Intermittent or variable: Are our wires crossed on renewables?

Energy Transition

As someone who publishes both in German and English, I have my work cut out for me when it comes to terminological niceties. Over the years, I have been told not to call renewables “intermittent,” but rather “variable.”

The use of “intermittent” certainly is well established for renewables; see this Wikipedia entry, for instance, which only mentions nuclear and coal in terms of reacting to intermittent energy – not as intermittent sources themselves. Likewise, “variable” sounds like the energy source is “capable of varying,” but wind and solar are not dispatchable. Taken literally, they are varying, but not variable.

But recently, someone finally took the time to explain to me why a clear distinction should nonetheless be made – with conventional plants being called intermittent and wind & solar called variable. The idea is that, while production of wind and solar power fluctuate (to use the German term), giant amounts of renewable generation capacity do not simultaneously go off-line.

Conventional plants can fail quickly. In a recent storm that hit Europe, the social media world was concerned about wind turbines being blown away, but I could not find any news of such a thing happening. We do know that the Ringhals nuclear plant, with a capacity of 878 MW, failed completely, however, as one of its blocks did again just a few weeks later.

In North America, the recent Arctic cold knocked out power plants across the country, with 39,500 MW going off-line in a single day within the PJM grid, 21 percent of PJM’s total generation capacity. Roughly 19,000 MW was coal plants, followed by 9,000 MW of natural gas turbines, 1,600 MW of nuclear (probably a single plant), and “nearly 1,500 MW of wind.” (One wonders whether it was the wind turbines themselves that failed or grid connections to the turbines.)

The PJM area was not alone, either. In Texas, 3,700 MW of conventional capacity had to be shut down during the polar vortex, but my favorite part of the report is where the official “declines to name” the specific plants that were affected. So much for market transparency, not to mention the public’s right to information. Our colleagues at Think Progress have more details on the matter in Texas.

A similar situation is reported for Australia, where solar power has helped curb demand for conventional power during the recent record heatwave.

I’m not arguing that wind and solar are more reliable than conventional plants. They are simply unreliable in different ways. Gigantic plants switching off suddenly can cause quite a ripple on the grid, and such events are more frequent than those who praise the reliability of fossil and nuclear wish to admit. Germany’s installed generation capacity has always been greater than its peak power demand, generally by around 20 percent (the situation is similar in France). And the Reuters report from which I took the failure figures above also mentions, without explicitly putting the two figures into the same context, that the PJM grid’s 189,654 MW of total generation capacity was built to serve a maximum demand of 141,312 MW – a clear sign that fossil and nuclear plants require considerable backup capacity in the US as well.

In a way, the German “fluctuating” best describes solar & wind; once again, German is more precise. But it is worth noting, as one colleague pointed out, that the IEA itself makes the above distinction and calls renewables variable, not intermittent. Variable renewables and intermittent conventional it is!

Craig Morris (@PPchef) is the lead author of German Energy Transition – where this post was originally published. He directs Petite Planète and writes every workday for Renewables International.

Comments

9 responses to “Intermittent or variable: Are our wires crossed on renewables?”

  1. Alen Avatar
    Alen

    Variable renewables and intermittent conventional, I like it!

  2. coomadoug Avatar
    coomadoug

    During the recent heat wave when the energy market struggled under the Victorian heat wave, annalysis done by various bloggers was disappointing.
    One respected writer highlighted the ramping down of wind generators in the middle of the day , on the wednesday as an indication of failure.
    In fact this was an indication of the benifit of the wind generation. The wind did ramp down and so allowed for the re introduction of a large steam generator. The large caol generator went suddenly off line causing the crisis. The wind was used to enable the re introduction of the coal generator after much trouble.
    The grid ,when stressed, has many complex calculations running to determine critical points of load. It is not possible to sit back in ignorance of these issues and constraints and make inteligent statements about performance. Certainly when a lage coal generator falls of the grid, as they do, this is very bad. This is certain. However, the fluctuations of wind and solar, when observed and scrutinised in full knowledge of the grid security control, has many aspects and most of them good.
    This is 2013. The technology of today is not that of the 1960s. It is madness to continue to think about the grid and to continue to manage it in that era of huge inefficient, bash and barge energy management.
    Solar and wind, by it’s very nature is a huge blessing to this nation in so many aspects of the economy. But the fact that it is so widly dispersed in small, easily managed load and energy sources, is the greatest benifit. To truely appreciate this gift, even the old hands of great experiance in the power industry, need to take a step back and look close. They will then recognise the security and stability that is so richly enhanced by the new plant and technology.

    1. DogzOwn Avatar
      DogzOwn

      If big generators present unscheduled outages, then big consumers should surely be taken off line. Portland Smelter wouldn’t suffer much inconvenience, not need much compensation considering it only pays 1.5cents/kWh.

      1. coomadoug Avatar
        coomadoug

        There are arrangements between large loads as you describe. This is the large bash and barge 1960s idea. It will be done in the future by switching small loads inside the home and small business. The new technologies have wonderful potential.

