WO2013005056A1

WO2013005056A1 - Apparatus and method for electrical energy storage

Info

Publication number: WO2013005056A1
Application number: PCT/GB2012/051604
Authority: WO
Inventors: Peter Fraenkel; Martin Wright
Original assignee: Fraenkel Wright Limited
Priority date: 2011-07-06
Filing date: 2012-07-06
Publication date: 2013-01-10
Also published as: GB2509437B; GB201405920D0; GB201111535D0; GB2509437A

Abstract

An energy storage system (2) comprising a weighted object (6) suspended from a cable (8), a lifting device (10, 16) connected to the weighted object by way of the cable and arranged to move the weighted object through a substantially vertical distance, and means for generating electrical energy (16) connected to the lifting device. The system preferably comprises a guidance system (18, 20) for guiding the weighted object.

Description

APPARATUS AND METHOD FOR ELECTRICAL ENERGY STORAGE

This invention relates to an energy storage device, and in particular a device intended for absorbing energy, particularly electrical energy, storing it without leakage or loss and then exporting it when required to do so.

So far as electricity storage is concerned, generally this involves options such as pumped (water) storage, flywheels, electro-chemical batteries, compressed air, hydrogen production, cryogenics or capacitors. A key problem with storing electricity is that most of these known methods involve poor cycle efficiency and in many cases there is loss of energy (internal discharge) while it is stored. Many methods, especially involving batteries, also suffer from a limited number of cycles being possible before performance deteriorates and the device needs complete replacement.

A concept of a method of energy storage in many physics or basic engineering text books uses a raised weight as an example of how potential energy can be converted to kinetic energy and vice-versa. However, this approach to energy storage has not so far been seriously developed and there are numerous practical engineering difficulties

According to one aspect of the present invention, there is provided an energy storage system comprising a weighted object suspended from a cable, a device for lifting and lowering the weighted object and connected to said weighted object by way of said cable, said device arranged to move said weighted object through a substantially vertical distance, and means for generating electrical energy connected to said lifting device so as to generate electrical energy during a lowering phase.. According to a second aspect of the present invention, there is provided a method of energy storage comprising lifting a weighted object through a substantially vertical distance by a lifting device, thereby storing potential energy, said lifting device connected to means for generating electrical energy, and lowering said weighted object, thereby activating the means for generating for transforming the stored potential energy into electrical energy.

Owing to these aspects, an efficient storage system can be provided in which there is relatively little waste of stored energy.

It is an object of the present invention to provide a method for using electricity at times when it is readily available to raise a massive weight (for example, where large numbers of wind farms are involved, at times when the wind is blowing strongly) and then to return the electricity to the grid or network by lowering the weight at times when there is a shortage of generating capacity. It can also be used as part of an uninterruptable power supply to produce power for a short period in the event of loss of power from the mains or to provide a powerful short-term local boost to an electrical power supply and moreover this can be generated at very short notice, a matter of a few seconds.

This system offers an exceptionally efficient means for storing electrical energy in such a way that there is no internal discharge (as with batteries or flywheels, for example), so that substantially all of the stored energy can be reclaimed at virtually any time and moreover the most costly components do not wear out and can be used indefinitely.

A key opportunity for applying this invention will be to use abandoned deep mine workings where the vertical shafts formerly used for the hoists can be used. In such locations the shafts may be wholly or partially water-filled but as will be disclosed, this invention can be applied within a water-filled shaft equally as well as in a dry shaft. In many cases where significant power is needed, such as for grid power supply applications, the weighted object may need to have a mass of thousands of tonnes, however the same principle can equally be used on a smaller scale. The weighted object is advantageously of a massive weight suspended from one or more cables and is mechanically interconnected to a windless or winch drum capable of reeling in and paying out the cable or cables and the winch drum is mounted on a robust shaft with equally robust bearings capable of carrying the necessary forces imposed by the massive weight. Said winch with its shaft and bearings will in turn be mounted on a structurally robust chassis frame resting across the top of the shaft (or possibly in some cases installed in a widened space in the top of the shaft (so that the entire system even including the winch and chassis can be below ground level) or it could be on the top of a tower extending above the top of the shaft).

