The Earth intercepts a lot of solar power: 173 thousand terawatts.
地球接收了很多太阳能：17.3万太瓦。
That's ten thousand times more power than the planet's population uses.
这一数字是地球上人类所使用能源总和的一万倍。
So is it possible that one day the world could be completely reliant on solar energy?
那么有没有可能有一天我们的世界会完全依赖太阳能？
To answer that question, we first need to examine how solar panels convert solar energy to electrical energy.
为了回答这一问题，首先我们需要明白太阳能板是怎样将太阳能转化为电能的。
Solar panels are made up of smaller units called solar cells.
太阳能板由更小的单位构成，称为太阳能电池。
The most common solar cells are made from silicon, a semiconductor that is the second most abundant element on Earth.
最常见的太阳能电池原料是硅，硅是一种半导体，也是地球上储量第二的物质。
In a solar cell, crystalline silicon is sandwiched between conductive layers.
在一个太阳能电池单元里，晶体硅像是三明治夹心一样被夹在两个导电层之间。
Each silicon atom is connected to its neighbors by four strong bonds,
每个硅原子通过四个强键和其他硅原子连接，
which keep the electrons in place so no current can flow.
能够固定电子，因此不会产生电流。
Here's the key: a silicon solar cell uses two different layers of silicon.
工作原理如下：一个硅太阳能电池有着两个不同的硅层。
An n-type silicon has extra electrons, and p-type silicon has extra spaces for electrons, called holes.
包括一层有着额外电子的n型硅层和拥有能存储额外电子的空隙的p型硅层。
Where the two types of silicon meet, electrons can wander across the p/n junction,
在两种硅层交界处，电子可以在p-n的连接处移动，
leaving a positive charge on one side and creating negative charge on the other.
在一个硅层上产生正电荷，在另一个硅层上产生负电荷。
You can think of light as the flow of tiny particles called photons, shooting out from the Sun.
你可以把光线想象成从太阳发射出的由微小的光子构成的粒子束。
When one of these photons strikes the silicon cell with enough energy,
当其中一些光子带着足够的能量撞击太阳能电池时，
it can knock an electron from its bond, leaving a hole.
它可以将一个电子撞离束缚它的强键，形成空隙。
The negatively charged electron and location of the positively charged hole are now free to move around.
带有负电荷的电子和带有正电荷的空隙现在可以自由移动了。
But because of the electric field at the p/n junction, they'll only go one way.
但是由于p型和n型硅层交界处存在电场，它们只会向一个方向移动。
The electron is drawn to the n-side, while the hole is drawn to the p-side.
电子被吸引至n型硅层处，空隙则跑到p型硅层处。
The mobile electrons are collected by thin metal fingers at the top of the cell.
这些自由电子被太阳能电池顶部的细小金属导体收集。
From there, they flow through an external circuit, doing electrical work, like powering a lightbulb,
从那里开始，它们留过一个外部电路，以电流的形式做功，比如点亮一盏电灯，
before returning through the conductive aluminum sheet on the back.
然后从背面的铝制导体薄片返回。
Each silicon cell only puts out half a volt, but you can string them together in modules to get more power.
每个硅太阳能电池只能产生半伏特的电压，但你可以将它们组装成模块来获得更多的能量。
Twelve photovoltaic cells are enough to charge a cellphone, while it takes many modules to power an entire house.
十二个光电太阳能电池可以为手机充电，但是为一个房间提供电能需要许多太阳能板。

Electrons are the only moving parts in a solar cell, and they all go back where they came from.
电子是太阳能电池中唯一的移动单元，而且它们工作后会沿原路返回。
There's nothing to get worn out or used up, so solar cells can last for decades.
没有组件会遭受磨损和损伤，因此太阳能电池可以使用几十年。
So what's stopping us from being completely reliant on solar power?
那么是什么限制了我们完全依赖太阳能提供能源呢？
There are political factors at play, not to mention businesses that lobby to maintain the status quo.
这里有着政治因素在起作用，更不要说商人为了维持利润现状而四处游说。
But for now, let's focus on the physical and logistical challenges,
但现在，我们只研究物理和后勤方面的障碍，
and the most obvious of those is that solar energy is unevenly distributed across the planet.
其中最主要的原因是太阳能不是均匀地分布在这颗星球上的。
Some areas are sunnier than others. It's also inconsistent.
其中一些地区比其他地区更晴朗。而且阳光也不是持续性的。
Less solar energy is available on cloudy days or at night.
在多云天气或是晚上，太阳能供给量就会下降。
So a total reliance would require efficient ways to get electricity from sunny spots to cloudy ones, and effective storage of energy.
所以对太阳能的完全依赖，需要依赖从晴朗地区将电能转移到多云地区的快捷方法，以及有效的能量储蓄技术。
The efficiency of the cell itself is a challenge, too.
太阳能电池本身的效率也是一大障碍。
If sunlight is reflected instead of absorbed,
如果阳光被反射而不是被吸收，
or if dislodged electrons fall back into a hole before going through the circuit, that photon's energy is lost.
或是失去束缚的电子在进入电路前掉进了空隙中，这时光子的能量就损失掉了。
The most efficient solar cell yet still only converts 46% of the available sunlight to electricity,
迄今为止最高效的太阳能电池也只能将阳光能量的46%转化为电能，
and most commercial systems are currently 15-20% efficient.
而目前大多数商业化的太阳能系统只有15-20%的转化效率。
In spite of these limitations, it actually would be possible to power the entire world with today's solar technology.
即使面临着这些障碍，使用现如今的太阳能技术为全世界提供能量依然是可行的。
We'd need the funding to build the infrastructure and a good deal of space.
我们只需要制造设施的资金和足够的空间。
Estimates range from tens to hundreds of thousands of square miles, which seems like a lot,
估计需要几万至几十万平方英里的土地，这看起来很多，
but the Sahara Desert alone is over 3 million square miles in area.
但光是撒哈拉沙漠就有三百万平方英里的土地。
Meanwhile, solar cells are getting better, cheaper, and are competing with electricity from the grid.
与此同时，太阳能电池正变得越来越高效，经济，而且正和输电网进行竞争。
And innovations, like floating solar farms, may change the landscape entirely.
像水上太阳能农场之类的创新设计可能会完全改变当今的格局。
Thought experiments aside, there's the fact that over a billion people don't have access to a reliable electric grid,
这不是异想天开，事实上超过十亿民众还没有可靠的输电网供给，
especially in developing countries, many of which are sunny.
尤其是在发展中国家，其中许多国家阳光很充足。
So in places like that, solar energy is already much cheaper and safer than available alternatives, like kerosene.
因此在这些地方，太阳能比起煤油等其它能源来说已经十分经济和安全了。
For say, Finland or Seattle, though, effective solar energy may still be a little way off.
但至于芬兰和西雅图，有效的太阳能应用可能还有很长的路要走。