0:00:01.571,0:00:03.586 Every summer when I was growing up, 0:00:03.610,0:00:06.992 I would fly from my home in Canada[br]to visit my grandparents, 0:00:07.016,0:00:08.816 who lived in Mumbai, India. 0:00:09.157,0:00:12.291 Now, Canadian summers[br]are pretty mild at best -- 0:00:12.315,0:00:16.022 about 22 degrees Celsius[br]or 72 degrees Fahrenheit 0:00:16.046,0:00:18.666 is a typical summer's day,[br]and not too hot. 0:00:19.276,0:00:22.315 Mumbai, on the other hand,[br]is a hot and humid place 0:00:22.339,0:00:25.557 well into the 30s Celsius[br]or 90s Fahrenheit. 0:00:26.364,0:00:27.950 As soon as I'd reach it, I'd ask, 0:00:27.974,0:00:32.384 "How could anyone live, work[br]or sleep in such weather?" 0:00:33.539,0:00:37.185 To make things worse, my grandparents[br]didn't have an air conditioner. 0:00:37.768,0:00:40.593 And while I tried my very, very best, 0:00:40.617,0:00:43.371 I was never able[br]to persuade them to get one. 0:00:44.220,0:00:47.315 But this is changing, and fast. 0:00:47.914,0:00:52.255 Cooling systems today[br]collectively account for 17 percent 0:00:52.279,0:00:54.652 of the electricity we use worldwide. 0:00:54.676,0:00:57.070 This includes everything[br]from the air conditioners 0:00:57.094,0:00:59.871 I so desperately wanted[br]during my summer vacations, 0:00:59.895,0:01:03.505 to the refrigeration systems[br]that keep our food safe and cold for us 0:01:03.529,0:01:04.760 in our supermarkets, 0:01:04.784,0:01:09.036 to the industrial scale systems[br]that keep our data centers operational. 0:01:09.624,0:01:13.109 Collectively, these systems[br]account for eight percent 0:01:13.133,0:01:15.383 of global greenhouse gas emissions. 0:01:15.823,0:01:17.434 But what keeps me up at night 0:01:17.458,0:01:22.379 is that our energy use for cooling[br]might grow sixfold by the year 2050, 0:01:22.403,0:01:26.684 primarily driven by increasing usage[br]in Asian and African countries. 0:01:27.284,0:01:28.982 I've seen this firsthand. 0:01:29.006,0:01:32.164 Nearly every apartment[br]in and around my grandmother's place 0:01:32.188,0:01:34.212 now has an air conditioner. 0:01:34.236,0:01:37.283 And that is, emphatically, a good thing 0:01:37.307,0:01:40.006 for the health, well-being[br]and productivity 0:01:40.030,0:01:42.675 of people living in warmer climates. 0:01:43.831,0:01:47.774 However, one of the most[br]alarming things about climate change 0:01:47.798,0:01:50.393 is that the warmer our planet gets, 0:01:50.417,0:01:52.729 the more we're going to need[br]cooling systems -- 0:01:52.753,0:01:56.512 systems that are themselves large[br]emitters of greenhouse gas emissions. 0:01:57.236,0:02:00.569 This then has the potential[br]to cause a feedback loop, 0:02:00.593,0:02:02.051 where cooling systems alone 0:02:02.075,0:02:05.062 could become one of our biggest sources[br]of greenhouse gases 0:02:05.086,0:02:06.428 later this century. 0:02:06.735,0:02:07.913 In the worst case, 0:02:07.937,0:02:11.557 we might need more than 10 trillion[br]kilowatt-hours of electricity every year, 0:02:11.581,0:02:14.269 just for cooling, by the year 2100. 0:02:14.846,0:02:17.711 That's half our electricity supply today. 0:02:18.198,0:02:19.348 Just for cooling. 0:02:20.862,0:02:24.777 But this also point us[br]to an amazing opportunity. 0:02:25.444,0:02:30.013 A 10 or 20 percent improvement[br]in the efficiency of every cooling system 0:02:30.037,0:02:33.521 could actually have an enormous impact[br]on our greenhouse gas emissions, 0:02:33.545,0:02:36.010 both today and later this century. 