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Manufacture and characterization of stealth wind turbine blade with periodic pattern surface for reducing radar interference
Composites: Part B 56 (2014) 178–183
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Manufacture and characterization of stealth wind turbine blade with periodic pattern surface for reducing radar interference Hong-Kyu Jang a,1, Won-Ho Choi b, Chun-Gon Kim b,⇑, Jin-Bong Kim c, Do-Wan Lim c a
Center for Composite Materials, University of Delaware, 201 Composites Manufacturing Science Lab., Newark, DE 19716, USA Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea c Composite Materials Laboratory, Korea Institute of Materials Science, 797 Changwondae-ro, Seongsan-gu, Changwon 642-831, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 15 April 2013 Accepted 12 August 2013 Available online 20 August 2013 Keywords: A. Polymer–matrix composites (PMCs) A. Smart materials C. Computational modeling E. Resin transfer molding (RTM) Electromagnetic properties
a b s t r a c t This paper presents a stealth wind turbine blade that reduces signal interference with electromagnetic wave based surrounding infrastructures. This study employed a method based on periodic patterns made of resistive materials to reduce the reflected radar signals from the wind blades. The wind blade with a lossy periodic pattern surface can absorb the incident electromagnetic energy and decrease the radar cross section. In this study, the periodic pattern surface was design with the radar absorbing characteristics at 10.0 GHz frequency. The fabricated lossy periodic pattern surface was applied to the outer surface of the wind blade in order to achieve radar absorbing characteristics. The measure results show that the radar cross section of the stealth wind blade section was reduced by more than 90% for whole azimuth angles at the target frequency. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Most developed countries are interested in renewable energy produced from natural resources, such as wind, solar, and water. With limitless sources and environmental-friendly characteristics, the renewable energy resources are considered as a promising alternative energy to supplant fossils and unclear fuels. About 16% of global energy consumption was supplied by the renewable energy in 2009. Among the sources of this energy, wind power is growing very rapidly, at a rate of 27% annually, and is widely used in many developed countries as well as growing countries [1]. The electric capacity of wind power has been continuously increased during the past 10 years. The power output of wind turbine is determined from a function of the blade’s size and the blade’s rotating speed, and thus wind blades are becoming larger. As a result, the wind turbine blades have a large radar cross section (RCS) and various linear velocities. However, the RCS and the rotation of the wind turbine blades can have significant impacts on the operational capabilities of surrounding infrastructures using electromagnetic (EM) waves, such as commercial and military radar or antenna systems. Because the rotating wind blades with a large RCS causes a Doppler frequency shift, shadowing, and clutter due to the reflected EM signals from the wind blade structures. Also, these effects degrade the target acquisition and tracking capabili⇑ Corresponding author. Tel.: +82 42 350 3719; fax: +82 42 350 3710. 1
E-mail addresses: [email protected] (H.-K. Jang), [email protected] (C.-G. Kim). Tel.: +1 302 831 8316; fax: +1 302 588 7225.
1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.08.043
ties of the radar systems. Consequently, a number of proposed wind farm projects across the globe have been postponed and canceled due to these signal interference problems [2–4]. Adverse effects such as Doppler shift, shadowing, and clutter are caused by unwanted reflected radar signals from the wind blade structures. These problems can be solved by application of radar absorbing structures, such as Salisbury screen and Dallenbach layer, to the wind blades in order to absorb incident EM waves [5–8]. However, these radar absorbers have an inherent limitation of increasing the structural weight because the Salisbury screen needs a thick substrate and the Dallenbach layer uses additional lossy materials such as dielectric and magnetic nano-fillers, to achieve the radar absorbing characteristics at the resonance frequency [9,10]. Above all, weight increment is a critical disadvantage in the wind blades, because the operating cost and efficiency of the wind power generation are closely related to the weight increase of the wind blades. Therefore, development and application of the lightweight radar absorbing structure for the wind blade systems are necessary to solve the radar interference problems. The purpose of this research is to present a wind turbine blade that offers reduced signal interference with the electromagnetic waves, which emitted by surrounding radar or antenna systems. In this study, we fabricated a stealth wind blade section to show the possibility of the proposed method to absorb incident EM waves. The applied radar absorbing technique is based on periodic patterns, which made of a resistive material that compromising a conducting polymer with a polyurethane binder. The fabricated
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H.-K. Jang et al. / Composites: Part B 56 (2014) 178–183
periodic pattern surface was applied to the outer surface of the wind blade to absorb the incident EM waves and to decrease the unwanted reflected signals form the wind blades. The radar absorbing characteristics of the developed wind blade section were evaluated and the feasibility of the stealth wind turbine blade is discussed.
2. Experimental 2.1. Concepts of stealth wind blade Wind turbine blades are made of lightweight structural materials, such as glass fiber cloths, carbon fiber cloths, urethane foam cores, and balsa cores, using a resin transfer molding (RTM) process [11]. The proposed stealth wind blade should be produced by the conventional production process using existing blade’s materials. Therefore, a radar absorbing technique is integrated into the current manufacturing process and applied to the structural component of the wind blades. Consequently, the applied technology allows the wind blade to absorb incident EM energy and to minimize reflected signals without compromising the structural strength. In this study, we applied a radar absorbing structure with a periodic pattern surface (PPS) made of resistive materials to the wind turbine blade in order to reduce the signal interference with radar systems. The RAS is based on the circuit analog absorber, which consists of lossy periodic patterns and a thin grounded substrate, can absorb the incident radar energy [12–14]. The radar absorbing mechanism is wave cancelation result from destructive interference between the reflected waves. In addition, heat dissipation by ohmic losses of induced currents is involved, which caused by constant field change of the EM waves at the resistive patterns [9,10,14]. In the concept of stealth wind blade, a resistive periodic pattern surface is embedded in the outer surface of the conventional wind blade. The wind blade structures can be roughly categorized into two types according to their construction: single structure and mixed structure. In brief, the PPS is placed above the skin layer composed of glass fiber fabrics, while the carbon fiber fabrics for the ground layer is inserted in the bottom of the skin layer in both of the structures, as depicted in Fig 1. In other words, the skin layer with a conductive sheet acts as a grounded substrate for the RAS with lossy periodic patterns. Consequently, the proposed wind blade with a periodic pattern surface can be achieved radar absorbing capability and reduced the reflected signals.
