Wind turbines with many small generators

[Zeynel]

Hello, This is not a homework question. I'm not a student. In fact I'm retired.

The other day, I was passing by a wind turbine. I thought "wouldn't it be better to have hundreds of small generators with small wings connected together instead of one huge generator with huge wings?"

But I did not know how to compute if small generators would be better. How can this be computed? I practically know nothing about electricity.

Thanks for any answer you may have

 

[mfb]

It's not about electricity, it is about aerodynamics and mechanics. The theoretical optimal efficiency (power harvested divided by power from the wind) is 59.3%, this is called https://en.wikipedia.org/wiki/Betz's_law]Betz's[/PLAIN] law. Modern wind turbines are not far away from this limit, and the fraction does not depend significantly on the size of the turbines.
Smaller turbines are stronger (Square-cube law) so their efficiency might be a tiny bit better, but then you need more separate parts, more infrastructure to mount all of them, and so on.

 

[russ_watters]

And more of them. It's all about how much air they capture: if you cut the radius in half, you need four turbines instead of one.

 

[Zeynel]

Thank you for the answers. This is very interesting for me. I will investigate further but can you also help with this question:

I am imagining really small wings. The radius may be as small as 3 inches. The idea is to try to capture the smallest wind. The calculation I'd like to make is to find out how many such small turbines I need to get the same power as one big turbine. How should I approach such a calculation?

 

[mfb]

What is the "smallest wind"? Turbulences? Those don't add much to the energy budget. And rotating every single turbine to be in the ideal direction looks like a huge mess. A support structure every 10 centimeters would reduce the efficiency of the setup significantly.

 

[Zeynel]

No, I don't mean turbulence. I mean wind strength, like in Beaufort scale: https://en.wikipedia.org/wiki/Beaufort_scale

İ'm trying to get some numbers. For instance what is the minimum wind strength that would turn a conventional turbine? What is the smallest wind strength that would turn a minuscule turbine?

Why rotate the turbines? If they are free to turn like a weather wane they automatically be in the ideal position.

 

[mfb]

No, I don't mean turbulence. I mean wind strength, like in Beaufort scale: https://en.wikipedia.org/wiki/Beaufort_scale

Where is the main point in replacing 1 turbine with area A by 100 turbines with area A/100 each then? You don't increase the total area. You just add complexity.

Why rotate the turbines? If they are free to turn like a weather wane they automatically be in the ideal position.

Doesn't work that well for wind turbines.

 

[Zeynel]

One benefit maybe to get more power from light winds. This is what I would like to calculate. Smaller turbines may be producing power in light winds while the big turbine is idle. But I don't how to compute this relation.

Another benefit would be in strong winds when there is too much power. Instead of turning off the entire turbine only a number of small turbines can be turned off to get as much power as needed.

It may be possible to have two layers of turbines. The second set can be downwind catching the wind that passed through the front turbine.

 

[mfb]

One benefit maybe to get more power from light winds.

Why? You keep claiming something like this without any justification.

Smaller turbines may be producing power in light winds while the big turbine is idle.

Why?
Also, power grows with speed to the third power, low wind speeds give a very small contribution.

Instead of turning off the entire turbine only a number of small turbines can be turned off

Why?

It may be possible to have two layers of turbines. The second set can be downwind catching the wind that passed through the front turbine.

That reduces the efficiency of the first one. More complexity, no gain. You cannot avoid Betz's law.

 

[gsal]

WWW.iflscience.com/technology/new-wind-turbine-looks-tree-coming-paris

 

[Jax Dax]

I think the main problem is us tbh. We tend to try build the big picture and forget 2 basic principles. Peak demand vs average demand. We have different uses and should have our circuits isolated and separated.
An average household uses 1000 W for 8 hrs a day, 2000 W for 4 hrs and 5000 W for less than 2 hours a day and on average 20kW a day. This shows we need the systems broken into 2 or 3 different supply sources. Solar creates power when we least need it. Wind is normally morning and evening ( when we need it most) storing the excess power will always be a problem due to current methods of storage
Wind turbines are quite complicated as the rotational speed is uncontrollable and much of the output can not be used.
We need to think out of the box as there is a solution, we have yet to find it.

 

[mfb]

Solar creates power when we least need it.

At least in Europe, electricity demand peaks during daytime, and it doesn't look so different for California, for example.
I don't see how this is related to the topic, however, which compares different ways to harness wind.

 

[DaveC426913]

Smaller turbines may be producing power in light winds while the big turbine is idle.

A 1 knot wind over a 100m² rotor is going to produce essentially the same power as 100 rotors of 1m².

