WO2011039749A1

WO2011039749A1 - Bi-rotor generator for efficient production of ac electricity

Info

Publication number: WO2011039749A1
Application number: PCT/IL2010/000784
Authority: WO
Inventors: Carmi Raz; Aharon Eyal
Original assignee: Re-10 Ltd
Priority date: 2009-09-29
Filing date: 2010-09-21
Publication date: 2011-04-07
Also published as: IL201221A0

Abstract

An electrical generator (100) and method of use for producing alternating electrical current at a predetermined frequency, wherein a first rotational element (110) is driven by a force in a first direction at rotational speed determined by the magnitude of the force and a second rotational element (120) is rotated in the same or an opposite direction relative to the first rotational element and is maintained at a predetermined net differential speed relative to the first rotational element. A control unit (130) governs at least one of the direction and the rotational speed of the second rotational element to maintain it at a predetermined net differential speed relative to the first rotational element.

Description

BI-ROTOR GENERATOR FOR EFFICIENT

PRODUCTION OF AC ELECTRICITY

FIELD OF THE INVENTION

The present invention, in some embodiments thereof, relates to an electrical generator for producing alternating electrical current at a predetermined frequency.

BACKGROUND OF THE INVENTION

Wind power represents the promising form of renewable energy. Wind turbines create electricity from flowing air and are environmentally clean. Wind farms are often located in empty plains or off a coast in high-wind regions. Wind power represents the most advanced form of non-fossil fuel electrical energy production. Gigawatt projects are on the drawing board or in the early stages of construction.

For wind power to be economically competitive, it must reach efficiencies in every aspect of its performance. Turbines must effectively respond to wind flow, the energy reaching the turbine must be efficiently converted to electricity production, and the electricity must be delivered to a grid for inclusion in the local/national electrical pattern. To date, wind power has become more closely priced to coal-based electricity, but additional efficiencies will be required to allow for truly competitive pricing patterns. Unlike coal, wind farms cannot produce electricity around the clock but rather during times of wind, and in many scenarios, only if the wind is at suitable conditions.

Attempts have been made to increase the overall efficiency of wind and water turbines by having propellers or other elements rotate in opposite directions. The goal has been to take existing conditions and extract a higher percentage of linear energy for electricity. U.S. Patent Number 4,039,848 to Winderl describes a wind operated generator employing counterrotating propellers with an alternating current generator positioned between the blades of the propellers and spacing the same. The propellers and generator parts are mounted on concentric shafts supported in a positive drive structure in which the shafts are tied together through gearing to insure positive starting and counter-rotation of the propellers in a positive equal drive system. A governor associated with one of the shafts governs the operational speed of rotation of both propellers to protect the propellers and generator.

U.S. Patent Number 6,504,260 to Debleser teaches a first capture unit including a first turbine rotor having a first hub. A second capture unit includes a second turbine rotor having a second hub. The rotors counter-rotate independently. A first electric generator includes a first rotor fixed to the first turbine rotor and a first stator fixed so as to face the first rotor. A second electric generator includes a second rotor having a rotor fixed to the second turbine rotor and a second stator fixed so as to face the second rotor. Power electronic means control the electric currents produced by the first and the second stators of the first and the second electric generators independently of each other thus regulating the rotational speed of the first and second turbine rotors.

U.S. Patent Number 2,153,523 to Roberts, et al. describes an improved construction of an electric generator in which the armature is rotated in one direction and the field of the generator is rotated in the opposite direction.

WO 2009/011637 to Israelsson describes a multi-rotor generator based on dividing the absorption of the wind energy on a number of counter rotating turbine rotors and to construct the rotors with many blades, made narrow and thin, which to a great extent are winning factors esthetically and acting on its surroundings. An electrical generator with counter rotating magnet- and inductor rotors and double sided air gaps, is integrated with the turbine rotors. Repelling magnetic fields are arranged between the turbine rotors and also by the radially outwards alongside the wings placed magnetic- and inductor rotors, which therefore can be arranged quite close to each other. By placing the wind turbine between two streamlined towers, interconnected by bearings for the turbine rotors and rotatable on a firm bedding or sub tower, the plant will resist very high bending moments. Together these contrivances mean that the plant can withstand very high wind-forces. U.S. Patent Number 3,974,396 to Schonball teaches an electric generator arrangement having at least two wind or water wheels rotatably mounted on a common stationary shaft and coupled such that they rotate in opposite directions with a given ratio or a progressively variable ratio. The wheels carry the two cooperating parts of a first generator and may also each carry the rotor of a second or third generator cooperating with a stator on the shaft, the field excitation or current loading of the second and/or third generator being used to control the speed of the wheels.

