GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 34

 - Renewable Energy


Renewable energy sources are discussed. These include wind energy, solar energy, biomass energy and geothermal energy. Energy from wind is acquired through the use of large wind turbines. These turbines ideally need to be located in areas where there is strong wind and low atmospheric turbulence. Solar power is collected using both photovoltaic solar cells and concentrated solar power. Energy from biomass can be produced in two ways: burning biomass to generate electricity or fermentation to produce fuel ethanol. Geothermal energy is produced by pumping water below the earth’s surface into areas of hot rocks which heats the water and creates steam. This steam is then run through a turbine to produce power.

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The Atmosphere, the Ocean, and Environmental Change

GG 140 - Lecture 34 - Renewable Energy

Chapter 1: Renewable Energy Sources [00:00:00]

Professor Ron Smith: So last time we talked about the more conventional energy sources and the way they relate human activity to the environment. I included hydropower in that, it’s renewable but it’s been around for a while. It’s a fairly big contributor already to what we’re–how we’re generating energy. So we’ve already discussed hydropower.

But today I’ll go on and talk about these other renewables. And in each case I’ve put for electricity or fuel. Because in principle you can compute—you can generate electricity from each of these sources, but if you don’t want to use it right away you could convert it to some kind of a fuel.

For example, the easiest thing to do in all these cases would be to dissociate water to make hydrogen and oxygen. You can do that with electricity. And then you can store the hydrogen and use it later on for fuel at any later time.

Now that requires at that time of course burning the hydrogen to get that energy back out. Either burning it in a flame or burning it in one of these cool fuel cells where you can take the hydrogen, oxidize it, convert it to energy, but without the hot flame. I don’t know if you realize this, but if you walk out that end of the building and look to your right there’s a fuel cell operating down in the parking lot there that’s running this electricity here. And I think that’s using natural gas, but you can design one using hydrogen as well.

And so any of these could be stored. And of course, there are other ways to store energy besides making fuel. But I’ve just listed—I’ve just mentioned the fuel there as one way to store that energy. So for the rest of the period I’ll pretty much just be talking about energy generation from these sources.

Chapter 2: Wind Energy [00:02:20]

Stop me if you have questions. There weren’t enough questions last time. OK, some of the terms we’re going to be dealing with today. I’m going to derive for you something called the wind power density, how much kinetic energy is in the wind. And that’ll tell us how much energy we can get out of the wind.

We’ll talk about the Betz efficiency limit, wind shear effects in the boundary layer because most of our wind turbines are put in the boundary layer, terrain effects for wind. So this all has to do is with wind power terminology. And I’ll define–this one I should define now.

Capacity factor is the actual energy you get from your wind turbine system divided by the maximum you could get if the wind blew strongly all the time. So in a way it’s a measure of how good is your site for getting steady strong winds. And it’s a very important measure of quality of a wind turbine system and the siting. We’ll talk about these other things.

Student: Can you repeat that really quickly?

Professor Ron Smith: The capacity factor is the ratio of how much energy you actually generate from your wind turbine system compared to what you could develop if the wind blew strongly all the time. And I’ll define what I mean by strongly in a moment. Wind turbine systems have a cut-off speed. If the wind blows stronger than a certain speed then the systems cuts out and you get no energy. So the maximum would be what you get just before you reach the cut-off speed for that wind turbine.

OK, so there is a conventional horizontal axis wind turbine with three blades. It’s like this little guy here. And in general it sweeps out a circular area that we’ll be working with. And the power that it can generate is roughly proportional to the cube of the wind speed, which is quite a big factor, right? Quite a big power. So this means if you double the wind speed, you get eight times as much power out of that wind turbine. So wind speed is very, very important to the action of a wind turbine.

There’s also this egg beater type. In technical terms it’s called a vertical axis wind turbine. It spins around this way and a number of them have been tried. They are losing favor and by this time about 90% of wind turbines that are constructed are of the horizontal axis type. So there are not too many of the vertical axis ones left, but in certain circumstances they do a good job.

So where is wind energy around the world? Here’s a global map derived from a computer model of the atmosphere, but it’s probably pretty accurate. It’s an annual average of the wind speed at 50 meters. Now remembering that the power goes like the cube of that, these differences are already strong even in wind speed, but then when you cube them there’s going to be an even more dramatic difference in wind power between the high wind areas and the low wind areas.

