Welcome back. We're talking about storage. Storage is one of a whole bunch of tools that can be used to ease grid integration of renewables. Storage is a bit tricky to think about, to understand because there's so many things it can do. Here's a figure showing the range of services. We've talked about some of these. Storage, doesn't have to be a battery, there are different types of storage. We'll talk about that in a minute, but it can act as we talked about several videos ago, essentially to enlarge a power plant. It's not really more power plant, but it can act like it. We've briefly talked about how storage can act to support what are called ancillary services to keep frequency in a tight band, for example. It can allow for transmission upgrade deferral. We've talked about how 20-megawatt battery located basically between the transmission distribution system for a village can essentially allow a 100-megawatt transmission line to provide essentially 120 megawatts of power by the battery being at the end of the transmission line. We talked a little bit how batteries can reduce demand charters at the household or end-user level. There's a whole range of batteries things can do in the off-grid's side. Which we're not going to talk about in detail, but I certainly want to mention. Let's imagine you're off the grid, which means you're not somewhere where there is an electricity system. This could be a cabin way off in the wilderness. But the more prevalent situation is in areas where the grid has not gotten. There are, roughly speaking, a billion people worldwide who do not have reliable access to electricity. Either the grid hasn't gotten there yet to a rural village or it has but there hasn't been enough investment to actually make it reliable. Well, an off-grid system with a solar panel on the roof is great when the sun's out. But what do you do at night when you need light? What does a rural medical clinic do to run the refrigerator? That of course has to run 24/7. Well, they add solar PV plus batteries. A solar home system can benefit from batteries. Better batteries can allow for electric vehicles, which we'll talk about briefly as well. Sure you've all heard that electric vehicles are increasingly getting a lot of attention. If you've read about them, you'll know that electric vehicle performance, in terms of durability, speed, comfort, particularly range, has improved dramatically because the batteries are better. There's lots of things batteries can do. Batteries can play lots of different roles in power systems from near the power plant to nearing the transmission time for what's called your bulk storage, to residential storage. There's just all sorts of things batteries can do and all sorts of places they can play a role. Did it make sense in any particular location? It's tough requesting an answer, but at least there's potential for batteries. I had a conversation some time ago with well, a vice president of a large battery firm. He described batteries as the Swiss army knife of the electricity industry. It can do lots of things. But that also he noted, challenge the marketing side because it does so many things but it's sometimes hard to explain what particular problems it can solve. Let's talk about the storage itself. The technologies, the things. There are several types of energy stored and there has been dramatic change in this. If I was giving a lecture about energy storage five or maybe 10 years ago, there'd be a long list of potential technologies that can provide storage. I'll just talk about a few of them. Capacitors and super-capacitors, where you can simply charge something up and then discharge it. Flywheels, something you can get spinning quickly by putting electricity in, and then turn it around and have it slow down but you extract that energy in the form of electricity. Compressed air. If you pack a lot of air in a confined space, that is a way to store energy, though not directly electricity. You can store thermal energy, heat energy, lots of different ways from molten salt, to heating up soil, there are different kind of magnets. There's lots of ways to do storage. Some of it is actual electricity electrons, more typically some other way it can be thermal heat, it can be even sensitive mechanical storage. But what's happening is pretty clear. Currently, the two storage technologies are dominated. We'll talk more detail about two of them, pumped hydro and batteries. Because these are essentially dominating the market. Almost all the storage that is functioning across the world is pumped hydro. There's a lot of attention and new construction and investment in batteries. Hydrogen's a bit behind. We'll talk about that in a separate course about energy futures, but there's a lot of interest in that as well. But if you look at the market, as I mentioned, what's a kind of storage we actually have in place on electricity systems. It's over 90, probably over 95 percent pumped hydro. There's a lot of talk about batteries and a lot of exciting things going on in battery world. We'll talk about that briefly. But where we're actually storing electricity, where we're actually doing this worldwide is almost entirely in the form of pumped hydro. I'll stop here. In the next slide, we'll pick up on pumped hydro.