Wind Turbine Maintenance and Noise Issue

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Wind Turbine Noise

Your neighbours' concerns relating to wind turbine noise are important. No matter the size of the wind turbine, the potential for turbine noise to bother other people always exists. Even if a wind turbine does not emit enough sound to violate any noise regulations, the noise it produces may still be objectionable to other people. Before building a wind turbine, familiarize yourself with the types of noise your wind turbine could make:
Aerodynamic noises may be made by the flow of air over and past the blades of the turbine. Such noises tend to increase with the speed of the rotor. For blade noise, lower blade tip speed results in lower noise levels. Of particular concern is the interaction of wind turbine blades with atmospheric turbulence, which results in a characteristic "whooshing" sound.
Mechanical noises may also be produced by components of a wind turbine. Normal wear and tear, poor component designs or lack of preventative maintenance may all be factors affecting the amount of mechanical noise produced.

Wind turbine noise 

How Loud Might a Wind Turbine Be?

At a distance of 250 m, a typical wind turbine produces a sound pressure level of about 45 dB(A) (decibels). As following figure shows, this sound level is below the background noise level produced in a home or office. Most small wind turbines, in fact, make less noise than a residential air conditioner.

Small Wind Turbines

The blades rotate at an average range of 175-500 revolutions per minute with some as high as 1150 rpm. Large turbines turbine blades rotate in the range of at 50-15 rpm at constant speed, although an increasing number of machines operate at a variable speed.

 Comparison of decibel levels from a hypothetical wind turbine (from 250 m away) with other sources of noise.

Maintenance

A wind turbine requires periodic maintenance such as oiling and greasing, and regular safety inspections. Check bolts and electrical connections annually; tighten if necessary. Once a year check wind turbines for corrosion and the guy wires supporting the tower for proper tension.
If the turbine blades are wood, paint to protect from the elements. Apply a durable leading edge tape to protect the blades from abrasion due to dust and insects in the air. If the paint cracks or the leading edge tape tears away, the exposed wood will quickly erode. Moisture penetrating into the wood causes the rotor to become unbalanced, stressing the wind generator. Inspect wooden blades annually, and do any repairs immediately.
After 10 years, blades and bearings may need to be completely replaced. With proper installation and maintenance, your turbine can last 20-30 years or longer. Proper maintenance will also minimize the amount of mechanical noise produced by your wind turbine.

Maintenance

Safety Concerns

All wind turbines have a maximum wind speed, called the survival speed, at which they will not operate above. When winds over this maximum occur, they have an internal brake and lock to prevent them from going faster than this survival speed.
For turbines operating in cold winter conditions, be prepared to de-ice as required, and store batteries in an insulated place.
Mounting turbines on rooftops is generally not recommended unless a wind turbine is very small (1 kW of rated output or less). Wind turbines tend to vibrate and transmit the vibration to the structure on which they are mounted. As a result, turbines mounted on a rooftop could lead to both noise and structural problems with the building and rooftop.

Safety Concerns

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Blade Parameters

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The blades should be designed so that the generator of the turbine rotates at the optimum speed at the desired range of wind speeds. The blades also affect the cut-in and cut-out wind speed of the turbines. The operation noise and vibration are also affected.

The parameters of the blades include shape, size, length, pitch, profile, material, weight, rigidity, number, etc.

Blades should be large in size, long in length, large in pitch and more in number at low wind speed.

The length of the blades can be modified to ones slightly larger for sites with low wind speeds or to ones slightly smaller for sites with high wind speeds.

Angle of Attack

The Critical Angle of Attack (αcritical) with Respect to the Blade

Tip Speed Ratio
Power Control Methods

Pitch Adjustment		Yaw Adjustment

The amount of surface area available for the incoming wind is key to increasing aerodynamic forces on the rotor blades. The angle at which the blade is adjusted is referred to as the angle of attack, α. This angle is measured with respect to the incoming wind direction and the chord line of the blade. There is also a critical angle of attack, α_critical, where air no longer streams smoothly over the blade’s upper surface. Figure 2 shows the critical angle of attack with respect to the blade.

The tip speed ratio λ (lambda) or TSR for wind turbines is the ratio between the rotational speed of the tip of a blade and the actual velocity of the wind.