Generally, we see that the highest wind areas are over the ocean in mid latitudes. Why would that be? Why in mid latitudes?

Student: Jet stream?

Professor Ron Smith: Gulf [correction: Jet] stream is too high, related to that though. Frontal storms, frontal cyclones. So the winds connected with mid latitude frontal cyclones are the cause of these maxima. And why over the ocean? It has to do with the fact that the ocean is smoother, so the winds blow much stronger over the ocean than they do over continents. So that roughness makes a huge difference as well.

Now there are some areas in other latitudes where you get significant amount of wind energy. And of course over land there’s some I’ll show you. I’ll zoom in on the US in just a second. But if you really wanted to get the maximum amount of wind energy, you would put your wind turbines offshore in the deep ocean.

Technologically we don’t know how to do that yet. We can put them in shallow water where we can anchor them. We can build foundations in a few tens of meters of water. We don’t know how to buoy them out in the deep ocean yet. But that presumably will come eventually and then you’ll be starting to develop the real wind energy resource on our planet. Questions on that? Yeah?

Student: Do wind turbines affect the wind?

Professor Ron Smith: They do, they slow down the wind locally. Usually it will recover within a few tens of kilometers downwind, that wind speed will recover once again. There have been a few studies to see whether massive wind farms, I mean really ones of scales like this, would influence the climate of the earth. And the answer is probably not. But they do have a local influence on the wind. And in fact, in just a moment when we’re talking about how to design a wind farm, we’ll have to take that into account. Was there another question?

Student: I just have a question, so if you’re going to put it in the deep ocean wouldn’t there be trouble or difficulty with getting the power, the energy to the shores?

Professor Ron Smith: Huge, huge problems, yeah. So you could drop a cable to the bottom of the ocean and then have a long distance transmission line. The idea that I like, I don’t think I invented it, but that is I would immediately convert the electrical energy.

Well I just mentioned this, take water, which you have because you’re out in the ocean, dissociate it to make hydrogen, store that on the ship in a big tank. And then once every month you just sail that ship with all the wind turbines on it to shore, pump off the hydrogen, go back and collect it again. That’s how I’m going to be rich someday, is to get that idea to work out.

Yeah, but that’s a huge problem. And on land it can be a problem too, how to get the energy to where you want it.

So in the United States it looks like this. This is wind speed. They should have been stated an altitude here. They haven’t which leads me to be confused because it does depend on altitude, how high you are above the ground. Perhaps this is at 50 meters or something like that, it’s an average wind speed.

But the point I can still make from this diagram is that there is this belt of wind energy in the center of the United States. And it’s pretty much weaker everywhere else overland. And why would that be, why that area?

Student: Flat land.

Professor Ron Smith: Smoother, yeah, if you’ve been there you know. So you’ve got forests, and you’ve got mountains and forests, and here you’ve got grasslands. So the wind blows much more strongly in those states. And so that’s where most of the wind energy is located over land over the United States. Not too many people live in that area, so that’s again a problem with having your energy resource not close to where your high population centers are. So transmission becomes a big issue there.

So this is to remind you that first of all in the boundary layer, this thing we’ve been talking about throughout the course, the atmospheric boundary layer, the first few hundred meters of the atmosphere that feels the effect of the surface of the earth, there is wind shear. Typically a wind profile might look something like that. This is meters above the ground and this is wind speed, it’s just an example.

If you cube that wind speed to get wind power, it increases even more dramatically as you go up. And so the higher you can get the turbine, the better off you are. You really want to ger it–most turbines that are put in today, the new ones, the recent ones in the last five years, are at altitudes of 50 or 60 meters.

But now they’re talking about 80 meters or even 100 meters for the hub height. Just picture that for a minute. That’s a football field tilted up for the hub height and then the blades go higher than that. But you want to do that in order to get up where the winds are stronger because of that cube effect. It’s really important to get up where the winds are a little bit stronger. So the industry is shifting towards higher and higher towers for these things.

So if you plot electrical output in kilowatts–I don’t know what turbine this is for but it’s typical–verses wind speed, it doesn’t cut in, start to rotate, until you get to about two or three meters per second. And then it increases rapidly like the cube of the wind. But then it reaches a maximum and the system shuts down or just feathers the blades to give you constant output beyond that.

