WO2012032368A1

WO2012032368A1 - Multi-tier redox flow cell stack of monopolar cells with juxtaposed sideway extended bipolar intercell interconnects on every tier of the stack

Info

Publication number: WO2012032368A1
Application number: PCT/IB2010/002231
Authority: WO
Inventors: Krisada Kampanatsanyakorn; Suradit Holasut
Original assignee: Krisada Kampanatsanyakorn; Suradit Holasut
Priority date: 2010-09-07
Filing date: 2010-09-07
Publication date: 2012-03-15

Abstract

A multi-tier stack is composed of monopolar cells having two flow compartments containing the positive and negative electrode respectively, hydraulically separated by a permionic membrane and defined by stacked electrically conductive plates of current distribution (briefly intercell interconnects) to electrodes of same electrical sign over both sides (i.e. belonging to two sequential cells of the multi-tier stack). Each tier of the stack includes a plurality of such monopolar cells, sideway juxtaposed one next to the other; and two or more of the intercell interconnects of the cells of the plurality, electrically not in contact among them, are juxtaposed side-by-side one next to the other and each of them extend sideway over the projected perimeter of the flow compartments of two adjacent monopolar cells of the plurality of cells laying on the same tier of the stack, defining flow compartments of electrodes of opposite signs in two sideway adjacent monopolar cells, of the electrodes of which they constitute bipolar intercell interconnects or alternatively even the working electrodes themselves, over unmasked active areas on their opposite sides, in contact with the relative electrolyte solution.

Description

"MULTI-TIER REDOX FLOW CELL STACK OF MONOPOLAR CELLS WITH JUXTAPOSED SIDEWAY EXTENDED BIPOLAR INTERCELL INTERCONNECTS ON EVERY TIER OF THE STACK"

TECHNICAL FIELD The present disclosure relates generally to electrochemical cells and in particular to multicell stack with internal ducting for the circulation of electrolyte solutions through respective cell compartments.

BACKGROUND ART

There are electrochemical processes that are conducted without gas evolution at the electrodes, apart from parasitic phenomena that may occur at certain operating conditions, and an important application among others is for energy storage.

The so-called redox flow battery or briefly redox battery stores energy in two electrolytic solutions containing a so-called redox couple (typically a multivalence metal in two distinct states of oxidation like, for example, vanadium) that are flown through respective flow compartments of each cell of a multi-cell stack, during charge and discharge phases. Most redox flow battery systems employ bipolar multi-cell stacks. The unlimited possibility of storing large volumes of positively charged (+) and negatively charged (-) electrolyte solutions (in electrochemical sense) in respective reservoirs make these systems exceptionally suitable for storing energy from renewable sources like wind and solar and other intermittent sources, as well as for load-leveling (peak-shaving), in the electric power generation and distribution industry.

Applications of this type, contemplating an ability of storing energy at power ratings in the order of MW or GW, raise technical problems that are not encountered nor considered when storing energy converted by relatively low power sources such as renewable source energy conversion installations operating with a relatively low voltage. As any electrochemical system, a redox flow battery system implies that besides the electrical connections to the bipolar stack, also the whole bodies of electrolyte solutions in the respective hydraulic circuits, including the volumes contained in respective reservoirs, may be at a DC voltage corresponding in practice the to the single operating cell voltage, multiplied by the number of cells of the bipolar stack. For example, in an all-vanadium redox flow battery system, a typical range of variation of the single cell voltage is from about 1.24 VDC to about 1.5 VDC.

Safety, practical considerations and attendant problems of anodic corrosions and of parasitic evolution of oxygen and/or hydrogen at "hot spots" on conductive elements, in contact with respective electrolyte solutions, that may become polarized at excessively large voltage difference, impose limits to the maximum number of cells that may be assembled in a bipolar cells stack, such that the total voltage at stack end terminals remains below a certain maximum value (generally between 40 VDC and 100 VDC). When dealing with extremely large power ratings as would be needed in renewable source energy plants requiring large energy storage capacities, in peak- shaving applications and alike, such a limitation would require implementation of tens or even hundreds bipolar stacks and individual hydraulic circuits of the two electrolyte solutions, depending on the ability to realize bipolar cell stacks with active cell area (width of the flow compartments hydraulically separated by a permionic membrane and of the electrodes of opposite polarity contained therein) as large as possible in order to enhance the power yield from each bipolar cell stack. Unfortunately, the size of flow compartments and of the electrodes (active cell area) is limited by technological fabrication problems and by other fluid- dynamical and electrochemical considerations concerning the need of ensuring conditions of uniformity of the current density across the whole active area of the cells.