0:02:38.080,0:02:41.629 And it could help us avert[br]that worst-case feedback loop. 0:02:42.928,0:02:46.608 I'm a scientist who thinks a lot[br]about light and heat. 0:02:46.632,0:02:50.228 In particular, how new materials[br]allow us to alter the flow 0:02:50.252,0:02:52.268 of these basic elements of nature 0:02:52.292,0:02:54.934 in ways we might have[br]once thought impossible. 0:02:55.315,0:02:57.728 So, while I always understood[br]the value of cooling 0:02:57.752,0:02:59.696 during my summer vacations, 0:02:59.720,0:03:01.983 I actually wound up[br]working on this problem 0:03:02.007,0:03:06.013 because of an intellectual puzzle[br]that I came across about six years ago. 0:03:07.149,0:03:12.766 How were ancient peoples[br]able to make ice in desert climates? 0:03:13.894,0:03:16.830 This is a picture of an ice house, 0:03:16.854,0:03:20.481 also called a Yakhchal,[br]located in the southwest of Iran. 0:03:21.006,0:03:24.760 There are ruins of dozens[br]of such structures throughout Iran, 0:03:24.784,0:03:28.474 with evidence of similar such buildings[br]throughout the rest of the Middle East 0:03:28.498,0:03:29.712 and all the way to China. 0:03:30.228,0:03:33.418 The people who operated[br]this ice house many centuries ago, 0:03:33.442,0:03:35.926 would pour water[br]in the pool you see on the left 0:03:35.950,0:03:39.022 in the early evening hours,[br]as the sun set. 0:03:39.046,0:03:40.855 And then something amazing happened. 0:03:41.442,0:03:44.204 Even though the air temperature[br]might be above freezing, 0:03:44.228,0:03:47.926 say five degrees Celsius[br]or 41 degrees Fahrenheit, 0:03:47.950,0:03:49.484 the water would freeze. 0:03:50.724,0:03:54.859 The ice generated would then be collected[br]in the early morning hours 0:03:54.883,0:03:57.518 and stored for use in the building[br]you see on the right, 0:03:57.542,0:03:59.478 all the way through the summer months. 0:04:00.133,0:04:02.852 You've actually likely seen[br]something very similar at play 0:04:02.876,0:04:06.328 if you've ever noticed frost form[br]on the ground on a clear night, 0:04:06.352,0:04:09.097 even when the air temperature[br]is well above freezing. 0:04:09.121,0:04:10.281 But wait. 0:04:10.305,0:04:14.036 How did the water freeze[br]if the air temperature is above freezing? 0:04:14.487,0:04:16.413 Evaporation could have played an effect, 0:04:16.437,0:04:19.752 but that's not enough to actually[br]cause the water to become ice. 0:04:20.064,0:04:22.150 Something else must have cooled it down. 0:04:22.761,0:04:25.309 Think about a pie[br]cooling on a window sill. 0:04:25.619,0:04:29.230 For it to be able to cool down,[br]its heat needs to flow somewhere cooler. 0:04:29.254,0:04:31.158 Namely, the air that surrounds it. 0:04:32.180,0:04:34.490 As implausible as it may sound, 0:04:34.514,0:04:39.890 for that pool of water, its heat[br]is actually flowing to the cold of space. 0:04:42.085,0:04:43.807 How is this possible? 0:04:44.434,0:04:48.006 Well, that pool of water,[br]like most natural materials, 0:04:48.030,0:04:50.180 sends out its heat as light. 0:04:50.506,0:04:53.323 This is a concept[br]known as thermal radiation. 0:04:53.792,0:04:58.260 In fact, we're all sending out our heat[br]as infrared light right now, 0:04:58.284,0:05:00.212 to each other and our surroundings. 0:05:00.608,0:05:03.077 We can actually visualize this[br]with thermal cameras 0:05:03.101,0:05:06.347 and the images they produce,[br]like the ones I'm showing you right now. 0:05:06.744,0:05:09.006 So that pool of water[br]is sending out its heat 0:05:09.030,0:05:10.712 upward towards the atmosphere. 0:05:11.379,0:05:13.474 The atmosphere and the molecules in it 0:05:13.498,0:05:16.053 absorb some of that heat and send it back. 0:05:16.077,0:05:19.899 That's actually the greenhouse effect[br]that's responsible for climate change. 0:05:20.435,0:05:22.958 But here's the critical thing[br]to understand. 