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The development of a graduated radar-absorbing multi-layer structure for wind turbine applications to enable their positioning in areas of optimal power generation

Final ReportSummary - OSGRAM (The development of a graduated radar-absorbing multi-layer structure for wind turbine applications to enable their positioning in areas of optimal power generation)

The particular aim of the OSGRAM project was to overcome the current limitations relating to the radar interference of onshore and offshore wind turbines by developing an innovative radar absorbing material (RAM) which can attenuate S-band (air traffic control) and X-band (marine navigation) radar signals by 20 dB. The RAM uses an intrinsically conductive polymer coating which can be applied to wind turbines towers, nacelles and blades at a target material cost of EUR 200/m2.

The radar absorbing properties of the intrinsically conductive polymer (polyaniline) have been achieved by developing a highly innovative continuous polymerisation process based on continuous oscillating baffled reactor (COBR) technology.

Polyaniline is currently manufactured on a limited scale using a batch process. The polymerisation reaction is slow and there is a limited control of the temperature which can be dangerous because the reaction is highly exothermic.

COBR technology is controlled by the generation and cessation of eddies that are created by the combination of the fluid oscillation and the presence of the baffles, not high-speed shear as in a tubular or batch reactor. The intensity of the mixing can therefore be precisely controlled, enabling the velocity components for both the axial and radial elements to be of similar orders of magnitude, which enables far greater control of temperature and mixing than in a batch reactor.

The technical work over the OSGRAM project duration (1 November 2009 to 30 April 2012) has been spread over the tasks in the following work packages (WPs):

WP1: Enhancement of scientific understanding
WP2: Enhanced COBR manufacture and optimisation
WP3: Multilayer film production
WP4: Testing and validation

As a result of computational fluid dynamics (CFD) studies in reporting period 1, a pre-production scale COBR was designed and manufactured. The polymerisation of aniline at a speed of 10 kg/h was achieved with the COBR and 200 kg of polyaniline water dispersion was produced as specified in the project plan.

Modelling of a theoretical RAM concluded that the most suitable RAM designs for the low-thickness, dual-band needs of the OSGRAM project are a Salisbury screen, or a frequency selective surface (FSS). The original project proposal envisaged the use of multiple layers of thin conductive polyaniline depositions as the radar absorbing material. However, analysis of the multilayer design and of the theoretical limits on thickness of RAM structures has led us to these alternative designs.

Polyaniline based Salisbury screen and FSS structures were developed and both showed promising microwave absorption at S-band and X-band frequencies.

For further details, contact:
Mr Neil Todd
Managing Director
NiTech Solutions Ltd
10 Cammo Place, Edinburgh, United Kingdom EH4 8EN
Mobile: +44-(0)78-79698990
Email: neil.todd@nitechsolutions.co.uk

Project website address:
http://www.osgram.eu

Project context and objectives:

The EU is committed to generating 20 % of its energy from renewable sources by 2020, and 50 % by 2050. Wind energy is the cheapest and technologically most straightforward way to generate the majority of this energy. As a result for this the amount of wind energy installed in the EU is predicted to increase from its current level of 56 000 MW to 300 000 MW by 2030, equating to the installation of over 80 000 wind turbines. Due to the very large radar cross section of the turbines and the motion of the rotating blades, these turbines have a detrimental effect on civilian radar navigations systems and there is growing concern about the potential problems which wind turbines may have on passenger safety. Due to space limitations, wind turbines are increasingly being located in areas close to the main shipping lanes and aviation flight paths and wind farm planning applications are increasingly being rejected.

As a result of the radar interference issues, wind energy companies are desperately looking for a solution. Although both electronic radar solutions and the use of radar absorbing materials (RAMs) coated onto the turbines have been considered, no cost-effective solution has as yet been produced. Current RAMs are far too expensive and their performance is not sufficient for wind turbine applications.

The particular aim of the OSGRAM project was to overcome the current limitations relating to the radar interference of onshore and offshore wind turbines by developing an innovative radar absorbing material (RAM) which can attenuate S-band (air traffic control) and X-band (marine navigation) radar signals by 20 dB.

The RAM would comprise an intrinsically conductive polymer coating which can be applied to wind turbines towers, nacelles and blades at a target material cost of EUR 200/m2.

The radar absorbing properties of the intrinsically conductive polymer (polyaniline) have been achieved by developing a highly innovative continuous polymerisation process based on COBR technology.

Polyaniline is currently manufactured on a limited scale using a batch process. The polymerisation reaction is slow and there is a limited control of the temperature which can be dangerous because the reaction is highly exothermic.

COBR technology is controlled by the generation and cessation of eddies that are created by the combination of the fluid oscillation and the presence of the baffles, not high-speed shear as in a tubular or batch reactor. The intensity of the mixing can therefore be precisely controlled, enabling the velocity components for both the axial and radial elements to be of similar orders of magnitude, which enables far greater control of temperature and mixing than in a batch reactor.

