WO2014162312A1

WO2014162312A1 - Controlling vertical axis rotor-type wind turbine

Info

Publication number: WO2014162312A1
Application number: PCT/IL2014/050324
Authority: WO
Inventors: Toms KOMASS; Alexander ZHIVETS
Original assignee: Windfire B.V.; Reinhold Cohn And Partners
Priority date: 2013-04-04
Filing date: 2014-04-02
Publication date: 2014-10-09
Also published as: IL225590A0

Abstract

A vertical axis wind turbine system is presented, the turbine system comprising a rotor connected to a load through a power transmitting shaft and a control unit configured to determined load level to thereby maintain a tip-speed-ratio. The rotor comprises one or more blade block, each comprising one or more blades, and is configured to rotate about a predetermined axis in response to wind. The control unit comprises a rotation speed indicator, a torque selection module and a load controller. The rotation speed indicator is connectable to the shaft and configured to determine a rotation speed of the rotor. The torque selection module is configured to receive data indicative of the rotation speed from the rotation speed indicator and to determine a torque value. The load controller is configured to receive said desired torque value from the torque selection module and to determine a corresponding load level and adjust said load accordingly to thereby provide the wind turbine operation within predetermined limit.

Description

CONTROLLING VERTICAL AXIS ROTOR-TYPE WIND TURBINE

FIELD OF THE INVENTION

The present invention relates generally to systems and methods for controlling wind power turbines and more particularly, to system and method for controlling wind power turbines controlling the load of a vertical axis wind power turbine maintaining a preconfigured, optimal tip-speed-ratio (TSR).

BACKGROUND OF THE INVENTION

Wind turbines are widely used to convert kinetic energy from the wind into mechanical energy. The mechanical energy is typically converted into electricity. The mechanical energy may also be used to drive machinery, such as for grinding grain or pumping water, the device is called a windmill or wind pump.

Today's wind turbines are manufacture in a wide range of vertical and horizontal axis types. The smallest turbines are used for applications such as battery charging or auxiliary power on boats; while large grid-connected arrays of turbines are becoming an increasingly important source of wind power-produced commercial electricity.

vertical axis rotor-type wind turbines include arrays of cascaded blades, arranged so that the blades are displaced in the rotary direction on the left or on the right of a virtual radius lines of each array of cascaded blades, wherein each blade is shifted with respect to the radius line and to the adjacent blade by a half of a blade's chord.

To obtain an optimal energy throughput from a vertical axis rotor wind turbine, for a given wind velocity, the load level of, for example, the generator that produces the electric energy, needs to be controlled. In the current, conventional systems, the control of the turbine in done in dependence on the wind speed. Wind speed value was average in preconfigured time intervals, for example, time intervals of about 10 seconds. This principle of control is not giving the best performance especially in unstable and/or turbulence wind conditions. There is therefore a need, and it would be advantageous to prove a system and method that do not depend on win speed measurements and provide efficient control of a vertical axis rotor wind turbine, independent of the wind stability.

SUMMARY OF THE INVENTION

There is a need in the art for a technique allowing control of operation of a wind turbine, to thereby enable the turbine to provide stable energetic output under varying wind conditions. Such output stability is largely affected by a relation between a rotation speed of the rotor and the wind speed generating said rotation. This relation has an important effect on the efficiency of vertical axis turbines, as the tangential movement of the rotors varies with respect to the wind direction. The principal intentions of the present invention are directed at providing an efficient control of a vertical axis rotor wind turbine, independent of the wind stability. The present invention utilizes a control unit (generally a computerized sub-system) associated with the wind turbine and configured and operable for maintaining a preconfigured tip-speed- ratio (TSR, denoted by λ). The TSR is defined as the ratio between the tangential speed of the

V

furthest tip of the rotor VB and the speed of the wind Vw- Thus λ =—— , and the tip speed of the rotor as defined by V_B = 2 * * co_g * R . Where ω_& is the angular speed of the rotor (rad/s); and R is the rotor radius.