The traditional bipolar cell stack configuration suits particularly relatively small power and relatively high voltage applications, but implies severe limitations for large power applications.

• Essential stack components, in particular: bipolar intercell interconnects, permionic membranes and porous mat or felt electrodes, commercially come in widths of about lm and would be difficult for manufactures to produce much larger sizes.

• It is increasingly difficult to ensure a uniform distribution of the two (undepleted) electrolyte solutions over an excessively large active cell (active electrode) areas.

On the other hand, wind and solar energy systems of several MW are already in operation and their size will grow to several GW. These systems will require storage systems of several MW.

With available bipolar cell stack technology and using the conductive plates, membrane and graphite felt porous electrodes of the largest commercially available sizes the maximum rated power of a 500 VDC battery circuit would be of only 30 kW (of just 3 kW for a circuit of 48 VDC rating). Therefore, in order to satisfy a requested storage capacity of 1 MW, 33 circuits at 500 VDC or 330 circuits at 48 VDC would be needed.

Redox flow battery systems of 1 to 50 MW or larger is what is required nowadays and the possibility of having a multi cell battery system with a rated storage capacity of 1 MW, wholly installed in a standard 40 feet container would be highly desirable, for flexibility and modularity reasons. However, such a containable redox flow battery system with a rated capacity of 1 MW operating at a safer low voltage of 48 VDC require a battery current of approximately 20,000 A. Operating at a reasonable current density of 800 A/m², hardly conceivable cells.

Monopolar cells have always represented the alternative option to bipolar cells when large currents are involved, because of the possibility of connecting each electrode to a respective DC rail that can be realized using thick copper buses for limiting ohmic losses. External connectivity of each electrode and associated external DC buses, render monopolar cell systems far more expensive than bipolar cells systems.

Moreover, in case of large power systems requiring a large number of cells, the compactness that can be achieved with bipolar cell stacks can hardly be matched with monopolar cells, even if organized, similarly to traditional bipolar cells, in a multicell stack having a "filter-press-like" structure alike a common bipolar cell stack, because of the dimensional allowances that must be made for providing an external electrical lug extension in each monopolar intercell interconnect to be connected to the respective rail of a DC bus. Numerosity of electrical connections due to the need of individually connecting the stacked intercell interconnects that distribute electric current to electrodes of same polarity of two sequential cells of the stack, besides imposing the use of DC rails of very large cross section, give rise to significant cumulative contact losses in view of the sheer numerosity of connections. In addition, a drawback of paramount importance, common to bipolar as well as to monopolar cell stacks remains the technical difficulty of making the active cell area as large as would be desirable, for reasons already mentioned above for the more usual case of bipolar cell stack assemblies.

SUMMARY Most if not all these limitations and drawbacks of common filter press type multicell stacks of bipolar or monopolar type are decisively reduced by a novel electrochemical flow redox cell multi-tier stack assembly of this disclosure, adapted for energy storage even at large power ratings, in the order of MW or GW. The novel multi-tier stack is composed of monopolar cells having two flow compartments containing the positive and negative electrode respectively, hydraulically separated by a permionic membrane and defined by stacked electrically conductive plates of current distribution (briefly intercell interconnects) to electrodes of same electrical sign over both sides (i.e. belonging to two sequential cells of the multi-tier stack).

Each tier of the stack includes a plurality of such monopolar cells, sideway juxtaposed one next to the other; and two or more of the intercell interconnects of the cells of the plurality, electrically not in contact among them, are juxtaposed side-by-side one next to the other and each of them extend sideway over the projected perimeter of the flow compartments of two adjacent monopolar cells of the plurality of cells laying on the same tier of the stack, defining flow compartments of electrodes of opposite signs in two sideway adjacent monopolar cells, of the electrodes of which they constitute bipolar intercell interconnects or alternatively even the working electrodes themselves, over unmasked active areas on their opposite sides, in contact with the relative electrolyte solution.

Electrical connection to an external DC bus is made through terminal intercell interconnects, purposely provided with a lug extension projecting out of opposite flanks of the stack assembly, belonging to side end cells of the plurality of monopolar cells of every tier of the stack, for connection to a respective rail the DC bus, extending alongside the stack.

