July 2012

Overlooking the scenic shores on a ridgeline in the West Maui Mountains, First Wind’s Kaheawa Wind Project is helping Hawai’i Clean Energy Initiative realize its goal to provide at least 70 percent of the power from clean energy sources by 2030.

“This is the second phase of a project on a very small island electric system where this wind project is quite large in proportion to the size of the electric system versus a similar site on mainland United States,” said Tom Siegel, vice president of Transmission for First Wind. “The challenge for us was the variability of wind power. It can significantly impact the reliability of the power system. As the wind increases or decreases along the trade wind path, operational frequency is affected.”

Initial studies for the project by the Maui Electric Company (MECO) asked how much energy could be taken by the utility without upsetting the balance of current electrical output, customer needs, and how a wind farm would impact the utility’s status with regulatory boards. The amount of energy could be supplied through wind energy were relatively low, so it was determined that the island and MECO could work in harmony if a 30 megawatt wind farm was constructed and connected to the grid.

The first phase known as Kaheawa Wind Project I (KWPI) was completed June 2006 and now KWPII, a 21 megawatt site is due for completion in the next few months.

“The first phase is the wind turbines, substation and operations support facilities, but has no energy storage system,” Siegel said. “Phase II includes 14 wind turbines rated at 1.5 megawatts for a total of 21 megawatts of wind energy. Phase II also includes the substation, integrated energy storage, and offices and facilities to monitor and maintain our integrated power system.”

KWPI provided First Wind the opportunity to learn how the island and wind energy would exist together, and because the systems are small, curtailing the resources was necessary to maintain the reliability of the MECO electric system. Figure 1

“People often talk about wind generation in terms of the average capacity factor. The Kaheawa wind project has an average capacity of 40 to 45 percent,” Siegel said. “The Kaheawa wind project provides about nine to 10 percent of all energy consumed on the island curtailing the energy was a concern for the Maui utility company who contracted us.”

Since curtailment was paramount to the success of both the KWPI and KWPII projects, First Wind had to manage the system frequency, transmission rates and also store energy for later use.

“When you add up the amount of utility-owned generation that has to be online and compare that to customer demand, there is often not enough of a difference to allow the utility to take all of the wind energy generation that’s available, so you curtail or back down and that’s where storage comes into play,” Siegel said. “Combining wind and energy storage give us the ability to store energy for use at a later time. We sought a way to smooth out moment-to-moment variability when the wind is gusting strongly or suddenly drops off. We needed a storage system and that’s how Xtreme Power’s system became involved,” he said. “The advantage of the energy storage system is it allows us to smooth out the variable wind generation and gives the utility company more time to make the decision of whether or not to start a generator when the wind generation drops unexpectedly.” Figure 2

This situation in Kaheawa was a significant concern for the local island utility provider, but investigational studies showed that by the electrical provider with continuous status updates on turbines, and substation and the energy storage system, it would be possible to deliver more renewable energy to a community eager to remain eco-friendly.

“As a wind farm developer, we had to ask ‘do we have the ability to control the energy we provide to Maui’s grid?’” Siegel said. “We found the Xtreme Power energy storage system was the right solution for the situation we faced.”

The solution for First Wind and Kaheawa was an alternative energy system that maximizes the winds on the island’s grid with a technology that integrates power management and energy storage into an intelligent package. Figure 3

“We found that DPR was the right solution in this instance,” he said. “It helps us meet stringent performance requirements and supply more energy to the residents of Maui than would have been possible without the energy storage.”

Providing integrated power management and energy storage for KPWII was a natural fit for both companies since they had already worked on the first Maui project in 2008.

“MPWI was our first renewable integrated site where they took wind turbines and one of our Dynamic Power Resource (DPR) systems and put them together in order to smooth the wind and match it to the grid,” said Alan Gotcher, president and CEO of Xtreme Power.

In Maui, Xtreme Power provided a 10 MW/20 MWh DPR to manage the renewable energy penetration rate increases where the likelihood of curtailment is higher. By combining the Dynamic Power Resource for the 10-megawatt / 20 megawatt hour facility, the DPR stores excess power during curtailment periods and delivers the power to the grid when MECO needs more energy. Figure 4

“Our system works with First Wind’s turbines to provide ramp control, frequency and voltage services, curtailment capture, and responsive reserves to help our customer, First Wind, make more money in its operation,” Gotcher said.

With ramp rate control through the DPR, the wind farm and the utility provider are able to smooth the wind to accommodate for wind variability so that it becomes much less of an issue.

“As an example, MECO might say ‘I need to count on you to control the rate of change or ramp and it can’t vary by more than one megawatt per minute up or down,” Gotcher said. “We do this through software controls or energy in the battery and we are able to add or subtract power relative to what the wind farm is capturing through our Dynamic Power Resource.” Figure 5

When the wind park generates too much energy, MECO and operators are able to ‘feather’ the blade props so they spin slower and capture less energy in the curtailment phase. Any excess energy is stored in batteries on site and used at a later date when usage is higher.