Core Layer
Skin Layer
2.2. Radar absorber with lossy periodic patterns Electromagnetic wave absorption of the stealth wind blade is accomplished by a radar absorbing structure comprised of a resistive periodic pattern surface and a thin grounded substrate. Thus, the design of the periodic patterns surface is directly related to the radar absorbing performance. The lossy periodic patterns can be described by an equivalent RLC circuit that contains elements of resistance (R), inductance (L), and capacitance (C). Therefore, the impedance of (ZPPS) of the lossy periodic pattern surface is represented through the RLC series combination, as shown in Eq. (1). In the equation, the resistance component is achieved with electrical conductivity of the used material and inductance; and capacitance components are associated with the pattern’s geometry [10,14].
Z PPS ¼ R jX ¼ R j
1 x2 LC xC
ð1Þ
According to the previous studies, the equivalent input impedance (ZRAS) of the RAS with the resistive periodic pattern surface can be interpreted as a parallel combination of the impedance (ZPPS) of the lossy PPS and the impedance (ZGDS) of the grounded dielectric substrate [14,15]. Consequently, the input impedance (ZRAS) of the RAS with resistive PPS can be derived into the both real and imaginary parts of Eqs. (2) and (3) as reported by Costa’s study [15].
Re ¼
RZ 2GDS 1x2 LC
xC
Im ¼
Z GDS
ðxLZ GDS Þ
2
ð2Þ
þ R2
1x2 LC xC
Z GDS þ 1x2 LC xC
1
xC Z GDS
Z GDS
2
1x2 LC xC
Z GDS þ R2 Z GDS
þ R2 ð3Þ
Theoretically, the input impedance (ZRAS) of the RAS should be matched the free space impedance (Z0 = 377 X) in order for the RAS to absorb the incident radar signals, that is a zero reflection condition. In the Eq. (3), when the grounded dielectric substrate’s impedance (ZGDS) and the reactance component (X) of the PPS impedance assume almost the same, the imaginary part of the RAS becomes nearly zero [14,15]. Consequently, by varying the substrate thickness and pattern geometry such as shape, array, and coating thickness, the input impedance of the RAS can be
Outer Surface
Conventional Wind Blade
Periodic Patterns Surface
Periodic Pattern Surface Skin Layer Ground Layer Core Layer Bottom Layer Single Structure
Mixed Structure Fig. 1. Design concept of stealth wind turbine blade.
H.-K. Jang et al. / Composites: Part B 56 (2014) 178–183
adjusted to that of zero reflection condition, and the RAS with lossy PPS can achieve good radar absorbing characteristics at the resonance frequency. In this study, the periodic pattern surface was designed by using a commercial EM field analysis program, CST microwave studio (CST-MWS). The design parameters and values of the periodic patterns, such as material properties and geometric information, are listed in Table 1. The radar absorbing target frequency was 10.0 GHz within the X band (8.2–12.4 GHz), which is widely used for military radar systems. In addition, the pattern shape was a square patch type, which is employed to achieve efficient design and wide radar absorbing bandwidth [15,16]. Other fixed design values were material properties, such as permittivity of the substrate and electrical conductivity of the lossy material. The variable design parameters of the PPS were pattern size, pattern gap, coating thickness, and substrate thickness. These parameters can control the values of the R, L, and C components and determine the RAS’s input impedance (ZRAS). In this study, the design values with optimal radar absorption were determined by the parametric studies using a numerical analysis method of the CST-MWS at the target frequency. Fig. 2 shows the detailed configuration of the RAS with the periodic pattern surface. The size of the pattern was 5 mm 5 mm 0.002 mm, and the gap between the elements was 2 mm. The unit cell of 7 mm 7 mm with the pattern and gap was arrayed in the periodic pattern surface with 3.0 mm substrate (input impedance ZPPS = R jX = 378.8 j6.4). According to the simulated resulted shown in Fig. 3, the reflection loss was 42.2 dB at 10.0 GHz frequency and the 10 dB bandwidth (90% radar absorption) was 4.2 GHz from 8.2 to 12.4 GHz frequency, covering the entire X band.
PG
PL
Periodic Patterns Surface PL
PC
SL
PH GH
SH
Substrate Ground
SL Fig. 2. Schematic design of RAS with lossy periodic patterns.
0 Simulation Experiment
-5
Reflection Loss (dB)
180
-10 -15 -20 -25 -30 -35 -40 -45
8.4
8.8
9.2
9.6 10.0 10.4 10.8 11.2 11.6 12.0 12.4
Frequency (GHz) Fig. 3. Reflection loss of RAS with a periodic pattern surface.
