While the need for renewable energy grows ever stronger and the energy market exploits wind power, the locations available for building windfarms without impacting airports and military bases due to radar interference are rapidly dwindling. Wind turbines can appear as clutter on radar screens; obscuring incoming aircraft, spawning false plots and even interfering with storm and precipitation predictions, making for unsafe landing conditions.Current efforts to mitigate these problems include new radar installations capable of distinguishing wind turbines from aircraft using software workarounds or stealth technology to reduce the radar cross section on the screen. Though still in its infancy, the latter of these, also known as ‘stealth windfarms’, has shown great promise. In order to solve the complex issue of wind turbine radar interference, Trelleborg’s applied technologies operation is developing next-generation radar absorbing materials for stealth windfarms to unblock gigawatts of wind turbine potential currently under objection across the world.
Trelleborg’s applied technologies operation is developing novel materials to reduce wind turbine radar reflectivity. Starting with polymeric tiles to coat the tower and nacelle, and diversifying to nanocomposites for wind blade manufacture, it aims to revolutionize the wind and aviation relationship.
As the world realizes the importance of renewable energy, radar interference is a major barrier to the realization of the ambitious renewable energy targets many countries around the world are committing to. It was against this background that Trelleborg’s applied technologies operation and Department of Materials at Loughborough University launched a Knowledge Transfer Partnership in 2018 with Innovate UK to develop FrameTM (Full Radar Absorbing Materials and Equipment) in answer to the multifaceted challenge of wind turbine radar interference. Since the beginning of the project, the focus has been on engaging with radar operators, wind developers and turbine manufacturers, to ensure that the development was in line with the needs of the wind and aviation industry, as well as the meteorological sector.
Wind Turbine Radar Interference
To better understand wind turbine radar interference, it is important to consider the impact of large, reflective, spinning wind turbines on highly sensitive electronic equipment such as radars. Typical clutter for radar is either highly reflective stationary objects, for example buildings, or moving but physically small objects, such as vehicles.
Windfarms present a unique challenge as they are both highly reflective and moving. This combination results in a high level of interference reflected back at the radar. Rotating blades can spawn false aircraft plots on the radar or even interfere with the tracking of genuine targets as they fly over the windfarm when the signal of the moving aircraft becomes confused with those of the moving blades. By reducing the reflectivity of the windfarm it is possible to remove the clutter from the radar screen.
The current state-of-the-art solutions can be largely grouped into two areas; radar-based solutions (radar hardware upgrades or new installations) and material solutions (‘stealth windfarms’). An example of the former would be the acquisition of new radar equipment capable of distinguishing windfarms from aircraft. However, the high upfront cost of this is not viable for all windfarm applications, particularly small windfarms or those in unique geographical locations. Trelleborg’s applied technology operation has chosen to innovate in the second of these groups and develop novel radar absorbing materials for next generation ‘stealth windfarms’.
Only with a complete suite of solutions available, will wind energy developers be able to mitigate all objections to windfarm applications. Indeed, many radar experts believe that the only solution to this emerging issue of wind turbine electromagnetic interference saturation is a combination of radar and material-based strategies, necessitating innovation in both areas.
The current leading technology in the ‘stealth windfarm’ sector was deployed in 2016 in Perpignan in Southwest France. It is known to be an effective solution for weather radar, and successfully reduced the average wind turbine radar cross section by 90%. However, the solution suffered from several drawbacks that prevented wider adoption within the wind energy industry, including single-band absorption, high upfront cost, narrow absorption bandwidth and insufficient absorption strength for large wind turbines.
In order for stealth windfarms to become truly viable for wind developers across the world, a solution must provide multi-band absorption at L&X-band and S&X-band (suitable for windfarms impinging on both civilian and military airports), wide absorption peaks (up to 1 GHz bandwidth) and high strength absorption (up to 40dB reduction), all designed from the ground up to minimize cost.
Masking the Tower
The initial goal of the Knowledge Transfer Partnership between Trelleborg and Department of Materials at Loughborough University was to develop polymeric tiles to coat the wind turbine tower. Made entirely out of metal, this is the most reflective part of the wind turbine, as opposed to the relatively less reflective composite blades.
As part of an ongoing commitment to improve sustainability, FrameTM tiles were developed as a versatile material with active radar absorbing fillers dispersed in a thermoplastic polyurethane for greater recyclability. Starting off as a material to absorb 90% of the incident radar wave at one frequency, the project swiftly developed to meet the stringent criteria needed to mitigate the largest of offshore wind turbines and windfarms situated in the vicinity of multiple radar. This resulted in a range of materials absorbing up to 99.99% of the radar wave across a range of frequencies from 1-12 GHz.
As the manufacturing methodology and mechanical environment of wind turbine blades are vastly different to that of the wind turbine tower, extensive research was required to take the tile technology and turn it into a format that is compatible with the blade manufacturing processes. While the versatile nature of the radar absorbing materials developed for coating wind turbine towers lent itself toward diversification, any increase in viscosity to the polymer by additional fillers can cause dramatic issues for resin infusion processes.
By making nanocomposite fiberglass material (Figure 2), which is fully compatible with both manual layup and resin infusion processes, it is hoped that this new option of radar interference mitigation will provide wind turbine blade manufacturers a value-added ‘stealth blade’ option to offer developers increased choice when faced with objections to planning applications.
Simulation and Demonstration
As a brand-new mitigation strategy with no current windfarms to demonstrate the performance of the material, computational analysis is vital to simulate how this technology can remove radar clutter. By measuring dielectric properties of the materials, it is possible to predict how a structure will appear to radar when coated in the radar absorbing material versus the ‘standard’ structure, something which is regularly validated to check the accuracy of the model.
Alongside this simulation for full-scale wind turbines, a pilot application of the tower tiles was undertaken at the Offshore Renewable Energy (ORE) Catapult, Blyth in the North of England, involving installation of the FrameTM polyurethane radar absorbing materials on a section of a wind tower to demonstrate methodology and radar absorption of the tiles when in a real-world environment (Figure 3).
Following the success of the pilot application, work is ongoing to manufacture and demonstrate a practical scale wind blade and fully working turbine with this material solution. In order to stay ahead of radar requirements as wind turbines become ever larger and wind farms, ever more ambitious, development of the next generation of radar absorbing materials, specifically for the electronic landscape of tomorrow, has begun.
Further research projects include the development of smart materials that can change their radar signature and absorption frequency, and advanced metamaterials to actively manipulate the radar wave at the point of contact. By developing these materials, Trelleborg’s applied technologies operation hopes to provide a vital part of the solution needed by the wind industry in order to solve the issue of wind turbine radar interference, which is rapidly becoming of paramount importance as countries across the world aim to reach their renewable energy targets.
Dr Adam Nevin, is the Innovation Lead within Trelleborg’s applied technologies operation. He obtained his doctorate at the University of Nottingham on highly functionalized nanomaterials and their potential for revolutionizing the renewable energy sector, before continuing to a subsequent post-doctoral position developing novel smart polymer materials at Cardiff University.