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← How we can turn the cold of outer space into a renewable resource

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Showing Revision 10 created 06/01/2018 by Brian Greene.

  1. Every summer when I was growing up,
  2. I would fly from my home in Canada
    to visit my grandparents,
  3. who lived in Mumbai, India.
  4. Now, Canadian summers
    are pretty mild at best --
  5. about 22 degrees Celsius
    or 72 degrees Fahrenheit
  6. is a typical summer's day,
    and not too hot.
  7. Mumbai, on the other hand,
    is a hot and humid place
  8. well into the 30s Celsius
    or 90s Fahrenheit.
  9. As soon as I'd reach it, I'd ask,
  10. "How could anyone live, work
    or sleep in such weather?"
  11. To make things worse, my grandparents
    didn't have an air conditioner.
  12. And while I tried my very, very best,
  13. I was never able
    to persuade them to get one.
  14. But this is changing, and fast.
  15. Cooling systems today
    collectively account for 17 percent

  16. of the electricity we use worldwide.
  17. This includes everything
    from the air conditioners
  18. I so desperately wanted
    during my summer vacations,
  19. to the refrigeration systems
    that keep our food safe and cold for us
  20. in our supermarkets,
  21. to the industrial scale systems
    that keep our data centers operational.
  22. Collectively, these systems
    account for eight percent
  23. of global greenhouse gas emissions.
  24. But what keeps me up at night

  25. is that our energy use for cooling
    might grow sixfold by the year 2050,
  26. primarily driven by increasing usage
    in Asian and African countries.
  27. I've seen this firsthand.
  28. Nearly every apartment
    in and around my grandmother's place
  29. now has an air conditioner.
  30. And that is, emphatically, a good thing
  31. for the health, well-being
    and productivity
  32. of people living in warmer climates.
  33. However, one of the most
    alarming things about climate change
  34. is that the warmer our planet gets,
  35. the more we're going to need
    cooling systems --
  36. systems that are themselves large
    emitters of greenhouse gas emissions.
  37. This then has the potential
    to cause a feedback loop,
  38. where cooling systems alone
  39. could become one of our biggest sources
    of greenhouse gases
  40. later this century.
  41. In the worst case,
  42. we might need more than 10 trillion
    kilowatt-hours of electricity every year,
  43. just for cooling, by the year 2100.
  44. That's half our electricity supply today.
  45. Just for cooling.
  46. But this also point us
    to an amazing opportunity.
  47. A 10 or 20 percent improvement
    in the efficiency of every cooling system
  48. could actually have an enormous impact
    on our greenhouse gas emissions,
  49. both today and later this century.
  50. And it could help us avert
    that worst-case feedback loop.
  51. I'm a scientist who thinks a lot
    about light and heat.

  52. In particular, how new materials
    allow us to alter the flow
  53. of these basic elements of nature
  54. in ways we might have
    once thought impossible.
  55. So, while I always understood
    the value of cooling
  56. during my summer vacations,
  57. I actually wound up
    working on this problem
  58. because of an intellectual puzzle
    that I came across about six years ago.
  59. How were ancient peoples
    able to make ice in desert climates?
  60. This is a picture of an ice house,
  61. also called a Yakhchal,
    located in the southwest of Iran.
  62. There are ruins of dozens
    of such structures throughout Iran,
  63. with evidence of similar such buildings
    throughout the rest of the Middle East
  64. and all the way to China.
  65. The people who operated
    this ice house many centuries ago,

  66. would pour water
    in the pool you see on the left
  67. in the early evening hours,
    as the sun set.
  68. And then something amazing happened.
  69. Even though the air temperature
    might be above freezing,
  70. say five degrees Celsius
    or 41 degrees Fahrenheit,
  71. the water would freeze.
  72. The ice generated would then be collected
    in the early morning hours
  73. and stored for use in the building
    you see on the right,
  74. all the way through the summer months.
  75. You've actually likely seen
    something very similar at play
  76. if you've ever noticed frost form
    on the ground on a clear night,
  77. even when the air temperature
    is well above freezing.
  78. But wait.
  79. How did the water freeze
    if the air temperature is above freezing?
  80. Evaporation could have played an effect,
  81. but that's not enough to actually
    cause the water to become ice.
  82. Something else must have cooled it down.
  83. Think about a pie
    cooling on a window sill.