DPR also incorporates a semi-responsive reserve capacity that turns on the alternating current when demand is high. Because this 10 MVA system has a dynamic range of 20 megawatts, the DPR can push or pull 10 megawatts of energy as needed.

When the wind is highly intermittent, the intelligent controls command the DPR to charge or discharge to smooth the power to the utility. If the wind ceases, the utility can command the storage system via Automatic Generator Control (AGC) to discharge electricity to the grid while the generator is coming online. Figure 6

“Our system has several building blocks,” Gotcher said. “Sometimes it is mounted in shipping containers that are 40-feet-long and situated at the base of the turbine or solar park. Sometimes it’s a smaller box. It depends on the numbers of turbines. It consists of a gray box of one IGBT’s with a silicon base with switches for high power demands. The system handles the power to the grid from the batteries and manages real and reactive power for AC that connects to transformers which step up the voltage to as much as 35,000 volts where the utility service takes the power.”

The purple box inside the main system is the master controller for the converter, which visually pushes data to the company’s site in Texas where technicians working 24/7 can control and monitor data lines from the farm to the utility provider as needed.

The DPR is bi-directional and can function in increments of five minutes, 15 minutes, 30 minutes, one hour, four hours or six hours depending on the energy needs in a particular location.

“The battery size is dependent on the duration of the energy cycle and we use copper bussing bars in sizes from 1-2 inches thick; 2-3 inches thick; and up to 2 to 3 feet long,” he said. “So the system acts like three components with energy storage, chemistry, power, an electronic system controller and software in one. Dynamic means we can do all of this quickly; Power is how the industry looks at the output; and Resources is to complement other things we manage. That’s DPR or Dynamic Power Resource.”

By managing the wind with the existing power grid using a system that offers curtailment capabilities, First Wind, MECO and Xtreme Power can provide a regulated amount of power at a given time, while still achieving a high return on the energy generated and sold to customers in an effective way.

“Our first project using DPR was in 2006 at the South Pole,” Gotcher said. “Of course that was in a remote location and we installed a DPR to a diesel powered generator to operate a radio telescope that is 20 feet in diameter. The telescope must rotate every minute to capture a signal, so the challenge was how to use our Dynamic Power Resource to rotate the telescope to generate power at regular intervals and then store the power during breaks. We do an alternating break and regeneration with capture so the break generates less power and stores the energy for later use. It does three million cycles per year and it pushes and pulls power as needed based on the available energy.”

Xtreme’s DPR is installed in 12 sites around the world with eight customers and 26 more projects in the works, and Gotcher speculates on how his company might generate more business as the industry evolves.

Siegel adds that his role with First Wind is making sure each wind farm interconnects and integrates to the existing power grid where he and his employees coordinate with each utility company to solve individual needs. “We have projects in Maine, Vermont, New York, Utah, Washington, and Hawaii and no project is the same.”

For more information on First Wind, visit  www.firstwind.com. For more information on Xtreme Power, visit  www.xtremepower.com. 

The major part of atmospheric wind energy is inaccessible to conventional wind turbines.
Computer-controlled kites provide an attractive solution to efficiently harvest this resource.

Wind generally gets stronger and more persistent with increasing altitude. For this reason, tower height is an important factor in the design of wind turbines and greatly affects their power output and capacity factor. However, even the largest turbines in the megawatt-range cannot exceed altitudes much beyond 200 meters due to the structural limits of tower-based designs. For offshore and particularly for deep-water deployment of such large turbines, the additional investments in foundations or mooring platforms are decisive cost factors. Figure 1

High Altitude Wind
Airborne Wind Energy (AWE) systems are designed to operate at higher altitudes. Common features of the many different concepts are flying devices such as wings, aerostats, or hybrid designs, which are tethered to ground stations and which can be controlled in altitude and flight path. Adjusting the operation to the prevailing wind conditions, significantly increased capacity factors can be expected.

Several concepts are already tested as prototypes and indicate that the technology is particularly attractive for areas where conventional wind energy systems cannot be operated economically.

AWE systems have other distinct advantages. Replacing the rigid tower of a wind turbine by a lightweight tensile structure directly translates into lower investment costs and a lower environmental footprint. The reduced visual and acoustic impact is an advantage for installations in ecologically sensitive areas or tourist destinations, while the low weight and compact dimensions is particularly suitable for mobile deployment.

Turbine or Traction Power?
The existing approaches can be classified by the position of the electrical generator. “Flygen” concepts use either propeller turbines on the flying device or the flow-induced rotational motion of the complete device to drive on-board generators. The electrical energy is transmitted to the ground by a conducting tether. Essential advantages are the continuous generation and the comparatively simple launch and retrieval of the flying device, using the generators as motors to provide thrust and lift for hovering away from and back to the ground station. Technological challenges are the development of lightweight generators with high power density and of conducting flexible tethers capable of withstanding high mechanical loads.

Fundamentally different, “groundgen” concepts are based on the conversion of traction power using cable drums and connected generators on the ground. Essential advantages are the positioning of the heavy system components on the ground and the possible optimization for maximum traction performance and controllability. However, a single flying device requires operation in periodic cycles, alternating between reel-out and reel-in of the tether. As a consequence, electricity generation is intermittent requiring buffering across the cycles. Continuous generation can be achieved by using multiple, individually controlled flying devices to drive a loop configuration.

