FIELD OF THE INVENTION

This invention relates generally to the field of energy storage and in particular to a system, apparatus and method of storing energy such as from an electrical power system.

BACKGROUND OF THE INVENTION

Gravity-based energy storage systems are increasingly being recognised as one method of energy storage and grid balancing that is reliable and efficient. Large scale pumped-hydro is well known, but recently innovations in raising and lowering weights (including in shafts or holes in the ground), and in particular weight and cable systems are offering efficiencies in energy storage as well as benefits in energy capacity to meet local and national grid needs and improved response times.

For example, UK patent no 2509437 describes an energy storage system having a weight suspended from a cable within a shaft to generate electrical energy during lowering of the weight into a shaft in the ground and to consume, for storage, electrical energy during lifting of the weight through the shaft.

A system utilising multiple weights (e.g. in a single shaft in the ground) could increase energy capacity and improve economic efficiency (having less capital expenditure on shafts and land cost), but would suffer from a particular shortcoming, which is that whilst it has an enhanced energy capacity, it is subject to a discontinuity in power input or output during storing (or charging) or releasing (or discharging) across its full energy capacity. The discontinuity is a consequence of there being end points in the vertical transport of individual weights and the associated cable disconnection and cable movement to connect to a subsequent weight and due to the need for the transport of a weight to slow down as it approaches an end storage location. Whilst such a multi-weight system may be effective in increasing the storage capacity of the system and is efficient in making additional energy storage use of a vertical shaft volume, it is not able to deliver continuous output over an extended period of time or across the extent of its storage capacity.

To be economically advantageous and provide high quality power receipt from or supply to an external power system, an energy storage system is advantageously providing a rapid response (i.e. short response time), a large energy capacity and a continuous input/output across that full capacity.

The present inventors have identified improvements in energy storage systems which address the shortcomings of such a system and which improve on the existing art.

PROBLEM TO BE SOLVED BY THE INVENTION

There is a need for improvements in energy storage systems to provide larger capacity and higher quality energy storage via cable and weight gravity-based systems. It is an object of this invention to provide an energy storage system which enables an enhanced energy storage capacity using a gravity-based energy storage system while providing extended duration energy storage or supply.

SUMMARY OF THE INVENTION In accordance with a first aspect of the invention, there is provided an energy storage system comprising: an input connection and an output connection with an external power system; a primary energy storage arrangement comprising a multi-weight gravity- based energy storage system having a primary energy capacity and characterized by discontinuities in power flow capability; and a secondary energy storage arrangement configured for cooperative and/or complimentary operation with the primary energy storage arrangement and having a secondary energy capacity, wherein, preferably, the primary energy capacity and secondary energy capacity together define the system energy capacity and wherein the secondary energy storage arrangement may operate cooperatively and/or in a complimentary manner with the primary energy storage arrangement in order to provide the energy storage system with one or more of: i) a continuous input or output power during a charge or discharge cycle across at least two energy events of the primary energy storage arrangement (an energy event in the primary energy storage arrangement being, for example, a charge or discharge of energy storage associated with moving a weight between its extreme upper and lower positions), which energy events are separated by a discontinuity in output from the primary energy storage arrangement; ii) an enhanced or faster system start-up response, whereby the system may reach a predefined or desired power input/output level by responding to a requirement of an external power system using both the primary energy storage arrangement and the secondary energy storage arrangement simultaneously for the requirement of the external power system; iii) an enhanced system discharge halting capability, whereby the system may rapidly halt discharge to an external power system without a power output surge above a desired system power output, the power output surge being directed for temporary energy storage in the secondary energy storage arrangement; and iv) a power surge input/output capability, whereby for short durations, the system can input or output power at a power level above a power rating of the energy storage system or the primary power storage arrangement.

In a second aspect of the invention, there is provided an energy storage system comprising: an input connection and an output connection with an external power system; a primary energy storage arrangement comprising a multi weight gravity-based energy storage system having a primary energy capacity and characterized by discontinuities in power flow capability; and a secondary energy storage arrangement having a secondary energy capacity and configured for cooperative and/or complimentary operation with the primary energy storage arrangement, the energy storage system having a system energy capacity and configured to provide continuous power input or power output, preferably at a continuous or constant level of power input or power output, during a charge or discharge cycle to the extent, preferably, of the system energy capacity at a pre defined system power rating.

In a third aspect of the invention, there is provided an energy storage system comprising: an input connection and an output connection with an external power system, the system having a system power rating; a primary energy storage arrangement comprising a multi-weight gravity-based energy storage system having a total energy capacity and a primary power rating, which primary energy storage arrangement is characterized by discontinuities in power flow capability during charge and/or discharge cycles; and a secondary energy storage arrangement having a secondary power rating at least equal to the system power rating and having an energy capacity sufficient to accommodate loss or reduction in power flow capability of the primary energy storage arrangement during its discontinuities, wherein the secondary energy storage arrangement is configured or configurable to charge and/or discharge according to requirements of the external power system during periods of discontinuity of the primary system.

In a fourth aspect of the invention, there is provided a method of storing and supplying energy to and from a power system, the method comprising providing an energy storage system as defined above and causing the energy storage system to import or export power to/from external power system in dependence upon the external power system’s requirements.

ADVANTAGES OF THE INVENTION

The energy storage system and method of the invention enable energy storage (charge) and discharge linked to an external power system such as an electricity grid to store energy using a multi-weight gravity-based storage arrangement across the full energy capacity of the system at a desired power level without interruption and also to provide intermittent surge storage/supply to the external system in an efficient and effective manner. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates an energy storage system of one embodiment of the invention;

Figures 2a to 2e illustrate the process in an energy storage system in an embodiment of the invention during a charge cycle;

Figures 3a to 3d illustrate the process in an energy storage system in an embodiment of the invention during a discharge cycle;

Figure 4 illustrates the process in an energy storage system in an embodiment of the invention during a charge event with a power surge;

Figure 5 is a representation of power against time for primary and secondary energy storage sub-systems of an energy storage system of an embodiment of the present invention;

Figure 6 is a representation of power against time for primary and secondary energy storage sub-systems of an energy storage system of an embodiment of the present invention in which the secondary storage sub-system is configured to enhance the system response;

Figure 7 is a representation of power against time for primary and secondary energy storage sub-systems of an energy storage system of an embodiment of the present invention in which the secondary storage sub-system is configured to enhance the sharp stop of power discharge of the system.

Figures 8A to 8G illustrate in front, side and cross-sectional and perspective views of a weight engaging and disengaging mechanism of a transporter linkage in disengaged and engaged configurations according to an embodiment of a system of the invention;

Figure 9 illustrates in cut-away perspective view a transporter linkage of Figures 8A to 8G;

Figure 10 illustrates in perspective view a weight of Figures 8 with transparency to illustrate a near side docking; and

Figure 11 is a perspective view of a suspended weight support system of one embodiment of the energy storage system. DETAILED DESCRIPTION OF THE INVENTION

The invention according to the first aspect (and each of the aspects referred to above, in relation to which the following features and description is written unless specific or clear to the contrary) is for an energy storage system. It comprises an input connection and an output connection, which may preferably be a single connection point, to an external power system. An external power system may be any system having a power demand, such as an electricity grid (e.g. local power grid or national grid), whereby the energy storage system can provide short term energy storage with quick response to both supply and demand from the grid. The energy storage system may typically be defined as having a system power rating (Ps) which is the power it can supply to (or receive from) an external power system at a continuous level across the full energy capacity of the system.

The energy storage system of this aspect comprises a primary energy storage arrangement which is a multi-weight gravity-based energy storage arrangement having a primary energy capacity and which is characterized by discontinuities in power input/output capability during charge/discharge cycles of the energy storage system. It further comprises a secondary energy storage arrangement, typically having a secondary power rating at least equal to the system power rating. The secondary energy storage system is configured for cooperative and/or complimentary operation with the primary energy storage arrangement and has a secondary energy capacity.

The primary energy capacity and secondary energy capacity together typically define the system energy capacity, which may be taken to be the sum of the primary energy capacity and the secondary energy capacity, subject to operational losses.

The primary energy storage arrangement may be any suitable multi-weight gravity-based energy storage system having a primary energy capacity. The primary energy storage arrangement as defined herein is characterized by discontinuities in power flow capability.

Preferably the multi-weight gravity-based energy storage system comprises a plurality of weights movable, successively (and/or sequentially), between respective first upper positions and second lower positions, the first upper and second lower positions defining respective vertical displacements for each weight. Depending upon how the weights are stored at the upper and lower positions, the respective vertical displacements for each weight may be the same or different. For example, the weights may be simply stacked on the base of the shaft in which case the respective vertical displacement for successively lowered weights will be less than the preceding weight, assuming they all start from a similar position at the top of the shaft. The primary energy storage system preferably comprises a transporter for transporting a plurality of weights individually respective pre-defmed paths between respective first upper and second lower positions. The pre-defmed paths of the respective plurality of weights define path volumes (being the volume of a weight swept along its pre- defmed path). The paths and/or path volumes associated with each weight preferably overlaps with that of at least one other weight and preferably all other weights in the multi-weight energy storage system. Preferably, all the weights follow the same paths (albeit potentially of different lengths), a single pre-defmed path. The transporter preferably comprises a transporter linkage which may be coupled to and decoupled from each respective weight of the plurality of weights for mechanically linking the respective weights to the transporter.

The pre-defmed path according to this embodiment may be vertical or an angle to the vertical whereby the weights may be moved between their upper and lower positions by way of a trolley and rail arrangement or otherwise in the case of an angled path to the vertical or, by suspended raising and lowering through a vertical path. Preferably the pre-defmed path is a vertical path directly corresponding to the vertical displacement of the weights between their upper and lower positions.