0:05:22.982,0:05:26.182 Our atmosphere doesn't absorb[br]all of that heat. 0:05:26.577,0:05:29.511 If it did, we'd be[br]on a much warmer planet. 0:05:29.982,0:05:31.490 At certain wavelengths, 0:05:31.514,0:05:34.966 in particular between[br]eight and 13 microns, 0:05:34.990,0:05:38.752 our atmosphere has what's known[br]as a transmission window. 0:05:39.402,0:05:44.919 This window allows some of the heat[br]that goes up as infrared light 0:05:44.943,0:05:48.276 to effectively escape,[br]carrying away that pool's heat. 0:05:48.895,0:05:52.665 And it can escape to a place[br]that is much, much colder. 0:05:53.633,0:05:55.601 The cold of this upper atmosphere 0:05:55.625,0:05:57.299 and all the way out to outer space, 0:05:57.323,0:06:01.133 which can be as cold[br]as minus 270 degrees Celsius, 0:06:01.157,0:06:03.877 or minus 454 degrees Fahrenheit. 0:06:05.242,0:06:08.606 So that pool of water is able[br]to send out more heat to the sky 0:06:08.630,0:06:10.401 than the sky sends back to it. 0:06:10.425,0:06:11.575 And because of that, 0:06:11.599,0:06:14.615 the pool will cool down[br]below its surroundings' temperature. 0:06:16.035,0:06:19.551 This is an effect[br]known as night-sky cooling 0:06:19.575,0:06:20.975 or radiative cooling. 0:06:21.369,0:06:24.823 And it's always been understood[br]by climate scientists and meteorologists 0:06:24.847,0:06:27.447 as a very important natural phenomenon. 0:06:28.879,0:06:30.371 When I came across all of this, 0:06:30.395,0:06:33.037 it was towards the end[br]of my PhD at Stanford. 0:06:33.061,0:06:37.490 And I was amazed by its apparent[br]simplicity as a cooling method, 0:06:37.514,0:06:38.780 yet really puzzled. 0:06:39.284,0:06:41.484 Why aren't we making use of this? 0:06:42.744,0:06:45.625 Now, scientists and engineers[br]had investigated this idea 0:06:45.649,0:06:46.887 in previous decades. 0:06:46.911,0:06:50.199 But there turned out to be[br]at least one big problem. 0:06:50.879,0:06:53.751 It was called night-sky[br]cooling for a reason. 0:06:54.109,0:06:55.283 Why? 0:06:55.307,0:06:57.676 Well, it's a little thing called the sun. 0:06:58.157,0:07:00.617 So, for the surface[br]that's doing the cooling, 0:07:00.641,0:07:02.815 it needs to be able to face the sky. 0:07:02.839,0:07:04.467 And during the middle of the day, 0:07:04.491,0:07:07.641 when we might want[br]something cold the most, 0:07:07.665,0:07:10.561 unfortunately, that means[br]you're going to look up to the sun. 0:07:10.585,0:07:12.441 And the sun heats most materials up 0:07:12.465,0:07:15.222 enough to completely counteract[br]this cooling effect. 0:07:16.409,0:07:18.417 My colleagues and I[br]spend a lot of our time 0:07:18.441,0:07:20.553 thinking about how[br]we can structure materials 0:07:20.577,0:07:21.989 at very small length scales 0:07:22.013,0:07:25.339 such that they can do[br]new and useful things with light -- 0:07:25.363,0:07:28.353 length scales smaller[br]than the wavelength of light itself. 0:07:28.377,0:07:29.957 Using insights from this field, 0:07:29.981,0:07:33.109 known as nanophotonics[br]or metamaterials research, 0:07:33.133,0:07:36.633 we realized that there might be a way[br]to make this possible during the day 0:07:36.657,0:07:37.830 for the first time. 0:07:37.854,0:07:40.910 To do this, I designed[br]a multilayer optical material 0:07:40.934,0:07:42.791 shown here in a microscope image. 0:07:42.815,0:07:46.196 It's more than 40 times thinner[br]than a typical human hair. 0:07:46.220,0:07:48.738 And it's able to do[br]two things simultaneously. 0:07:49.169,0:07:50.994 First, it sends its heat out 0:07:51.018,0:07:54.820 precisely where our atmosphere[br]lets that heat out the best. 0:07:54.844,0:07:56.977 We targeted the window to space. 