The present invention provides wind turbine comprising a rotor structure configured to rotate about a rotation axis and connectable to a load through a rotating shaft. The turbine is associated with a control unit, configured and operable to control operation of the turbine and to thereby maintain a desired TSR. To this end, the control unit is configured for continuously reading the torque generated by rotation of the rotor. Preferably, the control unit is configured to read data indicative of the torque level directly from the shaft of the rotor, and more preferably, to read the torque level without affecting power transmission through the shaft to the associates load. It should be noted that regarding of the rotation speed; the torque is directly affected by the wind speed as the torque corresponds to the moment forces acting on the rotor. The wind speed is reflected on the rotation speed of the tip of each blade (o_g. The TSR (λ), in this case, is determined based on the torque reading, which can be easily read from the shaft's rotational speed, and comparing a current torque read value with reference torque values varying in accordance with changes in the wind speed. An additional advantage of the torque measurements lies in the accuracy, which is generally significantly higher than measurement of the wind speed as torque measurement already takes into account the inertia forces of the turbine.

Generally, the control unit may comprise one or more processing units, storage utility and input and output ports. Additionally the control unit may comprise several modules, being computational and/or software modules and/or hardware sub-units, including a rotation speed indicator, a torque selection module and a load controller. The rotation speed indicator is configured to read from the shaft data indicative of rotational speed of the rotor. This is used by the torque selection module to determine a current torque level based on certain parameters which may be stored at the storage utility as the case may be. Generally various quantities associates with operation of the wind turbine are based on the following parameters:

1. P The theoretical maximum power what is designed to obtain from the generator, at the maximum wind speed (W);

2. V designates the designed maximum wind speed for the rotor;

3. p designates the air density - 1.225, kg m^" , on the average;

4. 5 designates the dimensional area of the combined wind turbine blades (m );

5. R designates the distance of a blade from the rotational axis (m). When referring to the turbine as a whole, R refers to the distance of the most distal blade from the rotational axis;

6. G designates the gear rational speed, associated with a gearbox when used to control power transmission through the shaft;

2P

7. C_p is the power coefficient defined by: C = ;

P *V_B ² * S

λο designates value of an optimal/desired tip speed ratio (TSR) of the chord of the blade for a given wind rotor;

Κχ defines a constant tip speed ratio line and determined by: n * p * R² * C_p

2 * A₀ ³ * G³

From the reading of the rotation speed the blade tip speed V_B may be calculated as:

(equation 1) using the blade rotational speed in round per minute, and where D defined the diameter of the rotor.

Generally the above parameters are known for a given vertical axis rotor wind turbine, or may be determined at a calibration stage, and may thus constitute data stored at a storage utility for use of the control unit according to the present invention.

It should be noted that in order to obtain the optimal power received from the generator, for an actual current wind speed, the torque of the generator ¾ may be used, thus eliminating the need for continuously measure the wind speed. The torque value may be calculated based on the rotation speed by the formula, as follows:

T _d = K_x * co_g ² (equation 2) where ω_& is derived from the rotational speed of the rotor.

Hence, the torque of the generator T_g is determined using the readings the tip speed of the blades and is used as reference torque value. Generally it should be noted that the torque value corresponds to the load applied on the generator.

Thus, according to one broad aspect of the present invention there is provided a vertical axis wind turbine system comprising a rotor connected to a load through a power transmitting shaft and a control unit configured to determined load level to thereby maintain a tip-speed-ratio; the rotor comprising one or more blade block, each comprising one or more blades, and is configured to rotate about a predetermined axis in response to wind; the control unit comprising a rotation speed indicator, a torque selection module and a load controller; the rotation speed indicator is connectable to the shaft and configured to determine a rotation speed of the rotor, the torque selection module is configured to receive data indicative of the rotation speed from the rotation speed indicator and to determine a torque value, and the load controller is configured to receive said desired torque value from the torque selection module and to determine a corresponding load level and adjust said load accordingly to thereby provide the wind turbine operation within predetermined limit. It should be noted that the load associated with the wind turbine may be an electrical generator. The electrical generator being selected from a group including: a synchronous generator, an asynchronous generator/motor, a permanent magnet synchronous generator/motor, a DC generator/motor. Alternatively the load may be a non-electrical generator or motor. According to some embodiments, the control unit may be configured and operable for periodically determine appropriate load level in accordance with the determined torque value, to thereby provide the vertical axis wind turbine operation at a desired tip-speed-ratio.

