[TED]：How we can turn the cold of outer space into a renewable resource | 如何把外太空的低 溫轉變為可再生的資源
What if we could use the cold darkness of outer space to cool buildings on earth? In this mind-blowing talk, physicist Aaswath Raman details the technology he's developing to harness"night-sky cooling"-- a natural phenomenon where infrared light escapes earth and heads to space,carrying heat along with it -- which could dramatically reduce the energy used by our cooling systems (and the pollution they cause). Learn more about how this approach could lead us towards a future where we intelligently tap into the energy of the universe. 如果我們能用外太空的寒冷黑暗來冷卻地球上的建築物，那會如何？在這場驚奇的演說中，物理學家奧斯瓦 斯拉曼 (Aaswath Raman) 詳細說明了他正在開發的技術，這項技術駕馭「夜空冷卻」這種自然的現象，讓 紅外線把熱帶離地球，送上太空前去，大大減少我們的冷卻系統所需要的能源。來進一步了解這種方式如何 能引領我們在未來能夠智慧地發掘宇宙的能量。
Every summer when I was growing up, I would fly from my home in Canada to visit my grandparents, who lived in Mumbai, India. Now, Canadian summers are pretty mild at best -- about 22 degrees Celsius or 72 degrees Fahrenheit is a typical summer's day, and not too hot. Mumbai, on the other hand, is a hot and humid place well into the 30s Celsius or 90s Fahrenheit. As soon as I'd reach it, I'd ask, "How could anyone live, work or sleep in such weather?" To make things worse, my grandparents didn't have an air conditioner. And while I tried my very, very best, I was never able to persuade them to get one. But this is changing, and fast.
Cooling systems today collectively account for 17 percent of the electricity we use worldwide. This includes everything from the air conditioners I so desperately wanted during my summer vacations, to the refrigeration systems that keep our food safe and cold for us in our supermarkets, to the industrial scale systems that keep our data centers operational. Collectively, these systems account for eight percent of global greenhouse gas emissions.
But what keeps me up at night is that our energy use for cooling might grow sixfold by the year 2050, primarily driven by increasing usage in Asian and African countries. I've seen this firsthand. Nearly every apartment in and around my grandmother's place now has an air conditioner. And that is, emphatically, a good thing for the health, well-being and productivity of people living in warmer climates. However, one of the most alarming things about climate change is that the warmer our planet gets, the more we're going to need cooling systems -- systems that are themselves large emitters of greenhouse gas emissions. This then has the potential to cause a feedback loop, where cooling systems alone could become one of our biggest sources of greenhouse gases later this century. In the worst case, we might need more than 10 trillion kilowatt-hours of electricity every year, just for cooling, by the year 2100. That's half our electricity supply today. Just for cooling. But this also point us to an amazing opportunity. A 10 or 20 percent improvement in the efficiency of every cooling system could actually have an enormous impact on our greenhouse gas emissions, both today and later this century. And it could help us avert that worst-case feedback loop.
I'm a scientist who thinks a lot about light and heat. In particular, how new materials allow us to alter the flow of these basic elements of nature in ways we might have once thought impossible. So, while I always understood the value of cooling during my summer vacations, I actually wound up working on this problem because of an intellectual puzzle that I came across about six years ago. How were ancient peoples able to make ice in desert climates? This is a picture of an ice house, also called a Yakhchal, located in the southwest of Iran. There are ruins of dozens of such structures throughout Iran, with evidence of similar such buildings throughout the rest of the Middle East and all the way to China.
The people who operated this ice house many centuries ago, would pour water in the pool you see on the left in the early evening hours, as the sun set. And then something amazing happened. Even though the air temperature might be above freezing, say five degrees Celsius or 41 degrees Fahrenheit, the water would freeze. The ice generated would then be collected in the early morning hours and stored for use in the building you see on the right, all the way through the summer months. You've actually likely seen something very similar at play if you've ever noticed frost form on the ground on a clear night, even when the air temperature is well above freezing. But wait. How did the water freeze if the air temperature is above freezing? Evaporation could have played an effect, but that's not enough to actually cause the water to become ice. Something else must have cooled it down.
