We're talking about enabling technologies. Before we dive into the depths of battery technology, I want to talk about storage conceptually, the basics of what it is and how it fits in. For those of you new to this field, haven't taken prior courses, this may be new to you. If not, you can skip this part, but it might be worth listening to just refresh yourself a little bit with what storage is and why it's relevant to renewable's futures. Let's start with what's called the daily load demand curve. This is for the US State of California on a random day, 15 October, 2020. The daily, what's called the load curve or demand curve shows how much electricity the state is using over the course of the day. It acts in a somewhat predictable way. It's very low, lowest at three or four in the morning, climbs at five or six or seven in the morning if people get up and turn things on, and start the oven, and turn on the television, and turn on lights, and so on. It varies over the course of the year, but California has a slightly typical peak in the early evening at 1700 hours, 5:00 PM or about there as everybody gets home from work, if they're out of work, or maybe they start turn on electric oven. This is October, the sun is starting to set about this time, maybe a little later turning lights on, turning on televisions. But you can see how the demand for electricity varies over the course of the day. Every region of the world has a demand curve, some look different. Some have morning peaks, some have midday peaks, some have evening peaks. It also varies over the season. For example, in a very hot climate with a lot of space conditioning, you'll see a late afternoon peak when all the air conditioners are running. But the concept is that electricity demand varies over the course of the day in a somewhat predictable manner. Now, let's look at a different curve also for the US State of California on the same day. We'll look at output of renewable electricity. US State of California has a lot of solar. This is mostly solar PV, and it comes up during the day, follows the sun, not surprisingly, and comes down when the sun sets about 1700-1800 hours at this time of year. You can see the problem here that the peak electricity demand is this time of day just when the solar is going away. This brings in the concept of what's called net demand. One way to think about this is if one needs to produce enough electricity to meet the demand. Now, the solar and the other renewables are just going to do what they're going to do. They're normally not dispatch. You don't normally turn them on or off, you just let them produce as much as they can because the marginal costs are close to zero. You're not buying that sunlight, it's just going. The net demand, which is this purplish line, is what's left over after the solar provides part of the demand or helps meet part of the demand. What's left over? The net demand is what all the other power plants and resources have to meet after the solar has already taken this bite out of the midday demand. Notice this net demand curve is quite different, it peaks in the morning then comes down in midday and then has a peak late in the day. This leads to a fundamental concept in electricity planning called the duck curve. Let me explain this for those of you who haven't seen it. At the top, we have a characteristic load curve for California. In fact, this is what it actually looked like in 2012. At that time, at this time of year, it actually peaked in the morning, came down a bit, and then went up during late day. Now what happened over time in California as more and more solar came on board, more and more solar PV was installed, that solar PV essentially took a bite out of the midday demand and brought the midday demand down. There were forecasts that by 2020, the midday demand will be way down here. Those forecasts made in 2014 vastly underestimated that dip in midday because so much more solar PV was installed than unexpected. In fact, by 2016, midday demand was all the way down here. Well, so what? The implications are, notice the way essentially net demand climbs very rapidly at say 4:00-7:00 PM or 1600-1900, and that's called the ramp. That's a challenge for electricity system because all of a sudden, all the power plants have to turn on all at once almost because demand for electricity is going up while the solar is going away. Notice as well that the midday demand is quite low, and that leads to another problem called overgeneralization risks. The power plants on the system weren't designed to turn off during the day and then turn back on immediately. They just weren't engineered or designed to do that. It's a challenge for the existing electricity system in the existing power plants to meet this different demand curve. That's called the duck curve. That's one of the challenges for electricity system operation in that essentially imposed or created by the success of solar PV. All that solar PV during the day in California is great for renewables implementation perspective for reducing carbon emissions, but does present some challenges for electricity system operation. We'll stop there and come back to summarizing these challenges and talking about how storage fits in.