So this would be what’s called the maximum capacity of that wind turbine. But to achieve that you’ve got to have a speed, in this case, equal to 12 meters per second, which is a pretty good wind. So every wind turbine has a slightly different curve but they all look something like this. Some of them actually shut down when you reach that maximum wind to protect against damage to the wind turbine.

So for siting then, favorable conditions are steady winds, low surface roughness. Top of a hill normally has higher winds than the low lands. Although sometimes in valleys you can get wind channeling through, that gives you high sustained winds that can be very, very good. Or off shore where the roughness is small, but then you’ve got to be close to transmission lines. So how do you optimize all of that?

Disadvantages, if the winds are too calm or two strong, if it’s a bird migration route, then you get in trouble with the birds getting killed by the blades. Although in the large ones the blades move rather slowly. But still there can be some bird mortality. Rough surface is bad because not only does it reduce the wind but it increases the turbulence and then those blades are getting hit by gusts and they don’t last as long. They’ll go for four or five years and then they’ll break off.

You can’t be near population because population–people don’t like, in many cases, to look at these. Although some people think they’re beautiful, others don’t. They are much quieter than they used to, but they’re not completely quiet. Sometimes if you’re nearby they can cast a flickering shadow depending on where the sun is in the sky, they can cast a flickering shadow on your house that’s extremely annoying. So you’ve got to watch out for that.

And then if you could put them in deep water, but the problem is how do you do that? I mean do you float them out there? What about the waves, and so on and so forth?

So Europe is a leader in this especially. This is wind power installed. The units are in megawatts, it’s a power unit, so the average power. But this is the installed, so this is the capacity not the actual generation. If you install a wind turbine, it will have a little plate on it that tells you what its capacity is. Maybe it’ll say 1 kilowatt or 10 kilowatts, that’s what we mean by installed, what its maximum capacity is and that’s what’s given here.

Germany has 18,000, or did in 2005, 18,000 megawatts, Spain, 10,000. For it’s size, Denmark is a leader. And Denmark generates the highest fraction of its total electrical usage from wind power of any country in the world. I think it’s more than 30% percent now of Denmark’s electrical usage comes from wind power. They are a leader and have been a leader in this field.

In the United States, again this is wind power capacity. It’s in the same units, it’s in megawatts. Texas, California, and then I guess North Dakota, Minnesota, Iowa, Illinois, quite a bit of activity back in that wind belt we were talking about. And that’s pretty much up to date, that’s the year 2010.

And here’s Connecticut, zero. It’s not quite true, as you recall from the top of KGL when we did our lab, if you look down towards the harbor there’s a windmill turning down there. If you drive across the Q Bridge, you see it. That was installed by the man who owns Phoenix Press, it’s a small printing press company there close to the harbor. And he generates most of the electricity he needs to run his printing operation off of that wind turbine. Right now it’s the only one in the state of Connecticut. So when you’re looking at that, you’re looking at it in terms of the state of Connecticut.

So we’re rather far behind, but there’s a lot of discussion these days about offshore wind along on the East Coast. I’ll show you that in just a moment. Any questions on this?

So here’s the world installed capacity. Once again, I’m using the word capacity not generation. You probably have to divide that by two or three. The capacity factor is usually about 30% to 40% for a good wind installation. Sometimes it’s lower than that, 20%, 15%, 10%. But to get to generation you probably have to divide this number by about three.

So in 2011 projected it’s 240,000 megawatts is the installed capacity worldwide and climbing rather rapidly. Small, on the scheme of things that’s still a pretty small number, but the good news is it’s climbing rather rapidly.

One of the things that’s been in the newspapers a lot the last couple of years is this proposed wind farm off of Cape Cod called Cape Wind. And these are some of the proposals of where they might be putting several hundred wind turbines offshore. And the politics of that are complicated and the economics too. But if you want to read more about it, just Google Cape Wind and you could learn all about the struggles they’ve had to get that installed. There’s nothing out there yet.

And now there’s a lot of discussion up and down the East Coast to put windmills offshore. This is an artist’s, I think, figment, it’s not a true picture. But basically put them a kilometer or two offshore where people don’t have to look at them, where the wind speeds are good, and then you just have to deal with the question of how to get the energy ashore.