This is an important parameter to evaluate the performance of a wind turbine. The tip speed of blade refers to the speed of rotation of the turbine. The higher the rotation speed of the turbine, the higher the output power of the turbine. Therefore if the TSR of a particular turbine is high, the efficiency will be higher.

The power output of the turbine can be optimized at the rated operating wind speed or limited at wind speed above the cut-out wind speed. The generator speed, blade angle adjustment, and rotation of the entire wind turbine can be controlled. Blade angle adjustment and turbine rotation are also known as pitch and yaw control, respectively. A visual representation of pitch and yaw adjustment is shown below: 

The purpose of pitch control is to maintain the optimum blade angle to achieve certain rotor speeds or power output. Stall and furl are two methods of pitch controls. By stalling a wind turbine, increasing the angle of attack, causes the flat side of the blade to face further into the wind. Furling decreases the angle of attack, causing the edge of the blade to face the oncoming wind. Pitch angle adjustment is the most effective way to limit output power by changing aerodynamic force on the blade at high wind speeds.

Yaw refers to the rotation of the entire wind turbine in the horizontal axis. Yaw control ensures that the turbine is constantly facing into the wind to maximize the effective rotor area and, as a result, power.  Because wind direction can vary quickly, the turbine may misalign with the oncoming wind and cause power output losses.

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Importance of the Location for Small Wind Turbines

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Wind is the fuel that drives a wind turbine. A windmill needs to be placed where the wind is; putting it on too short a tower is like installing solar photo voltaic panels in the shade. Neither will work very well. Not just any wind will do, a wind turbine needs air that moves uniformly in the same direction. Eddies and swirls, ‘turbulence’ in short, do not make good fuel for a wind turbine. The rotor cannot extract energy from turbulent wind, and the constantly changing wind direction due to turbulence causes excessive wear and premature failure of your turbine. This means that you want to place your turbine high enough to catch strong winds, and above turbulent air. Since the tower price goes up quickly with height there is a limit to what is practical and affordable. This section is intended to help you decide what tower height works best.

The quick-and-dirty rule of thumb for turbine height is a minimum of 10 meters (30 feet) plus the length of a turbine blade above the tallest obstacle (trees, house etc.) in a 150 meter (500 feet) radius, with a tower height of at least 19 meters (60 feet). If the obstacle is more solid than a few trees (for example a whole tree line) then even more distance than 500' is needed, as will be described below.

This should really be regarded as an absolute minimum for a wind turbine; at 10 meters above an obstacle there will still be some amount of turbulence and additional clearance is highly desirable. Keep changes in height of obstacles in mind as well. For example, if you have trees that are expected to grow up to 60 feet high, it is advisable to use a 100 feet tower. Likewise, a 60 feet tower should only be used when the terrain is very, very flat with no obstacles in a wide area around, for example at the edge of the sea, or on top of a cliff with a clear area around it, or in the tundra. For most situations a 60 feet tower will only save a little money up front, while short selling energy production in the long run.

To go beyond the rule of thumb, the airflow over any blunt obstruction, including a tree, tends to create a “bubble” of turbulent air of twice the height of the obstacle, extending 20 times the height of the obstacle behind it. So, your 30 feet high house disturbs the air up to 600 feet away. That tree line with 100 feet trees disturbs the air up to 200 feet high at a distance of 1000 feet away! The figure above illustrates this. Locate your wind turbine either upwind of the obstructions, or far enough downwind. Notice from the figure that preference should be given to a site upwind of obstructions, but keep in mind that tall features downwind of the turbine can also influence the wind going through the blades, as shown in the figure.

Upwind and downwind are relative to the prevailing wind direction; where the wind blows from most of the time. A wind atlas can sometimes tell you what your prevailing wind direction is, and if there is one at all. Some sites have winds that did not read the rule book, and there it is equally likely to blow from more than one direction.

When it comes to wind turbines, the bottom of a hill, valley, or ravine makes for a poor place to site a windmill. The wind tends to drop in speed at the bottom of a smooth hill, then speed up as it goes up the hill, reaching around twice the wind speed at the top of the hill. The figure below shows this. You can use this effect to your advantage if you have hills on your property.

