Why Plans to Replace Diablo Canyon With 100 Percent Clean Energy Could Fall Short

PG&E says it will close Diablo Canyon's two nuclear reactors in 2024 and 2025. (Lauren Sommer/KQED)

When PG&E announced plans to close California’s last remaining nuclear plant last week, it also announced a historic plan to replace it with clean energy, like solar or wind, and energy efficiency gains.

That way, the Diablo Canyon nuclear plant, which doesn’t put out carbon emissions, won’t be replaced with something that does, like natural gas, the utility said.

But a 100 percent clean energy transition is less certain than it sounds.

Some say that natural gas power plants could also play a role in replacing Diablo Canyon, because of the challenges of adding large amounts of renewable energy to the grid.

Solar and wind power fluctuate based on the weather conditions and time of day.  Natural gas plants are kept running to fill in the gaps.

In the short term, some say closing Diablo Canyon could actually boost greenhouse gas emissions, because there may not be enough renewable energy to cover the loss of the nuclear plant in the first few years after it retires.

PG&E says it’s committed to a greenhouse-gas free solution. Whether that happens will depend on how the utility and regulators at the California Public Utilities Commission put that plan in motion.

A Shrinking Gap?

Today, Diablo Canyon produces about 20 percent of PG&E’s electricity. That’s a lot of megawatts to replace. PG&E says not all of them be replaced, though, thanks to California’s “changing energy landscape.”

“You don’t need it,” said Geisha Williams, president of PG&E Electric. “There’s been so much energy efficiency. There’s been so much power that’s been generated by customers on their own private solar rooftop.”

California must double its energy efficiency by 2030 under a state law passed last year, SB 350, which means PG&E’s customers will need less electricity from their utility. Home solar systems are booming, which reduces the need for large power plants. Customers in Marin and Sonoma Counties, for example, have left PG&E and are forming their own local utilities, a process known as community choice aggregation.

In addition, PG&E is required to get more of its power from renewable sources, like solar and wind, in order to meet the state’s goal of 50 percent renewable power by 2030.

Add all that up and PG&E says it would only need to run Diablo Canyon at half-power in the future, which makes it financially infeasible to keep open.

When Diablo retires, PG&E will need to replace roughly 9,000 gigawatt-hours of electricity. (In California, it takes around 6,000 gigawatt-hours to power one million households for a year). Through improved energy efficiency, PG&E expects demand to fall by 2,000 gigawatt-hours by 2025.

That leaves a gap of roughly 7,000 gigawatt-hours to make up for the year that Diablo’s reactors go offline.

To bridge that gap, PG&E plans to significantly boost the renewable energy it uses by 2031. By six years after Diablo’s retirement, PG&E says it plans to be generating 55 percent renewable power, system-wide.

Even considering PG&E’s rapid renewable energy growth, some energy analysts say PG&E may need to turn to natural gas for the first few years after Diablo Canyon’s retirement, a move that could add to California’s greenhouse gas emissions.

Natural Gas as a ‘Bridge Fuel’

“They actually increase their dependence on gas-fired generation over a short time frame because that’s really the only option that’s available to them,” said Morris Greenberg, Managing Director of North American Power at PIRA Energy Group, an energy market research firm.

PIRA ran an analysis of the Western electric grid and found that while the use of natural gas is currently declining in Northern California, it could rise around 34 percent from 2023 to 2026 because of Diablo’s closure.

Grid operators balance California's transmission system on a minute-by-minute basis at the Independent System Operator.
Grid operators balance California’s transmission system on a minute-by-minute basis at the Independent System Operator. (Lauren Sommer/KQED)

Greenberg says the analysis takes into account PG&E renewable energy goals, but not the exact specifics of the plan announced last week.

PG&E says there’s a reason it hasn’t spelled out exactly how it will replace Diablo’s power: it’s hard to predict how energy technology will change.

“If I try to write down megawatt-hour by megawatt-hour what fills that gap in 2025 right now, that’s actually a losing strategy,” said Todd Strauss, PG&E’s senior director of energy policy planning.

“The proposal clearly recognizes there’s lots of other things that need to be done, but we shouldn’t specify it today,” he said. “We’re committed to having Diablo Canyon’s retirement not result in an increase in greenhouse gas emissions,” Strauss said.

