This invention relates to an energy storage unit having a grounding system, in particular, for energy storage modules comprising chemical energy storage, such as electrochemical cells, or batteries, providing electrical energy to an end user.

Stored electrical energy modules, or power units of various types are becoming increasingly common in many applications, in particular for use where there are

environmental concerns relating to emissions in sensitive environments, or public health concerns. Stored electrical energy power units are typically used to provide electrical energy to operate equipment, to avoid emissions at the point of use, although that stored energy may have been generated in many different ways. Stored electrical energy may also be used to provide peak shaving in systems otherwise supplied from the grid, or from various types of power generation system, including diesel generators, gas turbines, or renewable energy sources. Aircraft, vehicles, vessels, offshore rigs, or rigs and other powered equipment in remote locations are examples of users of large scale stored electrical energy. Vehicle drivers may use the stored energy power unit in city centres and charge from an internal combustion engine on trunk roads, to reduce the harmful emissions in the towns and cities, or they may charge up from an electricity supply. Ferries which carry out most of their voyage relatively close to inhabited areas, or in sensitive environments are being designed with hybrid, or fully electric drive systems. Ferries may operate with stored energy to power the vessel when close to shore, using diesel generators offshore to recharge the batteries. In some countries the availability of electricity from renewable energy sources to use to charge the stored energy unit means that a fully electric vessel may be used, provided that the stored energy units are sufficiently reliable for the distances being covered, with no diesel, or other non renewable energy source used at all. Whether hybrid, or fully electric, the stored energy units may be charged from a shore supply when docked. The development of technology to achieve stored energy units that are reliable enough for prolonged use as the primary power source must address certain technical issues.

In accordance with a first aspect of the present invention, an energy storage unit comprises at least one energy storage module and a grounding system for the or each energy storage module comprising a plurality of energy storage devices electrically connected together in series within a module housing, the module housing comprising at least one electrically conductive section; the system comprising a mounting for the or each energy storage module and a resilient conductive member fitted to the mounting and connectable with a surface of the electrically conductive section of the module housing, such that on mounting the module in the mounting, the electrically conductive section of the module housing is brought into electrical contact with the resilient conductive member.

The spring force of the resilient conductive members ensures electrical contact with the module housing to ground the module, helps to centre each module as it is inserted and also improves mechanical contact to hold the module in place on the mounting, without the need for connecting individual earth cables during installation.

The mounting may comprise a rack, the rack comprising two pairs of posts connected together by beams mounted substantially perpendicular to the posts, and at least one pair of crossbeams to join the two pairs of posts, the crossbeams having a length greater than the width of the module housing.

The mounting may further comprise sidewalls.

The resilient conductive member may be fitted to a surface of a beam, or to a sidewall.

The grounding system may comprise a plurality of resilient conductive members connectable to an electrically conductive section of the module housing, each resilient conductive member being mounted to a beam or sidewall.

In a rack, or mounting, for multiple energy storage modules, at least one resilient conductive member is provided for each module.

Resilient conductive members may be mounted in pairs, a first resilient conductive member of the pair being mounted on a beam or sidewall and a second resilient conductive member of the pair being mounted parallel to and facing the beam, or sidewall, on which the first resilient conductive member is mounted.

Pairs of equivalent resilient conductive members help to locate the module centrally and provide redundancy, so that in the event of damage on one side, the module is still grounded.

A plurality of beams may be provided in the rack, spaced by a predetermined distance from the next beam, in a direction orthogonal to the crossbeams, sufficient to allow the module to be mounted between two adjacent beams.

Pairs of beams parallel to one another in the rack may be provided with resilient conductive members on each beam on the surface of the beam that faces the other beam of the pair. The resilient conductive member may comprise a single or multiple leaf spring.

Leaf springs are inexpensive and widely available components.

The module housing may further comprise a back plate, the back plate comprising at least one of electrical, mechanical and fluid connectors.

