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Solar Curtailment: You Ain’t Seen Nothing Yet

Fereidoon P. Sioshansi, Ph.D.'s picture
President, Menlo Energy Economics

Dr. Sioshansi is President of Menlo Energy Economics, a consulting firm based in San Francisco, California, advising clients on the rapid transformation of the electricity sector and emerging...

  • Member since 2004
  • 44 items added with 55,494 views
  • Sep 17, 2021

When production from solar and wind generators exceeds demand – a frequent occurrence in places with vast amounts of renewables in the electricity generation mix – the grid operators have to resort to curtailment. This, they do more often when there is not enough transmission capacity to export the surplus, not enough flexible demand or electric vehicles to soak up the juice or storage to shift the extra supply for later use. Grid operator in California, Texas, as well as many parts of Europe and Australia are increasingly forced to resort to renewable curtailment as a last resort. And while wind is the main culprit in many cases, solar curtailment has recently been on the rise.

According to the Energy Information Administration (EIA), curtailments of solar-powered electricity generation have increased significantly in the California Independent System Operator (CAISO) in the last few years (visual). In 2020, solar curtailments accounted for 94% of the total energy curtailed in CAISO with the grid operator curtailing 1.5 million MWhrs of utility-scale solar, or 5% of its utility-scale solar production. The growth of small-scale rooftop solar – which cannot presently be curtailed – is making matters worse in sunny placed across the US and elsewhere.

Solar curtailments tend to be most pronounced in the cool but sunny spring months when electricity demand is relatively low – because moderate temperatures means little heating or air conditioning demand. In hot summer months, all solar generated power is needed to supply the large air conditioning load.

According to CAISO, in the early afternoon hours of March 2021, the grid operator had to curtail an average of 15% of its utility-scale solar output on many days. The opposite happens after the sun sets, forcing CAISO to replace the rapidly vanishing solar generation by increasing imports – when available – and relying on gas fired generation, the famous “duck-curve” problem.

Increases in renewable generation and curtailments of solar and wind have followed an increase in new renewable capacity additions – and this is only the beginning as California pushes ahead toward a carbon-free electricity grid by 2045, as do many other states.

To help meet California’s target of 50% renewable generation by 2025, the state is expected to add another 1.6 GW of utility-scale solar capacity plus another 0.4 GW of onshore wind in 2021. Combined, solar and wind represent 44% of CAISO’s capacity additions in 2021.

At the same time, small-scale rooftop solar PV capacity also continues to grow decreasing the need for CAISO-operated generation, leading to more solar curtailments. Wind is not as big a problem in California as it is in places like Texas, Iowa, Kansas, etc. – states where wind is a major contributor in the electricity generation mix.

Clearly the problem is growing more acute requiring longer-term solutions including a number of options considered by CAISO. One promising solution is to enlarge the physical size of the market, therefore increasing the diversity of the generation portfolio.

CAISO has been attempting to enlarge participation in the Energy Imbalance Market (EIM), a real-time market that allows participants outside of California – and non-CAISO stakeholders – to buy and sell energy to balance supply and demand in real-time. In 2020, 16% of total possible curtailments were avoided by trade within the EIM, according to participants in the EIM – a significant achievement.  

Aside from the EIM, everyone’s favorite solution is energy storage. California is expected to add 2.5 GW of battery storage capacity in 2021 alone. The surplus renewable generation can charge these batteries – electricity that would have otherwise been curtailed and wasted.

This, however, is scarcely sufficient. Current batteries are not well-suited to store large amounts of energy over long periods of time, as large pumped hydro storage technology does, shifting large volumes of capacity from the spring to the summer months, for example.

Aside from large duration batteries – now under development – other large capacity energy storage options are being studied including massive hydrogen-based energy storage, which could reduce future solar and wind curtailments.

Hydrogen can be produced by electrolysis using the surplus renewable electricity that would otherwise be wasted, stored for later use. Two projects in early stages of development include a power-to-gas-to-power and one by the Intermountain Power Project called IPP Renewed, which focuses on developing renewable-powered hydrogen production and storage capabilities as well as installing natural gas-fired generators capable of using hydrogen.