Converting the Traction Power of Kites
One of these designs is the “laddermill”. Patented by former ESA astronaut Wubbo Ockels in 1996, it is based on a cable loop, which runs through a pulley at the ground station several kilometers into the sky. Kites are attached to the cable at equidistant intervals and by individually adjusting their aerodynamic properties for high lift in the upward moving section and low lift in the downward moving section of the loop, where a net traction force is established driving the generator connected to the pulley. The aim of the original concept is to access the kinetic energy of high altitude wind, however, it is obvious that a realization of such a large-scale system with many connected airborne components will be an outstanding technical challenge.

Avoiding the complexity of the airborne cable loop, the German company, NTS Nature Technology Systems, is developing a prototype system based on a cable loop, which is integrated into a horizontal rail track for kite buggies.

Most of the current activities are focused on single-kite systems. A prominent example is the kite-based traction system for large cargo ships developed by the German company SkySails. The commercially available system can achieve fuel savings of up to 35% using kites of up to 320 square meters surface area with up to 160 kilonewtons (kN) of traction force. Single-kite systems for energy generation are based on the “groundgen” concept. To maximize the energy generated in the reel-out phase, the kite is flying fast crosswind maneuvers (see Figure 2 top). This substantially increases the aerodynamic forces, lift and drag, which depend on the square of the relative wind velocity that the kite experiences. Typical flight patterns are figure-of-eights or circles, which are both used by kite surfers to achieve high speeds.

In the reel-in phase, the generator is operated as a motor and the kite is pulled back towards the ground station. The angle of attack of the wing is decreased by rotating the kite into the relative wind to reduce the traction force (see Figure 2 bottom). As a result of this de-power manuever, the reel-in of the tether requires only a small fraction of the energy generated during reel-out.

Balancing the energy across the periodic pumping cycles requires a storage mechanism, which can be an integrated battery module or a mechanical flywheel module. For a group of interconnected systems buffer capacity is less of an issue as the systems can be operated with phase-shifted pumping cycles. The Kite Power Research Group of Delft University of Technology initially tested a first experimental prototype of 3 kilowatts (kW) traction power in 2007 and since January 2010 a 20 kW prototype. The developments are co-financed with one million Euros by the Rotterdam Climate Initiative (RCI) and with 136,000 Euros by the Dutch province Friesland.

The 20 kW Kite Power System
To minimize aerodynamic drag, a single cable is used to tether the Kite Control Unit (KCU), which is suspended some 10 meters below the kite wing (see Figure 3), to the ground station. The cable is made of the high-strength plastic fiber, Dyneema. It has a diameter of 4 millimeters and a total length of 1 kilometer with the option to extend to 10 kilometers. A custom-made, Leading Edge Inflatable (LEI) tube kite with a surface area of 25 square meters generates the traction force. The main components of this inflatable membrane wing are the front tube, defining the curvature of the wing, the connected strut tubes, defining the wing profile, and the canopy. The bridle line system connects the front tube to the structural frame of the KCU. The design of the bridle system incorporates pulleys to allow for deformation of the curved wing when rotating during a de-power manuever.

The KCU essentially is a cable robot incorporating two small but strong motor-winches for steering and de-powering of the wing using the two steering lines attached to the rear ends of the wing tips (see Figure 3). It is connected to the ground station by two redundant wireless links. The autopilot software runs on the ground station computer and uses data transmitted from the KCU–the control positions and status of the different actuators–and from two sensor units mounted on the kite. The software alternates between two control modes corresponding to the two phases of the pumping cycle: a figure-of-eight trajectory control during reel-out, and symmetry plane stabilization during reel-in. The system tests in December 2011 have confirmed the reliability of the autopilot approach.

The ground station incorporates the generator with a rated power of 20 kW and a connected drum. Both are mounted on a sled, which is part of the feeding mechanism for the tether (see Figure 4). A rechargeable battery module with a capacity of 18 kilowatt-hours allows for stand-alone operation and to cover periods of low wind by keeping the system employing reverse pumping cycles. The ground station controller uses measured velocity and force data to adapt the rotational speed of the drum such that the forces stay within the limitations of the system. An important feature of the development platform is the logging of all measurement data together with the video streams of various cameras at the ground station, the kite and the kite control unit. This data is used to analyse and improve the flight dynamics and structural dynamics of the kite as well as the performance of the complete kite power system.