The winch is also interconnected to means for generating electrical energy, such as an electric motor/generator either on the same shaft (direct drive) or via a gearing system which in most cases will involve a gearbox, but which could involve belts, chains or other means for speed-reduction between the electrical motor/generator and the winch.

The winch drum may be installed immediately above the weighted object like a hoist, or there may be a pulley system at the top of the shaft immediately above the weight to permit the winch to be offset to one side of the shaft.

If a single cable is be used, locked-coil or flat-stranded ropes are preferred to avoid a tendency for the weighted object to tend to unwind the rope. Alternatively, a plurality of cables in parallel could be used to allow individually smaller cable cross-sections and hence a smaller winch drum and/or pulley system radius as well as some redundancy in the event of a cable failure. Where a plurality of ropes is used this will tend to counter a tendency for the ropes to try to unwind but this can be further prevented by arranging for half the ropes to be twisted in one direction and the other half in the reverse direction.

Although stranded steel wire ropes offer the most obvious means of suspension, the system could make use of synthetic ropes, chains, or any other flexible rope-like material capable of carrying the necessary tensile loads or indeed of some combination of different types of material.

Instead of a simple arrangement where a single cable or a set of parallel cables are used, an alternative will be to use a plurality of pulleys on a common shaft suspended from the chassis at the top of the shaft and a similar set of pulleys attached above the weighted object also on a single shaft, sometimes known as a multiple pulley block, such that a single cable can be threaded around each pair of pulleys to gain mechanical advantage; in fact this is a preferred embodiment as although it involves a need for more cable length, it allows the winch drum to run faster thereby reducing the need for gearing between the winch drum and the electrical motor/generator. For example if there are three pulleys in each set there will be six vertical cable runs and the winch will not only haul in or pay out cable at six times the speed the weighted object moves at, but also with approximately one sixth of the tensile load. The reduction in load allows the use of a cable of smaller cross- section which in turn can be used on smaller diameter pulleys.

In order to generate power for a useful period of time and to maximise the energy storage capacity of the system, the depth of the shaft is important; the deeper the shaft the more energy storage capacity can be provided. In fact, energy storage is proportional to the product of the weight and the substantially vertical movement that is possible. Therefore shafts exceeding 1000m in depth are clearly to be preferred for grid connected electrical applications.

Two important factors are that the weighted object needs to be as dense as possible to minimise the size per tonne. Secondly, because the weighted object necessarily involves a large tonnage, it needs to be strong, yet low in cost. Therefore, the preference is to utilise scrap ferrous materials. Similarly, the weighted object cannot readily be pre-assembled and brought to site because it will be too heavy for conventional transport, so it is intended to assemble the weighted object on-site from either standard ferrous scrap components (for example used railway track rails bundled vertically, or rough cast iron or steel discs stacked around a central support column or placed in a robust prefabricated steel enclosure). A further possibility is to construct the weight in the form of a bucket-like container which may readily be filled with dense loose material such as iron ore or rocks. If more dense materials can be found at a cost that is not disproportionate to the weight, then they can also be used.

Clearly, the suspended weighted object may descend hundreds or even thousands of metres so it is necessary to prevent it from swinging and hitting the sides of the shaft. The use of aligned rails or a close fitting shaft lining as is needed for high-speed, but much lighter mine hoists or lifts could be possible but is likely to be too expensive. Also, to keep costs to a minimum, it is intended to avoid the use of human intervention in the shaft as would be necessary to build and maintain rails or other rigid guide systems. Therefore a preferred guidance system involves tightly stretching one or preferably two or more cables between the chassis at the top of the shaft and the floor at the bottom of the shaft. The weighted object can incorporate sliding sleeve-like devices, preferably lined with a low friction material, fitted around its periphery that loosely enclose the guide cable. The speeds of raising and descent are relatively slow, probably no more than 2m/s, and generally less than this so that friction between the sliders and guide cables will not be a serious issue.

