1 00:00:01,571 --> 00:00:03,586 Every summer when I was growing up, 2 00:00:03,610 --> 00:00:06,992 I would fly from my home in Canada to visit my grandparents, 3 00:00:07,016 --> 00:00:08,816 who lived in Mumbai, India. 4 00:00:09,157 --> 00:00:12,291 Now, Canadian summers are pretty mild at best -- 5 00:00:12,315 --> 00:00:16,022 about 22 degrees Celsius or 72 degrees Fahrenheit 6 00:00:16,046 --> 00:00:18,666 is a typical summer's day, and not too hot. 7 00:00:19,276 --> 00:00:22,315 Mumbai, on the other hand, is a hot and humid place 8 00:00:22,339 --> 00:00:25,557 well into the 30s Celsius or 90s Fahrenheit. 9 00:00:26,364 --> 00:00:27,950 As soon as I'd reach it, I'd ask, 10 00:00:27,974 --> 00:00:32,384 "How could anyone live, work or sleep in such weather?" 11 00:00:33,539 --> 00:00:37,185 To make things worse, my grandparents didn't have an air conditioner. 12 00:00:37,768 --> 00:00:40,593 And while I tried my very, very best, 13 00:00:40,617 --> 00:00:43,371 I was never able to persuade them to get one. 14 00:00:44,220 --> 00:00:47,315 But this is changing, and fast. 15 00:00:47,914 --> 00:00:52,255 Cooling systems today collectively account for 17 percent 16 00:00:52,279 --> 00:00:54,652 of the electricity we use worldwide. 17 00:00:54,676 --> 00:00:57,070 This includes everything from the air conditioners 18 00:00:57,094 --> 00:00:59,871 I so desperately wanted during my summer vacations, 19 00:00:59,895 --> 00:01:03,505 to the refrigeration systems that keep our food safe and cold for us 20 00:01:03,529 --> 00:01:04,760 in our supermarkets, 21 00:01:04,784 --> 00:01:09,036 to the industrial scale systems that keep our data centers operational. 22 00:01:09,624 --> 00:01:13,109 Collectively, these systems account for eight percent 23 00:01:13,133 --> 00:01:15,383 of global greenhouse gas emissions. 24 00:01:15,823 --> 00:01:17,434 But what keeps me up at night 25 00:01:17,458 --> 00:01:22,379 is that our energy use for cooling might grow sixfold by the year 2050, 26 00:01:22,403 --> 00:01:26,684 primarily driven by increasing usage in Asian and African countries. 27 00:01:27,284 --> 00:01:28,982 I've seen this firsthand. 28 00:01:29,006 --> 00:01:32,164 Nearly every apartment in and around my grandmother's place 29 00:01:32,188 --> 00:01:34,212 now has an air conditioner. 30 00:01:34,236 --> 00:01:37,283 And that is, emphatically, a good thing 31 00:01:37,307 --> 00:01:40,006 for the health, well-being and productivity 32 00:01:40,030 --> 00:01:42,675 of people living in warmer climates. 33 00:01:43,831 --> 00:01:47,774 However, one of the most alarming things about climate change 34 00:01:47,798 --> 00:01:50,393 is that the warmer our planet gets, 35 00:01:50,417 --> 00:01:52,729 the more we're going to need cooling systems -- 36 00:01:52,753 --> 00:01:56,512 systems that are themselves large emitters of greenhouse gas emissions. 37 00:01:57,236 --> 00:02:00,569 This then has the potential to cause a feedback loop, 38 00:02:00,593 --> 00:02:02,051 where cooling systems alone 39 00:02:02,075 --> 00:02:05,062 could become one of our biggest sources of greenhouse gases 40 00:02:05,086 --> 00:02:06,428 later this century. 41 00:02:06,735 --> 00:02:07,913 In the worst case, 42 00:02:07,937 --> 00:02:11,557 we might need more than 10 trillion kilowatt-hours of electricity every year, 43 00:02:11,581 --> 00:02:14,269 just for cooling, by the year 2100. 