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Residential Solar Power and the Duck Curve

Sometimes you can have too much of a good thing. Take a look at the growth of residential roof-top solar power in California. For individual customers who are blessed with sunny weather, installing solar panels can be a smart financial decision and a great way to escape the region’s high electricity prices. But for the utility companies and for the region as a whole, the results may not be what you’d expect. On sunny California afternoons, the state’s wholesale electric prices can actually turn negative. There’s a glut of electric generation, supply exceeds demand, but all that electricity has to go somewhere. So factories are literally paid to use electricity.

 
Negative Electricity Prices

What is going on here? It’s important to understand that all electric usage must be matched with electric generation occurring at the same moment. Aside from a few small-scale systems, there’s no energy storage capability on the grid. If customers are using 5 gigawatts, then there needs to be 5 gigawatts of total generation from power plants at that same time. If there’s not enough generation, you get brownouts. And if there’s too much generation, you either get over-voltage or over-frequency, damaged equipment, fires, and other badness.

Maintaining this balance is difficult. Electric demand is constantly changing, but it’s not so easy to switch a gas-fired power plant on and off. There’s some ability to adjust output, but it’s not perfect. For most residential solar installations, the utility company is obligated to buy any excess electric generation, whether they need it or not. There’s no way for the utility company to tell you “thanks but we already have enough power right now, please disconnect your solar panels”. So what happens when there’s too much electricity? You pay somebody to take it.

The challenge is illustrated by something called the duck curve, so-named because the graph supposedly looks like a duck. This curve shows the hour-by-hour net electricity demand for California, after excluding solar power. As the day starts, net demand is about 19 GW. Then around 8:00 AM, the sun rises high enough above horizon obstructions and solar power begins to flood the grid. Net demand from gas and coal power plants drops hard. Then around 6:00 PM the trend reverses. As the sun is setting, solar power generation declines to zero while everyone switches on lights and air conditioning. Net demand skyrockets, creating a huge demand wall peaking about 26 GW at 8:00 PM. It’s enough to make any grid operator swoon.

Every year, this problem gets worse as more and more residential customers install solar panels. This trouble is likely to grow even faster now due to California’s new first-of-its-kind solar mandate. As of January 1st 2020, the state requires that all newly-constructed homes have a solar photovoltaic system. Expect the duck curve to quack even more than before.

It’s not hard to understand why grid operators view residential solar as a mixed blessing. During the day it provides cheap clean electricity, and that’s a good thing. Gas and coal plants can be idled, saving fuel and reducing emissions. But solar power can’t eliminate those gas and coal plants entirely – they’re still critically important for evenings and night. So during the day many of those plants are just sitting there doing nothing. The power plants still need to be built, and staffed, and maintained, even if they’re only in use half the time.

 
Grid-Scale Energy Storage

Given the daytime production of solar power, there’s no amount of solar that could ever replace 100% of the state’s energy needs. By itself, it will never enable us to get rid of all those gas and coal power plants. That’s a big problem – a huge problem if you care about the long term and the environment. If you’re a young engineer casting about for a field in which to make your career, I believe you could do very well dedicating your career to solving this challenge.

The answer of course is energy storage. What’s needed is a way to take the dozens of GWh of solar power generated between 8:00 AM and 6:00 PM, and spread it out evenly across a 24 hour period. But how?

You might be thinking of solutions like the Tesla Powerwall, or similar home-based battery storage systems. The Powerwall 2 is a very cool device that can store 13.5 kWh. You can use it to store excess power generated by your solar panels during the day, and drain it at night to run your lights and appliances, helping to smooth out the duck curve.

But the Powerwall is too small and too expensive to really make economic sense. 13.5 kWh is not enough to get a typical home through the night, and the hardware costs about $13000 installed with a 10-year lifetime. You’d need to achieve massive cost savings for the Powerwall to pay for itself during those 10 years. It’s good for the utilities, and if every residential solar customer had a Powerwall, it would go a long way towards smoothing the duck curve and eliminating many of those gas and coal power plants. But it’s probably not a good value for you, the individual residential customer.


Image from arena.gov.au

Looking at the whole system, what’s really needed is grid-scale energy storage. Instead of individual customers storing a few tens of kWh with batteries, we need industrial-scale systems storing many MWh or GWh by pumping millions of gallons of water uphill or with other storage techniques. These are big engineering projects, and some effort has already been made in this direction, but it’s still early days. Aside from massive battery arrays and pumping water uphill, other options include heating water and storing compressed air underground. Nothing works very well yet, but watch this space. The world needs this.

TLDR: It matters not only how much clean energy can be generated, but when it’s generated. The variable output of solar power is a big problem that prevents eliminating more gas and coal power plants. A large scale energy storage solution that’s cheap and efficient would be a tremendous win.

Read 6 comments and join the conversation 

6 Comments so far

  1. scruss April 12th, 2020 2:43 pm

    Part of this problem is a small matter of geography. California has large cities and loads on the very west coast of North America. There’s no land further west of California to generate solar power, so when the sun goes down, the current stops flowing.

  2. Thomas April 14th, 2020 1:40 pm

    In the UK we’ve had to build grid-scale storage systems for entirely different reasons: Dinorwig power station is a 1.7MW pumped storage system that we built inside a Welsh mountain to handle everyone putting the kettle on at the end of Eastenders. Tom Scott did a good video about it at https://www.youtube.com/watch?v=6Jx_bJgIFhI .

  3. Thomas April 14th, 2020 1:51 pm

    Whoops, that should be 1.7GW.

  4. Alan Yau April 26th, 2020 8:44 pm

    Nice PT Cruiser in the parking lot 🙂

  5. Steve May 1st, 2020 1:01 pm

    Here’s another interesting option for large-scale energy storage: raise and lower a 1000 ton weight inside a vertical underground shaft. Claims 80-90 percent efficiency and the ability to generate up to 20 megawatts peak power. 1000 metric tons is 1 million kilograms; that’s a huge weight! https://www.gravitricity.com/

  6. Charles May 8th, 2020 4:01 am

    Omega Tau podcast had an episode about gravity storage that was rather informative:

    http://omegataupodcast.net/299-gravity-storage/

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