The technical work over the project duration (1 November 2009 to 30 April 2012) has been spread over the tasks in the following WPs:

WP1: Enhancement of scientific understanding
WP2: Enhanced COBR manufacture and optimisation
WP3: Multilayer film production
WP4: Testing and validation

The specific scientific objectives were to:

- enhance our understanding on the effect of the changing variables such as surfactant, temperature and pH on the chemical, structural and physical properties of polyaniline, including: degree of branching; level of conjugation along the polymer backbone; dielectric properties using design of experiments;
- enhance the understanding of the interaction between polyaniline materials and their radar attenuation properties to radar signals in the S- and X-bands (2-3 and 8-10 GHz);
- enhance the understanding of polyaniline / latex coating formations, their coating properties and the effect of these on the radar attenuation properties;
- develop a continuous, oscillating, baffled reactor capable of the controlled polymerisation of the semi-conducting polymer (polyaniline) in a continuous process, at an output rate of 10 kg/hr at a cost of less than EUR 250 per kg;
- develop a polymerisation process that has control of the polyaniline conductivity (between 5 and 15 S.cm-1) and dielectric properties to an accuracy of ±2 %;
- develop a multi-layer semi-conducting polymer film with the outer layer impedance equal to that of free space and exponentially reducing impedance through its thickness. The total thickness of the RAM will be 3 mm, most likely consisting of 9 layers, each 0.33 mm thick, sprayed onto a structural polymer film that is 100 µm thick;
- develop a coating method for the polyaniline and the carrier polymers to ensure a functionally graded material. This will require testing of the radar absorbance of the RAM material to ensure that it absorbs to 20 dB in the S and X bands;
- apply RAM to wind turbine blade and retest radar absorbance;
- consider the design of technologies that will allow the production of 500 m2 of OSGRAM material per day.

Project results:

A number of different RAM structures have been developed over the years and, after a thorough review, structures possibly suitable for incorporation into a typical wind turbine blade and wind turbine coating have been identified, these being:

- geometric transition RAM
- Salisbury screen
- Dallenbach absorber
- frequency selective surfaces.

To design a theoretical RAM material to achieve the optimum radar absorption from polyaniline formulations (20 dB in the S- and X-band regions) a number of absorber types were analysed using a unit cell approach which presents a pure assessment of the absorption properties of the various structures independent of any scattering phenomena. The simulations were carried out using the CST microwave studio 3D electromagnetic simulator.

In general, a thickness of about quarter wavelength in the S-band resulted in absorption better than 10 dB in the S-band and better than 20 dB in the X-band.

Frequency selective surfaces were modified to use resistive ink to introduce losses and these showed good wideband or dual-band performance depending on structure. The FSS structures were only 5 mm thick and resulted in peak absorptions of nearly 15 dB in the S-band and 20 dB in the X-band. To achieve this performance the modelling identified a required polyaniline conductivity of 500 S/m.

The modelling concluded the most suitable RAM designs for the low-thickness, dual-band needs of the OSGRAM project were Salisbury screen or frequency selective surface types.

The original project proposal envisaged the use of multiple layers of thin conductive polyaniline depositions as the radar absorbing material. However, analysis of the multilayer design and of the theoretical limits on thickness of RAM structures led the project to these alternative designs.

To enhance our understanding on the effect of the changing variables in the polyaniline polymerisation process, reaction condition optimisation for a water based polyaniline product was carried out in both a batch reactor and the oscillating baffled reactor (OBR).

The particle formation and conductivity properties of the polyaniline were found to be very much dependent on reaction parameters used during the processing. The main parameters affecting on the particle formation were mixing conditions, reaction temperature, concentration of the aniline in the reaction mixture, type and amount of dopant, type and amount of oxidiser and oxidiser addition time.

In the standard batch reactor the lowest resistance value was reached using a reaction temperature below 0 ºC, slow mixing speed, aniline concentration 0.4 % and slower ammonium persulphate (APS) oxidiser addition time. However below 0 ºC the reaction time was found to double. A good compromise for APS addition time, mixing speed and the reaction temperature (3 - 5 ºC) was then established.

Comparing the stirred vessel results to the OBR reaction, the OBR gave a lower resistance value at 3 - 4 ºC, a faster reaction time and improved particle size distribution. The work found that the good mixing conditions allowed the increase of the aniline concentration during polymerisation, which would increase the product yield and favour the use of COBR in aniline polymerisation.

Methods to increase the conductivity of polyaniline included optimisation of reaction parameters, selection of different aniline dopant ions and using hybrid systems with highly conductive materials. A study of new dopants and the use of conductive particles was conducted and potential formulations for polyaniline based RAM coatings determined.

