GB2465018A

GB2465018A - Electromagnetic shield for positioning between wind turbine and airport radar arrangements

Info

Publication number: GB2465018A
Application number: GB0820336A
Authority: GB
Inventors: Anne Marston; Matthew Hall
Original assignee: University of Nottingham
Current assignee: University of Nottingham
Priority date: 2008-11-06
Filing date: 2008-11-06
Publication date: 2010-05-12
Also published as: GB0820336D0

Abstract

A wind turbine assembly, wind farm assembly, airport radar system or their methods of manufacture or use comprise an electromagnetic shield 130, with a plurality of holes through which wind 120 may flow, which is suitable for being located between a wind turbine 21 and a radar emitter 60 to diminish the electromagnetic signals reaching the wind turbine 21 and/or being returned from the wind turbine 21 to the radar emitter 60. Also claimed is an electromagnetic shield 130 of a certain shape and size which is used for shielding an object. The said shield 130 including a plurality of holes through which wind 120 may pass and where the minor width of the holes is no larger than half a wavelength of the signal emitted from a radar emitter60. The shield 130 may use steel, aluminium, copper, carbon fibre, ionised gas or plasma to diminish an electric field and/or oxygen, sodium and aluminium and/or ferromagnetic materials including nickel to diminish a magnetic field. The holes may be circular or hexagonal and arranged to form a honeycomb structure. The shield may be connected to earth to protect against damage by lightning.

Description

IMPROVEMENTS IN AND RELATING TO WIND TURBINE

ASSEMBLIES

FIELD OF THE INVENTION

The present invention relates to an improvement in wind turbine assemblies. More particularly, the invention relates to minimising the radar cross section of a wind turbine or a wind farm.

BACKGROUND OF THE INVENTION

Generating power from wind turbines is becoming more popular. It is known to have wind farms where many turbines are sited together. They are typically on the top of hills, on coastlines and other windy places.

The turbines are often able to face into the wind so as to increase their * * . efficiency.

*..: One problem with wind turbines is that they have large, fast moving, blades. These have a significant radar signature which can be a problem in areas where radar clutter is not wanted. For example, at airports, *.**** * aircraft are tracked by air traffic control using radar, and radar is used to S..

* 5 assist their landing. The screen of an air traffic control radar system indicates "targets". It may do this using outgoing radar pulses, from the airport radar, and detecting returned reflections. It can assess the speed of targets by using Doppler shifts. It is often known to filter out objects that move slower than, say, thirty miles per hour, as indicated by the Doppler shift.

From the standpoint of the radar environment, objects in the illumination field of the radar system produce echo signals. Typically these objects

S

refer to actual radio frequency (RF) echoes returned from targets. Such targets mostly include natural objects such as ground, sea, precipitation (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections and meteor trails. These objects may also be returned from man-made objects such as buildings, wind turbine assemblies and wind farms.

The rotating blades of a wind turbine have a significant radar return and cannot be easily electronically filtered out of the signal because of changing speed of the turbine blades, the angle of the turbine blades, changes in the types of wind turbines, different size and shape of the turbine blades and also the number of turbine blades on each wind turbine. They interfere with the normal radar tracking of aircraft in the vicinity of airports. The solution to this has, to date, been to prohibit the sitting of wind turbines too close to airfields. "Too close" might be S'S. within fifteen miles or so. This requirement can rule out quite a lot of *:*. locations for the siting of wind turbines/wind farms, in particular *:*.. countries, such as the UK, where there are quite a lot of civilian airfields,

and military airfields.

*. *... * .

S....'

* SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a wind turbine assembly comprising: a plurality of wind turbine blades for generating electricity; an electromagnetic shield adapted to be interposed between the wind turbine assembly and a radar emitter, the electromagnetic shield is adapted to diminish radar signals from reaching the wind turbine assembly and/or being returned from the wind turbine assembly to the radar emitter; wherein the electromagnetic shield contains a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.

The plurality of holes may have a minor width which is no larger than � wavelength of the signal emitted from the radar emitter. The holes may have a minor width which may be less than 60mm for a radar frequency range of 2.0 to 5.0GHz and the holes may have a minor width of less than 300mm for a radar frequency range of 500MHz to 2GHz. The holes may also have a minor width which may be less than 37.4mm for a radar frequency of 3.3 GHz and the holes may have a minor width less than 142mm for a 1GHz radar frequency. The electromagnetic shield for a radar frequency range of 500MHz to 5.0GHz may have a thickness in the range of 1mm to 300mm.

In another example, for a radar emitter of 3GHz, the electromagnetic shield may have holes having a minor width of 42mm, a ratio of about 60% (say 62, or 63, or 62.8%) for a turbine having a blade diameter of 30m, of the number of openings/area of openings in the electromagnetic shield to the amount of solid area in the electromagnetic shield, and a physical electromagnetic shield size of 30m. In order to minimise the * loss of efficiency of the turbine the shield may be placed about, or at S.....

least, 108 m in front of the wind turbine assembly.

The electromagnetic shield may be used to diminish an electric field and/or may be constructed from electrical conducting materials, the materials comprising steel, aluminium, copper, carbon fibre, ionised gas or plasma. The electromagnetic shield may be used to diminish a magnetic field and/or may be constructed from paramagnetic materials comprising oxygen, sodium and aluminium and/or ferro-magnetic materials comprising nickel. The electromagnetic shield may be interposed between the wind turbine assembly and the radar emitter and may create a shadowing effect for at least 2km behind the electromagnetic shield therefore allowing the placement of a wind turbine assembly anywhere within the 2km area. In order to achieve the least loss of efficiency the electromagnetic shield may be placed a distance in front of the wind turbine assembly, the distance may be determined by: 1) a minor width of the holes in the electromagnetic shield; 2) a ratio of the number of openings in the electromagnetic shield to the amount of solid area of the electromagnetic shield; and 3) a physical shape and size of the electromagnetic shield.