Several of the environmental groups that signed on to PG&E’s closure plan say there is a risk of turning to natural gas.

“We’re determined to avoid that,” said Ralph Cavanagh of the Natural Resources Defense Council. He says there are alternatives like using energy efficiency programs.

“We’re all going to work together to ensure that the fears of our adversaries are not realized: that anytime you lose a nuclear plant you automatically get an uptick in fossil fuels and pollution,” he said.

Ups and Downs of Renewables

The solar and wind power that will eventually replace Diablo Canyon is carbon-free, but they only work when the sun is out and the wind is blowing.

As clouds cover the sun or the wind dies down, the power supply drops, creating fluctuations on the electric grid. To prevent widespread outages, the state’s grid operator, the California Independent System Operator (ISO), has to have another source of energy ready to fill in the gap immediately.

Generally, it’s been natural gas power plants, which can be turned up and down as needed. That means gas plants have to be run at the same time as renewables, because once turned off, they can take hours to turn back on.

But as more renewable energy comes online, running renewables, natural gas and other power sources all together sometimes creates more power than the state needs. At those times, the California ISO has to switch off solar farms to avoid overloading the grid.

Retiring Diablo Canyon could help with that problem. The nuclear plant is designed to run at a steady, constant level. Nuclear advocates have argued that the state needs this kind of “baseload” power. Others say shutting down Diablo could actually improve grid management.

“It will reduce the need to curtail our solar power plants,” said Cavanagh. “The nuclear plant can’t be ramped up and down, so it’s occupying space on the grid that solar farms could occupy at much lower cost.”


On a sunny day in March, some solar farms had to shut down because there was more power on the grid than Californians were using. 

SOURCE: California ISO
SOURCE: California ISO (Teodros Hailye/KQED)

Natural Gas At the Ready

But as California approaches the legislative mandate of getting half its electricity from renewable sources, the grid will need more backup power ready to fill in when solar and wind power fluctuate.

“We could be at risk of running the gas fleet higher than we would otherwise run it,” said Laura Wisland of the Union of Concerned Scientists.

Wisland says there are a number of strategies to avoid that. Spreading solar and wind farms across a wide geographic area would prevent localized weather conditions, like cloud cover or lack of wind, from creating such large fluctuations on the grid.

Some solar farms and natural gas plants are also less flexible than they could be, because their contracts limit how often they can be turned down or turned off. Wisland says having fewer limits on that would make it easier for the grid operator to balance the ups and downs of renewable energy.

PG&E says it plans to negotiate more flexible contracts with both new renewable projects and older ones whose contracts are expiring soon.

Energy storage projects, like large batteries, can also provide power to balance out solar and wind, so natural gas wouldn’t be needed as much.

California is also looking at joining its electric grid to other Western states, so power can be shared more efficiently throughout the West. Instead of turning on more natural gas plants here, California could hypothetically look for clean power from other states instead.

PG&E says it’s interested in those strategies, but state regulators must ultimately approve the decisions.

There remains “a lot that is actually beyond PG&E, to be worked out at the California Public Utilities Commission, California ISO, and other California discussions,” says PG&E’s Strauss.

“If action occurs too late, then there may be some challenge to the reliability of the system,” he says. “But with a planned, deliberate approach with plenty of foresight, we’re going get reliability in the system.”

“When they do sign up more renewables,” Wisland cautions, “they’re going to have to be much more aware of the integration challenges and be much more careful choosing renewables to minimize those challenges.”

PG&E says it will bring its plan to retire Diablo Canyon and replace it with clean energy before regulators later this year. When that happens, Wisland says, “It’s going to be up to everybody to hold them accountable.”

Why Plans to Replace Diablo Canyon With 100 Percent Clean Energy Could Fall Short 27 June,2016Lauren Sommer

  • I am amazed that there is no mention of geothermal energy in this article. California is the biggest producer of geothermal energy in the world. Just inland from Diablo Canyon, in the Imperial Valley there is over 2,000 MW of additional geothermal energy ready to be harnessed. It is available 24/7 365 days a year and has very low carbon emissions. There is enough geothermal energy in Southern and Northern California to make up for any loss of nuclear power and without the need for new natural gas plants.

