February 2012

In late 2011, Duke Energy’s commercial businesses presented the President’s Safety Leadership Award to the joint Duke Energy Renewables and Suzlon Wind Energy site team at the 29-megawatt Happy Jack and 42-megawatt Silver Sage wind farms near Cheyenne, Wyoming. Traditionally bestowed upon the operations teams at one of Duke Energy’s unregulated conventional power plants, this award recognized the exceptional team effort that took the sites’ safety performance to new heights in 2010.

In the narrative that follows, Bryan Stewart and John Valerius—Suzlon Wind Energy and Duke Energy Renewables’ respective site leads—share the lessons they learned along the way in the hope that they may aid operators at other wind farms throughout the industry.

The Evolution and the Goal
The Happy Jack Windpower Project, which reached commercial operation in 2008, was Duke Energy Renewables’ first wholly-owned commercial wind project. It is located in Laramie County, due west of Cheyenne. The adjacent Silver Sage Windpower Project came online in 2009. The sites are owned and operated by Duke Energy Renewables, which has an onsite team that oversees balance-of-plant operations and general site management. Suzlon Wind Energy oversees the maintenance of the sites’ 34, 2.1MW Suzlon turbines under a five-year full service agreement.

Productive partnerships require hard work, communication, and trust. From the beginning, site leaders from the companies sought to transform Happy Jack and Silver Sage into working laboratories where tried-and-true best practices and innovative ideas could be developed into safety protocols designed to protect both the workers and equipment at the two wind farms. Our collaboration continues to evolve, but the goal remains the same: create and continually enhance a zero-injury and illness culture, one in which the responsibility for hazard identification and mitigation is shared equally among all team members and everyone feels empowered to prevent accidents.

Lesson 1: Safety First
It takes more than perfunctory “safety moment” sharing and hardhat stickers to ensure employees and contractors make safety their top priority. To create a viable safety culture, site leaders first identified the unique strengths of each company and exchanged lessons learned at other power generation facilities. Many of the Duke team members relied upon their experiences designing, building, and operating fossil-fueled and hydroelectric power plants to ensure the wind farms leveraged best safety practices. This “utility mindset” encouraged rigorous standardization and measurement—two operational characteristics that are not always commonplace throughout the developing renewable energy industry. Figure 1

Suzlon Wind Energy’s intense focus on leading by example helped shape the way Happy Jack and Silver Sage site managers model the safety traits and behaviors they expect every employee and contractor to demonstrate each day. Specifically, Suzlon’s “Leading with Safety” training course provides managers with the tools and techniques they need to ensure safety remains the top priority, including recognition and reward mechanisms, employee engagement programs, and documentation templates to track key performance indicators. Leaders are held fully accountable for things that they can control and influence, including conducting safety meetings, inspections, and safety audits.

Lesson 2: Communication is Key
Open and candid lines of communication have also been critical to our success. Our joint team of 10 full-time staff members at the two wind farms conducts weekly safety briefings and delves deeply into topics such as turbine ladder accident prevention; slip, trip, and fall avoidance; and body stress mitigation techniques to counter working in extreme temperatures. At one point in 2010 we documented and analyzed a pattern of dropped tools at both sites. Brainstorming led to the implementation of a new training program that equips technicians with step-by-step guidance on how to properly handle certain types of tools during maintenance exercises. We reinforced the key aspects of the training program through new signage and message reinforcement at team meetings. Figure 2

Lesson 3: No Fault Safety Hazard Identification
To truly create a sustainable culture of safety, all site workers must feel empowered to identify and communicate potential hazards without fear of retribution for hampering operations. This concept is at the core of our shared safety philosophy. No matter who raises a safety concern, we work together to develop the solution.