For a radar emitter of 3.3GHz, the electromagnetic shield may have holes having a minor width of 37.4mm, a ratio of approximately 15% of the number of openings in the electromagnetic shield to the amount of solid area of the electromagnetic shield and a physical electromagnetic shield size of 5m, in order to achieve the least loss of efficiency the * *** electromagnetic shield may be placed at least 75m in front of the wind ** turbine assembly. The electromagnetic shield may have a circular physical shape with a plurality of circular holes. The electromagnetic shield may have a honeycomb physical shape with a plurality of hexagonal holes. The *....s * electromagnetic shield may be earthed to protect the electromagnetic shield from lightning strikes.

According to a further aspect the invention comprises a method of diminishing a radar emitter signal from reaching a wind turbine assembly and preventing the reflection of radar signals from the wind turbine assembly, the wind turbine assembly comprising a plurality of wind turbine blades, the method comprising: interposing an electromagnetic shield between the wind turbine assembly and the radar emitter; constructing the electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.

According to a still further aspect the present invention comprises a wind farm comprising: at least one wind turbine assembly comprising a plurality of wind turbine blades for generating electricity; at least one electromagnetic shield adapted to be interposed between the at least one wind turbine assembly and a radar emitter, the at least one electromagnetic shield is adapted to diminish radar signals from reaching the at least one wind turbine assembly and/or being returned from the at least one wind turbine assembly to the radar emitter; wherein the at least one electromagnetic shield contains a plurality of holes adapted to allow the flow of wind through the at least one electromagnetic shield to the at least one wind turbine assembly.

According to a further aspect the present invention comprises a method of producing electricity using a wind turbine assembly comprising a plurality of wind turbine blades for generating electricity, the method comprising: connecting the electricity generated by the wind turbine assembly through a transmission line to a substation; placing an electromagnetic shield * between the wind turbine assembly and a radar emitter, the * electromagnetic shield is adapted to diminish radar signals from reaching the wind turbine assembly and/or being returned from the wind turbine assembly to the radar emitter; constructing the electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.

According to a further aspect the present invention comprises an airport comprising a pulse radar system, the pulse radar system comprising an antenna system for transmitting and receiving frequency signals from the pulse radar system, a target detecting system to process the transmitted and received frequency signals and display the detected targets, the airport comprising: at least one electromagnetic shield adapted to be interposed between at least one wind turbine assembly and the antenna system, the at least one electromagnetic shield is adapted to diminish a radar signal from the antenna system from reaching the at least one wind turbine assembly and/or being returned from the at least one wind turbine assembly to the antenna system, wherein the at least one electromagnetic shield contains a plurality of holes adapted to allow the flow of wind through the at least one electromagnetic shield to the at least one wind turbine assembly.

According to a still further aspect the present invention comprises a method of modifying an existing wind farm, the wind farm comprising at least one wind turbine assembly comprising a plurality of wind turbine blades for generating electricity; placing at least one electromagnetic shield between the wind farm and a radar emitter, the at least one *:* electromagnetic shield is adapted to diminish radar signals from reaching the wind farm andfor being returned from the wind farm to the radar emitter; constructing the at least one electromagnetic shield to contain a * plurality of holes adapted to allow the flow of wind through the at least **SS6 * one electromagnetic shield to the wind farm.

According to a further aspect the present invention comprises a method of constructing a new wind farm comprising: placing the wind farm in line of sight of an airport; constructing an electromagnetic shield between the wind turbine assembly and a radar emitter, the electromagnetic shield is adapted to diminish radar signals from reaching a wind turbine assembly and/or being returned from the wind turbine assembly to the radar emitter; designing the electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.

According to a further aspect the present invention comprises an electromagnetic shield of a certain shape and size used for shielding an object, the electromagnetic shield comprising: a plurality of holes adapted to allow the flow of wind through the electromagnetic shield, a minor width of the holes being no larger than � wavelength of the signal emitted from a radar emitter.

According to a further aspect the present invention comprises an airport comprising a pulse radar system, the pulse radar system comprising an antenna system for transmitting and receiving frequency signals from the pulse radar system, a target detecting system to process the transmitted and received frequency signals and display the detected targets, the "S. airport comprising: placing at least one electromagnetic shield within the * VS.

grounds of the airport; adapting the at least one electromagnetic shield to diminish radar signals from being returned from an at least one wind turbine assembly to the antenna system; constructing the at least one electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the at least one electromagnetic shield to the at least one wind turbine assembly.

The effect wind turbines have on Air Traffic Control (ATC) Primary Surveillance Radar (PSR) is a considerable one. PSR works using the doppler shift principle. A frequency is emitted from the PSR; if the returning frequency is different the PSR can tell that this is a moving object. It can detect exactly how fast the object is travelling and pin point its position. Wind turbines in line of sight (LoS) of a PSR return a similar doppler shift return to that of aircraft and these appear on the PSR screen and so mask any aircraft flying over wind farms. The turbine blades give erratic returns to the PSR due to the difference in appearance in shape as they rotate. This produces a large quantity of potential doppler returns making it difficult to filter by the existing radar system.

