字幕表 動画を再生する 英語字幕をプリント The sun was humankind’s first source of power, and, with a little work, may be the last one we’ll ever need. A good desert collects more solar energy in six hours than the entire world uses in a year. The surface area of my body is about, uh, a meter and a half squared. If I laid out in the sun all day long every day for a year, I would collect about 1,500 watts of solar energy. And check it out, pretty much all of the power that we as humans use originally was solar power. And all coal is is the fossilized remains of plants and animals that died eons ago and have been buried in the earth, and they got their energy from the sun. Natural gas and oil, same thing, the sun. Nuclear power, which produces about 20% of our power, is one of the two sources that we have that isn’t originally solar power, the other being tidal, which is created by the moon. Hydroelectric power, how does that water get up in the mountains so that it has to run down the rivers, well, it gets evaporated from the ocean by the sun. Wind power, as you may have guessed by now, all weather on our planet is created by the sun. Burning tree and cornhusks and other bio-mass, which we do in bio-mass powerplants, all of those organisms originally got their power from our sun. And then, we have direct solar power, which yes, gets its energy from the sun and skips all those middlemen. And so it must be more efficient, right? Well, it turns out, it is more efficient, and you’d think that being more efficient, it would be less expensive, unfortunately, it is not. When we think about solar power, generally what we think of is photovoltaic cells, those big blue panels that people put on their roofs to generate electricity. When I was researching this, I was actually surprised to find we’ve known about the photoelectric effect for almost 200 years. It was discovered in 1839 by a 19 year old kid named Edmund Becquerel. Now, I have to totally go on a tangent here because this is really interesting. Edmund Becquerel is part of what we call a scientific dynasty. So Edmund Becquerel discovered the photoelectric effect. His father discovered that you can refine ores into their pure metals using electrolysis, and his son, along with Marie and Pierre Curie, discovered radioactivity. It’s just interesting to me that there can be that much scientific talent generation from generation in one family. It’s like Martin and Charlie Sheen, except with science and actually cool. Anyway, the most efficient solar cells that we have tend to find their way into outer space, because efficiency is expensive, but it doesn’t matter how expensive something is when you’re dealing with the International Space Station, ‘cause it’s not like you can run a wire up to it. The International Space Station has 16 115ft. long solar wings. All combined, at peak, these solar panels produce 120 kilowatts of electricity, which, is a lot. And now I can tell just by looking into your eyes that you’ve been filled with an insatiable desire to know more about photovoltaic panels. So if you hit a wafer of polysilicon with light, some of the electrons on that silicon will get knocked off and they’ll be free electrons. Now, this is something that’s normal, but it’s not anything like the amount of power that you would need to create a solar panel. But what scientists and engineers figured out is that if you dope the silicon, and that’s a technical term, it just means lacing it with impurities, if you dope the silicon with phosphorus, it suddenly has way too many electrons, and then you get what we call N-type silicon, ‘N’ because it’s negative. And then if you take another wafer of silicon, and you dope it with boron, that doesn’t have enough electrons, and so you get P-type silicon, for ‘positive’. A traditional solar panel is just a layer of N-type silicon sandwiched on top of a layer of P-type silicon, and then connected with a conductor, which we call a wire. Stick something on top of that wire and you can power it with a solar panel, and depending on the size of that panel, it could be a calculator, a house, or a frickin’ space station. The trick is, how do we either get solar panels to be so efficient that they can make up for their high costs, or find new, less expensive materials, that we can use to create photovoltaic panels. Now I have to get off topic a little bit here and talk about how solar power has an advantage that not a lot of people think about. In general, when we produce power as humanity, we do it at giant power stations that are often hundreds of miles away from where the power is actually used. In order to get the power from the power station to your house, you have to put it on these giant transmission lines, which are extremely expensive and also, having the power travel all that distance is pretty inefficient. You can lose as much as 30% of the power that you generate just getting it from one place to another, which, frankly, is embarrassing. We created all those mega-tons of carbon dioxide just so we can lose the power when we’re distributing it. And there’s a reason we do that, and that’s because with a coal-fired power plant, you don’t want to have a bunch of little inefficient ones scattering the landscape, you want to have one big one in one place where you can control the pollution and make it as efficient as possible. But with solar power, you can actually generate the power exactly where you’re using it. You can put the panel on your roof and use it in your house. We call it ‘distributed power’, and it’s great. It does sometimes make sense to use solar power in a centralized fashion, giant fields full of solar panels, especially if those giant fields are in places where the sun shines 364 days a year. But don’t get too excited, despite marvelous efficiency of distributed power, solar panels still remain much more expensive than centralized power stations. Photovoltaic panels now blanket rooftops all over the world, but while they make ecologic sense, they still don’t make economic sense. Getting a good value for your dollar from a solar panel is pretty much impossible which is why we’re still so reliant on coal and natural