Power Performance Upgrades to Wind Turbines

An evaluation of quantitative methods to measure effects of power performance upgrades to wind turbines.

Part 1 of 3: Mapping of methods to increase power performance.

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SummaryThe following document is a summary of the first deliverable in a project to evaluate quantitative methods to measure effects of power performance upgrades to wind turbines. The project is a Master Thesis in collaboration between the Royal Institute of Technology in Stockholm and Breeze.

The project consists of three deliverables.

1. Map upgrades for increased power performance from wind turbines

2. Describe the dynamics between manufactures and owners of wind turbines

3. Evaluate methods for measuring power performance upgrading

The purpose of this work is to incorporate methods of evaluating power performance upgrades into the Breeze Production wind farm management system.

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FocusThis documents presents a general overview of which methods and products are available for improving power performance, rather than presenting specific solutions of the individual actors. Focus is on wind turbines that have a nominal capacity greater than 1 MW.

Emphasis is placed on general upgrading techniques and concepts. Other means of improving production such improving power quality, availability and extending the lifespan, are not covered.

The upgrading methods presented are mainly based on the current product portfolios of OEMs and other actors.

Twelve of the largest wind turbine manufacturers established on the international market have been studied, along with other actors within the performance optimization field.

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Table of Contents

Summary 2

Focus 3

Definition of Power Performance 7

Upgrades to Different Regions of the Power Curve 8

Terminology & Concepts of Wind power Upgrades 9

Power Performance Upgrade Activities 10

Improve Wind Turbine Controls 12

Control System Updates 12

Wind Farm Control 13

Pitch Controls 13

Intermittent Wind Energy Capture 13

Handling of Special Conditions 14

Tuning and Optimization 14

Site Specific Tuning 14

Individual Turbine Tuning 15

Nacelle Misalignment 15

Aerodynamic Performance 15

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Blade Add-Ons 16

Blade Size 16

Blade Restoration 18

Modernization and Retrofitting 18

Overhaul and Modernization 18

Retrofit Entire Control Systems 19

Retrofit Drivetrain Components 19

Retrofit Electrical Systems 19

Grid Compatibility 20

Restoring Power Performance 20

Trends 21

Aerodynamic Performance in Focus 21

Site Optimization 23

Improved Anemometry 23

Power Uprating From Factory 24

Growing Fleet of Aging Wind Turbines 24

Combination of Technologies Into Packages 24

Marketing Approaches to Upgrading Packages 25

Relevance to Breeze 26

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Definition of Power PerformancePerformance is an ambiguous term that includes all sorts of improvements to the operation of wind turbines. Examples are reduction of load and wear, lower noise and increased energy production. A more accurate term to describe the power production capability of a wind turbine is power performance.

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Wind Speed

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The International Electrotechnical Commission (IEC) defines power performance as a “measure of the capability of a wind turbine to produce electric power and energy”. Hence, the power performance characteristics of a wind turbine are independent of availability. The power performance characteristics of a wind turbine are manifested by the power curve.

For this study, the power curve will be used as the basic framework for evaluation of power performance of a wind turbine. The power curve is a simple visual representation of a wind turbine’s performance, displaying net electric power output for every given wind speed.

The following terms will be used when referring to improvements to the different regions of the power curve:

1. Increase part-load efficiency – improve power curve in the partial load range.

2. Power up-rate – increase the rated power / nominal power output.

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3. Extend range of operation – extend range of operation by increasing the cut-out speed, often by gradual load ramp-down. The cut-in speed is often fairly set, and adjustments have marginal effect on the energy production.

Upgrades to Different Regions of the Power CurveThere are different limiting factors to the power performance at different regions of the power curve. A specific upgrade often targets a certain range of operation.

In the part-load range, the power performance is dependent on the overall efficiency of the whole wind turbine. Essentially every subsystem that impacts wind turbine operation can be upgraded to improve efficiency. Aerodynamic performance is one key focus area commonly targeted for improving part load efficiency.