By placing a stationary object, which can act as an electromagnetic shield, the size and shape of the wind turbine between it and the radar, the radar signal will not reach the turbine and therefore no doppler return will be produced and so nothing will appear on the radar screen as the existing filters within an ATC PSR can filter these returns from the point position indicator. This shield would be adaptive to new technology as it is independent of the turbine and the radar. The shield can be retrofitted and applied to new projects. This is especially good for small and building integrated wind turbines as it is relatively simple and the emphasis is on shielding the turbine relatively simply and cheaply. S4 * I* I. I

**.* BREIF DESCRIPTION OF THE DRAWINGS

II I * I I

Figure 1 is a schematic diagram of a typical airport with a ground based radar system in the form of an ATC (airport traffic control) radar system

according to the prior art;

S.,... * I

Figure 2 shows a schematic diagram of the electromagnetic shield in use according to one embodiment of the present invention; Figure 3 shows a schematic diagram of the electromagnetic shield in Figure 2 according to the present invention; Figure 4 shows a schematic diagram of a further embodiment of the electromagnetic shield according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

In Figure 1, a schematic diagram is shown for an airport 10 having a ground-based radar system in accordance with the prior art. Thus, an ATC radar system 70 employs a scanning, transmitting/receiving antenna to transmit radar pulse signals through an illumination space and to receive echo or reflection signals as well as any incident interference and noise signals. The antenna beam width, the antenna beam azimuth, the antenna scanning rate, and other antenna design features are determined in accordance with radar system design considerations.

The reflection signals include reflections from moving targets to be detected, i.e., an aircraft 50 as well as any other aircraft (not shown) * *** which may be within the target detection range of the ATC radar system 70. The reflection signals further include stationary and moving clutter reflections, such as those from a runway structure 90, a wind farm 20 and S.....

* from other ground objects 30 which are stationary, moving as a whole, or stationary with moving parts (such as a tree), and from other moving clutter 40, such as birds, rain cells or other weather systems or events.

The ATC radar system 70 processes received signals to detect moving target detection signals, which are to be passed, and to reject or block non-target signals. Thus, aircraft position reporting signals are transmitted to a display apparatus (not shown) located in an airport tower for airport controller use in traffic control. Wind turbines 21 in line of sight (LoS) of a PSR return a similar doppler shift return to that of aircraft 50 and these appear on the PSR screen and so mask any aircraft flying over wind farms 20. The turbine blades give erratic returns to the PSR due to the difference in appearance in shape as they rotate. This produces a large quantity of potential doppler returns making it difficult to filter by the existing radar system 70.

Figure 2 shows a schematic diagram of an electromagnetic shield 130 in use shielding a wind turbine 21 according to one embodiment of the present invention. A wind power plant or a wind farm 20 comprise a group of wind turbines 21 connected to generate electricity for the utility grid. The electricity is sent through transmission and distribution lines to homes, businesses, schools, and so on. On a smaller scale a single wind turbine 21 may be used to provide electricity to power a single household and may not provide power to the utility grid as described above.

Three-bladed wind turbines 21 are operated "upwind," with the blades * *** : facing into the wind 120. The other common wind turbine type is the two-* *** bladed, downwind turbine. The wind 120 turns the blades, which spin a shaft, which connects to a generator and makes electricity. Utility-scale turbines range in size from 50 to 750 kilowatts. Single small turbines, o.I * below 50 kilowatts, are used for homes, telecommunications dishes, or water pumping.

Electromagnetic shielding is the process of limiting the flow of electromagnetic fields between two locations, by separating them with a barrier made of conductive material. Typically it is applied to enclosures, separating electrical devices from the outside world', and to cables, separating wires from the environment the cable runs through. In an embodiment of the present invention an electromagnetic shield 130 is placed in between a wind turbine 21 and an ATC PSR 60.

The shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. The amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic field.

All of these factors will be further explained below.

In Figure 2, the electromagnetic shield 130 is placed upwind from the wind turbine 21 a certain distance in front of the wind turbine, the way in which this distance is determined will be described below. The electromagnetic shield 130 is placed in between the wind turbine 21 and a radar antenna or radar emitter 60 (PSR). The radar antenna 60 transmits radar pulse signals 110 through an illumination space and receives echo or reflection signals 100 as well as any incident interference and noise signals. In this embodiment the electromagnetic shield 130 is designed to .. : either absorb the transmitted signal 110 or reflect the reflection signal * S..

100. As the PSR works using the doppler shift principle. A frequency is emitted from the PSR 110; if the returning frequency 100 is different the PSR can tell that this is a moving object. It can detect exactly how fast S.....

* the object is travelling and pin point its position. In this case as the electromagnetic shield 130 is stationary, any reflected signal or frequency 100 will be disregarded or filtered by the ATC PSR 60 and therefore removing or hiding the wind turbines 21 which are located in the line of sight of a PSR 60 and which would normally return a similar doppler shift return to that of an aircraft 50 and therefore no longer masking any aircraft 50 flying over a wind farm 20 or a wind turbine 21.

Figures 3 and 4 show two different embodiments of the present invention.

Figure 3 shows the circular shaped 132 electromagnetic shield 130 with circular shaped 131 holes having a minor width and Figure 4 shows the honeycomb shaped 132 electromagnetic shield 130 with honeycomb or hexagonal shaped holes 134 having a minor width. As can be appreciated a-a number of different shapes and different shaped holes may be used to implement the present invention. Each design and type of shape require a number of different factors which need to be taken into account when designing a wind turbine 21 electromagnetic shield 130, and these are discussed below.

Referring to Figure 4, the axes of honeycomb cells 134 may be quasi-horizontal, and the non-angled rows of honeycomb cells 134 may be horizontally (not vertically) aligned. Thus, each cell has two vertical walls, with "floors" and "ceilings" composed of two angled walls. The cells may slope slightly upwards, between 9 and 14 degrees, towards the open ends. Honeycomb is composed of hexagons, rather than any other shape, due to the fact that the hexagon tiles the plane with minimal * ..S surface area. Thus a hexagonal structure uses the least material to create * a..

a lattice of cells within a given volume. *. a * * *

* Honeycomb is predominately used as a core in sandwiched structures to *.*.* * meet design requirements for highly stressed structural components. For example, when sandwiched between layers of carbon fibre, honeycomb exhibits extreme resistance to shear stresses. As a structural core material, it is used in all types of aerospace vehicles and supporting equipment where sandwich structure offers rigid panels of minimum weight, aerodynamic smooth surfaces, and high fatigue resistance.