44 00:02:14,846 --> 00:02:17,711 That's half our electricity supply today. 45 00:02:18,198 --> 00:02:19,348 Just for cooling. 46 00:02:20,862 --> 00:02:24,777 But this also point us to an amazing opportunity. 47 00:02:25,444 --> 00:02:30,013 A 10 or 20 percent improvement in the efficiency of every cooling system 48 00:02:30,037 --> 00:02:33,521 could actually have an enormous impact on our greenhouse gas emissions, 49 00:02:33,545 --> 00:02:36,010 both today and later this century. 50 00:02:38,080 --> 00:02:41,629 And it could help us avert that worst-case feedback loop. 51 00:02:42,928 --> 00:02:46,608 I'm a scientist who thinks a lot about light and heat. 52 00:02:46,632 --> 00:02:50,228 In particular, how new materials allow us to alter the flow 53 00:02:50,252 --> 00:02:52,268 of these basic elements of nature 54 00:02:52,292 --> 00:02:54,934 in ways we might have once thought impossible. 55 00:02:55,315 --> 00:02:57,728 So, while I always understood the value of cooling 56 00:02:57,752 --> 00:02:59,696 during my summer vacations, 57 00:02:59,720 --> 00:03:01,983 I actually wound up working on this problem 58 00:03:02,007 --> 00:03:06,013 because of an intellectual puzzle that I came across about six years ago. 59 00:03:07,149 --> 00:03:12,766 How were ancient peoples able to make ice in desert climates? 60 00:03:13,894 --> 00:03:16,830 This is a picture of an ice house, 61 00:03:16,854 --> 00:03:20,481 also called a Yakhchal, located in the southwest of Iran. 62 00:03:21,006 --> 00:03:24,760 There are ruins of dozens of such structures throughout Iran, 63 00:03:24,784 --> 00:03:28,474 with evidence of similar such buildings throughout the rest of the Middle East 64 00:03:28,498 --> 00:03:29,712 and all the way to China. 65 00:03:30,228 --> 00:03:33,418 The people who operated this ice house many centuries ago, 66 00:03:33,442 --> 00:03:35,926 would pour water in the pool you see on the left 67 00:03:35,950 --> 00:03:39,022 in the early evening hours, as the sun set. 68 00:03:39,046 --> 00:03:40,855 And then something amazing happened. 69 00:03:41,442 --> 00:03:44,204 Even though the air temperature might be above freezing, 70 00:03:44,228 --> 00:03:47,926 say five degrees Celsius or 41 degrees Fahrenheit, 71 00:03:47,950 --> 00:03:49,484 the water would freeze. 72 00:03:50,724 --> 00:03:54,859 The ice generated would then be collected in the early morning hours 73 00:03:54,883 --> 00:03:57,518 and stored for use in the building you see on the right, 74 00:03:57,542 --> 00:03:59,478 all the way through the summer months. 75 00:04:00,133 --> 00:04:02,852 You've actually likely seen something very similar at play 76 00:04:02,876 --> 00:04:06,328 if you've ever noticed frost form on the ground on a clear night, 77 00:04:06,352 --> 00:04:09,097 even when the air temperature is well above freezing. 78 00:04:09,121 --> 00:04:10,281 But wait. 79 00:04:10,305 --> 00:04:14,036 How did the water freeze if the air temperature is above freezing? 80 00:04:14,487 --> 00:04:16,413 Evaporation could have played an effect, 81 00:04:16,437 --> 00:04:19,752 but that's not enough to actually cause the water to become ice. 82 00:04:20,064 --> 00:04:22,150 Something else must have cooled it down. 83 00:04:22,761 --> 00:04:25,309 Think about a pie cooling on a window sill. 84 00:04:25,619 --> 00:04:29,230 For it to be able to cool down, its heat needs to flow somewhere cooler. 