So far we have covered the background of how and when various types of electrical plants are used, we have discussed the difference between capacity and generation, and we have observed the remarkable effects of squishing.
Next let's move on to something closer to what a grid operator is actually dealing with on a typical day. First let's look at how electrical demand typically varies over a 24 hour period. There are 2 distinct patterns when it comes to this demand -- one pattern that predominates during fall / winter / spring and another pattern during summer. First let's look at fall / winter / spring.
In the figure above, you can see that there are essentially 2 peaks (like the humps of a camel) of electrical use: one in the morning centered around 7-8am and another slightly higher peak centered around 6-7pm. These correspond to people getting up, turning on lights and microwaves etc... in the morning and then in the evening people are getting home from work or school and again turning on lights and TV's and computers and microwaves, etc.
Now let's look at summer.
In the summer figure you'll note that the double hump is now gone, replaced by one single bigger hump in the late afternoon, centered right around 4pm. This is because of (as you might guess) air conditioners. Most people have automatic thermostats, and when it starts getting hot inside buildings come early afternoon, all those a/c units come on. Combine that with people using power in the evenings in their homes and you have one big and broad hump that dominates from mid afternoon into the evening.
Now let's show the two patterns overlaid so we can make a couple of additional points.
We have fall / winter / spring just labeled as "winter" in blue and summer in red. First you will notice that the summer hump is significantly higher overall. The highest electrical demands of the year come on the hottest days because of the large amount of electricity that air conditioners consume. Also you will notice that summer has the lowest demands of the year too -- right around 4am. Therefore the grid operator has to provide a wider range of power during the summer (both a higher high and a lower low). By contrast during the winter power usage stays in a narrower range.
Finally I would be remiss if I didn't mention that the graphs I've presented don't show the whole range of demand starting at zero. In the lower left corner beneath where it says "Demand" you may have noticed the little double hash:
This is there to make the graph easier to read because the base load that never ever goes away would squish the curves toward the top of the graph. This base load averages somewhere between 50% and 75% of the total demand during an average day - we'll call it around 2/3 or 66%. If we take away that double hash the real curves actually look more like this:
Obviously that base load is pretty big! But it is rarely shown in graphs of power demand (to make the graphs easier to read). Now armed with all this knowledge we can finally show something more realistic as to what a grid operator deals with on an average day.
First let's start again with fall / winter / spring. I will be showing this on the full scale graph without the double hash to try and give the most realistic picture I can. Realize however that this is just a ball park average - any given single day may vary. For reasons I'll get to later I'm going to ignore Renewables - we'll add those in at the end. Also for this part I'm going to assume that the capacity of the power sources include:
- Nuclear 15%
- Coal 50%
- Combined Cycle Natural Gas 25%
- Simple Cycle "Peaker" Natural Gas 5%
- Solar 1 to 5% (I will show both what 1% and 5% look like)
These percentages will vary depending upon where you live. You can check out the current capacity and generation percentages in your state at: http://www.eia.gov/state/ or see my later post "Numbers by State." Here in Cincinnati Coal is nearly 70%! As for Solar I should say that somewhere around 1% is fairly common - but California is at 5% Solar generation as of 2016.
Let's Start then with Nuclear:
How simple! Because the Base Load is so big the Nuclear Plant can just run unchanged all day long. No need to turn that Nuclear Plant up or down, just "set it and forget it."
Next comes Coal:
Pretty easy here too. Between Nuclear at 15% and Coal at 50% we pretty much have the entire Base Load accounted for. The grid operator hasn't had to do much yet. Note that the Coal plant is essentially running at full capacity here.
Next comes Combined Cycle Natural Gas:
Note that for clarity I added the percentages right on there, and that I am using capacity. In the graph the capacity of the Natural Gas is 25% but the amount it actually generated was only about half of that (the shaded orange area) - maybe about 15% of this day's total.
Here is the first time here we see the grid operator actually turning up and down a power plant. Because the Combined Cycle Natural Gas Power Plant is fairly "agile" (it can increase or decrease its power output within minutes) the grid operator just turns the plant up along with the AM peak, then down around noon, then up again in the evening. The evening peak does not exceed the capacity of the Nuclear + Coal + Nat Gas (they total 90%) so there is nothing more to be done. Easy!
OK now it's time to add in renewables! First I am going to start with 1% Solar generation. Note that for solar the whole discussion of capacity vs generation isn't really that pertinent because solar just generates what it generates and that's it. The grid operator doesn't turn the solar up or down - that depends on the sun. That said, here I am using 1% Solar generation as that is usually how Solar is reported.
