Displacing CO2-emission with wind power
Dr John Etherington
Former Reader in Ecology in the University of Wales
August 12, 2004 (posted to windfarm.org)
Summary The main reason given for imposing wind turbines upon us is that they displace fossil fuel carbon dioxide (CO2) emission. The BWEA claims that wind generation displaces 0.86 tonne CO2 per megawatt-hour (MWh) of coal-fired CO2 emission. This is deliberately misleading. The true figure is the displacement of emission from the average of "coal-fired + gas-fired" generation. It is 0.58 t CO2 per MWh.* These two figures are absolute maxima as they are not corrected for the emission of CO2 by the reserve generation necessary to back up intermittence of wind. This is now accepted as at least 0.3 to 0.5 times the installed wind generating capacity.
Why do we need wind turbines? Supposedly because they may save emission of carbon dioxide (CO2) from fossil fuel which, allegedly, is causing "global warming" (Note 1).
How much CO2 is emitted by conventional generation?
The quantity of CO2 emitted by fossil-fuelled electricity generation can be expressed as weight of CO2 per unit of generated electricity.
To arrive at convenient numbers and to express industrial quantities, it is easiest to express the emission as tonne CO2 per megawatt-hour (t CO2/MWh) of generation (Note 2).
The quantities are not absolute but are variable in relation to fuel source, running efficiency and also to the proportion of CO2-free electricity (mainly nuclear) in the generating mix.
It is usually said that a unit of wind power displaces a unit of fossil fuel electricity, together with its CO2-emission. This is simplistically true -- 1.0 MWh of electricity, fed into the local electricity network from a wind "farm," would displace the consumption of 1.0 MWh of electricity from the Grid.
BWEA says wind saves 0.86 t CO2/MWh
The British Wind Energy Association claims that one MWh of wind electricity displaces one MWh of coal-fired generation (the "dirtiest fuel," in terms of CO2) which would otherwise have emitted 0.86 t CO2 (www.bwea.com/edu/calcs.html).
This is dishonest, because wind power is "embedded" in the distribution network beyond the high voltage National Grid and out of reach of its instrumental control. It is not possible for Grid controllers or power stations to choose whether to ramp-down coal or gas fired generation when wind comes on line (Note 3).
Generation in the UK includes c. 22% nuclear, which emits no CO2, but as this is baseload power, it is rarely ramped-down for any reason.
Thus, the only generating mix, which it is legitimate to equate with the windpower displacement, is the average of gas and coal contributions. Gas, burned in a gas-turbine, has a much smaller CO2 emission per MWh than coal.
The average CO2 emission saved by wind is about 0.58 t CO2/MWh
One MWh of wind generation thus displaces a mixture of coal-fired (0.86 t CO2/MWh) and gas-fired generation (about 0.37 t CO2/MWh -- Note 4). Coal represents 30% and gas 40% of UK generation (Note 5) thus the weighted average CO2 emission for coal + gas is about 0.58 t CO2/MWh.
Even this saving may be an exaggeration as the current operation of NETA favours the ramp-down of gas-fired Combined Cycle Gas Turbine (CCGT) generation to balance load fluctuation (see Note 3). The CO2 emission of CCGT is about 0.3 t CO2/MWh.
What about generation when wind fails?
Because wind power is intermittent it is necessary to top-up sudden shortfalls of generation, and also to cope with sudden excess of generation.
At present wind power provides only four thousandths of UK generation, and so surges can be corrected by "stealing" existing spare capacity (Note 6). Prior to the introduction of the New Electricity Trading Agreement (NETA) this spare capacity comprised about 25% of spinning reserve which insured against sudden losses or increments of load, for example by failure of a transmission line or shutdown of a large consumer. This "insurance" margin has now fallen to below 20% as a result of NETA economics.
Once wind power has increased to, say, 5% of UK generation, this could represent a sudden loss (or gain) of between 15% and 20% when windspeed decreases (or increases) suddenly if we assuming a load factor of 25% to 30% (see my Load factor -- mystery and magic mirrors). Patently we could no longer afford to jeopardise the insurance of the spinning reserve, and the wind power industry would have to provide dedicated spare capacity.