85 00:04:29,254 --> 00:04:31,158 Namely, the air that surrounds it. 86 00:04:32,180 --> 00:04:34,490 As implausible as it may sound, 87 00:04:34,514 --> 00:04:39,890 for that pool of water, its heat is actually flowing to the cold of space. 88 00:04:42,085 --> 00:04:43,807 How is this possible? 89 00:04:44,434 --> 00:04:48,006 Well, that pool of water, like most natural materials, 90 00:04:48,030 --> 00:04:50,180 sends out its heat as light. 91 00:04:50,506 --> 00:04:53,323 This is a concept known as thermal radiation. 92 00:04:53,792 --> 00:04:58,260 In fact, we're all sending out our heat as infrared light right now, 93 00:04:58,284 --> 00:05:00,212 to each other and our surroundings. 94 00:05:00,608 --> 00:05:03,077 We can actually visualize this with thermal cameras 95 00:05:03,101 --> 00:05:06,347 and the images they produce, like the ones I'm showing you right now. 96 00:05:06,744 --> 00:05:09,006 So that pool of water is sending out its heat 97 00:05:09,030 --> 00:05:10,712 upward towards the atmosphere. 98 00:05:11,379 --> 00:05:13,474 The atmosphere and the molecules in it 99 00:05:13,498 --> 00:05:16,053 absorb some of that heat and send it back. 100 00:05:16,077 --> 00:05:19,899 That's actually the greenhouse effect that's responsible for climate change. 101 00:05:20,435 --> 00:05:22,958 But here's the critical thing to understand. 102 00:05:22,982 --> 00:05:26,182 Our atmosphere doesn't absorb all of that heat. 103 00:05:26,577 --> 00:05:29,511 If it did, we'd be on a much warmer planet. 104 00:05:29,982 --> 00:05:31,490 At certain wavelengths, 105 00:05:31,514 --> 00:05:34,966 in particular between eight and 13 microns, 106 00:05:34,990 --> 00:05:38,752 our atmosphere has what's known as a transmission window. 107 00:05:39,402 --> 00:05:44,919 This window allows some of the heat that goes up as infrared light 108 00:05:44,943 --> 00:05:48,276 to effectively escape, carrying away that pool's heat. 109 00:05:48,895 --> 00:05:52,665 And it can escape to a place that is much, much colder. 110 00:05:53,633 --> 00:05:55,601 The cold of this upper atmosphere 111 00:05:55,625 --> 00:05:57,299 and all the way out to outer space, 112 00:05:57,323 --> 00:06:01,133 which can be as cold as minus 270 degrees Celsius, 113 00:06:01,157 --> 00:06:03,877 or minus 454 degrees Fahrenheit. 114 00:06:05,242 --> 00:06:08,606 So that pool of water is able to send out more heat to the sky 115 00:06:08,630 --> 00:06:10,401 than the sky sends back to it. 116 00:06:10,425 --> 00:06:11,575 And because of that, 117 00:06:11,599 --> 00:06:14,615 the pool will cool down below its surroundings' temperature. 118 00:06:16,035 --> 00:06:19,551 This is an effect known as night-sky cooling 119 00:06:19,575 --> 00:06:20,975 or radiative cooling. 120 00:06:21,369 --> 00:06:24,823 And it's always been understood by climate scientists and meteorologists 121 00:06:24,847 --> 00:06:27,447 as a very important natural phenomenon. 122 00:06:28,879 --> 00:06:30,371 When I came across all of this, 123 00:06:30,395 --> 00:06:33,037 it was towards the end of my PhD at Stanford. 124 00:06:33,061 --> 00:06:37,490 And I was amazed by its apparent simplicity as a cooling method, 125 00:06:37,514 --> 00:06:38,780 yet really puzzled. 126 00:06:39,284 --> 00:06:41,484 Why aren't we making use of this? 127 00:06:42,744 --> 00:06:45,625 Now, scientists and engineers had investigated this idea 128 00:06:45,649 --> 00:06:46,887 in previous decades. 129 00:06:46,911 --> 00:06:50,199 But there turned out to be at least one big problem. 