Now the properties of wind energy are it fluctuates with season. It fluctuates with the cycle of storms, a storm can get a lot of wind, then you get a high pressure system following that, the wind is weak. And there’s also a diurnal cycle, blows more perhaps during the day than during the night.

So we talk immediately about storage then because if this is going to be fluctuating so much it would be better if we could store some of that energy and use it when we need it. And the current proposals, I hope I have them all here, pumped hydropower, you take that wind generated electricity, use it to pump water up to a reservoir, and then when you need the energy, you let it run back downhill and through to a hydro turbine, like we talked about yesterday, but artificial in a way because you’re pumping the water up there.

Or make a chemical fuel like hydrogen. You can compress air into a cave and then run it back out through a turbine to make electricity when you want it. You could have a giant flywheel, big mechanical spinning mass storing the energy. Or you could put it into a battery. Those I think are most of the ways that have been talked about for smoothing out the fluctuations in wind energy.

Seasons, well I’ve already mentioned that there’s fluctuations in wind, but I wanted to mention this one in particular because it connects with things we’ve done in the course earlier. Most areas in mid latitudes have much more wind in winter. And that’s because of course of the differential heating between the equator and the pole, the generation of a temperature gradient, and then because of that the generation of these frontal cyclones.

But there are some exceptions and California is a nice exception to that. So there are a number of big wind farms along in California that look like this. And this is a typical wind speed diagram for the US as a whole, with larger in the winter, especially in the early spring, and reduced in the summer, and then climbing up again in the winter.

But here’s California, it has its maximum in the summertime. That’s because you generate–when you heat up the land compared to the ocean, you generate a big sea breeze that actually penetrates quite some distance into California. And it’s that breeze that gives you most of the wind energy from these sites in California. Now that’s kind of nice in a way because it turns out in California that the largest demand is in summer as well, and that’s an air conditioning demand. So that’s a rather good match as it turns out. Questions on that?

OK, so I’m going to stop for a minute and do a little bit of the math and physics of wind energy. The basic idea here is that you can imagine the air approaching this wind turbine as being in a kind of a cylinder. And I’ve chosen the cylinder to match the area of the sweeping blades of the wind turbine. And so we’re going to try to measure or compute how much air is passing in this tube that’s going to hit the blades and then how much kinetic energy there is in there to be extracted and converted into electricity.

So imagine I’ve got my little tube here with area A and wind speed U coming through it. The mass flux coming through that tube is going to be the product of rho U A (Mass Flux=ρUA). Rho is the air density of course, U is the wind speed, and A is the area. And the units on that are going to be kilograms per second because that’s a mass flux.

You remember from high school physics that kinetic energy is 1/2 mass times velocity squared (KE=½MU2). If I throw a baseball and I want to know how much kinetic energy is there, it’s the mass of the baseball times the speed of the baseball squared. If I divide out the M and define the kinetic energy per unit mass it’s 1/2 U squared (KE/M=½U2), I just divided out the M.

And now we’re ready to do the calculation. So the kinetic energy flux is the product of the mass flowing through the tube and the kinetic energy per unit mass (KE Flux=Mass Flux*KE/M). The mass flow rate and the kinetic energy per unit mass. So it’s rho U A times 1/2 U squared (KE Flux=ρUA½U2). That comes out to be 1/2 rho U cubed A (KE Flux=½ρU3A). So that’s where the cubed comes from, part from the definition of kinetic energy and part from measuring how much mass is flowing through that tube.

So that’s the kinetic energy flux. The quantity that’s usually talked about here is the wind power density, which is that thing per unit area (WPD=KE Flux/A). So I’m going to divide out the area and that’ll give me 1/2 rho U cubed (WPD=½ρU3). So that’s a property all of the wind, or of the air, the air density and then the cube of the wind speed.

If you want to know how much a particular turbine is going to generate, then you use this formula. It’s the wind power density times the swept area of the blades times an efficiency factor, which I’ve called epsilon (power generation=ε*WPD*A). Epsilon in modern wind turbines lies between 20% and 45%, I would say, 0.2 to 0.45.

The theoretical maximum efficiency is 0.59. That’s called the Betz limit. I won’t derive that for you. But no wind turbine can be, no matter how perfectly it’s designed, can have an efficiency better than 59%. But most are in the range of say 0.2 to 0.4, something in that range for the efficiency. I mean that’s pretty good, it’s getting 20%, 30%, 40% of the kinetic energy flowing past those turbine blades is being converted to electricity. That’s a pretty good design.

