How much electricity we make
The four wind turbines at Skelmonae Windfarm were limited in height because of planning rules at the time. Each one has the capacity to generate 800 kilowatts of electricity per hour.
The three turbines at Hill of Skilmafilly have taller towers and blades that are 11m longer and this makes them nearly 3 times more efficient than the shorter turbines – each with a generating capacity of 2,300 kilowatts (2.3 MW) of electricity per hour.
Skelmonae Windfarm is estimated to provide enough electricity over a year to meet the needs of approximately 1,800 households. The turbines at Hill of Skilmafilly boost this figure by another 3,900 households.
Balancing supply/demand – now and in the future
All forms of power generation require back-up and no energy technology can be relied upon 100%. The UK’s transmission system already operates with enough back-up to manage the instantaneous loss of a large power station. Variations in the output from wind farms are barely noticeable over and above the normal fluctuation in supply and demand, as seen when the nation’s workforce goes home or if lightning brings down a high-voltage transmission line. Therefore, at present there is no need to install additional back-up for using wind energy.
For the future, various electricity storage options are being researched, but one already in existence that seems likely to be a useful match to wind power is hydro-electric generation. When the wind is blowing and a lot of power is being produced, the dams at hydro-electric power stations can be closed and water stored behind them, to be released at times when power supplied by windfarms drops. Furthermore, two new Scottish hydro-electric power stations have been designed to allow power generated elsewhere at times of low demand, e.g. overnight, to be used to pump water back up to the reservoirs, so it can be released to drive the turbines and turned back into electricity when needed.
Less Carbon Dioxide – more electricity
Reduction in CO2 emissions
Every unit (kWh) of electricity produced by wind-powered generation displaces a unit of electricity which would otherwise have been produced by a power station burning fossil fuel. Wind-generated electricity does not replace electricity from nuclear power stations because these operate at “base load”, that is, they will be working for the whole time that they are available. It is the outputs from coal-fired and gas plants that are adjusted to meet the electricity demand on the system.
It is easy to calculate how much Carbon Dioxide (CO2 ) is emitted during the production of electricity from coal-fired, oil-fired or gas-fired power stations, as this information is available from the main generators in their annual Environmental Performance Reviews.
British Wind Energy Association (BWEA) calculations use a static figure of 430 grams of CO2 displaced for every kilowatt hour of electricity generated. Emissions reductions achieved by using wind energy as an alternative can therefore be calculated using the following formula:
CO2 (in tonnes) = (A x 0.3 x 8760 x 430)/1000
where A = the rated capacity of the wind energy development in MW and 0.3 is a constant, the capacity factor, which takes into account the intermittent nature of the wind, the availability of the wind turbines and array losses. 8760 is the number of hours in a year.
Skelmonae Windfarm has a total rated capacity of 3.2 MW (4 turbines x 0.8 MW each), so will contribute emission reductions of more than 3,600 tonnes of CO2 every year. The adjacent windfarm at Hill of Skilmafilly has a total rated capacity of 6.9 MW (3 turbines x 2.3 MW each), so will provide emission reductions of a further 7,800 tonnes of CO2 per year.
Electricity output
The amount of electricity produced by a wind energy development can be estimated using the formula:
Electricity produced = (B x 0.3 x 8760)
where B = the rated capacity of the wind energy development in kW and constants 0.3 and 8760 have the same meaning as above.
This is only an average estimation because in many places, particularly in northern Scotland, the wind speeds are higher than average and give a greater electricity production per turbine, as power output is a cube of the wind speed.
On average then, a typical onshore turbine in the UK, rated at 2.3 MW, produces 6 million units of electricity each year. This is equivalent to 6,044 MWh or 6 GWh.
Skelmonae Windfarm has a total rated capacity of 3.2 MW, so will produce approximately 8.5 million units of electricity per year. The windfarm at Hill of Skilmafilly has a total rated capacity of 6.9 MW, so will produce over 18 million units of electricity per year.
Homes-supplied equivalent
Perhaps a more meaningful measure of the amount of electricity that a wind project generates is the calculation of how many households this would supply, using the formula:
Number of households supplied = B x 0.3 x 8760/4700
where 4,700 is the average UK household electricity consumption in kW hours.
One typical 2.3 MW turbine would therefore produce enough electricity each year to meet the needs of 1,286 homes.
Skelmonae Windfarm has a total rated capacity of 3.2 MW, so will produce enough electricity each year to power approximately 1,800 homes. The windfarm at Hill of Skilmafilly has a total rated capacity of 6.9 MW, so will produce enough electricity to power approximately 3,900 homes.