Think about a pie cooling on a window sill. For it to be able to cool down, its heat needs to flow somewhere cooler. Namely, the air that surrounds it. As implausible as it may sound, for that pool of water, its heat is actually flowing to the cold of space.
How is this possible? Well, that pool of water, like most natural materials, sends out its heat as light. This is a concept known as thermal radiation. In fact, we're all sending out our heat as infrared light right now, to each other and our surroundings. We can actually visualize this with thermal cameras and the images they produce, like the ones I'm showing you right now. So that pool of water is sending out its heat upward towards the atmosphere. The atmosphere and the molecules in it absorb some of that heat and send it back. That's actually the greenhouse effect that's responsible for climate change.
But here's the critical thing to understand. Our atmosphere doesn't absorb all of that heat. If it did, we'd be on a much warmer planet. At certain wavelengths, in particular between eight and 13 microns, our atmosphere has what's known as a transmission window. This window allows some of the heat that goes up as infrared light to effectively escape, carrying away that pool's heat. And it can escape to a place that is much, much colder. The cold of this upper atmosphere and all the way out to outer space, which can be as cold as minus 270 degrees Celsius, or minus 454 degrees Fahrenheit. So that pool of water is able to send out more heat to the sky than the sky sends back to it. And because of that, the pool will cool down below its surroundings' temperature. This is an effect known as night-sky cooling or radiative cooling. And it's always been understood by climate scientists and meteorologists as a very important natural phenomenon.
When I came across all of this, it was towards the end of my PhD at Stanford. And I was amazed by its apparent simplicity as a cooling method, yet really puzzled. Why aren't we making use of this? Now, scientists and engineers had investigated this idea in previous decades. But there turned out to be at least one big problem. It was called night-sky cooling for a reason. Why? Well, it's a little thing called the sun. So, for the surface that's doing the cooling, it needs to be able to face the sky. And during the middle of the day, when we might want something cold the most, unfortunately, that means you're going to look up to the sun. And the sun heats most materials up enough to completely counteract this cooling effect.
My colleagues and I spend a lot of our time thinking about how we can structure materials at very small length scales such that they can do new and useful things with light -- length scales smaller than the wavelength of light itself. Using insights from this field, known as nanophotonics or metamaterials research, we realized that there might be a way to make this possible during the day for the first time.
To do this, I designed a multilayer optical material shown here in a microscope image. It's more than 40 times thinner than a typical human hair. And it's able to do two things simultaneously. First, it sends its heat out precisely where our atmosphere lets that heat out the best. We targeted the window to space. The second thing it does is it avoids getting heated up by the sun. It's a very good mirror to sunlight. The first time I tested this was on a rooftop in Stanford that I'm showing you right here. I left the device out for a little while, and I walked up to it after a few minutes, and within seconds, I knew it was working. How? I touched it, and it felt cold.
Just to emphasize how weird and counterintuitive this is: this material and others like it will get colder when we take them out of the shade, even though the sun is shining on it. I'm showing you data here from our very first experiment, where that material stayed more than five degrees Celsius, or nine degrees Fahrenheit, colder than the air temperature, even though the sun was shining directly on it. The manufacturing method we used to actually make this material already exists at large volume scales. So I was really excited, because not only do we make something cool, but we might actually have the opportunity to do something real and make it useful. That brings me to the next big question.
How do you actually save energy with this idea? Well, we believe the most direct way to save energy with this technology is as an efficiency boost for today's air-conditioning and refrigeration systems. To do this, we've built fluid cooling panels, like the ones shown right here. These panels have a similar shape to solar water heaters, except they do the opposite -- they cool the water, passively, using our specialized material. These panels can then be integrated with a component almost every cooling system has, called a condenser, to improve the system's underlying efficiency. Our start-up, SkyCool Systems, has recently completed a field trial in Davis, California, shown right here. In that demonstration, we showed that we could actually improve the efficiency of that cooling system as much as 12 percent in the field.
Over the next year or two, I'm super excited to see this go to its first commercial-scale pilots in both the air conditioning and refrigeration space. In the future, we might be able to integrate these kinds of panels with higher efficiency building cooling systems to reduce their energy usage by two-thirds. And eventually, we might actually be able to build a cooling system that requires no electricity input at all. As a first step towards that, my colleagues at Stanford and I have shown that you could actually maintain something more than 42 degrees Celsius below the air temperature with better engineering.