Therefore the advantages of honeycomb/cellular structure 134 include greater rigidity -maybe the shield can be self-standing/self-supporting. It is also possible to make sculptures out of honeycomb materials. Holes 134 of the honeycomb should be facing the airport radar 60. They could be easy to anchor to the ground, and stronger. They will also give a very good efficient electromagnetic shield 130. The holes/tubes 134 can also help to create a laminar flow -get the airspeed back up to fast speed on the other side of the shield.

The hole size 131 or minor width, it is, we believe, desirable to have holes 131 in the electromagnetic shield 130, and we believe that the holes 131 should have a minor width (or diameter) which is no larger than about half a wavelength of the signal emitted by the radar emitter (in their largest direction). Holes 131 make the structure lighter, and cheaper, and holes 131 allow wind 120 to flow through them. A solid wall would not allow wind 120 to flow through and will reduce the performance of a wind turbine 21 more than a electromagnetic shield 130 with holes 131 in it -where the wind 120 can flow through the holes 131 to power the wind turbine 21. * * * ** .

The optimum hole minor width size 131 in the electromagnetic shield 130 to continue to allow the electromagnetic properties of the electromagnetic shield 130 to hide the wind turbine 21 from the ATC PSR 60 can be ****** S experimentally determined. The maximum minor width or diameter of a circular opening/openings 131 in the electromagnetic shield 130, which will stop the frequency of the ATC PSR 60 penetrating the electromagnetic shield 130 is calculated by using equation (1): A([(c/f)/2]xl000))-8 (1) where A = minor width of the hole (mm) c = speed of light (299 752 485m/s) f = frequency of radar (Hz) Applying this to ATC frequencies currently in use at airports in table 1 below: Diameter must be under Diameter must be under this this size if there is one size if there are many circular Frequency circular hole in the shield holes in the shield to shield to shield from the from the frequency frequency 3.3GHz 45.4mm 37.4mm 1GHz 150mm 142mm

Table 1

: The thickness of the electromagnetic shield 130, is dependent on hole size 131 and the radar frequency. For example the thickness of the electromagnetic shield 130 may range from 1mm to 300mm for a radio frequency range of 500MHz to 5.0GHz. The larger the hole size 131 the thicker the electromagnetic shield 130. * *

The materials that may be used to make the electromagnetic shield 130 are dependent on whether the electric field or the magnetic field is being reduced or diminished. For an electric field, electrically conducting materials, such as sheet metal and metal mesh made from steel, aluminium, copper, carbon fibre, ionised gas, and plasma may be used.

The higher the conductivity of the material used will produce a stronger electromagnetic shield 130.

For the magnetic field, paramagnetic materials such as oxygen, sodium and aluminium, or ferro-magnetic materials for better shielding such as Nickel (magnetic permeability of 100) may be used to produce the electromagnetic shield 130.

The design of the electromagnetic shield 130 may produce a radar shadow or a shadowing effect for a wind turbine 21 in the range of at least 2km behind the electromagnetic shield 130. A radar shadow is the absence of radar illumination because of intervening reflecting or absorbing objects, for example terrain obstructions (mountains, cliffs) or by artificial features (buildings) within the illuminating radar beam. The electromagnetic shield 130 will have at least this shadowing effect and so as long as the wind turbine 21 is placed within at least 2km behind the electromagnetic shield 130 the wind turbine 21 will be shielded. S...

: The next design issue is the distance between the wind turbine 21 and * .** 15 electromagnetic shield 130. As the wind turbine blades have an effect upon the efficient working of the wind turbine 21, the wind 120 that passes through the electromagnetic shield 130 (through its holes 131) should be allowed to regenerate sufficiently in order for the wind flow to return to a sensible speed. The wind speed 120 should be allowed to regenerate to around 70%, 80% or 90% of its original speed.

In order to determine the optimum distance, the wind flow 120 is simulated through and around the electromagnetic shield 130. The optimum distance should be a distance which may allow the wind turbine 21 the least loss of efficiency. The positioning of the electromagnetic shield 130 from the wind turbine 21 is dependent upon a number of factors which will be discussed in turn below.

The first factor being the size of the hole aperture 131 in the electromagnetic shield 130. For many typical wind farms 20 the spacing of the respective turbines varies from 8 diameter lengths of the turbine, for prevailing wind hitting the turbines and 3-4 diameter lengths for turbines placed at 90 degrees to the prevailing wind. This gives respective turbines a chance to move into the wind and to regain wind speed which is required at each respective turbine to achieve efficient working speeds.

It is well-known in wind farms 20 to space wind turbines 21 apart. The electromagnetic shield 130 should be spaced at about the same sort of distance as the spacing between the wind turbines 21. Wind turbines 21 are spaced apart by a distance which has a similar consideration -the wind 120 needs to have got back up to a sensible speed when it encounters the next wind turbine 21.