Preferably, in the primary energy storage arrangement, a transporter is a winch and cable (or chain, preferably cable) arrangement and the pre-defmed path is along a vertical path, which preferably comprises a vertical path within a shaft, such as a shaft sunk or present in the ground (e.g. existing mine shafts, which may optionally be lined). Preferably, the winch and cable arrangement comprises at least one winch disposed at or in relation to a shaft opening at the top of a shaft. A cable may be wound and unwound by the winch to enable raising and lowering the weights. The cable may be mechanically linked to the weights by a cable linkage which may be clamp or latch system (or an electromagnetic latching or linkage system) which may be coupled and decoupled from the weights as required.

Preferably, the primary energy storage system comprises a single transporter arrangement capable of raising or lowering only one weight at a time. The single transporter arrangement may comprise a pair of winches, preferably two pairs of winches and optionally more disposed about the top of the shaft.

A winch and cable arrangement may comprise any arrangement of sheaves or pulleys in order to configure the arrangement as desired and as may be understood by a person of ordinary skill in the art.

By vertical displacement, it is meant the vertical distance between a first upper position and a second lower position.

In one embodiment, a cable linkage is a linkage member adaptable between a first configuration in which it is coupled with a weight, e.g. via a linkage dock on the weight (e.g. for receiving a gripper, clamp or other connecting member), and a second configuration in which it is decoupled. Preferably, when decoupled from a weight, the linkage member is capable of being raised and lowered by a winch and cable. In a preferred embodiment, the linkage member in its first configuration has a protruding member, for example in the form of a sliding plate, extendable from the side of a body portion of the linkage member by way of one or more extending arms or actuators so that preferably it moves laterally from the body portion. When aligned with a linkage dock of a weight, the protruding member may be actuated to cause the linkage member to adopt its first configuration whereby the protruding member enters into a recess formed in a side surface of the weight. Preferably, where the protruding member is in the form of a sliding plate or otherwise has upwardly orientated elements, the recess has an upwardly orientated slot such that when the linkage member is then raised, the upwardly orientated elements (e.g. top edge of the sliding plate) of the protruding member engage into the slot of the recess thereby coupling the linkage member with the weight. The linkage member can be decoupled by reversing the process. Preferably actuation and deactuation are enabled via a hydraulic actuator, although it may alternatively be enabled by electric actuators or other methods of attaching or detaching the weight from the lifting cables, such as servo-operated grabbers or hooks engaging with holes or other structural members/handles on the weight.

The primary energy storage arrangement preferably comprises a means for converting gravitational potential energy in a raised suspended weight into electrical energy during lowering of the weight and for converting electrical energy into gravitational potential energy by raising a weight. Any suitable means for doing this can be used.

Preferably, the energy storage system comprises a motor or motors to drive the one or more transporters, which are preferably winch and cable arrangements, to lift a weight along its path between a lower (discharged energy) position and an upper (stored energy) position. This can charge the system. Preferably, the energy storage system comprises a generator to generate electricity for exporting to an external power system from energy output resulting from lowering a weight between an upper position and a lower position. This can discharge the system. Preferably, the generator is coupled with a winch of a winch and cable arrangement. Optionally, the motor and the generator are a single configured arrangement coupled with one or more winches.

For a given mass of a weight, the power input and output from the primary energy storage arrangement is proportional to the speed the weight moves through a vertical displacement. Ideally, the speed may be varied to enable changes in the input or output of power (e.g. according to a requirement of an external power system).

Preferably, the winch has an unladen or decoupled winching speed (i.e. when not attached to a weight) in the range of from 2.5 m/s, preferably from 5 m/s and more preferably from 10 to 35 m/s, more preferably from 12 to 30 m/s, still more preferably from 15 to 25 m/s and optionally up to 20 m/s.

The multi-weight gravity-based energy storage system of the primary energy storage arrangement comprises at least two weights, preferably at least three weights and more preferably more than three weights, for example, at least 5 weights, preferably at least 10 weights and optionally at least 20 weights, such as at least 25 weights e.g. up to 50 weights. The appropriate number of weights can depend upon the depth of the shaft, the size (e.g. mass) of the weights and the energy capacity desired. In one embodiment, the system comprises from 6 to 20 weights. In another embodiment, the system comprises from 15 to 50 weights. For a given energy storage requirement, the primary energy storage arrangement may be configured by considering the relative consequences (or costs) of larger weights versus more weights versus degree of displacement (e.g. depth of shaft) and balancing those factors. Generally speaking, however, increasing the number of weights of a size that may be handled by the infrastructure used may be considered better since it results in an increase in energy storage capacity.

Preferably, the primary energy storage arrangement comprises a shaft, at least a portion of the path of which provides the pre-defmed path for the movement of weights between their respective upper and lower positions.

The shaft may be formed in a tower, built above ground, or in a shaft or hole dug into the ground or partly formed in a tower above ground and a shaft in the ground. Optionally, a shaft may be formed against a face, e.g. a face of a cliff or a face of a tall building, according to which embodiments, a shaft housing may preferably be built against such a face. Preferably, the shaft is an enclosed shaft. A shaft may be enclosed, for example, by the walls of shaft dug in the ground, which is preferably lined, or by a tower housing, or a housing formed against a face (of a cliff or building) or a combination thereof. Preferably, the shaft is formed in the ground. A shaft in the ground for use in the system may be sunk specifically for the system or may be a redeployment of a pre-existing shaft (e.g. mine shaft).

A shaft for use in any storage system of a preferred embodiment of the invention may be of any suitable depth, e.g. from 10 m or from 50 m, but is preferably in the range of from 100 m to 4000 m. For example, a relatively shallow shaft may be provided that has a depth of up to 500 m, e.g. 100 m to 350 m. Alternatively, and preferably, a relatively deep shaft may be provided that has a depth of greater than 500 m, preferably greater than 1000 m, e.g. from 1250 to 3000 m. The shaft may have any suitable width (or cross-sectional extent) provided it can accommodate the movement of the weights. Preferably, they are in the range 1.5 m to 20 m, preferably at least 3 m, more preferably from 5 to 15 m, more preferably up to about 10 m.

In one implementation of the energy storage system of the present invention, the primary energy storage arrangement may be deployed in pre existing or modified existing shafts. These may be in the region of 300 to 5000 m and preferably have a diameter of 3 to 10 m. For example, pre-existing shafts (e.g. for existing coal shafts), may have a depth of from 300 to 1200 m and preferably a diameter of 5 to 9 m. Alternatively (e.g. for disused metal mine shafts in the UK), the depth may be from 50 to 750 m and preferably having a diameter of from 2.5 to 6m, preferably 3 to 5 m (often a square or rectangular shaft of, for example 3 m by 5) m. In a further alternative, e.g. for pre-existing heavy metal/anhydrite mines in the UK, the shaft depth may be up to 1200 m and have a circular shaft of 5 to 8 m diameter. In a still further alternative, very deep shafts (e.g. pre-existing gold/heaving metal shafts, such as those in South Africa) may have a depth of, for example 1000 to 3000 m and preferably a diameter of 5 to 9 m.

In another implementation, new shafts may be sunk for the system. In one such embodiment, the new shaft may be from 30 to 120 m deep and preferably is sunk using traditional caisson technique. Such shafts may typically be from 6 to 25 m in diameter and generally circular in cross-section and preferably concrete lined. In another such embodiment, the shaft may be from 100 to 350 m deep and preferably sunk using automated vertical shaft sinking technology. Preferably these are generally circular in cross-section and have a diameter of 6 to 18 m. Again, the shaft is preferably concrete-lined. In a still further such embodiment, the shaft may be from 100 to 700 m and preferably formed by reverse circulation (e.g. pile top drilling) through hard rock.

Preferably, this is generally circular in cross-section and has a diameter of from 3 to 8 m. Preferably, the shaft is slip-form concrete lined.

Generally, it is preferred that new shafts sunk for the system have a diameter in the range from 4 to 10 m and a depth of from 50 to 750 m, preferably 6 to 10 m diameter and 50 to 250 m deep. For new shafts sunk in hard rock, it is preferred that the diameter is in the range of 4 to 6 m and the depth is from 400 to 700 m.

Preferably, the shaft has a cross-sectional area that is no more than 75% greater than the swept path area of the weights and the associated transporters (e.g. cables and linkages), preferably no more than 50% greater and optionally in the range of 5 to 30% greater, optionally at least 10% greater, e.g. up to 20% greater.

Preferably, the primary energy storage arrangement has a fully charged (that is, energy storage at capacity) configuration in which all the weights have been raised to their respective upper positions and fully discharged (that is, energy storage at a minimum) configuration in which all the weights have been lowered to their respective lower positions.

The weights may be stored at or in relation to the upper and lower positions by any suitable means. For example, the weights may be stored in their respective path volumes at the bottom of the shaft (e.g. stacked on top of one another) and/or at the top of the shaft (e.g. one below another and retained by some retaining mechanism that allows them to be retained in a suspended position and still allows, for example, cables to pass). Weights may be stored in an upper storage space associated with the upper position, but outside the respective path volume at or close to the top of the shaft and/or in a lower storage space associated with the lower position, but outside the respective path volume at or close to the bottom of the shaft.

According to one embodiment, which is preferred, when in the fully discharged configuration, a first weight, when decoupled from a transporter (e.g. cable arrangement), may be disposed on a base of the shaft and a second weight, when decoupled from the transporter (e.g. cable arrangement), may be disposed on top of the first weight at the base of the shaft and any third or further weights are disposed or stacked on the respective preceding weight at the bottom of the shaft. The path volumes for each weight in this case may thus be different (depending upon how the weights are stored at the top of the shaft) with (assuming a common upper position, outside of the shaft, for the weights) each successive lowered weight having a smaller path volume and a lower energy storage capacity (assuming the weights are of equal mass).

