Picking up on the topic we were just talking about electricity demand. Showing how it changes all the time due to things turning on, things turning off. As we've talked about, you have to supply pretty much, exactly as much as you consume. I had [inaudible] pretty much because we'll talk about storage, which is possible but challenging. For now, we'll talk about the problem of having demand equal supply. We talked about the demand. Let's talk about the supply side of the equation. The supply of electricity, electricity supply megawatts, must exactly equal the demand. The challenge is different types of power plants, have different characteristics. Some work better, some work worse at certain kinds of situations. I want to talk about the supply side where we produce the electricity and some of the challenges there. One of the fundamental things to understand about making electricity, the characteristics of the technologies. One thing that's not often recognized is that many types of power plants, which you might think are very different are actually pretty similar. Coal, nuclear, geothermal, most biomass plants, and somewhat obscure technology called concentrating solar power are all steam plants. Basically, these technologies are a different way of making the same stuff, steam. Here's an example. There's a former nuclear power plant about 60 kilometers from my home university that never worked very well as a nuclear power plant, and it was actually converted to a natural gas power plant that uses steam, and they took the nuclear side, shut that down and just use natural gas to make steam and kept the exact same turbine. All the technologies shown here use steam turbines. Why is that important? Well, because steam turbines have an operating characteristic of being big and inflexible. As noted here, nuclear, coal, geothermal, and some actual natural gas power plants, all use steam as what's called the working fluid, that transfer energy to make electricity. There's a curd, steam power plants they're reliable, they're relatively inexpensive. It's a technology that has been in use for at least a century, has lots of advantages. But one of the major disadvantages of steam power plants is, they're very slow to change output, and take a long time to start up. Which means, it's an indirect contrast. Think about your car. You get in your car, you step on the pedal, the engine speed RPM increases very rapidly. Well, steam turbines are the opposite, you can't turn them up or turn them down or more accurately, you can, but not much happened, it takes a really long time. Here's some data on, in fact, a coal-burning power plant in Germany, and the first row there shows ramp rate. Essentially, what percent increase in power do you get per minute, and it's one percent. Contrast your car, which will go up from almost a trivial power output to full power, in a matter of seconds, so very different. Notice that what's called the ramp rate, very slow. Notice the hot start-up time. You have a coal-burning power plant in this case. It's hot, ready to go, meaning all fired, but not turning on. It takes six hours to turn on if it's hot and the cold start-up time, meaning it's sitting cold, you have to warm it up first, is 10 hours. It's inflexible, slow to change, and take a long time to start up. They, in the context of an electricity system, they are something that's called the baseload. I'll explain in more detail what that term means in just a minute. Keep in mind, steam power plants, big, inflexible. Natural gas power plants, or more accurately, certain types of natural gas power plants are more flexible. Here's some data showing, first we have, gas and coal. I'm showing here is hot start-up time. The idea is that, let's say the plant is all heated up, ready to go, how long does it take until it actually makes electricity? As the essentially worst-case here, you have the Lignite, which is a certain type of coal like the plot we just showed, it can take up to six hours to get going. Different types of coal, so-called hard coal, it's still typically 2-3 hours start-up time. That means, if you're running a power grid, and you need electricity, you've got to know what you need. Two to three hours advanced if you want that coal plant to be there when you need it. Now, gas or natural gas does a lot better and the hot start-up time for what's called a simple cycle gas turbine can be minutes. You can see it's much more flexible technology and there's something in between called gas combined cycle, which doesn't do as well as simple cycle, but does a lot better than the hard coal. The idea here is that different types of power plants or different types of fuels are better. Some are better and some are not better, i.e. worse at being responsive, flexible, turning up and down quickly, being quick, being able to start off rapidly. The same idea, but a little more detail. This table uses the term responsiveness, some terminology is not very consistent, but it's the same idea. Notice that the coal plant could take hours to go from zero to 100 percent output. Now, this table claims 10 percent in 10 minutes, so it is true that some newer coal plants can tweak a little bit, but they can't go way up and down. Now, something called a combustion turbine, which is actually pretty similar to an aircraft engine, if you think about the engine [inaudible] in a big jet, you think of taking off, you're going through zero to full output in seconds. Well, the ones for electricity aren't quite as good, but they're not bad. They can go zero to 100 percent output in minutes and the newer ones can do even better. Nuclear, they don't really do this at all, because of how they're designed, and all the safety and operational concerns as they put it here. Nuclear plants don't really do much of this at all or very little. They're not what's called load followers. The final example I want to show just here is hydropower. Hydropower is really good at this. They're going from very low output to very high output really rapidly. The challenge for hydropower is that, that may not be appropriate for other regions, the fish downstream, agriculture, there's lots of competing interests for hydropower or different interests and different constituencies to serve in hydropower. From electricity perspective, you might say, "Oh my hydro plant, I can turn it up and down really quickly." That's true for electricity, but it might not make sense for downstream users, so there are other kind of constraints. The fundamental question we're getting at is, how do we achieve this balance? Given we have some power plants that are quick to change and some are not so quick to change? Well, here was the answer to that. We'll call this the way it was. For the 20th century, simplifying a little bit, but not that badly, this is how electricity systems operated, that there was something called base load, coal, and nuclear. Those were things you didn't mess with. You turn them on, you let them go. Maybe in the spring or the fall, so-called shoulder seasons, you might turn them down and do maintenance, turn them off, do maintenance, swap out fuel, fix things. But in peak times, winter and summer they'd run pretty much flat, not entirely flat, but they wouldn't be changed that much because they're steam systems, that's the way they're designed to work. Then the peaks in this typical system would be met with natural gas, because certain types of natural gas, particularly combustion turbines, can go from nothing to something very quickly. Maybe it's late afternoon and all the air conditioners are on and all of a sudden everybody wants electricity or you're in an area with an early evening peak, where everybody gets home from work, it's 1800 hours, and everybody turned on the oven and the lights, then you turn up on your natural gas turbines and then turn it back down. This is a model of how electricity systems were run. We'll talk about why that doesn't work so well anymore shortly. But this is how supply was used to meet demand. Here's another example of a different system, but with the same concept. This is a weekly curve with the same idea. Coal down here, didn't change much. You had a certain type so-called combined cycle, a certain type of natural gas technology that uses both steam and natural gas, not steam. It's somewhat better going up and down. Then you had, in this case, oil, which is unusual, but some countries still use oil and hydro and natural gas, CT combustion turbine, meeting those daily peaks. That's another way of operating a system. Now, reality running electricity system's much more complex. This is a screenshot from a control room at a large system where they run all the pieces together. We'll talk more about what these people do later in this course, but the idea here is that different types of power plants are used to meet varying demand. Some are better at meeting varying demands, some are better just running flat out. Again, the idea of base load and peak plants worked for the 20th century. Next, we'll talk about why that's not working so well anymore, and what are the challenges that renewables impose on that old model of operating electricity systems?