July 27, 2004

Wind's ability to raise CO2 emissions

An interesting paper by Robert J Bass and Peter Wilmot of the School of Mechanical and Manufacturing Engineering, Loughborough University (U.K.), has been brought to my attention. It shows that wind turbines supplying the grid may actually increase CO2 emissions. The latter part of the paper is reproduced below, with emphasis added.
[C]arbon dioxide liberated per GW generated continuously for one year (8760 hours) from different fossil fuels is:

Fuel type         CO2 tonnes/year

Direct Coal fired 10.8 m tonnes/year
Direct Oil fired 6.75m tonnes/year
Gas (open cycle) 4.5 m tonnes/year
Gas (CCGT) 2.85m tonnes/year
Wind turbine Nil

The new wind generators will come on stream during the next five years and may be expected to generate power for twenty-five to forty years. During this time the early years of the make-up power will come from the existing fossil fuel power plants. Many of these units, however, are already half way through their working lives and will have to be replaced during the next five to fifteen years.

Furthermore, the current Combined Cycle Gas Turbine (CCGT) units are not well suited to follow the demand load changes on the network as the boiler/steam turbine units respond slowly to major load swings. And when the transient output from the wind turbines is added to the fluctuating nature of customer demands, the picture of the network supply requirements becomes even more unpredictable.

The potential consequences of wind power

If a 1GW wind farm generates power to the grid during all the periods when wind is sufficiently powerful, it might be expected to deliver approximately 2,630 GWh per year. However, this would cause a shortfall against a 1GW base load demand (i.e. 8760 GWh) over the same period of approximately 6,100 GWh and this has to be generated by fossil fuels.

The consequences are summarised in the table below:

Annual tonnage of CO2 emitted

Wind turbine + CCGT station 2.0 m tonnes
Wind turbine + Open cycle station 3.15m tonnes
Wind turbine + Oil fired station 4.75m tonnes
Wind turbine + Coal fired station 7.5 m tonnes

The data demonstrates that at best a wind turbine farm of 1GW installed capacity would save approximately 0.85m tonnes of carbon dioxide annually if it displaced an efficient CCGT plant. By the year 2010 a number of the current CCGT stations will be more than twenty years old and approaching the de-commissioning phase. If the financial incentives are inadequate (as is the current position) and the base load market is not available to help defray capital and fixed operating costs, they will not be replaced. The technology of any such new plants will also need to have been developed to handle the transient nature to the demand after the wind farms have produced their volatile output. The supply of natural gas will need to be reliable and economically priced but by this time it will be imported from politically less stable sources.

If the gas fired units are not available, the supply would have to come from either oil or coal fired plant (or even new open cycle gas fired plants). This would cause carbon dioxide emissions to increase above their current best levels.

In the case of oil fired back-up, the increase is some 1.9 m tonnes greater than the current position would be where the whole load is supplied by a gas fired CCGT plant. If the comparison is made with a coal fired plant supplying the make-up, the increase in carbon dioxide would be 4.6m tonnes annually.

And these figures will be eight times greater if the wind turbine installed capacity reaches the government’s target of 8GW.

It is worth noting that the government is committed to reducing the carbon dioxide emissions by 26.5m tonnes annually by 2010. A significant proportion of this reduction is planned to be delivered by wind turbines. This analysis suggests that the current ‘Dash For Wind’ could actually make the situation worse.
Another article sent to me describes the same scenario in the U.S. It is by Richard Stevens and appeared in "Energy Pulse." It unfortunately devolves into a defense of nuclear radiation, but here is the interesting part, with emphasis added.
Operating a fossil fuel fired power plant in the cyclic mode, instead of operating at a constant power, has two very detrimental effects. First of all, cycling makes the power plant much less efficient. It must consume more fossil fuel to produce the same electrical output. Second, cycling produces thermal stresses that over time will cause material failures that will force the power plant to shut down to make repairs.

The failures produced by cycling is one of the reasons that has influenced most power plant operators to choose a power plant design that is relatively inefficient when they need to operate the plant in the cyclic mode. A simple combustion turbine is typically 40% efficient. A combined cycle power plant that includes a combustion turbine, a heat recovery steam generator, and a steam turbine, is typically 58% efficient. However, the combustion turbine is less likely to fail due to the thermal stresses induced by cycling.

The other major reason that a power plant operator would choose the inefficient combustion turbine over the efficient combined cycle is that the combustion turbine costs less to install. The power plant operator must operate his combined cycle generator longer than the combustion turbine to recover his investment. If he is forced to shut down or reduce power to make room on the electrical grid for a wind generator, he may never recover his investment.

Consequently, there are very compelling technical and financial reasons to choose a simple combustion turbine that is only 40% efficient if the power plant is forced to cycle because of the operation of a wind generator on the same electrical grid. Using the 26.0% wind capacity factor from California in 2000, one can calculate the amount of fossil fuel required to operate a combustion turbine for 74.0% of the time in order to replace the missing power from the wind generator, and compare it to the amount of fossil fuel required to operate a 58% efficient combined cycle power plant 100% of the time. The more efficient combined cycle can now be used since it does not have to vary its output to accommodate the wind generator. The result is that the combination of wind generator and combustion turbine uses 7.2% more fossil fuel than the combined cycle. That’s right. The introduction of the wind generator causes more fossil fuel to be burned not less. That means more pollution, not less. That means more carbon dioxide emitted into the Earth’s atmosphere, not less. That means more dependence on imported fossil fuels, not less.