[SOUND] One of the things you really notice about the sun is that it's warm. We've talked about how you can take a sunlight and make photo voltaic energy. You can turn it directly into electricity. But, what if you just want to get warm? Can we use the heat from the sun and substitute that for our energy use? And the answer is yes. Some people call it passive solar, some call it solar thermal. Let's go with that name. There are three scales on which you can do this. And first, we'll talk about the large scale, the power towers. Spain went into this in a big way. The made massive solar power towers where mirrors that acres and acres of land around it reflecting the sunlight up to one central location, and this central location gets really hot. I'm sure many of you have taken a magnifying glass and set a leaf on fire, the same principle. You take a lot of mirrors, now keep in mind these mirrors can't be fixed, you have to rotate them as the sun goes across the sky with some infrastructure. And you reflect that sunlight up to one central location. And boy does it get hot. You could just put it on water and it would flash into steam. But it's more efficient to have some type of solar thermal reservoir, and in most cases salt is used. We think of salt as a solid. You get hot enough, you get molten salt, and that molten salt will collect and store a huge amount of heat. Of course, you don't really just want to use hot salt. We, in the end, need to turn this into something useful. So if you take the hot salt, and now, in a separate area or by having pipes, take water through it, you can turn that water into steam. The steam turns a turbine, the turbine spins a generator. And you have electricity. The same way as if you had taken natural gas and you would boil water and then you had turned a turbine, and then the turbine spun the generator. Like all energy systems, it comes down in the end to economics. Is it cheaper to make whole bunch of rotating mirrors, a tower in the sky filled with salt? Or is it cheaper to have a few burners and pour some natural gas in it? I think you probably know the answer. It all depends on the price of natural gas. Today it's by far cheaper to make that same energy by burning the gas or by burning coal, but those make CO2. So some countries particularly Spain made the investment to say, yes this is not economical, but we believe this is a good thing to do. So now we will make a small percentage of our electricity in the solar power tower configurations. The future of power towers again comes down to an economic decision. With photovoltaic cells being extremely cost efficient, maybe not for the set up of them yet but at least for the cells themselves, my guess is that we will go towards the photovoltaics, turning the sunlight directly into electricity. In this system, with the large power towers, there are a lot of things that can go wrong. The mirrors can break, they can get dusty just like with any solar thing, it gets dark. So the solar power towers are probably an interesting niche but not something I think will ultimately run the world. There is another more personal way to utilize the sun's heat as an energy source, and that is in building design. Not too many people get to choose the building design that they'll live in, rent an apartment, or buy an existing house. But somewhere along the line, somebody made that house. Maybe you'll be in a position to add something on in addition and you can use some of these techniques or maybe you'll be a housing designer and you can make a whole suburb using the energy of the sun. You see, if you make a photovoltaic panel, you can very easily measure how much watts you save, you can put it on the chart. Look, this much energy from solar. But if you make a house that doesn't have to be heated because the sun is always warming it up, doesn't show on the charts. But man if you save both yourself, and your country, and the world a lot of carbon emission. So what are some of the tricks? One the most important tricks is that, if you live in the Northern Hemisphere, have your windows facing south. So let's take just some normal average house here, right? And we have a house and this area right here is going to be a window. All right, what side of the house should this window be on? The south side, south, that way. I want to focus on something very easy, and that's this overhang. This overhang of the roof. And the overhang with respect to where the window is. This ratio of where the bottom of the window is, the top of the window, and how far in the pitch of the overhang, depends on your latitude. Let me illustrate. Let's say it's the winter, and in the winter you want the sunlight to come into your home. The sun is lower in the winter in the Northern hemisphere. Let's say the sun is here, okay? And you notice that the sun is now shining into the house. Great, the Sun is warming up the house. The overhang is not in the way. The Sun is low in the sky. In the summer, the Sun is high in the sky, very bright. I made it bigger, it's not really bigger. Okay, and now the overhang intercepts the Sun's rays. In fact, no sunlight directly