经济学人下载:灌溉沙漠(1)

If you live in a desert, maintaining a supply of fresh water is a challenge.
如果你生活在沙漠中，保持淡水的供应是个挑战。

One answer is desalination, but that needs a source of brine from which to remove the salt—
一个答案是海水淡化，但这需要一个盐水源来去除盐，

which in turn requires that your desert be near the sea. Even in inland deserts, though, moisture is often present in the air as water vapour.
因此也要求你所在的沙漠靠近大海。即便是在内陆沙漠，空气中的水分也常常以水蒸气的形式存在。

The problem is extracting this vapour effectively and cheaply. And that is what two groups of researchers—
问题是如何有效而廉价地提取这些蒸汽。这就是两组研究人员—

one at the University of Connecticut, the other at the University of California, Berkeley—hope they have managed to do.
一组来自康涅狄格大学，另一组来自伯克利加利福尼亚大学—所希望他们能够做到的。

The ease with which water can be won from air depends on that air's relative humidity.
从空气中获取水分的难易程度取决于空气的相对湿度。

This is a measure of its current vapour content as a percentage of its maximum possible vapour content at its current temperature.
这是其目前水蒸汽含量的计算方式，作为其在当前温度下最大可能蒸汽含量的百分比。

A relative humidity of 100% means the air in question is holding as much water vapour as it possibly can.
100%的相对湿度意味着空气中含有尽可能多的水蒸气。

A good way to get air to give up some of its moisture is therefore to cool it to the point where its relative humidity exceeds 100%.
因此，让空气失去一部分水分的一个好办法是将其冷却到相对湿度超过100%的程度。

Sometimes this happens naturally at night, causing mist and dew to form.
有时，夜间会自然发生这种情况，造成雾和露水的形成。

These can be collected in special traps in areas where liquid water is otherwise rare.
在液态水稀少的地区，这些水可以用特殊的捕集器收集。

But if nocturnal cooling does not bring air all the way up to 100% relative humidity,
但如果夜间降温并不能将空气的相对湿度提高到100%，

building water traps out of special materials might give nature a helping hand.
那么用特殊材料制造脱水器可能会给大自然带来帮助。

Adsorption is a process which plucks water molecules from air
吸附是将相对湿度小于100%的空气中的水分子

that has less than 100% relative humidity by attaching them to the surface of a solid material.
吸附到固体材料表面的过程。

The molecules are held there by electrostatic connections called Van der Waals forces that link them with the molecules of the pertinent surface.
分子通过一种被称为分子间作用力的静电连接与相关表面上的分子相连接而固定在那里的。

To collect a lot of water this way therefore requires a material that has two properties.
因此，用这种方法收集水需要一种具备两种特性的材料。

One is a large surface area. The other is an appropriate Van der Waals response.
一个是很大的表面积。另一个是适当的分子间作用力反应。

Experimental traps that employ this principle have been made using substances called metal-organic frameworks.
利用这一原理的实验捕集器是用一种叫做金属-有机骨架的物质制成的。

These are porous molecular networks through which air can circulate. Their porosity gives them a huge surface area.
这些是多孔的分子网络，空气可以通过它们循环。它们的多孔性使它们具有很大的表面积。

And by picking the right ingredients, such as zirconium, they can be given the necessary Van der Waals properties. Zirconium is, however, costly.
通过选择正确的材料，如锆，它们可以获得必要的分子间作用力性质。但锆价格昂贵。

Moreover, once adsorbed, the water must then be released.
而且，一旦被吸附，水就必须被释放。

This means warming the adsorptive material—the warmth being provided by the sun, once it has risen.
这意味着加热吸附物质，即太阳升起后提供的热量。

Here, metal-organic frameworks present a problem. They tend to reflect sunlight rather than absorbing it, and so do not heat up well.
在此，金属-有机骨架出现了一个问题。它们更倾向于反射而不是吸收阳光，因此不能很好地加热。

To overcome this, engineers build solid "foams" made of copper into the system.
为了克服这个问题，工程师们在系统中加入了由铜制成的固体“泡沫”。

These heat up in the sun and transfer some of their heat to parts of the adjacent metal-organic framework
它们在太阳下受热，将一部分热量传递到邻近的金属-有机骨架上，

where adsorbed water needs to be released. This works, but adding such foams makes an already costly technology even dearer.
在那里被吸附的水需要被释放出来。这种方法起了作用，但添加泡沫这种方法让本就昂贵的技术更加昂贵。