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California’s Push for Carbon-Free Electricity

Gov. Jerry Brown has signed a new law requiring carbon-free power by 2045. Is that feasible?

The Altamont Pass Wind Farm in central California, part of the state's renewable energy mix. (Photo: John K / CC BY-NC-ND / Flickr)

California Governor Jerry Brown has signed a new law mandating that the electricity the state consumes not cause carbon emissions by 2045.

He also issued an executive order that goes even further: It commits California to “achieve carbon neutrality” across the board and not just for power generation by 2045. Together, these steps codify California’s ongoing transition away from relying on fossil fuels for energy. This effort has been ramping up since 2011, when Brown signed another law committing the state to deriving a third of its energy from renewable sources like wind and solar power by 2020 — not including big hydroelectric dams.

Based on more than 30 years of research related to solar energy, by my assessment, California can meet the law’s ambitious goal as long as it continues to implement programs that encourage the rapid expansion of renewable energy in the state.

A growth industry

The new law actually sets multiple targets rather than just one. It commits California to draw half its electricity from renewable sources by 2026, a share that would rise to 60 percent by 2030.

To take the next step, rather than mandating that all power be renewably sourced, state lawmakers established a 100 percent “zero-carbon” goal. They did not define this term, but it is understood as including wind and solar power, big hydropower plants, and other sources of electricity that do not generate carbon dioxide.

Utility-scale solar and wind electricity increased from 3%  in 2010 to 18% in 2017 in California, exceeding prior state targets, largely because solar prices have dropped sharply in recent years.

Being open to a wide range of technologies makes meeting the 2045 target easier and allowed State Senator Kevin de Leon, the original bill’s author, to amass broad support for the bill.

Where things stood in 2017

About 56% of the power California generated in 2017 came from sources that don’t emit carbon. That puts it more than halfway toward this new goal by 2045.

However, the Diablo Canyon plant, California’s last nuclear power station, is slated for decommissioning by 2025, and no other reactors are in the works. This closure would eliminate the 8.7% of the state’s carbon-free power that came from nuclear energy.

Nearly all of the remaining 44% of the state’s electricity is currently generated by burning natural gas, and virtually none comes from coal. Going completely zero-carbon would require phasing out the state’s natural gas power plants.

On top of wind and solar energy, other generation options include geothermal, small nuclear reactors, and carbon dioxide sequestration.

One quirk about this legislation is that it deals only with utility-scale power. It would not preclude private electricity-generation facilities such as the diesel generator a farmer might use to pump water. Nor would it count the power generated by a homeowner’s rooftop solar panels.

When the sun shines

One complication is that the state’s mix of energy sources can vary a great deal, even from one hour to the next.

Consider what happened on April 8, 2018, for example. It was a generally sunny and windy Sunday, with relatively low electricity demand. At night, about 40% of electricity was generated from renewable sources. But around noon that day, more than 80% came from renewable sources including large-scale hydropower.

If the electricity generated from these renewable sources is approximately doubled, as I estimate is necessary to meet the 2045 target, the power available in the middle of the day would greatly exceed the demand for electricity at that time.

This challenge shifts throughout the year.

On July 24 and July 25, 2018, Californians were asked to voluntarily use less electricity between 5 p.m. and 9 p.m. to avoid an outage because of hot weather. Prices spiked by more than a factor of 10, helping to keep demand within the supply.

On those days, renewably sourced electricity never met half of the demand for power.

Balancing act

Due to this degree of variability, relying heavily on renewable energy will require ample energy storage and big investments in grid-based technology.

Today, the expected demand for electricity is balanced by the Independent System Operator, an entity that controls the flow of electricity on the grid and selects the lowest-priced sources of electricity available.

Pumped hydro storage, electricity generated from water pumped to a reservoir, is the state’s most common form of storage today. While limited to locations with large dams, the amount of energy stored this way could be increased in California, as recently proposed for Hoover Dam.

Big lithium ion batteries are becoming more affordable and are now beginning to be deployed on the utility scale. As battery and solar prices drop, it may become attractive to disconnect from the grid and use electricity generated by a solar system and stored by a battery.

Lower battery costs are also spurring the sales of more electric vehicles. Ideally, these vehicles could be charged at times when electricity is plentiful and cheap. By 2045, I believe they could be helping make the grid more stable.

Other options are becoming available. One example is utility-scale compressed air storage, where energy is stored as pressurized air.

And there is growing interest in solar thermal plants, which generate electricity from sunlight’s heat and use high-temperature storage to continue generating electricity after the sun sets.

The University of California Merced and many other wholesale electricity customers are saving money by using thermal storage. They chill water when rates are low and use the chilled water for air conditioning when electricity prices are high.

Wiggle room

Because California’s new law does not require that every watt be generated within California’s borders, utilities could keep buying electricity from nearby states, as long as they verify its origins are in keeping with the new law’s requirements.

And because the law does not define “zero-carbon,” it provides flexibility in how the state can meet this new target.

For example, California would allow the continued operation of natural gas plants when their output is coupled with purchase of renewable energy certificates, credits issued for the generation of renewable electricity that may be sold, from a utility that generates solar or wind power.

These credits arise through several kinds of arrangements. Perhaps the most common is through rooftop solar systems. Small-scale solar energy supplied about 5% of California’s electricity in 2017. It is likely to grow because of California’s mandate for solar panels on most new homes, starting in 2020.

In assessing whether the goal of going zero-carbon by 2045 is realistic or not, it is worth considering that solar energy has grown for years at a pace that far exceeded projections thanks to technological progress, government policies like California’s new law, market forces and the public’s demand for renewable energy.

