“Stealth Blades” Get Turbines Under the Radar

After many years of research, the search to produce a ‘stealth turbine’ with low radar reflectivity is now bearing fruit in technical developments that could achieve stealth technology or make it easier to screen out interference from turbines.

In July 2011, when Vestas announced to the International Wind and Radar Forum in Canada that it had successfully tested a full-scale wind turbine utilising stealth technology, this was just the latest step in a long journey to try and unlock the full potential of wind power, and to overcome yet one more obstacle that stands in its way. Around the world, it is estimated that at least 20 GW of wind power is being blocked because of concerns over interference with radar systems, either military or civilian, with particular concern being expressed over the impact that wind farms could have on air traffic control (ATC) systems. In order to overcome these concerns, a series of technology projects have been run, both to find ways of reducing the radar impact of turbines, and to develop forms of radar that can cope with wind farms.

As far as developing stealthy turbines is concerned, this has been pursued first with, for example, the assistance of the UK government, and more recently as a partnership between Vestas and QinetiQ, a company founded to develop technology for military and strategic industries. Beginning with initial studies and computer simulations of the problem and finally graduating to prototype blades and rotors integrating radar absorbing material (RAM), finally these programs now appear to be coming to fruition.

Radar reflective structures

Wind turbines are large structures, and wind farms are large areas of large structures. Combined with their spinning blades and reflective surfaces they can present a significant Radar Cross Section (RCS). Put simply, this means that they show up as large static or twinkling objects on radar screens, obscuring other objects and raising concerns over a possible impact on the safety of air travel. This problem is particularly acute in countries like the UK and the Netherlands, with their limited space for wind farm development and large amount of air traffic. However, while these countries may have the most acute problems, concerns have also been raised in other countries, including the US. As a result, objections have been lodged against a great many wind farms, with some reports suggesting that up to 12 GW are stalled in the UK alone, and many more projects in the US and elsewhere.

This is far from a new problem, however. In 2003, what was then the UK government’s Department of Trade and Industry launched a study to better understand the complex interactions between wind installations and radar facilities, and to develop a series of guidelines and actions to enable the wind energy industry to minimise interference and predict the best locations for wind farm development. This initial research was contracted out to QinetiQ, the then recently privatised arm of the UK’s Defence Evaluation Research Agency (DERA), the very body that had first invented radar more than 60 years before.

Using a mixture of computer simulations, observations of existing wind farms and field testing involving an Enercon E66 turbine and an experimental radar system, the researchers were able to develop a model for wind farm operators, and also made some other useful discoveries. Perhaps most importantly they concluded that the RCS of the wind turbine blades could only be significantly reduced by using radar absorbing materials. They also concluded that the height of the tower made little difference to the RCS and that the radar footprint of the nacelle and tower could be reduced with careful shaping. This set the stage for the next set of research in which experimental blades and rotors would incorporate these materials.

Interference with radar

Radar systems work by sending out pulses of radio waves that bounce off objects. As the waves return to the radar station the variations in the incoming signal are processed by software to generate an image of what is in the path of the beam. Objects in the path of the radar will then show up as images or dots on the display. The larger the RCS of the object, the more visible it will be to the system and the larger or more clearly it will appear on the display. Commercial airliners, for example, have a larger RCS, making them easy to spot, whereas military aeroplanes are designed to have no or a very low RCS, making them difficult to ‘see’ by potential combatants.

Radar systems are further divided by the wavelength they use. Long wave radar systems use lower frequency signals and cast a ‘wider net’. While they are more likely to show up an object, they are also more likely to encounter interference or show up clutter. Shorter wave radar systems (used in most civilian applications) will show less clutter than long range systems, but are more likely to miss small and stealthy objects. Longer wave radar systems are therefore used in military applications to detect possible enemy aircraft.

A prototype of QinetiQ’s ‘stealth’ blade is fitted onto a Vestas V90 turbine at Swaffham Wind Park, Norfolk, UK (Source: QinetiQ)

Wind turbine towers and nacelles are a particular problem for radar systems because they are both large and moving. Many standard radar systems are calibrated to ignore stationary objects. They do this by seeking a Doppler shift, the shift in wavelength of radar waves returning from a moving object. By attuning themselves to detect Doppler shifts, they can focus on moving objects and ignore stationary ones.

With wind turbines, though, it is not so simple. As the blades spin, the radar systems may be swamped with Doppler-shifted reflections, or may pick up each individual blade, causing the farm to appear as a confusing mass on the screen, making it hard to identify aircraft or other targets, or obscuring important data from the radar screen. The result of this interference is to cause radar “black holes” in which aircraft moving of any height cannot be distinguished and so cannot fly safely. In addition, wind farms can create a ‘shadow’ in their wake in which low flying aircraft cannot be distinguished. Wind farms, especially large ones, may also interfere with the onboard navigational aids of ships and aircraft. This is of particular concern in regard to the very large offshore wind farms which are currently being developed in Northern Europe.

Although the interactions are complex, in general wind turbines reflect radio waves in a number of ways. The Glass Reinforced Plastic that makes up the majority of wind turbine blades is itself reflective, as are the metallic lightning conductors which snake through the inside of the blades and towers. The towers and nacelles themselves may also have large radar cross sections, particularly when there are many turbines located together (in comparison to their length, cylinders are said to have a large RCS). Other factors such as the pitch and yaw of the turbine and the individual pitch of the blades may also affect the RCS. Blades spinning to and away from the path of the radar system at a 90° angle to the receiver will have a greater impact than those in other positions as a result of the enhanced Doppler effects.

This pattern of interference points to a number of key physical factors which may be examined to try and reduce the RCS of an operating wind turbine. At the same time, there are number of possible solutions in terms of the RADAR systems themselves and the software used in them which may also resolve this issue.