Additionally or alternatively the rotor of the wind turbine may comprise at least two blade blocks, each blade block comprises one or more blades configured to collect wind power and to convert said wind power to rotation of said rotor. The blades of each blade block may be arranged in a cascaded configuration.

According to one other broad aspect, the present invention provides a method for controlling a vertical axis wind turbine comprising a rotor connected to a load through a power transmitting shaft; the method comprising: determining a rotation speed of the rotor, determining a torque level provided by the transmission shaft in accordance with rotation speed of the rotor and adjusting a corresponding load level to thereby maintain a desired tip- speed-ratio being indicative of a ratio of tangential speed of a rotor tip and a wind speed.

According to some embodiments said maintaining a desired tip-speed-ratio may comprise maintaining rotation of the rotor on a predetermine tip speed ratio line Κχ, being determined in accordance with geometrical parameters of the wind turbine and a load connected thereto.

According to yet some embodiments said determining a rotation speed of the rotor may comprise detecting a rotational speed of the rotor utilizing a rotational speed detector and determining a tangential speed of a tip of the rotor in accordance with V_B=Blade-rotational- speed* *D/60, wherein the blade rotational speed is given in round per minutes and D being a diameter of the rotor.

According to some embodiments said determining a torque level provided by the transmission shaft may comprise determining a power coefficient C_p in accordance with

P *p *R² * C C = ; , determining a ratio line Κ_λ in accordance with K, =

" 0.5 * p *V_B ³ * S 2* ^³ * G³ where p being a measure of average air density, R being a radius of the rotor, λο being a desirable Tip-Speed-Ratio and G being a gear ratio associated with the load, and determining said torque level T_dg as T_dg=Kx*co_g ^z, where ca_g being a rotation angular speed of the rotor.

The method may further comprise an initial releasing a brake system to thereby allowing the rotor to rotate in response to the wind. Additionally or alternatively said determining a torque level provided by the transmission shaft may comprise allowing the rotor to rotate while being disengaged from said load to determine a corresponding power coefficient, utilizing said power coefficient to determine a desired tip speed ratio line and determining a reference torque level in accordance with said desired tip speed ratio line.

BRIEF DESCRIPTION OF THE DRAWING AND FIGURES

The present invention will become fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration and example only and thus not limitative of the present invention:

Fig. 1A is a schematic illustration of an array-type vertical axis wind turbine, according to embodiments of the present invention, showing, by way of example only and with no limitations, a rotating shaft transferring the rotor motion to an the electro-mechanical receiving sub-system;

Fig. IB is a schematic illustration of the control unit associated with the wind turbine according to the present invention;

Fig. 2 is a top perspective view of an array-type vertical axis wind turbine, according to embodiments of the present invention;

Fig. 3 is a schematic block diagram of the system representing the array-type vertical axis wind turbine;

Fig. 4 is a schematic block diagram of the calibration sub-system of the array-type vertical axis wind turbine;

Fig. 5 is a schematic flow diagram that outlines the operational steps of the array-type vertical axis wind turbine; and

Fig. 6 is a schematic flow diagram that outlines a cycle of the torque computations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided, so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

An embodiment is an example or implementation of the inventions. The various appearances of "one embodiment," "an embodiment" or "some embodiments" do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.

Reference in the specification to "one embodiment", "an embodiment", "some embodiments" or "other embodiments" means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiments, but not necessarily all embodiments, of the inventions. It is understood that the phraseology and terminology employed herein is not to be construed as limiting and are for descriptive purpose only.

Methods of the present invention may be implemented by performing or completing manually, automatically, or a combination thereof, selected steps or tasks. The term "method" refers to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the art to which the invention belongs. The descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only.