In practice, alongside the two flanks of the multi-tier stack there will be external lug connections to the rail, respectively of one and of the opposite sign. If the number of juxtaposed monopolar cells of every tier of the stack is even, the terminal intercell interconnects will both lay on the same plane of planar intercell interconnects juxtaposed side-by-side one next to the other, whilst the number is odd, the terminal intercell interconnects will respectively lay on one and on the other plane of juxtaposed planar intercell interconnects of the monopolar cells of the tier defined there between. In every case, there will be only two such electrical lug connections to the DC bus for each tier of the stack, independently from the number of monopolar cells of each tier. This overcomes size constraints and associated problems in enlarging the active cell areas of the stack assemblies of the prior art.

The novel stack architecture makes possible to enlarge the active cell area that can be deployed at every tier of a classic "filter press type" stack assembly theoretically without limit of width while ensuring an undecremented uniformity of current density over the whole active cell area of every tier of the stack. The sideway extending bipolar intercell interconnects of each tier of the stack practically enforce a substantial uniform current density over the individual cell areas of a number of juxtaposed adjacent monopolar cells of each tier of the stack assembly. Multiple internal manifolds and/or internal ducting for each of the two distinct electrolyte solutions ensure an even distribution of electrolyte solution flow rate through the plurality of respective cell compartments of the juxtaposed monopolar cells of each tier of the stack.

Total number of external electrical connections to the respective DC voltage rails as typically required by a substantially monopolar cell stack architecture may be reduced by a factor ranging from 1/3 to 1/12 or even smaller.

The invention is defined in the annexed claims, the recitation of which is to be intended constituting part of the specification and the peculiar features and advantages of the stack architecture of this disclosure will become even more clear by going through the ensuing description of exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic exploded detail view of the stackable elements defining the electrochemical cells according to an embodiment of the stack architecture of this disclosure.

Figure 2 is an enlarged detail view of the internal constitution of flow compartments and electrodes of the individual cells of the multi-tier stack assembly. Figure 3 is a schematic cross sectional plan view of a four-tier stack showing the inlet and outlet pipes of the two electrolyte solutions.

Figure 4 is an exemplary layout view of a bipolar intercell interconnect with masked perimetral areas and central strip area.

Figure 5 is a replica of the cell stack of FIG. 3 for an alternative embodiment wherein a serial flow of the electrolyte solutions in series through the respective cell compartments of the cells belonging a tier of the stack is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The three-dimensional simplified exploded view of a four-tier stack of electrochemical redox flow cells of the present disclosure of FIG. 1 illustrates the novel architecture.

As observable in the drawing, the multitier stack assembly may be expanded for as many tiers of stacked monopolar cells as appropriate by introducing as many repetitive sequences of the stackable elements, and the stack is terminated by common end headers hi and h2 that define the flow compartment of the cells of the first and of the last tier of the stack. Distinct distribution and collection ports, or chambers in the headers or distinct inlet and outlet pipes, inP and outP, as illustrated in the exemplary embodiment, allow to circulate the solutions in the respective compartments of all the cells of the stack according to common practices in the art. In the considered embodiment, the flow mode of the two electrolyte solutions is "parallel" in all the relative cell compartments of the stack.

Alternatively, using a different through hole match between bas-relief patterned gaskets of the membrane assemblies and of the planar conductive intercell interconnects a "serial" (cascade) flow of the two electrolyte solutions in respective cell compartments can be implemented, as amply described in the prior patent application PCT/IB2010/001651, of the same applicants, filed on 29 July 2010, wherein an outstandingly compact and simplified (frame-less) structure of a stack of either bipolar or monopolar cells, with internal ducting for the circulation of the two electrolyte solutions in the respective compartments of all the cells of the stack is disclosed.

Looking at the exploded view of FIG. 1, it may be observed that each tier of the "filter-press like" stack assembly can be identified by the side-by-side juxtaposed permionic membrane (M) assemblies that in the example considered are in number of six of such identical permionic membrane assemblies though theoretically they may be in any number, preferably an even number.