Thomas in a baccalaureate dissertation at MIT ("Structural and Thermal Design of a Dual Rotor, Constant Frequency, Variable Speed Generator", 2003) describes a generator for more efficiently converting wind energy into AC electricity. Specifically, he describes a generator having a pair of rotors and a stator. One of the rotors is attached to a driveshaft and wound with excitation coils while the other, lying between the power coil and the stator, is free-spinning and supports a number of magnets. The generator employs axial flux in converting rotation of the rotor in response to linear wind flow.

US 20090164170 to Brondum, et al. describes an electric power generator system with improved power efficiency due to a reduced sensitivity to errors in the sensing of angular rotor position. The system includes a power generator with a rotor, and a position encoder connected to sense angular position of the rotor and to generate a position signal accordingly. A processor receives the position signal, calculates an angular position in response, calculates an estimated angular position based on earlier received position signals, and finally generates a processed angular position based on the calculated angular position and the estimated angular position. This processed angular position is a more reliable measure of the rotor position, reducing the influence of short-term errors in the position signal, allowing normal wind turbine operation during temporary position encoder failure, and allowing an orderly shutdown during complete position encoder failure.

SUMMARY OF THE INVENTION

It is an object of the present invention to describe methods and devices for improved efficiency in converting energy to electricity. Specifically, the invention includes structures and methods that allow for a more efficient generation of AC electricity, one in which the rotation of one or more rotational elements is controlled to provide an electricity at a constant, predetermined frequency.

According to one aspect of the invention there is provided an electrical generator for producing alternating electrical current at a predetermined frequency, the electrical generator comprising:

a first rotational element capable of being driven by a force in a first direction at rotational speed determined by the magnitude of said force;

a second rotational element capable of rotating in the same or in an opposite direction relative to said first rotational element and capable of being kept at a predetermined net differential speed relative to said first rotational element; and

a control unit capable of governing at least one of the direction and the rotational speed of said second rotational element to keep it at a predetermined net differential speed relative to said first rotational element.

In some embodiments, a field part is coupled to the first rotational element and an armature part is coupled to the second rotational element.

In some embodiments, an armature part is coupled to the first rotational element and a field part is coupled to the second rotational element.

In some embodiments, some components of an armature part and some components of a field part are coupled to first rotational element and some components of an armature part and some components of a field part are coupled to second rotational element.

In some embodiments, the first rotational element is driven by a first force, and second rotational element is driven by second force, which in some embodiments may be equal.

In some embodiments, the first rotational element, second rotational element or both are driven by a force selected from a group consisting of: wind, water, steam, gas, fuel and combination thereof.

In some embodiments, at least one of direction and rotational speed of the second rotational element is governed by at least one of a mechanical device and electric device.

In some embodiment, the control unit is realized as at least one of a mechanical unit and an electrical unit. In some embodiments, the control unit further senses at least one of the rotation speed and direction of at least one of first rotational element and second rotational element.

In some embodiments, the control unit further senses the magnitude of forces impinging on rotational elements and sets the one experiencing the greater force as first rotational element.

In some embodiments, the control unit further senses an output electrical frequency produced by the generator.

In some embodiments, the control unit further compares output electrical frequency to a constant frequency model.

In some embodiments, the control unit further compares output electrical frequency to an electric grid frequency.

In accordance with another aspect of the invention, there is provided a method for generating alternating electrical current at a predetermined frequency, comprising: providing a first rotational element capable of being driven by a force in a first direction;

providing a second rotational element capable of rotating in the same or in an opposite direction relative to said first rotational element;

exposing said first rotational element to a first force, wherein said first force determines the rotating speed magnitude of said first rotational element;

exposing said second rotational element to a second force, wherein said second force causes said second rotational element to rotate, whereby the first and second elements operate as an electrical generator; and

employing a control unit to govern at least one of rotational direction and the rotational speed of said second rotational element, so that the net differential speed between said first rotational element and said second rotational element is kept at a predetermined value.