In the full-load range, the power performance is dependent the wind turbine’s max capacity and safe operation under high loads. It’s important to minimize thrust and other forces, mitigate loads, and to operate close to the hardware limits to utilize the full potential of the wind turbine.

The nominal power is limited by the loading capacity of the hardware. If there is a certain system limiting the loading of the machines, upgrading key components might allow for power uprating. Given that a wind turbine is not fully optimized and there is untapped potential due to favorable conditions, the controls could be adjusted to allow operation above the original design limits.

Extending the range of operation is also limited by the hardware capacity. However, improving the operational range of a wind turbine is to a larger degree possible by adding enhanced or smart features to ensure safe operation and shut-down at high wind speeds. Loads and forces can be reduced by smart controlling of pitch and rotational speed, as well as gradual ramp-down and smart shut-down features at high wind speeds.

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Terminology & Concepts of Wind power UpgradesTo be able to discuss upgrades and impact on performance of a wind turbine, it is important share common a ground on basic concepts. The terminology of upgrading often overlaps, and the marginal differences are easily confused. These are some of the most commonly used expressions:

Upgrade To improve a system, either by updating or replacing key components.

Retrofit (fit in retrospect) A means of upgrading a system, which implies adding new components or features in retrospect to it. Retrofitting a system can denote either replacing it or adding additional equipment to it.

Recondition / refurbish To restore a system to original condition and functionality; worn-out and damaged components are replaced.

Overhaul To make an extensive inspection and reconditioning of a system, often associated with a complete disassembling and reconditioning of a wind turbine.

Modernization To modernize a system is to bring it up-to-date. A wind turbine modernization often denotes a complete system overhaul and reconditioning, as well as major retrofitting of control systems.

Re-power (re-equip) To remove old wind turbines and fit new wind turbines in their place.

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Power Performance Upgrade ActivitiesStudying the solutions available on the market and interviewing actors, the most common power performance enhancing activities of manufacturers and third parties were identified and categorized. The most pronounced effects on the power curve are indicated for each activity.

Categories ActivitiesPower up-rate

Part load efficiency

Ext. opera-tion range

Improve wind turbine controls

Control system updating

Wind farm control

Pitch control

Intermittent wind energy capture

Handle special conditions

Tuning and optimization

Site specific tuning

Individual turbine tuning

Nacelle misalignment

Aerodynamic performance

Blade add-ons

Increase blade size

Blade cleaning / restoration

Retrofits and modernization

Overhaul and modernization

Retrofit control systems

Retrofit drivetrain components

Retrofit electrical systems

Grid compatibility

Restoring power performance

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Improve Wind Turbine ControlsSmart control systems are integral in power performance upgrading, and control systems are becoming more and more flexible and powerful. Updating software and adjusting controls can make substantial difference to the operation and behavior of both individual wind turbines and the wind farm as a whole.

Control systems are can improve wind turbine power performance in all stages of a wind turbine’s life cycle. Software updates keep the modern machines’ controls up-to-date and efficient. Both new and old installations can be optimized and tuned, and new features can be added in retrospect according to need and site conditions. For aging machines, retrofitting control systems can make significant impact on the efficiency of operation.

Control System Updates

With regards to wind turbine controls, improvements can be made in all regions of the power curve. Smarter controlling can increase overall energy efficiency, and mitigate loads and reduce wear of mechanical systems. Such operational improvements may allow extended operational range or power up-rate.

Software updates are often issued on a regular basis by most manufacturers. These constitute improvements of the control algorithms and adjusting of control parameters for certain turbine models. The impact of software updates depends on the degree of delimitation of the original hardware and how well optimized the system was at commissioning. Well-proven and mature platforms have less room for improvement from software updates.

Hardware upgrading of controls may be required for making major updates or adding control features to the old system. Retrofitting of new auxiliary systems such as monitoring or lubrication might need new controlling hardware and integration with the present system. Major retrofitting is mainly carried out in connection with major control system modernization.

