Title of Invention

"A DEVICE FOR EXTRACTING POWER FROM TO-AND-FRO-WIND"

Abstract

This invention relates to a device for extracting power from to and fro wind comprising a rotatable care body (8) of circular cylinder with two identical hemi-spheres (9) fitted at two ends; said core (8) having plurality of blades in two airfoil cascades (10) mounted at diametrically opposed position; said blades of the airfoil cascades (10) mounted in a manner such that the axis or span of a blade is always along the radial direction of said core body (8); the chord of the blade being perpendicular to both the radial and axial directions of the core body (8); and and each blade in the said airfoil cascade (10) is of symmetrical section and its chord perpendicular to the duct wind (12) and the said duct wind (12) being alligned with the direction of rotation (11).

Full Text

This invention relates to a device for extracting power from to—and—fro wind. The term "to-and—fro wind" means a wind which alternates its approach from two diametrically opposed directions. For example, if the wind blows for a while from the North and then reverses direction to blow from the South, and then again from the North and so on, not permitting any other direction but North and South, then such a wind may be called to-and-fro wind. To-and-fro wind is created artificially inside a duct in some wave energy power plants which generate power from the sea waves. With the rise and fall of the sea waves, the wind in the duct alternates in direction and varies sinusoidally. Thus, when a wind turbine is placed in such a ducted artificial wind, it can act as a prime mover. An object of this invention is to propose a novel construction of a device for extracting power from to-and—fro wind. STATEMENT OF INVENTION According to this invention there is provided a device for extracting power from to and fro wind comprising: i) a rotatable care body (8) of circular cylinder with two identical hemi-spheres (9) fitted at two ends; ii) said core (8) having plurality of blades in two airfoil cascades (10) mounted at diametrically opposed position; iii) said blades of the airfoil cascades (10) mounted in a manner such that the axis or span of a blade is always along the radial direction of said core body (8); iv) the chord of the blade being perpendicular to both the radial and axial directions of the core body (8); and v) and each blade in the said airfoil cascade (10) is of symmetrical section and its chord perpendicular to the duct wind (12) and the said duct wind (12) being alligned with the direction of rotation (11). The nature of the invention its objective and further advantages residing in the same will be apparent from the following description made with reference to non limiting exemplary embodiments of the invention represented in the accompanying drawings. Fig. 1 shows forces acting on a single airfoil placed in a wind at an incidence angle below stall. Fig.2 shows the motion of air around a single airfoil at an incidence angle below stall. Fig.3 shows forces acting on a single airfoil with incidence angle above stall. Fig.4 shows the motion of air around a single airfoil at incidence angle above stall. Fig.5 shows a cascade of six blades at an incidence angle of about 25 degrees. Fig.6 shows two principal views of a currently used wells turbine. Fig.7 shows two principal views of the present invention cascade turbine. Fig. 1 shows the velocity diagram of an airfoil blade (shown hatched) travelling with a tangential velocity (1) in a duct wind (2) of velocity V. The tangential velocity is given by Q 7. r where Q is the rpm of the turbine which remains constant during operation and r is the radial location of the blade cross—section measured from the axis of rotation. The relative velocity (3) of the wind (with respect to the airfoil) is obtained by vectorially combining the opposite of (1) with (2) and is denoted W. The angle between W and the airfoil chord is the incidence angle (4) and is denoted by (1, as mentioned before. In this case (1 is about 13 degrees (smaller than stall angle ) because here the duct wind (2) is small compared to tangential velocity (1). Lift (5) and Drag (6) forces are shown and the resultant force (7) is inclined forward in the direction of the motion and this implies that the airfoil is producing power. Fig.2 shows the streamlines, that is, the motion of air around the airfoil when (1 is less than the stall angle, as in Fig.i. Air follows the contour of the airfoil and undergoes a smooth turn in the immediate vicinity of the airfoil. It should be noted that this is only an approximate description in order to explain the working principle of the present invention. Fig.3 shows a condition when the duct wind (2) is strong. It is comparable to the tangential velocity (1) and consequently the incidence angle (4) is larger than the stall angle. The lift force (5) is now much smaller, the drang force (6) is larger and the resultant force (7) is inclined backward implying that the airfoil is substracting from the power of the turbine. Fig.4 shows the streamlines when incidence angle (4) is larger than the stall angle, as in Fig.3. Air does not follows the lee surface (S) contour, and the