WO2010118777A1

WO2010118777A1 - Apparatus for generating current from natural and renewable energy

Info

Publication number: WO2010118777A1
Application number: PCT/EP2009/054538
Authority: WO
Inventors: Harshadkumar Maganlal Patel; Alain Van Ranst
Original assignee: PATEL RENEWABLE ENGINEERING Ltd
Current assignee: PATEL RENEWABLE ENGINEERING Ltd
Priority date: 2009-04-16
Filing date: 2009-04-16
Publication date: 2010-10-21
Anticipated expiration: 2011-10-16
Also published as: WO2010118905A3; WO2010118905A2

Abstract

The present invention uses natural, renewable energy sources to activate a torque driven servo permanent magnet generator (servo-PMG) (2) by generating a movement (e.g., rotational) of the permanent magnets relative to the windings of said servo-PMG. Regenerative braking means are combined with said torque driven servo-PMG to reduce the speed, ω, of the permanent magnets relative to the windings of the servo-PMG so as to increase the mechanical torque whilst recovering the kinetic energy thus released by the deceleration, and transforming both magnetic and kinetic energy into electric current. In order to increase the efficacy of the invention, a CPU is programmed to optimize the speed reduction as a function of the energy transferred to the servo-PMG to move the permanent magnets relative to the windings of the servo-PMG by the capturing means. Contrary to most energy generating systems of the prior art which tend to maximize the relative speed, ω, of the generator to increase current output (power driven systems), the present invention is torque driven and may function at low relative speeds. Several sources of natural, renewable energy may be used, alone or in combination, to perform movement of the permanent magnets relative to the windings of the servo-PMG such as thermal (e.g., sunlight, geothermal), wind, hydraulic (e.g., rain, tides, river) or gravitational.

Description

Apparatus for generating current from natural and renewable energy

Technical Field

The present invention relates to an apparatus for generating current from natural and renewable energies, such as gravity, wind, hydraulic, etc. In particular, the present invention combines a brushless servo-PMC activated by said natural, renewable energy with regenerative braking.

Background for the invention

Examples of conventional energy production means comprise thermal power plants which generate energy by the combustion of fossil fuels (e.g., petrol, coal), and nuclear power plants which use nuclear fission. In both cases, heat generated by combustion or fission is used to vaporize water and to drive a vapor turbine, thus generating electric current.

Although these methods can generate large amounts of energy, they have severe environmental impact, be it by the emissions of CO2 and sulfurous pollutants or radioactive waste. Aware of these problems, most governments raised drastically the emission control requirements, thus increasing the cost of running such plants, and stimulating research on alternative energy generation systems.

Hydroelectric plants and windmills farms are probably the most popular solutions for generating current based on natural, renewable energy. For example, in wind-power generators, rotational energy provided by the wind to the blades of the windmill's propeller is converted into electrical energy by a power generator. There are several types of power generators, including, for example, a type that uses coils and/or permanent magnets, and the so-called inrunners wherein a stator surrounds the out side of a cylindrical rotor, and the so-called outrunners wherein a stator faces a disc-shaped rotor in the axial direction, etc.

Since the generated voltage is proportional to the power generator's rotational speed according to the conventional wind-power generators, the voltage can be raised by raising the rotational speed. In order to increase the rotational speed of the generator, and thus the voltage, a gearbox is often interposed between the propeller shaft and the generator.

The use of a gearbox, however, has several drawbacks such as gear-induced torque losses, noise, reduced equipment reliability and life time, and cost. Examples of windmills using conventional DC generators with gearbox are described e.g., in US 4,31 8,01 9.

The voltage may be raised without gear by increasing the number of coils or magnets of a rotor because the generated voltage is proportional to the number of magnetic poles of the field magnet. Such method, however, actually increases the diameter of the rotor of a power generator increasing the overall diameter of the windmill. Examples of windmills using a generator with a large number of poles are described e.g., in DE3629872A1 and DE36381 29A1 . Alternatively, using an array of stators and rotors in parallel was proposed in US2008/0231 1 32 to increase the output of windmills.

Traditional DC motors/generators rely on a mechanical commutator system using a brush contacting different sections of the rotor. Alternatively, so-called servo-PMCs such as brushless DC electric motors/generators, use a solid-state circuit rather than a commutator/brush system. Different types of servo-PMCs exist: So-called delta-type which can run at high rotational speeds, and so-called wye-type which top rotational speed is limited but which generate high torque at low velocity. It follows that delta-type servo- PMCs can be mounted on windmills running at high rotational speeds. In WO03/049256, a servo-PMC was used in a windmill to advantageously replace mechanical braking systems. The energy generated by the braking torque is dissipated by heat and Eddy losses thus requiring specific design of the system to not overheat. In most existing energy generation systems based on natural, renewable energies the efficacy is limited. It therefore remains a need for new, more efficient concepts for generating energy. The present invention proposes a solution to make maximum use of the natural sources of energy with a novel apparatus for generating energy.

