The present invention relates to improvements in transmission units, and in particular to transmission units for renewable energy harvesting means.

The harvesting of renewable energy is a rapidly growing sector throughout the world. A known difficulty with many renewable energy sources, such as wind energy, is

fluctuation in the strength of the energy source. Taking the example of wind energy, wind speeds in a particular area will vary greatly and hence the power density of the wind will not be constant. Wind speeds are typically quoted as average speeds, but even an average speed may vary from day to day, month to month, seasonally and even yearly;

momentary or short-term wind speed predictions are

essentially impossible. Even in areas where there is typically very little wind, it is possible on a stormy day for there to be gusts of over 25ms ^"1 . Therefore, wind turbines need to be protected from such extreme conditions.

As a result of the fluctuations in wind speed, the energy produced by a wind turbine is not constant . If a wind turbine is coupled directly to a generator, these

fluctuations will be passed on to the latter. Electrical generators are designed to work optimally within a predetermined range of wind speeds, the performance of such generators (even so-called 'variable speed generators') being less than optimum at speeds which deviate from the rated value. The range of operation of a wind turbine is defined by its 'cut-in' speed and ⁴ cut -out' speed. The cut-in speed is the lowest wind speed at which a wind turbine begins

producing usable power. The cut-out speed is highest wind speed at which a wind turbine produces power. Conventional wind turbines have 'cut-in' speeds of more than 4ms ^"1 . At the cut-in speed the efficiency of the turbine is very low. The efficiency of the turbine gradually increases up to the optimum efficiency of the turbine at the rated speed, which is typically 10ms ^"1 to 12ms ^"1 . For higher wind speeds, the speed of rotation of the turbine is maintained at the level corresponding to the rated wind speed. If the wind reaches a very high speed, corresponding to the cut-out speed, brakes are applied to the turbine in order to curb the generator's rotational speed and/or to turn the turbine blades out of the direction of the casting winds for safety reasons

(commonly known as 'furling') . As a result, there is only a narrow band of wind speeds at which a wind turbine operates, corresponding to wind speeds which are close to the rated speed.

Although low speed winds have much lower energy density than high speed winds, they typically blow for a longer period than high speed winds. As a result, their energy content is not trivial. Extracting energy from these winds is therefore highly beneficial. At the other end of the spectrum, extending the usable wind speed spectrum upwards enables the harvesting of quite significant amounts of energy beyond what usual systems capture.

A further disadvantage is associated with smaller wind turbines. In such systems, which produce direct current (DC) , the generator operates at varying speeds depending on the wind speed. At the rated wind speed the efficiency is optimal, whereas at wind speeds which are either lower or higher than the rated speed the efficiency is reduced. Even 'variable speed' generators do not operate at their optimum efficiency at all speeds.

It is therefore an object of the invention to provide a transmission unit that can operate at over a wider range of wind speeds to enabling a generator associated therewith to operate at its optimum efficiency.

According to the invention there is therefore provided a transmission unit according to claim 1.

The transmission unit of the present invention enables energy from a wind turbine to be stored until a sufficient quantity is accumulated for an associated generator to operate at its rated values. The system additionally

obviates the need for decelerating the turbine, excepting deceleration due to safety considerations, which facilitates the optimisation of the operation of the generator.

As well as wind turbines, the transmission unit of the present invention is equally applicable to methods of harvesting other energy sources of time-variable power density. Examples of such energy sources are wave energy harvested from sea or lake waves (either when the waves hit the shore or in deep water) , water flow or currents in a river or stream, rainwater flowing down a roof, etc. As long as it is feasible to harvest the energy at a variable rate and store it in the energy storage system, the latter can be used to deliver the energy according to demand.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a transmission unit according to the invention mounted in a casing (cut-away view of the casing) , showing the system at full capacity; and

Figure 2 shows the transmission unit of Figure 1 when no energy is being stored in the system. Figure 1 depicts a transmission unit 10 according to the invention, which is in communication with a wind turbine (not shown) and a generator 11 mounted in a casing 12. The generator 11 may alternatively be an alternator. The turbine blades (not shown) are coupled to a shaft

13, which in turn is coupled to a flexible drive 14 which transfers the rotational motion to a step-down gearbox 15. The step-down gearbox 15 minimises the initial torque needed to rotate the turbine, which facilitates the capture energy from lower speed winds. The shaft 13 is preferably

flexible .

The flexible drive 14 may alternatively be replaced by any form of coupling for transferring rotational motion.

The output of the transmission unit 10 is optionally fed to a step-up gearbox 16 which increases the speed of the output before it is fed into the generator 11. The gear ratio of the step-up gearbox is designed to match its output RPM to the rated rotational speed of the generator 11. The generator comprises brake plates 21.

