# Wind power or energy waste?

Discussion in 'General Science & Technology' started by dixonmassey, Dec 8, 2006.

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1. ### dixonmasseyValued Senior Member

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I've made the simplest estimates of the energy efficiency of a windmill. Modern windmill weighs approximately 20 tonnes. Let's assume those 20 tonnes are just a big chunk of steel. To produce a tonne of steel one needs approximately 30GJ of energy. Let's assume winmill power is 2MW. Then in the best case a windmill produces 2*365*24=17.5 GJ per year. Therefore, just to get even a windmill must work for approximately 40 years uninterrupted. So where is energetical point of so called wind energy? Without abundant supply of fossil fuels to manufacture a mill wind energy is deadborn. It doesn't appear to be an energy "source". Am I wrong?

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3. ### guthrieparadox generatorRegistered Senior Member

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Thats nothing like the figures promulgated by wind power people. I dont have them to hand, but they suggest a pay back period of 2 or a few more years. Where do you get the 30Gj for the steel?
Also, is that 20 tonnes for a commercial 2MW wind turbine? I'm sure you could find out the actual weights of real turbines with their rated capacity online somewhere. More accurate figures makes a more persuasive calculation.

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5. ### dixonmasseyValued Senior Member

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It's not surprising.

http://www.eere.energy.gov/industry/steel/pdfs/theoretical_minimum_energies.pdf

Just add all the numbers in the first column of table 12 (and that's going to be rather low end estimate).

Windmill weighs approximately 20 tonnes, meaning total weight of everything. http://www.madehow.com/Volume-1/Wind-Turbine.html

That everything includes all the machined parts (machining is far more energy intensive than smelting), tower, etc. One must include the energy needed to transport, assemble, maintain a mill in the energy balance. Also, people who do the work need to be fed and transported (that's also energy). So not to go in the deep details of the energy balance I just assumed the wind mill to be a big chuch of steel (It's OK for a rough estimate, so I think). Also, I assumed that wind mill is going to work 24/7 for many years at maximum power and no breakdowns, which is really unrealistic. If anything my number should not be 100 times different than the real thing. My guess is that 40 years non stop work to get even is rather an optimistic number.

That's what professionals are for. Unfortunately, they just follow the money and will sing whatever that will unite them with that money.

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7. ### dixonmasseyValued Senior Member

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Darn, forgot about 3600 factor. If it's considered than one needs roughly 1 year of 0.5MW windmill operation to make 20 tonnes of steel. Getting old.

8. ### Baron MaxRegistered Senior Member

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You might try the National Geographic site ...they had a long, involved article about power generation in one of the last few issues. It was highly interesting, but the main conclusion was that nuclear energy was the "hope of the future" ...with all things considered. Check it out ...I don't know if it's on the web, but it's surely at your local library.

Baron Max

9. ### leopoldValued Senior Member

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first of all i seriously doubt if a airplane propeller weighs 20 tons.

10. ### scorpiusa realistValued Senior Member

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why make 20 ton windmill?
why not make a smaler one that would fit on the roof of a house, that way every household could be self sufficient and independent of the comercial grid and the supliers who will always charge way too much

Please Register or Log in to view the hidden image!

heres one for ya
www.windside.com

11. ### dixonmasseyValued Senior Member

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20 tonnes is kind of small, therer are 200 tonnes monsters. Why so big? Because windmill's maximum power is roughly a function of blade's length. Blade length of 40m will give you maximum power of 0.5MW. The thing you can fit on the roof of your house will have enough power for a light bulb.

12. ### draqonBannedBanned

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dixxonmassey...the point here is that windmills are not made of steel...the energy to make that which the windmilles are made of are applicable with understanding that all is made in large quantities and thus is cheaper to make.

13. ### RomanBannedBanned

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There are models that show that the current level of human consumption will never be inline with capturable, renewable energy. That means we can never live in harmony with the environment in our current capacity, or anything resembling it.

After we deplete fossil fuels, we turn to biofuels, and after those deplete, it's the end of civilization as we know it. Probably the end of civilization.

Those models are highly unpopular.