        1. DogzOwn Avatar
          DogzOwn

          Smaller loads used to be shed, wired on particular circuits from residential meter box, designated non-essential, like pool pumps and even aircon, have been switched since the 60’s by data tones injected on the power line by SCADA systems. Just interruptions of 10mins or so per hour, rotating around groups of houses in the neighbourhood. Pity our Smart Meters are dumbed down so much.

          1. coomadoug Avatar
            coomadoug

            This is very true what you say. However the only way we can tell if you are on line is if your lights are on. This is old old situation. In the future the survivors in the distribution industry will be in the home, the car and the factory. Very little energy will be wasted. They will be switching us in milli second response to disturbance. They will be storing energy in our cars and home batteries.

            The load will fall with this refining and product control and modern design by about the same ammount it will increase with the complete change of the transport industry to re newables. The grid will have loadings much less then half what it is now at peak time. The grid however will be essential and provide the link to large wind and solar installations.

            This transition is an absolute gold mine for this nation and our government needs to get on board. Trying to suppress innovation and green technology may be the biggest mistake in our history and it will fail anyway.

          2. DogzOwn Avatar
            DogzOwn

            Having observed gold mines of so many different types, they might exist in Australia but cultural cringe and tall poppy syndrome are alive and well. If anything significant can happen here, free traders(!) insist they relocate to USA eg catamarans and PBR. And/or international purveyors of capital become generous for large debt, which then needs to be serviced by diversification to low wage countries, like USA! If we’re ever lucky enough to get high speed trains, it will be the likes of Phoney Tony by mail order from China.

          3. coomadoug Avatar
            coomadoug

            Within three years the leaders of China and USA and Europe will be on a strong green agenda. They will know the chemical and thermal rules of survival on this planet. Poland, Australia, the worst offenders. will not continue to shit in the nest. Ecconomic sanctions will ensure it.
            With the right leader we can take advantage of the crisis response.

  3. PhilipKGlass Avatar
    PhilipKGlass

    But recently, someone finally took the time to explain to me why a clear
    distinction should nonetheless be made – with conventional plants being
    called intermittent and wind & solar called variable. The idea is
    that, while production of wind and solar power fluctuate (to use the
    German term), giant amounts of renewable generation capacity do not
    simultaneously go off-line.

    I have taken the time to download German solar production data from com.sunny-portal.de and convert the data into megawatt hours generated per day. In December installed German solar capacity did not change much. It is a low-insolation time of year but the day to day trends do not track with the winter solstice so much as with cloud cover. You can see changes from day to day of up to 3-fold or so.

    I also have data from California in December 2013 from caiso.com for solar and other renewables. There are some large solar production fluctuations, but note that solar is steadier day to day than wind. Values are also given in daily megawatt hours produced. Due to California’s more favorable location, there are actually some days where California outperforms Germany despite having less than 1/7 the installed capacity.

    I am not a meteorologist, so I don’t know if day-ahead forecasts for winds (wind power) or cloud cover (solar power) are roughly equal in achievable accuracy or if one is easier than the other. It seems to me that you need only a modest amount of storage for short-term changes in order to integrate a large renewable base with conventional generation, IF you can produce day-ahead production forecasts with good accuracy.

    Here is Germany, December 2013:

    2013-12-01 22490
    2013-12-02 45015
    2013-12-03 42365
    2013-12-04 11860
    2013-12-05 13623
    2013-12-06 16053
    2013-12-07 10510
    2013-12-08 14442
    2013-12-09 15035
    2013-12-10 23225
    2013-12-11 14115
    2013-12-12 19888
    2013-12-13 25723
    2013-12-14 12470
    2013-12-15 15061
    2013-12-16 47830
    2013-12-17 37428
    2013-12-18 17039
    2013-12-19 9211
    2013-12-20 15572
    2013-12-21 19993
    2013-12-22 14722
    2013-12-23 33677
    2013-12-24 21510
    2013-12-25 11162
    2013-12-26 9945
    2013-12-27 36964
    2013-12-28 17159
    2013-12-29 19983
    2013-12-30 44676
    2013-12-31 41539

    California, December 2013, solar followed by wind:

    2013-12-01 16056 1922
    2013-12-02 14724 21747
    2013-12-03 14682 64554
    2013-12-04 13787 29367
    2013-12-05 15901 8489
    2013-12-06 17430 12913
    2013-12-07 13381 44392
    2013-12-08 15503 21283
    2013-12-09 18331 16393
    2013-12-10 17455 4260
    2013-12-11 17122 3011
    2013-12-12 14516 5128
    2013-12-13 17855 17431
    2013-12-14 18513 8458
    2013-12-15 17129 3416
    2013-12-16 15012 2128
    2013-12-17 13783 894
    2013-12-18 7999 12633
    2013-12-19 10375 53417
    2013-12-20 16131 25872
    2013-12-21 18073 27330
    2013-12-22 18533 1424
    2013-12-23 18649 1147
    2013-12-24 17974 2540
    2013-12-25 18699 5098
    2013-12-26 19024 2938
    2013-12-27 18131 1115
    2013-12-28 13910 13798
    2013-12-29 19059 9736
    2013-12-30 18632 2561
    2013-12-31 18072 2698

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