For instance, the two companies’ employees jointly hold monthly fire inspections and hazard identification walkthroughs at Happy Jack and Silver Sage. The individuals responsible for conducting the inspections are rotated each month so that fresh eyes can uncover both obvious and subtle risks. In 2010, these site employees and contractors identified and addressed a number of seemingly minor but potentially harmful issues, including trip hazards from empty pallets and fire hazards from daisy-chained extension cords and fraying wires. Figure 3

Lesson 4: Safety Every Day
We developed a Job Safety Analysis (JSA) template for technicians to complete before each tower climb to ensure that safety remains at the forefront of everyday operations. At the beginning of our partnership, Suzlon’s employees shared their company’s wind safety procedures with Duke Energy Renewables employees. In turn, Duke’s employees provided safety procedures originally developed for Duke Energy’s sizable fleet of coal-fired power plants. Although it is impossible to guarantee that accidents will not happen, the JSA provides our technicians with a thought-provoking regimen that reinforces the importance of doing a job safely, not just quickly. Figure 4

Lesson 5: Leveraging Experience
A meaningful commitment to safety must involve innovation. This occurs by thinking about problems in new ways, and also by leveraging team members’ diverse experiences and skill sets. For example, John once served as a firefighter and emergency medical technician. We incorporated prevention and treatment techniques common to those professions into our site readiness and response plans. Happy Jack and Silver Sage purchased a compact backboard—often used by EMTs at the scene of an accident or crisis—and adapted it for high-angle wind turbine tower rescues. This makes it far easier to secure an injured individual in a confined space and lower them safely to the ground. John’s emergency medical response experience also led us to create medical trauma bags customized to meet the potential needs of wind farm technicians. Fortunately, we have not yet had the need to put these tools into service in a real emergency. Having them on hand, however, means our workers are trained and better prepared to contend with a great range of crises that might confront the operators of a large-scale wind farm. Figure 5

Another example: Both companies rely upon their respective meteorological expertise to identify and mitigate weather-related hazards. Meteorologists from both companies send automated and customized text messages to wind farm workers if the weather appears likely to take a dangerous turn. This ensures that our employees and contractors will have adequate time to suspend their activities and evacuate if necessary.

Spreading the Word
We believe the lessons we have learned and best practices we have developed through our partnership at the Happy Jack and Silver Sage wind farms in Wyoming can help leaders at other wind farms refine their safety protocols and programs. We regularly share these lessons and practices at other Duke Energy and Suzlon sites, and ask for their ideas in return. On numerous occasions, industry peers have generously provided us with food for thought related to safety excellence. In addition, both companies serve on the American Wind Energy Association’s Environmental Health & Safety Committee, which has proven to be an insightful forum in which to exchange information.

Although we are proud of our joint safety accomplishments, we know we can never rest on our laurels. Safety hazards are inherent in our line of work. When it comes to identifying and mitigating the risks that pose harm, our employees and our assets, vigilance, and creativity are essential. 

Carbon brushes are a small but crucial part of a wind turbine. Carefully selected brush grades, in combination with slip rings and intelligent brush holders, protect the turbine against lightning and the bearings against parasitic currents.

Many conveniences of our daily life would not be available without carbon ceramic materials, including the carbon brush for electrical sliding contact. Without this development, electrical motors and generators—and so, the industrial revolution—would not have been possible. Almost all electrical machines for household appliances like washing machines, vacuum cleaners, shavers, and power tools as well would not work without carbon brushes. An automobile without carbon brushes would be antiquated, since almost all components for convenience such as the power window motor, air conditioner, seat adjuster, sunroof, and fuel-injection would not be operational without carbon brushes. Trams, subways, and electrical locomotives with speeds above 350km/h wouldn’t have reached the present stage of development without adjusted, high-performance carbon ceramic components.

Special features of these materials include good thermal and electrical conductivity and a low friction coefficient. Carbon doesn’t have a liquid phase, but sublimes at 3500°C, so that there is no welding with metals. That feature makes carbon superior to other materials as a material for electrical sliding contact. Carbon ceramics are basically mixtures of defined carbon containing raw materials with organic binders. By means of different forming procedures, components of almost every shape and size can be fabricated (Figure 1).

At a subsequent temperature treatment up to 3000°C, the green bodies get their characteristic properties. With a temperature treatment up to 1000°C the materials are called carbon-graphite, and at a final temperature of 3000°C the result is electro-graphite. Besides graphitic components, metal-graphite contains metals such as copper or silver. Additional treatments involving impregnations with resins or metals allow further modifications of the physical properties.

The materials can be adjusted to all possible field conditions by means of using different raw materials, their conditioning, and by different shapes. The good thermal properties are useful in semiconductor manufacturing and for continuous casting. Burner nozzles for rockets or brake linings for automobiles and aircraft are future-oriented developments. Carbon components prove themselves as replacement for asbestos in the glass industry. The special tribological properties also show their advantages in sleeve bearings in pumps. Carbon-reinforced fiber materials rank among those with great future potential due to its high stiffness and thermal characteristics, and using such grades in medical applications is just beginning.