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Wind Farm Control

Aside from the individual turbine’s performance, an important factor for the overall energy production is how well the wind farm is managed as a unit. The operation of individual wind turbines can adjusted for the benefit of the whole farm. Depending on wake effects and turbulence, the turbine operation can be adjusted accordingly.

Wind Sector Management is a commonly used term, meaning that individual turbine’s operation is adjusted depending on turbulence or loading of different wind directions.

Up-rate turbines in low turbulence conditions – winds from flat terrain / water.

De-rate turbines subject to wakes / turbulent conditions, and reduce wake effects onto other turbines.

Pitch Controls

These days most machines are pitch regulated. Individual pitch control is today standard for megawatt wind turbine and enables advanced and rapid control adjustments in real time. Each blade is controlled individually during each rotation cycle to account for variations is wind speed at lowest and topmost positions of the blade. Smart pitching is key for improving efficiency and mitigating loads. Work lies in optimizing the operation in several aspects, including:

• Maximizing wind energy capture, accounting for wind intermittency, gusts and special conditions

• Minimize thrust forces and loads transferred to the main shaft and drivetrain

• Minimize loads and wear on the pitching mechanism itself to avoid fatiguing

Intermittent Wind Energy Capture

A recent focus area for controls and pitch development in particular is to improve energy capture of intermittent winds and gusts, by predicting or measuring the winds ahead of the turbine and making control adjustments in advance. The wind turbine needs a small window of time for adjusting rotational speed or pitching in a smooth and efficient

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manner. Several actors are looking into the possibility to use LIDAR technology for improved energy capture, by measuring of the free-steam wind in front of a wind turbine.

Handling of Special Conditions

Operating conditions out of the ordinary and environmental hazards can force wind turbines to shut down or operate inefficiently. Availability and operational efficiency may be improved significantly by adding features to control systems for handling special events.

The majority of special features address availability-related issues such as icing, lightning, power blackouts or grid instability. By implementing new features and hardware for handling such events, the wind turbine can stay operational and avoid damage. Power performance can be improved by adjusting operation to endure extreme wind speeds, and practically means extending the operational range of the power curve.

Tuning and OptimizationWind turbines which are designed for certain wind classes should meet certain standards and be able to endure certain conditions. Thousands of turbines have been installed world-wide in very different settings. Wind turbines which were designed for the same operating conditions, experience vastly different climates and temperatures, and even turbines within the same site can be loaded much differently.

Site specific and individual turbine tuning of wind farms can make considerable improvements to operation and performance. It allows each wind turbine to be operated much closer to its true limits, in the setting it is operation, reducing loading peaks and wear. Many old wind farms are targets for optimization projects as owners seek to improve the proficiency of their assets.

Site Specific Tuning

Control parameters of specific wind turbine models are often tuned with regards to the specific site conditions. Besides improving operation and part-load efficiency, favorable

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conditions may allow higher loading of electrics or drivetrain. The following factors are often taken into account:

• Wind characteristics (wind speed, turbulence)

• Weather, climate and atmospheric conditions (temperature, air density, icing)

• Seasonality and variations in time

Individual Turbine Tuning

Each individual turbine in a wind farm has its own operational characteristics. There are many factors making each machine individually different, such as:

• Settings with relevance to wind sector management, wake effects

• It’s siting with respect to surrounding terrain and topography

• Operational characteristics (condition, loading, historical data)

Nacelle Misalignment

Recent years, nacelle misalignment and related production losses have been given more attention. Several companies are working with detecting and correcting measurement errors in wind direction. Static off-sets in the measurements occur due to interference from the wake and nacelle geometry affecting the anemometry. Corrections can be made by measuring the free-steam wind in front of the turbine using a spinner-mounted ultrasonic sensor or a horizontally mounted LIDAR.

Aerodynamic PerformanceIn the part load range of operation, wind turbine power performance depends heavily the rotor swept area and its aerodynamic performance. Blade size have major impact, as it limits the wind energy available. Key factors affecting the aerodynamic performance are airfoil aerodynamics, and rotor speed and pitch controlling. Changing blades is not often

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feasible, and the main improvement areas are control improvements, blade add-ons, and keeping the blades in good condition.

