GB2293149A - Energy storage - Google Patents

Abstract

Energy, for example, that of a vehicle present on braking, is stored by drawing the piston of a cooperating piston and cylinder arrangement against at least a partial vacuum. The partial vacuum is suitably contained by a rolling diaphragm seal between the piston and cylinder. Embodiments are described which provide a near uniform force over the stroke of the piston. <IMAGE>

Description

ENERGY STORAGE This invention relates to energy storage and in particular is concerned with a novel means of storing energy suitable for example, for use with electric road vehicles.

It is to be understood, however, that the invention is not limited to the storage of energy in operation of electric road vehicles.

While electric road vehicles have had a long history, their wider utility has been severely restricted. The limited advances in battery technology which have taken place since electric vehicles were first proposed has meant that even today, the payload which such vehicles can achieve is relatively limited in relation to the weight of the batteries themselves.

As a result, attention has been focused more recently on the design of hybrid systems in which the battery may be supplemented by another source of energy, whether it be purely electric, as for example, solar panel systems, or by the use of a small petrol or diesel engine which may be operated only as and when required to top up the battery.

Attention has also been focused on the possibility of storing or recycling energy otherwise lost, for example in the course of intentional retardation. The normal stop-go cycle of acceleration and braking in town traffic places a heavy load on both motor and battery in electric vehicles.

Any system which can recycle the energy which otherwise would be lost in braking will have a significant effect on the efficiency of operation and may enable the use of a lower rated motor and/or lighter batteries.

Three proposals have been made for temporary energy 4storage systems useful in such vehicles. Primary attention has been directed to use of the vehicle's own main battery which can be charged by using the drive motor as a generator or by using a separate electrical generator. Both these systems have proved inefficient and to have only limited effectiveness.

Flywheel systems in which a relatively massive flywheel is spun by a transmission link from the road wheels have shown considerable promise, but to date in all practical embodiments, the system has proved heavy, complex and very temporary in its ability to store energy. The flywheel loses energy quite quickly due to air friction. Moreover, the incorporation of the massive flywheel in addition to the mass of the batteries further adds to the disproportionate weight of the vehicle in relation to its payload.

Gas spring systems have also been suggested, again to be driven by the road wheels either via a cable or rack and pinion drive or by an hydraulic pump so as to compress the gas spring mechanically. Again, this system, which has the merit over the flywheel system of not adding greatly to the mass of the vehicle, has shown promise; but it is inherently dangerous. In the event of transmission failure or crash impact of the vehicle, even when the gas spring is relatively "relaxed" there may be an explosive effect. In addition, the rate of the gas spring is very variable through its stroke and it suffers both from isothermal compression and near-adiabatic expansion losses.

Notwithstanding the evident advantages which electric road vehicles would have, if widely used, in terms of environmental impact as compared with internal combustion engines, because of the severe limitations in the available technology, to date the use of such electric road vehicles has remained restricted.

As will become clear from the description below, the present invention has addressed the limitations of the available temporary energy storage systems and proposes a novel energy storage device employing a gas spring which is substantially safer than the gas spring arrangements known heretofore.

Thus, in its broadest context, the invention is concerned with a method of storing energy by drawing the piston of a cooperating piston and cylinder arrangement against at least a partial vacuum. The partial vacuum is suitably contained by a rolling diaphragm seal between the piston and the cylinder.

The invention also provides in a second and alternative aspect thereof, an energy storage device comprising a tension gas spring consisting of a cooperating piston and cylinder arrangement in which the piston is adapted to be drawn against at least a partial vacuum contained within the cylinder to store energy.

More specifically, in a preferred embodiment, the energy storage device comprises a tension gas spring consisting of a cooperating piston and cylinder arrangement in which the cylinder has a first essentially closed end and the periphery of the piston is sealed to the cylinder at a position away from said first end by a rolling diaphragm seal so that a variable volume containing at least a partial vacuum is defined between said first end and said piston, the piston being coupled to a tension element adapted for drawing the piston away from said first end against the bias of said vacuum.

