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Slypork
11-30-2014, 11:07 PM
I have a couple quick questions about wind turbines used for generating energy and hope they can easily be answered.

1. I only see wind turbines in the 2 or 3 blade propeller design. It seems like a vertical axis turbine would be more efficient. It doesn't have to turn into the wind and could probably be enclosed in some kind of screening to prevent birds from being killed. Is the standard bladed version better or is there a cost reason why the vertical one isn't used?

2. Could wind turbines placed alongside highways generate enough electricity off the wind produced by passing cars to light streetlights?

engineer_comp_geek
12-01-2014, 01:54 AM
1. They do make vertical axis wind turbines. As they get larger, the variations in torque on the blades throughout the turbine's rotation get very significant. This requires much stronger materials than a conventional 3 blade turbine. Newer designs have a more helical shape which helps to reduce this, but you've still got the additional cost of stronger materials. They have also had problems in the past with variations in torque load on the blades causing them to eventually fail.

2. No. Cars and trucks produce "wakes" through air kinda like boats through water. The air moves back and forth a bit but they don't really produce a constant "wind". There's not much energy there to harness.

Reply
12-01-2014, 02:11 AM
I was going to page engineer_comp_geek, but he found the thread :D

bob++
12-01-2014, 04:05 AM
Think about it. Those guys back when they built windmills from stone and wood, found out, probably by experience, that the standard sails-facing-the-wind type gives you the most watts for your money. There were 'vertical' windmills in some countries, but they never caught on.

I just thought - I bet those 'vertical' ones were more likely to blow down in a gale too.

Ken001
12-01-2014, 04:55 AM
Think about it further. Blow a full breath of air at an unsuspecting friend. He'll be surprised but he might not even notice. Now throw a glass of water at him. The impact is obvious.

Water has magnitudes more energy than air.

Wind turbines have their place but only where there is a constant reliable wind which translates to only a few places around the world.

Solar energy has good possibilities now that more efficient panels are being made but only in sunny places and you better be up for the cost of batteries and inverters. The panels are the cheap bit.

Hail Ants
12-01-2014, 06:40 AM
Also, the 'screening to protect birds' is both impractical & unnecessary. Its impractical because because it would cause too much air resistance, and unnecessary because the number of birds killed is negligible,

ZenBeam
12-01-2014, 06:44 AM
Think about it. Those guys back when they built windmills from stone and wood, found out, probably by experience, that the standard sails-facing-the-wind type gives you the most watts for your money.They also had four blades, not the three we typically see. Just because they did something like use four blades back when doesn't mean it's the best approach given modern materials and design.

bob++
12-01-2014, 07:10 AM
They also had four blades, not the three we typically see. Just because they did something like use four blades back when doesn't mean it's the best approach given modern materials and design.

Some had more but I think that this is a reflection of the relative inefficiency of the sails themselves and the limitations of available materials. The arch bridge was the best design for stone - the development of high tensile steel allows us to build suspension bridges.

Slypork
12-01-2014, 08:09 AM
1. They do make vertical axis wind turbines. As they get larger, the variations in torque on the blades throughout the turbine's rotation get very significant. This requires much stronger materials than a conventional 3 blade turbine. Newer designs have a more helical shape which helps to reduce this, but you've still got the additional cost of stronger materials. They have also had problems in the past with variations in torque load on the blades causing them to eventually fail.

2. No. Cars and trucks produce "wakes" through air kinda like boats through water. The air moves back and forth a bit but they don't really produce a constant "wind". There's not much energy there to harness.

Thanks for the info. I always wondered about them, thinking you could place more in the same footprint as the standard turbines and increase energy generation/area. I mentioned the bird strikes because I keep seeing that as a complaint as to why some people don't want wind turbines to be built.

ZenBeam
12-01-2014, 08:54 AM
Some had more but I think that this is a reflection of the relative inefficiency of the sails themselves and the limitations of available materials. The arch bridge was the best design for stone - the development of high tensile steel allows us to build suspension bridges.Yes, just as the development of newer materials may make vertical windmills more feasible and more attractive. It may never happen that they become the best design, but you can't just look at old-fashioned wood and stone windmills and say that that's the case.