Student: The A in the power generation equation, is that the area of the blades?

Professor Ron Smith: No, it’s the area of the circle, the full area of that circle, not the blade area, but that full area. So it’s pi r squared (A=πr2). If the blade length is r, then that area is pi r squared.

So let’s do a sample calculation. Let’s see what I have here. Let’s say we’ve got a wind speed of 10 meters per second. And a density of 1.2, this is our old friend, sea level density, kilograms per cubic meter. So the wind power density is going to be rho U cubed (WPD=ρU3). And that turns out to be in this case 600 watts per square meter. Square meter of what? Well square meter of area facing the wind, right? So it’s the same area we were just talking about, this swept area.

Now let’s say that we have a turbine with a blade length of 50 meters. Now that’s a big turbine. That’s a blade length of 50 meters. This means that the area, which is going to be pi r squared, is going to be pi times 50 squared, and that turns out to be 7,850 square meters.

And let’s say we have an efficiency of that turbine of 0.4. Well then the power generated is going to be 0.4 times 600 times the area, which is 7,850 meters, and that is 1.9x106 watts. That’s about 2 megawatts from that one rather large wind turbine. Any questions on that? Yes?

Student: If 50 meter blades are long, what’s the average size? You said that’s a big one.

Professor Ron Smith: Yeah, that is big, yes. So I’d say typically–well of course they come in all sizes, but the ones we’re seeing here probably have blade lengths of about 20 meters, something like that, or maybe even slightly smaller than that. But the newer ones are going to be more like this example I’ve given you here. Yeah?

Student: When you have a wind turbine that large, do you get wind speeds that are very different in quantity from like the top of the turbine and the bottom?

Professor Ron Smith: You do. You remember that diagram I showed you with the wind shear in the boundary layer. That’s a problem. When it’s this large it’s got faster winds at the top and somewhat slower winds near the bottom which is causing a torque on that axis which can over time degrade the bearings and so on, on the turbine. So that’s a problem, wind shear is a problem. But on the other hand, you want to get big areas.

Again, the higher up you get, the better you are though because the wind shear is weaker aloft. Remember how that thing curved, that plot curved. So if you get it up high, there’s less wind shear up there.

But you know there’s another part of this calculation. I’m going to erase all of this, I hope you have it. And that is it doesn’t make any sense to put in one wind turbine, you want to put in a bunch. You want to put in what’s called a wind farm, 20, 50, 100, 200, 500 wind turbines.

And how closely can you place them? Because if the wind hits one turbine and the energy is extracted from that, then the winds are weaker behind it. And if you put a turbine right there it’s not going to have the same winds that the first turbine had. So that’s a real problem. And the rule of thumb then is that you want to space turbines roughly 10 times the diameter, that’s a D for diameter, I’ll write it out, the diameter of the rotor.

So let’s take this example I’ve done here where the blade length was 50 meters. That’s the radius, so the diameter is going to be 100 meters. And 10 times that is going to be 1,000 meters, that’s a kilometer. That means you’re spacing these things out by a kilometer.

So if you look down at the surface of the earth, if you put one here then you have to go a kilometer, and a kilometer. And you could put them in a grid all right, but there’s going to be one kilometer spacing between each wind turbine. And that then constrains, limits, how much energy you can derive per unit earth surface area. So now the area I’m talking about is not the A, which is the rotor spinning area, but rather on the surface of the earth.

Alright, so for this turbine we got 2 megawatts, 2x106 watts. And we can’t put them any closer than 1 kilometer apart. So that area is 103 times 103, that’s 106 square meters. So if I divide that here, 106 meters squared, I get 2 watts per square meter.

You see what I’ve done there? So I’ve taken how much energy each turbine is producing, but I’ve divided it by the area that it represents. I can’t put it closer than that because they will interfere with each other. So for a wind farm this is what you can expect for energy generation per unit area of that farm.

Now you can put this in another context. For example, the solar constant, how much solar energy reaching the earth, you remember is 1,380 watts per square meter. So if that sunlight was shining through on that same bit of land, there’s was a lot more energy reaching that directly from the sun than there is—than you’re picking up from the wind turbine. Yeah?

