Replacing an energy source by one with lower emissions doesn’t necessarily mean proportionally less emissions

The second myth from the Brochure “9 myths about the German Energiewende debunked” is: “The German nuclear phase-out leads to a greater use of coal and lignite”.

Most interestingly, it explained a statement used extensively in the debunking of previous myth:

The growth of renewable energy production more than compensates the nuclear phase-out.

This by using this graph of the evolution of the electricity production by source:

Evolution of German electricity production per source from 2003 - 2014

Evolution of German electricity production per energy source (2003 – 2014)

The graph made it clear what they were saying exactly and now I could understand where this statement came from. Not from the 26% total emission reduction, as seems to be alluded in the debunking of previous myth, but from the electricity production from the different sources. If I understand the graph correctly, increased production of renewable sources was higher (+115) than the decreased production of nuclear (-68). Not sure though what the units are (TWh? percent?), but nevertheless the conclusion seems to be clear that “the growth of renewable energy production more than compensates the nuclear phase-out”.

However, increased renewable electricity production is something different from actual emission reduction. But, could you say, if one takes an energy source with an emission of 66 g CO2/kWhe and replaces it with an energy source that only has a 32 g CO2/kWhe emission, then one could safely assume that emissions go down, no? Framing it that way, their statement could make sense.

Unfortunately, this is only part of the story.

The problem with this super simple reasoning is that one tries to compare two completely different ways of producing electricity. Wind and solar generated electricity are intermittent energy sources and this has consequences when integrating them in a continuous working grid. For example, its production doesn’t necessarily follows demand, so energy produced at moments that there was no need for it and could not be saved for later, is lost and will not contribute to a reduction in emissions. Another consequence is that this intermittency requires back-up capacity to be available. In most cases these are conventional power plants. They are needed to deliver electricity when wind or solar supply is insufficient to keep up with demand. This ramping up or down to compensate for the intermittency causes a reduction in efficiency of the backup turbine, meaning a higher fuel consumption and therefor more emissions. So it is a matter of how much extra fuel is required for this extra balancing.

A simple example to make the latter point clear. A company uses a 1 MW turbine to meet its electricity needs. That turbine produces 1 MW x 24 x 365 = 8,760 MWh in one year. If we assume that the production of 1 MWh needs 95.5 m3 natural gas, then we have a consumption of 836,580 m3 natural gas per year to produce this amount of electricity.

Now suppose that this company is going green and erects a windmill of 1 MW. We know that this windmill will only deliver power when the wind is blowing, so backup is needed to meet demand. Suppose the company will take that gas turbine to do just that. When the wind doesn’t blow, the gas turbine is going flat out and produces what it normally would do without the contribution of windmill. When wind picks up, the gas turbine scales back a bit. When it is very windy, the turbine will run at a slow pace. The assumed savings then would come from the fossil fuel plant having to work less, so using less fossil fuels.

If I look at the German production versus installed capacity in 2014, then an onshore windmill will produce on average 17.5% of its installed capacity. So one could say that on average there will be a production of 8,760 MWh x 0.825 = 7,227 MWh by the gas turbine and 8,760 x 0.175 = 1,533 MWh produced by the windmill. This will amount to a consumption of 690,178.5 m3 natural gas by the turbine and a savings of 146,401.5 m3 natural gas (the optimal gas consumption for the electricity that was delivered by the windmill).

That is how it is shown in the brochure and that is probably also how governments seem to see it. Basically assuming that every MWh produced by the windmill is displacing one MWh of fossil fuel.

What is wrong with this picture? It assumes that the gas turbine keeps working at the same efficiency as it does at optimal load and this is not the case. When the gas turbine adjusts its load to the intermittent production of the windmill, it is working at a reduced efficiency and therefor will need more fuel than when it would do it when it was running at optimal load. If we take that into account, we get this (assuming a turbine with an efficiency of 55%, all units in m3 natural gas):

Efficiency Consumption
by gas turbine
Theoretical
savings
by windmill
Total
consumption
Total
savings
55% 690,179 146,402 836,580 146,402
54% 702,960 146,402 849,361 133,620
53% 716,223 146,402 862,624 120,357
52% 729,996 146,402 876,398 106,584
51% 744,310 146,402 890,712 92,270
50% 759,196 146,402 905,598 77,384
49% 774,690 146,402 921,092 61,890
48% 790,830 146,402 937,231 45,750
47% 807,656 146,402 954,057 28,924
46% 825,213 146,402 971,615 11,367
45% 843,552 146,402 989,953 -6,971

In this example we see that when efficiency is reduced, less gas is saved and when efficiency drops to 45% it even goes negative. Meaning that the theoretical savings obtained by the windmill is overturned by the additional fuel use of the balancing gas turbine. There would be less emissions when the turbine ran at full load than when it had to adjust to an additional intermittent power source.

Okay, I know this is an incredibly simple example, just to show that the matter is much more complex than the popular every kWh of alternative sources is displacing a kWh of fossil fuels assumption.

The real world is of course much more complex. Energy production is much more than just one turbine, one windmill and a constant demand. That will make it hard to estimate the efficiency reduction and come to conclusions on how much extra fuel is used for balancing the intermittent load of wind and solar. But one thing is for sure: the emission reduction will be lower than what would be expected from the replacing of an energy source with low emissions by another energy source with even lower emissions.

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2 thoughts on “Replacing an energy source by one with lower emissions doesn’t necessarily mean proportionally less emissions

  1. poitsplace

    And as I mentioned before, the economics of wind are even worse. They simply can’t pay for themselves on their own…except when placed in exceptional areas (areas where wind remains almost constant). In a large scale deployment they cause spot prices of energy to spike and crash, providing strongly negative value. Their very use forces prices much higher than normal while they’re providing nothing. And if forced to take the energy by law (as is the case in germany) their inclusion crashes the price negative when they do produce. Without the purely ideological push for renewables, they simply would not be used.

    Reply
    1. trustyetverify Post author

      Of course there are other issues than the reduction of the efficiency of back-up turbines. Economical issues are also high on the list.
      In this post I just tried to explain that some of the emissions are wrongly booked on the fossil fuel side, while they should be attributed to wind/solar. This makes that fossil fuels come out worse than they are and wind/solar nicer than they. But probably more about that in some future post.

      Reply

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