airfoil fails to turn the air effectively. Conseqently there is a net backward thrust on the airfoil which substracts from the power of the turbine. Fig.5 shows a cascade of airfoils where the incidence angle (4) is 25 degrees approximately. Every airfoil in the cascade has its chord perpendicular to the duct wind (2). Air follows the contour of all airfoils except that of the last downstream airfoil. Most of the air undergoes a smooth turn and this is the reason the cascade as a whole can produce power for values of incidence angle (4) much larger than the stall angle. Fig.6 shows the currently used wells turbine which deployes several airfoil blades lined up along the circumference of the core body (8). In other words they are all deployed in a single plane perpendicular to the axis of rotation, where the sense of rotation is shown by (11). Here each blade acts more or less as a stand—alone as in Figs 1,2 3 & 4. When the duct wind is strong due to larger sea waves, it results in stall of the blades and loss of power. Even after the strong waves have subsided this turbine takes a long time to recover from stall and consequently the efficiency is poor (Ref. i & 2}. Fig.7 shows the details and the dimensions of the present invention. The core body (8) consists of a right circular cylinder fitted with two hemi—spheres (9) of the same radius (0.6 m). Two airfoil cascades (10) are mounted at diamertrically opposed positions on the core body(8). Each blade in the cascade has a symmertical section of NACA 0(312 airfoil. The spacing between two consecutive blades is 0.125 m which is half of the chord length. In other words, the space—chord ratio of the cascade is 0.5. The direction of rotation is given by (11). The direction of the to and fro duct wind is shown by (12) and this is aligned with the axis of rotation of the turbine. Each airfoil in the cascade has its chord perpendicular to the duct wind, as in Fig.5. For best operation the turbine should be placed concentrically in a duct with in radial clearnace of the order of 0.25 mm. It should rotate with a constant angular velocity of 2000 rpm. The duct wind speed varies sinusoidally with the rise and fall of the sea wave. The maximum value of the to and fro wind or oscillating wind depends on the wave height incident on the wave energy plant. The term "wind speed" hereafter will mean the amplitude or the maximum value of the sinusoidal to and fro wind. The power of the turbine, shown in Fig.7, goes on increasing with increasing wind speed till the latter reaches a value of about 40 m/s. for a further increase in wind speed, the power needs to be controlled to avoid damaging the generator. This automatically happens in two stages. Firstly when the wind speed reaches about 48 m/s, the last downstream airfoil of the cascade stalls providing a reduction in power. If the wind speeds up further, then an increasing fraction of the wind is allowed to pass through the turbine without being harnessed. This can be explained as follows: The two cascades together sweep the entire circle of the duct in 0.015 seconds. The axial diamension of the cascade (10) is 0.625 m (see Fig.7) and a 40 m/s wind crosses this distance in approximately 0.015 sec. Hence if the wind speed more and more exceeds this value, a greater and greater fraction of the wind passes through without interacting with the blades. In the currently used wells turbine, the blades are so closely packed in the front view (Fig. 6) that they almost touch each other where they join on to the core body. Hence no part of the wind can ever pass through without interacting with the blades. Thus the present invention offers a better power control feature compared to the currently used wells turbine. Returning to the feature of delayed stall due to the cascade effect in the present invention, it should be mentioned that all the blades will stall eventually when the duct wind speed is so high as to give an angle of attack of about 25 degrees. This would happen when the wind speed is about 70 m/s. The main advantage in the present invention is that for wind speed between 40 and 70 m/s, only one out of six blades stalls. Thus when the wind speed eventually decreases again below 40 m/s the recovery from this very partial stall will be quick. In the currently used wells turbine (Fig.6) the stall recovery is slow. The device according to the present invention, for extracting power from to—and-fro wind consists of a wind turbine, inside a cylindrical duct. The duct is open at both ends so as to permit to-and-fro wind. The turbine consists of a core body and airfoil blades mounted on it. The core body is a body of revolution and its axis coincides with the axis of the duct. The axis of the core body is supported at each end on bearings. The shape of the core body is obtained by fitting a right circular cylinder with a hemi-sphere of same diameter at both the ends so as to get a shape which is smooth and rounded everywhere. Instead of hemi-spheres, one may fit ellipsosids to the same effect. The airfoil blades are mounted in such a fashion that the axis or span of a blade is always along the raidal direction of the body of revolution (the core body); the chord of the airfoil blade is