In another embodiment, when in fully discharged configuration, the decoupled first weight may be stored in a base storage space associated with the shaft outside of the vertical path of the shaft and preferably the decoupled second and any third or further weights are stored in a storage space associated with the shaft outside of the vertical path of the shaft. Any suitable base storage space may be used, but preferably a base storage conveyor is provided to transport each of the first weight and preferably the second and any third or further weights from their respective lower positions to a storage space outside the vertical path of the shaft. The storage conveyor may take any suitable form, such as a rail track with movable platforms thereon, a conveyor belt, or a carousel which rotates a plurality of storage spaces through the shaft so that weights may be lowered into a free storage space at the base of the shaft and then rotated into a storage space revealing a further free storage space at the base of the shaft for receiving the next weight.

According to one embodiment, when in a fully charged configuration, a first weight, decoupled from the transporter (e.g. cable arrangement), is stored within its path volume at its upper position and typically within the vertical path of the shaft, the second weight, when decoupled from the transporter (e.g. cable arrangement), is disposed vertically adjacent to the first weight within its path volume at its upper position (typically within the vertical path of the shaft) and any third or further weights are disposed adjacent to the respective preceding weight within the same vertical path at their respective upper positions. This may be within the shaft or within a vertical path volume above the shaft opening.

According to another embodiment, when in a fully charged configuration, the first weight, decoupled from the transporter, is stored in an upper storage space outside of its path volume and outside the vertical path of the shaft, the second weight, decoupled from the transporter, is stored in an upper storage space outside of the vertical path of the shaft and any third or further weights are stored in an upper storage space outside of the respective vertical path volumes. Preferably, an upper storage conveyor is provided to transport each of the first weight and preferably the second and any third or further weights from their respective upper positions to a storage space. The upper storage conveyor may comprise any suitable means such as a gantry crane and matrix storage arrangement and/or a carousel and/or a rail track with flat wagons.

In one embodiment of the invention, where the weights are disposed and stacked on a base of the shaft in a discharged configuration, the base is effectively provided by a suspended platform suspended at a position above the actual base of a shaft. This is described in more detail below.

The weights may be of any suitable mass according to, for example, the total amount of energy storage required (having regard for the vertical displacement provided by the primary energy storage arrangement).

The second weight and any third or further weights are preferably identical or approximately the same in mass (and ideally in other factors, such as dimensions) as the first weight, but in some cases they might be different. For example, if different, it is still preferable that the second weight and preferably third and any further weights have a mass that is within 30% of the mass of the first weight, preferably within 20%, more preferably within 10% and still more preferably within 5% and most preferably within 1% of the mass of the first weight.

The weights may have any suitable mass, according to the particular requirements of the system. In one embodiment, the first weight has a mass in the range of from 25 to 1000 tonnes, preferably 50 to 500 tonnes. The weights are typically sized according to the particular application and that can be effectively handled by the system. Preferably, the weights are of similar or identical mass to one another. A typical weight, designed to provide a significant energy capacity in a suitable shaft may be, for example, from 250 to 750 tonnes, e.g. about 500 tonnes.

In one example, the primary energy storage arrangement comprises a vertical shaft (i.e. vertical passage such as a hole in the ground) and disposed in relation thereto a winch and cable arrangement as described above for raising and lowering multiple weights successively by way of storage of energy and release of stored energy and is applied in a shaft produced for this purpose. The dimensions selected for a newly sunk shaft may depend upon several geological and commercial factors. In one example, a newly sunk shaft, of generally circular cross-section, could be sunk with an inner diameter of 10 m and a depth of 200 m. The primary storage arrangement according to this embodiment may comprise weights of, say, 550 tonnes per weight. If 26 weights were incorporated into this primary energy storage arrangement, stored within the shaft (stacked on top of one another) at their lowest point when discharged and stored outside the shaft when fully charged, the primary energy storage arrangement could provide a deliverable energy storage capacity of 6.6 MWh assuming a generation efficiency of 94%. A similar energy capacity could also be achieved with a larger number of smaller, lower mass weights.

Discontinuities in the power input/output of the primary energy storage arrangement may be any interruptions or operations which result in interruption in power input/output between the primary energy storage arrangement and the external power system or grid to which the system is connected. During such discontinuities, no power is input or output to the external power system from the primary energy storage arrangement. Further, during the raising and lowering of weights in a primary energy storage arrangement, there is typically a short energy transition phase as the speed of movement of the weight is reduced from its input/output speed (associated with the desired power input/output) to zero when the weight comes to rest (and the input/output energy discontinuity period begins). A similar transition phase occurs from the point of starting from zero movement to movement at the desired speed.

The duration of a discontinuity is the period of time between one charging or discharging weight coming to rest and a successive charging or discharging weight starting to move. By charging or discharging weight, it is meant a weight in the primary energy storage arrangement being raised or lowered respectively.

The duration of a transition phase is the period of time from a weight at rest to reach its desired power input/output speed or the period of time from a weight at its desired power input/output speed to come to rest once it starts to reduce speed.

An energy event in the primary energy storage arrangement is preferably defined as a charge or discharge associated with moving a weight between its extreme upper and lower positions. An energy event may thus be a charge event or a discharge event.

The primary energy storage arrangement typically has an interrupted or inconsistent power input/output profile over two or more successive energy events as a result of the power input/output discontinuity between energy events and the energy transition phase at the beginning and end of each energy event. Preferably, in a system of the present invention, the primary energy storage arrangement is so configured that the duration of two transition energy phases of an energy event is not more than 20% of the duration of a discontinuity, preferably no more than 10% and more preferably no more than 5%.

In a system of the present invention, an energy gap may be defined as the energy shortfall in power input/output during a period in which the power input/output of the primary energy storage arrangement is below the desired power output of the system (e.g. the rated power of the system) across successive energy events (without the additional energy of the secondary energy storage arrangement). This may be considered to be a discontinuity energy shortfall derived from the duration of the discontinuity for which there is a required power output of the system (e.g. up to the rated power of the system) plus a transition phase energy shortfall, which transition phase energy shortfall is preferably no more than 20%, more preferably no more than 10%, still more preferably no more than 5%, more preferably from 0.5% to 3%, such as from 1% to 2% of the energy gap-

A maximum energy gap may be defined as the energy shortfall in power input/output (when theoretically relying solely on the primary energy storage arrangement) during a period in which the power input/output of the primary energy storage arrangement is below the rated power of the system across successive energy events. The duration of the discontinuity in a primary energy storage arrangement comprising a multi-weight gravity-based energy storage system having a winch and cable transporter comprises the duration of winching the decoupled cable between extreme upper and lower positions plus the duration of coupling and decoupling. The duration of the winching between upper and lower positions is a function of the winch speed when decoupled (and the distance between upper and lower positions, e.g. the depth of a shaft). Preferably, for most primary energy storage arrangements, especially with shafts that are sufficiently deep, the speed of winching is the rate determining step. Preferably, the sum of the duration of coupling and decoupling is no more than 20% of the duration of the discontinuity, more preferably no more than 10%, still more preferably no more than 5% and most preferably no more than 2% and ideally no more than 1%. Typically, the duration of coupling/decoupling will be at least 0.5% of the duration of the discontinuity and on occasion at least 3%.

The multi-weight gravity-based primary energy storage arrangement of the system of the invention preferably has a fast response time. Typically, the primary energy storage arrangement can go from zero to its desired or system rated power input/output in less than 1 second, e.g. from 0.8 seconds to 1 second. This lag may be a result of one or more of inertia, ramp up rate of weight/winch speed to full input/output speed, signal delays, motor coil magnetization periods etc.

A multi-weight gravity -based cable and winch primary energy storage arrangement may also be stopped rapidly during charge and discharge cycles, e.g. within up to 2 s and preferably within about 1 s. A rapid stop during discharge for example can result in a sharp, but short increase in power output.

Preferably, the system of the present invention is configured such that the secondary energy storage arrangement may operate cooperatively and/or in a complimentary manner with the primary energy storage arrangement in order to provide the energy storage system with one or more of: i) a continuous input or output power during a charge or discharge cycle across at least two energy events of the primary energy storage arrangement which energy events are separated by a discontinuity in output from the primary energy storage arrangement; ii) an enhanced (e.g. faster) system start-up response, whereby the system may reach a predefined or desired power input/output level by responding to a requirement of an external power system using both the primary energy storage arrangement and the secondary energy storage arrangement simultaneously for the requirement of the external power system; iii) an enhanced system discharge halting capability, whereby the system may rapidly halt discharge to an external power system without a power output surge above a desired system power output, the power output surge being directed for temporary energy storage in the secondary energy storage arrangement; and iv) a power surge input/output capability, whereby for short durations, the system can input or output power at a power level above a power rating of the energy storage system or the primary power storage arrangement.

In a preferred embodiment of the invention, the secondary energy storage arrangement may operate cooperatively and/or in a complimentary manner with the primary energy storage arrangement in order to provide the energy storage system with a continuous input or output power during a charge or discharge cycle across preferably at least two energy events of the primary energy storage arrangement which energy events are separated by a discontinuity in output from the primary energy storage arrangement. More preferably, the energy storages system is thus configured to provide continuous input or output power during a charge or discharge cycle across the full extent of storage capacity of the system.

The continuous input or output power during a charge or discharge cycle of this embodiment may be provided typically according to a desired power level or power level required by the external system (e.g. constant or variable requirements). Optionally the continuous input or output power is at a pre-defmed and optionally constant power level. Preferably, the system is thus capable of providing the continuous power output at a constant power level, which is preferably the level of a pre-defmed system rated power, Ps. The second energy storage arrangement, according to this embodiment has an energy capacity sufficient to accommodate loss or reduction in power input/output capability of the primary energy storage arrangement during its discontinuities and preferably for the duration of the energy gap of the primary energy storage arrangement.