    • Jesse Cane

      2,000 Mw in a state of nearly 40 million people?

      • flipperfeet

        As it is now, no single source will meet CA’s needs. To dismiss exploration or introduction of new sources because they will not meet the majority of the needs is setting requirements not placed on fossil fuel and nuclear power generation. Further, it ignores the advantages and security that comes with a blended portfolio of energy sources.

    • Props to geothermal, another source of carbon-free “baseload” power. The potential is certainly there, but can it be developed in the next nine years to the same degree as wind and solar? It seems like geothermal development has slowed in CA, compared to places like Nevada, where they’re pursuing it aggressively. Maybe you can share some insights into why that is.

    • EngineerPoet

      The available geothermal resource is trivial compared to demand.  If it was adequate to replace nuclear, it would already have done so.  Why hasn’t geothermal replaced the lost capacity of San Onofre?  Your failure to answer that question proves that your claims are false.  When you say geothermal can replace nuclear, YOU ARE LYING.

    • Joffan

      Why would you use geothermal development to replace Diablo Canyon when you could replace fossil combustion? It won’t be cheap to get started, it’s a thermal resource which isn’t “ready to be harvested” until you build your turbines etc, Sure, I have no problem with some effort going towards geothermal but it should be as well as, not instead of, its grown-up cousin, nuclear power.

    • Mike Carey

      Hi GRC,
      Have you folks solved the thermal depletion problems yet?


    • Starviking

      How difficult to access that 2 GW?
      How expensive to access it?
      What potential for the release of heavy metals like arsenic into the environment?

  • Andrew

    The projection that they will be using more than 20% less electricity in 2030 vs. 2017 seems…wildly optimistic, especially given that the population of the state is projected to grow by 20ish percent over that time period. It’s basically predicting that the average customer is going to be using only 2/3rds of the electricity they use now – I don’t see how we get there from here, especially if electric vehicles are much more widespread by 2030.

    • flipperfeet

      Most of these articles fail to even acknowledge the existence of consumer and commercial owned power storage as a way to mitigate the ebb and flow of renewable energy. The technology is moving so quickly here that it is conceivable that before this decade is out, a consumer could have several days, if not a week, of storage for under $2,000. The increase in electric vehicles has been proposed as part of the storage solution.

      I have yet to see a discussion where renewable excess power production is stored in the same way as base-line power production. In the case of PG&E this would include pumping water back up hill into reservoirs so it passes back through hydro plants in times of demand.

      • Jeff Stallman

        Articles don’t talk about this magical storage potential because no one has produced any proof of its concept. It’s all just an opinion that people are spreading as truth when there is no basis. If I am wrong prove it.
        As for your wonderful thought about pumping water back into reservoirs, do you know how detrimental that could be in downstream effects for your conventional dam? The fish that would die. The water shortages for communities that rely in downstream flows.
        Diablo Canyon actually sends excess power at night to Helms to pump water back into the man made reservoir above. And then in the day at peak times Helms let’s the water flow down hill to generate electricity.

        • flipperfeet

          So basically you referenced the solution I was referring to, but I know it was important for you to be a snarky a-hole. Congratulations! You succeeded.

          • Jeff Stallman

            I knew you couldn’t produce any proof. And I just didn’t want you to think pumping water up hill with excess power was new idea by fear mongering environmental activists seeing as it’s been around since the 80’s

          • flipperfeet

            Proof of home storage is provided with two links above. I never made a claim pumping water uphill was a new concept, I explicitly stated it was a solution already in use with current baseload systems, but as you have admitted, your goal was to try and make environmentalist look bad not discuss solutions and alternatives.

          • Jeff Stallman

            Just didn’t want people to think that it was a new idea during as it’s been around since the 80s. Plus renewables without nuclear won’t have excess power to support Helms. Gas plants will support Helms if they don’t just shutter it as well.

        • flipperfeet
          • Jeff Stallman

            Interesting. They have basically created a very large rechargeable battery that only the upper 10% can buy without the government subsidizing them. And then you get to replace them in 15 years if you are lucky or am I overestimating? Think of all the waste. It’s like thinking of all the waste soon to come from prius and hybrid dual battery systems; our are they just oversized special batteries. This will only cost 100s of billions of dollars to develop a viable large scale model and then billions more to implement.