Ethilrist
12-01-2014, 08:57 AM
Also, the 'screening to protect birds' is both impractical & unnecessary. Its impractical because because it would cause too much air resistance, and unnecessary because the number of birds killed is negligible

That turned out not to be the case in Altamont Pass:
The small turbines used at Altamont are dangerous to various raptors that hunt California Ground Squirrels in the area. 1300 raptors are killed annually, among them 70 golden eagles, which are federally protected; in total, 4700 birds are killed annually. Overall there has been an 80% decline in golden eagles in Northern California, with no golden eagles nesting near the facility, although it is a prime habitat. Considered largely obsolete, these numerous small turbines are being gradually replaced with much larger and more cost-effective units. The larger units turn more slowly and, being elevated higher, are claimed to be less hazardous to the local wildlife. This claim is supported by a report done for the Bonneville Power Administration.

As of 2010, a settlement has been reached between the Audubon Society, Californians for Renewable Energy and NextEra Energy Resources (who operate some 5,000 turbines in the area). Nearly half of the smaller turbines will be replaced by newer, more bird-friendly models. The project is expected to be complete by 2015 and includes $2.5 million for raptor habitat restoration.[6]

http://en.wikipedia.org/wiki/Altamont_Pass_Wind_Farm

Chronos
12-01-2014, 11:07 AM
Windmills are spaced the distance apart they are not because there's no room to pack them any tighter, but because at their current spacing, they're extracting as much energy from the wind as they can. If it takes more vertical-axis windmills to extract that amount of energy, then that's a point against them, not in their favor.

Ken001, you're right that it's a lot easier to extract energy from moving water than from moving air. That's why we've already tapped all of the good sources of moving water. But there's not enough moving water to meet all of our energy demand, so we have to turn to other sources as well, even though they're not as good.

johnpost
12-01-2014, 11:45 AM
when placed at low elevations and homemade or crude the vertical axis machines have value. you can use heavy material for the support, you could support both top and bottom.

this style gets used where lower amounts of power is needed and only heavy materials is available or lower costs are required. it is used worldwide.

height is important for higher yields of power. bladed machines do well if lighter weight high tech materials are available.

UncleRojelio
12-01-2014, 01:20 PM
There is an alternative energy firm here that has a bunch of its products arrayed on and around its building. One of the products is a rather large vertical wind turbine. Over a year ago one of the helical blades snapped off and the turbine has been motionless ever since. Apparently, even the prospect of the negative advertising generated by displaying a broken product in the front yard is not incentive enough to repair it.

Deeg
12-01-2014, 03:26 PM
I have a question: why are fewer, longer blades better than shorter, wider blades?

Machine Elf
12-01-2014, 03:55 PM
I have a question: why are fewer, longer blades better than shorter, wider blades?

Turbine blades are basically wings. Wings with high aspect ratio (large span, small chord) are more efficient than wings with a short aspect ratio:

http://en.wikipedia.org/wiki/Aspect_ratio_(wing)

Wingtip vortices (http://en.wikipedia.org/wiki/Wingtip_vortices) (or prop-tip vortices (http://en.wikipedia.org/wiki/File:DN-SD-06-03008.JPG), for a propeller/turbine)represent an inefficiency; if a wing or prop blade must have that tip vortex as a fixed waste value, then you maximize efficiency by making the wing/prop blade as long as possible.

This is why gliders have long wings with a short chord. Commercial airliners aren't too different from that idea (you need to be efficient to cover thousands of miles without refueling), but they make compromises for load-carrying ability and the aerodynamics of high-speed flight. Fighter aircraft need to tolerate the extremely high loads encountered during aerobatic maneuvers, so they pretty much have to go with low aspect ratio wings (small span, large chord), efficiency be damned. And so it is with wind turbines: longer turbine blades with shorter chords are more efficient.

As to the # of blades, there's a theoretical upper limit (http://en.wikipedia.org/wiki/Betz%27_law) for how much kinetic energy can be harvested from a stream of moving fluid. Modern 3-bladed horizontal-axis wind turbines are already achieving around 80 percent of this limit; I suspect the economics don't justify the addition of more blades to reach for just a thin slice of that final 20 percent.