2.3. Fabrication of lossy periodic pattern surface A radar absorbing structure with a periodic pattern surface should be produced by the conventional wind blade’s manufacturing process in order to facilitate application to wind blades. In this study, we suggested and used the modified manufacturing process such that the radar absorbing technique could be integrated into the current fabrication method of the wind blades, resin transfer molding (RTM) process. The manufacturing process for the RAS with the lossy PPS consists of two phases, a screen printing step for the periodic pattern surface, and a resin infusion step involved in fabrication of the radar absorbing structure. The periodic pattern surface was fabricated by a screen printing process using a metal mask and a conductive paste. The patterned metal mask was made of SUS 304 of 50 lm thickness by a photo etching technique. The conductive paste for the resistive patterns was synthesized using water-soluble polyurethane binder, NPC 3600 (Nanux Co., Ltd.) and a conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), Clevios
Table 1 Design parameters and properties. Symbol
Design parameters
Design values
PL PG PH SL SH GH PC
Pattern size Pattern gap Pattern thickness Substrate size Substrate thickness Ground thickness Conductivity Permittivity
5.0 mm 2.0 mm 2.0 lm 161 mm 3.0 mm 0.2 mm 3400 S/m 4.57 j0.05
er
PH500 (Heraeus Co., Ltd.). Conducing polymers are good candidates for the resistive materials of the radar absorbers due to the advantages of controllable electrical conductivity, ease of manufacture, easy surface coating, lightweight, and broadband radar absorption. For these various advantages, many research papers were reported on the radar absorbing capability of the conducting polymers [17–20]. In this study, the PEDOT:PSS complex was used as a conductive material for the periodic patterns and the electrical conductivity of the synthesized paste was 3400 S/m. Fig. 4 shows the fabrication process and final product of the lossy PPS sheet. The patterns were printed on a very thin and flexible glass fiber composites sheet of 80 lm thickness (GEP 110, SK Chemical Co., Ltd.) by using a bar coating device, and then cured at 130 °C for 30 min in a thermal oven. In the process, the coating thickness of the printed patterns was controlled and calculated by the preliminary experimental data on the thickness change of the conductive paste after curing. The radar absorbing structure of 3.0 mm thickness excluding the ground layer was fabricated by a RTM process using existing blade materials, as shown in Fig. 5(a). The prepared materials were stacked on an aluminum metal plate in the sequence of the printed lossy PPS sheet, glass fiber cloths (NCF E-glass fiber, Saertex Co., Ltd.), carbon fiber cloths (WSN 3K, SK Chemical Co., Ltd.), and a vacuum bag (Wrightlon 7400, Airtech Co., Ltd.). The mixed binding matrix (RIM 135, RIMH 134, Hexion Co., Ltd.) was infused into the vacuum bag through a connected flowing line, and then cured at room temperature for 2 days. The detailed material information and manufacturing steps were reported in the reference papers presented by Kim and Jang [8,21]. Fig. 6(b) shows the radar
H.-K. Jang et al. / Composites: Part B 56 (2014) 178–183
181
Metal Mask
Bar Coating Device
Periodic Pattern Surface
Fig. 4. Fabrication of lossy periodic pattern surface.
Infusion Resin (OUT) Bottom Plate Printed Sheet Glass Cloth Carbon Cloth
Vacuum Bag Infusion Resin (IN)
(a) Manufacturing process using RTM
Periodic Pattern Surface (PPS)
The 0.6 m long section used a wind blade shape of 34 mm from the root end of the blade and the two halves of the wind blade section mold were prepared by the Korea Institute of Materials Science (KIMS) in Korea, as reported by Kim [8]. Conventional wind blades are comprised of a skin layer made of glass fiber cloths and a core layer composed of a glass fiber mat and foam [11]. In this study, fabricated PPS sheets were applied on the outer surface of the skin layer to absorb the incident EM waves and decrease the radar cross section of the wind blades, as mentioned design concept in Fig. 1. Fig. 6 shows the manufacturing process of the stealth wind blade section using the resin transfer molding process. To fabricate the skin layer, the prepared materials were stacked on the two halves of the aluminum mold, a suction part and a pressure part, in order of fabricated lossy PPS sheets, glass fiber cloths, and carbon fiber cloths. The lightweight core materials, such as a glass fiber mat and foam were placed on the carbon fiber cloths, as depicted in the left-hand side of Fig. 6. Next, the mixed binding matrix was infused into the wind blade mold wrapped by the vacuum bag through the connected flowing line and then cured at room temperature. Finally, we assembled the fabricated suction and pressure parts with composite spars and epoxy glue to manufacture the stealth wind blade section, as shown in the right-hand side of Fig. 6. The detailed configuration of the stealth wind blade section with the resistive periodic pattern surface is shown in Fig. 7.
3. Results and discussion Glass fiber/epoxy Composite
3.1. Characterization of periodic pattern surface
Carbon fiber/epoxy Composite
(b) RAS with periodic pattern surface Fig. 5. Fabrication of RAS with a periodic pattern surface.
absorbing structure with the lossy periodic pattern surface made by the modified manufacturing process. 2.4. Manufacture of stealth wind blade The purpose of this study is to develop a wind turbine blade that offers minimized signal interference with the radar or antenna systems. In order to demonstrate a practical implementation, a stealth wind blade section with a resistive periodic pattern surface was fabricated by an aforementioned manufacturing process which is similar to the current wind blade’s production process. The wind blade section is based on the cross-section geometry of a 3 MW wind turbine blade KM 44 (3 MW IEC Class 1 A, KM).
A radar absorbing structure with lossy periodic patterns was designed and fabricated for application to a stealth wind blade. In this study, a reflection loss of the fabricated RAS was measured to evaluate the radar absorbing capability. The reflection loss is the ratio of the reflected energy power to the incident energy power and was tested by a free space measurement system (HVS technologies) at the KIMS, as shown in Kim’s paper [8]. Fig. 3 shows the simulated and experimental results of the reflection loss. The fabricated RAS has a minimum reflection loss of 34.6 dB at 10.1 GHz frequency. The 90% radar absorbing bandwidth (10 dB) was 4.2 GHz, covering the entire X band. According to the results, the resonance frequency and return loss values showed very small change relative to the simulation results, the difference was 0.1 GHz and 7.6 dB, respectively. The direct cause of the difference was manufacturing error of the pattern thickness; the coating thickness was 1.94 lm different from its design value of 2.0 lm. As a result, the fabricated RAS with the resistive PPS shows good radar absorbing performance at the target frequency.