Thank you.
So just imagine that -- something that is below freezing on a hot summer's day. So, while I'm very excited about all we can do for cooling, and I think there's a lot yet to be done, as a scientist, I'm also drawn to a more profound opportunity that I believe this work highlights. We can use the cold darkness of space to improve the efficiency of every energy-related process here on earth. One such process I'd like to highlight are solar cells. They heat up under the sun and become less efficient the hotter they are. In 2015, we showed that with deliberate kinds of microstructures on top of a solar cell, we could take better advantage of this cooling effect to maintain a solar cell passively at a lower temperature. This allows the cell to operate more efficiently. We're probing these kinds of opportunities further. We're asking whether we can use the cold of space to help us with water conservation. Or perhaps with off-grid scenarios. Perhaps we could even directly generate power with this cold. There's a large temperature difference between us here on earth and the cold of space. That difference, at least conceptually, could be used to drive something called a heat engine to generate electricity. Could we then make a nighttime power-generation device that generates useful amounts of electricity when solar cells don't work? Could we generate light from darkness?
Central to this ability is being able to manage the thermal radiation that's all around us. We're constantly bathed in infrared light; if we could bend it to our will, we could profoundly change the flows of heat and energy that permeate around us every single day. This ability, coupled with the cold darkness of space, points us to a future where we, as a civilization, might be able to more intelligently manage our thermal energy footprint at the very largest scales.