Blade Add-Ons

One currently popular type of blade retrofit is blade add-ons. Aerodynamic modules can be retrofitted for improving aerodynamic performance with regards to lowering noise and improving energy capture. These are the most common types:

• Vortex generators

• Blade cord extensions

• Trailing edge serrations

• Tip shape / winglets

Other blade add-ons aim to protect the blades, such as lightning protection and protective surfaces reducing erosion on the leading edge.

Most commonly, blade add-ons are attached at-site by accessing the blade by rope or by detaching the blades and mounting the add-ons at site. Since the installation is relatively fast and requires little downtime, this retrofit poses little risk to the wind turbine operation of economics.

Blade Size

Choice of blade size depends on wind characteristics. Larger rotors means higher wind energy capture, but also considerably larger loading forces. Commonly, R&D focuses on developing lighter and stronger blade structures and materials. Keeping the massive blades light is beneficial for logistics, and imperative for keeping down loads on the wind turbine.

Fitting wind turbines with larger blades could be an option, if the old blades need replacing due to damage. However, controls must be adjusted and structural and mechanical upgrades might be required. Just as retrofitting of other major components, it is not common practise to change blades for the sole purpose of improving power

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performance as the costs of new blades, production downtime, and installation are substantial.

There are upgrading concepts where the existing blades are lengthened. Blade extension is often a major procedure, which requires the blades to be detached, extended and reworked at a factory, before finally being reattached. For small wind turbines, there are less comprehensive procedures such as fitting extending add-ons.

Blade Restoration

For wind turbines in exposed conditions, the blades may over time be worn or build up dirt and debris to such a degree that it affects the operation. The need for blade maintenance very site-dependent. While some wind farms may never need blade maintenance, other site conditions may impose the need of regular blade cleaning, washing or repairs to maintain efficient operation and avoid blade failure. Blade maintenance can have noticeable impact on the aerodynamic performance.

Modernization and RetrofittingRetrofitting and upgrading is an interesting area with many different solutions for improving power performance. Power performance boosts can be achieved by increasing blade size, modernizing controls and making capacity upgrades to drivetrains. These are all major procedures, not commonly applied on a whole wind farm unless it is necessary. Owners do generally not change major components unless necessary, as such major procedures are both expensive, and causes long stops and downtime. The main interest lies in finding smaller retrofit and tweaks, for cost-effective and risk-free investments.

Overhaul and Modernization

Wind turbine modernization is a major procedure, including complete shutdown and at least partial dismantling of the turbine. All major components are reconditioned or exchanged, and the machine is often retrofitted with new up-to-date technology and functions. In connection with refurbishing components, design improvements and smart tweaks can increase the loading capacity of original components. Bearings, blades, pitch-

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control, gearbox and electrical systems are often revised. It is common that control systems undergo major retrofits or get completely exchanged.

Modernizing a wind turbine is a major reinvestment in the machine, and is mainly done to extend the lifespan of the wind turbine. Along with all the technological improvements, power performance can be improved in the whole range of the power curve.

Retrofit Entire Control Systems

Aging wind turbines often benefit substantially from retrofitting control systems, as outdated electric systems tend to become obsolete. Major retrofits of controls are often carried out in connection with other major overhauls to extend lifespan of the machine. Retrofitting control systems may implement completely new controlling mechanisms, such as converting old wind turbines from stall to pitch regulation. By more efficient, smart controlling, and load mitigation, improvements can be made in all regions of the power curve. Besides increased efficiency, smoother and more stable operation may enable safe operation also at higher wind speeds, extending the cut-out speed.

Retrofit Drivetrain Components

Increasing the power performance can be done by upgrading drivetrain components. The drivetrain consists of the rotor, gearbox, and generator, as well as the major mechanical components transferring the mechanical energy in between. By upgrading bottlenecking drivetrain components, the max mechanical loading can be increased, which may enable an increase of nominal power.