Each permionic membrane assembly includes a permionic membrane M, in form of a flexible film of an ion exchange polymer adapted to exchange anions, cations or both of the chosen electrolyte, depending on the destination of use of the electrochemical reactor (i.e. composition of the electrolyte solutions used), the perimeter portion of which is sandwiched between two identical parallelepiped elastomer gaskets Gl and G2, bas-relief patterned on their front side and disposed back-to-back (in the view of FIG. 1, only the front side (up-side) of one of the two identical gaskets is visible). The permionic membrane (M) assemblies may, as depicted, be similar to those described in said prior patent application PCT/IB2010/001651. The description of the peculiar effectiveness of the use of substantially planar electrical interconnects of a conductive resinous aggregate or of laminated metal sheets, having perimeter portions of their surfaces and of circulation though holes masked by insulating grommets and laminated surface films, and of elastomer gaskets having perimeter portions of a (front) surface bas-relief patterned, in achieving the goal of enhanced compactness and avoiding the need of rigid plastic frames and of the solutions of attendant technical problems are herein incorporated by express reference to the above identified prior disclosure of the applicants. The planar electrically conductive interconnects Γ and I" may be of an electrically conductive aggregate of particles of graphite and/or carbon and a resin binder that may be a thermosetting resin, for example an epoxy base resin, or even a hot moldable polyester or a polyolefin resin binder. In order to increase lateral conductivity, the conductive body, if made of an aggregate, may incorporate a metal plate, for example an aluminum plate, a metal wire or carbon fiber gauze or an expanded metal sheet as high conductivity core layer sandwiched between sheets of a conductive aggregate (by lamination) or embedded therein (by molding). Alternatively, the electrically conductive interconnects Γ and I" may be metal plates or laminates even of different metals or including conductive and non conductive layers.

The body of the electrically conductive interconnects I' and I" may be in the form of a relatively thin sheet of aggregate or a laminated article of sufficient stiffness once cut to size, through which flow holes are drilled, such to geometrically match (align with) through holes of the patterned elastomer gaskets Gl and G2.

Grommets or rings of a suitable plastic material, for example PVC, may be set into the holes to constitute an electrically non conductive lining of the flow passages through the electrically conductive interconnect.

Also the perimeter surfaces destined to press against all the seal areas of the bas- relief patterned front faces of the elastomer gaskets of the membrane assemblies belonging to two adjacently stacked cells are rendered electrically non conductive by laminating over the opposite sides of the electrically conductive interconnects I' and I", appropriate masking films of a suitable electrically insulating material, generally a plastic film. The electrically insulating mask film may be glued onto the surfaces of the electrically conductive interconnect or hot laminated thereon in order to bond to the plastic matrix of the aggregate body of the interconnect to the metallic surfaces in case of metallic interconnects. Alternatively, the same result may be obtained by applying an insulating enamel using an inverted application mask for spraying the insulating enamel. In any case, the insulating surface films overlay and are bonded onto end surfaces of the lining ring or grommets set through the holes, in order to secure isolation from contact with the electrolyte solutions in the so coated areas of the electrically conductive interconnects Γ and I". Two identical gaskets Gl and G2 (only the one having its bas-relief patterned front side looking upward being visible in the three-dimensional exploded view of FIG. 1) define a central aperture or window closed by the membrane M that has perimeter edge portions sealingly held between essentially flat seal surfaces of the unpatterned back side of the two identical gaskets, assembled back-to-back, thus providing for the required hydraulic separation between the flow compartments of the cell, on one side and on the opposite side, respectively, of the permionic membrane M. Therefore, the active area of each single cell will practically correspond to the area of the central aperture defined by the central parallelepiped window of the two identical bas-relief patterned gaskets, mounted back-to-back, which is closed by the membrane M, sandwiched there between.

In the case of the "sideway bipolar" interconnects I", the electrically insulating masking films include a central strip that besides rendering electrically insulating the seal surface areas along the adjacent two sides of two juxtaposed membrane assemblies, practically divide the planar surfaces on both sides of the sideway- extended bipolar interconnects I", in two unmasked areas, each of which substantially coincides with the geometrical projection of the central window defined by the two gaskets of the membrane assemblies (i.e. with the active cell area), thus defining the respective flow compartments of two adjacent cells of the tier. Said central masked strip of the bipolar interconnects I", pressing on perimetral seal surfaces of the patterned elastomer gaskets of distinct membrane assemblies of two adjacent cells of the tier, effectively shields from contact with the two electrolyte solutions such "blind" cross-over portion of the planar bipolar interconnects I". For the exemplary embodiment shown in FIG. 1, the gaskets Gl and G2 have four through holes 1, 2, 3 and 4 that, once the stack is completed and tightened, will form, together with aligned through holes in respective bipolar intercell interconnects I" or terminal monopolar intercell interconnects Γ, inner inlet and outlet manifolds of circulation of the two electrolyte solutions in parallel in the respective cell compartments of all the cells. As observable from the visible side of the gaskets Gl and G2, that is their "front side" as opposed to their flat (unpatterned) back side, has a bas-relief patterned perimeter seal area 5 that has loops adapted to contour completely a through hole 2 and a diametrically opposite through hole 4 (by-pass flow path of the electrolyte solution destined to flow in the other flow compartments, not visible in FIG. 1, of the cells).