Applications for Carbon Brushes
In the following passage numerous applications for carbon brushes on wind turbines are described, along with potential problems and approaches to solutions (Figure 2, Figure 3).

Generator Brushes
The majority of wind turbines are fitted with doubly fed induction generators (DFIG). The main application for carbon brushes on wind turbines is the current transfer on the slip ring of these generators (Figure 4).

From the extreme cold of Inner Mongolia or North Dakota to the heat in desert areas, wind turbines are exposed to harsh environments. Additionally, there are other challenges like changes in wind speed, low humidity, and on/off cycles of the generators, etc. Actually, the carbon brush is only a small part of a wind turbine, but a very crucial component for efficiency, maintenance periods, and output.

From the standpoint of carbon brushes, there are three core components for the current transmission: the carbon brush itself, the slip ring, and the brush holder with a spring system. Actual carbon brush grades consist mainly of graphite and metal powder, usually copper, and with a metal content of 40-60 percent. For difficult cooling conditions, even brush grades with a high silver content are in use. Additives and after-treatments allow adaptation to these conditions, especially low humidity. The grade selection and number of brushes depend on the rotor current, the cooling and ambient conditions, and the slip ring material as well.

During the past 10 years the industry has gone to great lengths to improve the brush performance and shorten maintenance intervals. Meanwhile, even the harshest environment is no problem for carbon brushes. The slip ring material (Figure 5) is stainless steel (e.g. X20Cr13 ) or bronze (e.g. GBz10).

Although the surface speed is in the normal range for carbon brushes, the rings are helically grooved to guarantee safe contact and to improve the cooling of the slip ring surface. During operation a protective film is formed on the slip ring surface, called a patina. This thin film is crucial for the current transfer and the friction behavior of the carbon brushes. The patina consists of oxides of the ring material, humidity, and graphite from the carbon brush. Any interference of the film formation process has a negative influence on brush performance. That, for instance, is ambient temperature, surface temperature of the ring, current, humidity, presence of oil or dust, and state of the ring surface, etc. An initially sufficient surface roughness (5-8µm Rz, 0,8-1.2 µm Ra) supports the film formation.

Slip rings are available in different designs: assembled, or molded. The diameter and size is a function of the rotor current. Variants up to 1500 amperes exist. Intelligent brush holder systems support the brushes in their function. Massive and stable, cast brush holders are the current state of the art. Referring to them as “intelligent” means that the holders have a brush wear monitoring device included. Holder systems with helical tension springs allow brushes with 100mm length and a brush lifetime of more than two years. The single holders are normally pre-assembled on brush rockers (Figure 6). Each component of the current transmission apparatus can also be adjusted for offshore usage.

Grounding Brushes
A special application for carbon brushes is the discharge of ripple voltages. Ripple voltages can arise due to different factors, with electrostatic effects on the electric circuit and backlashes of the inverter being the main reasons. Despite constructive measures ripple voltages or parasitic currents are not completely avoidable.

For shaft grounding, a double brush holder with two grounding brushes is usually used. The standard carbon brush design is a so-called sandwich design made of a silver metal and graphitic brush grade. The silver grade is for grounding the graphite parts, for film formation on the shaft, and the removal of existing oil. It has to be certain that in the turning direction of the shaft, the graphite layer is always in front of the silver layer.

This brush design is a safe solution, but also the most expensive one. Therefore, cheaper brush designs such as metal impregnated grades or metal graphite grades exist. The applicability has to be checked case by case (Figure 7).

Lightning Protection
Lightning strokes are a wind turbine’s worst enemy. Due to their height of over 100 meters, and mostly located in remote areas, wind turbines are exposed to lightning strokes up to 10 times a year. Wind turbines have the specialty that the blades and the nacelle rotate and change position during their function. Significant amounts of lightning current passes through or near to all wind turbine components, and it also passes electronic equipment containing control or measuring devices.

Besides systems based on lightning conductors, carbon brushes and brush holders are used to protect the bearings and the parts behind it, i.e. the gearbox and generator. They should allow the current to bypass the bearing, instead directing it from the blade via the shaft to the tower and then to the ground. Standard brush designs can carry up to 200kA during a lightning stroke. Lightning protection brushes are not only used on turbines with DFIG generators, but on turbines with synchronous or induction generators as well (Figure 8).