Summary of the invention

The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims.

In particular, the present invention concerns an apparatus for generating current comprising:

(a) Means for capturing a source of natural, renewable energy to activate; (b) A torque driven servo permanent magnet generator (servo-PMC) comprising regenerative braking means for reducing the speed, ω, of the permanent magnets relative to the windings of the servo-PMC according to the instructions received by a CPU and for transforming the kinetic and magnetic energy thus released into electrical current; and (c) A connection to the grid or to any energy storage or distribution unit,

Characterized in that, said CPU is programmed:

• to instruct the regenerative braking means to reduce the initial speed, ωi , to a reduced speed, G02, and to ensure that said reduced speed, G02, is at most equal to the transition speed, cot, below which maximal torque, τ_max(oot), is independent of speed as defined on the maximal speed-torque characteristic curve of said servo-

PMC; and

• to optimize the speed reduction, Δω, so that the braking torque, τtπ-ake(Δω), generated by braking adds up with the mechanical torque, τ_mec(ω2), as defined at said reduced speed, ω₂, on the nominal speed-torque characteristic curve of said servo-PMC to substantially the value, τ_max(oot), defined by its maximal torque curve. In a preferred embodiment of the apparatus for generating current, the capturing means transfer natural, renewable energy to the servo-PMC by rotation of a shaft thereof.

In another embodiment of the apparatus for generating current, the torque driven servo- PMC is a brushless servo permanent magnet generator, preferably a wye-type, and most preferably an outrunner.

In another preferred embodiment of the apparatus for generating current, the apparatus may further comprise a current converter positioned between the PMC and said connection to deliver current in a form compatible with the grid or with any energy storage or distribution unit to which it is to be delivered, said converter is preferably controlled by a CPU programmed to instruct said converter to ensure that the converted signal is permanently in a form according to IEEE 51 9 standard.

In another preferred embodiment of the apparatus for generating current, the source of natural, renewable energy is wind or hydraulic energy and the means for capturing it comprise at least one blade, preferably at least three, more preferably more than three blades.

In another preferred embodiment of the apparatus for generating current, the source of natural, renewable energy is liquid flow and the means for capturing it comprise a paddle- wheel or mill-wheel mounted on the shaft of the rotating servo-PMC.

In another preferred embodiment of the apparatus for generating current, the source of natural, renewable energy is gravity and different means for capturing it are proposed.

In another preferred embodiment of the apparatus for generating current, the source of energy comprises a hydraulic and a gravity components and the means for capturing it take advantage of the buoyancy. The present invention also concerns a method for generating current comprising the following steps:

(a) capturing a source of natural, renewable energy in order to activate a torque driven servo permanent magnet generator (servo-PMC) comprising regenerative braking means for reducing the speed, ω, of the permanent magnets relative to the windings of the servo-PMC according to the instructions received by a CPU and for transforming the kinetic and magnetic energy thus released into electrical current;

(b) by means of a CPU, instructing the regenerative braking means: (i) to reduce the initial speed, ωi , to a reduced speed, G02, and to ensure that said reduced speed, G02, is at most equal to the transition speed, cot, below which maximal torque, τ_max(oot), is independent of speed as defined on the maximal speed-torque characteristic curve of said servo-PMC; and

(ii) to optimize the speed reduction, Δω, so that the braking torque, τtπ-ake(Δω), generated by braking adds up with the mechanical torque, τ_mec(ω2), as defined at said reduced speed, G02, on the nominal speed-torque characteristic curve of said servo-PMC to substantially the value, τ_max(oot), defined by its maximal torque curve;

(c) delivering the thus generated current to the grid or to any energy storage or distribution unit in a form compatible therewith.

A preferred embodiment of the method for generating current according to the present invention comprises the following steps: (a) capturing a source of natural, renewable energy in order to drive the rotation of the shaft of a wye-type brushless servo permanent magnet generator (servo-PMC) comprising regenerative braking means for reducing the rotational speed of said shaft according to the instructions received by a CPU and for transforming the kinetic and magnetic energy thus released into electrical current; (b) through a closed loop connection to the CPU, instructing the regenerative braking means:

(i) to reduce the rotational speed of the servo-PMC shaft and to ensure that said speed is maintained within the range wherein torque is independent of speed as defined on the maximal speed-torque characteristic curve of said servo-PMC; and

(ii) to optimize the rotational speed reduction so that the energy regenerated by braking adds up with the mechanical torque as defined at said reduced speed on the nominal speed-torque characteristic curve of said servo-PMC to substantially the value defined by its maximal torque curve;

(c) preferably converting the thus generated current into a signal form compatible with the grid, or with any energy storage or distribution unit to which it is to be delivered; and

(d) delivering the thus converted, compatible current to the grid or to any energy storage or distribution unit.

Brief description of the Figures

For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:

Figure 1 shows three functional block diagrams of the major components of the apparatus for generating current using magnetic regenerative brake of the present invention.