  84. For it to be able to cool down,
    its heat needs to flow somewhere cooler.
  85. Namely, the air that surrounds it.
  86. As implausible as it may sound,
  87. for that pool of water, its heat
    is actually flowing to the cold of space.
  88. How is this possible?

  89. Well, that pool of water,
    like most natural materials,
  90. sends out its heat as light.
  91. This is a concept
    known as thermal radiation.
  92. In fact, we're all sending out our heat
    as infrared light right now,
  93. to each other and our surroundings.
  94. We can actually visualize this
    with thermal cameras
  95. and the images they produce,
    like the ones I'm showing you right now.
  96. So that pool of water
    is sending out its heat
  97. upward towards the atmosphere.
  98. The atmosphere and the molecules in it
  99. absorb some of that heat and send it back.
  100. That's actually the greenhouse effect
    that's responsible for climate change.
  101. But here's the critical thing
    to understand.

  102. Our atmosphere doesn't absorb
    all of that heat.
  103. If it did, we'd be
    on a much warmer planet.
  104. At certain wavelengths,
  105. in particular between
    eight and 13 microns,
  106. our atmosphere has what's known
    as a transmission window.
  107. This window allows some of the heat
    that goes up as infrared light
  108. to effectively escape,
    carrying away that pool's heat.
  109. And it can escape to a place
    that is much, much colder.
  110. The cold of this upper atmosphere
  111. and all the way out to outer space,
  112. which can be as cold
    as minus 270 degrees Celsius,
  113. or minus 454 degrees Fahrenheit.
  114. So that pool of water is able
    to send out more heat to the sky
  115. than the sky sends back to it.
  116. And because of that,
  117. the pool will cool down
    below its surroundings' temperature.
  118. This is an effect
    known as night-sky cooling
  119. or radiative cooling.
  120. And it's always been understood
    by climate scientists and meteorologists
  121. as a very important natural phenomenon.
  122. When I came across all of this,

  123. it was towards the end
    of my PhD at Stanford.
  124. And I was amazed by its apparent
    simplicity as a cooling method,
  125. yet really puzzled.
  126. Why aren't we making use of this?
  127. Now, scientists and engineers
    had investigated this idea
  128. in previous decades.
  129. But there turned out to be
    at least one big problem.
  130. It was called night-sky
    cooling for a reason.
  131. Why?
  132. Well, it's a little thing called the sun.
  133. So, for the surface
    that's doing the cooling,
  134. it needs to be able to face the sky.
  135. And during the middle of the day,
  136. when we might want
    something cold the most,
  137. unfortunately, that means
    you're going to look up to the sun.
  138. And the sun heats most materials up
  139. enough to completely counteract
    this cooling effect.
  140. My colleagues and I
    spend a lot of our time

  141. thinking about how
    we can structure materials
  142. at very small length scales
  143. such that they can do
    new and useful things with light --
  144. length scales smaller
    than the wavelength of light itself.
  145. Using insights from this field,
  146. known as nanophotonics
    or metamaterials research,
  147. we realized that there might be a way
    to make this possible during the day
  148. for the first time.
  149. To do this, I designed
    a multilayer optical material

  150. shown here in a microscope image.
  151. It's more than 40 times thinner
    than a typical human hair.
  152. And it's able to do
    two things simultaneously.
  153. First, it sends its heat out
  154. precisely where our atmosphere
    lets that heat out the best.
  155. We targeted the window to space.
  156. The second thing it does
    is it avoids getting heated up by the sun.
  157. It's a very good mirror to sunlight.
  158. The first time I tested this
    was on a rooftop in Stanford
  159. that I'm showing you right here.
  160. I left the device out for a little while,
  161. and I walked up to it after a few minutes,
  162. and within seconds, I knew it was working.
  163. How?
  164. I touched it, and it felt cold.
  165. (Applause)

  166. Just to emphasize how weird
    and counterintuitive this is:

  167. this material and others like it
  168. will get colder when we take them
    out of the shade,
  169. even though the sun is shining on it.
  170. I'm showing you data here
    from our very first experiment,
  171. where that material stayed
    more than five degrees Celsius,
  172. or nine degrees Fahrenheit, colder
    than the air temperature,
  173. even though the sun
    was shining directly on it.
  174. The manufacturing method we used
    to actually make this material
  175. already exists at large volume scales.
  176. So I was really excited,
  177. because not only
    do we make something cool,
  178. but we might actually have the opportunity
    to do something real and make it useful.
  179. That brings me to the next big question.
  180. How do you actually
    save energy with this idea?

