# Wind turbines with many small generators

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

## Answers and Replies

mfb
Mentor
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] [Broken] 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.

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russ_watters
Mentor
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.

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
Mentor
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.

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
Mentor
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.

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
Mentor
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.

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
Mentor
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
Gold Member
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
Gold Member
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.

Agree, sorry I am off topic, im 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 cant 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.

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 cant 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
Mentor
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
Mentor
...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 $/kWh. In what sense? Solar and wind are so different from each other, it is tough to compare them except perhaps by$/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 cant see the wood for the trees in this situation.

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 cant see the wood for the trees in this situation.

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
Mentor
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.

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
Mentor
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.

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 lets 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.