I think you're envisioning a tiny rotor being more able to be moved by a light wind. I'm not sure that's a valid assumption. (However, challenging assumptions is how we invent new things!)

 

[DaveC426913]

Also, power grows with speed to the third power, low wind speeds give a very small contribution.

Perhaps that's the key.

We have turbines for medium wind already; perhaps a complementary turbine - one attuned to light winds - could extract an additional fraction of energy on light wind days. Maybe we'll see wind turbines with one large rotor for strong wind days, and then an array of small rotors up and down the tower's trunk for light wind days.

 

[Jax Dax]

Agree, sorry I am off topic, I am working on the power generation side atm. I am presuming the discussion is about private generation.
During the day most households don't consume much electricity, it is normally 2 to 3 hours before work and after work, heating/cooling is not included as it is seasonal and area specific.
All big wind turbines adjust the blade angle to control the speed of the propeller, these are all expensive and computer controlled. There is the slow and high speed where power generation is impossible to control.
In the domestic market it is different however, and this is where my comments apply. Small users are the key to reduce our footprints, but we can't see the wood for the trees.
I have a design which I think is brilliant but am struggling to get it into the market. There are a number of reasons and one of them is its to small, yet it out performs solar.
It seems everyone wants a unit to produce 100kW all the time even if they are only consuming less on average.

 

[Jax Dax]

A 1 knot wind over a 100m² rotor is going to produce essentially the same power as 100 rotors of 1m².

I think you're envisioning a tiny rotor being more able to be moved by a light wind. I'm not sure that's a valid assumption. (However, challenging assumptions is how we invent new things!)

The problem here is not the wind or the blade, its the drag cause by the generator turbine. In idle mode the blades will turn, when the generator is switched on it causes drag on the propellers. So a smaller turbine with a lower output turbine will turn in the same wind that can't drive a big turbine. Its called the cutting in speed. A generator is sized to the RPM of the blades and wind conditions.

 

[russ_watters]

At least in Europe, electricity demand peaks during daytime, and it doesn't look so different for California, for example.
I don't see how this is related to the topic, however, which compares different ways to harness wind.

That's true in most places: the sun heats buildings, requiring the most air conditioning during the late afternoon.

 

[russ_watters]

...its to small, yet it out performs solar.

In what sense? Solar and wind are so different from each other, it is tough to compare them except perhaps by $/kWh.

 

[Jax Dax]

In what sense? Solar and wind are so different from each other, it is tough to compare them except perhaps by $/kWh.

each have negatives, solar daylight hours and consistency of the wind etc, solar is stable, however wind power is not. If both are in an optimal environment wind generation is the out-performer. I live near the ocean and am investigating and designing a different method of wind extraction for small power. It is a complicated subject and well debated. I however feel we can't see the wood for the trees in this situation.

 

[Jax Dax]

each have negatives, solar daylight hours and consistency of the wind etc, solar is stable, however wind power is not. If both are in an optimal environment wind generation is the out-performer. I live near the ocean and am investigating and designing a different method of wind extraction for small power. It is a complicated subject and well debated. I however feel we can't see the wood for the trees in this situation.

this thread will help you, https://en.wikipedia.org/wiki/Cost_of_electricity_by_source

 

[Zeynel]

WWW.iflscience.com/technology/new-wind-turbine-looks-tree-coming-paris

Thanks! This is exactly what I was imagining doing. Maybe not exactly, because I was imagining regular propellers not something like their nice vertical propellers. I was also thinking to have small turbines attached to each propeller.

But now I'm puzzled more because most of the replies I got told me that small propellers won't work because they make what is simple more complicated. But it seems that small propellers work well and it is good and feasible idea.

 

[russ_watters]

But now I'm puzzled more because most of the replies I got told me that small propellers won't work because they make what is simple more complicated. But it seems that small propellers work well and it is good and feasible idea.

Nobody said they wouldn't work, just that they wouldn't work better than conventional turbines. You should probably try comparing the cost and power output of one of those trees to a single turbine of comparable size.

 

[Jax Dax]

Just to give you an estimate, 1 m2 of air produces about 1kW of power in theory. There are many mitigating factors to be taken into consideration.
Betz law describes it well, for propeller systems. He says all systems can only use 59% of the energy out of the wind. However when it comes to HWAT It seems all theories are vague at best and the calculations mind boggling. It really is a chicken and egg scenario and the man with the money gets what he believes is the best even if its not.
Why do we want big turbines? Its what we taught at school, big is better. Our mindset is the fault not the manufacturers, 1000s of little turbine need a huge maintenance crew, 1 big turbine needs a handful. Would you rather control a work force of say 5 vs 5000? off subject I agree, I apologize.
According to the math a turbine irrespective of size will be as efficient as each other when mounted in good positions. All turbines will use 59% of the energy within reason of course. Therefore all blade driven turbines will operate within those perameters.
I have always wondered why they don't have a row of propeller followed by a row of Savonius turbines.

