WEBVTT 00:00:07.509 --> 00:00:09.317 We have some good reasons to completely 00:00:09.317 --> 00:00:11.235 switch over to solar power. 00:00:11.235 --> 00:00:12.859 It's cheaper in many cases, 00:00:12.859 --> 00:00:14.809 and definitely more sustainable 00:00:14.809 --> 00:00:17.213 than our dependance on traditional power plants 00:00:17.213 --> 00:00:18.906 that use resources like coal, 00:00:18.906 --> 00:00:20.950 which will eventually run out. 00:00:20.950 --> 00:00:22.035 So why don't we replace these 00:00:22.035 --> 00:00:24.570 traditional plants with solar energy? 00:00:24.570 --> 00:00:26.027 Because there's one factor that makes 00:00:26.027 --> 00:00:28.476 solar power very unpredictable: 00:00:28.476 --> 00:00:30.052 cloud cover. 00:00:30.052 --> 00:00:32.223 As the sun's rays move towards Earth, 00:00:32.223 --> 00:00:34.652 some get absorbed by the Earth's atmosphere, 00:00:34.652 --> 00:00:36.755 some are reflected back in to outer space, 00:00:36.755 --> 00:00:39.887 but the rest make it to the Earth's surface. 00:00:39.887 --> 00:00:41.436 The ones that aren't deviated 00:00:41.436 --> 00:00:43.516 are called direct irradiance. 00:00:43.516 --> 00:00:45.359 The ones that are deflected by clouds 00:00:45.359 --> 00:00:47.659 are called diffuse irradiance. 00:00:47.659 --> 00:00:50.509 And those rays that first get reflected by a surface, 00:00:50.509 --> 00:00:51.780 like a nearby building, 00:00:51.780 --> 00:00:54.379 before reaching the solar energy system 00:00:54.379 --> 00:00:56.610 are called reflected irradiance. 00:00:56.610 --> 00:00:58.256 But before we can examine how clouds 00:00:58.256 --> 00:01:01.281 affect the sun's rays and electricity production, 00:01:01.281 --> 00:01:04.500 let's see how these solar energy systems work. 00:01:04.500 --> 00:01:06.868 First up, we have solar towers. 00:01:06.868 --> 00:01:08.414 These are made up of a central tower 00:01:08.414 --> 00:01:11.120 surrounded by a huge field of mirrors 00:01:11.120 --> 00:01:13.034 that track the sun's path and focus 00:01:13.034 --> 00:01:16.651 only the direct rays on to a single point on the tower, 00:01:16.651 --> 00:01:19.434 kind of like an eager beachgoer. 00:01:19.434 --> 00:01:21.130 The heat generated by these rays 00:01:21.130 --> 00:01:24.706 is so immense that it can be used to boil water 00:01:24.706 --> 00:01:27.720 producing steam that drives a traditional turbine, 00:01:27.720 --> 00:01:29.545 which makes electricity. 00:01:29.545 --> 00:01:31.515 But when we say solar energy systems, 00:01:31.515 --> 00:01:34.252 we're usally talking about photovoltaics, 00:01:34.252 --> 00:01:35.704 or solar panels, 00:01:35.704 --> 00:01:37.325 which are the systems most commonly used 00:01:37.325 --> 00:01:38.945 to generate solar power. 00:01:38.945 --> 00:01:39.916 In solar panels, 00:01:39.916 --> 00:01:42.795 photons from the sun's rays hit the surface of a panel, 00:01:42.795 --> 00:01:44.465 and electrons are released 00:01:44.465 --> 00:01:46.818 to get an electric current going. 00:01:46.818 --> 00:01:49.818 Solar panels can use all types of irradiance, 00:01:49.818 --> 00:01:53.556 while solar towers can only use direct irradiance, 00:01:53.556 --> 00:01:56.218 and this is where clouds become important, 00:01:56.218 --> 00:01:58.290 because depending on their type and location 00:01:58.290 --> 00:01:59.537 relative to the sun, 00:01:59.537 --> 00:02:01.645 they can either increase or decrease 00:02:01.645 --> 00:02:04.122 the amount of electricity produced. 00:02:04.122 --> 00:02:06.236 For instance, even a few cumulus clouds 00:02:06.236 --> 00:02:07.690 in front of the sun can reduce 00:02:07.690 --> 00:02:10.042 the electricity production in solar towers 00:02:10.042 --> 00:02:14.461 to almost zero because of this dependence on direct rays. 00:02:14.461 --> 00:02:16.693 In solar panels, those clouds would decresase 00:02:16.693 --> 00:02:18.200 energy output as well, 00:02:18.200 --> 00:02:19.435 though not as much 00:02:19.435 --> 00:02:22.710 because solar panels can use all types of irradiance. 