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H.-K. Jang et al. / Composites: Part B 56 (2014) 178–183
Carbon Fiber Cloth Glass Fiber Cloth PPS Sheet
RTM Process
Foam Core Glass Fiber Core Aluminum Mould
Manufacture of Stealth Blades
Stacking Sequence of Stealth Blades
Fig. 6. Manufacturing process of stealth wind turbine blade.
Fig. 7. Configuration of stealth wind turbine blade.
Radar Cross Section (dBsm)
50
Stealth Wind Blade Compact Range
Feed Antenna
30 20 10 0 -10 -20
-30
30
60
90 120 150 180 210 240 270 300 330 360
Fig. 9. Monostatic RCS of conventional and stealth wind blades.
Suction Side Leading Edge
0
Azimuth Angle (°)
90º
180º
Conventional Wind Blade Stealthe Wind Blade
40
Trailing Edge
0º
Pressure Side
270º Fig. 8. Compact range system for measuring radar cross section.
3.2. Characterization of stealth wind blade The objective of this study is to develop the stealth wind blade that offers minimized signal interference with the sur-
rounding antenna and radar systems. In order to reduce the radar interference, the unwanted reflected signals from the wind blade structure should be decreased or eliminated. The magnitude of the signal reflection to the incident EM wave can be indirectly evaluated by measuring the radar cross section of the target. The radar cross section (RCS) is a measure of the energy scattered from a target following irradiation with EM energy of a given frequency. The RCS was measured by using a compact range system in the anechoic chamber of the Pohang University of Science and Technology (POSTECH) in Korea, as shown in Fig. 8.
H.-K. Jang et al. / Composites: Part B 56 (2014) 178–183
183
surface for reducing the radar signal interference was successfully manufactured.
Table 2 Comparison on monostatic RCS of wind blades. Azimuth angle
Conventional wind blade (dBsm)
Stealth wind blade (dBsm)
RCS reduction rate (%)
Leading edge (185°) Trailing edge (0°) Suction side (78°) Pressure side (280°)
1.5 2.5 30.2 31.1
7.1 19.7 15.2 19.3
86.2 98.1 96.8 93.4
In this study, the monostatic RCS of the wind blades according to an azimuth angles sweep given the frequency was evaluated to demonstrate practical implementation of the stealth wind blade. Fig. 9 shows the measured results of the conventional wind blade and the stealth wind blade. According to the results, the significant RCS reduction occurred at the whole azimuth angle ranges and the RCS was decreased by almost 90% compared with that of the conventional wind blade at 10.0 GHz frequency. In addition, the RCS reduction rates of the suction side (78°) and the pressure side (280°) was 96.8% and 93.4% respectively, as listed in Table 2. This indicates that the RCS of the stealth wind turbine blade constituted only 5–10% of the overall scattering from the conventional wind turbine blade. 4. Conclusions We have proposed a technique to decrease the reflected signals from a wind blade, and thereby minimize signal interference with electromagnetic waves emitted from surrounding infrastructures. The technique is based on a periodic pattern surface made of conductive paste. The fabricated lossy PPS was applied on the sink layer of the wind blade to achieve radar absorbing capability. Therefore, the wind blade with the lossy PPS can absorb the incident electromagnetic energy and reduce the radar cross section. According to the results, the minimum reflection loss of the RAS was 34.6 dB at 10.1 GHz frequency and 90% radar absorbing bandwidth was covering the entire X band. In addition, the radar cross section of the stealth wind blade with the periodic pattern surface was reduced by more than 90% for whole azimuth angle ranges, compared with the conventional wind blade. In conclusion, the stealth wind turbine blade with the lossy periodic pattern
References [1] Sawin JL. Renewables 2011: global status report. Renewable energy policy network for the 21th century; 2011. [2] Webster DM. The effects of wind turbine farms on air defence radars. Air Warfare Center; 2005. [3] Defense Research and Engineering. The effect of windmill farms on military readiness. US Department of Defense; 2006. [4] Bulter MM, Johnson DA. Feasibility of mitigating the effects of windfarms on primary radar. Report No. ETSU W/1400623/REP; 2003. [5] Bryanton M, Pinto J, Matthews J, Sarno C. Moore Z, Bellanger Y, et al. Stealth technology for wind turbines. Department for Business Enterprise & Regulatory Reform, Report No. MTSM/070348/109121 TES101865; 2008. [6] Tennant A, Chambers B. Signature management of radar returns from wind turbine generators. Smart Mater Struct 2006;15(2):468–72. [7] Pinto J, Matthews J, Sarno C. Radar signature reduction of wind turbines through the application of stealth technology. In: Proceedings of 3rd EuCAP. Berlin, March, 2009. p. 3886–90. [8] Kim JB, Lim DW. Reduction of radar interference – stealth wind blade structure with carbon nanocomposite sheet. Wind Energy 2013. [9] Knott EF, Shaeffer JF, Tuley MT. Radar cross section. Raleigh: SciTech; 2003. [10] Vinoy KJ, Jha RM. Radar absorbing materials. Boston: Kluwer Academic; 1996. [11] Brøndsted P, Lilholts H, Lystrup A. Composite materials for wind power turbine blades. Annu Rev Mater Res 2005;35:505–38. [12] Lee WJ, Lee JW, Kim CG. Characteristics of an electromagnetic wave absorbing composite structure with a conducting polymer electromagnetic bandgap (EBG) in the X-band. Compos Sci Technol 2008;68(12):2485–9. [13] Jang HK, Shin JH, Kim CG, Shin SH, Kim JB. Semi cylindrical radar absorbing structures using fiber-reinforced composites and conducting polymers in the X-band. Adv Compos Mater 2011;20(3):215–30. [14] Munk BA. Frequency selective surface: theory and design. New York: Wiley; 2000. [15] Costa F, Monorchio A, Manara G. Analysis and design of ultra thin electromagnetic absorbers comprising resistively loaded high impedance surfaces. IEEE Trans Antenn Propag 2010;58(5):1551–8. [16] Costa F, Genovesi S, Monorchio A. On the bandwidth of high impedance frequency selective surface. IEEE Antenn Wirel Propag Lett 2009;8:1341–4. [17] Harun MH, Saion E, Kassim A, Yahya N, Mahmud E. Conjugated conducting polymers: a brief overview. J Adv Sci Arts 2007;2:63–8. [18] Truong VT, Turner BD, Muscat RF, Russo MS. Conducting polymer based radar absorbing materials. In: Proceedings of SPIE 3241. Adelaide, December, 1997. p. 98–105. [19] Wong PTC, Chamber B, Anderson AP, Wright PV. Large area conducting polymer composites and their use in microwave absorbing material. Electron Lett 1992;28(17):1651–3. [20] Wright PV, Chamber B, Barnes A, Lees K, Despotakis A. Progress in smart microwave materials and structures. Smart Mater Struct 2000;9(3):273–9. [21] Jang HK, Choi WH, Shin JH, Song TH, Kim JK, Kim CG, et al. Radar absorbing structure with periodic pattern surfaces for wind turbine blades. In: Proceedings of 18th ICCM, Jeju Island, August, 2011.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Manufacture and characterization of stealth wind turbine blade with periodic pattern surface for reducing radar interference Hong-Kyu Jang a,1, Won-Ho Choi b, Chun-Gon Kim b,⇑, Jin-Bong Kim c, Do-Wan Lim c a