 

[mfb]

Just to give you an estimate, 1 m2 of air produces about 1kW of power in theory.

That would need more than 10 m/s wind speed on average, between 5 and 6 on the Beaufort scale. Certainly not the average wind speed in most areas.

 

[Jax Dax]

That would need more than 10 m/s wind speed on average, between 5 and 6 on the Beaufort scale. Certainly not the average wind speed in most areas.

Betz law describes it well, for propeller systems. He says all systems can only use 59% of the energy out of the wind.

So in theory it will produce 1kW minus Beltz law and only 0.59kW would be available. But let's not debate about windspeed, what might not be average to you is an average to me. It is all about the location. no windfarm is built in an area that has average windspeed of lower than 15 m/s and most cut in speeds are about 5 m/s.
So because we are not designing a turbine or building one we return to fundamentals. Why is so little effort put into small power wind turbines.
If you have the wind (which clearly Zeynal has) irrespective of the blade size power can be produced. Small turbines with a low cut in speed are available and have proven to be efficient. Correctly placed and installed they have a higher efficiency than the big MW generators.

 

[russ_watters]

Actually, what we are looking for is the rating point (yes, it is at an unusually high speed), since the article gives the power output for, but as far as I can tell, not the associated wind speed. It does say, however:

...each Wind Tree can produce an average of 3.1 kW of power, 280 days of the year, enough to power 15 streetlights.

Although traditional wind turbines can generate 5-6 kW, their large size means they require strong winds to get moving, meaning they generate power for fewer days a year. Also, their huge size - towering up to 120 feet (36 meters) tall with the blade rotary diameter of up to 43 feet - means many find them an unattractive option. In comparison the Wind Trees are a rather petite 36 feet (11 meters) high and 26 feet (8 meters) wide.

Without the wind speed for the rating point (unless someone can find it...?), it is hard to know for sure, but here is a turbine rated for 5 kW at 22 mph wind speed, with a diameter of 20 ft.

http://www.saferwholesale.com/Aero-...IlrY2ChPC11z70VFogXM20cfwmH6GOOrM3BoCparw_wcB
Also:

Each Tree costs $36,500 (€29,500)

The turbine I linked costs $10,000. So you can buy three of them for the cost of one Wind Tree. We can't know for sure how they compare without the rating point for the Wind Tree, but for less than a third the price for the Wind Tree's 5-6 kW you can get one conventional turbine to generate 5 kW while sweeping-out less area. Or buy three for the same price as a Wind Tree.

 

[Jax Dax]

When designing a wind turbine all things are possible. Costs are a grey area and many companies ride the gravy train and art has no realistic attachment to value. I would think to much money is spent to save to little electricity. Example why not put a bigger leaf on top of the light pole. Instead of supplying power for 15 streetlights you have power for 72 or a branch with 4 or 5 for that matter. There are many better options to chose from than the tree imo.
Imo the tree was a government funded project so money and cost were not the objective.

Actually, what we are looking for is the rating point (yes, it is at an unusually high speed), since the article gives the power output for, but as far as I can tell, not the associated wind speed.