00:02:22.710 --> 00:02:26.160 However, all this depends on the clouds exact positioning. 00:02:26.160 --> 00:02:28.468 Due to reflection, or a particular phenomeon 00:02:28.468 --> 00:02:30.344 called me scattering, 00:02:30.344 --> 00:02:33.125 the sun's rays can actually be focused forward 00:02:33.125 --> 00:02:36.566 by clouds to create a more than 50% increase 00:02:36.566 --> 00:02:39.681 in the solar irradiance reaching a solar panel. 00:02:39.681 --> 00:02:42.231 If this potential increase isn't accounted for, 00:02:42.231 --> 00:02:44.780 it could damage the solar panel. 00:02:44.780 --> 00:02:45.901 Why does this matter? 00:02:45.901 --> 00:02:47.731 Well, you wouldn't want this lesson to stop 00:02:47.731 --> 00:02:51.127 just because a cloud passed over the panel on your roof. 00:02:51.127 --> 00:02:54.429 In solar towers, huge tanks of molten salt or oil 00:02:54.429 --> 00:02:56.808 can be used to store any excess heat 00:02:56.808 --> 00:02:58.292 and use it when needed, 00:02:58.292 --> 00:02:59.736 so that's how they manage the problem 00:02:59.736 --> 00:03:02.525 of fluctuating solar irradiance to smooth out 00:03:02.525 --> 00:03:03.999 electricity production. 00:03:03.999 --> 00:03:05.345 But in the case of solar panels, 00:03:05.345 --> 00:03:07.905 there currently isn't any way to affordably 00:03:07.905 --> 00:03:09.730 store extra energy. 00:03:09.730 --> 00:03:11.717 That's where tradiational power plants come in, 00:03:11.717 --> 00:03:13.841 because to correct for any fluctuations 00:03:13.841 --> 00:03:15.815 in these solar powered plants, 00:03:15.815 --> 00:03:18.149 extra electricity from traditionatl sources 00:03:18.149 --> 00:03:20.429 always needs to be available. 00:03:20.429 --> 00:03:21.731 But then why aren't these tradtional power plants 00:03:23.033 --> 00:03:24.337 just used as a backup, 00:03:24.337 --> 00:03:26.075 instead of us humans depending on them 00:03:26.075 --> 00:03:28.296 as our main sources of energy? 00:03:28.296 --> 00:03:30.113 Because it's impossible for an employee 00:03:30.113 --> 00:03:32.248 at a coal fired or a nuclear plant 00:03:32.248 --> 00:03:33.507 to turn a knob to produce 00:03:33.507 --> 00:03:35.427 more or less electricity depending 00:03:35.427 --> 00:03:37.834 on how many clouds there are in the sky. 00:03:37.834 --> 00:03:40.521 The response time would simply be too slow. 00:03:40.521 --> 00:03:42.819 Instead, to accommodate these fluctuations, 00:03:42.819 --> 00:03:45.718 some extra electricity from traditional power plants 00:03:45.718 --> 00:03:48.036 is always being produced. 00:03:48.036 --> 00:03:49.496 On clear sky days, 00:03:49.496 --> 00:03:51.578 that extra electricity might be wasted, 00:03:51.578 --> 00:03:53.557 but when cloudy skies prevail, 00:03:53.557 --> 00:03:55.467 it's what fills the gap. 00:03:55.467 --> 00:03:56.930 This is what we currently depend on 00:03:56.930 --> 00:03:58.861 for a constant supply of energy. 00:03:58.861 --> 00:04:00.186 For this reason, a lot of researchers 00:04:00.186 --> 00:04:02.632 are interested in forcasting the motion 00:04:02.632 --> 00:04:05.835 and formation of clouds through satellite images 00:04:05.835 --> 00:04:07.883 or cameras that look up at the sky 00:04:07.883 --> 00:04:10.765 to maximize the energy from solar power plants 00:04:10.765 --> 00:04:13.038 and minimize energy waste. 00:04:13.038 --> 00:04:14.365 If we could accomplish that, 00:04:14.365 --> 00:04:16.484 you'd be able to enjoy this video 00:04:16.484 --> 00:04:18.486 powered solely by the sun's rays, 00:04:18.486 --> 00:04:20.067 no matter what the weater, 00:04:20.067 --> 00:04:21.845 although if the sun is shining, 00:04:21.845 --> 00:04:24.070 you may be tempted to venture outside 00:04:24.070 --> 00:04:27.034 to go and do a different kind of cloud gazing.