The transmission unit 10 comprises a connecting means comprising an endless chain loop 17, which is coupled to the step-down gearbox 15 by a first sprocket 18 and is further coupled to the step-up gearbox 16 by a second sprocket 19. Advantageously, the use of the gearboxes allows the endless chain loop 17 to move slowly enough that the springs 31 described below can extend/contract over a small operating range . Both the first 18 and second 19 sprockets are only able to rotate in one direction. This can be achieved by methods known in the art. For example, by using a ratchet. When the chain is arranged as in the figure, said direction is common for both sprockets 18,19. A third sprocket 20 couples the endless chain loop 17 to an energy storage means 31

comprising one or more springs 31. The third sprocket 20 is able to rotate in either direction.

The third sprocket 20 acts as a guide, through which the endless chain loop 17 can pass, but which can apply tension to the endless chain loop 17 under the action of the energy storage means. Alternative guides could be provided in the form of pulleys, eyes, etc. Other energy storage means could be used instead of springs 31, such as one or more suspended weight (s) movable between different heights to store gravitational potential energy, or a fly-wheel, any other suitable mechanical storage system, or a combination of two or more of these energy storage means. The exact form of the energy storage means used would depend upon the amount of energy to be stored, and the size and the materials of the system.

More specifically, the third sprocket 20 is mounted on a mounting plate 32, to which first ends of the springs 31 are coupled, for example by a first set of hooks 33. Second ends of the springs 31 are coupled to a base 34, for example via a second set of hooks 35. The springs 31 may comprise steel. An upper limit switch 50 and a lower limit switch 51 define the allowable limits of extension and contraction for the springs 31 by sensing the presence of the ends of the springs at two respective heights. In an alternative embodiment in which the energy storage means is formed as a suspended weight, the switches 50, 51 can sense the presence of the weight at an upper height and a lower height. In the case of a fly-wheel, the switches 50, 51 can sense the speed of angular rotation.

The springs 31 may store energy by extending, and then deliver their energy to the generator 11 when recoiling to their original shape. The stored energy is released when the springs reach the upper limit switch 50. Storage of energy is commenced when the springs reach the lower limit switch. The use of a pre-determined upper limit ensures that the elastic range of the springs 31 is not exceeded. Owing to the use of a step-down gearbox, the operative range of the springs can be small as compared with their elastic range. Advantageously, this allows the life of the springs to be longer as compared with equivalent springs having a greater operative range.

A first end of an idler arm 36 is coupleable with the endless chain loop 17 via a fourth sprocket 37, which is able to rotate in either direction. A second end of the idler arm 36 is coupled to a mounting structure 38 onto which the gearboxes 15,16 and the generator 11 are also mounted. The mounting structure 38 may comprise steel U- sections.

The idler arm 36 is movable between two positions.

Figure 1 shows a first position in which the idler arm 36 weakly engages the endless chain loop 17 (via the fourth sprocket 37) . Figure 2 shows a second position in which the idler arm 36 is used to take up slack in the endless chain loop 17 (via the fourth sprocket 37) .

The mounting structure 38 sits inside, and is coupled to, the casing 12. Also mounted in the casing 12 are battery compartments 39, a pivot for the shaft 40, and a box 41 containing a rectifier (not shown) from alternating current (AC) to direct current (DC) , which may be used to charge the batteries and inverter to AC 50Hz for grid tie-up (single or three-phase) . The casing sits on a steel plate base 42.

The transmission unit 10 is able to decouple the input to the system (from the shaft 13) from the output to the system (to the generator 11) .

The first sprocket 18 receives motion from the step- down gear box 15. Even at low wind speeds, the low torque produced by the step-down gear box 15 can be used to move the endless chain loop 17. The motion of the endless chain loop induces rotation in the second sprocket 19, which rotates the generator 11 (optionally via the step-up gear box 16) . In this mode the idler arm 36 is in its first position (as shown in Figure 1) and the springs 31 are fully contracted .

Braking means are applied to brake the generator 11 (specifically, by applying the brake plates 21) , the second sprocket 19 is thus prevented from rotating. Due to the fact that the second sprocket 19 is still while the first

sprocket 18 continues rotating, the proportion of the endless chain loop between the second sprocket 19 and the first sprocket 18 is reduced (i.e. in the clockwise

direction in Figure 1 - the lower part of the endless chain loop 17) . This results in the third sprocket 20 being pulled upwards towards the first sprocket 18. As the third sprocket 20 is pulled upwards, the springs 31 extend and potential energy is thereby stored in the springs 31. The slack in the endless chain loop 17 resulting from the increased

