Costs, Outputs and Outcomes

We found that it is fairly easy to estimate how much energy can be generated, at what cost and how much carbon reduction is likely to result. As a desk exercise, however, it can only be an estimate and people need to realise that actual performance may be slightly different.

Wind Speed

This is one of the most important factors affecting outputs. In the UK our prevailing wind is from the south west. Average wind energy tends to be greatest in the west and south west and to decrease as you go east. Also, average wind speeds increase with height above ground level (agl). Fortunately, there is a government wind speeds database you can consult to find out average wind speeds at any location and at various heights agl. This is available through the Department of Energy and Climate Change (DECC) – www.decc.gov.uk

In the case of Cookley and Walpole, our average wind speeds agl are as follows:

  • 10m agl 5.0m/s
  • 25m agl 5.7m/s
  • 45m agl 6.2m/s

Which Turbine(s)?

Choice of wind turbine(s) is, of course, a complex matter and depends crucially on how much money you can raise! But there are other factors to take into account. Importantly, different models from different companies have different ‘power curves’ – ie the amount of power they generate at different average wind speeds. In Cookley and Walpole, where our wind speeds are modest, it was vital to choose a model which performs well at low average wind speeds.

Most manufacturers’ websites and brochures will have graphs and/or tables showing energy outputs per year for their different models at various average wind speeds. Some manufacturers offer different tower height options for the same model – the higher the ‘hub height’ above ground level, the greater the wind speed and energy output.

Energy Outputs

In Cookley and Walpole, we are looking at an 11KW model with hub height options of 18m or 25m. To be on the safe side we have rounded down slightly the figures from the manufacturer’s website. Given our available wind speeds, and with two turbines, we expect to be able to generate between 50,000KWh per year (18m hub height) and 60,000 KWh per year (25m hub height).

According to Energy Watch, the average household uses about 3,300KWh of electricity per year – www.energywatch.org.uk This means we could hope to generate ‘ clean’ electricity roughly equivalent to the annual consumption of 15-18 average households, which is at least 10% of households in the parish.

Income Generated

There are two income components. First of all, from 2010 ‘Feed-in tariffs’ will be payable to generators of renewable energy who feed their electricity into the national grid. The amount payable will depend on the type and scale of technology used – see www.decc.gov.uk. For wind turbines rated at between 5KW and 15KW the proposed tariff is 26.7p per KWh. This gives a potential income in Cookley and Walpole of between £13,350 and £16,020 per year.

The second income component is what you can get through selling the electricity to a distribution company. There is no fixed price for this – it will depend on prevailing market conditions and what price you can get by shopping around. At the moment we believe that 6p per KWh is a reasonable estimate. In Cookley and Walpole this would yield additional income of between £3,000 and £3,600 per year.

Taking the two components together, we estimate that the Cookley and Walpole project could generate between £16,350 and £19,620 per year in total. We are proposing to allow 10% of gross income for maintenance and repairs. This would leave us between £14,715 and £17,658 per year to spend on further carbon reduction projects in the parish.

Carbon Reduction Outcomes

Currently, each KWh of electricity delivered via the national grid produces around 0.5kg CO2 in its production and distribution – the ‘carbon conversion factor’. However, we can expect this to fall over the coming years and decades with more renewable and, perhaps, nuclear power generation, as well as cleaner coal based generation. Over the hoped-for 25 year lifespan of our project in Cookley and Walpole, we are estimating an average carbon conversion factor of 0.45Kg CO2 per KWh of grid electricity delivered.

This means we can expect the ‘clean’ energy produced by our turbines to save between 23 and 27 tonnes of CO2 per year compared with average grid electricity (after the carbon footprint of the turbines’ production, delivery and installation have been offset).

Over and above this, we will have the annual net income from the project to spend on further carbon reduction initiatives within the parish.