All right, let's pick this up where we were, to remind you, we were talking about electricity demand changes all the time. Up down people turn light sound, people turn things off and you have to have electricity supply meat that exactly different types of power plants have different operating characteristics like coal and nuclear, basically anything powered by steam doesn't like to fluctuate all that much. So you might use natural gas to meet the peaks. And that was the 20th century model that's shown here. Were peeking demand was met typically by natural gas. This approach, as it says here, this approach works fine until it didn't, what happened? Well what happened was wind and solar PV. So let me explain, let's talk first about wind power. As you know, wind speed changes all the time. The wind might be totally still, it might be really strong, winds are gusty and changeable. That's just the nature of wind. And here's some data from a wind farm in Denmark showing wind speeds, average wind speeds, which fluctuate average over a certain time period and this is over an entire month. So just notice how much the wind speed changes every day. The wind changes goes up, goes down, there's a bit of a pattern you can kind of see in here, but not a real clear one. And this corresponds with your life experience. The wind goes up, the wind goes down and therefore the output of electricity from a wind turbine goes up and goes down with the wind. So just think about a wind turbine out in the wind, more wind means more power in the wind means more electricity or power that that turban can produce, but if the wind slows down or stops less power out of the wind turbine. And here's some data from China showing wind turbine electricity output over on two different days. And notice that first of all, it changes all the time that it looks like on the first day, which August 25th, it peaked about 2100 hours. But on the other day, the next day, August 26 there was a peak at 7 a.m. And it looks like it peaked again at 22, 2300. So this is a bit of a worst case. It doesn't always change that much. There is some predictability to the wind, but it's just the challenges that the output of a wind turbine will vary with the wind. And in the early days of wind, that really didn't matter very much. In fact, the electricity industry was sometimes characterized wind and solar PVS negative load, meaning the demand for electricity. The load changes all the time. We can deal with that couple wind turbine is not a big deal, we're not sure what we get from them, but we can manage that variability. But that has changed as wind power has become more prevalent and here's some data from 2019 showing wind as a percentage of total generation. So if you think about all the electricity made produced generated in a country in a year oops, this graph shows the percentage of that, that comes from wind. So clearly the market leader here at least in 2019 is Denmark. Almost half the electricity on an annual basis comes from wind, but Ireland's over 30%. Again, back in 2019. Portugal, Germany, UK, Spain all over 20% as you can see it goes down over time, but it's not trivial at all, is significant and is more and more countries get an increasingly large percentage of their total electricity from wind. They're having to deal with the variability of the wind. And a similar story can be told for solar and as you know, sometimes the clouds roll in, sometimes the clouds are out, sometimes it's partly cloudy, sometimes it's sunny and of course the end of the day the sun sets. And showing, here's some data from Pokhara Nepal, showing how solar radiation that is the sunlight coming in varies. This is over an entire year, but this is an average within each day. So making the point that even during the daylight, there might be some heavy over class clown and you won't get so much sunlight or might be very sunny, might be no clouds. And there's some predictability to that, but there's always going to be some variation in the amount of sunlight. And because the sun varies, the output of a solar photovoltaic system varies and this shows output from a solar photovoltaic system in Scotland. It's showing weekly generation over a course of a year. So I'm not just intermediate, very narrow time stance a minute to minute, but what you get over a week and then over the year and then multiple years as well from 2010-2019, there's no clear pattern in the graph. It's not as if overall Scotland is getting less or more sunny, but again, making the point that the amount of output you get from a solar PV system changes, it changes by the day and by the season and even possibly by the year, though the data on that are not so clear. And again, if you're looking at the perspective an entire electricity system meeting where supply has the equal demand and there's only a little bit of solar system. This is not that big a deal, but that's no wonder case. And shown here is a graph of global PV penetration. So percent of annual electricity generation that comes from solar football tax in the year 2019. So for example, Honduras, the world leader in 2019, got 15% of the electricity from solar football tax. [COUGH] But a number of countries, Israel, Germany, Chile, Australia and Greece were all at or above 8% of the electricity over the course you're coming from solar football Text and this was 2019, this number continues to grow rapidly. This is from a country perspective, if you look more narrowly at regions within a country, you find even higher numbers one example, we'll look at in fair depth is the US State of California has noted here in California where a country it's PV penetration would exceed Honduras. In fact the U S state of California got about 20 of its electricity from Solar poll tax in 2019. So let's talk a bit about California. This is a renewable supply curve in California for a random day in 2020. And this is real data. This is what actually was generated in the state on that day over the course of the day over 24 hours. So notice a few interesting things to notice here. The wind output didn't change that much over the course of the day. The geothermal was flat, biomass was flat, biogas, hydro some of those are too small to even see. But notice the solar by solar that's predominantly solar photovoltaics. It does in some ways what you would expect, right? The sun comes up in the morning at seven, the sun goes higher in the sky. There's more energy insulation, radiation, well insulation from the sun and it peaks midday and then comes down toward the end of the day. And that's not a surprising graph. Clearly that day in California, there was not a lot of clouds. There was a bit of a cloud peak at about midday noon, but it seemed like a sunny day in California. And now, if you think back on the plot we showed before of demand for electricity, it doesn't look like there's a bit of a mismatch, right? We'll get to that in California case study next. But to summarize the point is that renewables, wind and solar PV or what are called variable, they change with the resource with wind. There's not some more or less wind speeds. The wind blows more or blows less so the output of electricity from wind turbine is higher or lower. And similar there's more clouds. There's of course nighttime and both those factors mean less electricity coming out of the solar PV plant. So there were called variable sources. We'll stop there and then come back and look more detail the next video about the California story.