            Sounds great for remote applications but seeing as nuclear plants are already built it seems like a waste of money to pursue in place of nuclear. In place of natural gas would make more sense. Economically and environmentally.

          • flipperfeet

            Of course government subsidies for the development of nuclear and other industrial scale energy technologies as well as rural electrification and a national energy grid are perfectly fine, as is tax payer covered storage of private industry nuclear waste. But subsidies for energy sources PG&E and other PUCs do not yet have a majority stake in or can control the pricing are economically unsound. Please, its clear were you get your bread buttered.

  • Jesse Cane

    Diablo Canyon makes clean energy.

    • Yep. That’s certainly been one of the major arguments for keeping it.

    • flipperfeet

      Clean, except for the waste heat that changes the local ecosystem, the spent fuel rods and then there is disposal of the irradiated power plant at the end of its 40-50 life. Numerous scientific studies have shown these plants, especially those of Diablo Canyon’s scale and age do not pay for themselves. Further, the CO2 created during the construction and decommissioning are generally not part of the CO2 calculations when talking about these plants.

      • Jeff Stallman

        I’m not sure you realize but the waste generated by nuclear is far less harmful than that due to solar because of strict regulations. The waste from Diablo canyons full 40 year life will be stored in canisters with passive cooling (no power required) on a more than 8ft thick concrete slab the size of a football field. Diablo canyon power generating portion only sits on 12 acres and generates 2400 MW. The largest solar farm in the US sits on 3200 acres and generates about 590 MW if it is sunny. Imagine for a second the waste those panels will generate in 20-30 years (if they last that long). You will need to create a brand new landfill capable of preventing the leakage of fun things like lead, silver and arsenic. The government, last I checked, terminated their free recycling program. That doesn’t even touch on the production of the panels in China where the dump spent solvents into the nearest water supply killing everything downstream.

        Please do some research before you speak out again. Facts are what will help our climate; not promised of hope and fiction.

        • flipperfeet
          • Jeff Stallman

            I’m glad you referenced such a diverse group of highly unqualified “experts” for your defense.

            For your information. Plants must maintain fuel in the pools until they can be transferred to dry casks. Oh yeah we have been using dry casks for at least a decade considering that the government has failed to uphold its contract to provide a repository. It is also being sued for the same reason. Your overheating issue of the pools are well…Not an issue. Temperatures are highly controlled and regulated. And that terrorist comment is as foolish as the cancer claims. Both false. Nuclear plants all have highly trained security forces which make them probably the safest places in the US.

          • Jeff Stallman

            See my response to your other post. Those guys are just out to push a a liberal agenda based on lies and propagated by fear mongering.

            Do a quick Google search. I haven’t lied to you. Just exposed their lies.


      • EngineerPoet

        (This is a repost of this comment.  I’m logging the permalink in case it disappears.)

        You can recognize a load of Greenie bullfeathers by the distinctive smell.

        Clean, except for the waste heat that changes the local ecosystem

        The Pacific ocean, and the California current which is a part of it, is a vast heat sink which can dissipate thousands of gigawatts of heat with little effect.  It receives about 1 GW per square kilometer at noon on summer days from the sun; a little bit from nuclear plants is lost in the noise.

        the spent fuel rods

        “Spent” is a gross misrepresentation.  “Slightly used” is more accurate, as 95% of the available energy of enriched uranium fuel is left over after use in a light-water reactor.  Fast-spectrum reactors can consume the other 95%.

        The actual wastes, fission products, have a multitude of uses as they release their dying gasps of energy and become stable nuclei.  After 500 years they’re less radioactive than the uranium ore from which they came, and they don’t wind up as toxic lead.

        and then there is disposal of the irradiated power plant at the end of its 40-50 life.

        Let it sit for a few decades for the cobalt-60 to go away, then melt down its steel and make a new nuclear power plant out of it.  No sense in letting any of it go to waste, is there?

        Numerous scientific studies have shown these plants, especially those of Diablo Canyon’s scale and age do not pay for themselves.