Major component upgrading is not commonly seen for the sole purpose of upgrading power performance, but opportunity to upgrade is rather taken in connection with necessary component exchange as due to wear or failure. Most commonly, major drivetrain upgrading are carried out in connection with major turbine modernizations and restorations.

Retrofit Electrical Systems

All electrical power conversion and transmission is subject to losses, just as any other energy system. Major electrical components and systems are not commonly retrofitted

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for improving efficiency. Rather, electrical systems are upgraded if they come to bottleneck the power production, or in connection with other major overhauls if they are too out-dated and obsolete.

For example, upgrading the generator and associated electrical equipment is a costly procedure which is rarely seen as a cost-effective standalone upgrade. However in connection with major modernization of aging wind turbines, upgrading the generator may improve electrical efficiency, operation, and allow for power uprating. There are also cases where slightly more powerful electrical systems are fitted already at manufacturing for a slight power up-rate.

Grid Compatibility

Another feasible upgrading aspect to electrical systems of wind turbines is the interaction with the grid. High power quality is characterized by stable frequency and voltage, free of disturbance and interruptions. Grid codes of different markets are varying, as the grid stability and capacity, and power production looks very different. Markets may demand forecasting of wind power production due to the intermittent nature of wind power, or offer incentives for compensating reactive power.

These are examples on areas of which OEMs work to improve their wind farms:

• Smoother power delivery; Reduce step-up effects, Ramping up and down cut-in, cut-out, and emergency stops

• Compensating reactive power production

• Forecasting and electricity production

• Supporting grid frequency by controlling power output of the wind farm as a whole

Restoring Power Performance

In most cases, wind turbine power performance and availability decades over time. Restoring original functionality of long term degraded wind turbine systems, much resembles the characteristics of a power performance upgrading events. For service providers, the majority of the power performance work is not upgrading, but rather identifying and correcting underperformance.

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There are endless ways of repairing parts, rectifying malfunctions and otherwise restoring wind turbine systems. All O&M related services cannot be listed, but are simply acknowledged as possible power performance enhancing actions.

Trends All major turbine manufacturers work essentially with the same type of activities for improving power performance. There are differences in the technical solutions, but primarily in how services are marketed and offered to end customers.

Work is ongoing in aerodynamics performance, anemometry and site optimization. Bundling different upgrades into more comprehensive packages are commonly seen on the market. As the fleet of operational wind turbines grow, the outlook for the after-sales market looks promising.

Aerodynamic Performance in Focus

Much focus of power performance upgrading lie on the aerodynamic performance. Blade size have substantial impact on energy capture, and much effort is put into improving blade integrity and developing materials and structural strength. Blade size is integral for developing larger wind turbines, which continues to be a central development in the power business. For large machines, a small increase in tower height allows for substantially larger blade sweep and power output. Recent years, several wind turbine manufacturers have launched wind turbine in the 5-8 MW range for both on- and off-shore applications.

In terms of retrofits, aerodynamic add-ons are frequently seen from OEM and other actors. Vortex generators are frequently installed on both new and old machines, but also blade cord extensions and trailing edge serrations are seen from several actors as retrofits. Other blade add-on solutions aim to protect the leading edge and blade tip from lightning.

Control systems are central for aerodynamic performance, especially pitching. OEMs work with developing systems rapidly responding to wind intermittency, adjusting rotation speed and pitch for optimal energy capture and for reducing forces and loads. Horizontal

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measuring LIDAR for measuring the free-steam wind can provide upwind data to be fed into the control system.

Site Optimization

Site specific optimization of operational wind farms is getting increasingly popular, and many service providers offer services for optimizing wind power assets. Much of the OEM activities focus on fine tuning and adjusting controls for optimum performance to the specific site conditions, as well as retrofitting smart features.

Third party companies tend to get more involved in unbiased evaluation of performance. Inspections and follow-ups from an independent part are key to ensure the wind farm performance is in line with the availability and power performance of guarantees. Actors with new technologies are working for acceptance and standardization of new methods for verifying power performance.