Over two opposite perimeter sides of the central aperture there are two similar pluralities of patterned seal areas that define therebetween elongated split flow channels 6 that extend from a rim region 7 of non-contoured through holes 1 and 3 at the other diagonally opposite locations of the membrane assembly (inlet and outlet ports of the electrolyte solution flowing in the flow compartments visible in FIG. 1 of the cells).

As described in the above cited prior disclosure, all patterned seal areas at the top of the salient portions defined over the front side of the elastomer gaskets Gl and G2 have the same height, being destined to press against a substantially planar surface of the electrically conductive intercell interconnects I' and I", the core of which may be an electrically conductive aggregate of conductive particles, for example of graphite, or sheets of metal or even of a conductive laminated composite. Therefore, the salient patterned parts of elastomer over the front side of every gasket besides establishing a hydraulic seal over the counter-opposed surface of the interconnects Γ and I", define electrolyte flow ducting and the empty compartments void volume (eventually containing compressible porous electrode structures, not traced in FIG. 1) through which the respective electrolyte solution flows. In many important applications, typically for a redox flow storage battery system, the active electrodes may be compressible mats or felts of carbon fibers (not shown in FIG. 1) disposed in both flow compartments of every cell in electrical contact with the electrically conductive intercell interconnects Γ and I". The mat or felt electrodes constitute porous electrode through which the electrolyte solution may flow in a "lateral" direction from an inlet side of the flow compartment to the opposite outlet side of the compartment, providing for an augmented active electrode surface adapted to sustain the electrochemical reduction or oxidation (redox) reaction at the electrode at relatively large current densities, referred to the projected cell area. Though conductive adhesives may be used to enhance electrical conductivity through the bipolar electrode assembly composed of the mat or felt electrodes in contact with opposite surfaces of the electrical intercell interconnect, the electrical contact may also be ensured by a moderate compression of the mat or felt electrodes between the membrane separator and the intercell interconnects, upon tightening the stack.

In order to reduce the pressure drop caused by porous electrode mats or felts that are traversed sideway by the electrolyte solution flowing from an inlet port to an outlet port defined by the bas-relief elastomer gasket, usually at diagonally opposite points of the perimeter of the compartment void, a plurality of split flow distributing channels my be cut or formed in the mat or felt, oriented in the flow direction from an inlet side to an outlet side of the porous electrode mass. A particularly effective manner is to forcibly introduce in the porous mass thin walled tubes, alternately closed at their downstream end or at their upstream end, and having numerous holes pierced at regular intervals all along through their wall. The downstream-end plugged tubes acting as flow sources and the upstream- end plugged tubes interleaved to the downstream-end plugged tubes, acting as flow sinks. Of course the outer diameter of such flow distribution aiding foraminous tubes will be smaller or equal the "compressed" thickness of the porous electrode mat. Reduction of pressure drop through particularly dense porous electrode structures occupying the whole depth ("thickness") of the flow compartments of the cells, may become important particularly if a serial flow rather than a parallel flow is implemented in cells electrically in series (bipolar cells) that in the multi-tier cell stack architecture of this disclosure are such only along each transversal sequence of cells of each tier of the stack (i.e. cells juxtaposed cells sharing a sideway bipolar planar interconnect.

For stacks composed of a large number of multi-cell tiers, it could be difficult to guarantee that all the elastomer gaskets be equally and uniformly compressed. In order to ensure maintainment of parallelism between the planar conductive intercell interconnects Γ and I" when tightened against the bas-relief patterned front sides of the elastomer gaskets of the membrane assemblies interleaved there between, and a precisely defined identical void space in all the flow compartments of the cells upon tightening the stack, each two-gasket membrane assembly may be contoured by plastic spacers 9 having a thickness corresponding to a designed maximum compression of the elastomer gaskets between the planar interconnects, adapted to reliably secure that all hydraulic seals defined by the bas-relief patterned elastomer gaskets form substantially leak proof internal manifolds and split flow ducting 6, and at the same time avoid localized over compression of the elastomer gaskets and/or of the compressible mat or felt electrodes, if present in the compartment, making the planar interconnects I' and I" perfectly parallel to each other and equally spaced.

