David Lock

All this stuff at the moment on alternative energy sources. Everytime I see a wind turbine it strikes me as odd looking as I would have thought that the greater the surface area of the blades the more wind energy the turbine would collect. But they look more like a propeller than a windmill. Obviously they are designed for maximum efficiency so why are they so small? dl

Poor Guy

because you will get them turning with less wind there for more of the time creating electric. also, when the wind is strong it will spin faster. perhaps the radius of gyraion on the mast which supports it would become so large that it would fall down/shear with larger blades in the wind.

just guesses. i may be wrong.

sti-04!!

The blades are also made from fibreglass, the structure to support is steel.

Being fibreglass obviously makes them lighter, thus turning quicker & supplying more energy

ALi-B

iTrader: (1)

It is interesting though...After looking at the secondary stages of a gas turbine generator/pumping unit. It is driven by pure thrust i.e There no physical connection of the turbine shaft to the shaft of the gererator/pump shaft.

The blades of this unit although "driven" by thrust rather than providing it, resemble and aren't too different from that found at the instake stage of a conventional aircraft turbofan engine.

My reasoning, is that these units in comparison require high amounts of torque to be driven, which is provided by large amounts of thrust, due to the casing, this thrust cannot go anywhere except through the blades. With a wind turbine, wind doesn't provide enough thrust, so therefore you cannot use steep angled multiple blades, as a without a casing, a gentle breeze will just go around the all of blades altogether rather than going through them.

For a multiple high density blade setup to work you would need a large trumpet like ram pipe which would collect wind from a large area and funnel it to a smaller diameter, increasing speed and thrust, and also preventing wind from going around the blades. A hill full of huge trumpets would be an interesting sight to see The use of fewer, but much longer blades, allows wind which hits each blade to travel along the length of the blade, so maintaining a drive force on the blade, negating the need to channel and concetrate air into a smaller area.

Also it provides a more constant drive....as wind speed that is too fast (causing sheer stress, and overspeed etc )will just bypass the blades, yet low wind speeds will still provide enough power to turn them.


edit: Although it has just ocurred to me that they do use use high density small diameter multiple blades on water/irrigartion/well pumps on farms. Which goes against what I have typed.....but everytime I have driven past one..they are never spinning, which might say something

Flatcapdriver

Possibly a higher angle of attack with smaller surface area would prevent stalling?

David Lock

Thanks for that. I have more than a passing interest. Some years ago I was involved in a project using windmills for rural water supply extraction in Africa. Building the things with local materials where possible. But the big problem was getting them to work in light wind conditions.

When I drive over the Severn Bridge which is usually windy I think "why don't they suspend a couple of turbines under the bridge?" Would provide energy to keep the bridge lit and a bit extra and largely hidden from view.

Brendan Hughes

....and chew up passing river traffic

warrenm2

Right can actually answer this with some level of knowledge (good old Mech Eng degree!)

You describe a modern turbine with low solidity (where solidity = blade chord x no blades / 2 Pi R). Modern turbine blades are in fact aerofoils, their rotational effort is derived from the lift from the blades. Long thin blades have low drag and high lift (think albatross and round the world aircraft). In addition, blades need to be spaced out so as to avoid the wake from the preceeding blade. Wake interference reduces lift and hence power output. Lastly having fewer blades reduces costs. Power extraction at design speed is roughly 40% of wind energy (cf theoretical max [Betz] of approx 59%). However dont forget the wind speed is not uniform so designs have to operate over a wide range of wind speeds and so are a comprimise to some extent. Tip speed of the rotors is also very important in design, cost and efficiency. Concentrators and ducts are not really commercially advantageous, there is no real advantage in power output/cost - you might as well build a turbine with that swept area in the first place.

David Lock

Thank you very much. I'm impressed

farmer1

However the question should be why bother with windpower at all, its power production is negligable and they are only put up for private companies to make money and for the government to look like they are eco friendly.

David Lock

My preference is for solar energy but the basics (panels etc) seem to be so damn expensive.

