To remind you of the story so far, electricity demand changes all the time when people are turning things on or off. The characteristic load shape, what the demand is over the course of the day, varies by region, varies by season. But the challenge is to make as much electricity as is needed all the time, where supply has to meet demand exactly. Now, the way that's been done historically is certain types of power plants, steam-powered power plants such as coal and nuclear, have been operated what's called baseload. They aren't really designed to be turned up and down very much, so they just run. Then the peaks, maybe you're in an area with a peak at 1900 hours when everybody comes home in the evening and starts electric cookers, and ovens, and lights. If you're in that area, then the gas turbines will come on to meet the peak. That model of having baseload plants and peak plants and also what's called intermediate plants worked great for the 20th century. But all of a sudden it's gotten more challenging as wind and solar PV have become significant sources of electricity generation and electricity systems. Because wind and solar PV are what's called variable; the output varies with the sun and the wind. They're also called non-dispatchable. That's a technical term for the industry. Dispatch is called essentially turning the power plant on or off. A system operator might say, I'm going to dispatch this natural gas turbine. But you can't really turn wind and solar PV on or off, at least the same way you can a gas turbine. You can't say, "Hey, wind, blow stronger." It doesn't work that way. Those wind and solar PV are sometimes called non-dispatchable power points. The challenge is how do you make an electricity system work with growing penetrations of wind and solar PV? It's important not to overstate that challenge. It is a solvable challenge. We are figuring this out. Shortly in this course, we'll talk about ways we are addressing that challenge, but it's important to understand it as well as a significant challenge. How do we maintain system reliability in the context of lots of wind and solar PV? I want to talk about a case study now of the US state of California. Why California? Because they have an awful lot of solar PV. In 2019, 20 percent of their electricity on an annual basis came from solar PV. Very high. How are they managing that? What problems does that impose on the system? I'm talking about that case study of California. If California were a country, it'd be a big one. It's 40 million people. GDP of about three trillion dollars a year, about 35 gigawatts of electricity generating capacity. Let's talk about the daily demand or load curve in California on this random day, 15th October 2020. As we've talked about prior, their demand is a minimum at 3:00 AM. It climbs throughout the day and actually reaches a maximum, at least on this day, at about 1700, 5:00 PM. A slightly unusual demand trend, but that's just the way California is in the fall. That's what demand looks like. Notice it peaks out at, we'll call it 39,000 megawatts. Now, on that day, here's what renewables provided: the ones at the bottom, the wind, the geothermal, and so on were pretty steady. Wind went up a little bit and then went down a little bit, actually faded away. By 1900, wind was down to zero, and then it came back again. Solar did, as we talked about, what you would expect on a sunny day. It came up with the sun and went down with the sun. You're probably noticing that the demand curve and the renewables curve, they don't quite overlap. This leads to the important concept of a net daily load curve. The way wind and solar PV are typically run in electricity system is they just go. You get what you can out of wind and solar PV because the operating costs are close to zero. Generally, not always, one will typically operate an electrical system to take whatever electricity you can get from wind and solar PV. Because, again, you're not burning fuel to do that, so it just makes sense. But what's leftover, the so-called net demand, which is the purplish line you're seeing, is what is left over to be met with other types of power plants. I'll go through this again. This is the green total demand in California. Wind and solar PV provide some of that. Wind not very much in California, but solar a lot, and that peaks midday. What's left over here is the net demand curve. This net curve is what other so-called dispatchable power plants are left need to meet. The well-known representation of this idea is what's called the duck curve. This is a famous representation of the challenges that wind and solar PV can impose on electricity system. Why is it called the duck? Well, if you stand way back and squint, it looks like a duck. Not very much, but okay. What are we seeing here? The top line is the actual demand, the total demand in 2012 in California. This is for the US state of California. What happened is that more and more solar PV was installed in the US state of California and basically took a bite out of the middle, because remember the solar PV is just there. It's just taken because the operating costs are close to zero. It doesn't burn fuel. It doesn't make a lot of sense to just turn off solar PV, so you take what you get. What happens is the net demand curve got lower and lower every year. As solar PV provided a lot of midday electricity, the leftovers that have to be met with other resources showed this characteristic growing belly in essence. It happened much more quickly than anybody expected. The model was developed in, say, early 2010, it showed by 2020 that the net demand curve we look down here. But in fact, by 2016, four years early, it was all the way down here. So what? Well, this is a challenge for several reasons. We'll come back. Let me explain some of the major challenges this net demand curve imposes on electricity system. First of all, there are power plants in California and other places, particularly steam plants that you just can't turn down very much. You can turn them down maybe down from 100 percent to maybe 60 percent. They can't go lower than that unless you shut them down entirely. Then as we saw on some prior data, it might take days or at least many hours to turn them back up. They're constrained by so-called the minimum loading. If there's so much solar PV during the middle of a day that you can't maintain minimum load on your power plants, you've got a technical problem. What do you do? You could say, well, shut them down because you don't need them. But then what do you do in the afternoon and early evening when you do need them? That's the minimum load or sometimes called the overgeneration risk problem. A similar problem is the ramp. Notice how steep electricity curve is, how much we need. All of a sudden a bunch of power plants that all need to come aboard because the sun has set, the solar has gone away, everybody's coming home, they need a lot of electricity, so you have a very steep ramp. All these power plants that were designed to operate fairly steadily over the course of the day have to be turned way down. All of a sudden all of them turn on and just as soon they have to turn back off again. Now, in theory, you could design electricity system to do that. But that's not the way the California system was designed. It wasn't designed for that problem. It was designed in a different setting. The power plants in California weren't really ready to be off during the day and then on quickly in the afternoon and then off again at night. That wasn't how it was designed and this is a challenge to make electricity system work. As I noted, California as about 2019, it was at 20 percent solar PV. It's higher by now. Other countries and regions will likely see the same growth and will likely see this duck curve challenge. It's important to understand it. We'll talk shortly about how to address it. It's a solvable problem, but again, it requires some imagination, some thought, and we're still learning how to make this work. To summarize the grid integration challenge, demand for electricity has always been variable. But now, we've got this new situation where some supply, wind and solar PV, is variable as well, meaning you can't always control it. It can come and go for reasons you can't control. For example, the wind speeds up, the wind goes down, the clouds roll in. The challenge is to maintain system reliability, making sure the lights stay on, and power quality, which is a technical aspect of making sure the electricity signal to industry, to buildings is pure and clean as a kind of electricity that they need. How do we maintain system reliability and power quality as wind and solar PV grow? That's the challenge. We'll talk shortly about how we're addressing that.