Figure 2 shows two embodiments of the invention using (a) wind and (b) hydraulic flow as a source of renewable energy.

Figure 3 shows four embodiments of the invention using gravity as a source of renewable energy. Figure 4 shows two embodiments of the invention using both gravitational and hydraulic sources of renewable energy.

Figure 5 shows an example of characteristic nominal and maximal torque-speed curves of a torque driven servo-PMC.

Detailed description of the invention

The present invention uses natural, renewable energy sources to activate a torque driven servo permanent magnet generator (servo-PMC) (2) by generating a movement of the permanent magnets relative to the windings of said servo-PMC. Torque driven servo permanent magnet generators are capable of generating high torque at low speeds of the permanent magnets relative to the windings. Two main types of servo-PMCs can be defined,

(a) linear PMCs wherein the relative movement between the permanent magnets and the windings is linear with the static components (either the permanent magnets or the windings) being mounted on a rail, and the moving components being mounted on a shuttle able to move along said rail, and (b) rotational PMCs wherein the relative movement between the permanent magnets and the windings is rotative with the static components

(either the permanent magnets or the windings) being mounted on a stator, and the moving components being mounted on a rotor. Both types of servo-PMCs can be used in the present invention. For sake of clarity, the following description will focus on rotative

PMCs. It is clear, however, that the teaching applies to linear PMCs too.

According to the present invention, regenerative braking means are combined with a torque driven servo-PMC to reduce the speed, ω, of the permanent magnets relative to the windings of the servo-PMC so as to increase the mechanical torque whilst recovering the kinetic energy thus released by the deceleration, and transforming both magnetic and kinetic energy into electric current. In order to increase the efficacy of the invention, a CPU is programmed to optimize the speed reduction as a function of the energy transferred to the servo-PMC to move the permanent magnets relative to the windings of the servo-PMC by the capturing means. Contrary to most energy generating systems of the prior art which tend to maximize the relative speed, ω, of the generator to increase current output (power driven systems), the present invention is torque driven and may function at low relative speeds. Several sources of natural, renewable energy may be used, alone or in combination, to perform movement of the permanent magnets relative to the windings of the servo-PMC.

In a preferred embodiment according to the present invention, the torque driven servo- PMC is a wye-type brushless servo-PMC in a rotor-stator system. In this embodiment, natural, renewable energy sources are used to drive the rotation of the shaft of the wye- type servo-PMC. Wye-type brushless servo permanent magnet generators are capable of generating high torque at low rotational speeds. According to this embodiment of the present invention, regenerative braking means are used to reduce the rotational speed of the servo-PMC's shaft so as to increase the mechanical torque whilst recovering the kinetic energy thus released by the shaft deceleration, and transforming both magnetic and kinetic energy into electric current. In order to increase the efficacy of the invention, a CPU is programmed to optimize the speed reduction of the shaft as a function of the energy transferred to the servo-PMC's shaft by the capturing means. If necessary, the current thus produced may then be processed by a converter to render it compatible with the grid or with any energy storage or distribution unit to which it is to be delivered. Contrary to most energy generating systems of the prior art which tend to maximize the rotational speed of the generator's shaft to increase current output (power driven systems), the present invention is torque driven and may thus function at low rotational speeds. Several sources of natural, renewable energy may be used, alone or in combination, to drive the rotation of the servo-PMC's shaft.

(a) Sources of natural, renewable energy and means for capturing them

An apparatus according to the present invention comprises natural, renewable energy capturing means (1 ). Said means of course depend on the type of natural energy used to perform movement of the permanent magnets relative to the windings of the servo-PMC, preferably, to drive the rotation of the shaft of a rotational servo-PMC. In the context of the present invention, a source of natural, renewable energy provides energy generated from natural resources —such as thermal (e.g., sunlight, geothermal), wind, hydraulic (e.g., rain, tides, river) or gravitational— which are naturally replenished and requiring no chemical reaction (e.g., combustion, photovoltaic).

Preferably, the capturing means (1 ) are mechanically connected to a shaft (2a) of a servo- PMC (2) by means of mechanical connection (1 5) such that they can drive the rotation of the shaft upon capturing natural energy. Preferably, a CPU (4) is in closed loop connection with the means (1 ) for capturing a source of energy by means of control connection (14) and is programmed to instruct said means to capture maximum energy as a function of the varying parameters of said source of energy (cf. Figure 1 b&c).

Specific examples of capturing means are reviewed in section (e) below, particularly suitable for capturing various sources of natural, renewable energies.

