Wind Power: Developing the world’s first ‘stealthy’ wind turbine blades

Andrew Beck, QinetiQ Ltd, UK

To help meet ambitious new government targets for renewable energy in the UK, QinetiQ expertise is being applied to the various stages of wind energy site and system development, including the development of the world’s first ‘stealthy’ turbine blades.

Wind is an increasingly important source of energy in many countries, offering cost-effective solutions and the opportunity to reduce reliance on fossil fuels. As a result, plans to encourage the use of this sustainable energy resource through the development of wind farms are currently being actively encouraged by governments around the world. Within the specific context of renewable energy requirements for the UK, wind power has an important role to play.

The UK has over 33 per cent of the total European potential offshore wind resource. As a result, the UK has the potential to harness a significant proportion of its renewable energy generation from offshore wind farms.

Figure 1. The UK’s DTI, Qinetiq and NOI are working to develop the world’s first stealthy wind turbine blades
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The process of securing the various planning consents associated with new wind farm developments involves many considerations, including the potential interaction with civilian and military aviation radar systems. A potential area of concern for these systems is that wind turbines can generate radar returns, which are very difficult to distinguish from real target returns on radar displays. The returns from wind turbines have the potential to distract and confuse air traffic controllers and can effectively mask genuine aircraft returns in the vicinity of the wind farm. This is a major safety concern for both civilian and military radar operators. Additional concerns include the potential for low flying aircraft to ‘disappear’ in the radar shadow created by wind turbines and for indirect reflections off wind turbines to appear as spurious aircraft returns in false directions. These concerns have led to a large number of wind farm planning objections from civilian and military aircraft operators that treat as paramount the maintenance of air traffic safety.

QinetiQ, with its origins lying in the very roots of aviation history, and with an equally long and distinguished background in such fields of research as materials science, structural electronics and computer modelling systems, is well placed to contend with these problems. Funded by the UK’s Department of Trade and Industry (DTI), the company’s scientists undertook to predict the levels and types of interactions between wind farms and radar systems.

Stealth technology

The study, which has now been successfully completed, demonstrated that radar effects could be mitigated through one or more combinations, such as wind farm design, software fixes for the radar systems and the use of what has become known as ‘stealthy’ or radar-friendly turbine blades. QinetiQ, in partnership with NOI Scotland Ltd, a UK-based manufacturer of wind turbine blades, has now received funding from the DTI to proceed with a project to develop the world’s first stealthy wind turbine blades.

The planning of wind farms requires careful consideration of possible disturbances to local radar and communications systems that may be caused by these large rotating structures. Now, working with the DTI and Scottish firm NOI, QinetiQ is transferring its stealth technology to ameliorate the effects of wind turbines. Because of their size (turbines can be up to 180 m high) and movement, wind turbines can pose a serious problem for radar operators. The magnitude of the radar return, or clutter, from a turbine can be 1000 times that of a small plane, but unlike large static structures such as buildings, they are difficult to filter out.

Figure 2. The RCS for a single wind turbine for just under two-thirds of a complete revolution
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An essential element in considering the detection of a wind turbine by radar is the strength of the radar reflection from the turbine itself. This is measured by its RCS (Radar Cross Section), an area usually measured in square metres. It can have a wide range of values and is often quoted on a logarithmic scale. Figure 2 gives a sense of the scale of wind turbine RCS values. This shows why wind turbines are easily detected by radar, as their RCS is comparable with some of the largest aircraft targets. Aviation radars must be sensitive enough to see the small aircraft, which tend to have RCS values of about 1-10 m2, whereas the turbine return is up to 1000 m2 in some instances.

Twinkling effects

The radar returns from a wind turbine tower and nacelle are normally filtered out by aviation (air traffic control) radars, in the same way that returns from other large static structures are filtered (e.g. buildings). The returns from moving wind turbine blades are much more difficult to filter because their speeds can be comparable to real targets (in excess of 100 km/h). Each blade will only be seen by the radar when it is in a particular range of positions (the RCS varies significantly as a function of position), but the blades of a single turbine will always appear in the same place on the radar display. Therefore, when a number of turbines are present in a wind farm, the radar may detect a blade (or blades) from one turbine on one scan, but the blade(s) of a different turbine on the next scan. This can create the effect of the returns apparently moving within the area of the wind farm over time, which is sometimes described as ‘twinkling’.

This effect is highlighted in Figures 2 and 3. Figure 2 shows the RCS for a single wind turbine for just under two-thirds of a complete revolution. The radar is illuminating the turbine from the right hand side (equivalent to 90 degrees yaw angle) and the blades are rotating in an anti-clockwise direction. The contributions from static components, such as the tower and the nacelle, have been removed. The blade positions that generate the RCS peaks are identified in Figure 3. The large peaks (1 and 3) are when a blade is at the bottom of the rotation with the leading edge towards the radar, and the smaller peaks (2 and 4) are generated when a blade is at the top of the rotation with the trailing edge towards the radar.