The second energy storage arrangement is preferably sized to provide continuity in power input/output during a charge or discharge cycle across two energy events, preferably at a constant desired or pre-defmed power level, e.g. the system rated power Ps. Thus, the energy capacity of the secondary energy storage arrangement is preferably at least corresponding to the discontinuity energy (e.g. the duration of the discontinuity of the primary energy storage arrangement times the system rated power Ps) and preferably at least corresponding to the system energy gap, preferably the maximum energy gap. . The second energy storage arrangement may have an energy capacity of up to 10 times the maximum energy gap, preferably up to 5 times the maximum energy gap, more preferably up to 2 times the maximum energy gap and optionally up to 1.8 or 1.5 times the maximum energy gap. Preferably, the secondary energy capacity is at least 1.05 times the maximum energy gap and more preferably at least 1.1 times the maximum energy gap and optionally at least 1.5 in some embodiments. The maximum energy gap may be approximated for these purposes to be the maximum discontinuity energy (i.e. the maximum discontinuity energy shortfall) since the transition phase energy shortfall typically represents about 1- 2% and often no more than 5% of the maximum energy gap. Accordingly, the secondary energy capacity may be preferably at least 1.1 times the maximum discontinuity energy.

By minimizing the energy storage capacity of the second energy storage arrangement, such as within the aforementioned bounds, the levelized cost of storage of energy across the full energy capacity of the system can be minimized while preferably maintaining system performance, including continuity of power output across the full energy storage capacity of the primary energy storage system. In a preferred embodiment of the invention, the secondary energy storage arrangement may operate cooperatively and/or in a complimentary manner with the primary energy storage arrangement in order to provide the energy storage system an enhanced system start-up response, whereby the system may reach a predefined or desired power input/output level by responding to a requirement of an external power system using both the primary energy storage arrangement and the secondary energy storage arrangement simultaneously for the requirement of the external power system. Preferably, this is provided in addition to the continuous power input/output capability embodiment described above.

According to this embodiment, the system may preferably be configured to reach a predefined or desired power input/output level, e.g. system rated power Ps more quickly than can be achieved by the primary power storage arrangement alone, preferably within 0.75s, more preferably in up to 0.5 s, still more preferably in up to 0.4 s and still more preferably in up to 0.25 s.

Preferably, in an embodiment in which the system provides enhanced system start-up response in addition to the continuous input/output power capability, the system will be configured to ensure that any change in the energy stored in the secondary energy storage system is accounted (e.g. corrected) for during trickle charge or discharge of the secondary energy storage system during the following system charge or discharge cycle or occasion. Alternatively or in addition, the secondary energy storage system may optionally be sized to provide a little additional energy capacity to that in the embodiment above, such as a further 1 to 15% of the maximum energy gap as additional secondary energy capacity, more preferably up to a further 10% capacity, such as a further 3 to 8% additional energy storage capacity and most preferably an additional 5% of maximum energy gap as additional secondary energy capacity than in the embodiment above.

In operating the system according to this embodiment, when the system receives a demand for power (for example, but may equally well be for storage) at a power level Ps with an immediate requirement from an external power system, the system is configured such that both the secondary and the primary energy storage systems are operated to meet that demand, both of which are immediately ramped up in power output. In this case, the system power output is ramped up more quickly than either of the primary or secondary storage systems can ramp up individually and the system reaches the desired power output in a shorter time. In a typical situation in which the response time of the secondary energy storage system is faster than that of the primary energy storage system (and indeed in any situation), the system will be configured such that the secondary energy storage system ramps toward full power output (being responsible for the majority of the initial power output) and then once the combined power output of the system reaches the required power level, the secondary system will be ramped down again to zero. Thereafter, the primary energy storage system will provide the output power required by the external power system and preferably the primary energy storage system will output a marginal amount of additional power to trickle feed the secondary energy storage system by an appropriate amount.

In another preferred embodiment of the invention, the secondary energy storage arrangement may operate cooperatively and/or in a complimentary manner with the primary energy storage arrangement in order to provide the energy storage system with an enhanced system discharge halting capability, whereby the system may rapidly halt discharge to an external power system without a power output surge above a desired system power output, the power output surge being directed for temporary energy storage in the secondary energy storage arrangement. Preferably, this is provided in addition to the continuous power input/output capability embodiment described above and preferably also in combination with the enhanced system response start-up embodiment above.

An enhanced system discharge halting capability according to this embodiment , can preferably achieve a rapid halting of the power output from about 1 s power output halting of the primary energy storage system alone to up to 0.75 s, more preferably in up to 0.5 s, still more preferably in up to 0.4 s and still more preferably in up to 0.25 s.

This embodiment serves two functions which it may do together or independently. Firstly, it serves to accommodate a peak or surge in output above the desired system power output that may otherwise occur when the primary energy storage system is brought to a sharp halt. This is achieved by configuring the system to divert any halt-related surge in power output during the primary energy storage system halt into the secondary energy storage system. This way, the external power system is not delivered more power than has been required. Secondly, it serves to provide a significantly enhanced halt of system power output on demand, since in addition to the reduction of power output from the primary system as a result of halting the primary system, which can be achieved in up to or about Is, the secondary system ramps up its storage to accommodate power output from the primary energy storage system during the halt procedure, thereby enhancing the system power output halt response.

Preferably, in an embodiment in which the system provides enhanced system discharge halting in addition to the continuous input/output power capability, the system will be configured to ensure that any change in the energy stored in the secondary energy storage system caused by the implementation of the enhanced power discharge halting is accounted (e.g. corrected) for during trickle charge or discharge of the secondary energy storage system during the following system charge or discharge cycle or occasion. In particular, should the next energy cycle be a discharge cycle, trickle feed from the primary energy storage arrangement to the secondary energy storage arrangement may be slightly reduced until the imbalance is corrected. Alternatively, should the next energy cycle be a charge cycle, the flow of power from the secondary energy storage arrangement to the primary energy storage arrangement by way of additional trickle charge will be increased to ensure that the secondary energy storage level is adequately depleted to enable continuity in system charging.

In a preferred embodiment of the invention, the secondary energy storage arrangement may operate cooperatively and/or in a complimentary manner with the primary energy storage arrangement in order to provide the energy storage system with a power surge input/output capability, whereby for short durations, the system can input or output power at a power level above a power rating of the energy storage system or the primary energy storage arrangement. Preferably, this is provided in addition to the continuous power input/output capability embodiment described above and preferably also in combination with one or both of the enhanced system response start-up embodiment and the enhanced system discharge halting embodiments above.

Preferably, according to this embodiment, the system can deliver a surge power input/output capability that provides input/output power at a power level above a system continuous power rating, Ps (i.e. the power at which the system may be configured, according to a preferred embodiment, to provide continuous power input/output across the extent of the energy storage capacity of the system). Typically, a power surge capability may provide input/output power at a power level above the power rating of the primary energy storage arrangement. Optionally, a power surge may be in excess of 20% more than the system continuous power rating, e.g. in excess of 50%. The power surge may, for example, be up to 2 times system continuous power rating.

In operating the system according to this embodiment, in response to a requirement, e.g. for a surge power output to an external power system (such as a power grid), the system causes both the primary and secondary energy storage systems to output power to the external power system up to the surge power level and in excess of the system continuous power rating. The output will typically be configured such that the primary energy storage arrangement is operating at full power output (e.g. at its power rating) with any additional power to achieve the surge power demand being provided by the secondary storage arrangement, in order to maximise the duration at which the surge power output can be provided at that level. Respective power output from the primary and secondary energy storage arrangement may be configured to vary over the duration of the surge requirement while delivering the surge power from the system depending upon whether, for example, an enhanced response was required. Accordingly, (assuming the secondary power storage arrangement has a faster response time) in response to a surge power demand, the primary and secondary storage arrangements will be ramped up toward full power (with the secondary ramping up quicker) until the surge power level is reached at which point the primary power output will continue to rise toward its maximum of rated power output and the secondary power output will fall to the minimum output to achieve the surge power demand for the system. A corresponding process is available for surge storage requirements.

The duration of the surge power capability provided by the system will depend upon the surge power level, but the minimum duration will depend upon the secondary energy capacity available at the time of the surge request for power at, for example, 2Ps, while the maximum duration of a surge power output is the duration of an energy event of the primary energy storage arrangement.

Optionally, the provision of a power surge capability may be enhanced by providing an increase in the secondary energy storage capacity. For example, it may be provided with an additional 5 to 50% of the maximum energy gap over the capacity described for the continuous power capability embodiment above. Preferably, however, imbalances between the primary and secondary energy storage arrangements over what is ideal for providing continuous power input/output may be corrected for during the next charge/discharge cycle through adjusted trickle charge/discharge of the secondary energy storage arrangement from/to the primary energy storage arrangement or a correction between the primary and secondary energy storage arrangements while the system is dormant (not inputting or outputting power to the external system).

Features of the energy storage system of the invention (all of the aspects above) are described preferably in relation to the continuous power capability embodiment but also in relation to the other embodiments.

The energy storage system provides input and output power from and to an external power system via an input connection and an output connection. The system is preferably configured so that the primary energy storage arrangement can provide power to the external power system via the input/output connection and provide at least a portion of the primary energy storage power output to the secondary energy storage arrangement. Preferably power from the external power system to be stored may be stored in either the primary energy storage arrangement (e.g. during a charge event of the primary energy storage arrangement) or in the secondary energy storage arrangement (e.g. during a power input/output discontinuity of the primary energy storage arrangement). The secondary energy storage arrangement may be configured to supply power to the external power system via the input/output connector or to the primary power arrangement or to receive power from the external power system or the primary energy storage arrangement. Preferably, the primary energy storage arrangement and the secondary energy storage arrangement are connected to each other and to the system input/output connector via an interconnection arrangement.

