Cycling Coal to Balance Electricity Grids

Reader Jacob sent me a couple of emails about the mechanics of ramping coal plant up and down to load balance an electricity grid.

I send you two links to articles about “cycling” or “ramping down” coal power stations. I don’t see how they can “cycle” except by keeping the fires burning.

And so the question is if a coal fired power station is “switched off” when demand for coal fired electricity goes down are the furnaces extinguished thus eliminating CO2 emissions or are they kept burning, perhaps at reduced levels? This post poses the question and provides background data to the problem. I’m hoping that informed commenters may provide the answers.

Figure 1 Slide 70 from Professor Bruno Burger’s summary of electricity production in Germany 2013.

Figure 1 illustrates the problem but also shows that the question is rather more complex than originally framed by Jacob. The bulk of Germany’s electricity still comes from nuclear power and coal and it can be seen clearly that hard coal provides most of the dispatchable diurnal load balancing service. Every day hard coal fired power stations are ramped up and down. Are they switched off at night or merely turned down? Notably, Germany uses little natural gas in power generation.

Jacob’s question is more subtle and overprints the diurnal demand pattern. We can see three things happening:

  1. On many days the solar peak reduces demand for hard coal resulting in a mid day dip on hard coal generation. See for example 6th, 12th, and 13th of June. Is any CO2 emission actually saved during these dips?
  2. On windy days, see for example the 21st and 26th of June hard coal generation is significantly curtailed. At such times are power stations actually switched off or is the power generated simply spilled?
  3. On weekends, see for example 15th and 16th June, demand for hard coal generation is effectively zero and demand for brown coal generation is curtailed. Are power stations switched off for the weekend or are they simply turned down in some form of hot standby mode?

This article by John Kemp at Reuters covers many of the issues:

The operational and financial challenges of operating conventional plants in a more flexible mode are enormous.

Large-scale power plants take hours to warm up to operating temperature and synchronise their turbines with the grid.

“(Grid operators) may have to cycle resources on and off more than once a day,” the North American Electric Reliability Corporation explained in a recent report on integrated renewables in California.

“At times this may not be an option because the down time between shutdown and start-up of a resource may be too long, which would prevent the resource from being restarted in time for system peak,” NERC concluded.

While they are warming up, conventional plants waste huge amounts of fuel without producing useful output.

And repeatedly heating up and cooling down the boilers, economisers, pipework, turbines and other components shortens their life-span and requires more expensive maintenance.

On top of all this, cycling power plants must recover their more expensive operating costs as well as the expense of building them, while getting paid for fewer hours of generation each year.


In Germany, combined load and generation adjustments have risen as high as 50 gigawatts in an eight-to-10-hour period – equivalent to more than 60 percent of the country’s peak power consumption.

In response, production from coal plants has been successfully turned down to just 20-60 percent of normal output. The advisory board claims part-loaded coal-fired plants have been able to ramp down by as much as 3 percentage points per minute.

The board’s optimism is spoiled, however, by the financial woes of Germany’s big coal-fired generators. RWE this month announced yet more job cuts and said that 2014 would be a “valley of tears” for conventional power producers.

Rivals E.ON and GDF Suez have also warned of a prolonged crisis in the European power industry.

Enormous renewable generation, subsidised through feed-in tariffs, coupled with low wholesale power prices and the low number of hours in which coal and gas-fired power plants are able to operate mean that revenue is inadequate for conventional generators.

These are issues I covered in Parasitic wind killing its host and by Hugh Sharman in The balancing capacity issue: A ticking time-bomb under the UK’s Energiewende. It is quite clear that FF generators are being heavily penalised by the Energiewende with reduced operating efficiency, increased wear and tear and reduced market share while the opposition receive subsidies to produce electricity that increasingly is not used. All this is done in the name of reducing CO2 emissions and saving Earth’s climate from meltdown. Whilst evidence of meltdown is elusive we also need to ask if the Energiewende and strategies like it else where actually leed to a reduction of CO2 emissions? In Germany it seems likely that the answer is yes. Wind and solar have displaced some FF generation. The question is what portion of that displaced generation represents saved emissions?