Operation and Test Results
More than 400 pumping cycles (equivalent to 13 hours continuous operation)” have been recorded and analysed in detail. The maximum operational altitude depends mainly on local airspace regulations. At the present test site, the former naval airbase of Valkenburg, which is located in the controlled airspace of the international airport Amsterdam, the operational altitude varies between 150 and 300 m, with the length of the cable varying between 180 and 400 m. At a more remote test site, a maximum height of 500 m with a maximum cable length of 700 m was possible. Depending on the wind velocity, the flight velocity of the kite during reel-out is between 70 and 90 kilometres per hour. To maximise the net energy per pumping cycle, the reel-in phase has to be as short as possible with the tension in the cable as low as possible. However, de-powering the kite by rotating it into the relative wind negatively affects the flight stability and steering behaviour and in practice, a compromise between achievable de-power and diminished flight authority is required. For operation at a wind speed of 7 m/s, the cable force can effectively be lowered from 3.1 kN during reel-out to 0.6 kN during reel-in at a speed of 5 m/s.

The Figure 5 shows the instantaneous traction power at the drum over 5 consecutive pumping cycles, illustrating the alternating energy generation and consumption phases. The oscillations during reel-out are caused by the variation of the relative wind velocity during the figure-of-eight flight manoeuvres, between 4 and 5 per reel-out phase in this particular test. Considering the complete cycle, the traction power average is 4 kW. As a result of systematic optimizations on component- and system-level, affecting the kite and bridle system design, the responsiveness of the ground station winch and the flight trajectory, the traction power average has been increased to presently 6.5 kW, with the goal to achieve 9.6 kW in the near future. The tests have shown that the kite energy system can be operated even with very little wind on the ground. In this case, the kite is winch-launched, like a glider plane, until it picks up the wind at higher altitudes. The current system can be operated at wind velocities up to 9 m/s, resulting in a projected aerodynamic wing loading up to 30 kg/m2. For stronger wind a smaller kite can be used up to the maximum wing loading of 40 to 50 kg/m2 for kite designs based on Nylon.

Future Application Areas
The specific design of kite power systems is attractive for a number of application areas. With a rated power between 10 and 30 kW, commercial derivatives of the technology demonstrator system are suited for distributed generation of renewable energy in remote areas or in disaster areas, especially when deployment and start-up times are crucial and fuel supply is cost-decisive. For stand-alone systems of this power range, full automation is not strictly required and, consequently, ground personnel can assist launch and retrieval of the kite. Once in the air, the kite control unit and ground station winch switch to automatic operation. The production cost of a small-scale power system will be determined mainly by the ground station and the kite control unit, both incorporating mechatronic components with various sensors and embedded control systems. The flexible airborne system components – kite, bridle system and tether – are optimized to capture and transfer the aerodynamic force to the ground. Compared to the costs for structure and foundation of equivalent wind turbines, these components are inexpensive. Due to material degradation, a replacement in periodic intervals will be required, which will affect the operational costs of the system.

Offshore wind energy could profit in a major way from the cost advantage of kite power systems. Since the generator is close to the sea level, the moment induced by the traction force of the kite is a tiny fraction of the moment induced by a wind turbine tower rising to more than 100 m above sea level. For this reason, kite power systems can be deployed from inexpensive floating platforms, which are moored to the seabed to avoid drifting. The technology of semi-automated launch and retrieval of kites with surface areas of several hundred square metres from ships has been successfully developed and commercialized by SkySails. This demonstrates that large-scale offshore deployment of kite power systems in the MW-range is technically feasible. The environmental impact of an offshore wind park of kite power systems will be lower than that of a conventional wind farm. The cables leading into the sky are hardly perceivable from a distance and the same holds for the large membrane wings sweeping back and forth at higher altitudes. As consequence, the new technology has the potential to significantly alter the public perception of wind power and thus accelerate the transition from fossil and nuclear energy to renewable energy.

Current R&D Landscape of AWE Technologies
A key challenge for current research and development activities is robust automatic operation of tethered flying devices, which is a central requirement for reliable base load power generation. Recent advances in flight control algorithms, modelling of structural dynamics, aerodynamics and flight dynamics of flexible membrane wings, and the availability of hardware prototypes with sensor equipment have led to several successful demonstrations of automated flight. Similarly important are technological advances on high-strength, lightweight and UV-resistant materials to significantly increase the durability and lifetime of the airborne system components and thus decrease the maintenance effort and operational costs of AWE systems.

The emergence of AWE as a new technology complementary to conventional wind energy is relatively recent, mainly motivated by the drastically increased demand for renewable energy. For example, the number of institutions actively involved in AWE has increased from three in 2000 to presently over 40 (see Figure 6). The last 5 years have also seen some major investments. Since 2006, Google has invested $15 million in California-based start-up company Makani. In 2010, the company received an additional ARPA-E grant of $3 million from the U.S. government and it recently won the Popular Mechanic’s 2011 Breakthrough Innovator Award in the Energy Category.

In 2011 the high altitude wind power group of the University of Leuven in Belgium received an ERC Starting Grant of one million Euros from the European Union. Figure 7

The annual event for presentation of research and development achievements, exchange of ideas and development of new visions is the Airborne Wind Energy Conference (AWEC). After the successful launch in 2010 at Stanford University, in California, the event was held in May 2011 in Leuven, Belgium. Mid 2011, SkySails announced use of their technology base of kite-assisted ship traction as a starting point for venturing into power generation. Coincidentally, Garrad Hassan, a globally operating wind energy-consulting firm, has published a market status report on high altitude wind energy, critically analyzing more than 20 of the most advanced AWE projects. Figure 8

Business research and consulting firm Frost & Sullivan, and the German Fraunhofer Institute are preparing technology assessment reports. 