As explained below in more detail, in preferred embodiments, the tension gas spring is provided with a friction locking device in the piston so as to assist in ,making the system more fail-safe. It will also be tappreciated that the cylinder itself is clearly substantially safer than prior art gas springs as it contains no potential energy der se. The worst that can happen on catastrophic failure is withdrawal of the piston into the cylinder.

Use of a gas spring with a vacuum rather than a gas which is compressed in the course of use to store energy means that the variable rate of the prior art gas springs is effectively avoided and the losses which arise from isothermal compression and near-adiabatic expansion in the prior art compressed gas springs are avoided. The force needed to withdraw the piston can be virtually constant in operation.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings in which: Fig. 1 shows a first embodiment of energy storage device constructed in accordance with the present invention in a generally schematic longitudinal sectional view; Fig. 2 shows a second embodiment of energy storage device also in accordance with the present invention, in a similar schematic longitudinal sectional view, but with the piston in its most withdrawn position; and Fig. 3 is a generally schematic view illustrating how the embodiment of energy storage device illustrated in Fig.

2 may be coupled to the road wheels of a vehicle, being illustrated partly in section.

The embodiment of energy storage device illustrated in Fig. 1 comprises a tension gas spring consisting of a cooperating cylinder 1 and piston 2 arrangement. The ,cylinder has a first essentially closed end 15. In this embodiment the cylinder, which may be deep drawn, has a rounded and fully closed end 15. The piston has a similarly rounded profile at 16 and, as will be seen from Fig. 1 effectively fully fills the cylinder up to the closed end 15 when fully inserted. Attached to the periphery of the piston is a low friction rolling diaphragm elastomeric seal 3 which is coupled to the open end 17 of the cylinder. What little space there is between the piston and the cylinder in the configuration where the piston is fully inserted into the cylinder is effectively evacuated to eliminate almost all air.A tension element, here in the form of a tension wire or cable 4 is coupled to the piston at 18.

Since in practice, the space between the piston and cylinder cannot be absolutely evacuated, an embodiment such as that shown in Fig. 1 may have a small but measurable gas pressure in the space between the piston and the cylinder in the fully inserted configuration. This small, but residual effect can be largely overcome by increasing the volume of the cylinder, as shown in the Fig. 2 embodiment, so that the piston cannot be fully inserted into the cylinder so that the surfaces of piston and cylinder confront each other at the closed end 15. This is achieved simply by making the cylinder of additional length. The change in vacuum pressure within the cylinder is thus very little as the piston moves. As a consequence, the piston can be withdrawn with a virtually constant force. For a cylinder of approximately one metre length with a 300 mm diameter, we have found that the force needed to withdraw the piston can be a virtually constant 7kN. The stroke of the piston will depend upon the difference in radius of the diaphragm between its connection to the piston and its connection to the cylinder. In a typical case in which that difference in radius is 500 mm, the piston will have a stroke of one metre and so the energy stored will be 7kNm or in other words 7kj.

Neglecting drive losses, such a store of energy can ,accelerate a 500 kg vehicle to 5.3 m/s (approximately 12 mph).

The arrangement shown in Fig. 2 incorporates a friction locking device, generally indicated 5. This device comprises an angled lever 19 pivoted at its angle 20 but normally spring biased so as to bring finger 21 into contact with the interior surface of the cylinder. Tension applied to cable 4 in the direction illustrated rotates lever 19 so as to disconnected the friction finger 21 from the wall of the cylinder. Thus, if all tension disappears in the cable 4, as for example on some catastrophic failure of the vehicle of the system, the friction finger 21 will be engaged with the internal wall of the cylinder so as to slow the re-entry of the piston into the cylinder resulting from the vacuum.

