0:00:07.509,0:00:09.317 We have some good reasons to completely 0:00:09.317,0:00:11.235 switch over to solar power. 0:00:11.235,0:00:12.859 It's cheaper in many cases, 0:00:12.859,0:00:14.809 and definitely more sustainable 0:00:14.809,0:00:17.213 than our dependance on traditional power plants 0:00:17.213,0:00:18.906 that use resources like coal, 0:00:18.906,0:00:20.950 which will eventually run out. 0:00:20.950,0:00:22.035 So why don't we replace these 0:00:22.035,0:00:24.570 traditional plants with solar energy? 0:00:24.570,0:00:26.027 Because there's one factor that makes 0:00:26.027,0:00:28.476 solar power very unpredictable: 0:00:28.476,0:00:30.052 cloud cover. 0:00:30.052,0:00:32.223 As the sun's rays move towards Earth, 0:00:32.223,0:00:34.652 some get absorbed by the Earth's atmosphere, 0:00:34.652,0:00:36.755 some are reflected back into outer space, 0:00:36.755,0:00:39.887 but the rest make it to the Earth's surface. 0:00:39.887,0:00:41.436 The ones that aren't deviated 0:00:41.436,0:00:43.516 are called direct irradiance. 0:00:43.516,0:00:45.359 The ones that are deflected by clouds 0:00:45.359,0:00:47.659 are called diffuse irradiance. 0:00:47.659,0:00:50.509 And those rays that first get reflected by a surface, 0:00:50.509,0:00:51.780 like a nearby building, 0:00:51.780,0:00:54.379 before reaching the solar energy system 0:00:54.379,0:00:56.610 are called reflected irradiance. 0:00:56.610,0:00:58.256 But before we can examine how clouds 0:00:58.256,0:01:01.281 affect the sun's rays and electricity production, 0:01:01.281,0:01:04.500 let's see how these solar energy systems work. 0:01:04.500,0:01:06.868 First up, we have solar towers. 0:01:06.868,0:01:08.414 These are made up of a central tower 0:01:08.414,0:01:11.120 surrounded by a huge field of mirrors 0:01:11.120,0:01:13.034 that track the sun's path and focus 0:01:13.034,0:01:16.651 only the direct rays onto a single point on the tower, 0:01:16.651,0:01:19.434 kind of like an eager beachgoer. 0:01:19.434,0:01:21.130 The heat generated by these rays 0:01:21.130,0:01:24.706 is so immense that it can be used to boil water 0:01:24.706,0:01:27.720 producing steam that drives a traditional turbine, 0:01:27.720,0:01:29.545 which makes electricity. 0:01:29.545,0:01:31.515 But when we say solar energy systems, 0:01:31.515,0:01:34.252 we're usually talking about photovoltaics, 0:01:34.252,0:01:35.704 or solar panels, 0:01:35.704,0:01:37.325 which are the systems most commonly used 0:01:37.325,0:01:38.945 to generate solar power. 0:01:38.945,0:01:39.916 In solar panels, 0:01:39.916,0:01:42.795 photons from the sun's rays hit the surface of a panel, 0:01:42.795,0:01:44.465 and electrons are released 0:01:44.465,0:01:46.818 to get an electric current going. 0:01:46.818,0:01:49.818 Solar panels can use all types of irradiance, 0:01:49.818,0:01:53.556 while solar towers can only use direct irradiance, 0:01:53.556,0:01:56.218 and this is where clouds become important 0:01:56.218,0:01:58.290 because depending on their type and location 0:01:58.290,0:01:59.537 relative to the sun, 0:01:59.537,0:02:01.645 they can either increase or decrease 0:02:01.645,0:02:04.122 the amount of electricity produced. 0:02:04.122,0:02:06.236 For instance, even a few cumulus clouds 0:02:06.236,0:02:07.690 in front of the sun can reduce 0:02:07.690,0:02:10.042 the electricity production in solar towers 0:02:10.042,0:02:14.461 to almost zero because of this dependence on direct rays. 0:02:14.461,0:02:16.693 In solar panels, those clouds would decrease 0:02:16.693,0:02:18.200 energy output as well, 0:02:18.200,0:02:19.435 though not as much 0:02:19.435,0:02:22.710 because solar panels can use all types of irradiance. 