(b) Servo Permanent generator (servo-PMC)

A rotational brushless generator (servo-PMC) is a synchronous electric generator which generates electric current upon activation of the rotation of the shaft (2a) thereof and which has an electronically controlled commutation system, instead of a mechanical commutation system based on brushes as in conventional DC generators. Contrary to the conventional DC generators the windings in servo-PMC's are static whilst the permanent magnets are rotating. Examples of brushless generators are described in US5004976 and US5245238. Similarly, in a linear servo-PMC the relative movement between the permanent magnets and the windings is obtained by moving a shuttle loaded with one of the components along a rail along which are arranged an array of the other components. There are basically two main types of brushless generators: power generators sometimes referred to as delta-type generators because of the configuration of the windings serial connections forming a triangle, and torque driven generators which are embodied by wye-type generators thus called because of the Y-configuration of the parallel connections of the windings. A generator with windings in delta configuration yields low torque at low rotational speed, but can function at higher top speeds. Wye configuration yields high torque at low rotational speed, but is limited to lower top speeds than the delta type. The preferred torque driven generators for the present invention are brushless servo-PMC's, preferably of the wye-type.

Each torque driven servo PMC available on the market (e.g., wye-type) is characterized by its nominal and maximal torque-speed curves available from the supplier; an example is illustrated in Figure 5. The nominal torque-speed curves give the torque (hence, the generated current) as a function of the rotation speed of the shaft under normal working conditions. The maximum torque-speed curves define the limit of available maximum torque before the breaking point of the generator is reached. The maximum torque defined by the maximum torque-speed curves can be much higher than the torque defined by the nominal curves.

Each torque driven servo-PMCs is characterized by a family of maximal and nominal characteristic curves as illustrated in Figure 5. It can be seen in Figure 5 that the maximum torque increases linearly with decreasing rotational speed until a transition velocity, cot, is reached where it forms a plateau extending down to standstill (ω = 0) and defining a velocity range wherein maximum torque, τ_max(oot), is independent of speed. Both nominal and maximum curves vary with the magnitude of the natural energy transmitted to the shaft by the capturing means, thus defining a family of characteristic curves as illustrated Figure 5. Curves #1 refers to the nominal and maximum characteristic curves of a servo- PMC fed with a first magnitude of natural energy, whilst curves #2 refers to the same servo-PMC fed with a higher energy corresponding, e.g., to stronger winds in the case of windmills. Rotational torque driven servo PMCs can be so called inrunners or outrunners depending on the windings and permanent magnets relative position. An inrunner generator has stationary coils which surround the rotating magnet at the centre. An outrunner generator has stationary coils at the centre, and the rotating magnets on the outside. Outrunner generators generally produce higher torque at a lower speed then their inrunner counterparts, and are preferred according to the present invention.

To summarize, the servo-PMCs (2) suitable for the present invention are of the high torque, preferably wye-type and more preferably outrunners. Examples of suitable servo-PMC's are described and available from Rexroth (Bosch group) under the reference servo axe synchronous 4 quadrants motor/generator (e.g., Synchronous Motors MSK 1 01 E-0450).

(c) Regenerative braking means and CPU

Regenerative braking is already applied in the field of motorized vehicles, in particular with electrical trains. During braking, the traction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator and the motor armatures are connected across the load. The main traction generator now excites the motor fields. The rolling locomotive wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors (dynamic brake) or back into the supply (regenerative braking). For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque —herein referred to as "braking torque"— in a direction that is opposite from the rolling direction. A regenerative brake is a mechanism that reduces (rotational) speed by converting some of its kinetic energy into another useful form of energy.

The present invention takes advantage of the extra braking torque exerted on the generator shaft upon regenerative braking. The braking torque thus produced adds up with the mechanical torque obtained by the rotation of the shaft driven by the natural energy and defined by the nominal curve of the servo-PMC at the reduced speed. Through this additional braking torque the electrical current output of the servo-PMC can thus be increased accordingly. In other words, the additional torque applied to the servo-PMC upon reducing the speed, ω, of the permanent magnets relative to the windings of the servo-PMC (e.g., rotational velocity of a shaft (2a) of the servo-PMC (2)) from a high value (ωi) to a lower value (002) is used to generate additional current (Wake) which adds up with the current (IMEC (GC>2)) produced by the movement of the permanent magnets relative to windings of the servo-PMC (e.g., the rotation of the shaft) at the speed 002 and determined with the nominal curve thus yielding a high output.

The regenerative brake is controlled by a CPU (4) which is programmed to determine and control the extent of braking so as to optimize electrical current generation as follows. First, the CPU ensures that the relative speed, ω (e.g., rotational speed), is reduced to a value, 002, comprised within the range wherein maximum torque is independent of rotational speed (i.e., 002 < cot). Second, the magnitude of the speed reduction (001-002 = Δoo) obtained by braking is calculated so that the amount of braking torque thus generated adds up with the mechanical torque obtained at the reduced speed according to the servo- PMC nominal curve to substantially the value defined by its maximal torque curve at said reduced speed, 002. Said control may become more complex in cases where a variable source of energy such as wind is used, since we have seen that the nominal and maximum curves of a given servo-PMC vary with the magnitude of the energy collected by the capturing means (cf. Figure 5). In this case in particular, but also possible in other embodiments, a closed loop control to the CPU of the shaft load and speed may be required. Regenerative braking may be called as regenerative braking magnetic control regulation loop.