Preferably, the system comprises a control system for controlling cooperative or complimentary operation of the primary and secondary energy storage arrangements according to the requirements of the external power system (and in relation to one or more of the embodiments described above). The control system preferably coordinates the charge and discharge of each of the primary and secondary energy storage arrangements to ensure continuity of power input/output across more than one energy events of the primary power system and preferably at a continuous power level, such as the system rated power.

Preferably the controller controls the output of the primary energy storage arrangement to be at a power output higher than the system power rating by an amount in order that a portion of the power can be stored as energy in the secondary energy storage arrangement. Over the course of a discharge event, the amount of energy fed to the secondary energy storage system as a result of this supplementary primary power output is preferably sufficient to cover any energy gap shortfall and preferably is sufficient energy such that the secondary energy storage arrangement can output power to the external power system at the system rated power for at least the duration of the discontinuity of the primary energy storage system.

Preferably, the controller (e.g. a processor and software configured to control the system) controls the operation of the transporters (e.g. winches and cables) and movement of the weights in response to a requirement for storage of energy from or discharge of energy to an external system.

Preferably, a controller or control system is configured to sense or receive a signal in relation to the demand from the external system (e.g. power grid) and initiate raising or lowering and/or adjust the rate of raising or lowering of a weight to exactly match the demand (or requirement) from the external system contemporaneously plus the supplementary requirement to charge the secondary storage arrangement (in a primary discharge event) or discharge the secondary storage arrangement to the primary storage arrangement (in a primary charge event) over the duration of the primary charge/discharge event. As such, the controller or control system should be in signal communication with an external system (e.g. power grid) and configured to respond to the external system signals and with motors that drive the transporters, preferably winches, and direct power to the primary and secondary storage arrangements respectively, in a preferred embodiment.

Preferably, the controller or control system is configured to sense when a weight is nearing the end of its pre-defmed path, e.g. at the bottom of the shaft or near the bottom of the shaft close to the preceding weight, or near the top of the shaft, so that it can configure the system to slow the rate of raising or lowering of the weight as it approaches the extremity of its path and automatically start the output of the secondary storage arrangement, for example, to enable continuous power output/input level with the external power system. Thus, it will seamlessly substitute for the reduction in power input/output resulting from the first weight slowing and stopping at the end of its path. Thus, the input/output from the system at the external connection can be provided continuously according to the external system demand. Optionally, the controller or control system may be pre-programmed to match the input/output of the secondary storage arrangement as a first weight is slowed as it approaches the top or bottom of a pre-defmed path and/or it is configured to response to sensor signals such as sensors on the transporters (e.g. winches) regarding the speed of cable movement or sensors within the shaft for detecting location and speed of weight.

The controller preferably is configured to control the passage of a linkage member between a raised position and a lowered position and vice versa in order to collect a further weight in order to continue the input/output with the raising or lowering of subsequent weights. The controller preferably is configured to raise or lower such an unattached linkage member in order to arrive at the top or bottom of the pre-defmed path in sufficient time to engage a further weight.

By system power rating, it is meant a pre-defmed maximum energy input or output rate achievable by the system continuously over the full energy capacity of the system, and that is therefore normally delivered by the system to or from the external power system at the input/output connections. By surge power rating, it is meant an enhanced or preferably maximum energy input or output rate achievable by the system at any one point in time, which is higher than the system power rating (i.e. cannot be maintained continuously over the full energy capacity of the system). Typically, this will be up to the sum of the maximum power input/output capabilities of the primary and secondary energy storage arrangements. The primary power rating of the primary energy storage arrangement preferably corresponds to the weight of one of the weights of the multi-weight system (assuming the weights are all the same weight, otherwise it should be considered the largest or a typical weight) and the speed with which it can be vertically displaced (e.g. lifted or dropped) by, typically, winches or other vertical displacement means, in normal operation less any losses in the system, e.g. a generator/power converter. The system power rating is typically (and preferably) less than the primary power rating.

The total energy capacity is the total energy (storage) that may be provided by the primary energy storage arrangement and corresponds to the total mass of weights in the arrangement and the vertical displacement achievable by each weight in the arrangement, subject to operational losses.

The total system energy capacity could be considered to be the sum of the energy capacity of the primary energy storage arrangement and the secondary energy storage arrangement, being the total amount of energy being capable of being stored within the system, subject to operational losses. However, a system working energy capacity is the total energy capacity of the primary energy storage arrangement less a required energy capacity of the secondary energy storage arrangement, the required energy capacity of the secondary arrangement being the energy needed to compensate for discontinuities in input/output from the primary arrangement.

By ‘charge’ and ‘charge cycle’ it is meant the process of storing energy in the primary or secondary energy storage arrangement. By charge cycle it is meant the process of storing electricity imported from an external power system in the energy storage system, which may include storing electricity in the form of potential energy by using the electricity to lift successive weights across a vertical displacement from a respective lower position to a respective upper position by making use of a transporter, such as a winch and cable arrangement or other lifting means. It may also include storing electricity in one or more forms as part of secondary and optionally tertiary and further energy storage arrangements, such as in the form of a flywheel, a battery or a second gravity-based system.

The secondary energy storage arrangement may be any energy storage arrangement having a power rating at least equal to the system power rating and having an energy capacity sufficient to accommodate loss or reduction in power input/output capability of the primary energy storage arrangement during its discontinuities and capable of providing and configurable to provide energy at a desired rate to compensate for periods of discontinuity of the primary system.

The secondary energy storage arrangement may be one or a combination of, for example, an electrochemical-based storage system (such as a battery), a capacitor, a supercapacitor, a superconducting magnetic energy storage system, a compressed air energy storage system, a mechanical energy storage arrangement (such as a flywheel or a second gravity-based energy storage system) and a pumped heat energy storage system or any other appropriate energy storage system that may become available.

An electrochemical based energy storage system may be any suitable electrochemical cell or battery. A battery may be, for example, a lead acid battery or a bank of such batteries, lithium ion batteries or other solid state battery (such as post-use batteries from electric vehicles), or a flow battery such as a vanadium flow battery (or iron-chromium flow batteries, zinc-bromine flow batteries or others).

A secondary gravity -based energy storage arrangement may be any suitable gravity-based arrangement. The secondary gravity-based energy storage arrangement may be, for example, a pumped fluid (e.g. hydro) system, such as to pump a fluid from a lower position to an upper position (e.g. into a raised storage tank) and release it through a turbine mechanism to generate power as required. The second gravity -based energy storage arrangement may be a second single or multi-weight gravity-based system, preferably a single weight. This may be configured to be raised or lowered by a cable and winch arrangement (or other system) through a vertical displacement in a shaft which may be formed in the ground or in a shaft formed in a structure above ground. The weight may be a smaller weight than those used in the primary energy storage arrangement or may be a similar sized weight. The shaft may be sized to provide the energy storage requirement according to the needs of the system and the size of the weight.

Preferably, the secondary energy storage arrangement is an electrochemical based energy storage system or a capacitor or supercapacitor.

The secondary energy storage arrangement preferably has a response time that is at least as good as the primary energy storage arrangement. Optionally, for example to provide improvement in system response rate (e.g. to overcome inertia in a gravity system in going from zero to full charge/discharge rate), it is preferable that the response time of the secondary energy storage system is considerably shorter than the response time of the primary energy storage system.

Preferably, the secondary energy storage arrangement comprises a lithium ion battery or more preferably a flow battery.

The power rating of the secondary energy storage arrangement may be configured to be able to supplement the power input/output during a temporary shortfall in available power from the primary energy storage arrangement. Preferably, however, the power rating of the secondary energy storage arrangement is at least equal (and typically equal) to the system power rating so that the secondary energy storage arrangement can provide the power input/output for the system during discontinuity of the primary energy storage arrangement.

The secondary energy storage arrangement may thus have a power rating that is equal to or greater than the system power rating, optionally greater than the system power rating but preferably no more than 1.5 x, still more preferably no more than 1.3x or 1.2 x and more preferably no more than 1.15 x the system power rating. The primary energy storage arrangement has a primary power rating greater than the system power rating and typically greater than the secondary power rating, but preferably no more than 1.5 x, still more preferably no more than 1.3x or 1.2 x the secondary power rating. In one embodiment, the power rating of the system may be said to correspond to the power rating of the primary system less the sacrificial power demand of the secondary energy storage arrangement. Essentially, this is the system continuous power rating, whereby the sacrificial power demand of the secondary energy storage arrangement during a system discharge cycle is sufficient to enable the secondary energy storage arrangement to provide the system output power during discontinuity of the primary energy storage arrangement.

In the case of a single winch system in a multi-weight gravity- based primary energy storage system, the minimum energy capacity required for a secondary energy storage system corresponds to the amount of energy required to meet a customer power demand, which for the purposes of this calculation will be assumed to be the maximum power output of the energy storage system deliverable continuously across the energy capacity of the system. The amount of energy to be delivered to the external power system by the secondary energy storage arrangement will be determined by the period of discontinuity of the primary system, which corresponds to the period of time it takes to disengage cables from a first weight at the bottom of a shaft, draw cables to top of shaft, engage cables with new weight and begin process of lowering, plus the transitional portion arising from getting from zero to full output and full output to zero in the primary system.

The system power rating may be configured according to the particular requirements of a particular system. In some cases, for example, the system power rating may be from 1 MW to 10 MW, such as about 4 MW.