For obstructions that are not smooth, such as a cliff (i.e. a sudden rise in the landscape) it gets trickier: Sharp edges create turbulence, as illustrated in the figure below. The airflow at the top of the cliff can be stronger than the average wind speed in the area, but close to the cliff’s edge it may also be very turbulent, making it a poor site for a turbine. If you have a cliff edge on your property and want to use it for siting your turbine, you should still use a 60 feet high tower to get above turbulent air. Even if it seems that the wind is always blowing hard at the cliff’s edge.

The lee side (downwind of the prevailing winds) of a bluff object makes for a very poor wind turbine site. The bluff object will create large turbulence on its downwind side, and the average wind speed will drop off precipitously as well. This leaves no energy for the wind turbine to harvest.

There actually is a cheap way to visually find out at what height turbulent air ends, and smooth, laminar airflow begins. Just fly a kite at your proposed wind turbine location on a windy day, preferably when the wind is coming from the prevailing direction. To visualize airflow, use tape-streamers tied to the kite’s string every 15 feet or so (home improvement stores sell plastic marking tape in fluorescent colors for very little money). Wildly fluttering tape indicates turbulence, smoothly extended tape means smooth air. Be sure to take the angle of the kite’s string into account when calculating height.

The energy in the wind increases with the cube of the wind speed (P ~ v³), and wind speed increases with height. An increase of just 26% in wind speed means twice as much power available in the wind, and your wind turbine will produce almost twice as much. Double the wind speed and you an harvest almost eight times as much power! A small additional investment in tower height may therefore be well worth it, thanks to the increased energy production. If you know the annual average wind speed for your location (from weather data, a wind atlas, local weather station etc.) Weather data usually reports wind speeds at 10 meters above ground level, the spreadsheet can take care of translating that to a wind speed at turbine height. For a quick idea of how changes in tower height affect the power in the wind for an unobstructed site see the figure below.

The Danish Wind Power Association made a very nice, interactive, calculator that allows one to plug in various obstacles (for example, a row of trees), set their height and distance to the wind turbine, and visually show what effect this will have on wind speed and energy. The calculator shows the percentage of the wind speed at various distances and heights behind the obstacle. Keep in mind though that the effect of obstacles is not just to diminish wind speeds, but they also make the air swirl, creating turbulence. Turbulence is an energy thief when it comes to wind turbines.

If you have sufficient space for guy wires, we advise to use a tilt-up tower for your wind turbine. They are economical, costing only a little bit more than the cheapest type of tower (a fixed guyed tower), and allow the turbine to be installed on the ground. Maintenance can also be done on the ground, by tilting the tower down. This saves in crane expenses, and makes installation and maintenance much safer because the work does not have to be done at dangerous heights.

Another aspect of proper windmill siting is the distance from occupied buildings. All wind turbines produce some amount of sound. Even though the Scirocco is one of the most quiet wind turbines on the market (no, this is not just marketing hype, it really is quiet), it too produces sound. Some people find its sound soothing, since it tells them they are making energy, while it drives others absolutely bonkers. For that reason it is a good idea to place your wind turbine some distance away from your house, 100 feet is a good number for minimum separation. That is not to say that closer cannot be done, but you will have to honestly assess how the turbine’s sound will affect you. Generally, a Scirocco that is placed in smooth air will be almost inaudible unless the wind starts blowing hard. At that point the blades pitch to stall angle, causing the air to swirl across the blades instead of flowing smoothly, and this increases the audible sound. When this happens it can be heard over the wind when you are in close proximity (and downwind sound will carry further than upwind). There also is such a thing as too much distance, since the length and gauge of the wiring that is needed will increase. With the ever-increasing price of copper this makes it more expensive to install your turbine.

Since we are talking about buildings: Despite the current marketing pitch of many small wind turbine manufacturers and sales people, it is generally a very bad idea to mount a wind turbine (any turbine, not just a Scirocco) directly onto a building. The airflow that close to the building is generally very turbulent, leading to premature failure and poor power production. It is usually noisy too. Every wind turbine has some amount of vibration associated with it, and this too will be transmitted inside the house. We know, the thought of bolting a little turbine to the house, just over the roof line, to offset your electricity use (as that salesman put it) is appealing. The harsh reality is that it does not work: Several studies were done, involving dozens of roof-top-turbines. They all concluded that those turbines do not work. Their energy production is negligible, and some were even net-users of electricity (because their inverters draw power, even when nothing is going into the grid)!  Just say "no" to building mounted turbines!