Center for Composite Materials, University of Delaware, 201 Composites Manufacturing Science Lab., Newark, DE 19716, USA Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea c Composite Materials Laboratory, Korea Institute of Materials Science, 797 Changwondae-ro, Seongsan-gu, Changwon 642-831, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 15 April 2013 Accepted 12 August 2013 Available online 20 August 2013 Keywords: A. Polymer–matrix composites (PMCs) A. Smart materials C. Computational modeling E. Resin transfer molding (RTM) Electromagnetic properties
a b s t r a c t This paper presents a stealth wind turbine blade that reduces signal interference with electromagnetic wave based surrounding infrastructures. This study employed a method based on periodic patterns made of resistive materials to reduce the reflected radar signals from the wind blades. The wind blade with a lossy periodic pattern surface can absorb the incident electromagnetic energy and decrease the radar cross section. In this study, the periodic pattern surface was design with the radar absorbing characteristics at 10.0 GHz frequency. The fabricated lossy periodic pattern surface was applied to the outer surface of the wind blade in order to achieve radar absorbing characteristics. The measure results show that the radar cross section of the stealth wind blade section was reduced by more than 90% for whole azimuth angles at the target frequency. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Most developed countries are interested in renewable energy produced from natural resources, such as wind, solar, and water. With limitless sources and environmental-friendly characteristics, the renewable energy resources are considered as a promising alternative energy to supplant fossils and unclear fuels. About 16% of global energy consumption was supplied by the renewable energy in 2009. Among the sources of this energy, wind power is growing very rapidly, at a rate of 27% annually, and is widely used in many developed countries as well as growing countries [1]. The electric capacity of wind power has been continuously increased during the past 10 years. The power output of wind turbine is determined from a function of the blade’s size and the blade’s rotating speed, and thus wind blades are becoming larger. As a result, the wind turbine blades have a large radar cross section (RCS) and various linear velocities. However, the RCS and the rotation of the wind turbine blades can have significant impacts on the operational capabilities of surrounding infrastructures using electromagnetic (EM) waves, such as commercial and military radar or antenna systems. Because the rotating wind blades with a large RCS causes a Doppler frequency shift, shadowing, and clutter due to the reflected EM signals from the wind blade structures. Also, these effects degrade the target acquisition and tracking capabili⇑ Corresponding author. Tel.: +82 42 350 3719; fax: +82 42 350 3710. 1
E-mail addresses: [email protected] (H.-K. Jang), [email protected] (C.-G. Kim). Tel.: +1 302 831 8316; fax: +1 302 588 7225.
1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.08.043
ties of the radar systems. Consequently, a number of proposed wind farm projects across the globe have been postponed and canceled due to these signal interference problems [2–4]. Adverse effects such as Doppler shift, shadowing, and clutter are caused by unwanted reflected radar signals from the wind blade structures. These problems can be solved by application of radar absorbing structures, such as Salisbury screen and Dallenbach layer, to the wind blades in order to absorb incident EM waves [5–8]. However, these radar absorbers have an inherent limitation of increasing the structural weight because the Salisbury screen needs a thick substrate and the Dallenbach layer uses additional lossy materials such as dielectric and magnetic nano-fillers, to achieve the radar absorbing characteristics at the resonance frequency [9,10]. Above all, weight increment is a critical disadvantage in the wind blades, because the operating cost and efficiency of the wind power generation are closely related to the weight increase of the wind blades. Therefore, development and application of the lightweight radar absorbing structure for the wind blade systems are necessary to solve the radar interference problems. The purpose of this research is to present a wind turbine blade that offers reduced signal interference with the electromagnetic waves, which emitted by surrounding radar or antenna systems. In this study, we fabricated a stealth wind blade section to show the possibility of the proposed method to absorb incident EM waves. The applied radar absorbing technique is based on periodic patterns, which made of a resistive material that compromising a conducting polymer with a polyurethane binder. The fabricated
179
H.-K. Jang et al. / Composites: Part B 56 (2014) 178–183
periodic pattern surface was applied to the outer surface of the wind blade to absorb the incident EM waves and to decrease the unwanted reflected signals form the wind blades. The radar absorbing characteristics of the developed wind blade section were evaluated and the feasibility of the stealth wind turbine blade is discussed.