Meanings of technical and scientific terms used herein are to be commonly understood as to which the invention belongs, unless otherwise defined. The present invention can be implemented in the testing or practice with methods and materials equivalent or similar to those described herein.

Reference now made to the drawings. Figs. 1A and IB schematically illustrate an array-type vertical axis wind turbine (VAWT) 100, according to some embodiments of the present invention. Fig. 1A illustrates the wind turbine 100 and Fig. IB shows schematically the configuration of the turbine 100 in connection with a control unit 120 according to the present invention. It should be noted that although Fig. 1A and the following figured illustrate an array-type wind turbine, the wind turbine should be interpreted broadly and may also utilize rotor with single blade on each arm. The array-type VAWT 100 shown in Fig. 1A is illustrated by way of example only and with no limitations. As shown, the array-type VAWT includes an elongated rotating shaft that transfers the rotor motion to operate certain appropriate load, for example an electro-mechanical receiving sub-system (e.g. generator), which may be located at the ground level. Reference is also made to Fig. 2 showing a top perspective view of the upper section of array-type VAWT 100, according to some embodiments of the present invention. Fig. 3 also illustrates a schematic block diagram of wind turbine system 100 as shown in Figs. 1A and 2.

Generally, the array-type VAWT 100 according to the present invention includes a rotor 110, which may be configured of two or more arrays 112 of blades 114, such that each array includes one or more blades 114. The arrays 112 are operatively connected to an assembly of arms 140, which is rotatably connected to a vertical shaft 150 configured for transmission of the mechanical rotation. Array-type VAWT 100 further includes a control unit 120, and may generally connected, or be connectable, to an appropriate load 160. As generally such wind turbines may be used for production of electricity, the load may be an electro mechanical subsystem 160 that include, with no limitations, an electrical generator 164, a transmission unit 162 and a breaking sub-system (not shown). The breaking subsystem may be placed at the top of the tower.

Additionally, as shown in Fig. IB, the control unit 120 may include at least a speed indicator unit 122, a torque selection module 124 and a load controller 126, and is configured to control operation of the wind turbine to thereby provide a selected, preferably optimal, tip- speed-ratio (TSR). Generally the control unit may be configured as a computerized control unit including a processing unit 1220 such that at least part of the speed indicator unit 122, a torque selection module 124 and a load controller 126 may be software modules; or it may be configured as an electronic control unit utilizing predetermined electronic circuits configured for performing appropriate calculation as will be describe below. Additionally, the control unit may include a storage utility 1210 configured for storing data about operation of the wind turbine and/or physical parameters which may be utilized for appropriate calculations.

When operated, the wind may generally cause rotation of the rotor 110 and the shaft 150, which is connected to the rotor, transmits the power for operation of the associated load 160, e.g. for operation of the electrical generator 164. Generally, the load operation is regulated by a transmission unit 162 which may regulate the power transmission and accordingly the applied load level. According to some embodiments of the present invention, the control unit 120 may be connectable to the transmission unit 162 via the load controlling module 126 for allowing efficient operation of the wind turbine 100.

According to the technique of the present invention, to achieve the best performance of the wind turbine, an appropriate control on the operation of the wind turbine is important. To this end, the wind turbine of the present invention utilizes a control unit configured to control the load loaded on the turbine in accordance with torque level provided by rotation of the rotor 110. Generally, the speed indicator module 122 is configured for continuously reading rotation speed, either directly from the transmission shaft 150 or by connection to the load 160. It should be noted that the torque provided by the rotor is directly affect by the wind rotating the blades. The wind speed can be derived from the torque level in accordance with the rotation speed c¾ of the rotor. This allows efficient controlling of the tip-speed-ratio (TSR) λ of the rotor, defined by a ratio between the tangential speed of the tip of the rotor and the speed of the wind. According to the present invention, the TSR can be determined from the shaft's rotational speed while eliminating the need for measuring the speed of the wind. It should be noted that direct wind measurement are generally subject to statistical variations caused from unstable behavior of the wind in proximity with the blades of the rotor. Additionally, even if a wind-speed measuring device is located at a distance from the rotor it may be subjected to variations due to turbulence and other wind conditions. This is while speed measurements provided from the rotating shaft actually undergo filtering of high frequency variations and provide stable measurements.