OK we're going to talk about this graph for a while. First notice that because of "squishing" (always remember the squishing!) even though the solar production on this day only makes up 1% of the total power generated, the max power it provides is around 7%. You'll notice how that the solar "pushes" the nuclear and coal upward like the princess and the pea or something. The push really isn't too big - much like a pea under a mattress! The grid operator then has to adjust for this push by decreasing the power output from one of the other power plants. In this case the grid operator was able to decrease the Combined Cycle Natural Gas power plant, creating a "pinch point" between the two humps. The grid operator chooses the Natural Gas power plant as the one to turn down and then back up because it is the most agile. In this example the grid operator has to turn the Natural Gas power plant down to zero for an hour or so (OK they can't actually turn it down to "zero" but we'll get into that more in a second). Because the "push" of the solar is not too big (only pea sized!) the grid operator is able to compensate successfully.
As I said though the Natural Gas power plant cannot be turned down to zero - so how low can it go? The answer as always is that "it depends" but an average plant cannot run at less than 25% of capacity. We will get into all this in more detail in a later post, so just ride with me for now. Note that a Coal plant is even less flexible - it typically cannot run below 40% of capacity (remember that capacity is the max amount of electricity a plant can produce "pedal to the metal").
So in the graph above we once again have 1% of our electricity for the day coming from Solar. To compensate for this the grid operator turns down the Natural Gas plant down as far as they can (to its 25% minimum), then turns down the Coal Plant too. With these two adjustments everything continues to run smoothly for our grid.
In case you're having trouble seeing in the graph above that the coal plant gets turned down too, I flipped the graph upside down to make that easier to see. This hopefully makes is clear that by "pushing up" from below the solar results in the coal plant being turned down.
See above where it says "Coal turning down" in the upside down graph. Note that the coal plant doesn't have to turn down very much to accommodate the 1% Solar.
Obviously the next question is what happens when the generation from solar goes from pea sized to apple sized? Here we show the same graph but with 5% generation from Solar:
In this graph we have 5% of our electricity for the day coming from Solar, but again again because of squishing the 5% Solar maxes out at around 20% of the total at noon. See how much the Solar has pushed everything up? It's not pea sized anymore, it's more like apple sized. This is where the grid operator starts to get a headache - now they are forced to turn the Natural Gas plant down to it's minimum AND the Coal Plant down a bunch too! The act of turning a power plant up or down is called "ramping" (you can ramp up or ramp down). The reason that utilities don't like ramping is because it #1) is hard on the power plant itself and #2) causes the power plant to run less efficiently. Again we'll get into all this in more detail in a later post. In the graph above the grid operator has had to turn the Coal Plant down by 39%, and therefore the Coal Plant is running at 61% of its capacity (make sense? if it is at 100% to begin with and turns down by 39%, then 100 - 39 = 61). Recall that the lowest it can possibly go is 40% of capacity - so even with only 5% Solar we've pretty much used up all the flexibility the grid operator has to turn down their power plants!
You can pretty easily see that with 13% Solar the grid operator is going to be in trouble, and that trouble (and what a grid operator can do about it) will be the subject of later posts. But let's take a little peek anyway:
You might be thinking - geez that Solar looks huge - how is it only 13%? Remember the power of squishing my friend! In any case in the graph above the grid has blown out all of your electronics and then collapsed. Ok let me elaborate a little on that. You can see that both the Natural Gas and Coal Plants have been turned down to their minimums... but that wasn't enough! The solid black line showing electrical supply (or generation - same thing) has diverged from the hashed black line showing electrical demand. You might recall that this condition, called "overgeneration'" is not supposed to happen. When electrical supply is higher than demand then the voltage (and frequency for the electrical engineers in the audience) increase, which results in too much electrical energy coming out of your outlet, which results in all your electronics getting fried! In this scenario the grid operator (in addition to panicking) has no choice but to disconnect from the grid until the situation can be resolved, otherwise known as a power outage. Is a power outage what happens typically? No - as we'll get into later grid operators have ways to adjust. But has it happened? Absolutely. We would typically think of power outages happening because of a utility not making enough electricity, but it can happen because of making too much electricity as well.
Ok. Let's step back for a minute as we wrap up the "Overview" portion of this website. By now you should have a pretty good idea of how the grid is run in general, and what kinds of problems Solar electricity can cause for those poor grid operators. It should also start to become clear how home batteries (and other kinds of energy storage) are going to be crucial if we are going to having a grid that can handle a decent percentage of solar power. After all, if you could just store the solar energy then you could totally avoid the problems above! (You might ask yourself - well why aren't we doing that already? The answer is money. Batteries and other energy storage projects are very expensive, and without a price on carbon emissions, they don't make financial sense.)
In the posts that follow, I continue to elaborate on the ways in which Home Batteries are a crucial part of our future. I'm certain that you will clearly see that home batteries are absolutely essential to reducing your carbon footprint in a way that is sustainable for the grid. In other words, sustainable in real life, with the real grid, with grid that we actually have now. Any significant action taken by the government or the utilities to reshape the grid will take decades (or, ahem... even longer). But we don't have decades! Climate change is a crisis, and I feel people with the money to purchase a solar installation in the first place should install a home battery as well. Stay with me. Home batteries are available right now. Home Batteries are an inevitable part of our future - so why not make them part of our present?