perpendicular to both the radial and axial directions of the core body and lies in the plane of rotation of the turbine; the rounded leading edge of the airfoil blade faces the direction of rotation of the turbine. All the airfoil blades are of the same symmetrical cross—section, chosen from listed standard NACA airfoil series or some other standard series. Hereafter airfoil blades will be simply called "blades". They are all identical in shape and size and are arranged in cascade formations. Each cascade consists of for example six blades mounted, one after another in the direction of the wind with chords parallel to each other, on the core body. Thus each cascade is an array of blades stacked along the axial direction, with a gap of half chord length between two consecutive blades. There are two cascades at diametrically opposed points on the circumference of the core body. One can visualize a plane perpendicular to the axis of the core body cutting the core body into two equal halves. The turbine (consisting of the core body and the blades) is symmetrical about this plane. Before taking up the operation of the turbine it is necessary to discuss the behaviour of a single airfoil blade placed in a stream of air. The angle between the direction of the wind and the airfoil chord is known as the angle of incidence and will be denoted here by the symbol " components, one in the direction of the wind called "Drag" and the other perpendicular to the direction of the wind called "Lift". If The turbine described above is a prime mover and drives an AC generator and is required to rotate at a constant rpm. The wind relative to a rotating blade is the important parameter. This relative wind has two components, one is the wind along the axis of the duct (ie, axial wind or duct wind created by sea waves) and the other is the tangential velocity of the rotating blade itself. When the duct wind is extremely low (in the wave energy power plant very low speed wind is produced by extremely weak sea waves) the angel of incidence is extremely low (below 2 degree) and the blade produces no power. Such conditions are of no consequence and will be ignored hereafter. When the duct wind has a moderately low speed, cascade effect is dominant and enables the turbine to produce power from the high speed winds created by the large sea waves. According to a preferred embodiment of the invention there are six baldes in each cascade. Alternative embodiments with five or seven blades per cascade are also possible. According to another preferred embodiment of the invention, there are two cascades, equal angles apart on the circumference of the core body. In other words they are 180 degrees apart. An alternative embodiment with three cascades equal angles apart, that is, 120 degrees apart is also possible. According to another preferred embodiment, the gap or spacing between any two consecutive blades in a cascade is maintained constant and is equal to half the chord length of the airfoil. Alternative embodiment with lesser or greater constant gap are also possible. For instance, in a particular design the constant gap may be 1.8 times the chord length of the airfoil. Another alternative embodiment in which the six blades of the cascade are varyingly spaced is also possible. For instance, the gap or spacing between the 1st and 2nd blade may differ from that between 2nd and 3rd blade. According to another preferred embodiment, the airfoil cores-section chosen is NACA 0012. An alternative embodiment with another standard symmetrical airfoil is also possible. The invention described hereinabove is in relation to a non—limiting embodiment and as defined by the accompanying claims. WE CLAIM; 1. A device for extracting power from to and fro wind comprising: i) a rotatable care body (8) of circular cylinder with two identical hemi-spheres (9) fitted at two ends; ii) said core (8) having plurality of blades in two airfoil cascades (10) mounted at diametrically opposed position; iii) said blades of the airfoil cascades (10) mounted in a manner such that the axis or span of a blade is always along the radial direction of said core body (8); iv) the chord of the blade being perpendicular to both the radial and axial directions of the core body (8); and v) and each blade in the said airfoil cascade (10) is of symmetrical section and its chord perpendicular to the duct wind (12) and the said duct wind (12) being alligned with the direction of rotation (11). 2. A device as claimed in claim 1, wherein the turbine is placed concentrically in a duct with a radial clearance. 3. A device as claimed in claim 1, wherein the spacing between two consecutive blades is half the chord length and the space-chord ratio is 0.5. 4. A device for extracting power from to-and-fro wind as herein described with the help of accompanying drawings.

Documents:

2673-del-1996-abstract.pdf

2673-del-1996-claims.pdf

2673-del-1996-correspondence-others.pdf

2673-del-1996-correspondence-po.pdf

2673-del-1996-description (complete).pdf

2673-del-1996-drawings.pdf

2673-del-1996-form-1.pdf

2673-del-1996-form-19.pdf

2673-del-1996-form-2.pdf

2673-del-1996-form-3.pdf

2673-del-1996-form-6.pdf

2673-del-1996-form-7.pdf

2673-del-1996-gpa.pdf