In the embodiment shown in FIG. 1, the spacers 9 may be in the form of four strip spacers, adapted to be joined at the four corners, to constitute a perimetral spacer contouring the outer perimeter of the two gaskets Gl and G2. FIG. 2 is a detail partial sectional view of the four-tier stack of FIG. 1 that permits to observe the manner in which the repetitive sequence of stacked basic elements creates, once compressed between two headers hi and h2, the distinct flow compartments of the pluralities of cells of each tier of the stack.

The enlarged partial sectional view besides showing the relative position of the basic stacked elements described with reference to the exemplary embodiment of FIG. 1, shows the flow compartments of each cell as empty spaces separated by the membrane M.

Though the planar interconnects I' and I" may themselves provide a suitably activated or structured active surface facing the compartment void and thus in contact with the flowing electrolyte solution, practically adapted to act as a monopolar cell electrode, according to a preferred embodiment particularly effective for electrochemical cell stack destined to function as multicell electrochemical reactor of an all-vanadium redox flow battery system, the empty spaces of the respective flow compartments of each cell are at least partly filled by a porous mat or felt of carbon fibers slightly compressed between the permionic separating membrane M and the opposing surface of the respective intercell interconnect, whether a bipolar interconnect I" or a terminal intercell interconnect I'.

FIG. 3 is a complete (uninterrupted) cross sectional view of the four-tier exemplary stack of FIG. 1 , showing the plurality of tie rods r acting on opposite robust stress structures PI and P2 of the two headers hi and h2, for compressing the stacked elements therebetween, according to a common "filter-press" like electrochemical cell stacks architecture. The inlet and outlet pipes may be all branches of respective inlet and outlet manifolds of circulation of the two electrolyte solutions in all respective compartments of the cells manifolds in order to implement a parallel flow mode.

FIG. 4 is a layout view of a bipolar intercell interconnect I showing the flow holes with an insulating plastic lining 10 and the pattern of the insulating plastic film 11, laminated over both sides of the rectangular electrically conductive planar interconnect I in order to render electrically nonconductive the penmetral seal surfaces, destined to press against the patterned front side of the respective elastomer gaskets Gl and G2. The insulating plastic film 11 laminated over the surface on both sides of the electrically conductive planar body of the interconnect I overlies at least partially end surfaces, with which establishes o secure bond, of plastic grommets or rings 10 that line the hole wall, such to isolate the planar seal areas and the walls of the flow holes (internal manifolds of distribution of the two electrolyte solutions into the respective compartments of all the cells of the stack) from preventing the discharge of ions in the electrolyte solutions. The two unmasked areas 12a and 12b substantially coincide with the projection of the central window of the membranes Gl and G2 of the membrane assemblies and substantially represent the respective active areas of two adjacent monopolar cells, juxtaposedly defined one next to the other along each tier of the stack. Over the unmasked conductive areas 12a and 12b of the bipolar intercell interconnect I", may be disposed porous electrode structures, for example in form of a porous compressible mat or felt of carbon fibers, such to establish a good electrical contact with the unmasked conductive plate of the bipolar interconnect upon compressing the stack assembly. According to the multi-tier monopolar cell stack architecture of this disclosure, the electrode on the unmasked area 12a will result polarized as anode in respect to the electrolyte solution flowing in the cell compartment of the left-hand side monopolar cell, and the electrode on the unmasked area 12b will be polarized as cathode in respect to the other electrolyte solution flowing in the respective compartment of the right-hand side monopolar cell, or viceversa. Of course the unmasked areas 12a and 12b on the other side of the same intercell interconnect I" will have the same polarizations and eventually in contact with porous electrodes of corresponding monopolar cells of a next tier of the stack.

The plurality of intercell, interconnects Γ and I" destined to lay on a same plane of the multi tier stack may be singularly arranged one next to the other upon stacking the elements, exploiting the tie rods r as effective assembling guides by slipping each interconnect along respective rods r, passed through spaced perimetral holes or slots purposely present in the interconnect, eventually lined by insulating grommets. Alternatively, the plurality of intercell, interconnects Γ and I" destined to lay on a same plane of the multi tier stack may be joined together (mechanically) to form a stackable monolithic planar composite, for example by joining side-by-side the conductive interconnects with an epoxy resin adhesive or with purposely designed non conductive fasteners.