In such a method, the first force may be the same as the second force. BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. In the various embodiments disclosed herein, like elements have like reference numerals differing by multiples of 100.

In the drawings:

FIG. 1 is a schematic view of an electrical generator as per an embodiment of the present invention;

FIG. 2 is a schematic view of an embodiment of the present invention, in which a second rotational element moves in a direction opposite that of a first rotational element;

FIG. 3 is a schematic view of an embodiment of the present invention, in which a second rotational element moves in a direction identical to that of a first rotational element;

FIG. 4 is a schematic view of an embodiment of the present invention, in which a second rotational element does not move, while a first rotational element rotates in response to a linear energy source;

FIG. 5 is a schematic view of an embodiment of the present invention in which portions of the electrical generator machinery is associated with a first rotational element;

FIG. 6 is a schematic view of an embodiment of the present invention in which portions of the electrical generator machinery is associated with a second rotational element;

FIG. 7 is a schematic view of an embodiment of the present invention in which portions of the electrical generator machinery is associated with a first rotational element and a second rotational element;

FIG. 8 is a schematic view of an embodiment of the present invention in which the first and second rotational elements are experiencing different external forces; FIG. 9 is a flowchart of one of the suggested controlling unit of the present invention; and

FIG. 10 is a flowchart of a method associated with the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits and control logic have not been shown in detail in order not to unnecessarily obscure the present invention.

The present invention, in some embodiments thereof, relates to a bi-rotor electrical generation system and, more particularly, but not exclusively, to an AC electrical generator system that keeps two or more rotors at a net differential speed so as to guarantee output electrical frequency at a predetermined value.

Certain terms are now defined in order to facilitate better understanding of the present invention.

"Turbine", "stator", "rotor", "generator", "wind", "frequency", "AC", "alternating electrical", "armature part" and "field part" may have their generally used meaning as understood in the fields of electrical generation and motors.

"First rotational element" and "second rotational element" are terms used in the description and refer to components capable of rotation in response to wind, water, or the like. For ease of discussion, "first rotational element" generally refers to a concentric rotating element having a longer diameter than the "second rotational element", though either element can play the role of first or second rotational element. In the invention, a first rotational element rotates in a first direction in response to a force from wind, water, or the like. The second rotational element can rotate in the same direction or in an opposite direction or could even remain in the absence of any motion. A first rotational element and a second rotational element can rotate in either a clockwise or counterclockwise motion. When a first rotational element rotates in response to an external force such as wind or water, its rotation direction and rotation speed is generally determined by the flow of the wind or flowing water and the shape of its turbine. The first rotational element will rotate in the direction and in rotation speed of flow of the force that is acting upon it. A second rotational element can rotate in the same or in an opposite direction or it can remain motionless.

A "control unit" in the present invention controls the speed and direction of rotation of a second rotational element and in some embodiment selection of the first rotational element depends on the magnitude of forces. As a primary goal of the instant invention is to efficiently use prevalent flowing energy (wind, water, etc.) to drive an electrical generator, the control unit may use mechanical, electrical, or other means to define the direction and speed of the second rotational element.

Term "speed" as used herein means the magnitude of the speed.

The "net differential speed" refers to the speed (including direction) between the first and second rotational elements. For example, if both rotational elements are moving in the same direction, with the first rotational element at 90 cycles per minute and the second rotational element at 30 cycles per minute, then the net differential speed is 60 cycles per minute. If the first rotational element is rotating at 90 cycles per minute in one direction and the second rotational element is rotating at 30 cycles per minute in the other direction, then the net differential speed is 120 cycles per minute. If the first rotational element is rotating at 90 cycles per minute and the second rotational element is not moving, then the net differential speed is 90 cycles per minute. A control unit, or a plurality of control units, controls the speed and direction of the second rotational element so as to fix the net differential speed, in order to produce AC electricity at 50 Hz, 60 Hz or at some other predetermined frequency.

"Bi-Rotor" generator refers to an electrical generator in which a plurality of generating elements can move in a rotary manner. For example, if a rotor and "stator" both rotate, one around the other, then they are considered a "bi-rotor" for the present invention.