Improved Anemometry

Some wind turbine manufacturers have started using ultrasonic anemometry on wind turbines for improved low wind accuracy and icing resistance.

A recent trend is to correct nacelle misalignment, by measuring the free-steam wind and compensating the static off-sets of the vane. Increasing the accuracy of anemometry and measuring the free-steam wind is also useful for improving pitching.

Improvements to anemometry can be used as a tool to evaluate wind turbine performance. Testing wind turbine power performance is today a major procedure. The only IEC accepted method requires a met-mast of the same height as the wind turbine installed in its proximity. The disadvantages of this mast based method include high cost, relatively short time duration and lack of ongoing monitoring capability. New anemometry technologies may develop into powerful tools for wind power owners, for power performance evaluation and follow-up.

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Power Uprating From Factory

Wind turbines are normally tailored with different features for each individual client to suit their specific projects. Some OEMs offer ways of tailoring even the power output directly at manufacturing, to maximize wind farm capacity. Project permits allow a certain type of wind turbine and power rating, and in certain cases a marginal power uprating of 5% still fits within the same project permit. Several OEM have ways of modifying their wind turbines at manufacturing to fill out this power output margin, by for example fitting a slightly up-rated turbine generator and make adjustments to the electric systems to allow for the higher loading.

Growing Fleet of Aging Wind Turbines

The capacity of wind power have rapidly increased the last decade and the market is still growing. An increasing number of wind turbines are coming of age, and even closing in on their economical end of life. The O&M market is shifting from sub-megawatt machines to progressively larger megawatt machines. The trend points towards ever increasing numbers of megawatt wind turbines in need of servicing and eventually major overhauls.

The after-sales market is growing appears to have a bright future. Many independent companies are refurbishing and upgrading aging wind turbines of different brands, and also several OEMs are unconventionally offering solutions reaching beyond their own portfolio of wind turbines.

Combination of Technologies Into Packages

A noticeable trend amongst most OEMs, is the marketing of upgrade packages for their sales-leading turbine platforms. The upgrading packages are often a combination of different types of upgrades, which can be tailored to the customer. Most commonly the upgrades consist of several minor upgrades, often retrofits of technologies featured in the next generation of wind turbines such as vortex generators and special control features. The following improvements are frequently seen as components in upgrading packages:

• Turbine system updates

• Site specific optimization

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• Grid compatibility

• Blade aerodynamic add-ons

The upgrading packages most often affect several aspects of the power curve, sometimes the whole range. Packaging several upgrading events, makes for a more substantial and noticeable impact on power performance. Most of these upgrading packages promises results in the range of 3 – 5% increase of annual energy production.

Marketing Approaches to Upgrading Packages

Upgrading packages are often presented as risk free deals, where the customer pays depending on the performance increase. This reduces the financial risks and payback time for the customer, but also means that part of the revenue increase from the upgrade will be shared by the OEM.

There are several upgrading packages which are strongly marketed and publicly showcased. However, some OEMs choose a more subtle approach, avoiding public marketing or keeping information on performance upgrading completely undisclosed.

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Relevance to BreezeWith more turbines comes more data and a universal need to collect and understand this data.

Breeze is a system used by active owners and operators to increase energy production. A key component in increasing power performance is to identify under performing wind turbines and to discuss with the manufacturer what actions can be take to increase power performance of individual wind turbines or the wind farm as a whole.

Upon identification of under performing wind turbines and deciding on an action plan Breeze will have the capabilities for measurement and verification of the effects of such actions. By quantification of the ROI, the owner will know if actions make financial sense to implement on other wind turbines.

This is an exciting new field that will become more and more relevant as turbines are managed under production based availability contracts and turbines come out of warranty.

To learn more about Breeze, visit www.breezesystem.com.

Greenbyte AB

Kristinelundsgatan 16

411 37 Gothenburg, Sweden

Phone: +46 (0) 31 788 03 00

sales@breezesystem.com

www.breezesystem.com