Monday, August 25, 2008

Cippenham Wind-farm

There's a proposal for a new wind-farm just down the road, in Cippenham, about here.

A letter in the local paper last week (Slough Observer 22/8/08), pretty much the first comment there's been on the proposal, used figures from the local opposition group WindRats to say both that the developer would be making nearly £3m a year from the proposal (assuming full capacity) and that the wind speed is so low in the local area that it wasn't worth bothering with. Which is it?!

As usual, the truth lies somewhere these points (though in normal debate, different sides, not the same, tend to make the contradictory statements). Here's my letter and the workings behind it:

Tom Williams (Letters last week) states that his opposition to the proposed Cippenham wind turbine is based upon available sourced data, so perhaps his opposition could be turned into support. He rightly states that the average annual wind speed for the proposed site is just under 6m/s, a speed which would, from the manufacturers own statistics, produce about 0.25MW. This is only about one twelfth of it's maximum power (though Mr Williams and the website seem to assume 100% production when coming up with their £2.7 million figure), but this is only part of the story.

As any driver knows, wind-resistance increases significantly more the faster you get, requiring more petrol to accelerate from say 70-80 mph than from 20-30mph. This explains the difference in design between the Volvo and the Porsche. Happily, this effect also works in reverse - the faster the wind blows, then proportionally more energy a turbine can extract from the wind. In practice, what this means is that you cannot get an accurate estimate of the potential output of a turbine from the base average wind speed alone, but would need to survey the actual speeds on site over a period of months. I have no idea if this research has been done, but by using an estimation method used by the wind-energy industry, I calculate that this site and turbine would produce over 5000MW hours of electricity per year, enough for about 1200 houses and saving nearly 3000 tonnes of carbon dioxide emissions in the process (the exact workings available at ).

As these figures approximately match the developer's figures, I hope that Mr Williams will now join me in giving support to this proposal on economic grounds. Wind-power is going to become more common as we struggle to meet our rising energy demands from dwindling and polluting resources such as gas and coal, and to me are far preferable to the "energy-from-waste" incinerators currently operating on the Trading Estate and soon in Colnbrook.

OK, the maths.

This is based on the formula given in the Beurskins and Jensen article "Economics of wind-energy, Prospects and Directions" in the July/Aug 2001 edition of "Renewable Energy World". It looks to give an estimate of the energy available from a particular wind-turbine site. It's only an estimate, and you need to do a proper site survey and then match that to the exact specifications of the turbine you intend to fit. But it gives you an idea.

The turbine is sited at grid reference SU946797 (SU9479), which from the UK Wind Energy Database gives an average wind speed at 45m above ground level (agl). However, the actual turbine will be higher than that, so the developers suggest a speed of 6.4 m/s

The basic formula is :

Annual energy production (kWh) = K Vm3 At T

Nice. What this actually means is:
K = 3.2 = a factor based upon the relationship between standard turbine efficiency and wind-speed distribution.
V - is the cube of the average wind-speed for the site. This is the key, wind-energy is all based on the fact that the energy you get is all based on the cube of the wind-speed. This means you don't get much to start with, but it starts to rise rapidly as the wind-speed increases. For our basic speed of 5.8 m/s this is 195, 262 for the higher estimate.
A - the swept area of the blade. It's expected to be 90m diameter, so that's 45 m radius, which is (PIr2) 6352m2
T is the number of turbines, which in this case is one.

So, multiplying those together, we have either:
3.2 * 195 * 6352 * 1 = 3,972, 512 kWh (3,972 MWh / 3.972 GWh) per year, or
3.2 * 262 * 6352 * 1 = 5,337, 294 kWh (5,337 MWh / 5.337 GWh) per year

To put this in some sort of context, the average household uses 4478kWh p/year of electricity, so dividing the total amount of energy expected to be produced by this household average tells us how many houses could be powered by the turbine, between 887 and 1192.

Measuring CO2 emissions and what might be saved is a bit of dark art, depending mainly on which energy source you say you're replacing (measuring against coal makes you look good, hydro less so, the most recent energy mix is the fairest, but it's still a guess), but the latest figure I've a respectable source for is 0.527kg for each kilowatt hour of energy generated.

So, to calculate the theoretical amount of CO2 saved, multiply the expected energy generation from the turbine by this figure of 0.527kg, giving a figure of between 2,093,513.58kg (2094 tonnes) and 2,812,753.82kg (2,812 tonnes).

This is pretty much in line with what the developer estimates (section 3.12, page 9) for a Vestax 3MW turbine.

And it's both a lot smaller and cleaner than the Slough Estates CHP/incinerator and the now-legendary Colnbrook incinerator ...

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