Although connections may be made by cabling accessible from the front of the unit after insertion of the modules, preferably the mounting further comprises a back plate, the back plate of the mounting comprising electrical, mechanical and fluid connectors connectable with the or each connector of the housing.

An example of a grounding system for energy storage modules, according to the present invention will now be described with reference to the accompany drawings in which:

Ligure 1 is a block diagram illustrating an energy storage system in which the grounding system of the present invention may be used;

Ligure 2 illustrates a racking system in an energy storage unit, in which energy storage modules may be mounted and use the grounding system of the present invention:

Ligure 3 illustrates an energy storage module being extracted from an energy storage unit of the type using the racking system of Lig.2;

Ligure 4 illustrates part of an example of a grounding system according to the present invention;

Ligure 5 illustrates more detail of the system of Lig.4;

Ligures 6a to 6c illustrate examples of part of the systems of Ligs.4 and 5 in more detail.

Early large scale batteries were lead acid, but more recently, lithium ion batteries have been developed for electrical energy storage for large scale applications. Li-ion batteries are typically pressurised and the electrolyte is flammable, so they require care in use and storage. A problem which may occur with Li-ion batteries is thermal runaway which may be caused by an internal short circuit in a battery cell, created during manufacture. Other causes, such as mechanical damage, overcharge, or uncontrolled current may also cause thermal runaway, but the battery system design is typically adapted to avoid these. Manufacturing issues with the cells cannot be ruled out entirely, so precautions are required to minimise the effect should thermal runaway occur. In a large scale Li-ion battery system, the amount of energy that is released during a thermal runaway is a challenge to contain. A thermal event may increase temperatures in a single cell from a standard operating temperature in the range of 20°C to 26 °C to as much as 700°C to l000°C. Safe operating temperatures are below 60 °C, so this is a significant problem.

There are strict regulations in the marine and offshore industries regarding risk to the vessel or rig, there may be a requirement to prevent transfer of excess temperature between modules, or a requirement that there should be no transfer of excess temperature from one cell to another. If overheating occurs, then it should be contained in a single cell and not allowed to spread. In addition, for marine and offshore applications, weight and volume of any equipment is severely restricted, leading to compact, lightweight systems being preferred. It is a challenge to produce a compact, lightweight, system that achieves the required thermal isolation and cools the cell in which excess heating occurs, quickly and efficiently.

In a Li-ion battery system, it is very important that the temperature of the battery cells does not exceed the prescribed operating temperature and that the cell temperature in the entire system is uniform. Sustained operation outside the prescribed operating temperature window may severely affect the lifetime of the battery cells and increases the risk of thermal runaway occurring.

For marine applications, there is a particular focus on using energy storage modules, such as batteries, at their maximum charge or discharge rate due to cost of installation and the weight and space taken up by the modules when on a vessel or offshore platform.

Furthermore, maintenance and repair, or replacement is complicated and expensive compared to land based uses of stored energy systems, so extending the lifespan of stored energy modules is particularly important. For the example of Li- ion batteries, these are sensitive to high temperature, so it is important to ensure that the operating and ambient temperature are controlled for all cells of a Li-ion battery system to ensure the design lifetime is met. Local variations or hot spots on a single cell may also compromise the total lifetime achievable.

Energy storage modules comprising a plurality of energy storage devices, i.e. cells may be combined in an energy storage unit. Installation and maintenance of the modules of an energy storage unit needs to be possible without compromising the safety of any person coming into contact with the module or unit. An example of an energy storage system in which the present invention may be applied is illustrated in Fig.1. The system comprises a cabinet, cubicle, or unit 1, in which a plurality of energy storage modules 10 are electrically connected together by buses 2a to a cubicle controller 28 and by bus 2b to a central controller 3. Energy storage modules within a cubicle may be electrically connected in series, as shown, or in parallel, or a combination, such as some modules being connected in parallel and then that parallel grouping being connected in series with other modules in the unit.