FIG. 5 is a replica of the cell stack of FIG. 3 for an alternative embodiment of the stack architecture of this disclosure wherein a serial flow of the electrolyte solutions in series through the respective cell compartments of the cells belonging a tier of the stack, by feeding the solution into the respective compartment of a side end cell and from there into the respective compartment of the sideway juxtaposed cell and so forth as far as to exiting the relative compartment of the other end cell of the tier. Of course the membrane assemblies, the pair of front side bas-relief patterned gasket and the interconnects will have an arrangement of perimetral through holes adapted to implement the serial flow mode along the cells of each tier of the stack. Different bas-relief patterns and alternating match of through holes to implement zig-zag flow patterns of the two solutions across each tier of the stack, may be as described in detail in the above cited prior patent application of the same applicant.

EXAMPLE

Storage capacity of an 80-tier stack each tier comprising 1 1 unit cells electrically in series. The cells are operated with the membranes in vertical position.

Cell thickness:

Aluminum core plate of interconnects: 1.5 mm

Two conductive carbon aggregate sheets of 1.5 mm

laminated over the two sides of the aluminum core plate: 3.0 mm

Masking plastic films of 0.25 mm laminated over both sides: 0.5 mm

Two bas-relief patterned gaskets sandwiching the membrane: 7.0 mm.

Approximate cell thickness: 12 mm

Stack length:

Approximate overall thickness of 80 cells (tiers): 960 mm

Total width of the stack including end header structures: 1 ,100 mm

Stack height: Height of conductive carbon aggregate: 1 ,000 mm

Height of masked portions above and below (200 x 2): 400 mm

Height of the active area: 1000 - (2x200): 600 mm

Space required for tie rods above and below (100 x 2): 200 mm

Total height of the stack: 1,200 mm

Stack width:

Width of one cell: 500 mm

Thickness of PVC spacers (2 x 25 mm): 50 mm

Thickness of the rubber gaskets (2 x 15 mm): 30 mm

Width of the active area: 500 - 50 - 30 = 420 mm

Area of the electrodes (active area): 0.42 x 0.60 0.252 m².

Total area of 80 cells: 0.252 x 80 = 20 m².

Load @850 A/m² = 17 kA

Nominal voltage of 1 1 cells (@ 1.35 Volt/cell): 14.85 Volt

Nominal power of 1 1 cells: 17 x 14.8 = 252 kW

Approx. volume of the stack: 5.72 x 1.1 x 1.2 = 7.55 m³

Approx. power density 33.4-kW/m³ or

0.0334 kW/liter or 1 kW in 30 liters.

Two battery systems (for an approx. rating of 500 kW @ 29.7 Volt) could be installed in a 20' container.

For an all-vanadium redox flow battery system operating with a specific flow rate of the electrolyte solutions in the cell compartments of 0.1 liters/dm² (of active cell surface), the total flow required by a standard 20' container with 4 stacks (each stack with 1 1 cells) when all cells are operated in parallel (inner manifolds as in the exemplary stack structure described and illustrated in the drawings) will be about 132 x 4 = 528 mVh.

The expected pressure drop (using a graphite felt with distribution flow channels formed in the porous felted mass of carbon fibers) will be in the range 0.6 - 0.8 bar. By contrast, should the four stacks have a different internal ducting adapted to flow the two electrolyte solutions in series (cascade) as described as an alternative flow mode embodiment of the frame-less membrane assemblies disclosed in the cited prior patent application of the same applicant), the flow will be about 132 m³/h and the pressure drop of approximately 3 bar.

Four systems (1.0 MW @ 59.4 Volt) could be installed in series in a 40' container for a total rated power will be 1.0 MW at approximately 60 Volt.

By deploying ten 40' containers connected in series, the plant would have a rated voltage of 600 Volt and a rated power of 10 M W.

Thus by using commercially available materials (membranes, carbon felt, carbon aggregate plates), a 10 MW redox flow multi battery plant operating at 600 Volt can be realized.