        We know about the fake studies written by activists like Benjamin Sovacool and Storm & Smith.  They aren’t worth the paper they’re printed on.

        Further, the CO2 created during the construction and decommissioning are generally not part of the CO2 calculations when talking about these plants.

        Besides using about 1/10 the steel and concrete that wind uses per average watt (and thus 1/10 the CO2), energy and carbon balances for nuclear do include construction and decommissioning.

        So far you haven’t managed to say one thing that’s true.  Aren’t you embarrassed?

        • flipperfeet
          • EngineerPoet

            And your link is to an anti-nuclear propaganda organization.  No surprise.

          • flipperfeet

            Point of order, anything that opposes your pro-nuclear argument would be anti-nuclear. That does not make it automatically wrong or propaganda, but it is clear that is your stance.

            The plant at Diablo Canyon will be reaching the end of projected life, what remains is to retire and clean up its irradiated hulk. If, after babysitting its various components for decades, centuries and millennia, a safe use can be found wonderful. But it is equally impossible to take your arguments seriously when these time frames exceed the productive life of the original plant, in many cases by multiple orders of magnitude, especially when a core assumption of the economics of nuclear power relies on the externalization of the bulk of the costs on the shoulders of the public, not on the plant owners and operators.

            If the public is to the bear the brunt of these costs, then their larger number and collective burden imbue them with at a minimum a voice equal to engineers and industry, if not the right to determine the nature of the energy mix.

  • solodoctor

    There is enough time for prudent planning to implement increased conservation and efficiency measures as well as boosting the use of solar and wind. There are sites around the State which can provide lots of solar or wind power. I hope the state regulators work actively with PG&E to help the company move in that direction.

    • solodoctor

      It is true that nuclear energy does not produce greenhouse gases like fossil fuels do. But Diablo Canyon is sitting on an earthquake fault that is larger and potentially more powerful than was projected when the plant was built. It is not designed to withstand the kind of tremors which this fault could generate. It also has large amounts of spent fuel sitting in pools of water. In the event of a major earthquake this fuel could spew radioactivity into the sorrounding area. This is what happened at Fukushima Japan in March 2011. So, it presents other very significant hazards that merit its closure.

      • Jeff Stallman

        I’m sorry that you have been horribly misled and lied to.

        Yes Diablo Canyon sits NEAR the Hosgri fault which has a greater potential than was originally designed. This is actually the reason why the coastal permits did not match operating license. During early construction PG&E requested that the NRC place Diablo Canyon licenses on hold while they reevaluated and redesigned the plant to exceed the Hosgri fault potential. Furthermore the new shoreline fault potential is already enveloped by Hosgri and thus no additional modifications were required due to its discovery.

        Your understanding of the Fukishima events is also incorrect. The plants at Fukishima survived the earthquake perfectly. It was the Tsunami which flooded all but one emergency diesel generator which caused the meltdown. Throughout the entire event no person was killed as a result of nuclear related issues. All were killed by the earthquake or Tsunami in areas outside the plant. It sure was unfortunate that reporters ignored them in pursuit of the nuclear plant complications and they are starting to apologize for this.

        Lastly radiation doesn’t spew and, seeing as the spent fuel pools and the frames which hold the fuel are all seismically qualified to withstand Hosgri ground motions, the fuel will be fine under the 14 or so feet of water.

  • jimhopf

    There are many reasons to doubt that they will be able to replace all of Diablo’s output with renewables energy, but even if they can, it will mean that we will spend an enormous amount of money and effort, just to replace one non-emitting source with another (while leaving the fossil generation that provides ~60% of the state’s power intact). A huge amount of money and effort in order to make no progress at all in reducing emissions.

    And, in the likely event that they will not be able to fully replace Diablo (not to mentioned the now-closed San Onofre) with renewables, we will spend a huge amount of money and effort and have power-sector CO2 emissions actually *increase*.

    This is indefensible, given the seriousness of global warming. These are not the actions of a state (and “environmental” groups) that are genuinely concerned about global warming. Renewable energy must be used to replace fossil fuels, not nuclear.

  • jimhopf

    “It will reduce the need to curtail our solar power plants,” said Cavanagh. “The nuclear plant can’t be ramped up and down, so it’s occupying space on the grid that solar farms could occupy at much lower cost.”