The COBR design was developed using CFD code utilising FLUENT software. The reactor was modelled both in the continuous (COBR) mode and batch (OBR) mode, in order to determine optimal parameters for the polyaniline process. A number of different cases were simulated in order to study the effects of oscillation amplitude, oscillation frequency, size of baffle orifice, gap between wall and baffle, baffle supports and fluid viscosity. In addition the stirred tank reactor was simulated to obtain information of mixing performance and heat transfer for comparison to the COBR reactor. Quantitative measures of COBR reactor performance were used in the analysis based on the information available from the CFD simulations. These included the axial dispersion coefficient, mixedness index and axial to radial velocity ratio.

As a result of the CFD studies, a pre-production scale COBR design for polyaniline was generated. It was established that during the initial stages of the aniline polymerisation efficient mixing was very important, whilst in the later stages of the reaction gentle mixing was required to avoid breaking and agglomeration of the formed polyaniline chains. Two separate COBR systems were therefore proposed and design parameters were defined.

Due to the high costs involved in developing systems within the scope of the project, the Consortium agreed to develop a single COBR unit for the first four stages of the reaction, i.e. formation of the salt (at room temperature), cooling (to 4 °C), oxidiser addition (3 °C) and the initial stage of radical / polyaniline formation (3 °C). At the end of the continuous reactor, the overall flow of 10 litres/hour would then go to a stirred tank reactor with jacket for the 2 final stages of the reaction.

A single COBR 45 metres long with flow rate up to 10 litres per hour was manufactured. This was integrated with stirring and feeding tanks, pumping systems and temperature control units. Aniline polymerisation trials were then performed.

The acid-water concentration was fixed to give a 10 kg/h output from the reactor and mixing parameters selected to enable the salt movement through the reactor. The salt formation (aniline-counter ion) was observed one hour into the reaction and green end product was produced after two hours as expected. The reaction product was then post treated to purify from residual aniline and to concentrate it. The polymerisation of aniline at a speed of 10 kg/h was achieved with the COBR and 200 kg of polyaniline water dispersion was produced as specified. During the run the temperature stayed very consistent (5 - 7 °C) in the area good for the aniline polymerisation to be successful, which proved the good cooling efficiency of the reactor even with such an exothermic reaction as this.

A design of experiments matrix was produced to evaluate all of the influencing factors (including polyaniline conductivity, RAM thickness and substrate dielectric properties) on radar absorbance. Polyaniline formulations were produced and small-scale Salisbury screen and FSS structures prepared for testing by waveguide simulator method.

Measurements at S-band showed the following:

- The Salisbury screen structure to have a narrow bandwidth but polyaniline screens showed some promise.
- The FSS samples demonstrated larger bandwidth and showed good results with polyaniline filled with carbon black or carbon nanotubes.
- Combination structure with Salisbury screen and FSS showed the best results with absorbance > 20 dB and large bandwidth.

Coating methods were developed to produce large scale samples. A dipping method was used to impregnate a carrier substrate for the Salisbury screen, and a stencilling process was scaled up for the FSS. Measurements were carried out using a bistatic radar arrangement inside an anechoic chamber.

The measurements showed the following:

- The single layer 100 µm base polyaniline Salisbury screen placed on top of 9 mm neoprene spacer layer to have excellent radar absorbing properties with a peak absorption of 34 dB and a 10 dB bandwidth of over 60 % - the Salisbury screen performance appeared to be independent of incidence angle.
- The combination FSS and Salisbury screen structure had excellent absorption, with two 20 dB absorption resonances. These resonances could be tuned for any desired band by adjusting the geometry of the structure - the performance of this combination structure was affected by incidence angle but not significantly.

An important piece of information established during the course of the project was the importance of covering the lightning conductor within the blade with the radar absorbing material rather than covering the outer surface of the blade. Lightning protection is a major issue in the manufacture of wind turbines and the use of lightning conductors dramatically increases the RCS. If the conductor is inside the blade, the RCS can be reduced by covering the metal conductor with suitable radar absorption material and this was the construction the OSGRAM project agreed to develop. A simple prototype blade was designed and manufactured. Within the structure of a typical wind turbine blade a long central spar supports the outer skins. The spar caps provide stiffness and strength in bending and extension, while the spar webs provide shear stiffness and prevent buckling. For the OSGRAM blade design the spar supports also housed the lightning conductor and radar absorbing material.

Bistatic radar measurements were carried out on the blade after embedding the RAM layers within the blade. Calibrated measurements between 2.15 GHz and 3 GHz were carried out using an Agilent E5061B ENA series network analyser. The measured reflection coefficient was similar to the results obtained previously with the flat free-space configuration. Both sets of results showed multiple resonances in the sub-3 GHz band with peak absorbance around the 20 dB mark. Embedding the RAM layers inside the blade led to additional resonances due to multiple reflections inside the blade. It would be expected that, similar to the free-space configuration, the main resonant peaks of absorbance for this structure would be achieved at approximately 7 and 12 GHz. The resonant frequencies could then be altered by optimising the thickness and permittivity of the spacer layers.