gas for most of our electricity. To this day, we get more power from burning wood than we do from the solar panels. So you’re saying to yourself, there’s got to be a better way to do this. And maybe there is. If you were a particularly malevolent or scientifically-minded child, you may have experimented with this technique in the past, using your magnifying glass to create power, and you were probably using that power to kill small insects, which is not something I condone, but there you have it. Sunlight carries a lot of energy, and if you concentrate it into one place, you can do a lot of work and I prefer if we would be using that work to push electrons into your house so you can watch me on your computer screen, not to use it so you can vaporize small animals. Lenses like this are far too expensive to use in solar power plants, so instead, we use mirrors. These are called concentrating solar power plants, and in general, what’s done is we use the mirrors to focus light on a single point, and there’s two real ways that it’s done. One, you build a giant tower, and then you fill a field with mirrors, and you make sure that the mirrors are always focusing the sun on the top of that tower. Now, as you might expect, building a giant tower that can handle being heated to some ridiculous heat is kind of expensive, but it is cheaper than pure photovoltaic. The other way that concentrated solar power works is that they’ll build giant mirrored troughs, like parabolic sort of half-cylinders, and in the middle of those, they’ll put a pipe, so by the time the oil is finished traveling through this parabolic trough, it is so hot that as soon as it enters a vat of water, the water immediately vaporizes, and that’s generally how powerplants work, you vaporize water and the vapor takes up much more space than the liquid and so there’s a tremendous amount of pressure and they use that pressure to drive a turbine, which creates electricity. But even with all that fancy engineering, concentrated solar power plants still, in the best of circumstances, only produce power at about 11 cents per kilowatt-hour, which is about twice as much as a natural gas powerplant. But wait a minute, now we’ve got two solar solutions. One, photovoltaics where the capture of the energy is the most expensive part, and two, concentrated solar power, where the conversion of the energy into electricity is the most expensive part. What if we could have both of these technologies, and have the best of both worlds? Well, it turns out that we can, and it may just be the one solution that allows solar power to become cost-effective in our energy market. By using really sophisticated photovoltaic cells that can take in far more power than the one in your calculator, engineers and scientists are using mirrors to concentrate light on very small photovoltaic cells. Now, mirrors, which are actually capturing the light, are 10 times bigger than the solar panel, and thus the solar panel is taking in 10 times more sunlight and producing 10 times more energy, but the solar panel itself, the expensive part, stays the same size. Using this technique, which we call ‘concentrated photovoltaics’, we get the most cost-effective form of solar power that we currently have on the market today. They call it ‘CPV’, for ‘concentrated photovoltaics’, and there are several gigawatts of it getting ready to go online in the next 10 years or so. It’s important to note that a gigawatt is a lot of electricity, that’s about as much as produced by the largest nuclear powerplants in America. Going back to the Space Station for a moment, mostly just because I want to show more of the awesome graphics of the Space Station, as I said before, it doesn’t matter how expensive the panels on the Space Station are, ‘cause there’s no other way to get power up there. Now when I said that those solar panels create about 120 kilowatts of electricity, I was kind of lying to you. About half the time, the panels on the Space Station are producing 0 watts of power, and that’s because it’s in the shadow of the Earth. And here on Earth, when we’re standing here, we call that shadow ‘night time’, and it is the nemesis of solar power. And so unfortunately it would seem that solar power could never satisfy 100% of our energy needs. We’ll always need something else, whether it’s coal or nuclear or natural gas, to keep the lights on at night. Unless, of course, we could find some way to store the power up during the day and then let it all loose at night. Well, turns out, we kinda can. We can pump it up hills and then during the night, let the water fall down through turbines generating electricity. Or we can pressurize giant closed caverns in the Earth to thousands of PSI during the day, let the air escape to generate electricity. We can heat salt until it melts, and then use the molten salt to boil water at night. Or we can use the solar power during the day to split water into hydrogen and oxygen, and then use the hydrogen in fuel cells to generate power at night. But unfortunately, solar power is obviously already really expensive, and each one of these solutions to store that power adds to the price. So while it’s feasible and we have created the solutions, implementing them is just not in the near term. In physics, we have a thing called the Law of Conservation of Energy, which says that you can’t get more out of a system than you put in. Well, in economics, there might as well be a Law of Conservation of Dollars, which says that people aren’t going to put more in than they have to in order to get a certain amount of work done. Why would I climb over a mountain if someone’s already built a tunnel through it? i mean, aside from the obvious fact that that tunnel releases hundreds of megatons of carbon dioxide and other pollution into the atmosphere every single year. Meanwhile, the biggest powerplants in America produce around 1,000 megawatts of electricity. If we had 116 billion of those, it would be just enough to match the amount of power that the sun shines down on our little planet every single day. I’m Hank Green, solar power is awesome, and I hope you learned something.