http://www.scoraigwind.com/citywinds/ Wind Speed Measurement over a roof top in Edinburgh, Scotland by Hugh Piggott

WINDSPEED MEASUREMENT IN THE CITY

A comparison between real world data collected on an Edinburgh house, and data from the source used by most wind turbine suppliers to estimate windspeeds for their customers.

I have updated and tidied this page 6th January 2007 because it is attracting some notice and it was a bit of a rush bodge as usual in the first place.

On this page:

Address and data logger details (below right here)
the data
the database prediction used by the suppliers
some comments from Paul Gipe and from Swift turbines
some accepted wisdom in the industry about small wind turbine towers

page by :
Hugh Piggott
Scoraig Wind Electric

My agenda here is to protect the reputation of wind energy from disaster.

Wind energy works well if correctly applied, but rooftop mounting is not a suitable application.



Logie Green Road, Canonmills
Edinburgh EH7 4HD


I was given an NRG 'Windwatcher' logger to evaluate, and I decided to use it to measure the windspeed at this property.

My motivation was to check up on the wind in the city because I had a strong suspicion that there is not very much but I wanted to put numbers on it.

The data is below.

The Windwatcher reports an average windspeed for each month. But it is also cleverer than this. It actually records the energy potential of the wind in watts/square metre and reports that too.

From that data it then computes a new mean windspeed that would give the same energy if the windspeed distribution were of the standard Rayleigh type. All pretty arcane stuff, but low windspeed sites often have much more variable winds and therefore produce more energy for a given mean than would a site with the more typical Rayleigh distribution used for energy production reporting in wind turbine literature.

So the Windwatcher gives a very fair picture of the available power in the wind in spite of being quite a cheap logger.
Data collection with Maximum 40 anemometer
and NRG WindWatcher logger (no longer available




You could also use other low cost loggers for this sort of operation - for example
Windsurvey.co.uk

or the Wind prospector

Personally I have owned a prospector for about ten years, and I love it.

also aprsworld
http://www.aprsworld.com/wind2/



Month





Average wind metres/sec
Maximum windspeed m/s
average watts /sqmetre
Rayleigh equiv. average wind
ratio of averages



Mar-04
1.9
18
22
2.6
1.37



Apr-04
1.7
17
16
2.3
1.35



May-04
1.5
13
10
2
1.33



Jun-04
1.8
13
16
2.4
1.33



Jul-04
1.3
12
8
1.9
1.46



Aug-04
1.1
14
7
1.8
1.64



Sep-04
2.1
17
26
2.8
1.33



Oct-04
1.4
19
14
2.3
1.64



Nov-04
1.8
15
18
2.5
1.39



Dec-04
2.2
19
29
2.9
1.32



Jan-05
2.4
21
48
3.4
1.42



Feb-05
1.5
16
16
2.4
1.60



average for year
1.73
16.17
19.17
2.44
1.43




m/s
m/s W/m2
m/s









The annual mean is under 2 m/s. However the wind turbine could potentially deliver more than the expected energy of a 2 m/s site with Rayleigh windspeed distribution. If it can deal with the turbulence!

Average watts per square metre can help you to calculate the energy you could get from a wind turbine. Calculate the swept area (3.14 times radius squared). For example a 2 metre diameter machine has about 3 square metres of swept area. Over the year that would mean average power about 60 watts giving over 500 kWh units of electricity per year. But in fact wind turbines can convert less than 60% of this. In reality given the constraints of the technology about 20% would be good. 100 kWh with a value of about £10 would be likely.

Bottom line is about £10 worth of energy per year on this site.
Paul Gipe's comment:

Thus, manufacturers who say you will get an average of 6 m/s on the roof at your site and this will generate 1,000 kWh/year from a 2-meter diameter wind turbine will overestimate production by 10 (ten) times! Those that say you can get 2,000 to 3,000 kWh on your roof top will overestimate your production by 20-30 times! David E. Anderson of Renewable Devices Swift Turbines comments:

I can’t really comment on the validity of the data but I would have some concerns about where the anemometer is mounted – it seems to be very close to the chimney stack of the house and so will suffer from the localised turbulence effects created by the several chimney pots, aerials etc which are visible in the picture. Renewable Devices would never install a wind turbine in this location so the data cannot be compared directly. Further, it seems that there is no direction measurement on the meteorological array and so Hugh would have had no way of measuring the turbulent intensity of the airstream. This has a huge effect on energy yield from any wind turbine and has to be taken in to account when siting of any turbine (from Swift size up to multi mega Watts). Therefore, my main comment here is that the data presented seems to represent an airstream in which we would never consider mounting a turbine so, I suppose I am sort of agreeing with Hugh’s final conclusion. However, I would caveat this with the facts that the data has been measured in a location where a rooftop wind turbine should never be installed, and does not account for turbulent intensity, which has a large effect on energy yield.


(In fact the windwatcher does make allowance for turbulence but it makes little difference to the conclusion - hugh)
Just to show how the Renewable Devices Swift turbine is mounted on buildings totally differently from my anemometer,
here is a photo -

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Now the DTI windspeed data base (NOABL) gives the following computed windspeed data (m/s) for this square kilometre of Edinburgh and the ones immediately around it:

for the 1 km grid square 325 675 (NT2575)

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Wind speed at 45m agl (in m/s)

6.8 6.9 7
6.9 7.1 7.1
7.1 7.2 7.4


Wind speed at 25m agl (in m/s)

6.2 6.3 6.4
6.3 6.4 6.5
6.5 6.6 6.7


Wind speed at 10m agl (in m/s)

5.2 5.4 5.5
5.4 5.6 5.6
5.6 5.7 5.9

Blank squares indicate areas outside the land area of the UK - i.e. areas at sea or of neighbouring countries.
agl = above ground level.
Squares surrounding the central square correspond to wind speeds for surrounding grid squares.