2. Experimental 2.1. Concepts of stealth wind blade Wind turbine blades are made of lightweight structural materials, such as glass fiber cloths, carbon fiber cloths, urethane foam cores, and balsa cores, using a resin transfer molding (RTM) process [11]. The proposed stealth wind blade should be produced by the conventional production process using existing blade’s materials. Therefore, a radar absorbing technique is integrated into the current manufacturing process and applied to the structural component of the wind blades. Consequently, the applied technology allows the wind blade to absorb incident EM energy and to minimize reflected signals without compromising the structural strength. In this study, we applied a radar absorbing structure with a periodic pattern surface (PPS) made of resistive materials to the wind turbine blade in order to reduce the signal interference with radar systems. The RAS is based on the circuit analog absorber, which consists of lossy periodic patterns and a thin grounded substrate, can absorb the incident radar energy [12–14]. The radar absorbing mechanism is wave cancelation result from destructive interference between the reflected waves. In addition, heat dissipation by ohmic losses of induced currents is involved, which caused by constant field change of the EM waves at the resistive patterns [9,10,14]. In the concept of stealth wind blade, a resistive periodic pattern surface is embedded in the outer surface of the conventional wind blade. The wind blade structures can be roughly categorized into two types according to their construction: single structure and mixed structure. In brief, the PPS is placed above the skin layer composed of glass fiber fabrics, while the carbon fiber fabrics for the ground layer is inserted in the bottom of the skin layer in both of the structures, as depicted in Fig 1. In other words, the skin layer with a conductive sheet acts as a grounded substrate for the RAS with lossy periodic patterns. Consequently, the proposed wind blade with a periodic pattern surface can be achieved radar absorbing capability and reduced the reflected signals.
Core Layer
Skin Layer
2.2. Radar absorber with lossy periodic patterns Electromagnetic wave absorption of the stealth wind blade is accomplished by a radar absorbing structure comprised of a resistive periodic pattern surface and a thin grounded substrate. Thus, the design of the periodic patterns surface is directly related to the radar absorbing performance. The lossy periodic patterns can be described by an equivalent RLC circuit that contains elements of resistance (R), inductance (L), and capacitance (C). Therefore, the impedance of (ZPPS) of the lossy periodic pattern surface is represented through the RLC series combination, as shown in Eq. (1). In the equation, the resistance component is achieved with electrical conductivity of the used material and inductance; and capacitance components are associated with the pattern’s geometry [10,14].
Z PPS ¼ R jX ¼ R j
1 x2 LC xC
ð1Þ
According to the previous studies, the equivalent input impedance (ZRAS) of the RAS with the resistive periodic pattern surface can be interpreted as a parallel combination of the impedance (ZPPS) of the lossy PPS and the impedance (ZGDS) of the grounded dielectric substrate [14,15]. Consequently, the input impedance (ZRAS) of the RAS with resistive PPS can be derived into the both real and imaginary parts of Eqs. (2) and (3) as reported by Costa’s study [15].
Re ¼
RZ 2GDS 1x2 LC
xC
Im ¼
Z GDS
ðxLZ GDS Þ
2
ð2Þ
þ R2
1x2 LC xC
Z GDS þ 1x2 LC xC
1
xC Z GDS
Z GDS
2
1x2 LC xC
Z GDS þ R2 Z GDS
þ R2 ð3Þ
Theoretically, the input impedance (ZRAS) of the RAS should be matched the free space impedance (Z0 = 377 X) in order for the RAS to absorb the incident radar signals, that is a zero reflection condition. In the Eq. (3), when the grounded dielectric substrate’s impedance (ZGDS) and the reactance component (X) of the PPS impedance assume almost the same, the imaginary part of the RAS becomes nearly zero [14,15]. Consequently, by varying the substrate thickness and pattern geometry such as shape, array, and coating thickness, the input impedance of the RAS can be
Outer Surface
Conventional Wind Blade
Periodic Patterns Surface
Periodic Pattern Surface Skin Layer Ground Layer Core Layer Bottom Layer Single Structure
Mixed Structure Fig. 1. Design concept of stealth wind turbine blade.