  181. Well, we believe the most direct way
    to save energy with this technology
  182. is as an efficiency boost
  183. for today's air-conditioning
    and refrigeration systems.
  184. To do this, we've built
    fluid cooling panels,
  185. like the ones shown right here.
  186. These panels have a similar shape
    to solar water heaters,
  187. except they do the opposite --
    they cool the water, passively,
  188. using our specialized material.
  189. These panels can then
    be integrated with a component
  190. almost every cooling system has,
    called a condenser,
  191. to improve the system's
    underlying efficiency.
  192. Our start-up, SkyCool Systems,
  193. has recently completed a field trial
    in Davis, California, shown right here.
  194. In that demonstration,
  195. we showed that we could actually
    improve the efficiency
  196. of that cooling system
    as much as 12 percent in the field.
  197. Over the next year or two,

  198. I'm super excited to see this go
    to its first commercial-scale pilots
  199. in both the air conditioning
    and refrigeration space.
  200. In the future, we might be able
    to integrate these kinds of panels
  201. with higher efficiency
    building cooling systems
  202. to reduce their energy
    usage by two-thirds.
  203. And eventually, we might actually
    be able to build a cooling system
  204. that requires no electricity input at all.
  205. As a first step towards that,
  206. my colleagues at Stanford and I
  207. have shown that you could
    actually maintain
  208. something more than 42 degrees Celsius
    below the air temperature
  209. with better engineering.
  210. Thank you.

  211. (Applause)

  212. So just imagine that --

  213. something that is below freezing
    on a hot summer's day.
  214. So, while I'm very excited
    about all we can do for cooling,
  215. and I think there's a lot yet to be done,
  216. as a scientist, I'm also drawn
    to a more profound opportunity
  217. that I believe this work highlights.
  218. We can use the cold darkness of space
  219. to improve the efficiency
  220. of every energy-related
    process here on earth.
  221. One such process
    I'd like to highlight are solar cells.
  222. They heat up under the sun
  223. and become less efficient
    the hotter they are.
  224. In 2015, we showed that
    with deliberate kinds of microstructures
  225. on top of a solar cell,
  226. we could take better advantage
    of this cooling effect
  227. to maintain a solar cell passively
    at a lower temperature.
  228. This allows the cell
    to operate more efficiently.
  229. We're probing these kinds
    of opportunities further.
  230. We're asking whether
    we can use the cold of space
  231. to help us with water conservation.
  232. Or perhaps with off-grid scenarios.
  233. Perhaps we could even directly
    generate power with this cold.
  234. There's a large temperature difference
    between us here on earth
  235. and the cold of space.
  236. That difference, at least conceptually,
  237. could be used to drive
    something called a heat engine
  238. to generate electricity.
  239. Could we then make a nighttime
    power-generation device
  240. that generates useful
    amounts of electricity
  241. when solar cells don't work?
  242. Could we generate light from darkness?
  243. Central to this ability
    is being able to manage

  244. the thermal radiation
    that's all around us.
  245. We're constantly bathed in infrared light;
  246. if we could bend it to our will,
  247. we could profoundly change
    the flows of heat and energy
  248. that permeate around us every single day.
  249. This ability, coupled
    with the cold darkness of space,
  250. points us to a future
    where we, as a civilization,
  251. might be able to more intelligently manage
    our thermal energy footprint
  252. at the very largest scales.
  253. As we confront climate change,

  254. I believe having
    this ability in our toolkit
  255. will prove to be essential.
  256. So, the next time
    you're walking around outside,
  257. yes, do marvel at how the sun
    is essential to life on earth itself,
  258. but don't forget that the rest of the sky
    has something to offer us as well.
  259. Thank you.

  260. (Applause)