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All About Small Wind Turbines

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How do residential wind turbines work?

A wind turbine, which is installed on top of a tall tower, collects kinetic energy from the wind and converts it to electricity that is compatible with a home's electrical system. In a residential application with net metering, a home is served simultaneously by the wind turbine and a local utility. If the wind speeds are below cut-in speed (usually a minimum of 2 or 3 meters per second is required) there will be no output from the turbine and all of the needed power is purchased from the utility. As wind speeds increase, turbine output increases and the amount of power purchased from the utility is proportionately decreased. When the turbine produces more power than the house needs, the extra electricity can be sold to the utility if such arrangements are available. All of this is done automatically.

Don't I have to take wind measurements for a year or more? 
For many residential systems the cost of taking wind measurements is not justified. Wind resource data provided by Environment EU is often sufficient for an experienced evaluated to predict wind turbine performance. For larger turbines and larger investments, it may be wise to collect more detailed data by location.

What about towers? 

A rule of thumb for proper and efficient operation of a wind turbine is that the tower height (turbine hub height) should be at least 10 meters above anything within 100 meters of the tower. Typically, 25 to 37 meter towers may be supplied along with the wind turbine, which usually avoids turbulence from buildings and trees on most sites. Wind speed increases the higher you go above ground, and it also becomes less turbulent. In addition, electricity generation from a turbine increases exponentially with wind speed. Thus, relatively small investments in increased tower height can yield very high rates of return in electricity generation. For instance, installing a 10 kW generator on a 30 meter tower rather than an 18 meter tower involves a 10% increase in overall system cost but can result in ~30% more power. Several different types of towers are available, depending upon which manufacturer you select. Each type has its advantages; the most economical tower is the ‘guyed lattice’ tower, but a hinged tower may be easier for you to install yourself and provides easier access for maintenance.

How reliable are wind turbines? Much maintenance?

Most small turbines have very few moving parts and do not require any regular maintenance. They are designed for a long life (up to 20 years) and operate completely automatically.

How would I have a wind turbine installed at my home? Most dealers offer either complete turnkey (ready-to-operate) installations or the option to purchase direct from the factory and install the system yourself. The first option offers more customer support from the company. Self-installation offers significant savings and a hands-on understanding of the turbine. Prospective owners can discuss the options available with manufacturers to decide which method best suits their budget and technical skills.

Approach buying the equipment as you would any major purchase. You may begin this process on our Help with Planning page. You will need to weigh costs and various degrees of rugged/durable designs. Obtain and review the product literature from several manufacturers, and research those you want to pursue to ensure they are recognized businesses and their parts and service will be available when you need them. Find out how long the warranty lasts and what it includes, and ask for references of customers with installations similar to the one you are considering. Ask system owners about performance, reliability, maintenance and repair requirements, and whether the system is meeting their expectations.

How many turbines to power a household or farm? 

For a home or farm, one turbine is normally installed. The turbine's size is chosen to meet the energy requirements given the available wind resource.

What about new small wind turbines that run at very low wind speeds?

Many companies have developed turbines that run at low wind speeds. But because the energy available in the wind is a function of the cube of the wind speed, there is very little energy available to be harvested at wind speeds less than 3 or 4 m/s (10 or 14 kilometers per hour). If you are considering the purchase of a small wind turbine for use in a low wind speed location, shop for turbines with good low wind speed performance; this may mean turbines with larger-than-average rotor diameters for their rated power.

Is a windmill or an electrical wind turbine better for pumping water? 

To pump water using wind energy, you need to place a windmill directly above your pumping site since windmills pump water through mechanical motion rather than using electricity. While this may be appropriate for some users, the wind speed at their desired pumping site may not be strong enough or may be obstructed by trees, embankments, etc. or the foundation at the pumping site may not be stable enough to install a windmill. Therefore, for some users, it may be more desirable to erect an electricity-generating wind turbine where the wind is favorable and run electrical cable to the pumping location, where you use an electrical water pump. Which is more practical and economically feasible for you depends on your property and wind regime.