Term "component" as used herein means a part of the whole.

The term "same" as used herein with reference to forces, mean forces that have a common "natural source" but can be identified as coming from different orientations or different speeds.

"Linear flow" or "linear energy" refers to wind or water or other flow movement that can be harnessed to produce rotation by means of converting such energy sources to rotation, and are not limited to straight flowing systems. The terms "comprises", "comprising", "includes", "including", "having" and their conjugates mean "including but not limited to".

• The term "consisting of means "including and limited to".

As used herein, the singular form "a", "an" and "the" include plural references unless the context clearly dictates otherwise.

For purposes of better understanding some embodiments of the present invention, as illustrated in FIGS. 1-10 of the drawings, reference is first made to the construction and operation of a bi-rotor electrical generator as illustrated in FIG. 1. Without being bound by any particular theory, the following discussion is offered to facilitate understanding of the invention.

It is understood that the rotational elements described herewith may be attached to propellers or the like to harness air, wind, or other forms of linear energy. Propellers and other water/wind responsive elements are not shown in the figures so as not to obstruct the unique features of the instant invention.

First Embodiment

Referring now to the drawings, FIG. 1 illustrates an electrical generator 100 according to the present invention. The generator 100 minimally includes a first rotational element 110 and a second rotational element 120 and a control unit 130. The control unit 130 may be a part of the second rotational element 120 or it may be attached through means 140 including but not limited to electrical wires or the like.

FIGS. 2-4 schematically demonstrate one of the important features of the present invention. Currently, wind power is limited by the inability to control the speed of impending wind: sometimes the wind is too weak and other times its gusts are too strong. In the former case, a wind farm will not be able to generate electricity due to the weak winds, while in the latter case, much energy is wasted as the turbine propellers spin too quickly for the required output electrical frequency. In the following figures, the second rotational element 120 can be rotated in direction and speed so as to optimize utilization of impending external force, whether it be wind, water or other energy source.

Reference is now made to FIG. 2 which shows an electrical generator 200 in which a first rotational element 210 and a second rotational element 220 rotate in opposite directions as suggested by arrows 215 and 225. A situation as shown in FIG. 2 would arise in low-wind areas or conditions. As the rotation of first rotational element 210 is not enough to generate AC electricity at 50 Hz or 60 Hz, a control unit 230 directs the second rotational element 220 to rotate in an opposite direction 225 and at a speed that will yield a net differential speed that will allow for production of AC electricity at 50 Hz or 60 Hz (or some other predetermined value).

Attention is now drawn to FIG. 3 which shows an electrical generator 300 in accordance with an embodiment of the present invention. This figure shows a case of very high wind, with turbine rotations at a speed higher than that needed for a desired output electrical frequency. In this arrangement, first rotational element 310 and the second rotational element 320 move in the same direction, as suggested by arrows 315 and 325. Control unit 330 causes second rotational element 320 to move in the direction of the first rotational element 310 and at a speed such that the differential speed between rotational elements is constant and consistent with production of AC electrical current at a predetermined, grid frequency.

FIG. 4 shows a third potential relationship between rotational elements. In this electrical generator 400, first rotational element 410 rotates in response to wind, water or the like. Control unit 430 measures the speed of the first rotational element and determines that for a desired output electrical frequency, second rotational element 420 should not move. Thus, only the first rotational element 410 rotates, while the second rotational element remains fixed— similar to a standard generator in which only one element moves with respect to the other.

Attention is now drawn to FIGS. 5 - 7. It is understood that in the present invention the key portions of an electrical generator, such as magnets and wires, may be associated with either rotational element or both. Thus, in FIG. 5, field part 550 is associated with first rotational element 510 of electrical generator 500 while armature part 555 is associated with second rotational element 520. In FIG. 6, field part 650 is associated with second rotational element 620 of electrical generator 600, while armature part 655 is associated with the first rotational element 610. In FIG. 7, field part 750 and armature part 755 are associated with first rotational element 710 and second rotational element 720, respectively, of electrical generator 700. Field parts and armature parts may include magnets, wire, controllers, and any other elements required to convert energy associated with the rotating rotational elements into electricity. Second Embodiment

Attention is now drawn to FIG. 8 which shows a schematic view of an embodiment according to the present invention. Electrical generator 800 is shown in a perspective view. First rotational element 810 is rotated by the force associated with wind, represented by arrow 860. First rotational element 810 rotates in a first direction. Second rotational element 820 is rotated through the force associated with a flowing body of water 870, with the flow signified by arrow 890. An extension element 825 transmits the force of the body of water 870 to the second rotational element 820. It may rotate in the same or opposite direction of first rotational element 810. Control unit 830 measures either electrically or mechanically the rotational speed and direction of first rotational element 810 and adjusts the speed and direction of rotation of second rotational element 820 so as to allow for electricity production at a constant, predetermined frequency.

