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Rated Capacity

July 20, 2008, 11:35 am

One needs to be a careful consumer of information when reading about the "rated capacity" of certain alternative energy plants. 

Take a 1MW nuclear plant, run it for 24 hours, and you get 24 MW-hours, or something fairly close to that, of electricity.

Leave 1MW worth of solar panels out in the sun for 24 hours, you get much less total electricity, depending on where you put it.  On an average day in New York City, you will get about 4 MW-hours.  In one of the best solar sites in the word, my home of Phoenix, you get about 6.5 MW-hours per day.  The key metric is peak sun-hours per day, and some example figures are here.  So, even in the best solar sites in the world, solar panels run at only about 25-30% of capacity.

It turns out, not surprisingly, that the same relationship holds for wind.

It's not like it's a secret that wind turbines are an unreliable source of electrical power. Bryce points out that, "In
July 2006, for example, wind turbines in California produced power at
only about 10 percent of their capacity; in Texas, one of the most
promising states for wind energy, the windmills produced electricity at
about 17 percent of their rated capacity."

That means
that there has to be nuclear, coal-fired or natural gas power plants
functioning fulltime as a backup to the pathetically unreliable and
inefficient wind farms. Moreover, what electricity they do generate
is lost to some degree in the process of transmitting it over long
distances to distribution facilities.

Now, this should not outright dissuade us from these technologies, but since no one has really licked the night-time / not-windy storage proble, it's certainly an issue.   I have looked at solar for my house a number of times, and the numbers just are not there (even with up to 50% government subsidies!) without a 2-5x decrease in panel costs.  Low yields can potentially be tolerated, but capital costs are going to have to be a lot lower before they make a ton of sense.

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Category: Technology
  • anon in tx

    From the real world:

    "As can be calculated from the measured -not asserted -numbers in the excellent E.ON Wind Report 2005, the average production factor in 2004 for the more than 7000 E.ON wind turbines, distributed over thousands of
    square kilometres, was 18.3%. Due to the construction of several new turbines, this production factor had risen to around 19% by the end of 2004.

    In the month of July, 2006, the production factor for all the wind turbines in Germany was measured at 7.5%."

    from Wind Energy: Facts and Fiction by J.A. Halkema on the web. Also see the E.ON Wind Report 2005 itself.

    FWIW, Pickens is nothing but a rent-seeker: the windmill subsidy on rated power and then sales of his natural gas to provide the standby.

  • Larry Sheldon

    The devil is in the details, as they say.

    The unspoken issue here is system capacity as contrasted with component capacity and each statement of capacity includes (even if not voiced) a duty cycle parameter..

    Presumably a nuke is engineered such that all of the component capacities provide for a system capacity of 1MW with a system duty cycle duty cycle of 24 X 7 X 366.

    The solar array has a component with a duty cycle of less than 24 hours per day and that imposes a system limit unless there is a storage component that has not been mentioned.

    Similarly, the wind thing has duty cycle limits that have the additional limitation for being un-predictable.

  • davidcobb

    You're not wrong on wind power and solar. It's only good for dispersted generation. You are wrong on the calcs for nuclear power. It requiers large amounts of "free spinning" power and is slow to adjust to requirments. Case in point, a USN destroyer with an average requierment of 1500kwh has to run two 2000kw gas turbine generators or it will go dark.

  • Mike

    Even if one could store solar energy all night in the form of molten salt, for example, how much bigger would the plant have to be?

    That same 1MW of solar would have to store energy during the day to be used at night. That would lower its peak output as it produces energy just to be stored.

    There is a reason a hundred years ago we went from sail ships to ships powered by fossil fuel. Wind is not reliable, and neither is the sun when it comes to power consumption.

    I would love to have PV solar on my house, but as you stated, the capital costs are just too high. However, there is a company out there that is working that angle. If a home owner wanted to have PV solar on their house, they can contact this company.

    This company will install *their* panels on the house and sell the power back to the home owner at a price lower than what the utilities charge. There is an option to buy down the road. This actually sounds like a way to advance the technology and bring down the price at the same time.

  • Corky Boyd

    Shortly after Al Gore's speech I decided to check the current wind speed for the US. None of US east of the Mississippi had wind speeds over 9 mph (at 10 pm edt). None! One third of the country had insufficient wind to generate any power. I posted the maps of the coastal areas from Maine to Florida on my website that can be found here:
    http://islandturtle.blogspot.com/2008/07/al-gore-today-called-replacing-all.html

    The other problem with wind and solar is you must have reliable backup for these intermittent sources to satisfy the baseload. If a grid is delivering 50 gigs of power, of which 10 is wind, and the wind dies (as it did on my website) you must call up 10 gigs of backup power or shed load. Otherwise the entire grid will trip out. Essentially you have to double your windpower capital costs by installing a like amount of thermal backup.

    Pumped storage has been suggested, but there are only about 30 in the country and it is unlikely any more will be built. You have to create an artificial lake on a mountain top or high plateau above an existing supply of water. Not likely in the current NIMBY climate.

    Yikes Gore is calling for unlimited windpower and it's being blocked by another Democrat in coastal Cape Cod.

  • Leonard Huff III

    Between Corpus Christi & Brownsville Texas, they are 2 (Two) major wind farms being built at this time. 200-300 hundred towers (300-400 Foot each) being put in place as I write this. I have done some studies ( actual vist to sites, talk to workers, supersvision, ect.). Family has 500 acre block, with neighbors toal 3,000 acre block! Hot prosect now. Goverment grants, State Grants, Cheap Loans ect, . JOHN DEERE has a site on WIND FARMS on its business Farm site on WIND FARMS! Talk to them a couple of days ago about it. If they will paid for each tower site a year ($5,000 - $6,000) per year, + royalty ( % of GROSS PER TOWER of $ generate per month/Year) and here the gravy - FREE ELECTRICTY FOR EACH LANDOWNERS ON LAND! It worth a few telephone calls and dog & pony show.

