A lesson for Minister McArdle - read beyond The Australian

In taking seriously a report in The Australian that wind doesn't reduce CO2, the office of the Queensland energy minister has fallen victim to an urban myth about how power stations work.

According to Renew Economy yesterday, the office of Queensland Energy Minister, Mark McArdle, believes that wind turbines don’t save CO2, based it appears on reports from that oracle of accuracy on all things climate change - The Australian.

I’ll deal with the accuracy of their news source in another article, but thought it was worthwhile specifically busting a myth that fossil fuel power stations have little capacity to flexibly adjust their output or fossil fuel consumption. The urban myth is that if these power plants are forced to cut back their output to operate at part-load due to variation in the power output from wind turbines, then there will be huge efficiency losses. This apparently means the fossil-fuel plant ends up emitting more CO2 than is saved.

In reality most fossil-fuel plant, including baseload generators, have a reasonable degree of flexibility to alter fuel input and electricity output.  They certainly don’t just consume a constant amount of fuel and then regularly vent large amounts of steam into the atmosphere to vary their electricity output. Nor do they have just two settings of either 100% of full power plant capacity, or completely off.  

To help illustrate, the chart of the week below shows how the average greenhouse gas emissions intensity (GGE) for several power plant types vary with their load. This is taken from a publication produced by the Co-operative Research Centre for Coal in Sustainable Development. This organisation is of course constituted by a range of raving mad greenies who’d like to see a wind turbine on every hill like BHP Billiton, Xstrata, Rio Tinto, and - interestingly for the Queensland Minister for Energy - the Queensland Department of Mines and Energy and three Queensland Government-owned coal-fired generator businesses: Stanwell; Tarong; and CS Energy.  

Anyway onto the chart.

SC stands for a black coal supercritical power plant, which the chart illustrates at about 50% loading emits on average about 850kg of CO2 per megawatt-hour of electricity produced. This then reduces to about 800kg of CO2 per megawatt-hour at 100% load. For a modern 100 megawatt aero-derivative (this just means derived from turbines used on aeroplanes) gas turbine designed for meeting short peaks in demand, it emits 700kg of CO2 at 50% load, reducing to about 550kg at 100%. And the emissions from a gas turbine combined cycle (GTCC) power plant barely vary at 50% to 100% loading. Lastly they’ve provided an example of a very small 30MW gas turbine which does suffer large efficiency losses at part-load, but it’s irrelevant because there aren’t many (any?) installed in Australia’s main grids of the NEM and the SWIS.

Emissions intensity of different power plant types depending on loading 

Source: CRC for Coal in Sustainable Development (2008) GHG and cost implications of spinning reserve for high penetration renewables

So just like every urban myth, it contains a tiny sand grain of truth. If you back off a coal or gas power station it does increase its emissions intensity, but the impact is far outweighed by the fact that the electricity generated from wind has zero emissions. As the report which Minister McArdle's own department helped support, stated,

“The often held view that operating larger fossil plants at part load to provide spinning reserve would negate the GGE [greenhouse gas emission] benefits of wind is false.”  

In case the Queensland Energy Minister has trouble with maths as well as the basics of electricity generation, in the table below I’ve provided calculations on the CO2 impact of wind electricity substituting for fossil fuel plants. This involves wind substituting for 50% of the output from the other fossil-fuel power plants, taking into account losses in efficiency of the fossil fuel plants.

What it shows is that even though the fossil fuel generators’ lose some efficiency operating at part load, it doesn’t come remotely close to negating the emission savings from sourcing electricity from wind. For each MWh produced by wind it has avoided around 93% of the emissions associated with getting that same MWh from a supercritical coal or gas combined cycle plant, and 72.7% of the emissions from a gas peaking turbine.

CO2 emissions for 100% loading of fossil fuel generators versus 50% loading due to substitution by wind power  

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