0:07:57.519,0:08:00.950 The second thing it does[br]is it avoids getting heated up by the sun. 0:08:00.974,0:08:03.374 It's a very good mirror to sunlight. 0:08:04.315,0:08:07.029 The first time I tested this[br]was on a rooftop in Stanford 0:08:07.053,0:08:08.815 that I'm showing you right here. 0:08:09.339,0:08:11.720 I left the device out for a little while, 0:08:11.744,0:08:14.815 and I walked up to it after a few minutes, 0:08:14.839,0:08:17.633 and within seconds, I knew it was working. 0:08:17.657,0:08:18.815 How? 0:08:18.839,0:08:20.466 I touched it, and it felt cold. 0:08:21.395,0:08:26.053 (Applause) 0:08:26.862,0:08:30.846 Just to emphasize how weird[br]and counterintuitive this is: 0:08:30.870,0:08:32.600 this material and others like it 0:08:32.624,0:08:35.529 will get colder when we take them[br]out of the shade, 0:08:35.553,0:08:37.913 even though the sun is shining on it. 0:08:37.937,0:08:40.620 I'm showing you data here[br]from our very first experiment, 0:08:40.644,0:08:43.383 where that material stayed[br]more than five degrees Celsius, 0:08:43.407,0:08:46.683 or nine degrees Fahrenheit, colder[br]than the air temperature, 0:08:46.707,0:08:49.521 even though the sun[br]was shining directly on it. 0:08:50.855,0:08:53.990 The manufacturing method we used[br]to actually make this material 0:08:54.014,0:08:56.548 already exists at large volume scales. 0:08:56.903,0:08:58.060 So I was really excited, 0:08:58.084,0:09:01.125 because not only[br]do we make something cool, 0:09:01.149,0:09:06.202 but we might actually have the opportunity[br]to do something real and make it useful. 0:09:07.204,0:09:09.117 That brings me to the next big question. 0:09:09.141,0:09:11.728 How do you actually[br]save energy with this idea? 0:09:11.752,0:09:15.410 Well, we believe the most direct way[br]to save energy with this technology 0:09:15.434,0:09:17.093 is as an efficiency boost 0:09:17.117,0:09:20.180 for today's air-conditioning[br]and refrigeration systems. 0:09:20.561,0:09:22.720 To do this, we've built[br]fluid cooling panels, 0:09:22.744,0:09:24.315 like the ones shown right here. 0:09:24.339,0:09:27.006 These panels have a similar shape[br]to solar water heaters, 0:09:27.030,0:09:29.950 except they do the opposite --[br]they cool the water, passively, 0:09:29.974,0:09:32.041 using our specialized material. 0:09:32.815,0:09:35.275 These panels can then[br]be integrated with a component 0:09:35.299,0:09:37.867 almost every cooling system has,[br]called a condenser, 0:09:37.891,0:09:41.024 to improve the system's[br]underlying efficiency. 0:09:41.367,0:09:43.263 Our start-up, SkyCool Systems, 0:09:43.287,0:09:47.161 has recently completed a field trial[br]in Davis, California, shown right here. 0:09:47.649,0:09:48.831 In that demonstration, 0:09:48.855,0:09:51.903 we showed that we could actually[br]improve the efficiency 0:09:51.927,0:09:54.851 of that cooling system[br]as much as 12 percent in the field. 0:09:55.474,0:09:56.728 Over the next year or two, 0:09:56.752,0:10:00.656 I'm super excited to see this go[br]to its first commercial-scale pilots 0:10:00.680,0:10:03.823 in both the air conditioning[br]and refrigeration space. 0:10:04.260,0:10:07.847 In the future, we might be able[br]to integrate these kinds of panels 0:10:07.871,0:10:11.180 with higher efficiency[br]building cooling systems 0:10:11.204,0:10:14.006 to reduce their energy[br]usage by two-thirds. 0:10:14.030,0:10:17.688 And eventually, we might actually[br]be able to build a cooling system 0:10:17.712,0:10:20.275 that requires no electricity input at all. 0:10:20.966,0:10:22.482 As a first step towards that, 0:10:22.506,0:10:24.363 my colleagues at Stanford and I 0:10:24.387,0:10:26.413 have shown that you could[br]actually maintain 0:10:26.437,0:10:31.006 something more than 42 degrees Celsius[br]below the air temperature 0:10:31.030,0:10:32.418 with better engineering. 