14. ### draqonBannedBanned

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Wars take care of that.

15. ### dixonmasseyValued Senior Member

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As I said, it's a very rough estimate of the energy involved into a building a windmill. It's very conservative estimate, very crude, but that's what are estimates for. Sure windmill consists of thousands mostly nonsteel items, total weight of which is 20 tonnes, for example. How to estimate the energy which went into production of raw and processed materials, machining and assembly of parts, transportation, and all the rest. All those things take much more energy than required to produce equivalent mass of steel. So if one imagine a windmill as a big chunk of steel, the energy to produce that 20 tonnes chunk of steel will be skewed towards the minimal possible energy for construction of a real windmill. If one will consider that wind is not blowing 24/365 (let it be 35% of the time) + practical windmill efficiencey is around 42% my previous estimate will be corrected to 4 years of work (0.5MW windmill) just to make enough energy for its construction out of a big chunk of steel. That's minimal time. My estimate is really +/-10000% kind (but I'm betting on -).

"Are renewable sources viable without any input of fossils" - that's very important question, which is being ignored. People just forget that it takes humongous amount of energy to produce renewable sources, and that energy comes from fossils. For the energy savings sake, it's possible that life without renewable sources (on grand scale, application niches always exist) will spare environment more than those sources themselves.

16. ### dixonmasseyValued Senior Member

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There is no question in my mind, humanity will either invent the warp drive and spread the plague to other planets, or it will significantly adjusts its numbers (up to absolute zero). I don't blame humanity though. Any beast in the nature would do just the same. All we do is 100% natural, we just have more tools to fulfil grand nature design than the rest of maggots elking a share of sun's photons for themselves.

17. ### draqonBannedBanned

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yeah ok...so windmill takes ages to repay for itself...but ages pass...and there it is...free energy.

Am I the only one here...or has anyone thought of combining the fins of windmill and solar cells? so it captures solar energy and wind power. Also...I am sure efficiency of windmill increases every year or so...with all the technology developments...

18. ### dixonmasseyValued Senior Member

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Unfortunately, the maximum operation age of a modern windmills - 20 years or so. We can't count on ages. Windmill should make more energy (at least 5 times more) than it was spent on it in 20 years, or it shouldn't be built for the environment sake. Making eternal windmill is another option, but mother nature doesn't like anyting eternal.

I'm sorry to dissapoint you, but maximum theoretical efficiency of a windmill - 59%, modern windmills, as I said, have 42% efficiency. I doubt 17%efficiency boost will be critical for windpower success.

19. ### leopoldValued Senior Member

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untrue.
a unit no larger than a pair of shoes can generate enough power to use a CB base station by hand power alone.

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21. ### MetaKronRegistered Senior Member

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You are wrong. A watt-hour is exactly 3600 joules according to Wikipedia. That means that a 2 megawatt wind turbine produces 7.2 gigajoules of energy every hour. Using your figures, it's 600 gigajoules of energy for the steel for the windmill, so that's a payback time of about 83.3 hours at full capacity. That's less than 4 days.

Another way to look at it is that at 5 cents per kilowatthour, that wind turbine produces 100,000 USD worth of electricity every hour it is running at full power. In 83 hours it produces about 8.3 million dollars worth of electricity.

22. ### draqonBannedBanned

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I exist in this world for one purpose only, take away.

23. ### dixonmasseyValued Senior Member

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Who said that windmill on your roof can be much more powerful than your hand? Energy density of wind is a function of windspeed only. So, at a given wind speed, the energy density is a constant. It's not a very big constant of nature, no techology can change that. For example, to capture 0.5MW of power one needs mill blades wich makes a circle with radius of approximately 40m. In the roughest approximation, the mill power is proportional to the area of "blade" circle. So let's make an estimate. How big a blade of a roofmill can be made without turning a home into windmill? Let say it's 1m long (and even that most likele will be annoying as hell).

pi*40*40 - 500,000
pi*1*1 - X

X=500,000/(40*40)= 312.5W. It's enough of power for three good lightbulbs (that's assuming good wind is blowing).

Last edited: Dec 14, 2006