proportion of the endless chain loop 17 between the first sprocket 18 and the second sprocket 19 (again in the clockwise direction - the upper part of the endless chain loop 17) can be taken up by the idler arm 36, which moves into its second position (as shown in Figure 2) . The idler arm 36 is not essential, since it is acceptable to allow the chain simply to sag, however it is preferable to maintain a small amount of tension to keep the upper portion of the endless chain loop 17 taught. Tension can be achieved either by applying a spring to the idler arm 36, or simply by the weight of the idler arm 36. Conversely, if the brake plates 21 are not applied to the generator 11, the second sprocket 19 can rotate under action of the first sprocket 18. Additionally, the third sprocket 20 can fall as the stored potential energy in the energy storage means 31 is released, thereby drawing the chain loop 17 around the second sprocket 19 causing further rotation. This sequence of events results in the transfer of energy in the springs 31 to motion of the second sprocket 19, which will power the generator 11. The slack in the chain loop 17 is thus reduced to nothing, which causes the idler arm 36 to return to its first position (as shown in Figure 1) . The modes of operation of the transmission unit 10 will now be described.

During periods of operation having low wind speeds, the transmission unit operates in a decoupled mode. When the spring (s) 31 are contracted to the extent of their

predetermined operating range, the lower limit switch 51 is triggered and the brake plates 21 are applied to the

generator. As described above, the continued rotation of the shaft 13 and therefore of the first sprocket 18 causes the springs 31 to become tensioned, while at the same time the slack in the endless chain loop 17 is taken up by the idler arm 36. When the springs 31 are extended to the full extent of their chosen operating range, the upper limit switch 50 is operated and the brake plates 21 are released. This allows the springs 31 to contract, which causes rotation of the second sprocket 19 (as described above) . This in turn activates the generator 11, causing it to operate within its optimal range of speeds. When the springs 31 have fully contracted, the lower limit switch 51 is triggered once again and the process repeats. It can be seen that in this mode the turbine shaft 13 is decoupled from the generator 11 and electricity is produced intermittently, i.e. only when the springs are contracting.

During periods of operation having high wind speeds, the transmission unit operates in its coupled mode.

At high wind speeds the power generated by the turbine shaft 13 is greater than the power required to extend the springs 31 and therefore the springs 31 are kept fully tensioned with the upper limit switch 50 triggered, so that the brake plates 21 are not applied to the generator 11 and hence the generator 11 rotates continuously. Stops can be provided to prevent the springs 31 from extending beyond the upper limit switch and thereby prevent fatigue of the springs 31. In this mode the turbine shaft 13 is coupled to the generator 11 and electricity is produced continuously with the springs 31 fully extended. Only when the wind speed decreases will the springs 31 contract and the lower limit switch 51 be triggered, causing the brake plates 21 to be applied. At this point the system will operate in the decoupled mode. The upper limit switch 50 and the lower limit switch 51 can be triggered directly by sensing the portion of the ends of the spring (s) or weight (s) forming the energy storage means, or indirectly by sensing the position of the idler arm 36. The switches in that case may be formed as a single switching means. In practice, the decoupled mode may be used for wind speeds between 1.8ms ^"1 and 4 ms ^"1 . The coupled mode may be used for wind speeds above 4ms ^"1 . During operation, the first sprocket 18 and the second sprocket 19 can rotate at different rates, while the third sprocket 20 moves up or down depending on whether excess energy is being received or needs to be delivered. The step-down gearbox 15, the spring coefficient (5), and the spring's operating range can be chosen so that at a first wind speed (preferably 1.8 ms ^"1 ) the tension in the endless loop chain 17 is sufficient to extend the spring (s) 31 from the lower limit of their operating range

(corresponding to the lower limit switch) , and at a second wind speed, higher than the first wind speed, (preferably 4ms ^"1 ) the tension in the endless loop chain 17 (taking into account the resistance offered by the generator as the corresponding speed) is sufficient to maintain the spring (s) 31 at the maximum extension of the operating range

(corresponding to the upper limit switch) .

For any given energy harvesting system, the above- described transmission unit could match the rotational speed of the second sprocket 19 to the requirements of the

generator 11 so that the transmission unit will smooth the inhomogeneous input. This can be compared to the working of a smoothing capacitor used after a rectifier stage in a DC power supply.

The overall effect is that energy can be delivered at a stable rate, corresponding to the demand or capability of the generator, and that energy can be harvested even at low power levels.

Whilst the endless chain loop 17 has been described above, any connecting means 17 may be used, such as a belt or a cable.

Whilst the third sprocket 20 has been described above, any guide means may be used, so long as it is coupled to the connecting means 17 and can be displaced to store energy in the energy storage means 31.

Whilst the braking means discussed above are controlled by upper and lower limit switches 50, 51, it will be

appreciated that they can alternatively be controlled by a computer/controller in response to tension in the springs of the energy storage means 31 or in the endless chain loop 17. Such an embodiment may comprise a strain gauge arranged to output a signal indicative of the tension in the springs 31 or chain loop 17.