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More Damage to The Grid

December 8, 2015
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The damage being caused by wind turbines and solar power plants is not generally recognized and accounted for.

Mostly because it’s hidden from view.

Many people are now aware that wind and solar power plants produce electricity intermittently, and that natural gas power plants must be kept spinning, in reserve, to replace, on a moment’s notice, the power lost when the wind stops blowing or the sun stops shining.

Keeping natural gas power plants spinning and ready to go on-line costs money, and produces emissions unnecessarily.

What’s not generally recognized is that coal-fired power plants are now being asked to follow the load, something they were not designed to do.

Boilers are designed for steady operation as base load power plants, not for variable operation with shifting loads. Load following is what gas turbines are best suited for.

Load following requires boilers to operate at various temperatures, and these varying temperatures create stresses that cause considerable damage.

This problem will become worse as Obama’s Clean Power Plan is implemented, where coal-fired power plants will be required to be used for load following.

Load following can cause low-load, hot startup, warm startup, and/or cold startup situations.

Each of these conditions places unusual stress on boilers and their components.

These stresses lead to failures, that cost money, and which can cause unexpected additional problems in keeping the grid operating.

Low load requires the boiler to operate at well below design parameters. These conditions cause temperature variations within the boiler due to low water and steam flows, and also result in the boiler operating at lower efficiencies, which costs money and produces more emissions.

Hot startups occur when the boiler is shut down and then restarted. Warm start-ups occur when the boiler is shut down over the weekend when loads are lower, and then restarted on Monday morning.

Boilers are designed to operate steadily, and not for repeated starting and stopping, or for running at various loads.

Boilers are built using different materials and materials of differing thicknesses.

Different materials expand at different rates. These differences in the rate of expansion cause each material to expand differently creating stresses in each material, especially in the joints, such as with welds.

Components that operate at high temperatures, such as super heater tubes, are damaged by heat induced stresses known as thermal fatigue.

The relative movement between materials creates stresses that can lead to failure.

  • Headers in boilers are thick, expensive and hard to reach components that have been cracked as the result of thermal cycling.
  • The boiler structure that supports firewalls and water-tubes expands and contracts at different rates than the firewalls, etc. This uneven expansion and contraction causes severe damage to the firewalls and tubing.
  • Steam turbine rotors consist of huge, solid forgings. These rotors, unless designed for thermal cycling, must be brought up to speed slowly so as not to be deformed by changing temperature. The coefficients of expansion between components are also different, which requires steady operating temperatures under load. Clearances are tight and deformation could cause rubbing and damage.

When the boiler is required to quickly increase its output when the load increases, over-firing will occur causing components to operate at temperatures higher than for which they were designed.

John W. Turk, only U.S. ultra-supercritical power plant. Photo courtesy of SWEPCO.

John W. Turk, only U.S. ultra-supercritical power plant. Photo courtesy of SWEPCO.

The tallest building with conveyor belt leading to it, is the boiler.

Replacing heat exchanger tubes in marine boiler. Photo by D. Dears

Replacing heat exchanger tubes in marine boiler. Photo by D. Dears

It’s also more difficult to maintain water chemistry when the boiler is operated in load following mode. Operating the boiler with water oxygen content and pH values beyond normal can result in boiler tube failures.

These are a few of the adverse conditions created by requiring boilers to operate as load following rather than base load power plants.

With the government forcing the adoption of wind farms and solar power plants to cut CO2 emissions, costs are being increased and reliability is being reduced. Load following exacerbates cost increases and reliability problems.

Watch for my new book, which will be available in January.

* * * * * *


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15 Comments leave one →
  1. December 8, 2015 4:45 pm

    Donn – this was such a well written description of the problems associated with intermittent energy sources. I am looking forward to your book. Now isn’t this something that the rechargeable batteries are suppose to take care of? They will level the load and supply backup power.

    • December 8, 2015 4:54 pm

      Thanks for your comment.
      I don’t believe the batteries can handle the sudden ramping up and down.
      As the California CAISO Duck curve show, the ramping is large.
      Also, starting up cold or warm, will create the damage described in the article.

      • December 10, 2015 9:52 am

        Thanks. Then what is the purpose of grid storage battery systems?

      • December 10, 2015 10:06 am

        The purported advantage is to level out the sharp increases in load placed on traditional power generation sources when renewables cease generating during the evenings, with similar problems as renewables come on line during the morning.
        I don’t believe that storage will be successful in this regard, however that is what is being promoted.
        My book will explain this in greater detail using the Duck curves as the example.
        Thanks for your question.

    • Usurbrain permalink
      December 9, 2015 2:43 pm

      Look into battery fires. A good friend is into record setting hot air balloon flights. Recently he got a MillSpec LiIon high current low temperature battery, VERY expensive, to power his equipment in the well below 0 temperatures – -20 and lower. On his second flight with that battery it burst into flames. and almost caught his sleeping bag on fire (No basket,just a sleeping bag and a harness under the balloon – got to minimize weight.). then you had the EV fires, the aircraft fires and now the “hover board” fires. All due to high current applications. The science is not settled on these new batteries.

      • December 9, 2015 2:48 pm

        Thanks. A great example of why we have a lot to learn.

  2. December 11, 2015 8:03 am

    A comment about nuclear steam cycles was accidentally erased from the spam file. Would the person making that comment please restate it.

  3. December 14, 2015 3:11 am

    Donn: Surely much of what you say about boilers and turbines must also apply to CCGTs as well as coal plants. can you comment please?

    • December 14, 2015 8:51 am

      Yes. Thanks for pointing that out. The stresses in the heat recovery boilers aren’t as great, but they are still present. Rapid shifts in load will cause thermal stresses in them. Land based gas turbines are also subject to damage from sudden changes in load. Aero-derivitive gas turbines, however, are less prone to damage, since jet engines are designed for rapid load changes.
      Bottom line, all parts of the system are, in varying degrees, affected by rapid changes in load.

      • December 14, 2015 10:32 am

        Thanks Donn. It was my understanding that modern CCGTs are designed so that they could do normal load follow without overstressing, but trying to do rapid load follow caused by intermittent wind or solar would be a problem.

  4. December 14, 2015 1:01 pm

    Phillip: The latest land type gas turbines are better able to do load following, but the aero-derivatives are best in that regard. I don’t have the data on the latest land type gas turbines, but from what I see, they are still susceptible to thermal stresses. I realize manufacturers are making carefully worded claims, such as a 15 minute start-up, but more information is needed before anyone can tell whether the units will be subject to greater maintenance.

  5. Tom Stacy permalink
    January 7, 2016 9:14 pm

    Donn I really appreciate your knowledge in this area. Please advise me when the book will be released. I report on the economic implications of lower capacity factors on fixed cost recovery because it is easy to prove. If thermal stress and heat rate penalty costs were also easy to prove and monetize that would add a significant piece to the discussion. Tom Stacy


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