Referring now to Figure 5, the CPU ensures that the initial speed, 001 , is reduced to a value,

002 < oot, wherein oot defines the transition velocity between decreasing maximum torque with higher speed values and a constant maximum torque, τ_max(oot), independent of velocity for lower speed values. The CPU further ensures that the extra braking torque, τtπ-ake(Δω), generated by the speed reduction to ω₂ reaches substantially the value τ_max(oot) when added to the mechanical torque τ_mec(ω₂) generated by the servo-PMC at a reduced speed ω₂, as defined by:

Xmec(ω₂) + τbrake(Δω) ≥ Tmax(ff)t)

Indeed, if the sum of the two torques thus generated fell short of the maximal torque value Tmax(oot) the generator would not be used at its full capacity. On the other hand, if the sum exceeds τ_max(oot), the system will limit the torque converted into current to the value Xmax(oot) and the excess torque will be released as friction, wear and heat which is detrimental to the life of the generator. Actually, a certain amount of heat will anyway be generated upon braking. Said heat may advantageously be transferred to any calorific fluid such as water and/or cooling gas.

(d) Current converter and Grid

An apparatus according to the present invention further comprises an electrical connection (1 3) to the grid (5) or to any energy storage or distribution unit.

The "Grid" is meant here as a network of electric-power connections, in other words, a system of electric wires for sending power over a large area. Basically, any electrical appliance connected to a wall socket pulls its electricity from the grid. Local distribution units other than the grid, such as fed by a generator or photovoltaic cells may be connected to the apparatus of the present invention. Finally, the apparatus of the present invention may also be connected to a storage unit such as a battery.

The electrical connection (1 3) between the grid (5) or any energy storage or distribution unit and the apparatus according to the present invention not only feeds the former with current generated by the apparatus but may also serve to energize the apparatus for its functioning when needed. Of course, when this is the case, a positive balance between current generated and consumed by the apparatus is essential to the invention. Alternatively, the apparatus of the present invention may be provided with a small battery to supply the initial energy necessary to trigger the generation of current by the servo-PMC, and once running any power required for its functioning may be retrieved from the generated current or external source such as grid or battery.

The current generated by the apparatus of the present invention is rarely compatible with the grid (5) or any energy storage or distribution unit to which it is connected. For this reason, the apparatus may further comprise a current converter (6) located between the servo-PMC and the grid (5) or any energy storage or distribution unit.

Herein, a current converter is defined as any unit capable of transforming a current signal into a different signal compatible with the grid (5) or any energy storage or distribution unit. A particular example of current converter is an electrical apparatus for the interconversion of alternating current and direct current. The CPU (4) may be in closed loop connection by means of control connection (1 4) with the converter (6) and be programmed to instruct said converter to ensure that the converted signal is permanently in an acceptable form regardless of the variations in the magnitude of the natural energy transmitted by the capturing means to the shaft. In particular, if the apparatus is connected to the grid (5) the converter may ensure that the current transmitted thereto is according to IEEE 51 9 standard. Here again, a closed loop control by the CPU (4) may be necessary.

IEEE 51 9-1 992, which defines the IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power System, provides a basis for limiting harmonics. Harmonics are a concern because they can cause excessive heating and pulsating, and reduced torque in motors/generators; increased heating and voltage stress in capacitors; and misoperation in electronics, switchgear and relaying. In short, harmonics can lead to reduced equipment life if a system is designed without consideration for harmonics and if equipment is not properly rated and applied. Many grid providers would not allow, or only upon payment of a fine, that current not according to IEEE 51 9 be injected into their grids.

For example, current converters (6) for limiting harmonics suitable for the present invention are described and available from Rexroth (Bosch group) under the reference Drive Control Devices (e.g., RD 500).

(e) Examples of capturing means suitable for the apparatuses according to the present invention

• Means for capturing wind as a source of renewable energy

For instance, if the source of natural energy is wind then the capturing means are preferably mounted on a pillar and comprise at least one blade (I a) to capture the wind (cf. Figure 2a). Preferably, three or more blades forming a propeller can be used to increase the area exposed to wind. The blades can be distributed radially around the central hub either on a plane or on the surface of a truncated cone with the central hub defining the truncated base of the cone. The latter configuration allows a higher blade surface to be exposed to the wind thus increasing torque. Preferably, an apparatus according the present invention, comprises a CPU (4) controlling the direction of the shaft (2a) (i.e., by rotating the pillar) and/or the orientation of the at least one blade (I a) as a function of wind direction and speed. Preferably, the apparatus is equipped with a sensor giving feedback information to the CPU such as wind force and direction.