Aside from these, CAISO is also looking into a number of other promising but under-developed options such as:

  • Increasing demand response (DR), which would adjust consumer demand when warranted;
  • Encouraging time-of-use (TOU) rates, which better match consumer prices with real-time wholesale energy prices; and
  • Reducing minimum generation levels for existing generators, which would allow greater opportunity for renewable energy production.

Finally, there is the opportunity presented by the enormous and inherent flexibility to charge – and potentially discharge – millions of electric vehicle (EV) batteries expected to be available by 2030. EVs offer one of the best types of DR known to mankind if the charging – and potential discharging – can be properly managed. Geotab Energy claims that the existing 400,000 Teslas in California have the potential to be the largest distributed battery load resource in the world. They amount to more than 3 GW of manageable load. 
According to Geotab, ”By treating EV drivers as part of the solution, instead of part of the problem, we can stabilize the grid and create a more sustainable future.’

To avoid further renewable curtailment, all of the above options will be needed.n

Bob Meinetz's picture
Bob Meinetz on Sep 18, 2021

"The surplus renewable generation can charge these batteries – electricity that would have otherwise been curtailed and wasted."

Fereidoon, surplus renewable generation can charge these batteries, but it isn't. Not a single battery facility in California charges from the direct output of a solar or wind farm, but from a grid mix. And even at peak solar production, thousands of megawatthours of gas generation are contributing to that mix. People naively think batteries are used on the grid to shift renewable energy from daytime to nighttime - but far more lucrative is wasting energy on behalf of gas producers. After all, every kWh that's wasted in battery losses must be generated again. The more energy that's wasted, the more money gas producers make.

"In 2020, 16% of total possible curtailments were avoided by trade within the EIM, according to participants in the EIM – a significant achievement."

"Trade within the EIM" certainly wasn't a significant achievement on behalf of ratepayers:

• One would think solar producers would receive nothing for overgeneration - after all, it's worthless to electricity customers and worthless to California's grid, right? Wrong - to maintain system reliability, solar farms are actually paid, at the expense of electricity customers, to curtail their output. In essence, customers are paying solar producers to both generate and not generate electricity. How is generating too much electricity the fault of electricity customers?

• When system operators can't curtail solar electricity fast enough it must be sold on the EIM at "negative prices", i.e., out-of-state buyers must be paid to take California's unwanted electricity. In 2020, those trades made up at least 10% of trades on the EIM - and again, the bill goes to customers.

• As clouds pass over solar farms, or the wind dies at wind farms, gas-fired electricity must be ramped up and down to keep the grid in balance. Voltage and frequency must be constantly regulated; gas plants must be kept in "spinning reserve" to address these problems of intermittency quickly enough. Renewables, of course, are solely to blame for the emissions generated from these "ancillary" services, and the financial cost is not trivial. In Q3 of 2020, California electricity customers paid $96 million solely for the dubious honor of preventing renewables from taking their grid down!

Thanks for the comment, what would be your suggestion to reduce curtailment?

Bob Meinetz's picture
Bob Meinetz on Sep 23, 2021

Fereidoon, the answer is to generate clean electricity to meet demand, instead of attempting to match demand with available electricity. Makes no sense at all.

In 2021,  the only way to do it is with nuclear power. No other option makes any sense at all.

Jim Stack's picture
Jim Stack on Sep 20, 2021

With a very good and large battery storage system there should never be any curtailment. We finally have a way to balance the GRID and make it very efficient. Battery storage is here now and getting better each day. 

Thanks for your comment, agreed

Bob Meinetz's picture
Bob Meinetz on Sep 23, 2021

Orders of magnitude too expensive, Jim. Would need to be replaced every 7-10 years. Ask an electrical engineer - it's not even close. It's only being advanced by fossil fuel interests to secure a place for natural gas.

Joe Steinke's picture
Joe Steinke on Sep 23, 2021

Some perspective.  The California renewable curtailments annually are well less than 1% of the grid demand. Superimpose the actual monthly grid consumption over the curtailment, and the Y axis needs to be increased 50x.    Most of the curtailments occur during the spring months when electricity demand is low, which means that seasonal storage is needed to shift this generation 4 moths later in the year.  Storage methods such as hydrogen will return only 25% of the electricity generated and will leave major assets such as electrolysis plants unused most of the time.  One needs to factor that efficiency into the LCOE for building renewables, add the grid scale electrolysis facilities cost $1.5 million to $2 million per MW, and increased storage capacity (new and H2 compatible) required by low density hydrogen.    