As we confront climate change, I believe having this ability in our toolkit will prove to be essential. So, the next time you're walking around outside, yes, do marvel at how the sun is essential to life on earth itself, but don't forget that the rest of the sky has something to offer us as well.
Thank you.
在我小的時候，每到夏天， 我都會從加拿大的家裡飛去看望 住在印度的祖父母。加拿大夏季的氣候還算宜人—— 溫度通常在 22ºC （72ºF）左右， 並不算很熱。然而，孟買是個悶熱潮溼的地方， 夏天的平均氣溫大概是 30ºC （90ºF）。每次到了孟買，我都會好奇， 「人們怎麼能在如此的天氣 生活、工作和睡覺呢？」 更糟糕的是， 我的祖父母家裡沒有空調。但即使我用盡渾身解數， 也沒能說服他們買一臺。但是這種情況正在得到快速改善。
如今，冷卻系統的耗能總共佔到了 全球電力供應的 17%， 其中就包括我在暑假 熱切渴望擁有的空調系統， 在超市中保證我們的食品 安全新鮮的製冷系統。以及保證我們的數據中心 正常運行的工業級製冷系統。這些系統一共貢獻了 全球溫室氣體排放量的 8%。
但是令我夜不能寐的是， 我們用於冷卻的能源 可能在 2050 年之前增加 6 倍， 主要是由於亞洲以及 非洲國家能源消耗的增長。我親眼目睹了這一切。我祖父母家周圍的每個公寓 如今幾乎都安裝了空調。這對生活在中高溫地帶的 居民的健康、幸福以及生產活動 明顯是有益的。但是，對於氣候變化， 最應為我們敲響警鐘的是， 我們的地球越溫暖， 我們對冷卻系統的需求就越大—— 而這些系統本身 又是溫室氣體排放的源頭。這就有可能會引起反饋循環， 在本世紀晚些時候， 單單是這些冷卻系統就會成為 最大的溫室氣體來源。在最壞的情況下， 到了 2100 年底，每年我們 用來冷卻的電能就有可能 超過10 萬億千瓦時。這是如今全球電力供應的一半， 還僅僅是用於冷卻。不過，這也為我們 提供了一個絕佳的機會。把冷卻系統的效率提升 10%-20%， 就會對溫室氣體排放 產生巨大的影響， 不論是在今天，還是幾十年後。並且，還能幫助我們 避免最壞情況下的反饋循環。
我是一名科學家，致力於研究光和熱， 尤其是新材料如何能夠 以我們一度難以想像的方式 改變這些自然基本元素的流動方式。 所以，雖然我非常清楚 冷卻系統在炎熱的暑期 所扮演的重要角色， 但我之所以對這個問題非常感興趣， 是因為六年前我遇到的一個智力題。 古代人是怎麼 在沙漠氣候中製造出冰的？ 這是一座冰屋的照片， 也叫做「冰坑」， 坐落於伊朗的西南部。伊朗境內遍布著幾十處這樣的廢墟， 中東的其他地方也有 類似建築存在的證據， 並且一直延伸到中國。
幾世紀前操作冰屋的人 會在傍晚太陽落山的時候， 將水傾倒在圖中左側的水池裡。 隨後，神奇的一幕發生了。 即使周圍空氣的溫度可能在冰點以上， 比如 5ºC （41ºF）， 水依然會結冰。人們會在黎明時分收集生成的冰， 儲存在圖片右側的建築中備用， 整個過程一直重複到夏天結束。如果你在空氣溫度 高於冰點的晴朗的夜晚 注意過地面上的霜， 你就會發現二者的相似之處。但是，等等—— 水是怎麼在零點以上結冰的呢？蒸發可能在其中起到了一定的作用， 但是還不夠導致水變成冰。一定有些別的東西 降低了它的溫度。
想像在窗臺上有一塊 正在冷卻的餡餅。 想讓它冷卻，它自身的熱量 需要傳遞到涼爽些的地方， 也就是它周圍的空氣中。 聽起來可能難以置信， 那一池水的熱量實際上 擴散到了寒冷的太空中。
這怎麼可能呢？ 就像大多數天然材料， 這一池的水也會 以光的形式散發它的熱量。 這個概念被稱作熱輻射。 實際上，我們現在都在以紅外光的形式 向彼此以及我們周圍的環境 散發自身的熱量， 我們實際上可以利用 熱成像儀觀察到這一現象， 並生成像這樣的圖像。 所以，那一池水把自身的熱量 散播到了上方的大氣中。 大氣以及其中的分子 吸收並反射回了部分的熱量。 而導致氣候變化的溫室效應 就是這樣發生的。
但很重要的一點在於， 我們的大氣不會吸收所有的熱量。 如果吸收了，我們將會 住在一個更溫暖的星球。 在某些波長下， 尤其是在 8 和 13 微米之間的波長下， 我們的大氣相當於一個「傳輸窗口」。這個窗口允許某些熱量 以紅外光的形式上升， 或者說逃逸，同時帶走水池的熱量。並且它會逃逸到十分冷的地方。從上層大氣 到外太空的這段區域， 溫度可以低至 -270ºC， 或者 -454ºF。所以這一池水向天空釋放的熱量 比天空反射回來的熱量要多。因此， 這池水會冷卻到低於周圍的環境溫度。這種效應被稱作 「夜空冷卻」， 或者「輻射冷卻」。這一自然現象的重要性早已被 氣候學家及氣象學家所熟知。
