ELI5/ what is the “duck curve” and net load in solar power? i’m having trouble wrapping my brain around it.
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There are several peaks of power usage on the power grid. You might think that when the sun is shining its brightest is when power demand is at its highest for air conditioning, and businesses are running and so are all their machines of manufacturing and stuff.
That’s sorta true in that it’s a big usage time, but that’s not the whole story. At the end of the business day, people go home. Their homes have been not running the AC for a while for power savings, but must now make up the difference ahead of the occupants arriving. And what do they do? They turn on their electric stove to cook dinner, and their TV or other appliances. Lighting is also useful here since it might start getting dark. This is another peak of power usage during the day, and it’s pretty huge.
So the “duck” curve is because the curve looks sorta like a duck. There’s a slow hump during the day for that main power usage, and then this spike when people get home. The first half is the duck’s body, and then its head.
Unfortunately solar power doesn’t do much for that “head” spike. It does great for the main day usage, but come around 5pm the sun is going down and power consumption is moving from businesses to homes and spiking again. So now you need something else to take the power load, or you need to have stored power from the sun in batteries or something to be released now.
The normal demand and generation graph has a certain shape. Solar power reduces the generation required from other sources. This is the net load not covered by solar and thus must be supplied by other generation sources.
Duck curve= curve that kinda looks like a duck.
Night, no sun, baseload at night. This is the tail of the duck.
Sun comes up, solar kicks in. Now the net load is dropping because Solar kicks in. We’re going down the ducks bottom.
Noon, maximum solar output. Duck’s belly.
Now as the Sun drops the heat of the day and load climbs back up.
Late afternoon, low sun, high use because everyone got home and is cooking or turned stuff on. This makes the Duck’s head.
So, fundamentally the problem is an imbalance in supply and demand. Unlike a gas power plant which can be ramped up and down, solar produces what solar produces. Because it has no fuel cost, there’s no reason (or ability) to turn it down.
So, CA is a big place, and it’s almost always sunny in much of the state so lets’ just assume a standard production every hour of every day. So, starting at midnight we have the usual mix of non-renewables with some renewables like wind, geothermal, etc. being throttled to meet demand (the tail of the duck). Then in the morning solar ramps up and as we get toward noon, solar starts to dominate, and non-renewables need to throttle back (this is the drop in the back of the duck). This creates the first instability – price. If solar generation is able to exceed demand, wholesale energy prices go to zero, which creates two problems:
1) in the moment, a gas generator is losing money because their fuel costs money, so they might want to turn off, but that might take hours to do and take hours to ramp back up again. Put a pin in this.
2) all generation investments in this moment can’t pay off their investment. The gas generator was expecting a certain amount of revenue to pay off the investment in the plant, but it’s making zero dollars here. That’s also true for solar – they’re not making money either. This potentially strands assets – money invested in a plant that can’t ever pay itself off. It also discourages new investment even in things like solar because the time to pay off is now longer, and the more generation you add, the worse the problem gets.
Ok, so as we go later into the day, the sun starts to drop, solar generation drops and everything else needs to ramp back up, not just to the morning levels but much higher because the hottest time of day when AC demand is highest tends to be late afternoon when solar is starting to dip, people are driving home, turning on their AC at home, turning on appliances, plugging in their EV. This is the second instabiltiy – demand is highest a few hours after when solar generation is highest. So as solar is falling, everything else needs to ramp up extremely quickly. And if you turned of your gas generator because it couldn’t make money, it’s not available to ramp up quickly. Grid operators need to do a lot of very good forecasting and communicating to ensure that demand doesn’t outstrip supply – brownouts, etc. This is the head of the duck. Then as you get later into the evening demand falls and solar has been at zero for a while. The head of the duck creates a third problem:
3) You need to have enough generation capacity for this peak, which you don’t need the rest of the day. There’s going to be some generation that only needs to run 2 hours a day, and it needs to be able to pay off its investment in just those two hours. It has to charge a LOT for that power to pay off the investment. The solution here is to get rid of that peak.
In the case of California, we overgenerate a lot now. About 4TWh last year – ballpark of what Vermont needs for a year. That’s a LOT of $0 wholesale electricity, a lot of having to idle plants, and a lot of *really* rapid putting other sources online especially in summer when that AC demand is really high.
The solution is to shift demand. You can do this via policy. You can do this via incentives. And you can do this via storage. If you can get people to charge their EVs at work, when you’re mostly likely to be overgenerating, that’s a demand shift. But storage is the main solution – grid batteries and the like. Charge them up when you are overgenerating and discharge them when that demand is spiking. And batteries are very responsive, which is what you want. It keeps that wholesale price from hitting zero as often because there’s a place for the power to go. And because that demand peak always immediately follows the solar generation peak, you can solve a lot of the problem with just 4 hour batteries. Charge them up in the middle of the day, and discharge them between 4PM and 8PM and you knock down a lot of the peak discussed in 3, you buy time for other forms of generation to ramp up. We’re starting to see it working:
>On a recent Tuesday evening, battery storage marked a significant milestone in California’s energy sector by becoming the primary electricity source during the peak hours for the first time. This event was highlighted when battery storage output exceeded 6 gigawatts (GW), surpassing other energy sources such as gas, hydro, nuclear, and renewables. This peak output of 6,177 megawatts (MW) at 8:10 PM was a notable increase from the previous record of 5,625 MW.
>Over the past five years, battery storage capacity in California has seen a dramatic increase from a mere 120 MW to approximately 6.6 GW currently installed, with 5.2 GW of this being utility-scale. The state aims to expand this capacity further to around 52 GW by 2045 to support its goal of sourcing all power from carbon-free sources.
Having battery exceed all other forms of generation indicates that at least on that day, problem 3) was fully addressed. It probably addressed 1) in that it was enough capacity that other forms of generation had enough time to throttle up. 2) we just have to live with.
CA has the biggest problem with the duck curve now, but it’ll be a growing problem in most states at some point or another.
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