Sarah Mercer is a professor of materials science and engineering at the University of California, Merced. This post originally appeared at The Conversation.



  1. AlanB4 | | #1

    Its an attainable goal.
    The details will matter though, pumped hydro storage is a good one to exploit, solar power corresponds to most of the AC use and a few things from smart water heating to batteries to demand management will help things work out.
    In the end its about solar/wind and storage to iron out their variability.

    1. Expert Member
      Dana Dorsett | | #2

      California (unlike most of the US) has the seasonal peak PV and peak wind both happening in the warmer months, and are at their minimums in the winter.

      The major solutions to the seasonal space heating load problem isn't fixable by demand response or short-term storage. In addition to smarter load management it's going to take either...

      A: Massive storage capacity

      B: Overbuilding the renewables and curtailing/wasting output

      C: Better integration into the WECC grid markets (CAISO is somewhat islanded, constrained by the CA regulatory environment) and building out more transmission capacity to other regions.

      The most economic solution will likely be a combination of "all of the above."

      1. AlanB4 | | #4

        I mostly agree with you on this one

        I did not know about the seasonal peaks. But you are correct that dealing with it would require a great deal of overcapacity handling procedures and i also agree that demand management can only do so much. I was reading a Vox article a while back about not being afraid of curtailment, a concept that I don't agree with it but it does show people are working on future problems though i think the solutions will have to come either by discovery or innovation.

        I have been watching the grid in Australia recently, with South Australia's goal of 100% renewables being worked on. They have a huge amount of Wind, approaching peak grid usage, but with its intermittency they are at something like 50% net renewables (the remainder being met with gas). They often go above 100% renewable for short periods, sometimes half a day or more state wide and even over 100%, usually at night but whenever demand is relatively low on the daily cycle and wind is operating at very high capacity factor. They were at 120% wind for the whole state for a while in the past week was very interesting to see.
        Realtime energy generation by source, no historical data though

        So they could hit net zero in coming years rather easily but it only works because they are a part of larger grid who can absorb their overproduction and is not near 100% renewable penetration. And Net Zero will not work once the entire national grid gets closer to 100% renewables.
        I remember reading somewhere that the first 60-80% of carbon free electricity is not difficult but the rest will be increasingly harder to achieve.

        There is a Tesla solar/battery system on an island that has battery power for 3 days and can recharge in 6 hours (if i'm remembering numbers correctly). It must have high curtailment but is apparently much cheaper then the old diesel system.

        1. Expert Member
          Dana Dorsett | | #5

          I'm well aware of the Australia story ( been following the RenewEconomy blog for years), including the AEMO market design deficiencies and the political infighting. Rooftop solar is a lot cheaper (and better output per installed watt) than in California, and retail electricity pricing is more than 50% more than in CA too.

          What Australia really lacks (and California doesn't) is consistent policy support to allow them to converge on the right solutions. Changes in governments there result in crazy overnight shifts in policy, or sometimes just setting policy adrift (as it is under the current government in Oz.) The AEMO under Audrey Zibelman has the right vision & direction, but not the power to correct all of the regulation or policy support structures. Australia doesn't have a storage mandate, but they have sufficiently high electricity prices that private investors and state governments can make the financial case for storage & renewables, even as the Federal government is failing to come through. Australia at one time had a carbon tax, now they don't, and feed in tariffs & other policy measure change with every government, even when the party in power doesn't.

          The lack of consistency has left third party financiers on the sidelines, but the high prices are driving the shift to renewables anyway. Sanjeev Gupta isn't stupid enough to pay the going grid rate for unreliable electricity to suport steel making when his organization has sufficient capital to build their own renewable power & storage for a lower levelized cost, and even make money on the excess power production (for now).

          Tasmania has substantial hydro capacity and is planning it's own 100% renewables utilizing pumped hydro for the bulk storage, all without much guidance from the central government. Transmission capacity between Tasmania & South Australia is currently under major upgrades, and SA will be able to utilize Tasmanian storage to limit curtailment in much the same way that Denmark uses Norwegian hydro resources when the excess can't be simply exported elsewhere in Europe.

          New pumped hydro isn't very cheap (the economics of Snowy-2 in Australia isn't a slam-dunk), and as PV & wind get ever-cheaper, curtailment gets cheaper too. But better utilization of existing assets can be pretty cheap, if not exactly free. California is currently not able to get full use out of the wider WECC interconnect (due to CA's internally imposed regulations), which would go a long way toward solving both short term & seasonal energy deficits and curtailment issues.

          Simulations using historical weather data over the PJM grid control region have shown that 100% renewables takes even less storage (and less curtailment) than only 80% renewables, and improving transmission line improvements to adjacent grids can lower that even further. (Unlike CA the seasonal output of wind & solar are at least somewhat complemenary.) Transmission lines aren't cheap either, but they're cheaper than most long term storage schemes, and that's almost certainly going to be a part of the solution there & elsewhere.

          1. AlanB4 | | #6

            You obviously know your Australia stuff ;)

            Tasmania wants to become the battery of Australia and its a good chance they will. In general its likely Australia will need less storage then most countries because of their huge hydropower resources acting as storage. I commonly see 500MW or more being used to in pumped hydro storage, right now 200MW in NSW.

            I just read this article about carbon stripping, if curtailment energy was used it would make use of a discarded resource assuming the technology lives up to its promise (a big if)

            Though i find it interesting that you say 100% will need less storage and curtailment then 80%

  2. Deleted | | #3


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