I have always felt the best way to tackle both emissions and energy scarcity was to improve energy efficiency at every level of society. The current strategy appears to be taking us in the opposite direction.

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30 Responses to Cycling Coal to Balance Electricity Grids

  1. Hugh Sharman says:

    When a coal plant is closed down for the night, ready for a start the next day, fuel must be burned in the furnace, but at a relatively low level, in order for a smooth and rapid start. The furnace is well insulated to stop it warming the air, of course.

    I hate weasel words like “relatively low level” for their imprecision and had been hoping you would enlighten me with numbers!

    I still naively hope that one of your readers will oblige, of course!

    • Euan Mearns says:

      Darn Hugh, you were my best hope of providing the answer. There has to be information out there. The chart does not show the decline in nuclear power in Germany which is probably the main reason for recent rise in coal consumption.

      • Willem Post says:


        Some of that balancing of production with demand is done with imports and exports, especially when the sun shines in the south with exports to France, etc., and when it is windy in the north, mostly at night, with exports to Holland, etc.

        These import and export flows have become greater as wind and solar build-outs increased, AND they take place when grid prices are lowest, after having been subsidized, on average, at about 20 euro c/kWh, per ENERGIEWENDE.

        The net exports are increasing.

      • Jacob says:

        It is interesting that from about 2009 (before nuclear power was closed in 2011) coal consumption started rising, and it keeps rising despite the increasing amounts of renewables.
        Maybe the dip in consumption in 2008 was caused by the economic downturn, and not by renewables increase.

  2. ScottR says:

    I don’t know if this is still relevant, but I remember a visit to Longannet, I think around 1980. On the visit, the engineers taking us round explained that the furnaces could never be ‘switched off’. The refractory linings of the furnaces would crack if they cooled below a certain temperature, etc. A reline would cost millions and take many weeks.

    If the technology is still similar, that would set a limit on the de-powering of the furnaces and thus CO2 emissions.

    BTW, Thanks for avery interesting blog. Lang may yer lum reek, if that’s not the wrong thing to say these days.

  3. Joe Public says:

    Solid fuel (both coal & biomass) are very slow to respond to demand changes; both diurnal, and hour-to-hour. So there is a level of inefficiency built into those systems where heat is being produced, but not needed. i.e. 100% wasted.

    In addition, soot is formed within their combustion chambers which also reduces efficiency.

    The thermal mass of their heat exchangers is greater than those of gas- or oil-fired plant. So there are further efficiency losses.

    Coal, biomass & oil produce their maximum heat via radiation into the combustion chambers. Natural gas on the other hand, produces its maximum heat via convection, and this is distributed both into the combustion chamber and into the downstream flue-tubes. This means there is far greater surface area through which the heat transfer occurs. Whilst I have limited knowledge of power-stations, gas can be controlled virtually instantaneously, and ramped 0% to 100% in minutes rather than hours. The fact that Nat Gas is ‘clean-burning’ means its combustion chamber / flue tubes remain (virtually) spotless, preserving long term efficiency at the highest level. (c.f. oil-firing which has a slightly higher efficiency when new & with un-sooted combustion chamber.)

    These factors are often overlooked when comparing fuels.

  4. Coal seems to be the load-following fuel of choice in Germany:

    Why? A lack of alternatives? Or simply because it’s cheaper?

  5. Hugh Sharman says:

    Roger and Euan, will you kindly load up figure 14 from my paper in response to Joe Public’s assertion that “…gas can be controlled virtually instantaneously, and ramped 0% to 100% in minutes rather than hours.”

    Not true, unfortunately, which is precisely what motivated me to expose this £20 billion black hole in UK’s next 5 years growth of wind power from 10 GW to 25+ GW.

    Thanks in advance!

    Could we mere mortals be shown how to reproduce figures and quotations in these reply frames, just like you and Roger please?