Choosing the right wires and cables for wind farms may be one of the most important elements to consider in terms of longevity, reliability of service and cost of ownership for the system.

At the AWEA Windpower 2012 show in Atlanta, GA, many industry leaders lamented the fact that employing best practices in selecting equipment and materials are not always the reality.

Wire and cable manufacturers in particular say they are disappointed to see customers sacrificing projects by choosing lesser quality materials.

“Reliability and cost of ownership of all wind farm components should be the top priority in wind energy assets because the lifecycle cost of such a major investment is an important consideration,” said Jim Rosborough, North America Commercial Director, Dow Electrical &Telecommunications. “We see some developers choosing cheap and unreliable products that fail early and often when maintenance and replacement is not in the budget. This could have been avoided by using the right product from the beginning of the project.”

In the rush to cash in on wind energy, some developers trade low first costs for higher total cost of ownership over the life of the system. The lowest cost wind project is when you do it right from the beginning. Costly repairs to fix a project won’t be sustainable in the long term,” he said.

Potential failures and repairs can saddle owners, operators and ultimately the communities they serve with unexpected expenses, higher utility prices and premature equipment failure.

“Wire and cable components constitute a relatively small percentage of the total cost of the project – less than one percent. And the difference in cost between quality components and lower quality, lower price components can be less than 10 percent. Therefore it doesn’t make sense to endanger the reliability of a $500 million wind project by using off-quality materials,” said Ron Burchfield, director, Renewable Energy, Southwire Company. “The resulting cost of repairs from using off-quality materials is eventually passed to the owners and operators and ultimately the customers.”

Wire and cable suppliers and manufacturers see this as a problem with some of the developers that have only a short-term interest in a project before selling to a permanent owner. The realization of quick profits by using lower quality components will come at the expense of owners and users of the system.”

“Wind assets make money when they deliver energy to the grid. In other words, what comes out of the substation is what the wind farm gets paid for,” Burchfield said. “So all materials going into the system – from turbines to cable – need to be of a high standard from the beginning.”

Engineers say the rush to commission more wind farms has outstripped the usual development cycle of trial and error that results in mature technologies that define the equipment suited for the job. This may result in purchasing decisions based on lowest initial cost of ownership, not solutions that provide the best choice in terms of cost of ownership, network stability, higher maintenance costs and more downtime. Figure 1

Carol Godfrey, vice president of marketing and product development for Southwire’s Energy Division agrees. “If you deliver value with high performance assets and time, energy and engineering talent is considered and used properly, you’ll find longevity because of the pride and quality of workmanship throughout the system. Wire and cable is a key component of those systems.”

In a paper by the IEEE PES Wind Plant Collector System Design Working Group titled, Design and Application of Cables and Overhead Lines in Wind Power Plants, the authors provided a summary of the most important considerations for wind power plant collection systems. In their research, cable selection, cable properties, cable splicing and construction above or below ground were all considered. “Ease of installation and handling, as well as jacket material, insulation and rating, short circuit withstand ratings, and conductor material are all considered important in reliability and longevity of the wind power system.

“Making quality cable and using quality cable should be part of best practices for the wind industry. The industry currently lacks standards that are used in traditional power systems,” Burchfield says. “We’re confident in our reputation and our product because we’ve had cables, manufactured to meet or exceed power industry standards in the ground with major energy companies and our products are still doing the job.” Figure 2

Dow E&T also has a successful history with its products and Rosborough says Dow works with cable makers such as Southwire to deliver materials such as DOW ENDURANCE HFDC-4202 EC, an advanced performance tree-retardant cross-linked polyethylene (TR-XLPE) MV insulation to insure reliability.

“Dow E&T upgraded its medium voltage (MV) insulation to achieve a new level of performance for underground (UG) cable manufacturers, installers and utilities,” Rosborough said. “The HFDC-4202 EC provides easier installation, lower operating costs for the wind farm, enhanced reliability, longer life and optimized asset management.” Product Showcased

“Dow is excited to partner with Southwire because quality materials combined with quality cable manufacturing delivers the best outcome for customers,” Rosborough said. “Wind power producers must ensure reliability of the power connection for the duration of the wind farm lifecycle.”

Rosborough continued by saying that Dow has demonstrated that proper material selection is critical to the performance of cable in field applications by conducting rigid testing and surveys of utility cable engineers who identified several attributes they consider valuable for reliable performance.

Like Dow in the insulation of products, Southwire is the leading wire and cable manufacturer in North America, offering utility cable products, including overhead and underground transmission and distribution cable and other products for industrial applications. Southwire’s complete renewable energy solutions include wire and cable for turbines, towers and collection, overhead and underground transmission and substation wiring. Figure 3

“Wind is one of the fastest growing segments of our energy portfolio,” Burchfield said. “As a full-line cable supplier for this market, we’re excited to let our customers know how Southwire’s cables ensure reliable, cost-effective power throughout the life of renewable energy systems.”