    This statement, by the NRDC, makes it clear that Diablo will be replaced by a combination of renewables and gas (as opposed to all renewables). His argument is that the nuclear plant, that generates emissions-free power 24/7 may *sometimes* cause the curtailment of solar (during the small fraction of the time when those solar plants are producing maximum output). In other words, he wants to “curtail” a large block of emissions free power ALL of the time (i.e., close Diablo) to avoid curtailing another form of emissions free power (solar) some of the time. He’s saying that gas is better because it can ramp up and down and shut off when solar output is maximum, the clear implication being that the gas generation will be on most of the time (when solar is not at maximum). This will clearly increase emissions, vs. keeping Diablo open.

    Statements like these make it clear that their sole focus is maximizing renewable generation, as opposed to minimizing CO2 and air pollution. It is clear that they prefer a mix of natural gas and renewables (even if it’s mostly gas) to nuclear generation.

    Oh, and the “lower” cost statement is at best deceptive. It’s based on the presence of taxpayer subsidies, as well as the state’s 50% renewable mandate. The overall cost of new solar is significantly higher than the cost of continuing to run Diablo.

    Finally, it seems clear that if there is “no room” for Diablo on the grid, given the already-mandated amount of renewables, there clearly would be no room for any additional renewable generation that would replace Diablo’s output. Given the difference in capacity factors, for solar (or wind) to provide the same annual generation as Diablo, it’s maximum capacity would have to be 3-4 times higher. Thus, at the time of maximum renewables output, that extra renewable generation would have to 6-8 GW (compared to Diablo’s ~2 GW). If there is no room for Diablo’s ~2 GW of output at the times of maximum renewable generation, how could there be room for 6-8 GW of additional renewable generation?

  • Cesar Penafiel

    What a sham this whole deal is. To think that even by PG&E’s own estimates, by 2030 it will be burning 24 TWh of fossil fuels is incredible. How on earth can they justify killing their largest source of clean energy while extending the life of their fossil plants? This deal is a dirty backroom deal that will fail when the people of California find out that it undermines democracy, climate and progress.

    • flipperfeet

      By 2025, this plant will have been in service for 45 years, this is near the end of the projected service life for plants of this type and era. PG&E is not taking a young modern nuclear plant offline. This is framing the planned retirement of the plant in the best possible way to leverage PR for PG&E’s investments in renewables and to meet state requirements.

      • EngineerPoet

        By 2025, this plant will have been in service for 45 years, this is near the end of the projected service life for plants of this type and era.

        Hogwash.  Diablo Canyon is reaching the end of its initial 40-year license term.  Most plants have been maintained very well and have endured far less neutron embrittlement than the original worst-case projections, so they are good for another 20 or even 40 years.  Even 20 years puts it barely halfway through its life.

        Your fanatic opposition to Diablo Canyon, while climate change is supposedly the biggest threat facing humanity, proves that you don’t actually take climate change seriously.  You are not an environmentalist, you are a stealth shill for fossil fuels.

  • EngineerPoet

    KQED censors:  You should be ashamed of yourselves.  If you are allowed to represent the station it deserves to go broke.

  • Paul Swanee

    How is it that PG&E are increasing efficiency so much that the grid removes 19 TW/h in 12 years? Especially considering the fact that demand for electricity is increasing for population and the introduction of electric vehicles. I have not seen any evidence that the state is increasing their efficiency that much and this quickly.

    The numbers being put out have not made any sense to suggest that renewables make up even 50% of what was produced at Diablo Canyon in 7 years time. Theres also the historical evidence that the state claimed this exact same scenario in which renewables would replace 100% of production from the San Onofre plant, when even today 80% of the replaced production is made by natural gas.


Lauren Sommer

Lauren is a radio reporter covering environment, water, and energy for KQED Science. As part of her day job, she has scaled Sierra Nevada peaks, run from charging elephant seals, and desperately tried to get her sea legs – all in pursuit of good radio. Her work has appeared on Marketplace, Living on Earth, Science Friday and NPR’s Morning Edition and All Things Considered. You can find her on Twitter at @lesommer.

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