The rating point is relative, if you modify blade length, size or configuration the rating point is adjusted. The blade size and angle determines your cut in speeds. Its like a engine a 1000cc vs 1500cc vs a 2000cc motor.
Take the unit that you like for example,
-change the angle of attack of the blade and the rating point will change
-make the blade longer will change the rating point
A turbine will turn at 1 m/s but the generator will produce a 5v @ 100 W ( the numbers are for example purposes only) when it turns at 5 m/s it generates 110v @ 3kW and at 6 m/s it produces 115v @ 5kW, at 7 m/s 180v at which point the power generated is useless and is dumped and the wind turbine tilts.
Because of the aerodynamic effects of the blades, propeller racing can be avoided under all circumstances. (its a quote from the unit you are interested in) This is not possible with current engineering technology available. A mechanical system has to be employed, the turbine blade pitch must be adjusted to prevent propeller racing, thus making the blade smaller or the unit must be furled. There are techniques where a flexible blade is used. Or the generator is used to power an electrical brake system, a major stress to the system.
The turbine that they are selling you. It seems it is a fixed single wound motor. It does not have a gearbox. I would rather look for a dual or triple output wound motor that switches between windings at various speeds that crating a larger window of useful output. (Im sure they must be available, again it is one of my designs and might not be produced in the industry)
A triple wound motor ideally would the produce electricity over a wider range of wind conditions. At 3 m/s 110v @ 1kW , change windings at 5 m/s 110v @ 5 kW and change windings at 8 m/s @ 15kW. The generator side can get very complex with regard the windings and configuration and this will increase the performance 10 fold.
I have looked at a few website selling wind turbines none have impressed me so far, they are all selling a great idea incorrectly packaged. As this industry is extremely complex they are marketing an extremely average system (almost a backyard invention and very basic imo)
Lets break down what they are selling you, I will use top of my head figures here
a 5kW generator @ $ 1000
a propeller and hub @ $ 500
a tail fin @ $ 200
a mounting @ $ 100
slip ring @ $ 200
Charge controller @ $ 500 if that is included in the $ 10k
costs still to come
mast and guy ropes @ $ 2000
Inverter @ $ 1200
Batteries @ $ 100 ea
and an electrician for a few hours
im sorry about the rant but I could go on for hours, as long as we have companies selling fancy words, wind power will get a bad name. It is not the wind that's the problem, its how we use it. And as long as silly trees get funded with tax money and the market is full of bad systems I will continue to pull my hair out.
omg can believe I am posting this, lol

 

[mfb]

Why is so little effort put into small power wind turbines.
If you have the wind (which clearly Zeynal has) irrespective of the blade size power can be produced. Small turbines with a low cut in speed are available and have proven to be efficient. Correctly placed and installed they have a higher efficiency than the big MW generators.

Many costs don't scale well to smaller turbines. A 5 MW generator does not cost 5000 times as much as a 1 kW generator, the power electronics doesn't scale with a factor of 5000, installing 5000 smaller turbines costs more than a single large one, and so on.

1 kW of continuous power generates annual revenues of ~40€/MWh * 1 kW * 1 year = 350 € (give or take a factor of 2 depending on market details), the 1 kW peak turbine will produce less. To get cost-neutral over 10 years (so you can finally start earning money at some point), the whole system including installation and maintenance has to be cheaper than 3500 €. Every inspection is a serious cost factor.
It gets a bit better for 5 kW, but for 5 MW costs just scale better.

 

[Jax Dax]

With present systems on offer I tend to agree. Towers are frightfully expensive. That is why the units I am working on are low velocity roof mounted models. 5 x 1kW turbines excluding the towers makes the pricing very attractive. I think the price range will be about $ 350 - 500 each = max $ 2500 per 5 kW. The electrical installation and equipment, controllers, inverter and batteries will basically stay the same as the standard 5 kW system. As batteries are also a major cost, the time frame harvesting power is important as it can reduce the amount of batteries needed. It is also important to note that with every change in voltage results in a 20% loss of energy and this information is not published by the turbine suppliers, so in basis their 5kW will only provide 3.2kW of useful power. 220v > 24v >220v each phase based on a 80% efficiency on average. This loss is usually blamed on the wind. As its represented that a wind turbine is 40% efficient for example, the truth is the turbine is 60% efficient and current loss by converting the power is a staggering 40%. Another reason for losses is wind > electricity > heat, the 40% efficiency loss rears its head again. I don't think there are heating turbines on the market yet, but imagine how that will help your heating and electric bill.

1 kW of continuous power generates annual revenues of ~40€/MWh * 1 kW * 1 year = 350 € (give or take a factor of 2 depending on market details), the 1 kW peak turbine will produce less.

We need to look at how we consume our electricity. In effect we have all our eggs in one basket, all appliances are based on 220v even if they don't need it. A large percentage of electricity that we use wastes electricity, your downlighters are normally 12v, your computer is 12v, your Laptop is 19v, your TV 5 to 20 v, now add up all the 20% PF losses, that is power you are not using but you are paying for it. Wastage of transformed electricity could be as high as 15 or 20% of your monthly bill. As I said in an earlier post, we need to change our mentality and life style to effect the proper changes, imo its a pointless exercise to spend money on renewables while you are contributing to the problem, the source needs to change inline with the solution. ( If you understand what I mean.)
I suppose what I am trying to say, we need to change the way we consume power for it to be effective. If your minus the 20% + 20% efficiency to your 10 years, the payback will be reduced to 6.4 years, that makes a nice difference to the bottom line.
Hope my calculations are correct, please check them, lol

 

[mfb]

It is also important to note that with every change in voltage results in a 20% loss of energy

Only if the electronics is really, really bad. Even a laptop charger reaches 80% or more with multiple internal conversion steps.