0:02:22.710,0:02:26.160 However, all this depends on the clouds exact positioning. 0:02:26.160,0:02:28.468 Due to reflection, or a particular phenomeon 0:02:28.468,0:02:30.344 called Mie scattering, 0:02:30.344,0:02:33.125 the sun's rays can actually be focused forward 0:02:33.125,0:02:36.566 by clouds to create a more than 50% increase 0:02:36.566,0:02:39.681 in the solar irradiance reaching a solar panel. 0:02:39.681,0:02:42.231 If this potential increase isn't accounted for, 0:02:42.231,0:02:44.780 it could damage the solar panel. 0:02:44.780,0:02:45.901 Why does this matter? 0:02:45.901,0:02:47.731 Well, you wouldn't want this lesson to stop 0:02:47.731,0:02:51.127 just because a cloud passed over the panel on your roof. 0:02:51.127,0:02:54.429 In solar towers, huge tanks of molten salt or oil 0:02:54.429,0:02:56.808 can be used to store any excess heat 0:02:56.808,0:02:58.292 and use it when needed, 0:02:58.292,0:02:59.736 so that's how they manage the problem 0:02:59.736,0:03:02.525 of fluctuating solar irradiance to smooth out 0:03:02.525,0:03:03.999 electricity production. 0:03:03.999,0:03:05.345 But in the case of solar panels, 0:03:05.345,0:03:07.905 there currently isn't any way to affordably 0:03:07.905,0:03:09.730 store extra energy. 0:03:09.730,0:03:11.717 That's where traditional power plants come in 0:03:11.717,0:03:13.841 because to correct for any fluctuations 0:03:13.841,0:03:15.815 in these solar powered plants, 0:03:15.815,0:03:18.149 extra electricity from traditional sources 0:03:18.149,0:03:20.429 always needs to be available. 0:03:20.429,0:03:22.869 But then why aren't these tradtional power plants 0:03:22.869,0:03:24.337 just used as a backup, 0:03:24.337,0:03:26.075 instead of us humans depending on them 0:03:26.075,0:03:28.296 as our main sources of energy? 0:03:28.296,0:03:30.113 Because it's impossible for an employee 0:03:30.113,0:03:32.248 at a coal fired or a nuclear plant 0:03:32.248,0:03:33.507 to turn a knob to produce 0:03:33.507,0:03:35.427 more or less electricity depending 0:03:35.427,0:03:37.834 on how many clouds there are in the sky. 0:03:37.834,0:03:40.521 The response time would simply be too slow. 0:03:40.521,0:03:42.819 Instead, to accommodate these fluctuations, 0:03:42.819,0:03:45.718 some extra electricity from traditional power plants 0:03:45.718,0:03:48.036 is always being produced. 0:03:48.036,0:03:49.496 On clear sky days, 0:03:49.496,0:03:51.578 that extra electricity might be wasted, 0:03:51.578,0:03:53.557 but when cloudy skies prevail, 0:03:53.557,0:03:55.467 it's what fills the gap. 0:03:55.467,0:03:56.930 This is what we currently depend on 0:03:56.930,0:03:58.861 for a constant supply of energy. 0:03:58.861,0:04:00.186 For this reason, a lot of researchers 0:04:00.186,0:04:02.632 are interested in forcasting the motion 0:04:02.632,0:04:05.835 and formation of clouds through satellite images 0:04:05.835,0:04:07.883 or cameras that look up at the sky 0:04:07.883,0:04:10.765 to maximize the energy from solar power plants 0:04:10.765,0:04:13.038 and minimize energy waste. 0:04:13.038,0:04:14.365 If we could accomplish that, 0:04:14.365,0:04:16.484 you'd be able to enjoy this video 0:04:16.484,0:04:18.486 powered solely by the sun's rays, 0:04:18.486,0:04:20.067 no matter what the weather, 0:04:20.067,0:04:21.845 although if the sun is shining, 0:04:21.845,0:04:24.070 you may be tempted to venture outside 0:04:24.070,0:04:27.034 to go and do a different kind of cloud gazing.