Reducing turbines’ impact on radar detection

Recent studies have shown that there are a number of general methods than may be employed to try and reduce the radar visibility of wind turbines, aside from amending their physical location. Covering the turbine blades in claddings made of radar-absorbent materials like those used on stealth aircraft may significantly reduce the imprint, but this may also affect the aerodynamic performance of the blade, or come at great expense. The question is how to develop a workable, low-cost solution.

At a recent conference, Qinetiq presented some of the mechanisms they have been exploring with their experimental blades. To begin with, they discovered that not all parts of the blade need to be treated with RAM, and some are more crucial than others — particularly the leading and trailing edges, which may be coated in one or more layers of material designed to reduce the radar signal. Along with coatings, additional solutions have included incorporating RAM into the blade itself by substituting thin layers of radar-absorbent materials for sections of glass composites. Another option involves using layers of honeycomb foam inside the blade to help dampen the reflected signal.

The nacelle and tower can also be coated in RAM, or shaped to ensure that they present no flat surface to the radar waves, thereby reducing their RCS. The key, as with the blades, is to find a system that can be easily incorporated into existing manufacturing processes at a reasonable cost without significantly undermining the performance of the wind turbine system.

Building a stealth blade

The collaboration between Vestas and Qinetiq began in 2006, when work was initiated on developing and testing a single stealth turbine blade on an experimental site in Norfolk. This was a continuation of previous research conducted by Qinetiq and the UK government, during which a single turbine and three 34 metre blades (made in Scotland by NOI-Rotortechnik of Germany) were examined to identify methods for reducing radar imprint. It was this initial research into the materials used in wind turbine blades that first allowed for the development of the predictions and models necessary to designing alternatives. Initial findings from this research suggested a typical turbine radar signature of around 25 dB/m², so this was considered the baseline for reduction.

Since Vestas became involved the testing has moved to a 3 MW V90 turbine, with 44 meter blades, with the company announcing that it had tested a single blade in 2009, and a full rotor in 2011. According to reports put out at the time, this experimental rotor has been able to reduce the RCS of the turbine by more than 20 dB/m² using a range of methods.

Hardware is not the only solution to radar interference (Source: Vestas)

To begin with, the tower and the nacelle have been coated in a 5 mm thick layer of RAM. While the exact composition of such stealth materials is not usually made public, RAM may consist of ferrite paints or polymer layers incorporating crystalline graphite and it is possible that similar compounds are being employed in this case. These types of RAM contain tiny spheres of material that oscillate as the radar waves hit them, ‘converting’ the radio waves into heat, which is then dissipated over the structure, rather than reflected.

Although such materials could also be used to coat the turbine blades, it is reported that this would add an extra 1.2 tonnes to the blades. Given that the entire rotor of a V90 turbine has a weight of around 38 tonnes, this would be a three percent increase on blade weight. Instead, the Vestas blades incorporate RAM consisting of two layers of glass-reinforced epoxy and plastic foam built into their structure, which reflect and absorb waves, reducing their cross section. Since these layers simply replace existing layers in the blades, there is reported to be little or no difference to the overall weight of the blade. According to Qinetiq, the cost premium is expected to be on the order of 10% of the overall cost of the blade.

While Vestas and QinetiQ have hailed the success of their trial, studies elsewhere have cast doubt on the ability of these “treated” blades to appear stealthy to long-wave (low frequency) radar too, for instance in a 2008 study commissioned by the US Department of Homeland Security.

Alternative ways to reduce interference

While the idea of developing a “stealth” turbine is perhaps the most eye-catching of the strategies, several others are being explored.

One of these examples — telemetry transmission — was recently explored as a potentially cheap and easy option. Real-time data could be fed into a model which could then predict the RCS of the wind farm and account for it in its displays, effectively removing the interference problem. While this seems like a daunting challenge, its proponents maintain it would be relatively simple, requiring four sensors on each turbine, continuously transmitting information on speed, location, pitch and yaw to the radar. Altogether this would consist of just a few hundred bytes of data per second. It is not clear at the present time if this is one of the technologies being taken forward.

Another method involves developing so-called “holographic” or three-dimensional radar systems. Developed by Cambridge Consulting, this is a form of continuously tracking radar that can apparently tell the difference between turbines and aircraft by observing their behaviour. Slotted in alongside standard radar they avoid major disruption to operations. After testing by the UK’s Ministry of Defence larger-scale commercial trials are now due with a deal announced in September 2011 for a system at Prestwick Airport in Scotland.

At the same time the UK government, in association with the wind industry, is funding a £5 million (US$7.5 million) research and development project by Raytheon Canada to look at mitigatation. As part of this programme, trials were held in the Netherlands and the UK in 2010, where Raytheon claimed that using a mixture of additional hardware and alterations to the radar software allowed them to distinguish wind farms from other targets up to 90% of the time.

Following on from this, the US Department of Homeland Security awarded Raytheon a $22 million contract to develop a tool for predicting the interaction of existing radar sites with proposed wind farms. This will give the federal government the ability to much more rapidly process planning applications. This will also enable developers to determine, with a reasonable probability of success, whether their project is likely to be approved in its current configuration.

Seeking clear skies

Whether the Vestas-QinetiQ “stealth” turbine will see widespread application, or whether attention will turn to radar upgrades and software patches, is uncertain — but what is clear is that there will be a market for technologies that can allow turbines to be located near radar stations, and which can smooth objections in the planning and consent process. Perhaps in the end there will be room for both solutions. Not every location will be able to upgrade its ground systems, and it seems likely that a combination of “stealthy” turbines and ‘turbine friendly’ radar technology will be used to ensure that radar interference does not become a major stumbling block to future wind development, nor wind development an issue for air safety.

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