As its represented that a wind turbine is 40% efficient for example, the truth is the turbine is 60% efficient and current loss by converting the power is a staggering 40%.

There is no 60% efficient wind turbine. And even if there would and if the generator would be 100% efficient (it is not), 40% loss would leave at most 36% total efficiency.

Another reason for losses is wind > electricity > heat, the 40% efficiency loss rears its head again.

Electric heating is basically 100% efficient. There is no energy lost to heat ;). A heat pump can reach an even higher efficiency.

Wastage of transformed electricity could be as high as 15 or 20% of your monthly bill.

What do you suggest as alternative? A separate wind turbine and a house-wide grid for every possible voltage some device might need?

 

[256bits]

So in theory it will produce 1kW minus Beltz law and only 0.59kW would be available. But let's not debate about windspeed

At 10m/s 600 w is the power of the moving wind.
Theoretical Betz limit is 59% of that, or 354 w.

For a real windmill, efficiency might be 80% of that or 283 w. extracted as useful power.

With increased wind speed, efficiency of the machine will drop, but since power within the wind increases as V³, and extracted power can also increase, but only if the generator, usage devices, or storage devices can use it. If not, then the rotor speed has to be scaled back, or the excess energy dumped.

With decreased wind speed, efficiency also drops and so does power within the wind. - a double whamy.

 

[Jax Dax]

Only if the electronics is really, really bad. Even a laptop charger reaches 80% or more with multiple internal conversion steps.There is no 60% efficient wind turbine. And even if there would and if the generator would be 100% efficient (it is not), 40% loss would leave at most 36% total efficiency.
Electric heating is basically 100% efficient. There is no energy lost to heat ;). A heat pump can reach an even higher efficiency.
What do you suggest as alternative? A separate wind turbine and a house-wide grid for every possible voltage some device might need?

Short answer is Yes, a 12v and 220v (or something similar) should be standard imo, if we don't stop wasting we will continue overproducing to compensate and we pay for it.
With LEDs there is no reason why lighting should be 220v.
With specially wound generators 12 and 220v is possible (multi taps) generators can supply almost any voltage required.
Understand any power your inverter supplies is 40% less than generated (less but complicated 220>12 = 20% loss, 12> 220v = 20% loss = 37% loss in total and rounded to 40%)

Only if the electronics is really, really bad. Even a laptop charger reaches 80% or more with multiple internal conversion steps.There is no 60% efficient wind turbine. And even if there would and if the generator would be 100% efficient (it is not), 40% loss would leave at most 36% total efficiency.
Electric heating is basically 100% efficient. There is no energy lost to heat ;). A heat pump can reach an even higher efficiency.
What do you suggest as alternative? A separate wind turbine and a house-wide grid for every possible voltage some device might need?

When transforming electricity up or down there is a loss in efficiency, most PF is between 80 and 90%. So your 5kW unit produces 5kW 220v, goes to a transformer and reduced to 12v, only 4kW is received by the batteries, the loss over the transformer is 1kW, that is stored in the batteries, then the 12v is inverted to 220v again another 20% loss. This is all losses we can prevent.

 

[256bits]

I have always wondered why they don't have a row of propeller followed by a row of Savonius turbines.

Savonius turbines are only about 30% efficient, and if the wind has already had say, 50% of its power extracted ( an efficient propeller ) upwind, that leaves only 15% available that can be extracted with the 2nd row.

 

[Jax Dax]

At 10m/s 600 w is the power of the moving wind.
Theoretical Betz limit is 59% of that, or 354 w.

For a real windmill, efficiency might be 80% of that or 283 w. extracted as useful power.

With increased wind speed, efficiency of the machine will drop, but since power within the wind increases as V³, and extracted power can also increase, but only if the generator, usage devices, or storage devices can use it. If not, then the rotor speed has to be scaled back, or the excess energy dumped.

With decreased wind speed, efficiency also drops and so does power within the wind. - a double whamy.

Agree, wind turbine are extremely complicated with current technology, it all looks so easy until you do it. And the salesmen make it sound to good to be true

 

[mfb]

Short answer is Yes, a 12v and 220v (or something similar) should be standard imo, if we don't stop wasting we will continue overproducing to compensate and we pay for it.
With LEDs there is no reason why lighting should be 220v.

You cannot distribute 12 V over a city-sized power grid, losses in the cables will ruin that. It would be possible to have a central converter instead of many smaller ones, but I don't think that outweighs the additional effort of more cables everywhere.

Understand any power your inverter supplies is 40% less than generated (less but complicated 220>12 = 20% loss, 12> 220v = 20% loss = 37% loss in total and rounded to 40%)

It is not. Also, why should the generator output be transformed like that?