Flatcapdriver

At present, there is no single solution to our future energy demands whether it be natural alternatives such as wind, wave or solar production or gas, nuclear, oil or coal electricity production. Therefore, it is neccessary to invest in possible future solutions such as wind farms and at 2 megawatts apiece, they'll keep a few houses going.

Scrappy9

My understanding was that the two wind turbines in Norfolk on the A47 meet a significant amount of the electricity needs of nearby Swaffham.

I am not sure where I heard this, so I stand to be corrected.

Leslie

The blades can be compared to aircraft wings as mentioned above. Low speed aircraft perform more efficently with long thin wings. More lift and less drag is generated from this type of wing which is described as High Aspecr Ratio referring to the ratio between the length of the wing, or in this case aerofoil, and its chord length, ie the distance from the leading edge of the blade to the trailing edge.

The lift/drag ratio is improved thus improving efficiency at the low speed that the rotors travel at.

Angle of attack is not affected with respect to the stalling angle very much. A slender delta wing will stall at a higher angle of attack due to the spanwise vortex that is generated, ie the Concorde, or the Vulcan to a lesser extent and some of the modern delta wing fighters which can still fly at much higher apparent angles of attack. Straight wing aircraft,or in this case the rotor blades, will all stall at around 15 degrees angle of attack or thereabouts unless they use artificial methods such as boundary layer control to delay the stall.

Les

jbryant

Scoobynet - the font of all knowledge. I used to use www.wikipedia.com, but don't need to any more

You guys rock !

cheers
Joolz

106rallye

here is another question - how much energy does it take to make a wind turbine + put it up and plug it in to the grid?

or in other terms how long before it has broken even?

As a side note you can not imagine how big these 'blades' are till you have seen them on the docks in Southampton!

Andy

warrenm2

break even is 9-12 months

JamieMacdonald

I heard the other day that these are designed to keep the propellor tips below the speed of sound - any truth in that?

David Lock

When parked out at sea which is the best place for them they do an enormous amount of damage to sea birds as one can imagine

OllyK

Errr - no.

OllyK

My slightly bigger concern would be the cost. I assume we can't put them close to the coast, due to the effect they may have on silting patterns and such. So we need to put them out to sea a good bit. That's going to involve an awful lot of concrete to get them out of the water, or some massive floating peirs.

David Lock

May be there will be some 1/2 price oil platforms available when the oil runs out

ALi-B

iTrader: (1)

Indeed, I would like to re-roof the house entirely with solar panels. But the cost is far too much and the companies providing it insist that you sell ALL power generated to the national grid at a fixed rate (not via a metre). So you still have to remain connected to the national grid and buy back your own power as well as any extra via the usual metre Sounds like a poor deal to the investing customer, because I'm sure they buy your electricty at a much lower rate than what they charge you to buy it back again - and you even get the power cut off when there is a power cut

David Lock

Didn't know that Ali-B. I guess they must subsidise it then to give them those rights? It's not their bloody sunshine is it

And yet here we are in 2005, half of Africa is starving and the place is bathed in sunshine for nearly half the time and I've hardly ever seen a solar panel on any of my trips there. Just a bit of power, albeit intermittent, could make so much difference. Water extraction, communication, may be transport, power for small health centres etc. And all we can do is give then some wind-up radios. dl

OllyK

Things may have moved on a bit since I looked in to, but basically you have 2 types of solar panel. The type that you pump water through and hook up to heat exchangers and such to provide a hot water store for heating and hot water. This works great in the summer in this country, but not too well in the winter when you really need it. Cost wise this isn't too prohibitive.

The other option is the type of cell you have on a solar powered calculator. These are comparatively expensive, innefficient and degrade pretty quickly - especially when outside. When I looked in to it, they used more energy to produce then they could ever get back from the sun, the efficiency dropped very rapidly unless the panels were cleaned frequently (we're talking weekly here), they had an end user pay pack period of about 90 years and an effective life of about 10. So in terms of environmental salvation you'd be better off burning wood. I expect there have been gains in efficiency over the years - but I suspect the climbing on the roof to polish the panels and the lack of good sun light will remain an issue.

David Lock

Thanks Olly,

This is quite interesting as a starting point.

http://www.darvill.clara.net/altenerg/solar.htm