Amateur Barbarian
12-01-2014, 04:05 PM
They also had four blades, not the three we typically see. Just because they did something like use four blades back when doesn't mean it's the best approach given modern materials and design.
It's interesting how many times the same thing gets re-invented. The last great heyday of wind generators was the 1970s, when every commune was rediscovering what the makers of Winchargers in the 1930s had already learned. I realize that things like computer modeling and engineering analysis can carry things further and perhaps more easily, but the basics are still old news - if any bright young UC researchers care to look.

The optimal number and orientation of the blades haven't changed, for example. The Albers did extensive experimentation to discover that three blades were optimal and caught as much wind as could be caught - more blades just dragged down efficiency and the vertical orientations were only suitable for small-scale because of the material strengths needed.

They knew all this in the early 1930s. :) Original Winchargers are still highly prized.

engineer_comp_geek
12-01-2014, 04:07 PM
I have a question: why are fewer, longer blades better than shorter, wider blades?

It's all about maximizing efficiency.

You need at least two blades to create a balanced turbine. As you add more blades, you basically add in the cost of each blade, but the gain in performance you get decreases with each additional blade. In other words, if one blade costs X, then your minimal two blade system costs 2X just for the blades. Adding a third blade costs 3X and gives you a decent boost in performance, but adding a fourth blade costs 4X and does not give you anywhere near double the performance of the two blade system.

Additionally, each blade disturbs the air for the blade that follows it. The more blades you have, the more you lose efficiency due to the turbulent air.

These things push towards fewer and thinner blades.

However, there are some design considerations that push things the other way as well. Fewer and longer blades may be more efficient, but you can't just keep making longer and longer blades out of the same material or it will flex more and eventually break. Longer blades also require a longer mast so they don't hit the ground (which again requires stronger materials or a different design), and if the blades are too long the tips can start to go supersonic, which adds drag and totally screws up your efficiency. So these considerations push for a larger number of shorter blades.

The sweet spot that tends to give you the best efficiency for the least cost is the current three-blade design with fairly long and narrow blades.

Deeg
12-01-2014, 04:29 PM
Interesting. Thanks Machine Elf!

ETA: and ecg!

The Second Stone
12-01-2014, 04:54 PM
There are roughly a dozen pairs of nesting golden eagles in the Altamont pass area. The figure of 70 killed annually is bullshit, even when the windmills were the small very fast moving variety. That figure multiplies to total population by three and then assumes annual full extinction. I go bird watching on the pass the past 5 years on a monthly basis, and all the birds know how to handle the newer and larger windmills and even play and harass each other in their wakes. Goldens are very territorial and their number limited by area.

Propeller type windmills have a significant advantage over vertical windmills in that they can "feather" so that the blade may rotate on its axis to be more or less efficient in different wind conditions.

Xema
12-01-2014, 04:58 PM
That figure multiplies to total population by three and then assumes annual full extinction.
Or, possibly, that Golden Eagles migrate.

The Second Stone
12-01-2014, 06:57 PM
Or, possibly, that Golden Eagles migrate.

Some migrate, but most remain in a territory. http://en.wikipedia.org/wiki/Golden_eagle

Xema
12-01-2014, 07:52 PM
Some migrate, but most remain in a territory. http://en.wikipedia.org/wiki/Golden_eagle
From your link:
the entire population of golden eagles from northern and central Alaska and northern Canada migrates south. At Mount Lorette in Alberta, approximately 4,000 golden eagles may pass during the fall, the largest recorded migration of golden eagles on earth. Here the mountain ranges are relatively moderate and consistent, thus being reliable for thermals and updrafts which made long-distance migrating feasible.Birds hatched in Denali National Park in Alaska traveled from 818 to 4,815 km (508 to 2,992 mi) to their winter ranges in western North America. These western migrants may winter anywhere from southern Alberta and Montana to New Mexico and Arizona and from inland California to Nebraska.