Small Wind Economics

Will a small wind turbine save me money? 

Since energy conservation is usually less expensive than energy production, making your house or farm more energy-efficient first will likely reduce the amount of investment in a wind system to meet your needs. Most wind system purchasers have done all the reasonable efficiency measures first.

A wind turbine typically lowers a household electricity bill by 50% to 90%. It is not uncommon for wind turbine owners with total-electric homes to have monthly utility bills of only $8 US to $15 EU for nine months of the year in the EU. In northern parts of the EU. where less air conditioning is used the bills can be very low year-round. The amount of money a small wind turbine saves you in the long run will depend upon your electricity costs, the amount of electricity you use, the average wind speed at your site, and other factors.

How much does a wind system cost? 

It depends on the size of the turbine and electric demands. But you can also build your own homemade wind turbine easily... See: homemade 1 kW wind turbine

What should I watch out for in buying a small wind turbine? 

"If it seems too good to be true, it probably is" are words to keep in mind when shopping for a small wind turbine. Over the years, a steady stream of "breakthrough" wind turbines has promised exceptional performance at an incredibly low price. Sometimes the claimed performance violates the laws of physics, promising more power than the total kinetic energy available in the wind-stream that is intercepted by the rotor's swept area. Most of the popular models of small wind turbines operate at about the same efficiency. The energy production you should expect will be closely related to the swept area of the rotor blades, which is based on the diameter of the rotor. If you are offered a product that promises to run your whole house with a turbine that is much smaller than conventional products, it's time to start asking hard questions.

How do small turbine costs compare to other alternatives? 

Small wind turbines (ranging in size from 250 watts to 50 kW) are often the least expensive source of power for remote sites that are not connected to the utility system. Hybrid systems -- wind/photo-voltaic, wind/diesel, and other combinations -- can often provide the most efficient and cost-effective option for rural electrification. Photo-voltaic (PV) -- the direct conversion of sunlight into electricity -- is often used to supplement wind power since PV tends to operate best in low wind months. Diesel generators or batteries can be used for backup power and to maintain power production during low wind seasons.

One study of an Arctic community with annual average wind speeds of 15 mph (24 kilometers per hour) compared the cost of a 500-kW diesel system to that of a 200-kW diesel generator and four mid-sized wind turbines. It found that the wind/diesel combination cost considerably more to install ($378,000 EU versus $125,000 EU), but would deliver fuel savings of $90,000 EU per year, paying for itself in less than three years.

Small Wind and Your Environment

Do wind turbines make noise or interfere with TV reception? 

Small wind turbines do make some noise, but this is not a problem as long as the turbine is well-sited and located at least 200 meters from occupied buildings. Small wind turbines do not interfere with TV reception. See our Social, Environmental, and Other Considerations page for more details

Do small wind turbines kill birds? 

Anecdotal evidence indicates that birds occasionally collide with small wind turbines, as they do with any other type of structure. However, such events are rare and very unlikely to have any impact on bird populations. House cats in the EU, by contrast, are estimated to kill roughly one billion birds each year. Statistically, a single house cat is a much greater threat to birds than a small wind turbine. See our Social, Environmental, and Other Considerations page for more details

Are small wind turbines safe? 

·         Yes. However, neighbors who are uneasy about a nearby homeowner installing a small wind turbine may raise all sorts of questions about safety. Brief answers to some of these concerns:

• Falling tower: Thousands of wind turbines are installed in the EU, and their safety track record is excellent. Trees are much more likely to fall than a properly installed wind turbine, but no setbacks or minimum property sizes are required for trees.
• Safety of utility repair personnel during a power outage: Small wind systems shut down automatically in the event of a power outage, and will not energize a dead power line.
• Ice throw from rotor blades: Ice buildup makes wind turbine blades less aerodynamic, so that they turn more slowly. Typically, ice will drop to the base of the turbine tower instead of being thrown.
• Children climbing the tower and falling: Possible, but wind turbines should be treated no differently than other climbable structures such as water towers or amateur radio antennas.

Small Wind Grid Connection and Legal Issues

Will my utility allow me to hook up a wind generator? 

More and more provincial and regional utilities in EU have been developing and implementing interconnection and net metering policies to allow customers to buy/sell electricity to/from the national grid. Please contact your utility directly to find out their most recent news regarding interconnection and net metering.