This was spelled out by Dr D. Helm (DTI Energy Advisory Panel) in a recent TV appearance: "the paradox of building windmills is that you have to build a lot of ordinary power stations to back them up" (Note 7).
The need for spare capacity has recently been quantified by ELTRA, the West Danish grid operator: "300-500 MW of system resources for ramping up and ramping down will have to be made available if an additional 1,000 MW of windpower are to be installed" (Note 8).
Given a Danish load factor of less than 25%, this indicates that a greater megawattage of spinning spare capacity is required than the wind generation actually achieves! It must represent a substantial reduction in the amount of CO2 emission saved! (Note 9).
West Denmark has more wind turbines per head of population than any other country on earth. We should learn from the growing predicament of its wind power industry.
1. My previous article, Global warming and windpower, makes the point that whether or not one accepts the arguments that man-made CO2 emission is the cause of "global warming," it is a matter of simple calculation to show that wind power cannot significantly alter atmospheric CO2 concentration. Climatic change models suggest that a reduction of at least one third (back to pre–Industrial Revolution concentration) would be necessary to halt warming, assuming the basic premises of the models are correct. Wind power could not approach this contribution.
2. Units, definitions, and abbreviations. Generating capacity is expressed as power: e.g., 1.0 megawatt (MW) = 1000 kW (= 1000 domestic units). Energy is expressed as (power × time): e.g., kilowatt-hour (kWh) or megawatt-hour (MWh). To relate CO2 emission to domestic consumption: 1.0 tonne/MWh is equivalent to 1.0 kg/kWh (i.e., 1.0 kg/unit).
3. The current operation of the New Electricity Trading Arrangements (NETA) generally favours the ramping down of gas-fired Combined Cycle Gas Turbine (CCGT) units when generation is reduced, because coal is favoured as a cheaper fuel. In July 2002, Reuters pointed out that this had caused more CO2 emission in the UK than wind electricity had saved! Furthermore present day CCGT plant was not designed for continuous start-ups and load change, and is being damaged by this process.
4. Bass, R. J., and Wilmot, P. (2004). UK Power, Issue 2. "Wind Power may not be the answer." These authors give 0.5 t CO2/MWh for open-cycle gas and 0.3 t CO2/MWh for CCGT-gas, giving a weighted mean of about 0.37 t CO2/MWh.
5. UK Energy in Brief (DTI, 2003).
6. Spare capacity is on "hot standby," i.e., connected to the network and operating at part load or available for instant start-up and connection as is the case for hydro- and gas-turbine plants. Whilst windpower represents only a few thousandths of UK generation (as now) this spare capacity can be "stolen" from the existing spinning reserve. Once wind exceeds a few percent of total generation it will become necessary to provide dedicated spare capacity for its failure or sudden change of velocity.
7. Dr Dieter Helm of the DTI Energy Advisory Panel, has explained that provision of dedicated spare capacity will become necessary and that it would be "almost certainly gas in the short to medium term." He added, "If you ask the question, who's making sure that there's enough gas stations out there to back up the windmills, the answer is nobody" (BBC2 programme, "What if the lights go out?," 2004).
8. ELTRA 2003 System Report. Until recently, West Denmark has been able to take its wind-balancing "backup" from neighbouring countries via its very generous provision of interconnection. The capacity of these transmission lines is now becoming overloaded and Norway, Sweden, and Germany also have various power problems of their own, which are now forcing West Denmark to provide its own "backup" -- from fossil fuel.
9. At present, with only four thousandths of UK generation from wind, the wind power companies receive three times market value for their electricity (Renewables Obligation and Climate Change Levy premium). However, for much of the time their wind turbines require support from existing spinning spare capacity. Hot spinning spare capacity has its own CO2-emission cost in reduced plant efficiency (even "quick-start" CCGT plants were not designed for such operation and incur both a CO2 price and a maintenance price). Arguably the support currently taken from Grid reserve capacity should be purchased at similar rates to the premium payment for wind generated electricity. It is not.
*The U.K.'s total emission of CO2 was 150 million tonnes in 2002 (DTI). --ER
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