Fig. 3 schematically indicates how the arrangement of Fig. 2 may be incorporated in a vehicle. The tension cable 4 is wrapped around a drum 6 which is coupled to a drive axle 7 mounting road wheels 11 of a vehicle by an epicyclic reduction gearbox generally indicated 8. Sun gear 22 is mounted on the axle 7 while planet gears 23 are carried by a planet gear carrier 9 supported from the drive axle 7 by a bearing 14. Also fixed to the planet gear carrier 9 is a disc 24 of a disc brake 10. Applying the brake fixes the planet gear carrier in position and allows the road wheels 11 to wind cable 4 on to drum 6. The drum 6 is also provided with a ratchet and pawl system 12 preventing the vehicle from being inadvertently reversed by the gas spring as the vehicle comes to a halt, and also prevents automatic acceleration when the brake is released.For acceleration, the planet carrier 9 has to be unbraked and the pawl released by the accelerator pedal, allowing the vehicle to be driven by the tension gas spring via a one way roller clutch 13 mounted between the axle 7 and the drum 6.

In practice, an emergency braking system would also be effected in parallel by means of a conventional braking system acting directly on the road wheels or axle.

Because the illustrated system does not rely on energy being stored by an increase of pressure within a gas spring, the illustrated system enables relatively low strength material to be used for its construction. Care must be taken, of course, to ensure that the cylinder is not too thin that it buckles under the vacuum internally.

Stiffening webs may be incorporated in the exterior of the cylinder to aid in preventing this phenomenon and as a consequence the cylinder may be constructed in what is in effect a relatively lightweight form, thereby providing a system for energy storage which adds little to the overall mass of the vehicle whilst providing a substantial increase in efficiency of use of energy.

Claims (12)

1. A method of storing energy by drawing the piston of a cooperating piston and cylinder arrangement against at least a partial vacuum.

2. A method according to Claim 1, wherein said at least partial vacuum is contained by a rolling diaphragm seal between the piston and the cylinder.

3. An energy storage device comprising a tension gas spring consisting of a cooperating piston and cylinder arrangement in which the piston is adapted to be drawn against at least a partial vacuum contained within the cylinder to store energy.

4. A device according to Claim 3, wherein the cylinder has a first essentially closed end and the periphery of the piston is sealed to the cylinder at a position away from said first end by a rolling diaphragm seal so that a variable volume containing at least a partial vacuum is defined between said first end and said piston, the piston being coupled to a tension element adapted for drawing the piston away from said first end against the bias of said vacuum.

5. A device according to Claims 3 or 4, wherein the piston is provided with a friction locking device.

6. A device according to Claims 4 and 5, wherein the friction locking device comprises a lever pivoted on the piston and associated with a finger spring biased into contact with the interior wall of the piston, the lever also being coupled to the tension element whereby a predetermined tension present in the tension element is sufficient to maintain the finger out of contact with the interior wall of the cylinder, the arrangement being such that loss of tension in the tension element is effective to bring the finger into engagement with the internal wall of the cylinder.

7. An energy storing device according to any of Claims 4, 5 or 6, which is adapted to release stored energy with a generally constant force, the cylinder having a longitudinal length significantly longer than the stroke of the piston whereby to define a space in which said at least partial vacuum is present into which the said piston cannot be extended even when fully withdrawn.

8. A vehicle incorporating an energy storage device according to Claim 4 or any claim appendant thereto, wherein the end of said tension element remote from the piston is adapted to be wound upon a drum as the piston is drawn against said at least partial vacuum, the vehicle being adapted to run on land and having wheels adapted to run on a track or on the ground and a braking system, the vehicle including means for coupling said wheels to the drum when said braking system is applied whereby to store kinetic energy of the vehicle in said energy storing system by winding said tension element upon said drum and drawing said piston from said cylinder, and being arranged so that upon acceleration of said vehicle, said drum is coupled to said wheels in a manner to release energy stored in the energy storing device by rotation of said drum which rotation is transmitted to said wheels to drive the vehicle in a predetermined direction.

9. A vehicle according to Claim 8, wherein said drum is provided with a ratchet and pawl system adapted to prevent inadvertent reversing of the vehicle by the energy storing device as the vehicle comes to a halt and to prevent automatic acceleration of the vehicle merely on release of the brake system.

10. A method of storing energy substantially as hereinbefore described with reference to the drawings.

11. An energy storage device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

12. A vehicle substantially as hereinbefore described with reference to and as shown in Fig. 3 of the accompanying drawings.