This is specific to small shields whose radius is measured in cm. The shield sizes tested when there were no holes in the shield were 25cm, 37.5cm and 50cm radius shields. When wind turbines are spaced from each other their diameters are taken into consideration. In this case the distance the turbine is placed from the shield can be estimated at : approximately 5 diameters. ... * *

Distance shield should be from turbine when average wind speeds reach 2.5m/s Predictions Shield hole % Results using Radius diameter Open from (Equation Predictions (cm) (mm) Area simulations 6.4) Using 5D 0 0% 0.8m im 2.5m 37.5 0 0% 3.3m 3.3m 3.75m 42 0 0% 3.8m 4.lm 4.2m 0 0% 5.6m 5.6m 5m

Table 2

Using the distance five diameters will be more useful when looking at larger turbines. Five diameters places the turbine further from the shield than the simulated results suggest for all the shields accept 50cm. It is better to predict a slightly further distance than a closer distance at this point. When judging the distance a solid shield should be from the turbine it is suggested that equation (2) is used. Five diameters will be used later and will be referred to as the shield size factor (SSF) Ds(sjze) =[(2r50)x2.3]+i (2) The diameter of the holes in the shield affects the distance the shield should be * placed from the turbine. This affected the shield in two different ways depending * on whether the open area was greater or less than 50%. The equation formulated . : 15 relied on small shields with specific openings. This will look at changing that equation to percentages rather than specific numbers and distances. This will look S **.S * at the effect that hole sizes on a whole have on the distance the shield should be * : * from the turbine, the next section will incorporate the open area percentage and * : * the hole diameter. The first part of each of the percentage equations starts with a starting position for the shield using the variables for shield size and hole diameter. The next two equations will incorporate both of these for the beginning of the two equations for percentage open area. This will be referred to as the hole size factor (HSF) To complete the part the Hole shield factor SSF will be added Under 50% 1 d HSF<50%=l+I I X5D open \l500,/ Over5O% I d HSF>50%=l-I I X5D open l5OOj Table 3 showing the new substitution of HSF factors and SSF factors The open area of the shield provided the greatest decrease and increase in the distance the shield can be from the wind turbine. These equations will be :::: percentage increases and decreases so that the equations can be applied to all situations. This will be known as the percentage open factor POF. * S* * S.

*S.S.. * .

________________________________________________

5. Percentage open factor --105+d d 1 (I-so)+14o-_JJ.Iloo POF>50% = 1 -{[ _50)_[2�JJ]} Table 4 showing updated parts of the equations The final equations are shown in equation (3) and (4) below: D<50% = HSF<50% x SSF x POF<50% (3) D>50% = HSF>50% x SSF x POF>50% (4) * * * ** I * ** I * I I... *1 I * I * * I* * * I

I

I...,.

S

* I. SI * * The following table shows the predicted results from equations 3 and 4 in comparison to the results obtained from the simulations.

Predicted Distance Shield Hole % HSF POF SSF SSFxHSFxPOF Radius Diamete Open (% (% (m) For average wind speed (cm) r (mm) Area factor) factor) at 83% of initial speed 0 0 2.5 1 1.05 2.6m 3.7 37.5 0 0 1 1.05 3.9m 42 0 0 4.2 1 1.05 4.4m 0 0 5 1 1.05 5.3m 30 12 5 1.02 1.11 5.Zm . : 50 45 12 5 1.03 1.098 5.Zm * ** . 60 12 5 1.04 1.086 5.6m 30 22 5 1.02 1.16 5.9m 60 22 5 1.04 1.116 5.8m 30 32 5 1.02 1.21 6.2m S..... ___________ ______________ _________ _______ ___________ __________ ___________________________________ * 50 60 32 5 1.04 1.146 6.Om **.*.I ___________ ______________ _________ _______ ___________ __________ ___________________________________ * 50 30 41 5 1.02 1.255 6.4m 60 52 5 0.96 1.02 4.9m 30 53 5 0.98 1 4.9m 30 58 5 0.98 0.9 4.4m 60 58 5 0.96 0.78 3.7m 45 59 5 0.97 0.81 3.9m 30 62 5 0.98 0.82 4.Om 60 65 5 0.96 0.5 2.4m

Table 5

The relationship between the distance of the shield from the turbine blades, its hole sizes and percentage of area hole vs solid can be expressed in the following distance equations:-Distance Equations Distance if open percentage is under 50% D<50% = HSF<50% x SSF x POF<50% (5) Where: POF<50O {_105�a(P _5o)�14o_i)}/1oo HSF<50% _i+(J SSF = 5D * * * S. * * .* * *....S 15 : Distance if open percentage is over 50% D>50%=HSF>50%XSSFXPOF>50% (6) *.S* * S Where: d 30 * 20 POF>50% 1_{15OO[ _50)_(2+))]} HSF>50% SSF=5D

A Worked Example

This example will look at shielding a 15m blade length turbine for ATC PSR which works on a 3GHz frequency.

Using equation 1, where c 299 792 458 rn/s and f 3 GHz d = ([(c/f) / 2] x 1000)) -8 d= ([(299,792,458 I 3,000,000,000) / 2] x 1000)) -8 42mm The hole diameter will be 42mm. In order to minimise the distance between the turbine and the shield the open area of the shield will be set to greater than 50%. The holes in the shield will be set 5mm apart. The shield will have a radius of 15m to cover the circular area swept by the blades. To find the number of holes, 3000cm diameter / 4.7cm hole and space 638 holes along the diameter, 319 along the radius, gives an inner square of holes, 4512 this leaves 8 triangles of holes which turns into 4 squares, half the inner square long (225) by the inner square -*...

radius (94). From this the open percentage can be worked out and gives approximately 70.69% open area, this is an approximation of hole number and so the calculation and can be made much more accurate by finding the exact number of holes in the shield. *.I

* 20 Using equation 4 as the open area is over 50%, each part can be now calculated. HSF> 50% D>50% = HSF>50% x SSF x POF>50% (d HSF>50% HSF>50% ll5OO,) = 0.97 SSF X5D = 75m POF>SO% POF>50% = l{[F _50)_[2+JJ]} = 0.456 = 0.97 x 75m x 0.456 = 33.2m For a 30m diameter swept area for a turbine the shield should be placed 33.2m from the turbine.