The top end of the or each tension cable can be readily affixed to the chassis across the top of the shaft and can also incorporate some tensioning devices such as screws, hydraulic jacks or other such means of producing a strong tensile force. At the bottom the most effective way to fix the or each tension cable reliably to the floor of the shaft will be to fix heavy weights or a heavy template that is of a width slightly smaller than the width of the shaft. Either the template needs to be heavy enough to permit adequate tensioning due to its inherent weight or else concrete or the like can be dropped into the shaft to bury the template. If the guide cables subsequently need replacement, this process can be repeated several times, although on each occasion the depth of the shaft will be reduced by a few metres corresponding to the depth of concrete or the like, if used. But with a shaft that is hundreds of meters deep loss of a few metres at the bottom will not make much of a difference.

An alternative or possibly an addition to the use of tensioned guide wires to prevent the weighted object from swinging would be to use resilient means, such as rubber-tyred wheels or part thereof, or spring loaded skids with low- friction pads attached extending radially outwardly from the weighted object such that they can roll or slide down the walls of the shaft. These wheels or skids could also be set so as to press against the walls of the shaft by way of radially inwardly acting suspension springs. Nevertheless, the use of a guidance system simply checks any swinging movement and acts as a buffering mechanism as the weighted object ascends or descends. The choice of guidance system depends on the relative smoothness and evenness (or otherwise) of the shaft walls and the verticality of the shaft. Very rough, crumbling and/or uneven walls may preclude the use of method guidance system.

A further possibility to prevent instability while raising and lowering the weighted object is, that if the weighted object is circular cylindrical in cross- section (which is the preferable shape in any case) and suspended from a rotary bearing, then it could be made to spin by reacting a small drive motor against the suspension cable(s) or by adding helical strakes around the outside of the weighted object which would cause axial rotation through interaction with the surrounding fluid in the shaft, be it air or water. This can impart some gyroscopic stability although, of course, in this case neither guide wires nor wheels could be used for guidance.

Many disused mine shafts or other such "holes in the ground" that may prove suitable for this technology and even newly drilled shafts especially prepared for this will be susceptible to flooding due to the penetration of sub-terrain aquifers. It is unlikely to be economic to seek to line such shafts and pump them dry, so it would be preferable that the system could also be applied in a flooded shaft. In such a case, clearly the moving parts will need to be made compatible with submerged operation. Lack of oxygen in deep water combined with suitable anti-corrosion precautions such as high quality paint finishes, use of non-corrodible fittings and possible use of impressed current cathodic protection or sacrificial anodes can prevent serious problems. In fact, the presence of water can be used as an advantage if the lower face of the weighted object is shaped so as to achieve dynamic stability as it descends and ascends and, moreover, water will tend to dampen any lateral movements of the weighted object. However, if the weight is to be submerged in water it will have an inherent buoyancy force and therefore high density of the weighted object is especially important, steel for example has a density of around 7.5 tonne/cubic metre in air but this is reduced to around 6.5 tonnes/cubic metre in water. Thus, a submerged weighted object would need to be approximately 7.5/6.5 tonnes/cubic metre or around 15% larger in volume (and cost) than one to create the same effect in air. The situation for lower density weight materials such as concrete or rock would be proportionately worse. However, the cost of the weighted object will be a more critical factor than its overall size, especially in very deep shafts where its vertical size is a relatively small proportion of the depth of the shaft.