Student: Why is it 2x106? Because there are four there, wouldn’t you multiply by four?

Professor Ron Smith: Well it’s this area I’m computing. So it’s a thousand—it’s a kilometer each way. So that’s 10 to the 3 meters by 10 to the 3 meters.

Student: Yeah, but like the actual energy, like the watts. Because there are four turbines there, so would you multiply that by 4?

Professor Ron Smith: Well, no not really because this one I’d have to assign to that area, and this one I’d have to assign to that area. I think I’ve done it right. Yeah because each of these then–I could have put them at the center, but you have to remember each one has its own area. So is that clear then? This is a different calculation, a different quantity. This watt per square meter is different than this watt per square meter.

Chapter 3: Solar Power [00:35:19]

OK, so much for wind energy. Solar, so there’s two basic types of electrical generation from solar power. One is a photovoltaic, abbreviated PV. And the other is concentrated solar power, abbreviated CSP. This is a direct generation of electricity, in this case you make steam and you put it through a turbine and you generate electricity that way.

The things you have to consider to understand solar power are the solar constant, I’ve just put it on the board, the cloudiness, how much comes through and reaches the surface of the earth where your solar power system is located, and then the seasons, the latitude, and this all important solar zenith angle. So we’ll have to deal with all of that in the next couple of minutes here.

Remember so this is our geometry, North Pole, South Pole, the Equator. New Haven is at 41 degrees latitude. I’ve just taken New Haven as an example. So at the summer solstice, June 21, the sun at noon is here. And so the sun is to the south of you here in New Haven, but it’s only 18 degrees to the south. The solar zenith angle at noon on June 21 is 18 degrees. That’s the solar zenith angle.

Six months later, we’re coming up on this date of December 21 pretty soon, the sun will be in the Southern Hemisphere, 23 degrees below the Equator. Now the solar zenith angle is going to be 64 degrees. So at noon, that’s the highest it gets, is 64 degrees. So in terms of energy being received, you have to take these angles into account.

You’re going to want to tilt your solar panel. If you want to change it with season, then do it this way. Or if you want to keep it fixed, then you probably want to choose a tilt to your solar panel that’s somewhere between 18 and 64, to get to your annual optimum solar panel angle.

Well this makes that same point. So here’s the solar zenith angle. You know the amount of energy reaching the horizontal surface of the earth goes like the solar constant times the cosine of the solar zenith angle. So the lower the sun is in the sky, the less radiation is being received by a horizontal surface. But you’re not that dumb, right? You can just tilt that solar panel up and aim it right at the sun and you get a lot more energy that way than you would if you lay it flat, especially if the solar zenith angle is large. So remember that, you’re going to want to tilt that solar panel to be aiming at the sun all the time to get rid of this cosine effect.

So this is where solar energy is located in the United States. And this takes into account the tilt. So they’ve removed the latitude effect here by assuming that you’re always tilting your solar panel to catch the direct rays of the sun. So this is mostly cloud climatology you’re seeing here. This is how many clouds there are in the atmosphere. And there are fewest in this southwestern corner of the country.

The units on this diagram are kilowatt hours per square meter per day. OK, it’s kind of a bastard unit. But kilowatt hours is this popular electrical unit of energy, it’s got the time in there, kilowatt hours. Per square meter, that makes sense. And then per day, sun rises and falls, you’ve averaged over the whole day. So that’s where the solar energy could best be. That’s a bit different than the wind energy which was in here or offshore.

So you can put these solar panels in at a fixed angle because that’s happens to be your roof angle, or you could put them in in a way that would allow you to adjust the angle. Even better would be it tracked the sun from the morning to the afternoon. But if you don’t want to do that, you can at least adjust that fixed angle towards the south to get optimum performance.

Here’s the PV installed capacity worldwide if you sum all this up. In 2009, it was 22,000 megawatts. How does that compare with wind? It’s an order of magnitude less. You go back and look at that earlier diagram, order of magnitude less. Most of it is in the European Union, the rest of the world’s not doing that great mostly because these panels are expensive. The price is the big factor here.

The other way to do it is with CSP, concentrated solar power. So you take reflectors which are motorized. So they’re tracking the sun as it rises and goes through the day and changes from season to season. And then you aim them at a tower of power they call it. And you heat up a liquid, sometimes it’s sodium [correction: sodium nitrate], it could be water, I guess.