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adjusted to that of zero reflection condition, and the RAS with lossy PPS can achieve good radar absorbing characteristics at the resonance frequency. In this study, the periodic pattern surface was designed by using a commercial EM field analysis program, CST microwave studio (CST-MWS). The design parameters and values of the periodic patterns, such as material properties and geometric information, are listed in Table 1. The radar absorbing target frequency was 10.0 GHz within the X band (8.2–12.4 GHz), which is widely used for military radar systems. In addition, the pattern shape was a square patch type, which is employed to achieve efficient design and wide radar absorbing bandwidth [15,16]. Other fixed design values were material properties, such as permittivity of the substrate and electrical conductivity of the lossy material. The variable design parameters of the PPS were pattern size, pattern gap, coating thickness, and substrate thickness. These parameters can control the values of the R, L, and C components and determine the RAS’s input impedance (ZRAS). In this study, the design values with optimal radar absorption were determined by the parametric studies using a numerical analysis method of the CST-MWS at the target frequency. Fig. 2 shows the detailed configuration of the RAS with the periodic pattern surface. The size of the pattern was 5 mm 5 mm 0.002 mm, and the gap between the elements was 2 mm. The unit cell of 7 mm 7 mm with the pattern and gap was arrayed in the periodic pattern surface with 3.0 mm substrate (input impedance ZPPS = R jX = 378.8 j6.4). According to the simulated resulted shown in Fig. 3, the reflection loss was 42.2 dB at 10.0 GHz frequency and the 10 dB bandwidth (90% radar absorption) was 4.2 GHz from 8.2 to 12.4 GHz frequency, covering the entire X band.
PG
PL
Periodic Patterns Surface PL
PC
SL
PH GH
SH
Substrate Ground
SL Fig. 2. Schematic design of RAS with lossy periodic patterns.
0 Simulation Experiment
-5
Reflection Loss (dB)
180
-10 -15 -20 -25 -30 -35 -40 -45
8.4
8.8
9.2
9.6 10.0 10.4 10.8 11.2 11.6 12.0 12.4
Frequency (GHz) Fig. 3. Reflection loss of RAS with a periodic pattern surface.
2.3. Fabrication of lossy periodic pattern surface A radar absorbing structure with a periodic pattern surface should be produced by the conventional wind blade’s manufacturing process in order to facilitate application to wind blades. In this study, we suggested and used the modified manufacturing process such that the radar absorbing technique could be integrated into the current fabrication method of the wind blades, resin transfer molding (RTM) process. The manufacturing process for the RAS with the lossy PPS consists of two phases, a screen printing step for the periodic pattern surface, and a resin infusion step involved in fabrication of the radar absorbing structure. The periodic pattern surface was fabricated by a screen printing process using a metal mask and a conductive paste. The patterned metal mask was made of SUS 304 of 50 lm thickness by a photo etching technique. The conductive paste for the resistive patterns was synthesized using water-soluble polyurethane binder, NPC 3600 (Nanux Co., Ltd.) and a conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), Clevios
Table 1 Design parameters and properties. Symbol
Design parameters
Design values
PL PG PH SL SH GH PC
Pattern size Pattern gap Pattern thickness Substrate size Substrate thickness Ground thickness Conductivity Permittivity
5.0 mm 2.0 mm 2.0 lm 161 mm 3.0 mm 0.2 mm 3400 S/m 4.57 j0.05
er
PH500 (Heraeus Co., Ltd.). Conducing polymers are good candidates for the resistive materials of the radar absorbers due to the advantages of controllable electrical conductivity, ease of manufacture, easy surface coating, lightweight, and broadband radar absorption. For these various advantages, many research papers were reported on the radar absorbing capability of the conducting polymers [17–20]. In this study, the PEDOT:PSS complex was used as a conductive material for the periodic patterns and the electrical conductivity of the synthesized paste was 3400 S/m. Fig. 4 shows the fabrication process and final product of the lossy PPS sheet. The patterns were printed on a very thin and flexible glass fiber composites sheet of 80 lm thickness (GEP 110, SK Chemical Co., Ltd.) by using a bar coating device, and then cured at 130 °C for 30 min in a thermal oven. In the process, the coating thickness of the printed patterns was controlled and calculated by the preliminary experimental data on the thickness change of the conductive paste after curing. The radar absorbing structure of 3.0 mm thickness excluding the ground layer was fabricated by a RTM process using existing blade materials, as shown in Fig. 5(a). The prepared materials were stacked on an aluminum metal plate in the sequence of the printed lossy PPS sheet, glass fiber cloths (NCF E-glass fiber, Saertex Co., Ltd.), carbon fiber cloths (WSN 3K, SK Chemical Co., Ltd.), and a vacuum bag (Wrightlon 7400, Airtech Co., Ltd.). The mixed binding matrix (RIM 135, RIMH 134, Hexion Co., Ltd.) was infused into the vacuum bag through a connected flowing line, and then cured at room temperature for 2 days. The detailed material information and manufacturing steps were reported in the reference papers presented by Kim and Jang [8,21]. Fig. 6(b) shows the radar
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181
Metal Mask
Bar Coating Device
Periodic Pattern Surface
Fig. 4. Fabrication of lossy periodic pattern surface.
Infusion Resin (OUT) Bottom Plate Printed Sheet Glass Cloth Carbon Cloth
Vacuum Bag Infusion Resin (IN)
(a) Manufacturing process using RTM
Periodic Pattern Surface (PPS)
The 0.6 m long section used a wind blade shape of 34 mm from the root end of the blade and the two halves of the wind blade section mold were prepared by the Korea Institute of Materials Science (KIMS) in Korea, as reported by Kim [8]. Conventional wind blades are comprised of a skin layer made of glass fiber cloths and a core layer composed of a glass fiber mat and foam [11]. In this study, fabricated PPS sheets were applied on the outer surface of the skin layer to absorb the incident EM waves and decrease the radar cross section of the wind blades, as mentioned design concept in Fig. 1. Fig. 6 shows the manufacturing process of the stealth wind blade section using the resin transfer molding process. To fabricate the skin layer, the prepared materials were stacked on the two halves of the aluminum mold, a suction part and a pressure part, in order of fabricated lossy PPS sheets, glass fiber cloths, and carbon fiber cloths. The lightweight core materials, such as a glass fiber mat and foam were placed on the carbon fiber cloths, as depicted in the left-hand side of Fig. 6. Next, the mixed binding matrix was infused into the wind blade mold wrapped by the vacuum bag through the connected flowing line and then cured at room temperature. Finally, we assembled the fabricated suction and pressure parts with composite spars and epoxy glue to manufacture the stealth wind blade section, as shown in the right-hand side of Fig. 6. The detailed configuration of the stealth wind blade section with the resistive periodic pattern surface is shown in Fig. 7.
