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The Duck has Landed

May has arrived and days are getting longer and warmer. This is good news for baseball fans, barbecue enthusiasts, and grid operators concerned about integrating unprecedented levels of solar energy onto the California grid.


Source: Solar panels at Busch Baseball Stadium

Plugging lots of solar into the power system creates challenges, particularly on days when electricity demand is relatively low and renewable generation is high. Here in California, this happens in March and April when solar intensity is up (relative to the winter months), but air conditioning demand has yet to kick in.

Back in 2013, some California energy analysts with an eye for aesthetics were looking at how projected increases in renewable energy generation might affect power system operations. They plotted actual and projected hourly net load profiles (i.e. electricity demand minus renewable generation) over the years 2012 to 2020, focusing on late March when integration concerns loom large. The result was remarkably duck-like.


The California ISO “duck chart” made a big splash for a number of reasons. For one, a graph that looks like a duck makes an otherwise dry, technical issue more fun to talk about.  Conversations about renewable integration become more engaging when sprinkled with fowl word plays.

Perhaps more importantly, the graph highlights two related integration challenges. First, the long duck neck represents the steep evening ramp when the sun sets just as Californians are coming home and turning on their lights and appliances. Accommodating this ramp requires maintaining a fleet of relatively expensive generation resources with high levels of flexibility. Second, the duck’s growing belly highlights the near-term potential for “over-generation”. As solar penetration increases, net load starts to bump up against the minimum generation levels of other grid-connected generators, such as the state’s remaining nuclear power plant. At some point, system operators have to start curtailing solar to balance the grid.

How’s the duck shaping up?

The CAISO duck chart predicts that we should see increasingly duck-like net load profiles in March and April. So I’ve been keeping an eye on the great data that CAISO makes readily accessible. This year, the duck showed up. The graph below plots average net load profiles for late March/early April since 2013 (I averaged across seven days around March 31 to smooth out the variation that comes with random weather, week days versus weekends, etc.).

duck_graph.fwNote: All data taken from CAISO website. Graph summarizes hourly data, March28-April 3, 2013-2016.

In the 2016 duck season, we saw mid-day net loads at or around predicted levels. Increased solar penetration on both sides of the meter (utility scale and distributed)  has been driving net loads down when the sun is up. Fortunately,  the ramp from 5 – 8 pm has not been quite as steep as projected because electricity demand in the evening hours has  been lower than projected. Perhaps this is due to unanticipated demand-side energy efficiency improvements. I could not easily find hourly curtailment data. The data I could find on plant outages indicate that March 2016 saw the highest forced solar plants outages on record, but these outages could  be due to factors other than curtailment.

My after-the-fact duck chart suggests that renewables integration challenges are showing up more or less on schedule (although ramping requirements are somewhat less than projected). So far, these challenges are quite manageable without major changes to grid operations. But the duck of the future – especially given California’s new target of 50% renewables by 2030 –  will present a more formidable challenge.

Renewables integration strengthens the case for regional coordination

California is not alone in creating and confronting unprecedented renewable integration complications. Take Hawaii, for example, where a 100% renewables target makes California’s 50% look timid. Our colleagues at University of Hawai’i, Michael Roberts and Mathias Fripp, have been thinking hard about how Hawaii can pull this off at least cost. The charts below illustrate a hypothetical 100% day in Oahu in April (no more duck when all load is served by renewable energy!):


Source: Fripp (2016)

The broken line in the right graph represents the “traditional”, business as usual demand profile. To hit the 100% target, wind and solar generation increases to nearly double current levels of the traditional peak.  Differences between the timing of renewable energy production and traditional demand are reconciled primarily by EV charging and other demand-side response  programs (although batteries and pumped storage also play a role).

When you’re an island in the middle of the ocean, you’re pretty much on your own when it comes to tackling these grid integration challenges. Thus, Hawaii is preparing to demonstrate how significant renewable energy integration can be achieved with demand response, grid management, and storage. In contrast, California has more options to leverage.

Although California fancies itself a different world, it is physically connected to (but not perfectly integrated with)  a larger western power system.   From an economic perspective, expansion of the energy imbalance market and improved coordination of the western grid looks like an obvious and important piece of California’s renewable integration puzzle.  A regionally coordinated western grid would integrate mandated renewables across a larger area, thus reducing the likelihood of over-generation. Coordination across balancing areas should also provide increased flexibility.

In the past, economists have documented the efficiency gains of improved regional coordination and bemoaned the inefficiencies of the balkanization that persists.  Looming renewable integration challenges could provide the needed additional impetus for grid integration.  To be sure, there are some important details that need to be better understood. But if done right, a fully coordinated regional grid could help clip the duck’s wings.


75 thoughts on “The Duck has Landed Leave a comment

  1. I said before nuclear does not have to be base loaded and Per Peterson has the perfect design for load following with nuclear. Stop saying that all nuclear has to be base loaded. Its just not true.

    • Sorry, Gene.

      Did not mean to imply that an $8,000 / kilowatt nuclear resource needs to be baseloaded for technical reasons. It only needs to be baseloaded to have any chance of being cost-effective.