• Means for capturing hydraulic energy (first embodiment: turbine & paddle- wheel)

Conventional turbines used in hydro-electric plants may be used as capturing means to drive the rotation of the shaft according to the present invention. Alternatively, a paddle- wheel or mill-wheel (I b) used for a traditional water-mill may be mounted on the shaft of the rotating servo-PMC as capturing means (cf. Figure 2b).

• Means for capturing hydraulic energy (second embodiment: tide)

Alternatively the capturing means may comprise a buoy (9a) rigidly connected to the shaft (2a) by an arm (7a) as illustrated in Figures 4a and 4b, said buoy being suitable for floating on sea water and for driving the rotation of the shaft (2a) in one direction as the tide rises (cf. Figure 4a) and for driving it in the opposite direction as the tide recedes (cf. Figure 4b). In practice, the buoy must have sufficient buoyancy (i.e., volume) to raise with the tide and sufficient mass to drive the arm down upon receding tide to generate maximum torque both ways.

• Means for capturing hydraulic energy (third embodiment: immersed buoy)

Alternatively the capturing means comprise a buoy (9a) rigidly connected to the shaft (2a) by an arm (7a) as illustrated in Figure 4c, said buoy being suitable for floating on water. In the present embodiment, the buoy is maintained under water and the rotational velocity of the shaft is reduced to a very low value, G02 ≠ 0, thus generating a braking torque which adds up with the mechanical torque, τ_mec(ω2 ≠ 0), defined by the nominal curve of the servo-PMC. To ensure that the sum of these two torques substantially equals the value of maximum torque, τ_max(oot), and to allow the buoy to be brought down once it has reached its top position, it is necessary to control the volume (i.e., the level of buoyancy) of the buoy which will determine the shape of the nominal curve as illustrated in Figure 5. It is possible to vary the buoyancy of the buoy by controlling the injection therein of water or air depending on whether buoyancy is to be reduced or increased, respectively. • Means for capturing gravitational energy (first embodiment: seesaw)

In a preferred embodiment illustrated in Figure 3, the source of natural, renewable energy is gravity and the means for capturing it comprise: (a) an elongated arm (7) mounted at its centre on the shaft of the rotating servo-PMC

(2), to allow a seesaw movement thereof defining a top, respectively bottom position of the arm ends; (b) mass transfer means (8) for bringing a mass (9) to the arm end located at its top position; and (c) means for allowing the arm to rotate so as to drive by gravity the arm end provided with said mass from its top position to its bottom position, and thus rotating the shaft (2a) of the servo-PMC (2); the servo-PMC (2) and mass transfer means being selected such that more electric energy is generated by the rotation of the shaft upon one seesaw movement than it is consumed by the transfer means to drive the mass to the top arm end.

Embodiment (A): linear PMC

Preferably, in this embodiment of a gravity driven apparatus as described above, the mass (9) is movingly mounted on the elongated arm (7) and the mass transfer means (8) consist of either:

• a conveying belt (8a) spanning the full length of the arms (cf. figure 3a); or

• a low torque high speed servo-PMC (8b), wherein the windings thereof are either distributed along the elongated arm, or form the stator in a stator-rotor system or, alternatively, the permanent magnets are distributed along the elongated arm(cf. figure 3b).

The choice of a suitable motor (8b) to drive the mass to the top arm end depends on the torque-driven servo-PMC (2) used to generate current and must consume less current to drive the mass up than is generated by the torque-driven servo-PMC (2) by one swing down of the seesaw. For example, suitable linear type servo-PMCs (8b) for transferring the mass to the top end position of the arm which may advantageously be used with e.g., a torque driven servo-PMC (2) of the type RD 500 (Rexroth (Bosch group)) are described and available from Rexroth (Bosch group) under the reference Synchronous linear motors (e.g., MLP070B-01 00) and of the stator-rotor type can be found in Synchronous Motors MSK (e.g., MSKl OOC-0200).

Embodiment (B): flowing material

In another embodiment of a gravity driven apparatus as defined above,

• the mass (9) is a flowing material selected among liquids and solid particulate materials; for instance, water or sand are suitable materials for the present embodiment;

• each end of the elongated arm is provided with a vessel (1 0) suitable for containing a given mass of said flowing material and comprising a drain, preferably with closing means (1 1 );

• the transfer means (8) comprise either: o a pump (8c) suitable for transferring the flowing material from the vessel located at its bottom position to the top vessel and wherein said pump is controlled by a CPU (4) (cf. figure 3c); or o a storage tank (1 2) for storing said flowing material, in fluid connection with each vessel (1 0), each vessel being provided with a drain with closing means (1 1 ), and further comprise means for controlling the flow of said flowing material from the storage tank to one or the other vessels (1 0) (cf. figure 3d); and wherein the CPU (4) controls that the drain (1 1 ) is open when the vessel is at its bottom position and the fluid connection between the storage tank and the vessel at its top position is open to transfer thereto said given mass. • Means for capturing gravitational energy (second embodiment: one arm swing)