The Second Stone
12-01-2014, 10:29 PM
and if any by coincidence came to the Altamont Pass, they had to keep moving because the range is occupied the entire year around by full time inhabitants. An eagle that took more than a hour to move through the area at less than can't be seen by human altitude is going to find a territorial fight on its hands.

I see a lot of birds of prey dead at the bottom of electrical utility poles. Not around wind mills.

Take a look at the map on the article. Virtually no goldens are California part time residents.

levdrakon
12-01-2014, 10:37 PM
Wind turbines have their place but only where there is a constant reliable wind which translates to only a few places around the world.

Solar energy has good possibilities now that more efficient panels are being made but only in sunny places and you better be up for the cost of batteries and inverters. The panels are the cheap bit.1970's called. Higher altitude winds are increasingly accessible and that's most of the planet, not "a few places."

Germany is sunny all the time? Who knew? I lived there and never noticed.

Ken001
12-02-2014, 01:57 AM
1970's called. Higher altitude winds are increasingly accessible and that's most of the planet, not "a few places."


Yes quite right and my bad. :smack:

I was thinking about tidal currents going through straits which could be used for generating electricity. There are not many places around the globe where the flow is strong and consistent enough and close to population centers.

Ethilrist
12-02-2014, 07:21 AM
I have a question: why are fewer, longer blades better than shorter, wider blades?

From the point of view of the birds, slower blades are easier to spot & dodge, as indicated by The Second Stone. Also, with the axis higher off the ground (and above the bird's cruise altitude), the blades spend more time in air that won't affect the birds.

Xema
12-02-2014, 08:54 AM
and if any by coincidence came to the Altamont Pass, they had to keep moving because the range is occupied the entire year around by full time inhabitants.
Moving is what migrants do. You surely aren't saying they can't pass through areas where year-round birds are found, are you?[/QUOTE]

Xema
12-02-2014, 09:22 AM
You need at least two blades to create a balanced turbine.
Not really - here's a single-blade wind turbine (http://www.mnn.com/earth-matters/energy/photos/9-ingenious-wind-turbine-designs/powerhouse-thinair-single-blade-turbine).


if the blades are too long the tips can start to go supersonic
Probably impossible, in any practical sense. Most wind turbines operate with tip speed ratios under 10 (i.e. tip speed is less than 10 times wind speed) - and of course they must feather when wind gets really strong.

Here's a link to a list of common industrial wind turbines (http://www.aweo.org/windmodels.html). Max blade tip speeds range from 138 to 232 mph. I'd guess that the blades of every one of these would destroy themselves at less than 1.5 times max speed. (It would make no sense to incur the cost of producing a blade able to operate far outside its turbine's design limits).

Chronos
12-02-2014, 09:48 AM
While a one-blade turbine might be possible, it sounds like a lose-lose proposition to me. You still need some sort of counterweight, and that counterweight is going to be adding moment of inertia and drag without adding any collection area to make up for it.

Stranger On A Train
12-02-2014, 02:07 PM
Rotor blades are not only designed to balance the "rigid body" (non-flexing) moments of inertia, but also balance the modal (flexing) modes to reduce fatigue stress and unbalanced torsional modes. In other words, a single blade is going to flap like a flag as it rotates with the torque at the root being only resisted by the geometric section of the pylon. With two or more evenly spaced blades and appropriate aerofoil design the parasitic and potentially destructive modes can be minimized by making the rotational center also the inflection point for the primary modes. It is possible to design a balanced single blade rotor as show, but it compromises maximal energy recovery for other benefits (lower parts count, less effective mass, et cetera).

Stranger

eschereal
12-02-2014, 03:12 PM
I guess one big drawback to vertical axis turbines is that they are a little less than 50% effective, as the part that captures the wind force must move upwind on the back side. Horizontal axis turbines are capturing wind power more or less evenly for the full cycle of each blade.

Xema
12-02-2014, 03:43 PM
Rotor blades are not only designed to balance the "rigid body" (non-flexing) moments of inertia, but also balance the modal (flexing) modes to reduce fatigue stress and unbalanced torsional modes. In other words, a single blade is going to flap like a flag as it rotates with the torque at the root being only resisted by the geometric section of the pylon.
In the case of the single-blade turbine mentioned above, this issue is neatly handled by means of a "teetering" hub (http://www.powerhousewind.co.nz/products.htm) - the blade is free to fold in or out as the airflow dictates, so forces on the pylon are low.

