QinetiQ and Vestas recently demonstrated a prototype stealth blade that is a classic example of a radar absorbing structure (RAS). Tested on a Vestas V90 turbine at Swaffham Wind Park in Norfolk, UK, in October 2009, the stealth product reflected much less radar energy than a standard version of the 44 m long blade, according to Steve Appleton, now a researcher at Vestas' R&D centre on the Isle of Wight, UK (he previously worked on stealth materials at QinetiQ).
QinetiQ has provided the technology with which Vestas aims to start producing and marketing stealth blades, probably some time next year.
According to Dr David Moore, business development manager for QinetiQ’s Materials and Energy Business, the solution arrived at with Vestas results in a marginal mass increase and has no adverse effect on structural integrity. As he tells Reinforced Plastics: "It is a purely passive fit-and-forget solution that is an integral part of the composite structure. It has a minimal effect on blade weight and will require no on-going attention."
The solution is also said to be affordable.
"In addressing the issue of blade reflectivity for S-band radars, we have tackled the most difficult aspect of the wind turbine/radar problem," Moore points out. "There were technical solutions available, but the biggest challenge lay in finding one that the wind energy sector would be able to afford, bearing in mind that it could never contemplate cost levels normally associated with stealth in defence markets. Many experts said it couldn't be done."
The new stealth material is used as fabric interlayers within laminates produced by standard blade production processes – primarily wet lay-up, resin infusion or prepreg. While QinetiQ is guarded over details, it describes its composite-friendly solution as basically standard glass cloths modified to confer the required characteristics. As such, the stealth fabrics are no thicker, heavier or more difficult to handle than the composite materials normally used in blade manufacture. They are readily included in solid laminate and are thin enough for use in sandwich skins as well.
For the static parts of a wind turbine (tower, nacelle etc) the collaborative partners have also developed a radar absorbing material (RAM) coating that combines both microwave absorption and destructive interference to combat the frequencies used by aircraft tracking radars. A substrate loaded with conductive and/or magnetic particles provides the absorption, while applying the appropriate coating thickness secures destructive interference. The loading is more likely to be carbon than iron or ferrite based to avoid risk of future corrosion.
While conceding that a stealth turbine is bound to be more costly than a non-stealth equivalent, Vestas and QinetiQ claim to have established that the industry will consider the extra cost bearable when it enables a potential wind farm to be sited where it would otherwise be barred.
Game changer
David Moore believes that RAM and RAS solutions will a game changer, transforming the prospects for onshore and offshore wind farms sited within line of sight of radars. He suggests that there are three zones of risk around a radar:
- a far zone where stealth solutions are not necessary;
- a less distant zone where RAM and RAS are called for; and
- a near zone where RAM/RAS measures must be combined with sophisticated electronic filters added to the radar.
There is also an inner zone extremely close to the radar where no combination of measures will sufficiently counter the interference problem, and here wind turbine siting would be inappropriate. Now that solutions are available for S-band aircraft tracking radars, QinetiQ says it will be easier to develop RAM and RAS solutions for radars that operate at higher frequencies, such as marine and weather radars.
Now the partners are moving on to a further demonstration phase in which they will trial a full stealth wind turbine, complete with tower, nacelle and three-bladed rotor, designed for use with S-band radars. Reinforced Plastics understands that this could happen by the end of this year.
Radar interference
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Plans for wind farms are routinely vetoed by defence ministries and air traffic control services on the grounds that they interfere with radar, making it difficult to track aircraft.
Large rotor blades are particularly challenging. Even though their predominantly GRP skin and foam core construction has low radar reflectivity, internal features such as metal lightning conductors, carbon reinforcement and solid laminate sections give rise to significant reflections. Moreover, blade movement can defeat radars that combat interference by filtering out all targets that are not moving. (The technique is known as moving target indication, or MTI). This is especially the case when blades are rotating in a plane edge on to the radar, creating Doppler effect.
Wind farm interference can create ghosting, false targets and clutter, causing target confusion or loss. Hostile aircraft could use wind farms to their advantage, for example hiding in the radar 'shadow' or clutter produced by a large farm. UK research and technology company QinetiQ has developed a software model that can predict the effect on radar of a wind turbine as a function of blade yaw and pitch.
The radar signature of towers and nacelles can be minimised with radar absorbing material (RAM) coatings – paints, foams etc – and similar solutions have been tried on some rotor blades. But RAM coatings tend to be thick and, in the case of blades, would add mass and bulk to elements for which low weight and precise shaping are important. They may be insufficiently flexible to accommodate blades' natural bending, and they can spoil aerodynamic flow by being less smooth than the moulded finish. Coating operations also add to production costs. It would be far better to be able to integrate a radar absorbing layer within the original laminate, at the point of lay-up.
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Although, naturally, other organisations are addressing the wind turbine/radar problem as well – for instance, the UK's Blade Runner initiative in which a consortium including microwave materials specialist HITEK Electronic Materials has been working on a blade solution along with a layered broadband absorber for use on towers and metal structures – the Vestas/QinetiQ team believe they have a viable answer that is working now, is already in demand, and could soon be available in the market place. If their confidence turns out to be well founded, large wind turbine blades could soon become the highest profile examples of radar absorbing structures, constituting a promising niche market for very specifically engineered composites.
This article is an extract of the feature Going stealthy with composites, published in the November/December 2010 issue of Reinforced Plastics magazine. Read the complete feature here.