A further feature which needs to be identified is the need for lightning protection. As lightning bolts are common in clouds during rainstorms, and on average 6000 lightning bolts occur between clouds and the Earth every minute, the possible damage caused by lightning is a problem that needs to be taken care of. The earthing of the electromagnetic shield 130 *: provides a protection system which distributes the lightning current in the soil without causing dangerous potential differences. The potential S. .*** * : increases on the earthing and on all earthed metal parts of the object *...S.

* 15 relative to the zero potential at a distant point. It may reach a very high value but it does not cause any danger if the potential differences inside the object to be protected are limited. Potential equalization is realized by the bonding of all extended metal objects.

An advantage of the use of placing a stationary object, which can act as an electromagnetic shield 130, the size and shape of the wind turbine 21 between it and the radar 60, the radar signal will not reach the wind turbine 21 and therefore no doppler return will be produced and so nothing will appear on the radar screen. This electromagnetic shield 130 would be adaptive to new technology as it is independent of the wind turbine 21 and the radar 60. The electromagnetic shield 130 can be retrofitted and applied to new projects. This is especially good for small and building integrated wind turbines as it is relatively simple and the emphasis is on shielding the wind turbine 21 relatively simply and cheaply.

As commercial wind energy is one of the most economical sources of new electricity available today. Wind turbines 21 can be set up quickly and cheaply compared with building new coal-fired generating stations or hydroelectric facilities. Modern wind generating equipment is efficient, highly reliable, and becoming cheaper to purchase. The environmental impact of large wind turbines 21 is negligible compared with an open pit coal mine or a reservoir, and during their operation produce no air .. : pollution. Because of these factors, wind energy is recognized as the world's fastest-growing new energy source. * . * SS

Small, highly efficient wind turbines 21 are becoming popular as a source of electricity for rural homes. The cost of installing one comes close to that of putting up poles, overhead power lines and other equipment necessary to connect to the electrical grid. The advantage is that the homeowner owns the generating equipment and is freed from paying monthly electrical bills! Electricity is now being generated on commercial scale at large installations called "wind farms" in several places around the world.

By way of an example for the ever increasing need for wind farms 20 and the requirement for the wind farms 20 to include an electromagnetic shield 130 in order to make the wind farms 20 invisible to the ATC PSR 60. In the United Kingdom plans to meet up to a third of Britain's energy needs from offshore wind farms are in jeopardy because the Ministry of Defence (MoD) objects that the wind turbines 21 interfere with its radar 60.

A wind farm 20 which is in the line of sight of any radar station makes it impossible to spot aircraft 50. The wind turbines 21 create a hole in radar coverage so that aircraft 50 flying overhead are not detectable. This obscuration occurs regardless of the height of the aircraft 50, of the radar and of the wind turbine 21.

The use of the electromagnetic shield 130 provides for the ability to place a wind farm 20 in any area, be it near an airport, close to a military installation or even in an off-shore installation. By placing a stationary object, which can act as an electromagnetic shield 130, the size and shape * *.* : of the wind turbine 21 between it and the radar 60, the radar signal will *s.

not reach the wind turbine 21 and therefore no doppler return will be produced and so nothing will appear on the radar screen.

S

*SSe*S It should also be noted that an N x M array of wind turbines 21 in a wind farm 20 you will not need an N x M electromagnetic shields 130 -electromagnetic shields 130 are only needed for the first row, nearest the airport. After that, the other wind turbines 21 are shielded behind the same electromagnetic shield 130. We do not need one electromagnetic shield 130 per wind turbine 21. The electromagnetic shield 130 should be fixed -that it does not move with the wind turbine 21 when the wind turbine 21 points into the wind 120.

Although the present invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omission and additions may be made therein and thereto, without departing from the scope of the invention. Therefore the present invention should not be understood as limited to the specific embodiment set Out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalent thereof with respect to the features set out in the appended claims. * . S S. S *. * C * .** * *. * SS * * .. * S

* S. SS S

Claims

CLAIMS1. A wind turbine assembly comprising: a plurality of wind turbine blades for generating electricity; an electromagnetic shield adapted to be interposed between the wind turbine assembly and a radar emitter, the electromagnetic shield is adapted to diminish radar signals from reaching the wind turbine assembly and/or being returned from the wind turbine assembly to the radar emitter; wherein the electromagnetic shield contains a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.* .* * * 0 * ***
2. The wind turbine assembly according to claim 1, wherein the ** plurality of holes have a minor width which is no larger than � wavelength of the signal emitted from the radar emitter.

*
3. The wind turbine assembly according to claim 2, wherein the holes S.....