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NOABL is the DTI based source used by most wind turbine suppliers when estimating customer site potential.
It has worked well for me in rural locations.

Here where I live on Scoraig we get annual mean windspeeds between 5 and 7 m/s on most sites.
It is a lot windier here than the city.
The anemometer study just confirms what I already know.


The message is beware of using NOABL in the city. Get some real data first.

There are some pretty ignorant and over enthusiastic sales people out there!


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WINDSAVE - A WARNING MESSAGE

The image below is from Windsave website about 3-4 years ago - this was their product!


These are the same people who now supply B&Q with their very popular turbine. It has matured a bit over the few years of experimentation but there is little evidence that the makers have learned much about siting small wind turbines.
These guys came out of nowhere a few years back claiming they had all the answers but their web pages do not make any sense.
One does not know whether to laugh or cry.
Marketing is their strong card but they cannot deliver, because the wind is not there.
They are offering a product that everyone wants, but sadly it is not technically possible to supply.

COPY OF AN EMAIL TO THE BWEA (British Wind Energy Association) 05 January 2007


CEO, British wind Energy Association
Dear Maria McCaffery,

I am writing to you because I am very concerned about the claims made by Windsave and other rooftop wind turbine manufacturers and their agents. I suggest that BWEA would do well to distance itself from these people before the resulting fiasco reflects on the windpower industry as a whole.

For example, last night on TV I saw the boss of B&Q confidently assert that a Windsave turbine would save a typical householder 25-40% of their electricity bill. I was very pleased when the interviewer did ask if there was enough wind. This is a big worry for me. I know there is not enough wind in most city locations to make this possible. He replied that maybe 20% of sites would not be suitable. I don't think he has a clue what he is getting into.

There does not appear to have been any systematic assessment of the wind resource at rooftop level in an urban environment. This seems ludicrous given that Windsave attracts a 30% grant from LCBP. My own tests over one year in Edinburgh revealed a huge shortfall of wind compared to the annual mean that might be expected based on NOABL. The Windsave web site (which is full of glaring errors) states that "According to the Department of Trade and Industry (DTI), the average wind speed across the UK is approximately 5.6 metres per second [m/s] at 10m above ground level (agl)." This nonsense is widely believed. Of course it is not true because of localised obstacles, especially in built-up areas where the turbines will be sold.

My experience is that the general public have highly inflated expectations of small wind turbines. I have been in the business for nearly 30 years and I am constantly dealing with people who wildly overestimate the amount of energy they will get from small wind turbines. I spend much of my time educating them and explaining that windpower will not do what they expect. There is a massive opportunity for marketing here, but only for the unscrupulous (or ignorant). I am very surprised that BWEA are willing to even recognise organisations such as Windsave and Renewable Devices that make wild claims for their rooftop products. This can only end in tears. There are very few sites where the expected energy can be produced. Siting a wind turbine below the ridge of a building (at bedroom window level) in the back garden of an urban dwelling as shown in the product literature is absolutely crazy!

I strongly suggest that BWEA make some sort of public statement warning of the dangers of this latest marketing racket. Otherwise wind energy as a whole will take a massive hit to its credibility. I suggest that you demand evidence from Windsave and Renewable Devices to support their improbably energy production claims. I suggest that you contact the BRE and the EST and get them to do their homework before they offer grants for these installations.

What I find is that when people learn that small wind turbines cannot deliver on their assumptions, they go on to assume the wind energy as a whole is a waste of time, or worse a complete scam. We need to educate the public to understand what wind energy can do and what it cannot. My experience is that when people learn how little energy is available at rooftop level, they immediately extrapolate this lesson to windfarm level, and decide that wind energy is completely irrelevant (see for example http://www.timhunkin.com/a125_arch-windpower.htm).

I really hope that you will take this request seriously. If nothing else, please try to find out more about the wind at rooftop level in the UK. I think you will find that there is not enough to do anything useful. This is a matter of some urgency now, and soon it will be too late to avoid a complete disaster for the industry.

All the best, and let's hope for a windy new year :-)
--
Hugh Piggott
Scoraig Wind Electric

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Wind turbines should be installed on suitable towers
Here is a photo of a suitable tower for small wind turbines in a built up setting
(thanks to Bergey Windpower)

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"A wind generator tower is very often more expensive than the turbine. The tower puts the turbine up in the “fuel”— the smooth strong winds that give the most energy. Wind turbines should be sited at least 30 feet (9 m) higher than anything within 500 feet (152 m)." Ian Woofenden - Home Power magazine
Since 1987, we've dedicated more than 100 issues to home-scale renewable energy and sustainable living solutions. That means comprehensive coverage of solar, wind, and microhydro electricity, home energy efficiency, solar hot water systems, space heating and cooling, green building materials and home design, efficient transportation, and much, much more.

Whether you're a do-it-yourselfer or not, off-grid or on-grid, Home Power is here to help you make informed decisions about your home energy use. We provide extensive product information, homeowner testimonials, buyer advice, and "how-to" instructions.
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Some useful tips from Westwind about tower height selection are here

Built up areas have very high surface roughness, so it is very important to use a tall tower if you want your wind turbine to work.
"A good rule of thumb is that the tower should be 2.5 times the height of any nearby buildings or trees. "

Westwind are relocating their manufacturing to Northern Ireland.


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