    • Hugh: If you could post a link to your Figure 14 I would be happy to bring it to life for you. 🙂

    • Euan Mearns says:

      Figure 14 Source:

      Unfortunately only “users” can post charts. Although over on Peak Oil Barrel they have a facility for posting charts – Dennis might be able to illuminate. You can use basic html tags like blockquote, b, i etc.

      As Roger pointed out, if you went to your post and clicked the chart it would open in its own window with a link that you could have posted. I need a site engineer.

      • Gridwatch data show that between the hours of 0605 and 0740 on January 16, 2013, UK CCGTs ramped up by 10,000MW (from 7,000 to 17,000MW) in the space of 95 minutes. They must have been pedalling as fast as they could.

        • Hugh Sharman says:

          Roger, demand rises predictably every workday morning as UK wakes up, has tea and goes to work. So the bulk of this increase was already “market-ready” and already ramping up – see my curve. Did wind output fall at the same time?. What makes all this “exciting” is when wind output rises and falls outside the central prediction.

          • Hugh: Right. The Jan 16 ramping rate is a close match to your starting cycle plot between ~32 and ~106 minutes.

            Wind output was constant at around 700 MW over the period.

  6. Hugh Sharman says:

    Ta very much Euan/Roger!

    Joe Public, you also wrote that “The thermal mass of their (coal-fired) heat exchangers is greater than those of gas- or oil-fired plant. So there are further efficiency losses.” The first statement is certainly true. That means that in a well insulated boiler, (and they all are) the heat is retained longer reducing the necessary inefficiency (unnecessary heat losses) the next time the boiler is restarted and starts delivering electricity.

    As regards Scott’s visit to Longannet in 1980, coal-fired power stations have access to a higher quality refractories now and the operators have learned how to close down and restart their power plants without excessive damage to their boiler refactory linings.

    During the summer, coal took its share of demand reduction and averaged 6 – 8 GW in UK, if I recall correctly. In December we can expect coal base load to hover around 15 – 18 GW, ie virtually everything that can run, will run!

    • Joe Public says:

      Thanks Hugh for your additional information in both this & your 4:36 posting.

      The gas burner can be ramped rapidly; it’s the plant into which it fires that is the laggard.

      The flame temperature of Nat Gas, being lower than that of HFO and coal, means that plant and flue-gas heat losses (with equivalent insulation), are lower.

      Regarding my comment, you state:

      “The thermal mass of their (coal-fired) heat exchangers is greater than those of gas- or oil-fired plant. So there are further efficiency losses.” The first statement is certainly true. That means that in a well insulated boiler, (and they all are) the heat is retained longer reducing the necessary inefficiency (unnecessary heat losses) the next time the boiler is restarted …

      Yes, heat is retained longer reducing, but not eliminating, the necessary inefficiency the next time the boiler is restarted. A proportion of any heat not used immediately is a disadvantage, because it’s never known for certain when or if the boiler will next be fired. This means my second statement is also true.

  7. Euan Mearns says:

    Reader Andrew sent me this chart by email and the following text:

    reading the latest on Cycling Coal to Balance Electricity Grids, I see the question is raised about how ramping affects CO2 production. I have accumulated Scottish data for 2002-12 [attached] for published CO2 emissions and electricity generation from various fuels. I have then plotted CO2 intensity [t CO2/GWh] for coal and gas against the fraction of Scotland’s electricity generation derived from non-thermal renewables. The burgeoning wind fleet seems to have had no effect on coal CO2 intensity, but is very probably increasing that of gas – which I assume is the main balancing fuel in Scotland. Graph below

    In Scotland balancing is achieved by variable imports / exports to England and via hydro and pumped hydro. But as renewables penetration grows cycling the Peterhead CCGT become ever more intense and uneconomical. Andrew’s chart actually shows an increase of 300 to 500 tCO2/GWh. And this is for 17% renewables penetration. So will the relationship be linear or exponential? And what will happen when Peterhead goes into zombie CCS mode?

