# The Naked Scientists Forum

### Author Topic: How much power can I generate with a small wind turbine?  (Read 6438 times)

#### bunker

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##### How much power can I generate with a small wind turbine?
« on: 11/03/2009 20:10:04 »
can anyone help me? I am trying to ascertain how much power I can generate from a small wind turbine at high wind speeds?

diameter of wind fans no more than 12 inches.

Wind speed 40 mph+

Device needs to be lightweight and portable

Thanks
« Last Edit: 12/03/2009 09:42:49 by chris »

#### Vern

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##### Re: How much power can I generate with a small wind turbine?
« Reply #1 on: 11/03/2009 21:17:52 »
Aircraft supply sources used to sell a small wind generator to power the instruments of small home-built aircraft. They were powerful enough to run a couple of radios.

#### syhprum

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##### Re: How much power can I generate with a small wind turbine?
« Reply #2 on: 11/03/2009 21:22:24 »
Small wind turbines are normally built with about 12 blades, some idea of the power available could be calculated by the mass of air flowing thru and the difference in velocity of the incoming air and the air going out.
There may well be algorithms to do this calculation if you hunt for them.
PS dont expect a lot about 1 watt at a guess
« Last Edit: 11/03/2009 21:33:38 by syhprum »

#### syhprum

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##### Re: How much power can I generate with a small wind turbine?
« Reply #3 on: 11/03/2009 21:37:47 »
Sadly I was over optimistic if everything was 100% efficient you would get 0.5 watts, in the real world half of that

#### Don_1

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##### Re: How much power can I generate with a small wind turbine?
« Reply #4 on: 12/03/2009 09:07:51 »
The big problem is, how can you guarantee you will get wind at all, let alone 40mph+

Even this lot in the Thames estuary can't guarantee anything at all.

#### lyner

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##### How much power can I generate with a small wind turbine?
« Reply #5 on: 12/03/2009 10:20:38 »
I have a 'small' turbine on my boat. It is very rugged, because it is out in all weathers and exposed to saltwater spray etc..
It is 50cm diameter and produced 25W (@12V) with a reasonable wind. It reliably gives me enough to charge batteries when I'm not on the boat or when sailing. It is pretty useless in light winds, though - with, perhaps 200mA, which isn't enough to actually run anything significant. If there's too little wind then I use the engine, which provides much more, of course. It is reassuring to know that I don't actually need to start the engine if the batteries should happen to be flattened - I just have to wait for some wind to charge them . . . .

I have seen a 'vertical axis' turbine, made by Forgen, which is a bit smaller.

You can see the Rutland on this link:

Wind speed and diameter are critical - the power available is proportional to the square of the speed and the square of the diameter over quite a wide range so, if your application is a real practical one, you need to investigate the sort of wind speeds you are likely to expect and consider going bigger.

#### syhprum

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##### How much power can I generate with a small wind turbine?
« Reply #6 on: 12/03/2009 20:11:45 »
I based my calculation of the maximum power available as that of a cylinder of air 17.88 meters long and 0.3048 m diameter of mass 1.957 Kg moving at 17.88 m/s.
applying MV^2/2 Gives 208.5 J/sec or Watts.
This seem rather high I would like to see other estimates.
This assumes that the turbine absorbs all the available power from the wind but even so if one puts in a realistic assumption for the efficiency it still seems high.
If I base the likely output on the figures quoted for the 'Rutland 503 Wind Turbine' I get 31 watts an efficiency of 15% which seems reasonable

« Last Edit: 13/03/2009 10:29:41 by syhprum »

#### lyner

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##### How much power can I generate with a small wind turbine?
« Reply #7 on: 12/03/2009 22:42:21 »
Seems fair enough. I reckon you've got it right.