130 00:06:50,879 --> 00:06:53,751 It was called night-sky cooling for a reason. 131 00:06:54,109 --> 00:06:55,283 Why? 132 00:06:55,307 --> 00:06:57,676 Well, it's a little thing called the sun. 133 00:06:58,157 --> 00:07:00,617 So, for the surface that's doing the cooling, 134 00:07:00,641 --> 00:07:02,815 it needs to be able to face the sky. 135 00:07:02,839 --> 00:07:04,467 And during the middle of the day, 136 00:07:04,491 --> 00:07:07,641 when we might want something cold the most, 137 00:07:07,665 --> 00:07:10,561 unfortunately, that means you're going to look up to the sun. 138 00:07:10,585 --> 00:07:12,441 And the sun heats most materials up 139 00:07:12,465 --> 00:07:15,222 enough to completely counteract this cooling effect. 140 00:07:16,409 --> 00:07:18,417 My colleagues and I spend a lot of our time 141 00:07:18,441 --> 00:07:20,553 thinking about how we can structure materials 142 00:07:20,577 --> 00:07:21,989 at very small length scales 143 00:07:22,013 --> 00:07:25,339 such that they can do new and useful things with light -- 144 00:07:25,363 --> 00:07:28,353 length scales smaller than the wavelength of light itself. 145 00:07:28,377 --> 00:07:29,957 Using insights from this field, 146 00:07:29,981 --> 00:07:33,109 known as nanophotonics or metamaterials research, 147 00:07:33,133 --> 00:07:36,633 we realized that there might be a way to make this possible during the day 148 00:07:36,657 --> 00:07:37,830 for the first time. 149 00:07:37,854 --> 00:07:40,910 To do this, I designed a multilayer optical material 150 00:07:40,934 --> 00:07:42,791 shown here in a microscope image. 151 00:07:42,815 --> 00:07:46,196 It's more than 40 times thinner than a typical human hair. 152 00:07:46,220 --> 00:07:48,738 And it's able to do two things simultaneously. 153 00:07:49,169 --> 00:07:50,994 First, it sends its heat out 154 00:07:51,018 --> 00:07:54,820 precisely where our atmosphere lets that heat out the best. 155 00:07:54,844 --> 00:07:56,977 We targeted the window to space. 156 00:07:57,519 --> 00:08:00,950 The second thing it does is it avoids getting heated up by the sun. 157 00:08:00,974 --> 00:08:03,374 It's a very good mirror to sunlight. 158 00:08:04,315 --> 00:08:07,029 The first time I tested this was on a rooftop in Stanford 159 00:08:07,053 --> 00:08:08,815 that I'm showing you right here. 160 00:08:09,339 --> 00:08:11,720 I left the device out for a little while, 161 00:08:11,744 --> 00:08:14,815 and I walked up to it after a few minutes, 162 00:08:14,839 --> 00:08:17,633 and within seconds, I knew it was working. 163 00:08:17,657 --> 00:08:18,815 How? 164 00:08:18,839 --> 00:08:20,466 I touched it, and it felt cold. 165 00:08:21,395 --> 00:08:26,053 (Applause) 166 00:08:26,862 --> 00:08:30,846 Just to emphasize how weird and counterintuitive this is: 167 00:08:30,870 --> 00:08:32,600 this material and others like it 168 00:08:32,624 --> 00:08:35,529 will get colder when we take them out of the shade, 169 00:08:35,553 --> 00:08:37,913 even though the sun is shining on it. 170 00:08:37,937 --> 00:08:40,620 I'm showing you data here from our very first experiment, 171 00:08:40,644 --> 00:08:43,383 where that material stayed more than five degrees Celsius, 172 00:08:43,407 --> 00:08:46,683 or nine degrees Fahrenheit, colder than the air temperature, 173 00:08:46,707 --> 00:08:49,521 even though the sun was shining directly on it. 174 00:08:50,855 --> 00:08:53,990 The manufacturing method we used to actually make this material 175 00:08:54,014 --> 00:08:56,548 already exists at large volume scales. 