, you wouldn't want this lesson to stop just because a cloud passed over the panel on your roof.

我想你應該不希望這堂課就這麼結束了，只因為雲層通過頂樓的太陽能板。

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.

在太陽塔中，有一個大缸子的熔鹽或油可以用來儲存多餘的熱能在需要的時候使用，所以這就是他們如何處理這個問題：利用照度的起伏不定，來調整發電量。

But in the case of solar panels, there currently isn't any way to affordably store extra energy.

但在太陽能板這方面，目前沒有任何可行的方法來儲存多餘的能量。

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.

這就是為何傳統發電廠存在因為可以補足任何太陽能電廠的浮動，傳統發電多餘的電量來源必須保持隨時可以供給的狀態。

But then why aren't these traditional power plants just used as a backup, instead of us humans depending on them as our main sources of energy?

但是不用來當做備用就好，而人們卻倚賴他們成為主要能量來源呢?

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.

因為對在火力發電廠或核能發電廠的員工而言，依據目前天空有多少雲，來調整應該輸出的電量，是不可能的。

The response time would simply be too slow.

反應時間就會太慢。

Instead, to accommodate these fluctuations, some extra electricity from traditional power plants is always being produced.

取而代之，為了補足這些浮動，傳統發電廠多餘的電力是必須隨時保持供應的。

On clear sky days, that extra electricity might be wasted, but when cloudy skies prevail, it's what fills the gap.

天氣晴朗時，多餘的電力或許浪費了，但當陰天來臨，這就是派上用場的時刻。

This is what we currently depend on for a constant supply of energy.

這就是我們現在依賴一個能持續供給的能源。

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.

就這個理由看來，有很多的研究者對於利用衛星雲圖或是拍攝天空的相機來預測雲的動態和形成非常有興趣，因為可以最大化使用太陽能發電廠並最小化能源的浪費。

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.

如果我們可以完成這些，你也就可以純以太陽能所供的電，來享受這部影片不論在任何天氣狀況下。當然如果太陽很大，你也可以到戶外欣賞雲層。