Owing to this system being designed to raise the weighted object using electricity during periods when it is abundant (or at a low cost), and lowered to generate electrical energy when there is a high demand, the system can lend itself to being applied in a so-called "intelligent grid", where demand management techniques are being used. It can, for example, have a control system associated with it, which senses electricity supply and demand (and possibly variations in price) and can therefore be programmed to store and supply electrical energy under predetermined conditions on a completely automatic basis. In this way, if a multiplicity of these storage systems is used, it can assist to stabilise the grid when increasing levels of intermittent generation are in use, such as wind, solar, wave or tidal energy. It can also enable high levels of power to be generated at very short notice with no more than a few seconds delay, and as such, could play an important role in ensuring future grid stability. The control system may also be provided with the ability to vary the speed of movement of the weighted object 6, either to increase or decrease it, in the event that the weighted object 6 starts to oscillate either laterally or torsionally in order to stabilise the weighted object 6.

In order that the present invention may be clearly and completely disclosed, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a schematic view of an energy storage system, Figure 2 is a perspective view of a system similar to Figure 1 ,

Figures 3a and 3b show perspective and plan views respectively of an alternative suspension arrangement to that shown in Figures 1 and 2, Figure 4 shows a perspective view of a further alternative of the suspension arrangement, and

Figure 5 is a perspective view of the suspension arrangement of Figure 4, b shown in more detail.

Referring to Figure 1 , a storage system 2 used for the storage of potential energy and the subsequent generation of electrical energy requires a deep hole 4 in the ground defining a shaft or the like, or other location with scope for considerable vertical movement of a suspended weighted object 6.

Inside the hole (or vertical space) 4 the weighted object 6, generally circular cylindrical in form is suspended from at least one cable 8 (or other flexible tensile load carrier means such as chains, ropes, fibres or wires). The cable(s) 8 or other tensile support hang from a pulley 10 and/or a winch drum 16 mounted above the weighted object 6 on a suitably robust frame 1 2 and bearings 14. Where the pulley 10 is used, the winch drum 1 6 may be mounted to one side, as shown. The winch drum 16, whether mounted to one side or directly over the weighted object 6 is connected to an electric motor/generator (not illustrated in Figure 1 ) either directly by being on the same shaft (possibly with a coupling between them) or by way of a gearbox speed changer between the winch drum 10 and the electric motor/generator.

A key problem is to control the movement of the heavy weighted object 6 so that it does not swing and damage the walls of the hole 4 while being raised or lowered. To this end, one or more guidance cables or other tensile members 18 stretched tightly substantially vertically between the frame 12 and the floor of the hole 4. This can be achieved by having tensioners 20 attaching the guidance cables 18 to the frame 12 and weights or a weighted template 22 that can, if necessary, be buried in concrete 24 or the like poured down the hole 4.

The weighted object 6 could have, for example, tubular low-friction guides 26 that can enclose the guidance cables 1 8 and thus prevent more than minor lateral movements of the weighted object 6 as it slides up and down the guidance cables 18. A key engineering issue will be to avoid resonant lateral or torsional excitation by tensioning the guidance cables 18 correctly. An alternative or an addition to the guidance cables 18 would be to have spring- loaded wheels, rollers or skids (not illustrated) that are deployed radially around the weighted object 6 to engage either continuously or occasionally with the walls of the hole 4 in order to dampen any undesirable lateral movements of the weighted object 6. This latter arrangement will only be possible where the shaft walls are relatively smooth and in relatively good condition, for instance when bored through solid rock.

Alternatively, the weighted object 6 may be in the form of a large container or bucket device (see Figure 5) readily tillable with heavy loose material such as broken up or ground iron ore, or loose rock, such materials being simply dumped into the container in a suitable manner. Instead of securing the guidance cables 1 8 at the bottom of the hole 4 with concrete as previously mentioned, an alternative procedure would be to lower the guidance cables 1 8 attached at their lower ends to a bucket-like chassis frame and when that bucket-like chassis frame is located on the floor of the hole 4 to then tip in a similarly dense material such as iron ore or broken rock in sufficient quantity to weigh down the bucket-like chassis frame such that the guidance cables 18 can sufficiently be tensioned. For example, 1 000 tonnes of heavy material tipped down the hole 4 will allow the guidance cables 18 to be tensioned to 250 tonnes each with a factor of safety of substantially 100% where two guidance cables 18 are used. It may also be useful to use three or possibly four guidance wires 18 arranged, preferably, uniformly around the periphery of the weighted object 6 at a suitable radial distance such that they do not interfere with the lifting cable(s) 8. This has the advantage of inhibiting torsional movement of the weighted object 6 as well as controlling lateral movements more effectively by applying symmetrical correcting forces.