176 00:08:56,903 --> 00:08:58,060 So I was really excited, 177 00:08:58,084 --> 00:09:01,125 because not only do we make something cool, 178 00:09:01,149 --> 00:09:06,202 but we might actually have the opportunity to do something real and make it useful. 179 00:09:07,204 --> 00:09:09,117 That brings me to the next big question. 180 00:09:09,141 --> 00:09:11,728 How do you actually save energy with this idea? 181 00:09:11,752 --> 00:09:15,410 Well, we believe the most direct way to save energy with this technology 182 00:09:15,434 --> 00:09:17,093 is as an efficiency boost 183 00:09:17,117 --> 00:09:20,180 for today's air-conditioning and refrigeration systems. 184 00:09:20,561 --> 00:09:22,720 To do this, we've built fluid cooling panels, 185 00:09:22,744 --> 00:09:24,315 like the ones shown right here. 186 00:09:24,339 --> 00:09:27,006 These panels have a similar shape to solar water heaters, 187 00:09:27,030 --> 00:09:29,950 except they do the opposite -- they cool the water, passively, 188 00:09:29,974 --> 00:09:32,041 using our specialized material. 189 00:09:32,815 --> 00:09:35,275 These panels can then be integrated with a component 190 00:09:35,299 --> 00:09:37,867 almost every cooling system has, called a condenser, 191 00:09:37,891 --> 00:09:41,024 to improve the system's underlying efficiency. 192 00:09:41,367 --> 00:09:43,263 Our start-up, SkyCool Systems, 193 00:09:43,287 --> 00:09:47,161 has recently completed a field trial in Davis, California, shown right here. 194 00:09:47,649 --> 00:09:48,831 In that demonstration, 195 00:09:48,855 --> 00:09:51,903 we showed that we could actually improve the efficiency 196 00:09:51,927 --> 00:09:54,851 of that cooling system as much as 12 percent in the field. 197 00:09:55,474 --> 00:09:56,728 Over the next year or two, 198 00:09:56,752 --> 00:10:00,656 I'm super excited to see this go to its first commercial-scale pilots 199 00:10:00,680 --> 00:10:03,823 in both the air conditioning and refrigeration space. 200 00:10:04,260 --> 00:10:07,847 In the future, we might be able to integrate these kinds of panels 201 00:10:07,871 --> 00:10:11,180 with higher efficiency building cooling systems 202 00:10:11,204 --> 00:10:14,006 to reduce their energy usage by two-thirds. 203 00:10:14,030 --> 00:10:17,688 And eventually, we might actually be able to build a cooling system 204 00:10:17,712 --> 00:10:20,275 that requires no electricity input at all. 205 00:10:20,966 --> 00:10:22,482 As a first step towards that, 206 00:10:22,506 --> 00:10:24,363 my colleagues at Stanford and I 207 00:10:24,387 --> 00:10:26,413 have shown that you could actually maintain 208 00:10:26,437 --> 00:10:31,006 something more than 42 degrees Celsius below the air temperature 209 00:10:31,030 --> 00:10:32,418 with better engineering. 210 00:10:33,165 --> 00:10:34,315 Thank you. 211 00:10:34,339 --> 00:10:38,394 (Applause) 212 00:10:39,196 --> 00:10:40,347 So just imagine that -- 213 00:10:40,371 --> 00:10:43,774 something that is below freezing on a hot summer's day. 214 00:10:45,927 --> 00:10:50,410 So, while I'm very excited about all we can do for cooling, 215 00:10:50,434 --> 00:10:53,688 and I think there's a lot yet to be done, 216 00:10:53,712 --> 00:10:57,180 as a scientist, I'm also drawn to a more profound opportunity 217 00:10:57,204 --> 00:10:59,220 that I believe this work highlights. 