Pitch Control
Pitch control motors are DC motors with standard brush grades. Because of the low speed and short operating intervals, the lifetime of these carbon brushes is extremely long.

Hub Control Slip Rings
The hub control slip ring is used for communication between the generator and the pitch control unit. Modern wind turbines adjust rotor blade pitch angle for two important reasons, safety and control. The control function improves turbine efficiency by adapting the blade aerodynamics to wind conditions. The safety function performs as an aerodynamic brake and rotates the turbine blades to a non-rotating (feather or stall) condition in wind speeds that exceed safe rated speeds or in other fault conditions. These slip ring units consist of a high voltage part and a low voltage/low current part for data and signal transfer. Gold wire technologies are commonly applied for current transfer. Due to increasing demands (blade heating) and increased blade size, however, this technology hits the wall, so the “old” brush technology enjoys a revival in this application. Silver brush grades are the main choice when it comes to data transfer as well as power supply.

Auxiliary Drives
There are also some minor applications like pumps driven by induction or DC motors using carbon brushes.

Maintenance Checklist
Brushes, holders, and slip rings must be checked regularly. A problem detected at an early stage can be addressed before failure occurs. Recognized too late it may give rise to heavy damages, stopped turbines, and expensive repairs. The following should be used as a checklist for regular inspections:

Carbon Brushes
• Brush length: Too-short brushes should be replaced.
• Shunts: Discolored shunts are an indication for uneven current distribution, possibly caused by different brush temperature. It is recommended to replace the complete brush set, since single brushes could already have a defect. If shunts are frayed by vibration, they should also be replaced.
• Vibration markings: If carbon brushes show a shiny side-surface it is an indication of radial movement of the brushes in the brush box. This can be caused by out of round slip rings, bearing failures, or a too-smooth ring surface. If the side surface shows markings of current, the connection between brush and shunt could be damaged.
• Brush contact surface: Rough, broken contact surfaces may appear in the case of brush sparking. Brush sparking of the power brushes is a rather rare phenomenon, while sparking of the grounding brushes is observed quite often.

Brush Holder
• Brush box: It must be ensured that brushes can move freely.
• Electrical connections: All electrical connections must be fitted accurately.
• Springs: The holder springs should be checked in longer periods. Springs with a deviation of more than 10 percent from the nominal value should be replaced.
• Brush holder distance: It is important for a proper guiding of the brushes in the brush box, and that the distance between holder and ring is 2-3mm.

Slip Rings
• The slip ring surface should be checked with a strobe light for damages. The surrounding should be checked for oil. If bearing grease or oil gets to the ring or commutator surface, it forms an insulation layer hampering the current transfer and leads to high brush wear. The porous brush material absorbs the oil. So in case of an oil leakage, all brushes should be replaced.

Flashover Markings
Heavy brush dusting and the deposition of brush dust on the insulators of the brush holders are the main reasons for flashover on the power slip rings, so it is essential to remove brush dust regularly. That must be carried out with a sucking method, not with a blowing method. Slip rings with a flashover mark must be replaced on the spot. 

References:
1) Volkmann – Kohlebürsten – Schunk GmbH (1980)
2) Nippes – Shaft Voltage Current Monitoring System for early Warning and Problem Detection – US Patent 6 460 013 B1, Oct.1 2002
3) Technische Information Nr. 8901 – Wellenspannungen ind Wellenströme an Turbosätzen – Siemens TI-Nr. 8901/1 (Aug . 1989)
4) Ulrich Wiessler – Historische Entwicklung und heutige Bedeutung des Kohlebürstengleitkontaktes – Bericht aus Forschung und Entwicklung No 42 – Schunk Kohlenstofftechnik 1988
5) Bachinger, Ringleb, Wiessler , Weigel – Einsatz von Kohlenstoffkeramik für die elektrische Traktion – ZEV+ DET Glasers Annalen, 125 (2001) 11

The world continues to pursue wind as a source of low-cost, renewable, zero-emissions electricity. With worldwide annual growth through 2020 expected to average 22 percent, wind becomes a significant percentage of total electricity sourcing. As the amount of electricity supplied by wind reaches 15 percent and higher in some areas—with others deriving 50 percent from wind—the reliability of this resource becomes critical.