Referring to Figure 2, the winch drum 16 is shown connected to an electric motor/generator unit 17, and further detail of the frame 12 is shown. The cable 8 is shown to extend from the winch drum 16 to the pulley 10, which is in the form of a pulley with a plurality of sheaves (i.e. a plurality of rollers each having a circumferential groove to receive the cable 8) fixed between the bearings 14. A similar pulley 28 with a corresponding number of sheaves to the pulley 1 0 is attached to the upper end region of the weighted object 6, the pulley 28 being similarly mounted between bearings 30. The motor/generator 17 can act as a motor to raise the heavy weighted object 6, and thus uses electrical energy and it can be arranged that when the weighted object 6 is allowed to descend then the unit acts as a generator, thereby storing potential energy and releasing it as electricity on demand. Figures 1 and 2 show a single winch drum 16 with the cable 8 that is looped through sheaves at ground level and attached to the weighted object 6 such that the plurality of loops of the cable 8 tend to reduce the tension in the motor/generator 17 by the number of loops and sheaves provided. This allows the motor/generator 17 to run faster with a reduced load by a factor corresponding to the number of loops/sheaves.

Referring to Figure 3, an alternative embodiment in which, instead of using the single cable 8 looped around sheaves, one or more cables 8' attached to the weighted object 6 are lifted by separate winches 16', one per cable. The winches 1 6' may be located in a row or in an echelon offset arrangement from the hole 4 as illustrated in Figure 3b. The power electronics provided (not illustrated) can be used to equalise the tensions and speeds of the individual winches and cables.

Although the winches 16' are illustrated as being offset in a substantially horizontal plane with the pulleys 1 0' directly over the weighted object 6, it would be possible, as an alternative arrangement, to achieve the desired results by mounting the winches 1 6' across the top of the hole 4 on a suitable beam or other support means directly above the weighted object 6. It would also be possible to have a single pulley 10' from which to hang all the cables 8' to be driven by a single drive, possibly equipped with a gear-type speed reducer to handle the high torque to be expected if one pulley 10' is to carry the entire weight, albeit shared by a plurality of cables.

Referring to Figures 4 and 5, a further alternative to the suspension arrangement to that of Figure 3 is shown in which the winches 16" can be arranged around the periphery of the top of the hole 4 so that the cables 8" and the respective winch drums 1 6" are distributed equi-angularly around the periphery of the heavy weighted object 6. This arrangement may reduce any tendency to swing or twist and thereby reduce the loads on the guidance wires 1 8 (which are not shown in Figures 4 or 5 for clarity). Figure 5 shows how the hole 4 is located in surrounding ground and where the weighted object 6 is moved substantially vertically through the hole 4 while suspended from the cables 8" attached to the weighted object 6 by fixing means 32 and may be lifted or lowered by individual winches 1 6" with cable reels or drums which are supported on the frame 12 spanning the opening at the top of the hole 4.

Claims

1 . An energy storage system comprising a weighted object suspended from a cable, a device for lifting and lowering the weighted object and connected to said weighted object by way of said cable, said device arranged to move said weighted object through a substantially vertical distance, and means for generating electrical energy connected to said lifting device so as to generate electrical energy during a lowering phase.

2. An energy storage system according to claim 1 , wherein the means for generating electrical energy is mounted on a frame.

3. An energy storage system according to claim 2, wherein said means for generating electrical energy is located substantially directly over the weighted object.

4. An energy storage system according to claim 2, wherein said means for generating electrical energy is offset horizontally with respect to said weighted object.

5. An energy storage system according to claim 4, wherein said means for generating electrical energy is offset via at least one pulley.