#### paul.fr

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##### How much power can I generate with a small wind turbine?
« Reply #8 on: 16/03/2009 04:36:40 »
A few weeks ago the United Kingdom Meteorological Office along with the Carbon Trust announced the launch of a new web site to estimate site specific potential for wind generated power systems. The on-line service will estimate the potential for small scale wind power generation based on local climatic data, local land use, and topography, along with modern technology in the deployment of small wind turbines. You can view more at the web site

http://www.carbontrust.co.uk/default.ct

#### Don_1

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##### How much power can I generate with a small wind turbine?
« Reply #9 on: 16/03/2009 10:05:11 »
The United Kingdom Meteorological Office seems to have some considerable trouble predicting the weather in the next hour or two in my experience!

"Earlier on today apparently a lady rang the BBC and said she heard that there was a hurricane on the way. Well don't worry if you're watching, there isn't." said Michael Fish in October 1987.

#### paul.fr

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##### How much power can I generate with a small wind turbine?
« Reply #10 on: 16/03/2009 12:55:21 »
But that was correct, there wasn't a hurricane in the UK. And the lady in question, was a friend of the family embarking on her holiday and was enquiring after the weather there, not at home in the UK.

Edit:
spelling corrected before the fog rolls in ;-)
« Last Edit: 16/03/2009 13:13:36 by Paul. »

#### dentstudent

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##### How much power can I generate with a small wind turbine?
« Reply #11 on: 16/03/2009 12:57:28 »
I thought that it officially was a hurricane?

#### paul.fr

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##### How much power can I generate with a small wind turbine?
« Reply #12 on: 16/03/2009 13:11:10 »
There have been a few references to "the great storm", on the forum over the years and that's what it was "a great storm"

from the Met office:
The Great Storm of 1987 did not originate in the tropics and was not, by any definition, a hurricane - but it was certainly exceptional.

#### lyner

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##### How much power can I generate with a small wind turbine?
« Reply #13 on: 16/03/2009 22:22:01 »
The United Kingdom Meteorological Office seems to have some considerable trouble predicting the weather in the next hour or two in my experience!

"Earlier on today apparently a lady rang the BBC and said she heard that there was a hurricane on the way. Well don't worry if you're watching, there isn't." said Michael Fish in October 1987.

That is truly libellous!!!
I plan my sailing on the available Met information and it is excellent, in general, and pretty good all the time. If you are obsessed with actual precipitation (landlubber, eh?) then you may have problems with the way the BBC etc. say what will happen - too global, I imagine. the weather may take several hours to travel over one TV Region so timing can never be accurate. Go to the BBC local forecast is you want better info.
If you have to go back to 1985 to find a gross error then you have no serious case.

(I am not a member of the Met Office and nor is any of my family)
« Last Edit: 16/03/2009 22:26:01 by sophiecentaur »

#### John Chapman

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##### How much power can I generate with a small wind turbine?
« Reply #14 on: 17/03/2009 00:05:54 »
What I've never understand about wind transferring energy to a turbine is this:

If the wind in front of the turbine is travelling at 30mph then the wind behind the turbine must also be travelling at the same speed or it would back up and have nowhere to go. If the wind behind the turbine is travelling as fast as the wind in front then the turbine must have taken no energy from it.

How does the energy picked up by the turbine translate to a change in wind movement?

#### paul.fr

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##### How much power can I generate with a small wind turbine?
« Reply #15 on: 17/03/2009 01:16:53 »
I plan my sailing on the available Met information and it is excellent, in general, and pretty good all the time...

Andrew, you have an ipod touch or iphone, don't you? If you do there are some tide apps (not sure how good they are) or with a twitter app you can get general weather warning at http://twitter.com/ukmetweather and the shipping forecast at http://twitter.com/shippingcast/

For those interested, this feed contains job adverts and general news releases.

Quote
Don_1
The United Kingdom Meteorological Office seems to have some considerable trouble predicting the weather in the next hour or two in my experience!

Our three-day forecasts are now as accurate as our one-day forecasts were 20 years ago. This improvement in forecast accuracy stems from investment in research, faster supercomputers and greater coverage by observations.