Third Embodiment

Attention is now drawn to FIG. 9, which shows flowcharts of one of the suggested controlling unit of the present invention. Electric govern of the second element rotational speed and direction 900 is shown in a schema diagram. Electric control unit 960 receives three inputs; first element rotational speed 910, generator output electric frequency 920 and desired electric frequency 925. A "stage A" rotation speed module 915 produce a simulated rotation speed that represent the rotation speed in which the generator produce the desired frequency when no movement of the second element is needed. First element rotational speed 910 is compared to "stage A" rotation speed 915 and have three possible results; first element slower then "stage A" 916, first element equal to "stage A" 917 and first element faster then "stage A" 918.

The generator output electric frequency 920 is compared to desired electric frequency 925 and has three possible results: generator frequency is lower than desired 926, generator frequency is equal to desired 927 and generator frequency is higher than desired 928. According to the rotational speed and frequency comparison results, the control unit directs the second element power extractor on the rotation direction and amount of power needed. For example, if the first element's rotational speed is lower than "stage A" and the generator output frequency is lower than desired, then the control unit will govern the second element 950 by causing the second element to rotate in the opposite direction to the first element 936 thus gaining more power to the second element 946.

Fourth Embodiment

Attention is now drawn to FIG. 10, which shows a flowchart of a method of the present invention. One provides a first rotational element, wherein the first rotational element may rotate in a first direction in response to a force. One next provides a second rotational element, wherein the second rotational element may rotate in the same or in an opposite direction relative to the first rotational element. The two rotational elements are concentric, with the first rotational element generally being the ring having the greater diameter. Electrical generator equipment such as wires and magnets may be associated with one or both— or neither— of the rotational elements. The first rotational element is exposed to a force such as, but not limited to wind, flowing water, fuel, or the like. The first rotational element begins to rotate. The second rotational element is exposed to a force, either the same or different than the one acting on the first rotational element. A control unit determines the direction and speed of rotation of the second rotational element so that the net differential speed between the first and second rotational elements is constant and appropriate for a predetermined frequency of output AC electrical current. The resulting electricity may be delivered to a home, grid, machine, ship, car, or other element or device.

It is expected that during the life of a patent maturing from this application many relevant turbines will be developed and the scope of the term of the patent is intended to include all such new technologies a priori.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. Various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below find experimental support in the following examples. EXAMPLES

Reference is now made to the following examples, which together with the above descriptions illustrate some embodiments of the invention in a non limiting fashion.

Example 1

The present example is for a birotor generator attached to two elements that harness wind power. A wind turbine generator with two sets of propellers, wherein the first set of propellers has fixed blades that cannot change the blade angle and is attached to a first rotor of the generator. The second set of propellers has controlled angle blades. This control angle blade allow changing a blade angle so it can rotate in both directions in response to a specific wind flow and wherein the second propeller set is attached in a way that the flow coming out from the first propeller does not interfere with the second propeller set's air flow. The second propeller set is attached to a second rotor of the generator.

The first propeller harnesses power from wind flow and rotates to one direction according to the blade assembly. The rotation speed of the first propeller is governed by the magnitude of the wind flow.

When the magnitude of the wind flow is weak, the first propeller cannot reach the desired rotation speed that allows the generator to produce electricity at a desired frequency.

A control unit detects that the rotation speed of the first propeller is too weak and adjusts the blade angle of the second propeller to a blade angle that causes the second propeller to rotate in a direction opposite that of the first propeller, and at a selected speed. This arrangement allows for the two rotors of the generator to move at a predetermined net differential speed and thus generate electricity at a predetermined, desired electrical frequency.

