[SOUND] Let's now turn to medium scale. Not large utility level but rather something you could do on a farm or maybe for a group of houses. One of the most interesting ways to use solar thermal power is called a salt pond. And let me draw the basics. Basically, you first have to dig a pond, okay. Then, depending on where you live, you want to make sure that the stuff you put in the pond doesn't just drain out the bottom. So most places will have to put some kind of liner. And it will also turn out that this liner needs to be black. It needs to absorb sunlight. So we'll put a black liner. The black liner also prevents what we're going to put in next, from getting into the ground and that is salt water. I'm not talking about your ocean level salt water. Which by the way is the same as the salt content in your blood, which is the same as the salt content in your tears. We clearly evolved from ocean creatures. I'm talking about supersaturated salt water. Take water, get it boiling, put in salt, keep putting in salt, keep putting in salt until nothing else dissolves. At that point you're up to 20%. So supersaturated salt water. This is about 20% salt. Now if I just had the pond like this, what would happen? Fortunately water is clear, so here we have our nice sun. Okay, I'll give him sunglasses and color. All right, and the sun is radiating down heat and the sunlight will of course, go right through the water and it will hit the bottom of the pond. The bottom of the pond is black after all. It won't go through the bottom of the pond, it will warm up the bottom of the pond. So, we now have little chunks, little groups of hot water. Imagine that I now have a little chunk of hot water from the bottom of the pond. Hot water rises, just like hot air rises. Think of a hot air balloon. You heat up the air in the balloon. It becomes less dense than the air around it, and therefore it floats. Likewise the salt water will float up to the top. If this is all we had, it would then cool with the air rushing by. And that air would cool it off and the pond would come uniformly to some temperature, not terribly useful. The cooler salt chunks that have come up here and been cooled by the air, will fall back to the bottom. This is convection. So next step is we have to add fresh water. And you've gotta do that carefully. Can't just pour it in, it's going to all mix. But you very gradually start pouring in a layer of fresh water. No matter how careful you are. This, remember, is at 20%. But freshwater floats. So after some level, some gradient layer. Some layer over which the water will go from very, very salty to very, very fresh, will be established. And on top of this, we will have freshwater. Now, what happens? You might think that the convection, the convection of this hot water at the bottom of the pond would just keep rising up, but it doesn't. Because there is a gradient of the salt layer, because there is each individual part of the water is less dense, this salt water chunk here can not float in a layer where there is a gradient of density. Salt water makes something heavy. There is no convection. This gradient layer insulates the bottom of the pond. Indeed, the water that's up here, the little chunks of water, they go up and they go back down. And fresh water, like any other has convection. [COUGH] This non-convective layer insulates the bottom of the pond. So when the sunlight comes down, the bottom of the pond can get really hot. And I'm not talking, just hey, it's hot water from the tap hot, I'm talking virtually boiling hot. In fact, that's a danger. If the water gets too hot and it boils, then you've got gas bubbles and that mixes everything up, and that's a real disadvantage for the salt pond. But if we just get this hot to 70 degrees Celsius, 160 degrees Farenheit, this becomes a way to store energy. Because at night when the sun goes away, this water is still warm. In the winter, when you don't have sun, you have a cloudy day for a week at the time, this water is still warm. Now what are you going to do with it? One thing you can do with it, is you can run in pipes. Imagine that I have some pipe going in here, right? And inside this pipe, I have nice cold water and then coming out of this, I have some pipe now that has hot water. What can I do with hot water? I can run it through radiators in a building, and I could heat up barns. Gotta do this someplace you have some land area. I could heat up buildings. This could be my district hot water. Not hot water necessarily take a shower with, but hot water to heat up your building. Or, if you really wanted to, you could then heat exchange this with a fluid like an alcohol, or something that's at a low pressure, and you could turn it into a vapor, a steam, not the water steam, but say an alcohol steam, and then spin a turbine and make electricity. The efficiency will be low, because the temperature difference is not that great, but it is possible to do. This type of salt pond is a passive solar device that gets you through the day and night cycle, gets you through the summer, winter cycle, and can produce hot water year round. There are disadvantages, don't go swimming, very bad, dive down here, you get boiled practically. Something else falls in and mixes up these layers, that's not very good either because then, of course, you'll lost your establishment of the gradient layer. Rain's all right, after all it's fresh. But silt, dust, dirt blowing in the wind, very bad. If this water becomes cloudy, well the sunlight doesn't penetrate to the bottom. If the heat doesn't come in at the bottom, then the whole pond concept doesn't work. They've tried it here in the Midwest of the US, we've had one here at the University of Illinois for some time, it heated up some agricultural barns, Chinese wine actually, and it worked quite well. But every year or so they had to drain the water, re-establish it, because the dirt from the fields blowing in the breeze would make the water cloudy. That's what you need to know about salt ponds. [MUSIC]