  • dearieme

    A few years ago we travelled by train across the North German Plain. It was littered with windmills, mostly not turning. Occasionally one would twitch. I realised they were twitching shortly after the train went by: they were drawing power from our slipstream!

  • Leonard Huff III

    P.S. To earlier post.
    CPL (Central Power & Light Company) now know as A&EP
    built a major north/south tranmission line in the early 1980 to run from north of houston to mexico broder near Brownsville texas to ship power to Mexico. Bet they did not anticipate WIND POWER back in there ecomics decisions to spend millions to built the line then. As the old saying in the oil & gas business - I would rather be luckly than consider a genius when you strike a major oil & gas field in this business.The power line runs right through my family land holdings.Easy substation connect for a WIND FARM!

  • rxc

    The comment about nuclear requiring "spinning reserve" is completely untrue. It does not even make sense. Nuclear plants are generally operated as "baseload" facilities, at 100% power, because the cost of the fuel is so low, and it is generally more efficient to load-follow with coal plants or gas (last resort). The French have so much nuclear capacity on-line that they actually run some of their plants in load-follow mode. The factors that limit nuclear load-following relate to highly techncal (and proprietary) fuel performance factors (the way that the fuel pellets interact with the fuel cladding during transients), and strategies to use the uranium and plutonium in the fuel most efficiently as it burns during its multi-year lifetime.

    And, BTW, there are no nuclear-power destroyers in the Navy any more. The only Navy nukes are submarines and aircraft carriers, and they load-follow a LOT, as the ship goes faster or slower. But that is a different type of fuel, designed specifically for such transients

  • http://neubranderinc.com/blog Nobrainer

    rxc has it right on nuclear power. davidcobb is right that nuclear is slow to adjust, however, as part of the entire electrical grid, they don't need to adjust. They can run all out all the time.

  • http://neubranderinc.com/blog Nobrainer

    Let me add the information quoted is a bit misleading.

    "That means that there has to be nuclear, coal-fired or natural gas power plants functioning fulltime as a backup to the pathetically unreliable and inefficient wind farms. Moreover, what electricity they do generate is lost to some degree in the process of transmitting it over long distances to distribution facilities."

    With respect to backup, a system will have some generators which are on and spinning with extra capacity to produce more electricity if needed. Other backups are quick-starting units. The quick-start units operate as a backup fulltime, but in this case their operation means little more than sitting around waiting. But to be clear, all electrical grids operate under these conditions now. And the amount of backups available is typically compared to the biggest plant which could fail. So if the biggest plant in the system is a 2GW nuclear reactor and there are 2GW of wind turbines available, the amount of backup scheduled and available at any time will be the same.

    With respect to losses, again large amounts of generation are already far from their end users and the electricity has to be transported. I highly doubt that the losses incurred due to wind will vary substantially from what we already see.

    Finally, to call wind farms "pathetically unreliable and inefficient" is to be both honest and unfair. Any grid operator with any sense can easily assess the nature of the turbines and the nature of the wind in the area and have a pretty good idea of when to count on wind turbines and for how much power.

  • rxc

    The comment about the same capacity being needed for nuclear backup vs wind, does not take into account that the backup units must (1) be spinning, (2) have people ready and trained to operate them, and (3) have fuel available to burn. This last item is quite important, because coal these days is sold on long-term contracts. The production and transportation of coal is a very non-trivial process, which cannot just be turned on and off. It takes an enormous infrastructure to deliver coal cheaply, and to support the technology to burn it, and it is not amenable to being a "boutique" business. It requires a LOT of experienced people with quite large machinery.

    Right now, the backup for the largest nuclear unit in a system comes from several coal plants of about the same size that run at less than capacity. Since they are normally on-line and making power, the incremental cost of running them a bit harder to make up for the temporary loss of one nuclear unit for a few days is not high. However, if you have to have large units sitting around at very low loads for long periods of time, you are going to see all sorts of problems - the machinery does not want to operate at 5% load - the feedwater systems don't work well at that power level, so you have to have specialized systems to run the plant at lower load. The turbine is operating at a different state-point, so it is less efficient. And, do any of you have any experience moving piles of coal in the winter that have been sitting in the rain/snow/ice for a long period of time?

    Gas units will have the same fuel problem, but it will be worse, because gas for utilities is sold on a take-or-pay basis, now. No one wants to drill a well for gas unless they have a buyer lined up for a long-tern sale. And then you have to oversize the gas transmission system to provide for the infrequent times when the gas demand will be MUCH higher than normal, instead of for a slightly higher expected peak demand.

    I have read some comments on this blog about using pumped storage or molten salt to store the energy from solar/wind units for the time when they are not running. If the society is not willing to permit new hydro facilities, it is certainly not willing to support the construction of pumped storage, which will flood large areas (or small deep (i.e., pictureque)ones. And you need to do some arithmetic about how large the molten salt facility needs to be to store the amount of energy that is needed.

    Finally, I don't think there has been much talk about electrical transmission systems that would be needed to support a diverse wind/solar economy. It would be enormous, and right now, no one want any more transmission lines erected anywhere near them. You want to bury them? Well, the price will go up accordingly - high voltage buried lines are QUITE expensive.

  • TKM

    Just curious, what precisely does a "2-5x decrease in panel costs" mean, mathematically? One half to one fifth of the current price?

  • Noumenon

    According to this article, gas turbines do not deal very well with providing a fluctuating amount of power. Using them to back up wind turbines could actually result in increased carbon emissions due to inefficient operation.