  8. Alfred says:

    It seems to me that a novel way of burning coal to make electricity needs to be tried out. The making of steam to turn turbines has to be replaced by something more speedy to turn on and off.

    I am not making suggestions here, only pointing out that coal is awfully cheap compared to other fuels and that the current way of turning it into electricity is unsuitable for use in conjunction with renewables.

    • Alfred says:

      Here is one possible approach:

      “Coal-fired gas turbine and ash separation system”

    • Euan Mearns says:

      The efficient way to use coal is either as supercritical base load or as CHP base load. The best way to solve the load following problem is to eliminate wind from the mix. Its kind of like the best way to treat small pox is to not catch it in the first place.

      In sunny climes I am perhaps tolerant of solar that naturally follows diurnal load. Comment subject to a positive ERoEI.

      • Hugh Sharman says:

        Euan, you are right, of course. But wind capacity on line and under construction cannot be wished away; the further growth of wind capacity (in Ireland and UK, as elsewhere around the globe) is inevitable.

        So my “engineer’s” response is to design and deliver capacity, in the form of storage and super-fast response thermal plant that can do this with the least energy wasted.

        This also minimizes CO2 emissions as a side effect. I won’t elaborate on whether that is so super-duper-wonderful. I try to keep out of the AGW shouting match!

        By the way, a paper was published at yesterday that highlights the absurd waste inherent in Danish energy/”climate” policy of doing everything that is possible to minimize CO2 emissions.

        At its most expensive, energy/climate measures are costing Danich citizens to pay DKK 3700/ton (>£400) of CO2 avoided.

        We (Danes) are driving the world’s most efficient coal-fired power stations, all designed as CHPs that also heat our cities and are utilizing 93% of the fuel used, out of business through the crass ignorance and stupidity of our political classes.

  9. Hugh Sharman says:

    Alfred, it has has been tried. The first diesel engines were also coal-dust fired. But filtering out 100% of the ash was/is impossible. The tiniest ash fraction, melting temperature about 1200°C will quickly screw up turbine blades, hot gas passages and/or cylinder liners.

    • Alfred says:

      I realise that the ash is a huge problem, and I am not pretending that the thermodynamic efficiency will be the same. However, it seems to me that the possibility of using cheaper replaceable ceramic blades and a lower rotational speed might make it economic. Certainly, it should start and stop within minutes rather than hours. I am no material scientist, but I suspect there could be an angle there for someone with the knowledge.

  10. Leo Smith says:

    What a refreshing post with articulate intelligent and – that are thing – informed, comments.

    Not much to add, but a few observations from a more detached and philosophical viewpoint.

    First of all, beware the optimal solution to the wrong problem.

    ‘more efficient coal burning’? ‘renewable energy’ ? what is the problem we are actually supposed to be solving?

    The chain of reasoning was supposed to be CO2 scary/dangerous->less coal and other fossil->less emissions ergo windfarms and solar panels.

    The problem of course being that more wind didn’t mean less emission, and that’s been known for ages, but brushed aside, leading to the conclusion that TPTB were more interested in cosmetic political solutions than real ones, which also prompts another thought, namely if they themselves were content with a cosmetic solution, how much did they really believe AGW in the first place? Or was it simply a convenient lie to achieve political control of the lucrative energy market to dish it out to crony capitalists?

    That is, renewable energy is a highly effective solution to a political and commercialproblem that has nothing to do with generating low cost electricity in an environmentally optimal way or achieving energy security.

    All we can say from the comment flow here, is that ultimately intermittent renewable energy compounds a problem that already exists – namely dispatching supply to meet fluctuating demand. Intermittent supply merely increases the problem. It is a spectacularly bad way to generate electricity.

    If we now look at ways to improve plant and grid dispatch as an entirely separate issue, it becomes a matter of cost alone really. For example, take a nuclear power station. Its typically running at 30% thermal efficiency. Maybe more, maybe less. Its fuel is almost free. The enormous cost is in building it, and manning it to meet unbelievably strict regulations. The fuel rods are comparatively cheap, and the actual uranium in them, cheaper still.