The OSGRAM project has successfully developed:

- a continuous oscillating baffled reactor (COBRTM) for the continuous production of conductive polyaniline;
- a polyaniline based radar absorbing material (RAM) which can attenuate S-band (air traffic control) and X-band (marine navigation) radar signals by 20dB.

Potential impact:

The EU is committed to generating 20 % of its energy from renewable sources by 2020, and 50 % by 2050. Wind energy is the cheapest and technologically most straightforward way to generate the majority of this energy. As a result for this the amount of wind energy installed in the EU is predicted to increase from its current level of 56 000 MW to 300 000 MW by 2030, equating to the installation of over 80 000 wind turbines. Due to the very large radar cross section of the turbines and the motion of the rotating blades, these turbines have a detrimental effect on civilian radar navigations systems and there is growing concern about the potential problems which wind turbines may have on passenger safety. Due to space limitations, wind turbines are increasingly being located in areas close to the main shipping lanes and aviation flight paths and wind farm planning applications are increasingly being rejected.

As a result of the radar interference issues, wind energy companies are desperately looking for a solution. Although both electronic radar solutions and the use of radar absorbing materials (RAMs) coated onto the turbines have been considered, no cost-effective solution has as yet been produced. Current RAMs are far too expensive and their performance is not sufficient for wind turbine applications.

The innovative RAM developed by the SME consortium will enable wind turbine manufacturers and wind energy suppliers to overcome the key barrier of radar interference by wind turbines that are currently restricting the growth of the wind energy sector. The drivers (political, environmental, commercial and societal) which are currently driving the expansion in the wind power sector, therefore represents a significant market opportunity for our SME consortium.

The European Wind Energy Association (EWEA) is the voice of the wind industry, actively promoting the utilisation of wind power in Europe and worldwide. It is EWEA's objective to facilitate national and international policies and initiatives that strengthen the development of European and global wind energy markets, infrastructure and technology in order to achieve a more sustainable and cleaner energy future.

Some of the many benefits associated with wind power include the following: (Source: http://ww.ewea.org online).

- Economic growth and job creation. In 2010, investment in new European wind farms reached EUR 13 billion, including EUR 2.6 billion offshore. In 2010, over 40 % of all new electricity-generating capacity installed in the EU was from renewables. Wind power alone represented 17 %. About 190 000 people in the EU were in wind energy-related employment in 2010. The wind industry could create up to 271 000 new jobs in the EU by 2020.
- Cleaning up the environment. The 84 GW of wind power installed at the end of 2010 in the EU avoided the emission of 119 million tonnes of CO2. Moreover, the expected EUR 192 billion of investments in wind power from 2011 to 2020 will avoid EUR 85 billion worth of CO2 emission costs during the same period.
- Energy independence. Europe now imports more than half its energy, a figure that is expected to climb to 70 % in the next 20 to 30 years. The wind power capacity installed by the end of 2010 will, in a normal wind year, produce 5.3 % of the EU's electricity. From 2011 to 2020 wind power will avoid EUR 138 billion in fuel costs.

A project website (see http://www.osgram.eu online) was set up at the start of the project to disseminate the project objectives, activities and results.

15 seminars and talks have been given during the project on the benefits of COBR. A paper entitled 'Evaluation of axial dispersion and mixing performance in oscillatory baffled reactors using CFD' has been written by Manninen, Mikko; Gorshkova, Elena; Immonen, Kirsi; Ni, Xiong-Wei and submitted to Journal of Chemical Technology & Biotechnology in May 2012.

The NiTech COBRTM technology will be presented at Achema 2012 Exhibition, Frankfurt, Germany, June 2012, and 'Flow chemistry technologies handling solids including crystallisation' presentation given by Professor Xiong-Wei Ni.

A presentation entitled 'Dual band radar absorbing material using polyaniline' was given at the CST-Computer Simulation Technology AG, 7th European User Conference, Mannheim, Germany, 23 to 25 May 2012, and papers are planned for Electronic Letters, The Institution of Engineering and Technology. Seminar September 2012 - University of Liverpool, UK.

The consortium are is presently in contact with the Renewables Sector in Scotland through Scottish Enterprise and approaching Dupont who have taken over the Intellectual Property Rights of the original project partner, Panipol OY.

Project website:
http://www.osgram.eu

Mr Neil Todd
Managing Director
NiTech Solutions Ltd
10 Cammo Place, Edinburgh, United Kingdom EH4 8EN
Mobile +44-(0)78-79698990
Email: neil.todd@nitechsolutions.co.uk