Bob Meinetz's picture
Bob Meinetz on Sep 29, 2021

"The California renewable curtailments annually are well less than 1% of the grid demand."

Joe, that isn't surprising - solar is generating nothing at least half of every day. But "demand" typically refers not to energy, but power - the rate energy is delivered to customers, measured in megawatts.

Between 11 AM and 1 PM on May 23, 2021 in California solar was generating too much electricity, and it was a problem. To maintain system reliability, electricity customers had to pay California solar developers to curtail 5,600 MW of unneeded electricity, or 35% of total demand. In return, all customers got for their money was preventing solar from destroying their grid.

Though paying solar developers to not generate electricity is bad for customers, it's even worse for grid reliability. Gas power plants never get paid to not generate electricity, and with gas to buy and engineers on staff they can't even afford to generate electricity for free. So they close - then, when there's a week or two of cloudy weather and there's no baseload electricity left on the grid, the lights go out.

Despite all the hype surrounding storage and hydrogen, they'll be useless for filling in. Storage is two orders of magnitude too expensive, and hydrogen generates more carbon emissions than burning natural gas. So yes, "one needs to factor that efficiency into the LCOE for building renewables"  - and when one does, renewables are a waste of money - and more importantly, time.


John Simonelli's picture
John Simonelli on Sep 23, 2021

The article hits all the high points. Personally, from what I've seen people are going to have to just accept the fact that we need significant additions of pump storage hydro (terawatts worth). It provides the volume of storage, generating flexibility, inertia, primary frequency response, and voltage and reactive control that will be necessary to deal with the variability of the inverter-based renewables. Also, we need a national macro grid. We need the ability to move massive amounts of energy long distances to take advantage of the long generation conditions in one area and short generation conditions in another. Lastly to reduce carbon emissions, we need to produce hydrogen when energy is abundant/cheap to use in CTs that can then be used to help address the variability in the renewable generation. The problem is no one seems to be able to move the ball forward on any of those fronts. People have been talking about it for the last 5 to 10 years and yet not much is being done. The ultimate problem is going to be how are we going to pay for all of this.

Bob Meinetz's picture
Bob Meinetz on Sep 23, 2021

"The ultimate problem is going to be how are we going to pay for all of this."

John, the ultimate problem is nobody will pay for it. It would be ridiculously impractical and expensive.

"...people are going to have to just accept the fact that we need:
• significant additions of pump storage hydro
• a national macro grid
• the ability to move massive amounts of energy long distances
•  to produce hydrogen when energy is abundant/cheap to use in CTs that can then be used to help address the variability in the renewable generation..."

Or, people are going to have to get over their irrational fear of nuclear energy. Not money, not time, not technology is as much of an obstacle to solving climate change. And If we have to wait until people are more afraid of climate change, it will already be too late.

Joe Deely's picture
Joe Deely on Sep 24, 2021

Solar curtailment on CAISO is down YTD...  EIA guys are not that good.


Bob Meinetz's picture
Bob Meinetz on Sep 24, 2021

What source are you referencing to create your homemade charts, Joe? Links, please...specific ones.

Joe Deely's picture
Joe Deely on Sep 24, 2021

Not my charts Bob those are CAISO...


Here is 2021:


Here is 2020:


100,000 less MWh of curtailment so far this year vs last.  Plus Solar is up 15% on CAISO thru August. More solar and less curtailment so curtailment share is down even more.


Thats the real story that EIA guys missed. Pretty bad miss.


Thanks for the feedback, comments, suggestions, corrections etc

Clearly we need new solutions, be it long duration storage &/or more demand flexibility


John Simonelli's picture
John Simonelli on Sep 29, 2021

Never wise to put all your eggs in one basket, we need both.

Fereidoon P. Sioshansi, Ph.D.'s picture
Thank Fereidoon P. for the Post!
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