By contrast, a plant of same power rating realized with bipolar cell stack architecture of the prior art using commercially available materials (membranes, carbon felt, carbon aggregate plates) would require 46 circuits, each comprising 440 cells, for a total of 20,240 cells. Besides making feasible multicell stacks of outstandingly large power rating, without requiring technological advances in the commercial processes of manufacture of essential functional materials of constitution of the cells, the dramatic reduction from would-be 46 circuits to 1 circuit as afforded with the novel hybrid (monopolar stack-wise-bipolar width-wise) multicell stack architecture of the applicant, achieves an outstandingly large saving of space and weight.

Claims

1. Electrochemical redox flow multi-tier cell stack, composed of cells having flow compartments containing an electrode, respectively for a first and a second electrolyte solution, hydraulically separated by a permionic membrane and defined by electrically conductive intercell interconnects, characterized in that each tier of the stack includes a plurality of stack-wise monopolar redox flow cells, sideway juxtaposed one next to the other;

each tier of the stack includes a plurality of distinct membrane cell assemblies juxtaposed one next to the other and sandwiched between two pluralities, first and second, of planar conductive intercell interconnects juxtaposed across the width of the stack one next to the other, all having the same sign of polarization on opposite sides thereof that respectively define the flow compartments for the same electrolyte solution of adjacent stacked cells belonging to distinct tiers of the stack; each of said pluralities of planar conductive intercell interconnects juxtaposed across the width of the stack one next to the other includes at least a planar conductive bipolar intercell interconnect extending sideway such to lie over the projected perimeter of flow compartments of two adjacent monopolar cells juxtaposed one next to the other of the tier, respectively for said first and said second electrolyte solution, changing its sign of polarization versus the two distinct electrolyte solutions;

at least two terminal monopolar intercell interconnects having an electrical connection lug portion projecting out of opposite flanks of the stack assembly at ends of said pluralities first and second or at the opposite side ends of every second plurality of planar conductive intercell interconnects juxtaposed across the width of the stack one next to the other.

2. The electrochemical redox flow cell multi-tier stack of claim 1, wherein said two electrolyte solutions are flown in respective flow compartments of all the cells through internally defined inlet and outlet pairs of manifolds or through internally defined ducting, implementing a parallel or a serial flow in the respective cell compartments.

3. The electrochemical redox flow cell multi-tier stack of claim 1, wherein each of said membrane assemblies comprises:

two identical parallelepiped elastomer gaskets defining a central aperture and having at least two through holes along two opposite perimeter sides thereof, a flat perimeter seal surface on a backside and on the opposite or front side thereof: a bas-relief patterned perimeter seal area contouring one every two through holes along said two opposite perimeter sides and two similar pluralities of patterned seal areas defining there between flow channels extending from a rim region of non-contoured through holes to the edge of the nearest juxtaposed side of said central aperture of the gasket;

the permionic membrane having perimeter edge portions sealingly held

between said flat perimeter seal surfaces of the two identical gaskets, disposed back-to-back.

4. The electrochemical redox flow cell multi-tier stack of claim 2, wherein surfaces of said planar conductive intercell interconnects destined to be pressed against all raised seal areas of the bas-relief patterned front of the elastomer gaskets of said membrane assemblies are rendered electrically nonconductive by laminating over the opposite sides of the electrically conductive interconnect masking films of an electrically insulating material.

5. The electrochemical redox flow cell multi-tier stack of claim 1, wherein a porous electrode, through the pores of which flows an electrolyte solution, is disposed over each unmasked surface area of a planar interconnect closing a cell compartment and bonded to or held in electrical contact with the conductive interconnect for acting as an ion charging or ion discharging electrode of specific surface area greater than the projected area of said unmasked surface area.

6. The electrochemical redox flow cell multi-tier stack of claim 5, wherein said porous electrode is a compressible porous felt or mat of carbon fibers.

7. The electrochemical redox flow cell multi-tier stack of claim 1, wherein each unmasked surface area of a planar interconnect closing a cell compartment has an active coating adapted to act as an ion charging or ion discharging electrode surface.

8. The electrochemical redox flow cell multi-tier stack of claim 1 , wherein said electrically conductive planar intercell interconnects comprise a conductive aggregate of graphite particles and/or carbon fibers and of a resin.

9. The electrochemical redox flow cell multi-tier stack of claim 1, wherein said electrically conductive planar intercell interconnects comprise a conductive laminated including a metallic core sheet.

10. The electrochemical redox flow cell multi-tier stack of claim 9, wherein said electrically conductive planar intercell interconnects comprise outer sheets of conductive aggregate of graphite particled and/or carbon fibers and of a resin, laminated together with said metallic core sheet.