6. An energy storage system according to any preceding claim, wherein said means for generating electrical energy is mounted over a hole in the ground in which said weighted object is located.

7. An energy storage system according to claim 6, wherein said means for generating electrical energy is mounted on a tower construction.

8. An energy storage system according to any preceding claim, wherein said lifting device comprises a pulley and a winch drum.

9. An energy storage system according to claim 8, wherein said pulley includes a plurality of sheaves.

10. An energy storage system according to claim 9, and further comprising a second pulley including a plurality of sheaves, the second pulley being attached to an upper end region of the weighted object.

1 1 . An energy storage system according to any one of claims 1 to 8, wherein a plurality of cables are used to suspend said weighted object.

12. An energy storage system according to claim 1 1 , wherein said lifting device further comprises a plurality of winch drums corresponding to the plurality of cables.

1 3. An energy storage system according to claim 1 1 or 12, and further comprising a corresponding number of pulley wheels.

14. An energy storage system according to any one of claims 1 1 to 13, wherein the plurality of cables and respective winch drums are distributed equi-angularly around the weighted object.

15. An energy storage system according to any preceding claim, and further comprising a guidance system for guiding the weighted object.

16. An energy storage system according to claim 15, wherein said guidance system includes one or more substantially vertical guide wires.

1 7. An energy storage system according to claim 1 6, wherein said guide wires are attached by tensioners at the top region of the vertical space through which the weighted object is moved.

18. An energy storage system according to any preceding claim, and further comprising a buffering mechanism.

19. An energy storage system according to claim 18, wherein said buffering mechanism includes resilient means.

20. An energy storage system according to claim 18, wherein said buffering mechanism comprises skids deployed radially around the outer periphery of said weighted object.

21 . An energy storage system according to any preceding claim, wherein said weighted object is shaped so as to enable efficient movement through a fluid.

22. An energy storage system according to any preceding claim, wherein said weighted object comprises scrap metallic items.

23. An energy storage system according to any preceding claim, wherein said weighted object comprises an enclosure that is filled with dense loose materials.

24. An energy storage system according to any preceding claim, and further comprising a swivel bearing for stabilising the weighted object gyroscopically.

25. An energy storage system according to claim 24, and further comprising an electrical drive mounted at the region of attachment of the or each cable to the weighted object

26. An energy storage system according to any preceding claim, wherein the weighted object includes fins or strakes to stabilise it when moving.

27. An energy storage system according to any preceding claim, and further comprising a control system.

28. A method of energy storage comprising lifting a weighted object through a substantially vertical distance by a lifting device, thereby storing potential energy, said lifting device connected to means for generating electrical energy, and lowering said weighted object, thereby activating the means for generating, for transforming the stored potential energy into electrical energy.

29. A method according to claim 28, wherein said substantially vertical distance is a hole in the ground in which said weighted object is located.

30. A method according to claim 28 or 29, and further comprising subsequent to said lowering, lifting the weighted object once more for further lowering stages.

31 . A method according to any one of claims 28 to 30, wherein said weighted object is suspended by a plurality of cables.

32. A method according to any one of claims 28 to 31 , and further comprising guiding the weighted object along the substantially vertical distance.

33. A method according to any one of claims 28 to 32, and further comprising buffering the weighted object against impacts.

34. A method according to any one of claims 28 to 33, and further comprising loading said weighted object with scrap metallic items.

35. A method according to any one of claims 28 to 34, and further comprising providing a swivel bearing for gyroscopically stabilising the weighted object.

36. A method according to any one of claims 28 to 35, and further comprising providing a control system to control input and output of electrical energy.

37. A method according to claim 36, wherein said control system senses electrical energy supply and demand.

38. A method according to claim 36 or 37, and further comprising programming said control system to store and supply electrical energy under predetermined conditions.

39. A method according to any one of claims 36 to 38, and further comprising programming said control system to vary the speed of movement of the weighted object.