The secondary energy storage arrangement may preferably be sized in dependence upon the maximum energy gap as discussed above. Depending upon the particular system, the secondary energy storage gap may be appropriately sized. The maximum energy gap may vary depending upon the particular system, including the system rated power (assuming the secondary energy storage arrangement has a power rating the same as the system rated power), the depth of the shaft in the primary energy storage arrangement and the speed of unladen winch. Thus, with a system power rating, for example, of 0.5 to 20 MW, an unladen winch speed of 10 to 25 m/s and a shaft depth of 100 m to 4000 m, a maximum energy gap may in one embodiment be approximated to from 1.11 kWh (for a 1 MW rating in a 100m shaft at 25 m/s an unladen winch speed) to 1.11 MWh (for a 10 MW rating in a 4000m shaft at 10 m/s winch speed). Preferably, for a system power rating of, for example, 2 MW to 5 MW, an unladen winch speed of, for example, 15 to 20 m/s and a shaft depth of, for example 250 to 750 m, the maximum energy gap may preferably be approximated to a range from 6.9 kWh (e.g. for a 20 m/s unladen winch speed in a 250 m shaft for a 2 MW system power rating) to 69 kWh (e.g. for a 15 m/s unladen winch speed in a 750 m shaft for a 5 MW system power rating).

The primary energy capacity may be made up of the cumulative energy capacity of all the weights in the primary energy storage arrangement, subject to operational losses. The energy storage capacity associated with each weight in the primary energy storage arrangement may be termed the energy capacity per weight. Subject to operational losses, the maximum energy capacity per weight may be calculated simply as the gravitational potential energy of each weight at the top of the shaft (assuming the maximum path length of the weights). In one embodiment, where for example the weights are selected from the range of 20T, preferably 250 T to 750 T and the shaft selected to have a depth of 100 m to 4000 m, the energy capacity per weight may be from about 5 kWh (e.g. for a 20T weight and a 100 m shaft) and preferably from about 70 kWh (e.g. for a 250T weight and 100 m shaft) to about 8 MWh (e.g. for a 750 T weight in a 4000 m shaft). Preferably, for example where the weights are selected from the range 250 T to 750 T in a shaft depth of about 250 m to 750 m, the energy capacity per weight may be approximately from 170 kWh to 510 kWh (e.g. for a 250 T weight in a shaft depth of from 250 to 750 m) to the range from 510 kWh to 1.5 MWh (e.g. for a 750 T weight in a shaft depth of 250 m to 750 m).

Thus, in certain embodiments, e.g. in an energy storage system having a system power rating of from 2MW to 5 MW, weights in the primary energy storage arrangement of from 250 T to 750 T, a shaft depth of 250 m to 750 and an unladen winch speed of 15 to 20 m/s and where, for example, the secondary energy capacity is approximated to the maximum energy gap (notwithstanding the ranges of secondary energy capacity set out above), the ratio of the secondary energy capacity to the energy capacity per weight of the primary energy arrangement, may optionally be from 0.05 to 0.15.

The primary energy storage system as mentioned above may be composed of any suitable number of weights, e.g. from 6 to 50, for example from 10 to 20. Thus, the ratio of secondary energy capacity to primary energy capacity may be for example, for the circumstances in the paragraph above, from 0.001 to 0.025 (e.g. with 6 to 60 such weights) or preferably from 0.0025 to 0.015 (e.g. with 10 to 20 such weights).

Thus an energy storage system can be provided with a large energy capacity at a desired power rating with enhanced and flexible performance (such as a continuous power capability) provided by a secondary energy storage arrangement which is relatively small (in energy capacity) compared with the primary energy storage arrangement and even compared with the energy capacity per weight of the primary energy storage arrangement, which can thus be achieved at a low levelized cost of storage.

Optionally, there is provided in the primary energy storage arrangement a suspended platform system for providing a load bearing suspended platform, the system comprising a platform element, a plurality of suspension members engaged with the platform element by which the platform element is suspended and an anchoring mechanism for anchoring the suspension members at a raised position relative to the platform element.

The suspended platform system is preferably suitable for disposing within a vertical shaft so that the platform element provides a load-bearing platform upon which weights in a gravity -based energy storage system as defined above may be disposed. The suspended platform should be sized to fit within a shaft and the suspension members have a length to allow the platform element to be suspended at a point within the shaft a distance above the actual base of the shaft.

Preferably, the suspended platform system may be installed from the top of an existing shaft. This could help remove the need for any down-shaft workings and negate the need for workers to descent the shaft for installation work.

A suspended platform system of this aspect may also have the advantage that it can be used in existing shafts which have disadvantageous features at their lowermost region, such as abandoned machinery or water. Using this platform method could enable the energy storage system as defined above to operate without being affected by these features.

In another embodiment, sealing features may be installed to the side of or below the suspended platform to isolate the section of utilised shaft from the volume below the platform. This will be beneficial in scenarios where the lower section contains gasses which are dangerous for people, equipment or the environment such as the methane found in coal mines.

The platform element may be any suitable platform for supporting weights for use in an energy storage system as defined above. The platform element may, for example, be a frame element having engaging members or support members for receiving a weight and supporting the weight or may be a grid arrangement. In one embodiment, the platform element is a planar, rigid member and preferably a solid support member.

The suspension members may be any suitable members that may suspend a platform element within a shaft from a raised position, such as the top of a shaft. Preferably, a plurality of suspension members are provided, such as at least three, more preferably four or optionally five or six or more. The suspension members are preferably elongate members which may be engaged, preferably attached or mounted to the platform element which may be suspended therefrom in a shaft and which are preferably anchored via an anchoring mechanism at or near the surface.

The suspension members may optionally be rigid members, such as rod members, or preferably are flexible members such as cables.

Preferably, suspension members may extend into the shaft and thus have a length according to the depth to which the platform element is desired to be disposed. For example, the suspension members may have a length of, for example at least 50 m, preferably 100 to 1000 m, more preferably up to 500 m, e.g. 200 to 400 m.

The suspension members may be engaged with the platform element by any suitable means. For example, the suspension members (e.g. when cables), may be engaged with receiving channels disposed in an underside of the platform element or through sheaves disposed on the underside of the platform element whereby two opposing suspension members may be formed of a single cable extending from the top of a shaft down the shaft and engaged with the underside of the platform element and back up the shaft to the top. Preferably, however, the suspension members are affixed or otherwise securely engaged with connection points on an underside of the platform element, on the peripheral rim of the platform element or on an upper surface at a peripheral portion of the platform element.

Optionally, when the suspension members are cables, they may be provided with tensioning members, which may be adjusted to shorten the cable lengths in order to maintain a desired flat orientation of the platform element.

Optionally, the suspension members may act as guides to the weights being raised and lowered in the shaft when used with an energy storage system as defined above. The weights may be installed with rollers or skids to engage with the cables for this purpose. The roller or skids may include buffering means.

The anchoring mechanism may be any suitable means for anchoring the suspension members to allow them to suspend a platform element within the shaft. In one embodiment, the anchoring mechanism comprises one or more anchors in relation to each suspension member. For example, the anchor mechanism may comprise at least one ground engaging anchor such as a threaded anchor for threading into the ground to which a suspension member may be attached. For example, where the suspension members are cables, each cable may be anchored by at least one ground engaging anchor in the ground adjacent the opening of a shaft, for example a number of metres (e.g. 5 to 10 or even up to 20 metres from the opening of the shaft, e.g. rim of the shaft opening). In this embodiment, there may be further provided sheaves disposed in relation to the shaft opening or rim thereof across which the anchored cables may pass and then extend down into the shaft.

In an alternative and preferred embodiment, the anchor mechanism may comprise a load spreading anchor support, preferably disposed on the ground about the shaft opening and having or defining an aperture over (small or larger or the same size as) the opening of the shaft. The anchor support may optionally be a frame member. Preferably, the anchor support is a planar and more preferably plate member, e.g. a plate of steel or other metal, or of reinforced concrete (e.g. metal mesh reinforced concrete). Such an anchor support may be simply disposed on the ground over or about the opening of a shaft or may itself be anchored to the ground, e.g. at peripheral edges thereof. The lateral extent of the anchor support may be any suitable amount, but may typically be a total of from 5 to 25 m, preferably 10 to 20 m. Optionally, there may be a skirt member extending from the anchor support, typically from a rim of an aperture formed in the anchor support, preferably extending downward and may effectively line the upper portion of a shaft over which the anchor support is disposed. For example, the skirt may extend up to 15 m, e.g. 3 to 10 m into the shaft. The skirt may be made of any suitable material but is preferably rigid and more preferably is made of a similar structure to the anchor support, e.g. a steel tubular skirt or reinforced concrete skirt.

Suspension members, according to this general embodiment, may be affixed or secured to the anchor support member. They may, for example, be secured to mounting means on an upper surface or engaged with a peripheral edge of the anchor support (and provided with sheaves at a rim of an aperture in or defined by the anchor support), or mounted on or affixed to a rim of an aperture in or defined by the anchor support and/or, where present a skirt thereof.

Optionally, lateral projecting members may be provided for engaging shaft walls in order to stabilize the platform element or suspension members and/or to enable a degree of load bearing support by the shaft walls of the system. The lateral projection members may take any suitable form. Preferably, the lateral projecting members are rigid and optionally deployable from a retracted position (to facilitate installation) to a deployed position (in which they are engaged with a shaft wall). The lateral projecting members may be, for example, fold out, telescopic, hydraulic or threaded members or a combination thereof.

The lateral projecting members may be configured to abut against a shaft wall or may be configured to affix into a shaft wall.

Preferably, the lateral projecting members are disposed in association with the platform element. For example, they may be disposed or mounted beneath or on an underside of the platform element. In one embodiment, the lateral projecting members are configured to project from lateral edges of the platform element.

