In the last post, we looked at how Solar electricity integrates with the fall / winter / spring demand curves. Now we turn to summer. First let's take a look at the summer curve all on its own:
The double "camel's hump" is gone, replaced by a single mountain centered right around 3pm. This is due to the fact that as the temperature outside warms up around midday, there is a lag of a couple of hours until peoples' A/C units kick on, leading to high demand. As the day cools off, demand then falls. Note that I drew the curve approaching 100% - so this curve would represent one of the hotter days of the year - for example a July day here in Cincinnati with a high around 90 degrees Fahrenheit. Recall that during these super hot days the grid operators have to draw from special Natural Gas "Peaker" plants to meet the peak demand.
The graph above has Nuclear 15%, Coal 50%, Natural Gas 25%. Add that all up and it equals 90%. But because this is summer and demand goes higher than in winter, we need that extra 10% - this is where the Natural Gas "Peaker" plant comes in. These "Peaker" plants are only used during these times of high demand, so they don't bother making them super efficient in terms of their fuel use. The "Peaker" plants make a lot of CO2 for the amount of electricity they make, and therefore anything that cuts back on "Peaker" plant use is cutting back CO2 emissions even better than normal.
OK now let's run through the scenarios of 1%, 5%, and 10% Solar Generation using a Summer Demand Curve. You'll see pretty quickly that the news is better than in Winter!
The graph above has 1% Solar with 15% Nuclear, 50% Coal, 25% Natural Gas, and the 10% Natural Gas "Peaker" at the top. You can see that the 1% Solar pushes up, but there is plenty of room for the Natural Gas to turn down and accommodate it. In fact what has happened is that it is the "Peaker" plant that has been needed less - and that is good! Also note that I have shifted the top of the solar hump so that the top is right at 1pm, which is because of daylight savings. The net effect is good for solar because the 1pm peak of solar is pretty close to the 3pm peak of demand. You will soon see that this makes a world of difference.
This graph has 5% Solar with 15% Nuclear, 50% Coal, and 25% Natural Gas. Unlike what happened with the winter curve, the Natural Gas is able to turn down enough to accommodate the Solar. Between 9am and Noon the Natural Gas is turned down to its minimum, and that was all that the grid operator had to do in this case. Once again, the amount of power from the "Peaker" plant is less than before. Again, because the peak of Solar electrical production (1pm) and the peak electrical demand (3pm) are so close, this really simplifies things. But onward and upward!
This graph has 13% Solar with 15% Nuclear, 50% Coal, and 25% Natural Gas. As you might recall, when we went up to 13% with the winter demand curve things got pretty crazy - there was a risk of overgeneration which required either the Solar being turned down (i.e. curtailed) or turning some coal blocks off (i.e. going offline). In this case however, turning both the Natural Gas and the Coal plants down to their minimums just squeaks us through. From around 8am until 11am Natural Gas is at its minimum (25%) and from around 11am until 3pm Coal it at its minimum (40%). As you can see, going any higher than 13% for our Solar would require turning either Natural Gas or Coal off. Finally we can see that we don't need our "Peaker" plant at all.
In summary accommodating Solar electricity in summer is easier than in fall / winter / spring. In winter we were only able to accommodate around 10% Solar without needing a power plant to go offline, but in summer we can get up to 13% without a problem. In summer, Solar is also able to displace use of the "Peaker" plants which is certainly beneficial in terms of reducing CO2 emissions. That said, in the grand scheme of things (as in avoiding catastrophic climate change) whether we're talking about 10% Solar or 113% Solar... that's simply not going to do it. There is an irony to adding more Solar to the grid - if the Solar isn't enough to make much difference (say 1 or 2%) then it works fine, but if the Solar is enough to make a real difference (say 30%) then it's a mess. There will be a window of time where we are adding Solar generation but energy storage won't be necessary yet. But if we're really going to reduce our use of fossil fuels for electrical production it won't be long until we blow past that 10 to 13%. Let's take Ohio as an example: as of 2016 we only get a fraction of a percent of our electricity from Solar. But if the state and federal governments were to alter their policies to favor Solar, we could be getting 7 to 10% of our electricity from Solar in 5 to 10 years. After all, California went from 2% Solar in 2013 all the way up to 7% in 2015! (see http://www.energy.ca.gov/renewables/tracking_progress/documents/renewable.pdf) And California uses a lot more power than Ohio.
For anyone familiar with the industry you will recognize that the problem I am describing is well known as the "Duck Curve" issue, which is already a real problem in Hawaii and quickly becoming a problem in California during springtime (remember that spring has the winter demand curve). The original paper describing this issue of integrating Solar is short and I encourage you to check it out if you're interested: https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf
And to understand why they call it the "Duck Curve" check out: http://www.vox.com/2016/2/10/10960848/solar-energy-duck-curve
Throughout this post and the "Solar in Winter" post I kept the sources of electricity constant at Nuclear 15%, Coal 50%, and Nat Gas 25%. When those percentages change so too does a grid's ability to accommodate Solar electricity. As you might guess, having more Coal makes this harder, and having more Natural Gas makes it easier. California has retired all of its Coal plants partly for this reason. In the next post, I look at how demand curves affect utilities bottom line, and after that look at the power mix in different states across the US.