In a preferred embodiment of a gravity driven apparatus, the source of natural, renewable energy is gravity and the means for capturing it comprise: (a) an elongated arm (7a) mounted at one of its ends on the shaft of the rotating servo-PMC (2), to allow a rotating movement thereof, and defining a top, respectively bottom position of the free arm end; (b) mass transfer means (8) for bringing a mass (9) to the free arm end when it is located at its top position; and (c) means for allowing the arm to rotate so as to drive by gravity the free arm end provided with said mass from its top position to its bottom position, and thus rotating the shaft (2a) of the servo-PMC (2); the servo-PMC (2) and mass transfer means being selected such that more electric energy is generated by the rotation of the shaft upon one rotational movement of the arm than it is consumed by the transfer means to drive the mass to the arm end at its top position.

• Means for capturing gravitational energy (third embodiment: lift&crane)

The present invention may also be combined with existing systems designed to drive a weight up a given height to generate current from gravitational energy upon bringing said weight down. For example a crane or a conventional lift e.g., for carrying people or goods, may be combined with a torque driven servo-PMC and a CPU according to the present invention to generate current as the crane hook or lift is driven down. The servo-PMC may be mounted on the pulley or on the winch of a crane. Alternatively, depending on the type of lift the servo-PMC can be mounted on the return pulley over which the cable holding the lift and counterweight is passed, or on the winch of the motor, or attached to an external wall of the cabin and running along a rail provided either with magnets or windings in case a linear servo-PMC is used, or with a gear in case a rotational servo-PMC is used. It is clear that the servo-PMC must be idle when the crane hook and lift go up and is activated only when they go down. In this embodiment, the energy balance is not necessarily positive as more energy may be required to drive the crane hook or lift up than is generated by the servo-PMC as the they come down, but since that energy would have been wasted anyway and additional energy would be required to slow the loaded crane hook or the lift down during its descent, adapting the present invention to a crane or a lift can be highly advantageous.

1 : means for capturing a source of renewable energy

2: PMC SERVO generator

2a: shaft

3: regenerative braking means 4: CPU

5: grid / storage / distribution unit

6: current converter

7: elongated arm (seesaw)

7a: elongated arm (lever) 8: means for transferring the mass

8a: conveying belt

8b: low torque high speed servo-PMC

8c: pump

9: mass 1 0: vessel

1 1 : drain

1 2: tank

1 3: electrical connection

1 4: control connection 1 5: mechanical connection

Claims

1 . An apparatus for generating current comprising: (a) Means (1 ) for capturing a source of natural, renewable energy to activate;

(b) A torque driven servo permanent magnet generator (servo-PMC) (2) comprising regenerative braking means (3) for reducing the speed, ω, of the permanent magnets relative to the windings of the servo-PMC according to the instructions received by a CPU (4) and for transforming the kinetic and magnetic energy thus released into electrical current; and

(c) A connection to the grid (5) or to any energy storage or distribution unit, Characterized in that, said CPU (4) is programmed:

• to instruct the regenerative braking means (3) to reduce the initial speed, ωi , to a reduced speed, G02, and to ensure that said reduced speed, G02, is at most equal to the transition speed, cot, below which maximal torque, τ_max(oot), is independent of speed as defined on the maximal speed-torque characteristic curve of said servo- PMC (2); and

• to optimize the speed reduction, Δω, so that the braking torque, τtπ-ake(Δω), generated by braking adds up with the mechanical torque, τ_mec(ω2), as defined at said reduced speed, G02, on the nominal speed-torque characteristic curve of said servo-PMC (2) to substantially the value, τ_max(oot), defined by its maximal torque curve.

2. An apparatus according to claim 1 , wherein the capturing means (1 ) transfer natural, renewable energy to the servo-PMC (2) by rotation of a shaft (2a) thereof.

3. An apparatus according to claim 1 or 2, wherein the torque driven servo-PMC (2) is a brushless servo permanent magnet generator, preferably a wye-type, and most preferably an outrunner.

4. An apparatus according to any of the preceding claims, further comprising, a current converter (6) positioned between the servo-PMC (2) and said connection to deliver current in a form compatible with the grid (5) or with any energy storage or distribution unit to which it is to be delivered, said converter being preferably controlled by a CPU (4) programmed to instruct said converter to ensure that the converted signal is permanently in a form according to IEEE 51 9 standard.

5. An apparatus according to any of the preceding claims, wherein the CPU (4) is in closed loop connection with the means (1 ) for capturing a source of energy and is programmed to instruct said means to capture maximum energy as a function of the varying parameters of said source of energy.

6. An apparatus according to any of the preceding claims, wherein the source of natural, renewable energy is wind or hydraulic energy and the means for capturing it comprise at least one blade (I a), preferably at least three, more preferably more than three blades.

7. An apparatus according to claims 2, 5 and 6, wherein the source of energy is wind and the CPU (4) controls the direction of the shaft (2a) and/or the orientation of the at least one blade (1 a) as a function of wind direction and speed.