The present invention provides a control sub-system to control the VAWT 100, by continuously, or at least periodically, reading the torque on the shaft 150 transmitting power from the rotor 110 to the load 160. As indicated above, the torque is directly affected by the wind speed and thus can be used for determining the average wind speed rotating the rotor. The TSR (X) can be derived from the torque value calculation, which, as described above can be easily determined from the shaft's rotational speed. To this end the rotation speed indicator/measuring device 122 may utilize for example an incremental encoder TTL, an absolute encoder, a resolver, an optical sensor, an inductive sensor, capacitive sensor or any other rotational speed measuring device known in the art.

The rotation speed indicator 122 transmits data about the shaft's 150 rotation speed ω_& to the torque selection module 124, which is configured to determine the torque level T_g provided by the rotor 110. The torque computed can generally be determined as that of the generator while taking into account losses of the mechanical system, according to the formula:

T _d = K_x * {equation 2)

Hence, control unit 120, via its rotation speed indicator 122 is adapted to compute co_g from the rotational speed of the rotor and determine co_g , e.g. by calculation module (note specifically shown). Control unit 120 is further adapted to compute T_g by the torque selection module utilizing a predetermined constant tip speed ratio line Κχ, which is determined by various parameters of the wind turbine, including value of the optimal TSR /lo as follows:

J _ ¹ P_

¹ 2 * * G³

^¾ (equation 3) The determined torque level is transmitted to the Load controlling module 128, which is configured to determine the required change in load for causing the rotor to maintain optimal TSR. The load is determined in accordance with the computed torque T_g . According to some embodiments, the load controlling module 128 is configured to compare currently determined torque with previous torque values stored at the storage utility 1210. This allows for determining required changes in the load based on known efficient operation of the turbine 100. It should be noted that the level of the load directly determines the output power generated by electrical generator 164 when used, and generally determined the power provided by the turbine and converted to operate the load. It should also be noted that the load controlling module may generally control the transmission unit 162 for varying the load level.

For example, at initial operation of the wind turbine 100, before the VAWT 100 is started-up, the breaking sub-system should be neutralized. Additionally the load is to be detached from rotor 110 to allow easy rotation with negligible friction. This allows the rotor to rotate freely at a rotational speed resulted from the current speed of the wind V_W- When, the rotational speed of the blades VB is stabilized, the control unit 120 may set an initial reference torque value for the system, based on the rotational speed of rotor 110 as measured by a rotational speed measuring device 122. This may be provided by operatively engaging the load at a predetermined load level in accordance with the determined rest rotation speed, e.g. by selectively engaging electrical generator 164 to be active.

It should be noted that rotational speed measuring device 122 may typically, with no limitations, be positioned after generator 164. However, the rotational speed measuring device 122 may be positioned directly onto the shaft 150 between the rotor 110 and the load, or together with the load 160, after transmission unit 162.

It should be further noted that the load 160 may include, as indicated above, a generator 164, which may be a synchronous generator, an asynchronous generator/motor, a permanent magnet synchronous generator/motor a DC generator/motor or any other type generator.

It should be further noted that the electrical electricity produced by the generator may be used to upload the general grid or to charge a battery or for any other usage.

It should be further noted that the generator may be operatively coupled with a Regenerative frequency converter (back-to-back drive), a frequency converter, a DC motor, a PLC or any other load control unit.

Reference is made to Fig. 4 schematically illustrating in a way of a block diagram, the operation of the control unit for pre-calibration 102 of the wind turbine as described above. It should be noted that the pre-calibration may be provided by the control unit 120 and/or by a calibration sub-system associated therewith (which is note specifically shown). As shown, in response to wind flow, the rotor 110 starts rotating and speed of rotation is measured by the speed indicator 122. The torque selection module 126 determines the initial torque value by selectively engaging the load 160 (e.g. generator) as described above.