As the pictures show, this also allows for passive 'feathering' in strong wind.

Xema
12-02-2014, 03:58 PM
You still need some sort of counterweight, and that counterweight is going to be adding moment of inertia and drag without adding any collection area to make up for it.
The counterweight looks plenty small enough that the drag would not be a big deal. And additional moment of inertia probably isn't a big problem in a device that is typically trying to operate at a constant rpm.

Offsetting these issues are the cost savings of a single blade and the fact that it gets to operate in air that hasn't been disturbed other blades.

The Second Stone
12-02-2014, 04:14 PM
Moving is what migrants do. You surely aren't saying they can't pass through areas where year-round birds are found, are you?[/QUOTE]

If they are not seen, yes. Goldens are highly territorial and they will fight each other. Golden eagles were used in Asia to fight wolves. Everything else in the sky backs off when they assert dominance. If they wish to pass through, they had best be quick about it and not seen and have a long head start and altitude if they are seen "passing through". Sedentary golden eagles are in pairs that mate for life, so there is at least one ally nearby.

Ethilrist
12-03-2014, 07:54 AM
Okay, that's good to know... however, the number of dead golden eagles found at Altamount has dropped since they stopped using the older, shorter, faster turbines. Are you saying that all the migrating eagles got killed by locals and the reports that they're migrating are wrong?

Xema
12-03-2014, 10:22 AM
Goldens are highly territorial and they will fight each other.
They will if they perceive a valid threat - e.g. another eagle hanging around, poaching prey or looking to encroach on their territory. But it would be of less than no value to pick fights with birds that are moving through anyway, posing no threat.

The article you linked makes it clear that plenty of migration passes through areas where some golden eagles are resident.

MikeF
12-03-2014, 01:04 PM
Back to blade design - why do some blades have a "dip" in the face of the blade towards the tip? I don't know how else to describe it. I couldn't find an image that shows it well but it pretty obvious on the turbines near me.

Bill Door
12-03-2014, 01:46 PM
I have a question: why are fewer, longer blades better than shorter, wider blades?

Not disagreeing with what engineer_comp_geek said, just expanding on it. It's a question of solidity, which is the ratio of the total area of the blades compared to the area swept by the blades. A high solidity turbine, think of the old Dutch windmill, spins slowly with high torque. Very useful for applications like pumping water where high torque is needed. Low solidity turbines spin rapidly with less torque which is what you want for generating electricity.

I suppose you could use a slow speed high torque arrangement and a gearbox, but that introduces extra expense and inefficiencies in power transmission.

Machine Elf
12-03-2014, 02:05 PM
Back to blade design - why do some blades have a "dip" in the face of the blade towards the tip? I don't know how else to describe it. I couldn't find an image that shows it well but it pretty obvious on the turbines near me.

Something like the raked wingtips on the Boeing 787? (http://en.wikipedia.org/wiki/Raked_wingtips#Raked_wingtip) For aircraft, it reduces inefficiencies associated with tip vortices, and iincreases lift without increasing span (causing a smaller increase in wing bending loads than one would observe with a simple increase in wingspan). Presumable the same is true for wind turbines: reduced tip-vortex losses, and increased rotor torque without the bending loads that would be associated with using just a larger-diameter turbine.

Chronos
12-03-2014, 05:27 PM
You can also generate electricity using high torque and low speed without a gearbox, if you just put more windings in your generator. I believe some offshore rigs use a setup like this, because fewer moving parts means less maintenance required.

MikeF
12-04-2014, 01:24 PM
Re: Dip in the blade - maybe for tip vortices but I don't know how. I am a glider pilot and have some understanding of the aerodynamics of wings. The picture of the glider in the link you posted shows winglets and the non-linear leading edge of the wing but the shape that I am seeing isn't either of those things. Maybe l'll ask this question on the glider forum I frequent. There are a lot of aerodynamic experts on there.

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