Optionally, the suspension members are provided with stiffening or support members at pre-defmed positions on the length of the suspension members. Preferably the stiffening or support members are inter-connected so as to define a peripheral support frame, which preferably has an aperture to allow passage of a weight (as defined in the above energy storage system). Optionally the peripheral support frame is provided with lateral projecting members whereby the peripheral support frame may be engaged with a wall of the shaft in which the system is disposed to stabilize the peripheral support from and indeed suspension members and optionally may extend into the wall of the shaft, whereby the wall may provide load support to the system via the peripheral support frame. The lateral projecting members may independently be as defined above.

The platform element, when disposed in a shaft, preferably has a lateral dimension (excluding any projecting members) of 50 to 95% of the corresponding cross-sectional dimension of the shaft in which it is disposed.

The suspended platform system may preferably be incorporated, as a preferred embodiment, into an energy storage system as described above.

The invention will now be described in more detail, without limitation, with reference to the accompanying Figures.

In Figure 1, an energy storage system 1 according to an embodiment of the invention comprises a primary multi-weight gravity energy storage sub-system 3 and a secondary energy storage battery 5 (where battery may be a lithium ion battery, for example, but is used here to represent any secondary energy storage arrangement or sub-system) working together to provide energy storage in the system 1 for an external power grid 7 via input/output connection 9. An interconnection and control arrangement 11 controls power supply to and from the external grid 7 via input/output connection 9 according to the power demand from or storage requirements of grid 7.

The interconnection and control arrangement 11 controls the distribution of energy to and from the external grid 7 to the primary energy storage sub-system 3 and secondary battery 5 in order to ensure a constant power input/output to and from the grid 7 across the full depth of the energy storage capacity of the system 1.

Primary multi-weight gravity-based energy storage sub-system 3 has weight wl 17 suspended in shaft 19 by two cables 21 from winches 23 configured to raise or lower weight wl 17 according to whether energy storage or supply is needed. At the bottom of shaft 19, further weights w2 and w3 27,29 are stacked, ready to be raised by the winches 23 during an energy storage or charge cycle. Multiple weights 25 are stowed at the top of the shaft ready to be deployed in the shaft 19, e.g. by an overhead gantry (not shown), and lowered by the winches 23 during an energy supply or discharge cycle. Primary energy storage sub-system 17 is shown with weight wl 17 disposed part way down the shaft, ready to be raised (for charging) or lowered (for discharging) as required.

Energy is stored in secondary energy storage battery 5, which has a much smaller energy capacity than the primary sub-system 3 and can be utilised to ensure that the supply of power to the grid 7 (or power from the grid) remains constant during a discharge or charge cycle even while primary sub-system 3 is not able to charge or discharge during the period between raising or lowering of successive weights. Secondary energy storage battery 5 is shown on scale 31 as being partially charged.

The energy storage system 1 has a power rating Ps, being the maximum power at which it may supply or receive power from the grid 7 continuously across the full depth of the energy storage capacity of the system 1. Secondary storage battery 5 has a power rating P2 _MAX that is typically equal to Ps, while primary storage sub-system 3 has a power rating Pi _y of a little greater than Ps.

Figures 2a to 2e illustrate the process and indeed the configuration and cooperation of primary energy storage sub-system 3 and secondary battery 5 controlled by interconnection and control system 11 in order to provide continuous uninterrupted service (whether charge or discharge, storage or supply) to the grid 7.

In Figure 2a, the system 1 is shown operating during a charge cycle with power input 33 from the electricity grid 7 of power level Ps being taken into the system 1 via input/output connection 9. This is supplied via interconnection and control system 11 to provide, along with a secondary power output supplement 35 of power level P2 _y (considerably less than Ps), to primary supply cable 13 to provide cumulative primary charge power 37 of power level Pi _y , being the sum of Ps and P2 _y . This causes weight wl 17 to be raised as indicated by arrow 39 (thus storing energy from the grid 7) while the secondary energy level 31 in secondary storage battery 5 becomes depleted.

As can be seen in Figure 2b, when the weight wl 17 reaches the top of shaft 19 and is disconnected from cables 21 (for transport to weight storage 25), which begin to descend (as shown by arrows 43) to collect the next weight w2 27 from the bottom of shaft 19, the energy in the secondary storage battery 5 is depleted. The interconnection and control system 11 directs the power 33 of amount Ps incoming from the grid to secondary storage battery 5 as secondary power storage input 41 in an amount P2 _MA being equal to Ps. This causes the energy level 31 in the secondary energy storage battery 5 to increase (in this case to a maximum level, although the secondary energy storage battery 5 may have some excess capacity) by the time the cables are returned to the bottom of the shaft 19 and ready to connect to weight w227, as is shown in Figure 2c.

Once the cables 21 are connected to weight w227 at the bottom of the shaft 19, the interconnection and control system 11 re-directs the incoming power 13 at level Ps to primary input cable 13 toward the primary energy storage sub-system and draws power from the secondary storage battery 5 in a secondary power output supplement 35 to provide also to the primary energy storage sub system, as previously, as is shown in Figure 2d. The charge cycle continues in this pattern until weight w3 29 has also been lifted and the primary energy storage sub-system 3 is fully charged, then one remaining charge of the secondary storage battery 5 leaves this battery as fully charged again as indicated by secondary energy level 31, as shown in Figure 2e. The system 1 is thereby fully charged and storage was provided for excess power from the grid 7 at a constant and continuous input power Ps for the full energy storage capacity of the system 1, despite the intermittent nature of multi-weight gravity -based primary energy storage sub-system 3.

Figures 3a to 3d illustrate the process and indeed the configuration and cooperation of primary energy storage sub-system 3 and secondary battery 5 controlled by interconnection and control system 11 in order to provide continuous uninterrupted discharge or supply of power to the grid 7 across the full extent of the energy capacity of the system 1.

In Figure 3a, a fully charged system 1 starts to discharge by the lowering of a first weight w3 29 and supplying a reduced power output 47 of Piyred to provide a grid supply power output 49 through output connection 9 to the grid 7 at a power level of Ps (the maximum continuous output achievable by the system) being the same as Piyred. No supply to or from secondary storage battery 5 is necessary during this first phase of discharge from a fully charged system 1.

Once the weight w3 29 has been lowered to the bottom of shaft 19, empty cables 21 may be raised again toward the top of the shaft 19 as indicated by cable raise arrows 51. While the primary energy storage sub-system 3 is not discharging, the secondary storage battery 5 discharges its energy as secondary power supply output 53 at power level P _2MA equal to Ps. This is shown in Figure 3b.

By the time the cables 21 have been raised to the top of the shaft 19 and the subsequent weight w227 attached and lowering begun (as indicated by arrow 45), the energy level 31 in the secondary storage battery 5 has been depleted, as illustrated in Figure 3c. The primary energy storage system outputs power at an increased primary power output power 55 of Pi _y being greater than Ps and of which a sacrificial portion 57 P2 _y is supplied to secondary energy storage battery 5 to recharge that battery 5 ready for the next discontinuity in discharge of the primary energy storage sub-system 3 in order to continue the supply of power to the grid 7 at system power Ps for as long as necessary to the extent of the energy storage capacity of the system 1.

Once all the weights 25 have been lowered and are stowed at the bottom of shaft 19 and sub-system 3 fully discharged and the secondary storage battery 5 is also fully discharged, the system 1 is in fully discharged state and can no longer export to grid 7, but is ready for a charge requirement, as shown in Figure 3d.

In Figure 4, a particular feature of the system is illustrated in which it can provide storage and supply in response to surges or temporary depletions in grid power. Thus, as shown in Figure 4, should there be a surge in power on the grid 7, it can require storage of a surge power input 59 at a power level Psurge which is any power level above the rated continuous system power Ps that the system 1 can accommodate. The surge power level can by any power level the system is configured to accommodate. For example, the surge power level may be up to 2Ps. The duration of the surge power input or output depends upon the surge power level and the capacity of either the primary storage sub-system 3 or the secondary battery 5The surge power, e.g. P _surge equal to 2Ps is supplied by interconnection and control system 11 to both the primary energy storage sub system 3 by way of a primary charge power 37 of level Pi _y causing weight w2 to be raised as indicated by arrow 39 and to secondary storage battery 5 as a secondary power storage input 35 of power level P2 _MA , which is the same as the system power rating of Ps _. Thus, the power surge storage capability is more than double the system power rating Ps which it can sustain at this level for a period limited by the energy capacity of the secondary battery 5. This is a very effective further feature for grid balancing and surge absorbing applications.

Equally, the system can provide an enhanced output of power to the grid above system power rating Ps for a limited period of time.

Figure 5 shows an illustrative graph 61 of Power (output power) against Time in a discharge cycle of the system 1 such as illustrated in Figure 3c in which the power output over time from the primary energy storage sub-system 3 is illustrated as a dashed line 63 while the power output over time from the secondary storage battery is shown as solid line 65. In the discharge phase illustrated in Figure 3c, the primary energy storage sub-system 3 is discharging at a power Pi _y , of which power Ps is exported via output connection 9 to grid 7 (in Figure 3c), while the difference in power between Pi _y and Ps is supplied to secondary storage battery 5 via secondary supply cable 15 in order to trickle charge it at power P2 _y. The energy supplied by the primary sub-system 3 to the secondary storage battery 5 during the energy discharge event (that is the discharge from one weight in the primary energy storage sub-system 3 for the full extent of its path) is the hashed area 67 under the primary sub-system line 63 that is above the system output power Ps.

As the weight w2 in Figure 3c begins to approach the base of the shaft 19, the winches 23 need to slow down the rate of descent in order that the weight w2 can be gently rested at the base of the shaft 19. This portion of the discharge cycle is a transition portion 69 of the power curve 63 during which the power output from the primary energy storage sub-system 3 reduces from Pi _y to zero.