218 00:10:59,760 --> 00:11:02,895 We can use the cold darkness of space 219 00:11:02,919 --> 00:11:04,569 to improve the efficiency 220 00:11:04,593 --> 00:11:07,847 of every energy-related process here on earth. 221 00:11:09,204 --> 00:11:12,521 One such process I'd like to highlight are solar cells. 222 00:11:12,934 --> 00:11:14,379 They heat up under the sun 223 00:11:14,403 --> 00:11:16,887 and become less efficient the hotter they are. 224 00:11:17,276 --> 00:11:21,153 In 2015, we showed that with deliberate kinds of microstructures 225 00:11:21,177 --> 00:11:22,724 on top of a solar cell, 226 00:11:22,748 --> 00:11:25,592 we could take better advantage of this cooling effect 227 00:11:25,616 --> 00:11:29,315 to maintain a solar cell passively at a lower temperature. 228 00:11:29,708 --> 00:11:32,029 This allows the cell to operate more efficiently. 229 00:11:32,627 --> 00:11:35,595 We're probing these kinds of opportunities further. 230 00:11:35,619 --> 00:11:38,864 We're asking whether we can use the cold of space 231 00:11:38,888 --> 00:11:40,967 to help us with water conservation. 232 00:11:41,316 --> 00:11:43,665 Or perhaps with off-grid scenarios. 233 00:11:43,689 --> 00:11:47,856 Perhaps we could even directly generate power with this cold. 234 00:11:48,522 --> 00:11:51,475 There's a large temperature difference between us here on earth 235 00:11:51,499 --> 00:11:53,189 and the cold of space. 236 00:11:53,213 --> 00:11:55,341 That difference, at least conceptually, 237 00:11:55,365 --> 00:11:57,959 could be used to drive something called a heat engine 238 00:11:57,983 --> 00:11:59,173 to generate electricity. 239 00:11:59,967 --> 00:12:03,570 Could we then make a nighttime power-generation device 240 00:12:03,594 --> 00:12:05,991 that generates useful amounts of electricity 241 00:12:06,015 --> 00:12:07,919 when solar cells don't work? 242 00:12:07,943 --> 00:12:10,477 Could we generate light from darkness? 243 00:12:11,872 --> 00:12:16,261 Central to this ability is being able to manage 244 00:12:16,285 --> 00:12:19,396 the thermal radiation that's all around us. 245 00:12:19,420 --> 00:12:22,220 We're constantly bathed in infrared light; 246 00:12:22,666 --> 00:12:25,118 if we could bend it to our will, 247 00:12:25,142 --> 00:12:27,872 we could profoundly change the flows of heat and energy 248 00:12:27,896 --> 00:12:30,629 that permeate around us every single day. 249 00:12:31,190 --> 00:12:34,531 This ability, coupled with the cold darkness of space, 250 00:12:34,555 --> 00:12:37,864 points us to a future where we, as a civilization, 251 00:12:37,888 --> 00:12:43,126 might be able to more intelligently manage our thermal energy footprint 252 00:12:43,150 --> 00:12:44,950 at the very largest scales. 253 00:12:45,904 --> 00:12:48,118 As we confront climate change, 254 00:12:48,142 --> 00:12:50,745 I believe having this ability in our toolkit 255 00:12:50,769 --> 00:12:52,569 will prove to be essential. 256 00:12:53,428 --> 00:12:56,626 So, the next time you're walking around outside, 257 00:12:56,650 --> 00:13:02,994 yes, do marvel at how the sun is essential to life on earth itself, 258 00:13:03,018 --> 00:13:07,694 but don't forget that the rest of the sky has something to offer us as well. 259 00:13:08,533 --> 00:13:09,684 Thank you. 260 00:13:09,708 --> 00:13:13,818 (Applause)