Reference is now made to Fig. 5, illustrating a schematic flow diagram 200 that outlines the operation of an array-type vertical axis wind turbine 100 according to embodiments of the present invention as may be provided by the control unit 120. Generally, method 200 includes two main segments in the operation of array-type vertical axis wind turbine 100, a calibration segment 202 and an operation segment 204. Calibration segment 202 is performed when starting-up array-type vertical axis wind turbine 100. After calibration segment 202 is concluded, method 200 proceeds with cyclic operation segment 204.

Operation of the wind turbine 100 begins by releasing the brakes (step 210); generally a short time is provided for the rotor to reach a stable rotation speed 220. After the rotor reached a stable rotation, an initial torque value is determined (step 230). Generally, the Control unit 120 delays action to allow for the breaks to release rotor 110 and to further allow the rotor to pick up speed. The control unit determined the initial torque value (Step 300) in accordance with the rotation speed as described above. Additionally, the control unit may be configured for manual setting of the torque level (step 230), if so, an operator may set the torque level manually or in accordance with factory pre-setting. If the torque level is not set manually, the control unit operates to determine an initial torque value (step 300) and may be configured to verify if the torque level is stable (step 240) by performing and additional measurement after a short (generally a few seconds) delay. If the rotational speed of rotor 110 has not yet been stabilized, the rotor may be allowed additional time until it reached stable rotation. If the torque level is found to be stable, the determined torque level is considered as reference torque and may be stored in the storage utility 1210 (step 250). At this stage the calibration process ends and the wind turbine is ready for operation.

Operation of the wind turbine, after pre-calibration, is based on application of load to the shaft (Step 260). Generally, the Control unit 120 may operatively engage the load, e.g. electrical generator 164, with a load level which may be derived from the initial torque reference, or in accordance with a data table for appropriate load levels. While operating the control unit is configured to periodically or continuously determine torque values as defined above, based on rotation speed of the shaft (step 300) and to update the stored torque values if needed (step 270).

At any operation stage, an operator may issue a stop command in order to end operation of the turbine, e.g. for maintenance etc. If a stop command has been issued (Step 280) the control unit may activate the brake system to thereby stop rotation of the rotor and end operation of the turbine (step 290).

As indicated above, Fig. 4 schematically illustrates operation of the control unit to determine a torque level. Fig. 6 specifies certain calculations which may be used for determining the torque level according to some embodiments of the present invention. As shown in the figured, the torque level may be determined by determining a power coefficient C_p of the system (step 310) in accordance with:

P

C = ; (equation 4) p 0.5 * p *V_B ³ * S where P, p, V_B, and 5 are as defined above.

Accordingly, the ratio line Κχ is determined, based on a predetermined desired optimal tip speed ratio (step 320), as indicated in equation 3. It should be noted that all the parameters are generally known for a given vertical axis rotor wind turbine in accordance with it design. It should also be noted that Κχ needs to be calculated computed only when the optimal TSR λο changes. This is while the other parameters are constants, for a given Array-type VAWT 100. These determined parameters may be stored in the storage utility for use during operation of the turbine.

During normal operation, the control unit reads the rotation speed of the rotor by the rotation speed indicator 120 (Step 330). The rotation speed may generally be read directly from the shaft, upstream or downstream of the connection of the load, with respect to power transmission direction. The torque selection module 126 may than determine the torque level (step 340) as follows:

T _d = k_x * co_g ² (equation 2)

It should be noted that the determined torque level preferably corresponds to the generator torque T_g and is thus marked accordingly taking into account losses of the mechanical system. It should also be noted that the torque level may correspond to the torque transmitted by the shaft.

Hence, control unit 120 is adapted to determine a torque level provided by the rotor to the load, in accordance with rotation speed of the rotor. Generally, the control unit may perform certain calculations such as: compute co_g , derived from the rotational speed of the rotor, by a co_g ^z calculation module 124. Control unit 120 is further adapted to compute T_gd, after computing Κχ as a function of the optimal λ by a torque calculation module 126. Load controlling module 128 determines the required change in the load according to the newly computed torque T_gd. Optionally, load controlling module 128 compares the computed torque with the previously computed torque, to determine the required change in the load. The level of the load directly determines the output power generated by electrical generator 164.