Rosborough said unplanned cable repairs are costly in terms of maintenance and downtime is a profit killer that could be avoided by installing cables that exceed utility industry standards in UG cable for wind power applications. Produced Showcased

“TR-XLPE insulated MV cables made with DOW ENDURANCE products have been operating in-ground for years,” Rosborough said. “We know our products will last because they’ve already been tested in traditional utility applications. We have done studies with utilities to dig up cables to assess long-term performance. In one study, after 17 years in service, the insulation exceeded in providing electrical strength and long wear well above NA-ANSI / ICEA minimum requirements. These same cables, now 30 years in service, says a lot about the product; same thing with Southwire’s cables and wires. They have been in business 60 years in traditional utility applications and since the beginning with wind turbines in the U.S., and their products are still performing as expected.”

Rosborough said the insulation materials are produced at the company’s plants in North America and reduce dielectric losses to increase the amount of energy delivered, while eliminating failure and repair costs. The insulation results in clean and consistent performance, retention and magnitude of electrical strength efficiency. The jacketing’s physical characteristics resist moisture absorption, the conductor shield is fully bonded and the insulation shield is easy to strip.

He attributes the success of his company’s products to extensive testing and validation processes and a commitment to produce wire and cable products that exceed current power industry standards.
Burchfield agrees that being committed to developing the best products available is important to meet the unknown challenges of the renewable market, both on and offshore in a variety of demanding applications.

Southwire also offers a complete line of products for the wind market, including its TR-XLPE medium voltage cables, overhead CAMV cables, 2KV power cables, fiber-in-duct and grounding conductors, all used in the wind farm collection system.

Additionally, they produce the cables used within the wind turbine and tower, the cables used withstand the substation and a full line of both overhead and underground high voltage transmission cables.

As a single source providerwind turbine DLO, kV Cable, for transferring power in wind turbines in wet or dry areas. This cable resists oils, acids, alkalines, heat, flame and has abrasion resistance.
As a single-source provider for cables from generation to transmission, Burchfield says the company’s customizable solutions may help to improve costs, reliability and compliance throughout the energy chain. 

For more information, call 770-832-4403 or visit www.southwire.com. Also call 1-800-441-4369 or go to  www.dow.com/electric.

How can we develop wind turbines that reduce the overall cost of electrical power generation? When answering this question, investment costs as well as operating and maintenance.

(O&M) costs have to be taken into consideration. One of the focal areas of this paper is load reduction as it can play a key role in increasing turbine efficiency and lifetime. When trying to reduce loads on turbine structures, designers focus on pitch control systems. We present recent technological developments and research results in this field, especially concerning Individual Pitch Control (IPC).

Given the opportunities and challenges in wind energy, we see the need for close cooperation between turbine designers and key system suppliers. This means companies combining their strengths and expertise by jointly engaging in research, as well as the development of prototypes and systems. Our partnership approach for developing future wind energy solutions is further explained at the end of this White Paper. Figure 1

Developments in the Wind Energy Sector
How are we going to meet our energy demands in the future? When governments and businesses worldwide draft strategies to answer this question, renewable energies play an increasingly important role. Among renewable energies, wind energy has gotten a head start in the race towards competitiveness. Electrical energy generated by wind power can already compete with energy generated by fossil or nuclear sources. [1]

According to a study of the Fraunhofer ISE from December 2010, wind energy plants in locations with favorable wind conditions can already compete with conventional power plants. The costs for electricity generated by onshore wind parks are currently 0.06 to 0.08 €/kWh, which is in the range of electricity generation costs of conventional power plants using coal, lignite or nuclear fuels. [1] Consequently, wind parks are emerging from the state of research or pilot projects to commercially attractive investments. Large energy companies are increasingly investing in wind power. Among these investors are companies specialized in wind power but also electrical power suppliers who are relying on a mix of fossil, nuclear and renewable energy sources. When planning new wind parks, especially in Europe, one of the main hurdles is getting approval from authorities. Due to regulations with respect to landscape protection and noise control, the sites available for new wind parks are increasingly located offshore or, in the case of onshore installations, in remote areas. These sites are often characterized by poor access, extreme climatic conditions or non-ideal wind conditions. For the manufacturers of wind turbines this means facing new challenges: their wind turbines need to withstand extreme temperatures and work efficiently for a wider range of wind speeds. The installation of turbines in areas with poor access also places increased emphasis on operating and maintenance (O&M) costs. Figure 2

The growing importance of wind energy as a source for electrical power generation leads to stricter requirements concerning reliability and predictability of the power supply.  Meeting grid code requirements with respect to frequency and voltage becomes more and more important for operators of wind parks. Large energy companies engaged in the wind power business are likely to focus their development efforts on meeting these requirements. [2]

The necessity to lower the overall cost of electrical power generated by wind turbines (cost per mega watt) has led to a trend towards larger turbine sizes. Especially for offshore installations the cost of the foundation represents a substantial part of the overall investment cost. It proved to be more cost efficient to build wind parks with fewer large size turbines than a large number of smaller turbines. [3]