      If you spread $8,000/kW for new nuclear over only 1,000 hours/year of generation (about what is needed to fill in the gaps from a wind/solar generating system), the cost per kWh rises to about $0.70/kWh. At that price, it’s cost-effective to install solar or wind plus batteries for peaking needs.

      The fixed O&M alone, of $135/kW/year, works out to $.135/kWh. The capital costs, fuel costs, and waste disposal costs are on top of that. According to Lazard, taken together, at a 90% capacity factor, it adds up to $.097 – $.136/kWh. Cut the capacity factor by a factor of 9, and the capital and fixed O&M (which are 85% of the total cost) increase by a factor of 9.

      Wind and solar are now available for about half the cost of new nuclear. See Lazard 9.0 at

      So the problem is economic, not technical. I have no doubt that a nuclear unit can be designed to ramp quickly. But WHY? To spend three times as much money as necessary to keep the lights on, the beer cold, and the coffee hot?

      • “Can be designed to ramp quickly.” That may be true for unbuilt nukes, but unfortunately it appears not to be true for already built ones. Here in California, where market clearing prices already fall below the marginal cost of nuclear fuel in a few percent of the hours (and are predicted to do so in far more hours in the 2020s), it would make economic sense to ramp down nuclear generation at the already built Diablo Canyon nuclear plant during those hours. Such ramping would alleviate the “belly” part of the “duck curve” problem, and in Jim Lazar’s apt phrase, help the duck to fly. But a variety of technical and safety objections have been raised by the plant owner to doing so. Does Mr. Preston have any comment on how fast or how frequently 10-20% output reductions could be implemented for an existing PWR?

        • The duck curve is a severe limitation of renewables, mostly associated with a lot of solar power, but during light load periods wind can also cause the same problem at any time. Even if you did cycle Diablo Canyon it would hardly make a dent in the inherent problem with renewables which is they produce too much power when its not needed and not enough power when its needed. The obvious solution but an expensive one is storage to capture excess renewable energy when its produced and then supply it to load when its desired. Cycling Diablo Canyon does not solve the problem. Even if you drove Diablo Canyon completely off the grid the problem still exists and that would only drive up the price of electricity since Diablo Canyon energy price is about 2 cents per kwh at this time. Of course the solar folks say their energy cost is zero but this is just playing games with numbers. The solution is to curtail the solar and wind when it cannot be used. This forces the issue to decide how to best utilize the over production of renewable energy. There are two solutions. 1) increase load, i.e. the opposite of conservation, find loads that can be increased and put then on line to absorb the excess power, and 2) install storage, like the home tesla battery system would be a good idea. Those people who want to promote renewable power need to provide their own solutions to management of this problem, i.e. use the excess power or lose it. This is where home microgrids come into play. last page.

          • Gene, the long term plan IS to increase load–electrify building space conditioning (to reduce methane emissions) and to electrify transportation (to reduce oil use.) And the EVs can double as “mobile” storage which provides storage services virtually for free as an ancillary servie to the prime transportation service. Much of this has already been thought through.

  2. Is the CAISO really getting 50% of its energy from renewables? You have to be careful to not to think 50% of the power in CAISO translates into 50% of the energy supplied. They don’t supply as much of the overall percentage of energy as their capacities might suggest.

    • Yes, I’m talking energy, not capacity. I reported the data in Mw form because it was hourly average data. 1 Mw averaged for one hour is 1 Mwh. The CAISO energy supply was over 50% renewables for 1 hour on 4/24, 5 hours on 5/14, and 7 hours on 5/15. (Counting large hydro as non-renewable, and counting imports as 100% non-renewable; with other definitions the renewable percentage would be even higher.) Data sources: hourly CAISO load from Hourly renewable generation from

      • Oh come on man, we are talking about 8760 hours, the entire year. One hour makes not one iota difference. Its the energy for the whole year. What is that percentage for CAISO?

        • Last year CAISO was got 31% of their electricity from internal renewables. A quick examination of their imports shows they imported ~20 TWh from hydro heavy Balancing Areas like BPA and BC. This pushes their renewable sourcing up to around 39% and leaves 45 TWh imports from CFE, TIDC, PACW, NEVP, LDWP, WALC, IID, SRP, AZPS and BANC… I can’t say definitively how much renewables may have come from these BAs but off the top of my head BANC, LDWP and SRP all have plenty of hydro that could be wheeling through to CAISO.

          In 2017 CAISO’s internal production and sourcing from BPA/BC shows a mix that’s 45% renewable year to date. I wouldn’t be terribly surprise if the mix is 50% year to date.

          • Follow up – 10 TWh of net hydro wheeled from BPA to LDWP to CAISO in 2016. This bumps their renewable sourcing up to ~43%.

            It’s impressive how renewable CA already is when you break down the imports.

          • BPA hydro doesn’t qualify as RPS eligible renewables, so LADWP doesn’t get credit for it. Also behind the meter solar doesn’t count toward the RPS either.

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