Each of the energy storage modules is cooled by cooling fluid, circulating from cooling system 5 through inlet pipes 6 and outlet pipes 7. The cooling fluid is typically water which is inexpensive and easier to source and dispose of than synthetic coolants. Additives may be provided, for example to inhibit freezing, biogrowth, or corrosion. Typically the proportion of additive is determined by the additive chosen, for example 20% frost inhibitor. The cooling fluid is typically supplied to each module in parallel, although it is possible, but less effective for the later modules, to supply the cooling fluid in series. Each energy storage module 10 comprises a plurality of energy storage devices, for example battery cells, electrically connected together in series. A modular system of this type, incorporating cooling, is particularly applicable for Li-ion cells.

Within a module 10, on at least one side of each cell, a battery cell cooler is provided which receives cooling fluid from the cooling system 5 via the inlet pipes 6 and outlet pipes 7 to cool the battery cell. The cell cooler comprises tubing for the cooling fluid to flow through, which may be metal tubing, but more typically is a synthetic material, such as polymer plastics, for example polythene, polyamide, such as PA66 plastics, or thermoplastics such as TCE2, TCE5, or other suitable materials, which may be moulded or extruded to the required shape, or generated by additive manufacturing techniques and is able to withstand normal operating temperatures of the energy storage modules 10. Individual modules may be stacked on one another, with or without additional layers between the side of the cell away from its cooler and the surface of the next cooler in the stack. Assembly and installation of such modular energy storage systems may be improved. When lithium - ion battery cells are used as the energy source in energy storage systems, they have an inherent disadvantage that they cannot be turned off. Li ion cells also store large amount of energy. These aspects need to be taken into account in designing systems for short circuit protection.

Power systems typically apply a grounding philosophy and type which allows for the possible fault currents that may arise in the system. This contrasts with grounds for electronic components, which do not have any particularly well defined requirement. Typical marine and offshore market regulations require that the cross sectional area of the bonding, or grounding, connection must be at least 50% of the power system cross section per phase, the cross sectional area being related to the current flowing in the system and the material used, for example, copper. If copper is not use, the total cross sectional area may need to be higher, based on the conductivity of the material used and the expected maximum short circuit current. In a large electrical system, which may have more than 1000 modules, the cost of providing high quality grounding for all of the modules is relatively high and the time taken to manually connect grounding systems, often on site, adds to the cost. It is also a safety requirement that the module is grounded before power is connected. Without sufficient cross sectional area of grounding, there is a risk that bonding cables will burn off during a short circuit and cause an arc flash. In a battery system, an arc flash may be extremely dangerous and destructive.

A common approach is to use high inductive, thin cables that are screwed on the outer casing of the battery module, but this takes a lot of commissioning time. Additionally, if high inductive bonding, i.e. thin cables with no added protection, is used, there may also be electromagnetic compatibility issues with the design.

The present invention addresses the problems discussed by using a conducting resilient member, such as an integrated leaf spring, to provide grounding for the battery modules. In a modular energy storage unit 1, each energy storage module 10 may be mounted on a shelf (not shown) in racking 30, or directly on the parts of the racking 30, within the unit 1, as illustrated in Fig.2. The modules may be slid into the racking 30, or onto the shelf supported in the racking. A connector in a back plate of each module 10 may connect to a connector 31 at the rear of the racking of the cubicle by means of a push- fit, or other type of connection. Alternatively, the modules may be connected with cabling at the front of the unit (not shown). Cooling fluid inlets and outlets 6, 7 at the rear of the module are aligned with the equivalent component in the racking at the rear of the unit and connectors for power cables by which the batteries are charged and discharged are also provided in the unit and at the rear of each module to connect to the buses 2a, 2b. Fig.3 shows an example of an energy storage unit 1 with walls 33 and door 32, as well as the modules 10 mounted on their racks 30. The modules have a module housing 35 including an electrically conductive section. In Fig.3, the top module 10 is shown being extracted.