Will I have to change any of the wiring in my house? 

No. A wind turbine can easily be installed at virtually any existing home without the need to change any wiring or appliances..

Will my local government allow me to install a wind turbine? 

A wind turbine is a tall structure that normally requires a building permit. Zoning regulations often limit the height, placement, and other characteristics of "appurtenant" structures, so a conditional (special) use permit or variance may be necessary. It's usually best to let your neighbors know about your installation. Be prepared to answer questions and clear up common misconceptions with well-documented facts about small wind turbines.

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Cost Saving with Small Wind Turbines

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Small wind turbines have a high potential to be part of a decentralized renewable energy system. Though costs are high and profitability is still low. For several years now the small wind industry has been expected to reach a tipping point with an increase in sales volume and a drop in production costs. This assumption is being analyzed in this paper and further saving potentials in small wind turbine production costs are outlined. Based on small wind market reports in several countries a global market analysis for small wind turbines in the range of 1 kW to 5 kW has been performed. From the findings a relevant set of manufacturers have been selected to study manufacturing methods and development trends.

The global market segment has a size of 22 000 small wind turbines annually with China, the United States and Great Britain as leading countries. The sales of market leaders can be estimated to be between 1000 and 2000 units a year. For the main components potential future cost savings are being discussed.

The tower is found to be the part with the major saving potentials. Measures to lower production and installation cost include centrifugal casted, segmented fiberglass towers and slip-joint techniques to connect the tower segments.

This article focuses on grid-tied systems with capacities from 1 kW up to 5 kW rated power and takes on the U.S. Small Wind Industry Road map's thesis that the industry “is close to the ‘tipping point’ where production volumes would skyrocket, causing production costs to plummet”. There is a lot of empirical evidence proving further high expectations on imminent production and sales increase such as a “huge potential” and an “age for the industry to boom and prosper“ or an potential annual energy yield of 1.5 TWh for Great-Britain. However, the expectations on plummeting production costs need to be backed by specific measures in design, manufacturing, logistics or installation cost.

Present prices for small wind turbines range from 2300 €/kW to 4600 €/kW compared to about 1000 €/kW in large wind . Amortization of investments without state subsidies is only viable in off-grid applications and on sites with high wind yields. The objective of this poster is to presents cost saving potentials of small wind turbines.

Blades: Savings of about 70% with change to large scale industrial processes such as matched metal molding, pultrusion or RTM-Molding, DLR has developed a filament winding process for small wind blades manufacturing at an industrial scale which has not been put into practice so far.

Generator: Production costs for permanent magnet generators are made up to 70% of material with rare earth elements prices being the main cost driver. Fixed cost degression is possible though it only has limited effect on the overall price.

Bearings: Early small wind turbines used bearings with additional bearing flanges in their machine housing. These flanges were casted and machined parts and drove costs up. Present bearing concepts have integrated the bearings into the housing and thereby cut costs by approximately two thirds.

Tower: The tower is the component with the main effect on the overall price of the small wind system. Manufacturing choices concern raw material, joining processes and surface protection. Furthermore the tower design and quality has effects on installation, logistic and life cycle cost.

Tower design using new structure materials is under development and has large potential to reduce cost. Significant cost savings can come from centrifugal casted, segmented fiberglass tower and slip-joining concepts for the tower segments.

Controller and inverter: Inverter technology is largely derived from photo-voltaic applications and costs are estimated at 0.50 €/W [9]. Therefore small wind inverter technology benefits from manufacturing and R&D investments that have been driven by the larger PV industry and will continue to do so.

Logistics: To facilitate shipment and work sharing in international value chains worldwide the length and width of a 20' sea freight container as an international standard should be met. 

The article that production costs may plummet has been partly proven right. The large cost potentials by industrial composites manufacturing processes for rotor blades have been realized by a few large small wind and component manufacturers in the 1-kW- to 5-kW segment with annual production volumes near 1000 units. Further cost savings have been realized by improved bearing concepts in turbine design and by benefiting from R&D efforts on inverter by the PV-industry.

After all annual sales volume is a limiting factor for smaller companies to invest in cost cutting technologies. Future potentials will come from innovations in tower design and manufacturing technology.

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