* 20 have a minor width less than 60mm for a radar frequency range of 2.0 to 5.0GHz and the holes have a minor width of less than 300mm for a radar frequency range of 500MHz to 2GHz.
4. The wind turbine assembly according to claim 2, wherein the holes have a minor width less than 37.4mm for a radar frequency of 3.3GHz and the holes have a minor width less than 142mm for a radar frequency of 1GHz.
5. The wind turbine assembly according to claim 1, wherein the electromagnetic shield for a radar frequency range of 500MHz to 5.0GHz has a thickness in the range of 1mm to 300mm.
6. The wind turbine assembly according to claim 1, wherein the electromagnetic shield is used to diminish an electric field.
7. The wind turbine assembly according to claim 6, wherein the electromagnetic shield is constructed from electrical conducting materials, the materials comprising steel, aluminium, copper, carbon fibre, ionised gas or plasma.
8. The wind turbine assembly according to claim 1, wherein the electromagnetic shield is used to diminish a magnetic field. S...
9. The wind turbine assembly according to claim 8, wherein the electromagnetic shield is constructed from paramagnetic materials comprising oxygen, sodium and aluminium and/or ferro-magnetic S.....* materials comprising nickel. * 20
10. The wind turbine assembly according to claim 1, wherein the electromagnetic shield when interposed between the wind turbine assembly and the radar emitter creates a shadowing effect for at least 2km behind the electromagnetic shield therefore allowing the placement of a wind turbine assembly anywhere within the 2km area.
11. The wind turbine assembly according to claim 1, wherein in order to achieve the least loss of efficiency the electromagnetic shield is I, placed a distance in front of the wind turbine assembly, the distance is determined by: 1) a minor width of the holes in the electromagnetic shield; 2) a ratio of the number of openings in the electromagnetic shield to the amount of solid area of the electromagnetic shield; and 3) a physical shape and size of the electromagnetic shield.
12. The wind turbine assembly according to claim 11, wherein for a radar emitter of 3.0GHz, the electromagnetic shield has holes having a minor width of 42mm, a ratio of approximately 63% of the number/area of openings in the electromagnetic shield to the amount of solid area of the electromagnetic shield and a physical electromagnetic shield size of 30m, in order to achieve a reduced or minimised loss of efficiency the electromagnetic shield is placed *:*. 15 at least 108m in front of the wind turbine assembly.
13. The wind turbine assembly according to claim 11, wherein the electromagnetic shield has a circular physical shape with a plurality of circular holes.
14. The wind turbine assembly according to claim 11, wherein the electromagnetic shield has a honeycomb physical shape with a plurality of hexagonal holes.
15. The wind turbine assembly according to any one of the preceding claims wherein the electromagnetic shield is earthed to protect the electromagnetic shield from lightning strikes.
16. A method of diminishing a radar emitter signal from reaching a wind turbine assembly and preventing the reflection of radar signals from the wind turbine assembly, the wind turbine assembly comprising a plurality of wind turbine blades, the method comprising: interposing an electromagnetic shield between the wind turbine assembly and the radar emitter; constructing the electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.
17. The method according to claim 16, wherein the plurality of holes have a minor width which is no larger than � wavelength of the signal emitted from the radar emitter.
18. The method according to claim 17, wherein the holes have a minor width less than 60mm for a radar frequency range of 2.0 to * S..5.0GHz and the holes have a minor width of less than 300mm for a radar frequency range of 500MHz to 2GHz.

S..... * S
19. The method according to claim 17, wherein the holes have a minor e....* . width less than 37.4mm for a radar frequency of 3.3GHz and the * 20 holes have a minor width less than 142mm for a radar frequency of 1GHz.
20. The method according to claim 16, wherein the electromagnetic shield for a radar frequency range of 500MHz to 5.0GHz has a thickness in the range of 1mm to 300mm.
21. The method according to claim 16, wherein the electromagneticshield is used to diminish an electric field.
22. The method according to claim 21, wherein the electromagnetic shield is constructed from electrical conducting materials, the materials comprising steel, aluminium, copper, carbon fibre, ionised gas or plasma.
23. The method according to claim 16, wherein the electromagneticshield is used to diminish a magnetic field.
24. The method according to claim 23, wherein the electromagnetic shield is constructed from paramagnetic materials comprising oxygen, sodium and aluminium and/or ferro-magnetic materials comprising nickel. * * *
25. The method according to claim 16, wherein the electromagnetic * .,* shield when interposed between the wind turbine assembly and the radar emitter creates a shadowing effect for at least 2km behind the electromagnetic shield therefore allowing the placement of a wind * turbine assembly anywhere within the 2km area. *S** * *SS.....

* 20
26. The method according to claim 16, wherein in order to achieve the least loss of efficiency the electromagnetic shield is placed a distance in front of the wind turbine assembly the distance is determined by: 1) a minor width of the holes in the electromagnetic shield; 2) a ratio of the number of openings in the electromagnetic shield to the amount of solid area of the electromagnetic shield; and 3) a physical shape and size of the electromagnetic shield.
27. The method according to claim 26, wherein for a radar emitter of 3.0GHz, the electromagnetic shield has holes having a minor width of 42mm, a ratio of approximately 63% of the number/area of openings in the electromagnetic shield to the amount of solid area of the electromagnetic shield and a physical electromagnetic shield size of 30m, in order to achieve a reduced or minimised loss of efficiency the electromagnetic shield is placed at least 108m in front of the wind turbine assembly.
28. The method according to claim 26, wherein the electromagnetic shield has a circular physical shape with a plurality of circular holes.
29. The method according to claim 26, wherein the electromagnetic shield has a honeycomb physical shape with a plurality of hexagonal holes. * *.*
30. The method according to any one of claims 16 to 29, wherein the electromagnetic shield is earthed to protect the electromagnetic * shield from lightning strikes.S..... * S

* 20
31. A wind farm comprising: at least one wind turbine assembly comprising a plurality of wind turbine blades for generating electricity; at least one electromagnetic shield adapted to be interposed between the at least one wind turbine assembly and a radar emitter, the at least one electromagnetic shield is adapted to diminish radar signals from reaching the at least one wind turbine assembly and/or being returned from the at least one wind turbine assembly to the radar emitter; wherein the at least one electromagnetic shield contains a plurality of holes adapted to allow the flow of wind through the at least one electromagnetic shield to the at least one wind turbine assembly.
32. The wind farm according to claim 31, wherein the plurality of holes have a minor width which is no larger than � wavelength of the signal emitted from the radar emitter.
33. The wind farm according to claim 32, wherein the holes have a minor width less than 60mm for a radar frequency range of 2.0 to 5.0GHz and the holes have a minor width of less than 300mm for a radar frequency range of 500MHz to 2GHz. * *.* * * .
34. The wind farm according to claim 32, wherein the holes have a * *** minor width less than 37.4mm for a radar frequency of 3.3GHz and the holes have a minor width less than 142mm for a radar frequency of 1GHz.*.* ..* *
35. The wind farm according to claim 31, wherein the electromagnetic S..?.,.