And then you run that through a turbine, a single turbine that generates electricity just like lots of the other electricity generation we’ve talked about. In this case you’re just heating up the fluid with solar radiation and then putting it through a turbine to generate the electricity. Any questions on solar power? Yeah?

Student: What does the heating of the liquid do that’s better than just running normal water through it?

Professor Ron Smith: It has to do with whether it handle the very high temperatures. I think you could do this with water, you could generate the steam and put it through a turbine. But I don’t know why they might use something other than water, but I think it’s because they want to be able to handle even higher temperatures. You don’t want the–even steam if you get it too hot it will begin to dissociate and then you haven’t got steam anymore. [correction: Heated salt is used to make steam for a turbine.]

Chapter 4: Biomass Energy [00:42:18]

Biomass energy, of course, is hydrocarbons from photosynthesis. The inputs are sunlight, CO2 from the atmosphere, and you need fertilizer because plants need nutrients. And it requires some kind of processing before either electricity is generated or a storage-ready fuel product is created.

Here’s the basic idea. So you grow something, it takes in sunlight and CO2 through photosynthesis. One thing you could do would be to take those logs to the plant and burn them, generate steam, put them through a turbine, and generate electricity. That would be one thing you could do with the biomass.

The other thing you could do is to distill it into a fuel product such as ethanol. I won’t go through all these boxes. But you receive the grain, mash preparation, fermentation, dehydration, generating a fuel product ethanol. And also something called DDGs, which I had to look up, it’s called Dried Distillers Grain, it’s a high value livestock feed. So you get another value off of this, you can feed this to your cattle and put the ethanol in cars. Connecticut requires that you have some ethanol in your gasoline and that’s where it would come from.

That’s what an ethanol plant looks like. And they can–some of them can use corn others can use woody products from trees, for example fast growing poplar trees. That’s ethanol. Here’s the installed–this is the production capacity, again, increasing very rapidly with time. There’s no energy unit on this. There’s bushels of how much corn you’ve put in, and there’s billions of gallons of how much ethanol you’re producing. But I don’t have a diagram that shows you the energy equivalent of these quantities.

Where is it being done? Mostly in this part of the country, somewhat coinciding with the wind belt. Iowa seems to be the leader, but Minnesota, Nebraska, Illinois, Indiana does a lot of it as well. That’s ethanol plants. Ethanol production in millions of gallons per year per state is what those numbers represent.

Chapter 5: Geothermal Energy [00:45:14]

And the last thing I’ll do, this is perfect timing, geothermal energy. Basically you have some hot rocks down below. You put water down into it, it gets heated, comes back up, you put it through a heat exchanger, you generate steam. And then the story is the familiar one, the steam goes through the turbine, the turbine runs the generator, and the electricity comes out. And that’s what a geothermal energy plant looks like.

And where is that located? Each of these things has their own geographical distributions. Where is the geothermal energy located? Well it’s in the red areas here. This is in–what’s plotted is the heat coming out of the earth because of local hot rocks beneath the earth.

So this is in units of milliwatts per square meter, heat conducting out of the earth, pretty small numbers. Remember we neglected this when we were talking about climate. We didn’t worry too much about the heat being generated internal to the earth changing climate because these numbers are so small, they’re milliwatts. But this is a measure of how much geothermal energy you can get if you put pipes down to get that hot water and bring it out and use it to run turbines.

Chapter 6: Electricity Sources [00:46:45]

And then I’ll just summarize with this slide. So the characteristics then of electrical sources, think of all the things we’ve just spoken about today and last time. You want a source that’s steady and reliable. You need baseload and peakload. Baseload is defined as electricity production that’s constant through time. And certain technologies are really good at that, like nuclear plants, hydro plants, coal-fired plants.

But you also need peakload because your demand may change. You need something that you can start up rapidly to handle a sudden increase in load. A good example of that would be a gas turbine, for example, run off natural gas or run off hydrogen.

Cost is a huge factor. Proximity to demand or at least to a transmission line. You want to know whether it’s renewable or fossil, whether it’s going to pollute the region locally, or whether it’s going to emit greenhouse gases globally. Those are the factors by which you would judge any of the energy systems that I’ve described the last two periods. OK, we’re done. See you on Friday.

[end of transcript]

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