Figure 3. The blade positions that generate the RCS peaks are identified above (1 and 3)
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The potential for false returns to have detrimental effects (masking of real targets, false returns and shadowing) on aviation radar is a major concern, which has resulted in a large number of planning objections in the UK. To get around the problem, QinetiQ is developing turbine blades that are far less visible to radar. By modifying the composition of glass fibre reinforced polymer (GFRP) blades, which can be some 50 m or more in length, blades can be made to absorb radar signals without compromising their structural strength or adding significantly to the costs of production. In turn, the technology reduces, or can remove altogether, spurious signals received by radars, without adding significantly to the cost. The idea is to reduce the radar return sufficiently so that it can produce an acceptably low impact on the radar system’s overall effectiveness and compatibility.

A world first

QinetiQ has received funding from the DTI to proceed with a project to develop the world’s first stealthy wind turbine blades. The project will develop and prove absorbing blade materials to counter the range of frequencies over which aviation, marine and weather radar systems operate. The first phase of the project is focusing on a detailed radar cross-section assessment on NOI Scotland’s existing 34 m blade design. The assessment is using sophisticated computer modelling tools, developed by QinetiQ, to identify ‘hot-spot’ radar problem areas and assess the benefits of different levels of stealth.

The results of the assessment will then be used to identify the benefits of various levels of stealth in terms of reducing the radar impact. Linking back to the earlier explanation of the ‘twinkling effect’ with aviation radars, the challenge for stealth technology is to remove or significantly reduce the twinkling effect returns from the radar operator’s display.

The third phase of this project will then concentrate on manufacturing actual stealth blades which contain radar absorbing materials (RAM), using NOI Scotland’s standard manufacturing methods. The absorbing structures will be specified to require only small modifications to the blade composition and, importantly, to ensure that no structural detriment is introduced.

QinetiQ is confident that the negative impact of wind farms on radar systems can be mitigated through a combination of stealthy wind turbines designs and enhanced radar filtering. The UK’s government is clear in its vision and targets for renewable energy in the UK and also clear in its commitment to wind energy significantly contributing to this. Although committed, the timescales are challenging, and both the UK government and the industry need to work closely together to accelerate this overall process.

Technology can greatly assist with this process and can provide solutions to many of the challenges that are currently experienced. For example, the company has the ability to accurately model and simulate the impact that proposed wind farms may have upon radar and communications systems. This modelling process scientifically quantifies the interaction and provides the data necessary for determining the most appropriate mitigation options ” stealthy turbines, enhanced radar filters or a combination of both solutions.

Solving the radar issue is only one way in which technology can be employed to assist with achieving the UK’s and other countries’ renewable energy targets. Reducing the planning process and site development time, increasing the installed capacity through volume and turbine size, increasing the efficiency of that installed capacity and ensuring maximum generation time in terms of low maintenance and life expectancy, are all ways to actively assist realising these targets. QinetiQ has additional technologies, originally developed for military programmes, that can now be applied to address these challenges. Our laser-based LIDAR technology can now be employed to increase turbine efficiency and increase turbine life, provide rapidly deployable on and offshore wind resource sensors, and provide wind behaviour data to an extremely high level of fidelity.

Satellite data can be interrogated to provide climatic histories of potential offshore sites to reduce risk when assessing annual variations in wind resource. Composites and materials technology developed for demanding aerospace applications, such as advanced helicopter rotor blade structures, can be applied to wind turbines to produce stronger, lighter and smarter structures. This is of increasing importance as the trend for increasing turbine size continues. Condition monitoring developed for demanding military programmes could be applied to wind turbines to facilitate a more effective, proactive approach to maintenance and as a result consequently ensure maximum generating time and turbine life.

These are examples of existing technologies that can be readily transferred to the wind energy industry, given appropriate support, priority and investment. It has been said that wind energy is not ‘rocket science’; it now appears that a little of this ingredient would greatly assist in tackling climate change in the UK.

Tapping into the growing need for wind measurement

QinetiQ is making its mark in another, major growth area ” wind measurement. “Wind farm development requires massive up front capital investment. It is vital that the right location is found and the site is thoroughly assessed to provide confidence in the wind resource,” explains Ian Locker, a business development manager for QinetiQ.

For the past 20 years, QinetiQ has been a pioneer in LIDAR, a laser-based wind measurement system that offers a viable alternative to traditional methods. The technology, which is easily deployable, requires no costly installation and can take readings up to 200 m, works on a similar principal to radar.

A laser beam collects back scattered radiation from particles (e.g. water, dust and pollen) moving along with the wind. Wind velocity can be detected from the shift between the frequency of the outgoing and returning radiation ” known as the Doppler effect. Further measurements are taken at varying angles and heights to provide a comprehensive picture of wind behaviour.

In November 2003, QinetiQ launched ZephIR ” a new product that takes the principal of LIDAR a step forward and replaces costly ‘free space’ optics with low-cost off-the-shelf telecommunications components.

ZephIR provides comprehensive data on wind speed, direction, sheer and turbulence at a range of heights remotely. It also operates well in cluttered environments where previously predictions have proved difficult. “ZephIR can be used to undertake initial site survey at a prospective wind farm and it can also be fitted to the turbine to provide detailed information on wind speed changes to allow intelligent blade control,” says Locker. “In this way, the technology can increase energy efficiency, provide advance warning of the unexpected gusts and increase the lifetime of the blades”, he adds.

ZephIR was officially launched at the British Wind Energy Exhibition in Glasgow in November 2003 and has already generated considerable interest among leading industry players. The first unit has already been purchased by Risàƒ¸ ” the Danish Wind Energy Institute and a global authority on wind energy.

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