Evolution of Standard Rotor Diameters
However, larger turbines with longer rotor blades and higher tower structures are creating technical challenges for turbine designers. The longer the rotor blades, the stronger the effect of any in homogeneities of the incoming wind field. Examples of such in homogeneities are lower wind speeds close to the ground and higher speeds with increasing distance from the ground (wind shear). Such a gradient of the wind speed translates into an asymmetric load on the rotor blades. The feasibility and technical challenges of large wind turbines has been the subject of recent studies and publications, notably the upwind project sponsored by the European Commission. While the power output of commercial wind turbines is usually in the range of 1 to 7 MW, the upwind project evaluates the feasibility of a 20 MW turbine. [4] [5] Figure 3

Individual Pitch Control
How can designers build wind turbines with longer lifetimes? Recent economic and technical developments such as the pressure to reduce the overall cost of electricity generated by wind turbines, the necessity to reduce O&M costs as well as increased emphasis on reliability and predictability of power production make it urgent to find a technical solution to that question. Load reduction is a key element of the solution. In addition, load reduction gains an increasing importance due to the trend towards larger wind turbines. Individual pitch control (IPC) plays a key role in compensating loads. So what is IPC? Any pitch control system allows control of the turbine speed and consequently the power output. It also acts as a brake, stopping the rotor by turning the blades. Moreover, pitch control, especially an IPC system, has a role in reducing fatigue loads on the turbine structures. Recently developed wind turbines are variable speed turbines capable of adapting to various wind conditions. This adaption is realized via new generator concepts on the one hand, and a pitch control system on the other hand. Pitch control means the turning of rotor blades between 0° and 90°. When wind speeds are below rated power, typically below 12 m/s, the rotor blades are turned fully towards the wind which means that the pitch is positioned at 0°. Figure 4

At increasing wind speeds the pitch of the blades is controlled in order to limit the power output of the turbine to its nominal value. When wind speeds reach a predefined threshold, typically 28 m/s, the turbine stops power production by turning the blades to a 90° position.

Collective pitch control adjusts the pitch of all rotor blades to the same angle at the same time. In contrast, IPC dynamically and individually adjusts the pitch of each rotor blade. Based on current individual loads this pitch adjustment is carried out in real-time. The main benefit of IPC is the reduction of fatigue loads on the rotor blades, the hub, and mainframe and tower structures. In order to compensate these loads, especially symmetric loads caused by inhomogeneous wind fields, the pitch of each rotor blade has to be adjusted independently from the other blades. A reduction of fatigue loads has two considerable advantages: It allows lighter designs and translates into longer lifetimes of wind turbines. What is meant by lighter designs? In cases where components are designed according to fatigue loads, a reduction of these loads allows savings in cost and material notably for the rotor blades and the tower structure, which are the most expensive elements of a wind turbine. Moreover, lighter rotor blades enable a more efficient turbine, especially in low wind conditions. Finally the load reduction through IPC gives designers the option to develop low wind turbines from existing designs, which means a reduction of time to market.

Load Reduction – A Technical Overview
During start-up, regular operation (power generation) and shutdown a wind turbine is subject to various forces causing peak loads and fatigue loads. In the context of this paper we shall focus on fatigue loads because they can be influenced by pitch control systems most effectively. Let us consider a wind turbine in operation at nominal wind speed and above, and have a look at the forces acting on the main elements of the turbine e.g., rotor blades, hub, mainframe, and tower. The rotor blades are subject to periodic bending forces. These bending forces are acting in two ways: First, edgewise bending in the direction of the rotor movement and second flap wise bending in the direction perpendicular to the plane of movement. There are also forces on the hub, mainframe and tower structures. These forces have two effects, in particular on the tower: The yaw moment (Myaw) is twisting and the tilt moment (Mtilt) is bending it. Figure 5

When designing strategies to counterbalance the forces discussed above, a first step is an analysis: The Fourier analysis gives what is usually called the 1p, 2p, 3p, components of the loads. Classic IPC, which is most often used, only compensates for the 1p component. Other components can also be addressed by IPC but their compensation requires increased pitch activity and more dynamic control systems. Highly dynamic control systems would also be necessary to fully benefit from recent developments in wind measurement. The newest measurement technologies such as LIDAR (Light Detection and Ranging) provide real-time information on wind conditions and forecasts for the next few seconds. Based on this information it becomes possible to prevent peak loads by using IPC to develop preventative load alleviation strategies. [4]

Advanced blade sensing systems provide information about the load condition of the rotor blades in real-time. These monitoring systems rely on sensor technology using e.g., optic fibers embedded in the rotor blade material. Using IPC and blade sensing systems to adjust pitch actions to the actual loads measured for each rotor blade individually becomes an obvious choice. Figure 6