The issues caused by varying winds and the time of use versus generation have been well documented. However, the reliability of wind turbine generators connected to weak electrical grids has received less attention. With wind farms typically found in remote locations, a weak grid to which turbines are connected can have severe effects on the wind turbines. Grid voltage dropouts can frequently occur, and the wind turbine must stay connected—i.e., controls and output must be maintained—despite perturbations in the grid.

Several occurrences of grid disconnect have been experienced in recent years. In the case of a grid disturbance, the wind turbines would disconnect from the grid as a safety precaution to protect the turbine. While these events may have occurred previously, their effect was relatively minor since wind generation was not a major portion of the overall electricity supply. At base load conditions, which typically occur at night, wind may provide 50 percent of the total power demand, so a disconnect is not an acceptable reaction to a transmission disturbance.

Typical wind turbine generator designs are shown in Figure 1. Grid disturbance problems can occur along the main power line or in the controls, which are powered directly from the grid. Along the main power a voltage drop or spike can cause an over-current, over-voltage, or over-speed, all of which could damage the generator. A voltage dropout in the power to the controls can also cause a grid disconnect.

Grid Requirements: Low Voltage Ride Through
Technical requirements for low voltage ride through vary by country and by grid operation. A few key requirements are shown in Figure 2. Typical dropouts up to 0.625 seconds must be fully covered and full generator output returned two to three seconds after recovery of the grid.

Varying blade angle, i.e. pitch control, can help control generator speed in extreme wind conditions, and the applicability of ultracapacitors for this function has been described elsewhere. While pitch control does effectively protect and optimize the wind turbine, it does not help with grid disconnect issues. When a low voltage event occurs this is effectively a low resistance to the generator, which leads to an over-speed situation. The reaction of the wind turbine is to use pitch control to slow down and eventually bring the blades to a halt, causing a disconnect from the grid. Pitch control is very effective in its purpose to protect a catastrophic failure of the wind turbine. We are left with electronic solutions to low voltage ride through to avoid this disconnect.

Solutions: Passive Devices
Crow bar devices and resistor banks have been used for low voltage ride through and to protect the wind turbine. However, the energy they consume is wasted. They are effective due to their ability to provide active and reactive power, but they are quite limited in the timescale that can be covered. They require sophisticated electronics and software, which can run in the $30,000-$40,000 range per turbine.

Static synchronous compensators (STATCOM) have recently come into use for power quality purposes. They are effective at providing active and reactive power as required by the grid. However, their particular effectiveness is in the short duration of a few cycles. Sizing a STATCOM to cover a few seconds becomes economically unfeasible.

Ultracapacitor energy storage can provide ride through for the main power conversion as well as the control electronics. They are scalable in time and power, but can cost effectively provide power from seconds to a few minutes. They have long been used as backup power for pitch control, so their reliability and lifetime are proven in similar applications and environments. Additional benefits of ultracapacitor energy storage include:

• Long life—typically in the 15 year range, depending upon operating voltage and temperature;
• Green solution—recyclable materials, no toxic chemicals;
• Scaleable—with modular designs, systems can be scaled up depending upon the wind turbine output;
• Maintenance free—capacitors do not require any regular maintenance and can be fully discharged for safe handling, and;
• Easy monitoring—simple discharge time measurements can be used to determine system health along with voltage monitoring.

Ultracapacitors function similarly to other types of capacitors, but they have tremendous surface area (3,000 m2/g) and molecular scale plate spacing (1 nm). These parameters allow their use as an energy storage device and essentially a DC component. They cannot be used in typical filtering applications. With no moving parts or chemical reactions, they have lifetimes up to 15 years, and potentially longer with predictable degradation and simple monitoring techniques. End of life is defined as 80 percent of rated energy storage capacity (measured as an 80 percent reduction in capacitance) and 50 percent of rated power capability (measured as a doubling of the rated equivalent series resistance, ESR). With this in mind, a system can be easily sized using a 1.5MW wind turbine as an example.

Required Power: the maximum power required is determined peak power output. In the extreme, this is the rated 1.5MW specification. In the assumption that, on average, a wind turbine operates at 80 percent output, the power rating is:
P_required = 1.5 MW x 80% = 1.2 MW

Required Energy: using the China grid requirement, the wind turbine must support a 0.625 second dropout plus an additional two seconds of recovery. The ultracapacitors must provide voltage support for this entire period. Additionally, the power output of the wind turbine is assumed to be constant power.