The invention being thus described in terms of embodiments and examples, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A vertical axis wind turbine system comprising a rotor connected to a load through a power transmitting shaft and a control unit configured to determined load level to thereby maintain a tip-speed-ratio;

the rotor comprising one or more blade block, each comprising one or more blades, and is configured to rotate about a predetermined axis in response to wind;

the control unit comprising a rotation speed indicator, a torque selection module and a load controller; the rotation speed indicator is connectable to the shaft and configured to determine a rotation speed of the rotor, the torque selection module is configured to receive data indicative of the rotation speed from the rotation speed indicator and to determine a torque value, and the load controller is configured to receive said desired torque value from the torque selection module and to determine a corresponding load level and adjust said load accordingly to thereby provide the wind turbine operation within predetermined limit.

2. The system of Claim 1, wherein the control unit is configured and operable for periodically determine appropriate load level in accordance with the determined torque value, to thereby provide the vertical axis wind turbine operation at a desired tip-speed-ratio.

3. The system of Claim 1 or 2, wherein the rotor of the wind turbine comprises at least two blade blocks, each blade block comprises one or more blades configured to collect wind power and to convert said wind power to rotation of said rotor.

4. The system of any one of Claims 1 to 3, wherein the blades of each blade block being arranged in a cascaded configuration.

5. The system of any one of Claims 1 to 4, wherein wind turbine is associated with a load being an electrical generator.

6. The system of Claim 5, wherein said electrical generator being selected from a group including: a synchronous generator, an asynchronous generator/motor, a permanent magnet synchronous generator/motor, a DC generator/motor.

7. The system of any one of Claims 1 to 4, wherein wind turbine is associated with a load being a non-electrical generator.

8. A method for controlling a vertical axis wind turbine comprising a rotor connected to a load through a power transmitting shaft; the method comprising: determining a rotation speed of the rotor, determining a torque level provided by the transmission shaft in accordance with rotation speed of the rotor and adjusting a corresponding load level to thereby maintain a desired tip-speed-ratio being indicative of a ratio of tangential speed of a rotor tip and a wind speed.

9. The method of claim 8, wherein said maintaining a desired tip-speed-ratio comprising maintaining rotation of the rotor on a predetermine tip speed ratio line Κ_λ, being determined in accordance with geometrical parameters of the wind turbine and a load connected thereto.

10. The method of Claim 8 or 9, wherein said determining a rotation speed of the rotor comprising detecting a rotational speed of the rotor utilizing a rotational speed detector and determining a tangential speed of a tip of the rotor in accordance with V_B=Blade-rotational- speed* *D/60, wherein the blade rotational speed is given in round per minutes and D being a diameter of the rotor.

11. The method of any one of Claims 8 to 10, wherein said determining a torque level provided by the transmission shaft comprising determining a power coefficient C_p in

p

accordance with C = ; , determining a ratio line K> in accordance with

p 0.5 * p *V_B ³ * S

* p * R² * C

K_x =— 2 * _y^₃ * ' ^wnere P b^em§ ^a measure of average air density, R being a radius of the rotor, λο being a desirable Tip-Speed-Ratio and G being a gear ratio associated with the load, and determining said torque level ¾ as T_dg=Ki*(o_g ² , where co_g being a rotation angular speed of the rotor.

12. The method of any one of Claims 8 to 11 , further comprising an initial releasing a brake system to thereby allowing the rotor to rotate in response to the wind.

13. The method of any one of Claims 8 to 12, wherein said determining a torque level provided by the transmission shaft comprising allowing the rotor to rotate while being disengaged from said load to determine a corresponding power coefficient, utilizing said power coefficient to determine a desired tip speed ratio line and determining a reference torque level in accordance with said desired tip speed ratio line.