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Wind Power may not be the answer

The government has determined one of its major solutions to reducing global warming as the installation of a large number of wind farms to generate up to 8GW of electricity and so reduce the emissions of the green house gas, carbon dioxide. To encourage the installation of wind power generators, the price paid per unit of power will be approximately three times that currently paid to fossil fuelled stations.

When the new wind farms start to generate for the national grid, we know they will deliver power inconsistently. As soon as the wind speed drops below that required to generate maximum output, (nominally 15 m/s), the power flow starts to reduce significantly. When the wind reaches a minimum speed (nominally 4 m/s) all power stops flowing (Figure 1). This deficit has to be immediately replaced by power from other generating plants. Presently, there are no other credible sources of renewable energy, which can be marshalled to pick up this deficit. It will fall to the fossil fuel powered stations to generate the shortfall well into the foreseeable future.

As the wind is a fickle source of energy and will, on occasions, deliver no power at all the alternative standby capacity will need to equal the total installed wind turbine capacity (see Figure 2). This will require, in effect, the provision of double the installed capacity of electrical generating equipment. All this plant must remain economically viable if the network is to provide security of supply.

Considering the predictable performance of a wind farm over a period of one year, the actual output is approximately 30% of its installed capacity. Although the standby power plants must be capable of producing 100% of the power requirement, they, will need to recoup their fixed overhead costs from generating just 70% of their capacity. Hence the price of the power they generate will need to rise in comparison with today’s current base load price to meet the short fall in revenue. When this is taken into account the real cost of wind power will be even higher than the incremental price paid to wind generators.

At the present time there is sufficient installed capacity to meet these transient requirements but much of this plant is well into its useful life and will need to be replaced during the next decade. It is at this point that a major problem arises.

The real carbon dioxide reduction achieved by the use of wind turbines is not a simple calculation. While it is true that the wind delivers energy, without emitting carbon dioxide, it does not take into account the process which replaces it when the wind fails.

As an example, consider 1GW of installed wind turbine capacity, which as we have seen would deliver an average clean output of approximately 300MW over the period of one year. Clearly, it would cycle between 1GW and 0GW during the period and this would require a back up power station of 1GW capacity to deliver the required make–up on demand.

The environmental impact of this replacement generation will depend upon the type of fuel used and the efficiency of the power plant.

Comparing carbon dioxide emissions

The approximate carbon dioxide liberated per GJ of heat output for specific fuels.

Fuel CO2 per GJ of heat output
Coal 120 kg
Oil 75 kg
Natural gas (methane) 50 kg

(source: Renewable Energy, Boyle 1996)

Type of Power plant Plant efficiency
Direct Coal Fired 34-35%
Direct Oil Fired 34-36%
Open Cycle Fired 34-36%
Combined Cycle Gas Turbine (CCGT) 54-56%

Hence the quantity of CO2 liberated by generating a continuous output for a period of one year can be calculated for various fuels:

e.g. For a power station continuously generating 1GW of power (where 1 GWh = 3.60 × 103 GJ and 1 year = 8760 hours) the total energy produced is:

Total energy generated = 3600 × 8760 = 3.15 × 107 GJ

And for a coal fired power plant of 35% efficiency the total quantity of CO2 produced is therefore:

CO2 produced = 3.15 × 107 × 120 × 10-3 = 10.8 million tonnes/annum

Similarly, carbon dioxide liberated per GW generated continuously for one year (8760 hours) from different fossil fuels is:

Fuel type CO2 tonnes/year
Direct Coal fired 10.8m tonnes/year
Direct Oil fired 6.75m tonnes/year
Gas (open cycle) 4.5m 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 farms 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 short fall against a 1GW base load demand (ie. 8760 GWh) over the same period of approximately 6,100GWh 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 2.0m tonnes
Wind turbine + Open cycle station 3.15m tonnes
Wind turbine + Oil fired station 4.75m tonnes
Wind turbine + Coal fired station 7.5m 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.

Robert J Bass and Dr Peter Wilmot
School of Mechanical and Manufacturing Engineering, Loughborough University
UK Power, Issue 2 (2004)

For further information, contact Mr Robert J. Bass on Tel: 01780 763024 or Dr Peter Wilmot on Tel: 01509 227 555.