* 20 shield for a radar frequency range of 500MHz to 5.0GHz has a thickness in the range of 1mm to 300mm.
36. The wind farm according to claim 31, wherein the at least one electromagnetic shield is used to diminish an electric field.
37. The wind farm according to claim 36, wherein the at least one electromagnetic shield is constructed from electrical conducting materials, the materials comprising steel, aluminium, copper, carbon fibre, ionised gas or plasma.
38. The wind farm according to claim 31, wherein the at least one electromagnetic shield is used to diminish a magnetic field.
39. The wind farm according to claim 38, wherein the at least one electromagnetic shield is constructed from paramagnetic materials comprising oxygen, sodium and aluminium and/or ferro-magnetic materials comprising nickel.
40. The wind farm according to claim 31, wherein the at least one electromagnetic shield when interposed between the at least one wind turbine assembly and the radar emitter creates a shadowing effect for at least 2km behind the at least one electromagnetic shield therefore allowing the placement of the at least one wind turbine assembly anywhere within the 2km area.
41. The wind farm according to claim 31, wherein in order to achieve the least loss of efficiency the at least one electromagnetic shield is placed a distance in front of the at least one wind turbine assembly, S...* S the distance is determined by: * 20 1) a minor width of the holes in the at least one electromagnetic shield; 2) a ratio of the number of openings in the at least one electromagnetic shield to the amount of solid area of the at least one electromagnetic shield; and 3) a physical shape and size of the at least one electromagnetic shield.
42. The wind farm according to claim 41, wherein for a radar emitter of 3.0GHz, the electromagnetic shield has holes with a minor width of 42mm, a ratio of approximately 63% of the number/area of openings in the electromagnetic shield to the amount of solid area of the electromagnetic shield and a physical electromagnetic shield size of 30m, in order to achieve a reduced or minimised loss of efficiency the electromagnetic shield is placed at least 108m in front of the wind turbine assembly.
43. The wind farm according to claim 41, wherein the at least one electromagnetic shield has a circular physical shape with a plurality of circular holes.
44. The wind farm according to claim 41, wherein the at least one electromagnetic shield has a honeycomb physical shape with a plurality of hexagonal holes. * * **I*
45. The wind farm according to any one of claims 31 to 44, wherein the at least one electromagnetic shield is earthed to protect the at least one electromagnetic shield from lightning strikes.

S.S***
46. A method of producing electricity using a wind turbine assembly comprising a plurality of wind turbine blades for generating electricity, the method comprising: connecting the electricity generated by the wind turbine assembly through a transmission line to a substation; placing an electromagnetic shield between the wind turbine assembly and a radar emitter, the electromagnetic shield is adapted to diminish radar signals from reaching the wind turbine assembly and/or being returned from the wind turbine assembly to the radar emitter; constructing the electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.
47. An airport comprising a pulse radar system, the pulse radar system comprising an antenna system for transmitting and receiving frequency signals from the pulse radar system, a target detecting system to process the transmitted and received frequency signals and display the detected targets, the airport comprising: at least one electromagnetic shield adapted to be interposed between at least one wind turbine assembly and the antenna system, the at least one electromagnetic shield is adapted to diminish a :::: radar signal from the antenna system from reaching the at least one * wind turbine assembly and/or being returned from the at least one * *** wind turbine assembly to the antenna system, wherein the at least one electromagnetic shield contains a plurality of holes adapted to allow the flow of wind through the at least one electromagnetic * shield to the at least one wind turbine assembly. * ******.

* 20
48. A method of modifying an existing wind farm, the wind farm comprising at least one wind turbine assembly comprising a plurality of wind turbine blades for generating electricity; placing at least one electromagnetic shield between the wind farm and a radar emitter, the at least one electromagnetic shield is adapted to diminish radar signals from reaching the wind farm and/or being returned from the wind farm to the radar emitter; constructing the at least one electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the at least one electromagnetic shield to the wind farm.
49. A method of constructing a new wind farm comprising: placing the wind farm in line of sight of an airport; constructing an electromagnetic shield between the wind turbine assembly and a radar emitter, the electromagnetic shield is adapted to diminish radar signals from reaching a wind turbine assembly and/or being returned from the wind turbine assembly to the radar emitter; designing the electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the electromagnetic shield to the wind turbine assembly.
50. An electromagnetic shield of a certain shape and size used for a:::': shielding an object, the electromagnetic shield comprising: a plurality of holes adapted to allow the flow of wind through the * *** electromagnetic shield, a minor width of the holes being no larger a. than � wavelength of the signal emitted from a radar emitter.*55sSS * *
51. An airport comprising a pulse radar system, the pulse radar system comprising an antenna system for transmitting and receiving frequency signals from the pulse radar system, a target detecting system to process the transmitted and received frequency signals and display the detected targets, the airport comprising: placing at least one electromagnetic shield within the grounds of the airport; adapting the at least one electromagnetic shield to diminish radar signals from being returned from an at least one wind turbine assembly to the antenna system; constructing the at least one electromagnetic shield to contain a plurality of holes adapted to allow the flow of wind through the at least one electromagnetic shield to the at least one wind turbine assembly. * * . S. * * S * *** *. S * S * * **I *.S * S * * SSS.....S