0:10:33.165,0:10:34.315 Thank you. 0:10:34.339,0:10:38.394 (Applause) 0:10:39.196,0:10:40.347 So just imagine that -- 0:10:40.371,0:10:43.774 something that is below freezing[br]on a hot summer's day. 0:10:45.927,0:10:50.410 So, while I'm very excited[br]about all we can do for cooling, 0:10:50.434,0:10:53.688 and I think there's a lot yet to be done, 0:10:53.712,0:10:57.180 as a scientist, I'm also drawn[br]to a more profound opportunity 0:10:57.204,0:10:59.220 that I believe this work highlights. 0:10:59.760,0:11:02.895 We can use the cold darkness of space 0:11:02.919,0:11:04.569 to improve the efficiency 0:11:04.593,0:11:07.847 of every energy-related[br]process here on earth. 0:11:09.204,0:11:12.521 One such process[br]I'd like to highlight are solar cells. 0:11:12.934,0:11:14.379 They heat up under the sun 0:11:14.403,0:11:16.887 and become less efficient[br]the hotter they are. 0:11:17.276,0:11:21.153 In 2015, we showed that[br]with deliberate kinds of microstructures 0:11:21.177,0:11:22.724 on top of a solar cell, 0:11:22.748,0:11:25.592 we could take better advantage[br]of this cooling effect 0:11:25.616,0:11:29.315 to maintain a solar cell passively[br]at a lower temperature. 0:11:29.708,0:11:32.029 This allows the cell[br]to operate more efficiently. 0:11:32.627,0:11:35.595 We're probing these kinds[br]of opportunities further. 0:11:35.619,0:11:38.864 We're asking whether[br]we can use the cold of space 0:11:38.888,0:11:40.967 to help us with water conservation. 0:11:41.316,0:11:43.665 Or perhaps with off-grid scenarios. 0:11:43.689,0:11:47.856 Perhaps we could even directly[br]generate power with this cold. 0:11:48.522,0:11:51.475 There's a large temperature difference[br]between us here on earth 0:11:51.499,0:11:53.189 and the cold of space. 0:11:53.213,0:11:55.341 That difference, at least conceptually, 0:11:55.365,0:11:57.959 could be used to drive[br]something called a heat engine 0:11:57.983,0:11:59.173 to generate electricity. 0:11:59.967,0:12:03.570 Could we then make a nighttime[br]power-generation device 0:12:03.594,0:12:05.991 that generates useful[br]amounts of electricity 0:12:06.015,0:12:07.919 when solar cells don't work? 0:12:07.943,0:12:10.477 Could we generate light from darkness? 0:12:11.872,0:12:16.261 Central to this ability[br]is being able to manage 0:12:16.285,0:12:19.396 the thermal radiation[br]that's all around us. 0:12:19.420,0:12:22.220 We're constantly bathed in infrared light; 0:12:22.666,0:12:25.118 if we could bend it to our will, 0:12:25.142,0:12:27.872 we could profoundly change[br]the flows of heat and energy 0:12:27.896,0:12:30.629 that permeate around us every single day. 0:12:31.190,0:12:34.531 This ability, coupled[br]with the cold darkness of space, 0:12:34.555,0:12:37.864 points us to a future[br]where we, as a civilization, 0:12:37.888,0:12:43.126 might be able to more intelligently manage[br]our thermal energy footprint 0:12:43.150,0:12:44.950 at the very largest scales. 0:12:45.904,0:12:48.118 As we confront climate change, 0:12:48.142,0:12:50.745 I believe having[br]this ability in our toolkit 0:12:50.769,0:12:52.569 will prove to be essential. 0:12:53.428,0:12:56.626 So, the next time[br]you're walking around outside, 0:12:56.650,0:13:02.994 yes, do marvel at how the sun[br]is essential to life on earth itself, 0:13:03.018,0:13:07.694 but don't forget that the rest of the sky[br]has something to offer us as well. 0:13:08.533,0:13:09.684 Thank you. 0:13:09.708,0:13:13.818 (Applause)