當我了解到這些時， 我在斯坦福的博士研究已經接近尾聲了。 我為如此簡單的冷卻方法所震驚， 但也十分的困惑。 為什麼我們還沒有 充分利用這一現象呢？ 在過去的數十年中， 科學家和工程師已經對這一想法 展開了研究。 但是他們發現，至少還需要 解決一個大問題。 這個現象被稱作「夜空冷卻」是有原因的。為什麼呢？因為有種小東西叫做太陽。當地球表面正在冷卻時， 它需要朝向天空。在中午時分， 在我們最需要冷卻什麼東西的時候， 很遺憾，這些東西也需要面朝太陽。而太陽會將大多數材料加熱， 足以完全抵消這種冷卻效果。
我和同事花費了很多時間 思考如何打造 一種微型材料—— 它的尺寸比光本身的波長更小—— 從而利用陽光 做一些新奇且實用的事情。利用這個領域的知識， 即納米光子學，或超材料研究， 我們首次發現了在白天 讓這個構想成為現實的方式。
為了實現這個目的， 我設計了這張顯微圖片中的 多層光學材料， 它比人的頭髮絲細 40 多倍， 並且能夠同時實現兩種功能。首先，它可以恰到好處的 散發出大氣能夠向外傳導的熱量。我們把這扇窗開向宇宙。其次，它不會被太陽加熱， 而是可以像鏡子一樣 高效的反射太陽光。我第一次測試這種材料 是在斯坦福的樓頂， 就在圖片裡的這個位置。我將這個設備放置在室外， 幾分鐘後，當我走上前查看， 就立刻知道它湊效了。我是怎麼知道的呢？它摸起來挺涼快的。
為了強調這多麼的違反直覺： 把類似這樣的材料 放置在陽光下， 它們的溫度反而會降低。 這是我們第一次的測試數據， 即使把它放在太陽光下， 材料表面的溫度始終維持在 比周圍大氣溫度 還要低 5 ºC （9 ºF）的水平。我們用來製造這種材料的方法 實際上已經規模化了。這讓我激動萬分， 因為我們不僅做出了很酷的東西， 並且很可能有機會 實現大規模的實際應用。這也引出了下一個大問題。
怎麼通過這個想法來節約能源？ 我們相信，這項技術最直接的節能方式 是提高當今 空調和製冷系統的效率。 為了做到這一點，我們 已經建造了流體冷卻板， 正如圖片中展示的一樣。 這些節能板看起來很像 太陽能熱水器， 卻有著截然相反的功能—— 使用我們發明的材料 被動的冷卻水。然後，這些面板可以與一個 幾乎每個冷卻系統 都擁有的部件，冷凝器 進行結合，以提高該系統的潛在效率。如圖所示，我們的初創公司 SkyCool Systems 如今已經在加利福尼亞州的 戴維斯市完成了實地測試。在這項演示中， 我們證明了可以將現場 冷卻系統的效率提高最多 12%。
在未來的一兩年內， 我非常期待這項技術 能夠在空調和製冷領域 開展第一個商業規模的試驗。 在未來，我們也許能夠將這類面板 與更高效的建築冷卻系統相結合， 從而減少三分之二的能源消耗。 最終，我們也許能夠打造 一個完全不需要 電力供應的冷卻系統。 作為邁向這一目標的第一步， 我和斯坦福的同事 已經證明了，通過更好的工程設計， 我們可以讓材料維持在比氣溫 低 42ºC 以上的溫度。
謝謝。
不妨想像一下—— 在炎熱的夏天，擁有一些 低於冰點的東西。我對於冷卻技術的 巨大潛能感到十分激動， 並且我認為仍然有 許多事情需要完成。作為科學家，我也被 這項工作所凸顯的 意義深遠的機會所吸引。我們可以利用寒冷的黑暗太空 加速地球上每一個 與能源相關的過程。我想強調的其中一個工程 就是太陽能電池。它們會在太陽光下被加熱， 但隨著溫度升高，效率會逐漸下降。在 2015 年，我們展示了 在太陽能電池的頂部 加上一些精心設計的微觀結構， 就可以更好的利用冷卻效果 來使太陽能電池被動的保持低溫。這保證了電池更高的操作效率。我們正在探索更多類似的機會。我們想知道，是否可以 使用太空中的低溫 來幫助我們節約用水， 又或者在離網的狀態下實現。也許，我們甚至可以 直接利用這種低溫來發電。地球表面與寒冷的太空之前 存在著很大的溫差。至少在概念上，這種差異 能夠被用來啟動所謂的「熱力發動機」 進行發電。我們能不能製造 一個夜間發電裝置， 當太陽能電池不工作的時候， 來產生大量的替代電能？我們能不能從黑暗當中產生光？
這種能力的核心在於 管理我們周圍的熱輻射。 在我們的四周，紅外線輻射無處不在； 如果我們能夠讓它為我們所用， 就可以徹底改變遍布在我們身邊的 熱量和能量流動。 這種能力，再加上 宇宙的寒冷黑暗， 能夠指引我們的未來—— 作為一個文明， 我們或許能夠 在非常大的尺度上更智能的管理 我們的熱能足跡。
在面對氣候變化時， 我相信，在我們的工具箱中 擁有這樣一種能力 將被證明是至關重要的。因此，下一次你在戶外散步， 在驚嘆太陽對地球的生命 如此重要的同時， 也不要忘記，天空的其他部分 也可以為我們提供一些東西。
謝謝。
（掌聲）
-End-