    Since the cooling system is already capable of dumping 66% of the total reactor output, a 50% increase in that cooling system would result in the whole reactor output being dumpable into e.g. a cold bit of sea. WE achieve dispatch simply by throwing the total reactor output away. It’s highly fuel inefficient, but who cares?

    I only put this up as a lateral thinking example to illustrate the fact that once you have actually worked out what you are trying to achieve, the best answer may surprise you.

    With coal burning, there are basic thermodynamic equations that govern efficiency. And the temperature of superheated steam is really the limit. Using some form of gas turbine with something other than steam as the working fluid is of course possible, as are higher combustion temperatures by using other oxidants than impure air.

    But at what cost?

    Energy generation is ultimately a cost-benefit exercise. The triumph of political interference has been to divorce it entirely from any considerations of cost, allowing far higher profits to be masked in a plethora of ‘green’ disinformation.

    Examined from a cost benefit perspective, modern coal with SO2 scrubbers is the cheapest way to generate electricity. It’s cheaper without the scrubbers but the real genuine smog and acid rain that you then get is arguably a higher social cost than money saved.

    Beyond that, the best form of dispatchable short term power is hydro, pumped or otherwise. IF you have the mountains and the rain/snow.

    If not then gas is the next best thing, and as Hugh points out, its not that good but could be better.

    (Nuclear currently is not competitive with coal or gas, not because it couldn’t be, but because regulation (deliberately? ) ensures that it isn’t).

    What this post does is to highlight the argument made my many, myself included, that renewable energy is, not to mince words, pants. In terms of meeting what we thought was the reason for its introduction.

    So my first response is to say that what you thought was the reason for its introduction was not the real reason. Given a choice between utter incompetence at government level, and deceit for profit and political power, I pick the latter every time as the more likely hypothesis.

    My second response is to say that (sorry Hugh!) in longer time-scales than a few years, its would be arguably cheaper if we can gain the political traction to scrap renewable energy rather than try and build expensive efficient dispatchable generation capacity to balance it.

    That is the real problem is not how to achieve efficient dispatch, its how to achieve low cost secure reliable energy.

    And if carbon emissions are the issue, replace coal with nuclear or gas. It is way cheaper than wind or solar.

    Beware the straw man, and the double straw man, and the triple straw man.

    The straw man is climate change.

    The double straw man is pretending that renewable energy is a ‘solution’.

    The treble straw man is pretending that ‘smart grids’ ‘batteries’ ‘pumped storage’ ‘carbon capture’ will solve the problems of the renewable energy that didn’t actually work, that we didn’t actually need to meet the climate change problem that didn’t actually exist, in the first place.

    Trillions of dollars of money have been diverted into an industry that has absolutely no rational reason to exist, that has been invented, for various reasons by various people, to suit their individual selfish ends.

    Please lets not get involved more than we have to in how to make it better. The true answer is to get rid if it altogether.

    • Euan Mearns says:

      Leo I agree. But understanding the root cause of the political aims etc is not simple. I don’t have time to write what I would like to write today. Will simply observe the seismic shift in UK politics, and in EU politics. I think the public are sick fed up of having this minority view, politically correct Green crap thrust down their necks 24/7. The false logic of climate science has been cast onto energy policy and the result is a catastrophe for society.

    • A C Osborn says:


    • jacobress says:

      “The false logic of climate science has been cast onto energy policy and the result is a catastrophe for society.”

      There are two, unrelated (apparently) questions: 1. Is there dangerous, man made, warming? 2. What to do about it (if there is)?
      Even if there is CAGW, we must not embrace fairy-tale solutions, that don’t work. Even if there is CAGW, it does not justify ignoring simple physical and engineering truth, and embarking upon irrational schemes that cannot achieve the sought goals (emission reduction).

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