at the heat of the day at noon is coming into the house at all. Simple design, how big do you put the overhang, how high up and low down do the windows go can make a big difference in passive solar heat to your home. Here's another trick. This one's a little weirder, all right? Let's say I once again have my nice house, maybe this doesn't have a peek roof, right? And I have a window and in this house, again south is this way, so we've got the sun light coming in to the window. And I'm going to build a brick wall in front of the window. All right, I'm not really sure anyone would really have the guts to do this. Let's put this make beautiful picture window and the break it up. But notice I didn't break it up all the way. I left this gap, right, because this is the window. And there's some vents in the wall, the wall is really not floating. But you can imagine that this, in various spots, right? If I look along here, right, you'd have some vent paths, right? So this might be your brick wall and you have the same thing up on the top as well, right? You got some spots up here. That's what these gaps are for. Now what's going to happen is that the sun light is going to come in, and it's going to warm up this wall, this trombe wall. And the air from a house is going to start self circulating, and hot air is going to come in, cold air is going to be sucked up through here. This brick walls at thermal mass that absorbs the sun heat all day, and we now have heated the entire interior of this room merely, by having this brick wall in front of a window, making a little green house right here. The diagram you see now is another example of this which is a bit more useful. You can see that they took this space that was between the window and the brick wall and they actually made it very large. It's like a solarium. You could live in or use during a nice sunny day. At night, of course, it'll get colder but the thermal mass will keep this system going at night. You just put some drapes over the window. You can even play with the patio outside and make it reflective, so that light it doesn't quite make it in the window will still bounce off and come in. Passive solar designs for buildings can be extremely helpful. But it's not just the buildings. What are our other energy uses in a building? One of them that uses maybe 15% of the energy in a typical building is heating water. Nobody likes a cold shower, nobody wants to cook or wash your hands in cold water all day long, you've gotta heat some water. Why not use the sun? You can make a solar collector very easily. You can just take say some plywood, put some black tubing in it, put on some plexiglass or glass on the top, all right, just like this example. Here's the idea, you put this on your roof, right. And then sunlight comes through the plastic, and it heats up those black pipes inside that have water going in it. If you want to get a little more sophisticated, you can purchase them and here are some pictures of some really nice commercial solar heaters. When I've traveled around in fairly warm climates, the south of China, southern Europe, Mexico, these things are everywhere. People do not spend fossil fuels to make their hot water, they use the energy of the sun. I know in Northern climates when this roof gets full of snow and so forth, this isn't going to work. But for most of the planet and most of the population, hot water can be done from the sun with a fairly simple collector. Other personal uses of thermal solar energy, one of the best swimming pools. I realize not everybody has a swimming pool, but if you do have a swimming pool, why not heat it using sunlight. I'm not talking about just the sunlight that comes down on the pool, but rather, use the same type of solar hot water collectors. Run the pool water through them, it gets warmer, it comes back in. You say, it's not going to work in the winter. You're not going to use your pool in the winter, okay? So therefore you can use this at the times when it's the sunniest, that's when your pool will get nice and warm. Gets too warm, you can turn it off. And finally one other personal use of thermal solar energy, and that is cooking. I think this is terribly impractical, but I do want to show you a solar thermal cooker. You notice that it has a shelf in it, it insulates from the rest of the body, and it's painted black inside. Sunlight comes through the glass, reflected by these mirrors, gets absorbed in the black area. There's thick insulation, and there you can put your roast. And if you like your roast really, really rare five hours later you can eat it. I've tried it, all right? Maybe it's just because I live here in Central Illinois, but is still a potentially useful device to be able to heat things up to a point where you don't have to do any work with it. You just have to put there in it. I doubt they're really going to catch on. It takes a lot of time. You got a cook in the middle of the day when most people are working. But it is, once again, a creative idea of how to capture the sun's heat directly and thermal energy to be able to help us have a renewable energy system. [MUSIC]