E_required     = P_required x (0.625 seconds + 2 seconds)  = 3.15 MJ

System Configuration: a system must be configured to meet both the power and energy requirement. Capacitor system power and energy is calculated as follows:
Pcap = 0.12 x V² / ESR
Ecap = ½ C x V²  

Additionally,
ESRsystem = ESRmodule x Ns / Np
Csystem = Cmodule x Np / Ns

Where Np = number of modules in parallel
Ns = number of modules in series

The capacitor voltage drops as it is discharged, the energy output is the difference between the starting energy at V_charged and the ending energy at V_discharged. The value for V_discharged is dependent upon the power electronics between the capacitors and the DC bus. In this example, we assume that the V_discharged can be as low as 30 percent of V_charged, the normal DC bus voltage. A common wind turbine ultracapacitor module is rated at 75V, 94F, and 13 mohm. Assuming the DC link operates at 450V, Ns = 6. We can now calculate a system configuration.

For power requirements,

1.2 MW = 0.12 x 450² / (0.013 x 6 / Np)
Then Np = 1.2 MW / (0.12 x 450²) x (.013 x 6) = 3.85 → 4

For energy requirements,

3.15 MJ = ½ x (Cmodule x Np / 6 ) x (450-150)²
Then Np = 3.15 MJ / (½ 94 / 6 x (450-150)2 ) = 2.2 → 3

Thus, an example system for a 1.5MW wind turbine will contain six modules in series with four such strings in parallel. The calculations above are an example only and detailed sizing calculations should be made for each system and region. However, ultracapacitor energy storage would cost $20,000-$35,000 per wind turbine, less than the $30,000-$40,000 reported for other solutions which require a combination of electronics and software. With a demonstrated reliable history of installations in the nacelle for pitch control, ultracapacitors have proven their value as a maintenance-free energy storage solution. Figure 3

Market Potential
Although wind energy today contributes approximately 2 percent of the world’s total electricity supply, it is estimated that its contribution will grow to over 4 percent by 2020. Therefore, within a decade another 230GW of new capacity is expected to be installed, which represents a market potentially worth $250 billion. Ultracapacitors will play a major role in this expansion and have already been installed in approximately 20,000 wind turbines. While the requirements for LVRT will grow along with overall wind installation, the rate of penetration will be determined by the percentage of total electrical energy coming from wind turbines in local regions and the frequency of grid disconnects causing reliability issues. Figure 4

Other requirements will also be place upon wind energy for “firming” of its output to reduce short-term variability. Grid operators and those responsible for reliability will place specifications on wind generation to control ramp rates and short-term spikes or sudden drops in ouput. Ultracapacitor energy storage will play a role in these functions as well as LVRT to provide cost effective, reliable solutions to grid reliability with renewable resources. 

There has been significant growth over the past few years in the size of wind turbines, which went from hundreds of kilowatts to several megawatts. This change in size has also imposed very strict conditions for wind turbine components, including steel towers. The demand generated by the limitations of the current technologies of the steel towers has led to the development of technologies for precast concrete wind towers of high-energy performance that can overcome heights and weights not achievable by steel towers. In this field, the Spanish company Inneo Torres has positioned itself as one of the leading international manufacturers of precast structural concrete towers.

Tower Characteristics
The exceptional experience acquired by the construction of numerous vertical concrete structures is not directly applicable to the design and construction of concrete towers for wind turbines, which have very specific requirements including: speed on construction rates; demanding fatigue loading; specific requirements concerning dynamic behavior and vibration; visual quality and finishing; visual integration requirements in the landscape, and more. Figure 1

Usually the “in situ” construction techniques for vertical concrete structures—by means of climbing or sliding formwork—have essential limitations: its geometry is strongly influenced by technology and cost of the formwork, and the time required for its implementation is quite extensive and dependent on the weather conditions. It is logical, therefore, that this technique has been used only with specific wind towers or on an experimental basis in towers where the costs or lead times have been far-reaching. Figure 2

On the other side, the precast concrete structural wind towers have all the conditions that may be required, and a single constraint that limits its scope: the amortization of the initial investment in technology and equipment, which requires a sufficient number of identical towers. Figure 3