3. Results and discussion Glass fiber/epoxy Composite
3.1. Characterization of periodic pattern surface
Carbon fiber/epoxy Composite
(b) RAS with periodic pattern surface Fig. 5. Fabrication of RAS with a periodic pattern surface.
absorbing structure with the lossy periodic pattern surface made by the modified manufacturing process. 2.4. Manufacture of stealth wind blade The purpose of this study is to develop a wind turbine blade that offers minimized signal interference with the radar or antenna systems. In order to demonstrate a practical implementation, a stealth wind blade section with a resistive periodic pattern surface was fabricated by an aforementioned manufacturing process which is similar to the current wind blade’s production process. The wind blade section is based on the cross-section geometry of a 3 MW wind turbine blade KM 44 (3 MW IEC Class 1 A, KM).
A radar absorbing structure with lossy periodic patterns was designed and fabricated for application to a stealth wind blade. In this study, a reflection loss of the fabricated RAS was measured to evaluate the radar absorbing capability. The reflection loss is the ratio of the reflected energy power to the incident energy power and was tested by a free space measurement system (HVS technologies) at the KIMS, as shown in Kim’s paper [8]. Fig. 3 shows the simulated and experimental results of the reflection loss. The fabricated RAS has a minimum reflection loss of 34.6 dB at 10.1 GHz frequency. The 90% radar absorbing bandwidth (10 dB) was 4.2 GHz, covering the entire X band. According to the results, the resonance frequency and return loss values showed very small change relative to the simulation results, the difference was 0.1 GHz and 7.6 dB, respectively. The direct cause of the difference was manufacturing error of the pattern thickness; the coating thickness was 1.94 lm different from its design value of 2.0 lm. As a result, the fabricated RAS with the resistive PPS shows good radar absorbing performance at the target frequency.
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Carbon Fiber Cloth Glass Fiber Cloth PPS Sheet
RTM Process
Foam Core Glass Fiber Core Aluminum Mould
Manufacture of Stealth Blades
Stacking Sequence of Stealth Blades
Fig. 6. Manufacturing process of stealth wind turbine blade.
Fig. 7. Configuration of stealth wind turbine blade.
Radar Cross Section (dBsm)
50
Stealth Wind Blade Compact Range
Feed Antenna
30 20 10 0 -10 -20
-30
30
60
90 120 150 180 210 240 270 300 330 360
Fig. 9. Monostatic RCS of conventional and stealth wind blades.
Suction Side Leading Edge
0
Azimuth Angle (°)
90º
180º
Conventional Wind Blade Stealthe Wind Blade
40
Trailing Edge
0º
Pressure Side
270º Fig. 8. Compact range system for measuring radar cross section.
3.2. Characterization of stealth wind blade The objective of this study is to develop the stealth wind blade that offers minimized signal interference with the sur-
rounding antenna and radar systems. In order to reduce the radar interference, the unwanted reflected signals from the wind blade structure should be decreased or eliminated. The magnitude of the signal reflection to the incident EM wave can be indirectly evaluated by measuring the radar cross section of the target. The radar cross section (RCS) is a measure of the energy scattered from a target following irradiation with EM energy of a given frequency. The RCS was measured by using a compact range system in the anechoic chamber of the Pohang University of Science and Technology (POSTECH) in Korea, as shown in Fig. 8.
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surface for reducing the radar signal interference was successfully manufactured.
Table 2 Comparison on monostatic RCS of wind blades. Azimuth angle
Conventional wind blade (dBsm)
Stealth wind blade (dBsm)
RCS reduction rate (%)
Leading edge (185°) Trailing edge (0°) Suction side (78°) Pressure side (280°)
1.5 2.5 30.2 31.1
7.1 19.7 15.2 19.3
86.2 98.1 96.8 93.4
In this study, the monostatic RCS of the wind blades according to an azimuth angles sweep given the frequency was evaluated to demonstrate practical implementation of the stealth wind blade. Fig. 9 shows the measured results of the conventional wind blade and the stealth wind blade. According to the results, the significant RCS reduction occurred at the whole azimuth angle ranges and the RCS was decreased by almost 90% compared with that of the conventional wind blade at 10.0 GHz frequency. In addition, the RCS reduction rates of the suction side (78°) and the pressure side (280°) was 96.8% and 93.4% respectively, as listed in Table 2. This indicates that the RCS of the stealth wind turbine blade constituted only 5–10% of the overall scattering from the conventional wind turbine blade. 4. Conclusions We have proposed a technique to decrease the reflected signals from a wind blade, and thereby minimize signal interference with electromagnetic waves emitted from surrounding infrastructures. The technique is based on a periodic pattern surface made of conductive paste. The fabricated lossy PPS was applied on the sink layer of the wind blade to achieve radar absorbing capability. Therefore, the wind blade with the lossy PPS can absorb the incident electromagnetic energy and reduce the radar cross section. According to the results, the minimum reflection loss of the RAS was 34.6 dB at 10.1 GHz frequency and 90% radar absorbing bandwidth was covering the entire X band. In addition, the radar cross section of the stealth wind blade with the periodic pattern surface was reduced by more than 90% for whole azimuth angle ranges, compared with the conventional wind blade. In conclusion, the stealth wind turbine blade with the lossy periodic pattern
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