During the transition phase 69, the interconnector and controller 11 causes the power output of secondary storage battery 5 to ramp up so that the system power output to the grid 7 remains at the required power Ps. This continues until the weight w2 comes to a rest at the bottom of the shaft 19. The transition phase lasts a period of time _s at the end of which the power output from the primary sub-system 3 is zero and the power output from the secondary storage battery is P2 _MA which is equal to Ps. In fact, depending on the particular arrangement, P2 _MA may be slightly larger than Ps, with a small portion of the power used to power the raising of the winch cable 21 back to the top of the shaft 19 (alternatively, the winching of unloaded winch cable 21 may be independently powered). The power output of the secondary storage battery remains constant at P2 _MA to meet the grid 7 demand for as long as it takes for the winch cable 21 to be disconnected from weight w2, the winch 23 to winch the cable 21 back up to the top of the shaft 19 and to connect to the next weight wl ready to be lowered for further discharge. This time period is referred to as the tdiscontinuity. The primary storage sub-system 3 is then ready for its next power discharge event at which point the output power from the primary storage sub-system ramps up (see line 69a in Figure 5) and at the same time the controller 11 causes secondary storage battery to correspondingly lower its power output, maintaining a system output power of Ps. When the primary storage sub-system 3 power output exceeds Ps, it starts again its trickle feed to recharge secondary storage battery 5.

The total energy required by the secondary storage battery 5 to maintain a constant and continuous system output of Ps during the transitions and the period of discontinuity (zero output) of the primary energy storage sub-system 3 is the area 71 under graph 65 in Figure 5. The area 71 should be at most equal to the hatched area 67. This energy may be termed the primary energy gap or EG. The minimum energy capacity required (subject to allowing for operational losses) for the secondary storage battery is therefore EG. EG is made up of the transition energy (the energy shortfall during the transition phase before and after the discontinuity) plus the discontinuity energy (the energy shortfall during the discontinuity). The energy shortfall during the discontinuity is the system power output Ps x tdiscontinuity. The value tdiscontinuity is a function of the depth of the shaft 19 and the speed of the winches 23. In a typical example where the shaft may be 400 m deep and the load free winch speed is from 15 to 20 m/s, tdiscontinuity may be somewhere in the region of 25 to 30s. For a 4 MW rated system 1, the discontinuity energy may be between 27 and 33 kWh. The transition periods _s are typically for such a system much shorter than the discontinuity periods, being typically less than Is and up to 3s. Two discontinuity periods may be assumed to have a total duration of 3s. The average power shortfall may be assumed to be about half of target of Ps over the duration of ttrans. So, for a 4 MW system, the transition energy (for two transitions in an energy gap) is equivalent to a 4 MW output over 1.5s, which is 1.67 kWh. The transition energy in this case is around 5% of the discontinuity energy and may be anywhere from 1 to 10 % typically, depending on the system. Thus, the Energy Gap, EG is roughly equivalent to the discontinuity energy or up to 10% more. Figures 6 illustrates in a graph 73 of Power (output power) against Time in a discharge cycle of the system 1 in a further embodiment of the invention in which the system 1 is configured such that the secondary energy storage battery 5 has a faster response time than the primary multi-weight gravity- based energy storage sub-system 3. In a discharge event, in particular shown in Figure 6 a discharge event from a fully charged system 1, in which an external power system such as grid 7 demands energy from the energy storage system 1 at output power Ps, the primary energy storage sub-system, which is a multi-weight gravity -based system has a lag before it reaches the desired output power due to inertia associated with the system and other factors (such as signal or electrical delays). For a typical commercial system (e.g. a 4 MW output system), the lag may be about Is. It is often desirable to have a faster response time. Thus, where the secondary storage battery 5 is a storage system with a faster response time than a multi -weight gravity -based energy storage system 3, such as a lithium ion battery, a flywheel or a super capacitor, for example, the system may be able to provide a faster response than the primary storage sub-system 3 alone, while the primary storage sub-system 3 ramps up its output. For example, in a scenario in which a grid requirement is for power Ps (of 4 MW) within 0.3 seconds, the interconnector and controller 11, will cause the secondary storage battery 5, with its fast response time, to ramp up toward a power output of Psin a response of 0.3 s and to moderate the output of the secondary storage battery 5 as the output of the primary energy storage system 3 ramps up. Thus, the secondary storage battery power curve 75 ramps up rapidly then descends back to zero over a 1 second time frame, while the primary storage sub-system power curve 77 ramps up at a lower rate than curve 75 to a power of Ps (of 4 MW) in about Is. The cumulative power output curve 79 of the system 1 ramps up smoothly to output power Ps (4 MW) to deliver a faster combined response than the primary energy storage sub-system alone..

Such an arrangement requires an additional energy storage capacity in the secondary storage battery 5 than is required for the energy gap EG described above. For an example system with a 400 m shaft, 500T weights and a 4MW output power rating, the additional energy may be in the region of 0.25 kWh for a secondary energy storage battery with a capacity for supplying power for an energy gap of approximately 30 kWh.

Figure 7 shows an illustrative graph 81 of Power (output power) against Time in a discharge cycle of the system 1 in which the primary storage sub-system power curve 83 is shown, along with a system power curve 85 as a result of an intervention from the interconnection and controller system 11.

When, during a discharge cycle, the external power system requires a sharp stop in output, the halting of the descent of the weight suspended from winch 23 results in a small surge in power output from the primary storage sub-system 3 illustrated by peak 87. The primary energy storage sub-system may then take up to a second to reduce its power output to zero. By coordination with the secondary storage battery 5, which is a fast response system, the interconnection and control system 11 may cause the peak energy between curve 83 and 85, shown as hatched area 89, to be used to charge up secondary storage battery 5, thereby giving a combined, system power output curve 85 which almost instantly falls to zero without any output surge, which thereby provides for a much more responsive discharge profile that particularly suits certain grid applications. This arrangement can enable the secondary storage battery to store the excess peak energy as well as enabling an instant/fast stop.

The additional energy storage requirement for secondary storage battery 5 for a similar typical commercial system as discussed above is in the region of 0.5 kWh (compared with the energy storage capacity of 30kWh required for the energy gap in such a system).

Figures 8 A to 8G illustrate linkage members 91 for use in the system above to enable rapid coupling and decoupling o the cable 21 with the weight 17. Rapid coupling and decoupling contributes to reducing the energy gap and thus reduces the required capacity of the secondary energy storage arrangement or sub-system. Figures 8 A to 8G illustrate the linkage members 91 in disengaged configuration (Figures 8A to 8C) and engaged configuration (Figures 8D to 8G) with a first weight 17.

In Figures 8 A to 8C, the linkage member 91 comprises body 95 to which the cable 21 is mounted and cooperating sliding plate 97 disposed on the inner (weight-side) face of body 95. When linkage member 91 is aligned with a weight 17 and, in particular, a linkage dock 93 in the form of a recess on the weight 17, the sliding plate 97 may be caused to be displaced laterally from the body 95 and into the recess 99 of the linkage dock 93. The linkage member 91 may then be raised and the upper portion of sliding plate 97 engages slot 101 in the upper portion of recess 99 thereby locking the linkage member 91 into the linkage dock 93.

As can be seen in cross-sectional views in Figures 8C and 8F and also in Figure 9, the sliding plate 97 is provided with six hydraulic actuators 107 that extend through bushes 109 formed in the body 95 and serve to control the movement of the sliding plate 97 away from and back to the body 95 and retain the sliding plate 97 in cooperative engagement with the recess 99 and slot 101. A lower edge of the sliding plate 97 is provided with lateral wings 103 which serve to inhibit rotation of the linkage member 91 when engaged into linkage dock 93 by cooperating with lateral slots 105 associated with the recess 99.

To release the linkage members 91 from the weight 17, the linkage members 91 are lowered relative to the weight 17 thus disengaging sliding plate 97 from slot 101 (and disengaging lateral wings 103 from lateral slots 105). The hydraulic actuators 107 are then operated to draw the sliding plate 97 back to contact with the body 95 at which point the linkage members 91 may be safely raised or lowered relative to the weight 17 without further contact.

Figure 10 illustrates in transparency a perspective view of a weight 17 showing one of the linkage docks 93 for receiving sliding plate 97. As can be seen, once the sliding plate 97 is disposed in the recess of linkage dock 93 and the linkage member 91 raised, the upper edge of the sliding plate 97 engages into slot 101 and, at the same time, lateral wings 103 on the sliding plate 97 engage into lateral slots 105 of the linkage dock 93. Abutting face 111 of each lateral slot 105 serves to abut lateral wings 103 and serves to counter potential rotational motion of the linkage member 91 once engaged with and lifting the weight due to the offset position of the sliding plate 97 and its engagement with the weight 17 from the resting centre of gravity of linkage member 91 suspended from cable 21. In Figure 11, which illustrates a suspended platform system 113 of one embodiment of a first gravity-based energy storage sub-system used in an embodiment of the invention, which platform system 113 is for disposal in a shaft (not shown) for the raising and lowering of weights 115. A platform element 117 is suspended (typically in a shaft) by ten cable suspension members 131 affixed to peripherally disposed brackets 119 themselves affixed to the underside of the platform element 117 and extending about the periphery of the platform element 117. The cable suspension members 131 may be affixed at their opposing ends to an anchor support 121 in the form of a spreader plate for disposing on the ground on the surface over a shaft (not shown) and having an aperture 123 formed therein which should generally coincide with a shaft opening (not shown). A rim 125 about aperture 123 may be provided with a plurality of fixings or brackets 127 to which the cable suspension members may be secured. Weights 115 may be disposed, in a discharged configuration of an energy storage system, on the upper surface 129 of platform element 117.

The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.