In Fig.4, more detail of the invention can be seen. Where the module 10 has been extracted, it can be seen that on each side of the unit 1 are grounding strips comprising conducting resilient members 34, sprung mounted such that they are pushed in by the action of the module being slid into the unit on the rack and spring back when the module is removed, or constructed with a bowed form to achieve the same effect. This ensures a good contact of the grounding strips and an electrically conductive section of the housing of the module. The electrically conductive section of the housing of the module is typically metal. Simply relying on there being electrical contact between a metallic housing of a base of a module and a metallic shelf onto which the base of that module is placed is not deemed sufficient by the regulatory authorities. However, positive contact made in the grounding system of the present invention addresses this issue. The grounding strips may be mounted to a vertical wall 36 of each of the racks 30, which form the racking system of the unit, if the racks comprise L-shaped, or angled, brackets 37, as illustrated in Fig.5, or the strips 34 may be mounted to the walls 33 of the unit, for example if the brackets support shelves across the full width of the unit, onto which the modules are mounted.

One example of an implementation of the conducting resilient member 34 is a leaf spring. A leaf spring in this example may comprise a single strip of a conducting material having an arced or curved profile, or multiple strips of conducting material of different lengths to allow curvature when mounted under compression. The leaf spring may take many different forms, for example a single curved piece of conductive material of constant thickness, or be made up of multiple layers that have different lengths, or be a single piece with varying thickness. A conductive geometry is used for the conducting resilient member that allows for a module to be inserted into the unit and applies pressure to the enclosure of the module once it has been inserted

If multiple strips of different lengths are used, they are typically mounted one above the other and clamped together. The length of each strip forming the leaf spring varies, so that the spring can compress and expand. For a single strip, a central portion of the arc may be made thicker than the ends. Figs. 6a to 6c illustrate examples of leaf springs as described. In Fig.6a, a single bowed strip pre-bent from a strip of substantially constant thickness is shown. In Fig.6b, an example in which the strip is formed as an arc with a thicker central portion than its ends is shown. In Fig.6c, a leaf spring formed by clamping strips of different length together and applying a bend to the composite member is illustrated. Other designs having the same effect may be used.

By providing a resilient conductive member 34 in contact along a substantial part of the length of the housing 35 of the module 10 on each side, the requirement to have a minimum contact surface area can be more easily and reliably met than using cables connected to the housing. By implementing leaf springs in a shelf or racking system 30 of the energy storage unit, the module 10 can be automatically grounded upon insertion, so that when the connectors at the rear of the module makes their connection to the unit electrical connections, the grounding of the module has already been carried out. No extra time is required during commissioning to deal with grounding and power connection separately. The arrangement ensures that the module is grounded before power connections are made, as the power connections only occur when the module is fully inserted in to the racking, or shelf of the unit, when the full area of the conductive grounding member is in contact with the module housing. In some embodiments, the connections may be made at the front of the module after the module has been inserted. The spring force from the resilient conductive members 34 on the sides of the module housing 35 helps to keep the module fixed in place and provides vibration and shock absorption. A high cross sectional area is ensured during the transition of the module between its removed and fully installed states within the cubicle.

The grounding system of the present invention replaces conventional cables, making use of the racking system in which the modules are mounted. Although an alternative would be to use snap-on connectors and copper busbars, these are far most costly and increase production time. The design of the present disclosure requires fewer parts than in

conventional systems, reducing manufacturing cost, time spent on commissioning and the purchase cost for parts. Vibration damping is improved compared to existing solutions and the grounding cross-sectional area is also increased, which in turn increases performance in a safety-related event, as compared to cable connections on a chassis.

Although the detailed examples have been given with respect to electrochemical cells, such as batteries, for example Li-ion, alkaline, or NiMh batteries, or others, the invention applies to other types of stored energy units, in particular non-cylindrical capacitors, ultracapacitors, or supercapacitors, fuel cells, or other types of energy storage which have a surface that can be cooled by a cooler and which may also suffer if the temperature of modules of the stored energy units regularly goes outside a preferred operating range, reducing the overall lifetime and increasing maintenance costs.