In addition to improving the accuracy of the one- to five-day forecast, research has enabled us to make forecasts that were previously impossible. We can now forecast further into the future allowing regular seasonal forecasts to be produced, and predictions of climate change are continually improving. New research is expected to provide further significant improvements in our ability to forecast heavy thunderstorms a few hours in advance.
http://www.metoffice.gov.uk/science/creating/improvements.html

Back to the original question
Will a wind turbine work for you?
5 March 2009

A new online service highlighting the potential for small-scale wind power generation at specific locations has been launched, following scientific input from the Met Office.

The innovative web tool has been developed by applying our science and follows a study commissioned by the Carbon Trust. By simply entering a postcode, users can choose the relevant type of building and local environment to understand the total power yield of installing a small wind turbine.

The effectiveness of wind generation has been calculated by using historical Met Office climate data, local land use and large-scale orography of the land. If 10% of the population installed turbines, a saving of 0.6 million tonnes of carbon dioxide is possible, generating 1.5 TWh of green electricity. However, it is possible that some urban sites will not pay back the carbon used as a result of the manufacturing, production and installation of the turbine, and this new tool will help people evaluate the potential benefits of using local wind generation.

The carbon prize for rural sites is about four times that of urban sites due to higher wind speed conditions. This is because of the increased friction caused by buildings and other urban structures.

Cathy Durston, Head of Met Office Consulting, said: "By using our historical climate data, people will be able to assess the potential to place a small wind turbine at their location.

"Applying scientific expertise from the Met Office has provided the Carbon Trust with a new way of serving businesses and the public with specific wind data for their property."

Mark Williamson, Director at the Carbon Trust, added: "In the right location, small-scale wind turbines can provide both cost and carbon savings, but anyone considering installing a turbine should use the Carbon Trust wind estimator to check before proceeding with actual on-site wind monitoring. There has been some confusion around the true performance of small wind turbines but this online tool should help to clear that up."

The launch of the new web feature comes as scientists from around the world gather in Copenhagen, for the conference 'Climate Change: Global Risks, Challenges and Decisions'. The Met Office will be presenting work at the conference which has investigated climate model information and how it can be used by the wind energy industry. This includes how climate change may impact upon wind power generation within the UK and across Europe.

http://www.metoffice.gov.uk/corporate/pressoffice/2009/pr20090305.html

« Last Edit: 17/03/2009 01:35:46 by Paul. »

#### lancenti

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##### How much power can I generate with a small wind turbine?
« Reply #16 on: 17/03/2009 03:03:51 »
What I've never understand about wind transferring energy to a turbine is this:

If the wind in front of the turbine is travelling at 30mph then the wind behind the turbine must also be travelling at the same speed or it would back up and have nowhere to go. If the wind behind the turbine is travelling as fast as the wind in front then the turbine must have taken no energy from it.

How does the energy picked up by the turbine translate to a change in wind movement?

I've had trouble with this too, but I think the answer goes something along the lines of the wind doesn't have the same speed before and after the turbine. After all, as the wind collides with the blade, it strikes it at an angle so that part of the energy goes into turning the blade in a certain direction and deflecting the air in another. So it's more of a conservation of momentum thing than speed.

#### syhprum

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##### How much power can I generate with a small wind turbine?
« Reply #17 on: 17/03/2009 09:07:23 »
I did mention in my first post of 11/03/2009 21:22:24 on this matter that the power that could be harvested from the moving mass of air was dependent on the difference in velocity of the incoming air flow and the out going air flow although in later posts I was only interested in calculating how much energy was present in the air stream.
I think the situation is analogous to the electrical power flowing into a resistor from a current source with internal resistance, when the load and source Resistance are the same the maximam power transfer occurs which seems to imply that the maximum efficiency of the turbine is 50%

I feel the situation may be a little more complex as the energy in the air stream is proportional to V^2.
« Last Edit: 17/03/2009 09:20:33 by syhprum »

#### lyner

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##### How much power can I generate with a small wind turbine?
« Reply #18 on: 18/03/2009 00:21:35 »
Where does the air go, instead of backing up?
It just needs to spread out after passing by the turbine.

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##### How much power can I generate with a small wind turbine?
« Reply #18 on: 18/03/2009 00:21:35 »