Tower Technology
Inneo’s product range is composed of three models of towers for heights of 80 m, 100 m, and 120 m that are suitable for onshore and offshore wind farms and turbines from 1.5 to 4.5MW. Thanks to its versatility the precast tower solution developed by Inneo Torres is suitable for all types of wind turbine hub heights and under any wind conditions, and it can be tailored to meet the specifics requirements of any wind turbine (geometry, dynamic behavior, special loads, etc.). Inneo can also adapt the towers, which have a high ductility, to seismic risk areas. Figure 4

Main Features
We will outline some of the primary tower attributes here, including: behavior and structural capacity; transport logistics and assembly; durability; foundations; environmental constraints; and functionality:

Behavior and Structural Capacity
• Adoption of a system that is not conditioned by transport limitations and therefore provides freedom in the choice of the geometry of the tower, allowing the structural optimization both in its resistant capacity and on the control of its natural frequency and dynamic response.
• The ability to achieve great heights and support large-scale wind turbines both on land and at sea.
• Significant improvement of structural damping, and therefore of the dynamic behavior, reducing fatigue loading and thus contributing to an increase of the equipment life and the reduction of maintenance requirements.
• Joints between ductile parts are reliable, maintenance-free, tested and certified. Simple and quick implementation in the field.
• Optimal response to seismic actions thanks to the high ductility of all sections of the tower, including the unions and the high structural damping that increases in extreme load situations. This enables the structure to absorb and dissipate high amounts of energy in the event of an earthquake, in clear contrast with the behavior of existing steel towers.

Transport Logistics and Assembly
• Sensitivity to stimuli that can produce aeroelastic phenomenon of vortex shedding on the isolated tower. This makes it possible to finish the assembly of the tower and to install the nacelle at any time, unlike metal towers that have to assemble the gondola immediately after placing the last section of the tower.
• High rate of assembly through the use of a few pieces of great length and simple and rapid systems for the use of the joints. Onshore construction rates of 2.5 towers per week, equivalent to the existing steel towers and unreachable for in-situ building systems of concrete towers or hybrid ones.
• Easy and reliable transportation of large pieces of reduced height.

Durability
• There is practically no need of maintenance for the tower or the unions.
• Greater durability of concrete structures, particularly in marine environments, thanks to the adequate protection of the concrete lining in the inner reinforcement corrosion. This increased durability is enhanced by the high-performance of the concrete used in the wind towers.
• Increased tolerance to damage from impact or accidental actions, simpler and cheaper in case of future repairs.
 
Foundations
• The increased stiffness and frequency of vibration of the towers greatly reduces stiffness requirements of the foundation. This helps to reduce uncertainties associated with the deformation of the ground and allows significant savings in the foundation, particularly in places with soft ground.
•  The connection with the foundation is made without interfaces; it’s simpler, cheaper, and more reliable.
• The great weight of the tower makes it more stable and allows for a significant reduction of the foundation, making it more economical.
• The larger diameter at the base of the tower reduces flights of the shoe, as well as the effort applied, and allows less amount of armor.
• The foundation is 25-35 percent cheaper than equivalent metal towers, and it is faster to execute.

Environmental Constraints
• Generation of less noise due to the concrete damping effect.
• Lower CO2 emissions to manufacture the tower, between 55 and 65 percent less than the emissions spent in a metal tower.
• Recyclable materials. Once completed the useful life of the structure, the concrete can be used as recycled aggregate for new concrete preparations.

Functionality
• Large interior space at the base of the tower, thanks to its larger diameter (maximum diameter for steel towers 4.3 m, for concrete towers the diameter can be equal or superior to 6.8 m), which favors the assembly of equipment inside the tower at one level and without constraints of space.
• Increased size of the door, which allows for the passage of the equipment easily and facilitates its installation or eventual repair.
• Facility to anchor the elements and interior equipment necessary in the walls of the tower.
• Improved safety facilities in case of lightning strikes.

Summary
Inneo Torres was founded in 2004 and has installed more than 150 towers of 80 and 100 m in Spain and recently in Brazil. In fact, the company recently signed several contracts with some of the main wind turbines manufactures of the market and in 2012 will start serial production of its120-meter model. Also, Inneo will launch new installations of its patented mobile factory, already used in Brazil last year, in other Brazilians states in the course of this year.