When the wind power gets stronger, causing the first propeller to rotate faster (to the point that the first propeller alone could produce electricity at the desired frequency - "Stage A") the control unit causes the blades of the second propeller to come to position that provides negative force and the second rotating element will halt. In this arrangement, the generator will continue to produce alternate current electric at the desired frequency.

When the wind power becomes even stronger, causing the first propeller to rotate faster, the control unit moves the blade angle of the second propeller farther then the feather position so that the second propeller rotates in the same direction as the first propeller but at a slower speed. This arrangement keeps the rotors at a constant net differential speed, thus producing alternating electrical current at the grid frequency.

Example 2

This example includes a bi-rotor generator attached to two elements that harness water waves and ocean currents power. A bi-rotor generator assembled on a pole at the sea wherein the first rotor is attached to a turbine sunk in the water that can harness the sea currents, while the second rotor is attached to a turbine that is on the surface of the water and can harness the power from the sea waves. As described in Example 1, the currents turbine have a fix blade angle and will rotate responsive to current flow power, causing the first generator rotor to rotate at different speeds. The wave turbine has a changing blades capacity and can change the blades to cause the generator motors to move in predetermined net speed and produce alternate current electricity at a constant frequency.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Claims

CLAIMS:

1. An electrical generator (100) for producing alternating electrical current at a predetermined frequency, the electrical generator comprising:

a first rotational element (110) capable of being driven by a force in a first direction at rotational speed determined by the magnitude of said force;

a second rotational element (120) capable of rotating in the same or in an opposite direction relative to said first rotational element and capable of being kept at a predetermined net differential speed relative to said first rotational element; and a control unit (130) capable of governing at least one of the direction and the rotational speed of said second rotational element to keep it at a predetermined net differential speed relative to said first rotational element.

2. The electrical generator (500) according to claim 1, wherein a field part (550) is coupled to said first rotational element (510) and an armature part (555) is coupled to said second rotational element (520).

3. The electrical generator (600) according to claim 1, wherein an armature part (655) is coupled to said first rotational element (610) and a field part (650) is coupled to said second rotational element (620).

4. The electrical generator (700) according to claim 1, wherein some components of an armature part (755) and some components of a field part (750) are coupled to said first rotational element (710) and some components of the armature part and some components of the field part are coupled to said second rotational element (720).

5. The electrical generator according to claim 1, wherein said first rotational element is driven by a first force, and said second rotational element is driven by second force.

6. The electrical generator according to claim 5, wherein said second force is the same as said first force.

7. The electrical generator according to claim 1, wherein said first rotational element, said second rotational element or both are driven by a force selected from a group consisting of: wind, water, steam, gas, fuel and combination thereof.

8. The electrical generator according to claim 1, wherein at least one of said direction and said rotational speed of said second rotational element is governed by at least one of a mechanical device and an electric device.

9. The electrical generator according to claim 1, where said control unit is realized as at least one of a mechanical unit and an electrical unit.

10. The electrical generator according to claim 1, wherein said control unit further senses at least one of the rotation speed and direction of at least one of said first rotational element and said second rotational element.

11. The electrical generator according to claim 11, wherein said control unit further senses the magnitude of forces impinging on said rotational elements and sets the one experiencing the greater force as said first rotational element.

12. The electrical generator according to claim 1, wherein said control unit further senses an output electrical frequency produced by said generator.

13. The electrical generator according to claim 12, wherein said control unit further compares said output electrical frequency to a constant frequency.

14. The electrical generator according to claim 12, wherein said control unit further compares output electrical frequency to an electric grid frequency.

15. The electrical generator according to claim 12, wherein said control unit further sense the magnitude of forces impinging on said rotational elements and sets whichever one of the rotational elements that experiences the greater force as the first rotational element.

16. A method for generating alternating electrical current at a predetermined frequency, comprising:

providing a first rotational element (110) capable of being driven by a force in a first direction; providing a second rotational (120) element capable of rotating in the same or in an opposite direction relative to said first rotational element;

employing a control unit (130) to govern at least one of rotational direction and the rotational speed of said second rotational element, so that the net differential speed between said first rotational element and said second rotational element is kept at a predetermined value.

17. A method for generating alternating electrical current according to claim 16, wherein said first force is the same as the second force.