8. An apparatus according to claim 2 and any of claims 5 to 6, wherein the source of energy is liquid flow and the means for capturing it comprise a paddle-wheel or mill-wheel mounted on the shaft of the rotating servo-PMC.

9. An apparatus according to any of claims 2 to 4, wherein the source of natural, renewable energy is gravity and the means for capturing it comprise:

(a) an elongated arm (7) mounted at its centre on the shaft (2a) of the rotating servo- PMC (2), to allow a seesaw movement thereof defining a top, respectively bottom position of the arm ends; (b) mass transfer means (8) for bringing a mass (9) to the arm end located at its top position; and

(c) means for allowing the arm to rotate so as to drive by gravity the arm end provided with said mass from its top position to its bottom position, and thus rotating the shaft (2a) of the servo-PMC (2); the servo-PMC (2) and mass transfer means (8) being selected such that more electric energy is generated by the rotation of the shaft (2a) upon one seesaw movement than it is consumed by the transfer means to drive the mass to the top arm end.

1 0. An apparatus according to claim 9, wherein the mass (9) is slidingly mounted on the elongated arm (7) and the mass transfer means (8) consist of either:

• a conveying belt (8a) spanning the full length of the arm (7); or

• a low torque high speed servo-PMC (8b), wherein the windings thereof are either distributed along the elongated arm, or form the stator in a stator-rotor system or, alternatively, the permanent magnets are distributed along the elongated arm (7).

1 1 . An apparatus according to claim 9, wherein:

• the mass (9) is a flowing material selected among liquids and solid particulate materials; • each end of the elongated arm (7) is provided with a vessel (1 0) suitable for containing a given mass (9) of said flowing material;

• the transfer means (8) comprise either: o a pump (8c) suitable for transferring the flowing material from the vessel located at its bottom position to the top vessel and wherein said pump is controlled by a CPU (4); or o a storage tank (1 2) for storing said flowing material, in fluid connection with each vessel (1 0), each vessel being provided with a drain with closing means (1 1 ), and further comprise means for controlling the flow of said flowing material from the storage tank (1 2) to one or the other vessels (1 0); and wherein the CPU (4) controls that the drain (1 1 ) is open when the vessel (1 0) is at its bottom position and the fluid connection between the storage tank (1 2) and the vessel (1 0) at its top position is open to transfer thereto said given mass.

1 2. An apparatus according to any of claims 2 to 5, wherein the source of natural, renewable energy comprises gravity and the means for capturing it comprise:

(a) an elongated arm (7a) mounted at one of its ends on the shaft (2a) of the rotating servo-PMC (2), to allow a rotating movement thereof, and defining a top, respectively bottom position of the free arm end; (b) mass transfer means (8) for bringing a mass (9) to the free arm end when it is located at its top position; and

(c) means for allowing the arm to rotate so as to drive by gravity the free arm end provided with said mass (9) from its top position to its bottom position, and thus rotating the shaft (2a) of the servo-PMC (2); the servo-PMC (2) and mass transfer means (8) being selected such that more electric energy is generated by the rotation of the shaft (2a) upon one rotational movement of the arm than it is consumed by the transfer means to drive the mass to the arm end at its top position.

1 3. An apparatus according to claim 1 2, wherein the source of energy comprises a hydraulic, a gravity components and wherein the apparatus comprises a buoy (9a) rigidly connected to the shaft (2a) by an arm (7a), said buoy being suitable for floating on sea water and for driving the rotation of the shaft (2a) in one direction as the tide rises and for driving it in the opposite direction as the tide recedes.

1 4. A method for generating current comprising the following steps:

(a) capturing a source of natural, renewable energy in order to activate a torque driven servo permanent magnet generator (servo-PMC) (2) comprising regenerative braking means (3) for reducing the speed, ω, of the permanent magnets relative to the windings of the servo-PMC according to the instructions received by a CPU (4) and for transforming the kinetic and magnetic energy thus released into electrical current;

(b) by means of a CPU (4), instructing the regenerative braking means (3):

(i) to reduce the initial speed, ωi , to a reduced speed, G02, and to ensure that said reduced speed, G02, is at most equal to the transition speed, cot, below which maximal torque, τ_max(oot), is independent of speed as defined on the maximal speed-torque characteristic curve of said servo-PMC (2); and (ii) to optimize the speed reduction, Δω, so that the braking torque, τtπ-ake(Δω), generated by braking adds up with the mechanical torque, τ_mec(ω2), as defined at said reduced speed, G02, on the nominal speed-torque characteristic curve of said servo-PMC (2) to substantially the value, τ_max(oot), defined by its maximal torque curve;

(c) delivering the thus generated current to the grid or to any energy storage or distribution unit in a form compatible therewith. 5. A method according to claim 14, wherein an apparatus according to any of claims 1 1 3 is used.