Economic Considerations and Technical Challenges
Compared to collective pitch systems, IPC systems require higher investment costs. These investments relate particularly to a more complex control strategy, higher requirements for the pitch motor and increased fatigue loads on the pitch bearings and pitch gears.  However, as shown in the graphic below, the cost of any pitch system is low in comparison to the overall cost of a wind turbine. The savings in other components due to reduction of fatigue loads have the potential to compensate for the extra investment for an IPC system. When creating the business case, turbine designers would therefore need to consider the entire cost of a wind turbine. When not only investment costs, but also O&M costs are taken into account, IPC could be even more beneficial. This is due to load reductions, which translate into an increased lifetime of turbines. Experts predict increasing potential for reducing the total cost of ownership for turbines using IPC systems. To achieve this it would be necessary to optimize and adjust existing systems through careful evaluation of, not only the components of the IPC system, but of the entire turbine design. Focus should be on the evaluation of extreme loads during start-up and shut-down, fatigue loads on the main shaft and main bearing, the use of new types of pitch bearings and the optimized utilization of load sensors.

For these reasons, the best and fastest results are expected through close cooperation between manufacturers of turbines and IPC systems.

Expertise and Partnership Approach
In the field of pitch control systems for wind turbines, Moog has in-depth experience highlighted by an installed base of more than 25,000 systems. We are a supplier of all necessary products for pitch control systems, including software and hardware. This means that critical products such as Pitch Servo Drive, Pitch Motor, Slip Ring and Blade Sensing System are all designed and manufactured by Moog. A partnership approach in adapting to our clients’ needs is especially important. This approach is characterized by the flexibility to adjust to the technology chosen. We can tailor our products for example to the needs of electric as well as hydraulic pitch systems. Figure 7

Demand For Renewable Resources
Demand for electrical power generated by renewable sources is increasing on a global scale. The prospects in the wind energy sector are especially promising. However, this development also brings new challenges including: Larger turbines, wind parks in remote areas with difficult climatic conditions and higher expectations on reliability, flexibility and predictability of electrical power generation. When turbines are getting larger, load reduction, especially for asymmetric loads caused by inhomogeneous wind fields, becomes more and more important. Consequently, the manufacturer anticipates that IPC will play an increasingly important role as the most common technology capable of compensating asymmetric loads. The ultimate aim of this cooperation is to reduce the overall cost of electrical power generated by wind turbines. To achieve this it is necessary to reduce the total cost related to the design, construction and operation of wind turbines to develop the wind energy solutions for the future.

Moog has supplied more than 27,000 systems and products to many of the world’s top-ten wind turbine manufacturers. The company’s wind industry products and expertise span both electric and hydraulic technologies. For example, by precisely monitoring wind loads on blades, the rotor monitoring system improves the turbine’s life span and maintenance costs. Predictive maintenance is vital to wind park operators because the cost of a shutdown and subsequent turbine repairs is high.

The Pitch Systems also improve safety when the wind turbine loses electrical power. The pitch system puts the turbine blades off-wind into a safe operating mode that protects the wind turbine from damage. When the wind blows at 25 meters per second (50 mph) or higher, a wind turbine needs a failsafe to put its blades at an angle where the load is reduced and the wind turbine stops. Found in the hub of the wind turbine, the Moog Pitch System consists of: control boxes containing Moog Pitch Servo Drives; Wind Pitch Servo Motors; and, a control system including software for remote diagnostics and back-up power. Figure 8

The company also offers slip ring solutions, which are critical to operation. Found inside the wind turbine’s nacelle, slip rings provide electrical signals and energy for blade pitch power and control. The fiber brush slip rings offer wind turbine owners a minimum of 100 million revolutions of operational life with no maintenance.  

REFERENCES
[1] Kost, C., Schlegl, T. Studie Stromentstehungskostenerneuerbare Energien. Fraunhofer ISE, Freiburg,Germany. December 2010
[2] El-Henaoui, S. Wind Farm Requirements are Changing. Moog Research Report. Unna, Germany. April 2010
[3] Spada, S. The Reliance on Wind Energy Depends on Advancements in Blade Pitch Control. ARC Insights, Insight # 2010-23MPH. July 2010
[4] UpWind, Design limits and solutions for very large wind turbines. The Sixth Framework Programme for Research and Development of the European Commission. EWEA, Brussels, Belgium. March 2011
[5] Bossanyi, E., Wright, A., Fleming, P. Further Progress with Field Testing of Individual Pitch Control. Garrad Hassan & Partners Ltd., Bristol, UK
[6] Shan, M., Duckwitz, D. Study on Individual Pitch Control, Internal Report. Moog & Fraunhofer IWES, Kassel, Germany. March 2011
[7] Rösmann, T., Adelt, S. Kollektive- und Individuelle Pitch Regelung von Rotorblättern. Moog & VDI Wissensforum, Unna, Germany. April 2011
[8] Engelen, van T.G., Hooft, van der E.L. Individual Pitch Control Inventory. ECN-C–03-138. June 2005
[9] Hauptmann, S. et al. Deliverable D1: State-of-the-Art-Report. PROTEST project, University of Stuttgart,Germany. April 2009
[10] Rösmann, T., Adelt, S. Moog IPC Integration – Technical Assessment of MLS Approach. Unna, Germany. June 2010