An energy storage system for maintaining the maximum amount of electrical power delivered over short and long periods to electrically driven equipment or machinery.

Abstract

Invention details;

Description

[0001] ENERGY STORAGE SYSTEM FOR SUSTAINING POWER DELIVERY PEAKS OF SHORT AND OF PROLONGED DURATION TO ELECTRICALLY DRIVEN EQUIPMENTS OR MACHINES

[0002] STATE OF THE ART

[0003] Electrically driven machineries, when started and in many cases also during intermittent or variable phases of operation, when their mechanical load, from an idle or stand-by condition, raises to a full operative level, draw a proportionate high current from the power supply albeit for a limited time. The augmented current absorption from the supply source may often last for a prolonged time after a relatively short-timed initial peak. In frequent situations, the effects of these temporary augmented current absorptions are unacceptable voltage drops, due to the impedance associated to the power supply grid. The drop of supply voltage may in turn cause malfunctions in other equipment connected to a local and often private voltage stabilization and energy storage DC power circuit derived from the power grid.

[0004] Banks of rechargeable batteries are often employed for stabilizing the voltage on a local DC power network. Although batteries can store and release a large amount of energy over extended time periods, most of them are hardly able to provide the short-timed, high-power levels required for voltage stabilization, because of electrochemical limits to the sustainable current density over the active area of cell electrodes and other causes of significant internal resistance.

[0005] Recent commercial developments of so-called supercapacitors or ultracapacitors (UC) with capacitances of thousands of Farads, exploiting the outstanding properties of graphene, promotes their use as rechargeable means of energy storage of considerable capacity that, differently from batteries, may be released at high power levels, though for a comparatively short, period of time (in the order of minutes).

[0006] Association of the two rechargeable means of energy storage has been pursued for exploiting their distinct properties. In particular, the redox flow batteries differently from lead acid and lithium batteries, offer a practically unlimited storage capacity that may be easily increased in case of an eventual future Increased demand. A chain or string of UCs connected in series is frequently connected in parallel to a multicell battery or to a bank of multicell batteries connected in series and the parallel of battery or batteries and or UC is connected to a DC rail of the local power network. Such a combination has several implications as the UCs store energy linearly proportionally to their voltage (from 0 to a maximum value of 2.7 Volt) while, most of tire rechargeable batteries operate at a substantially constant voltage. Moreover, lithium batteries as well as lead acid batteries operate at a constant voltage that is close to the maximum voltage of a UC. The constancy of voltage of the battery limits the amount of energy dischargeable from the UCs. Therefore, in case of a prolonged energy demand after the eventual short-timed initial peak, most of the considerable energy stored in the UCs may never be eventually exploited to a reasonable extent. The limited amount of its stored energy that a UC would contribute during a prolonged augmented power request to the electrical supply, often in the order of several tens of minutes, hardly justifies its cost, notwithstanding the advent of graphene has significantly mitigated it. UCs have a limit of the voltage at which can operate, that in general is 2.7 Volt, although it may extend up to 3.0 V in some commercial models. Above its limit voltage the capacitor is rapidly damaged and become unsafe. For most voltage stabilization use, large numbers of capacitors in series need to be employed, but uniform distribution of relatively large DC voltages (up to 800 - 1 ,500 V required by typical high-power inverters) is elusive as the capacitance of individual UCs may significantly differ. Reliance and durability' reasons make mandatory the incorporation of individual voltage limiting means of active or passive type in commercial UCs, adding significantly to their cost. Moreover, there are significant practical limits to the number of UCs to be installed in electrical series as would be required by large storage systems operating at high DC voltages (800 V to 1500 V).

[0007] OBJECTS OF THE INVENTION

[0008] It is an object of the invention to provide a system of energy storage and of uninterruptahle power supply of large storage capability, capable of delivering power peaks of short and of prolonged duration, as required by electrically driven machinery of an electricity grid and / or local and / or private power distribution network, comprising one or more multieell redox flow batteries, electrically connected in series, and one or more strings of serially connected supercapacitors, of enhanced cost effectiveness, efficiency and reliability.

[0009] THE INVENTION

[0010] For describing the invention, certain peculiarities of supercapacitors and of the kind of supereapacitors pertaining to the practice of this invention should be recalled by referring to drawings and diagrams, so as for illustrating an exemplary practical embodiment thereof. FIG. 1 is a simplified view of a double-layer of negative ions at an electrode and solvated positive ions in the liquid electrolyte, separated by a layer of polarized solvent molecules;

[0011] FIG. 2 is a simplified view of a double-layer with specifically adsorbed ions which have submitted their charge to the electrode, explaining the faradaic charge-transfer occurring in a so- called pseudo-capacitance;

[0012] FIG. 3 depicts the voltage excursions of charge and discharge of a supercapacitor (on the left) and of a redox cell (on the right);

[0013] FIG. 4 are typical voltage vs. current density characteristics of a redox cell;

[0014] FIG. 5 is a block diagram of a power distribution grid that includes an energy storage system for voltage stabilization of the type and capabilities as those remarked in this disclosure.

[0015] FIG. 6 shows an exemplary embodiment of this invention whereby the individual capacitors of the string of capacitors in series are prevented to reach overvoltage conditions that could damage or destroy them while implementing useful exchanges of electric charge with the plate electrodes of a redox flow battery.

[0016] FIG. 7 is a fragmentary, exploded, three-dimensional illustration of functional construction elements of a redox flow battery.

[0017] Electricity is stored in capacitors in two ways: electrostatically in traditional two plates capacitors; or electrochemicaliy, wherein ions are absorbed on the electrode and provide electrons without forming chemical bonds and without changing their oxidation state.

[0018] A typical construction of a supercapacitor is depicted in FIG. 1, showing a polarizing DC power source 1, positive and negative collectors 3, polarized electrodes 3, the Helmholtz double layers separated by a layer of solvent molecules 4, the electrolyte 5 with positive and negative ions and a separator 6.

[0019] By applying a voltage to the terminals of the depicted electrochemical capacitor, ions in the electrolyte, move towards the opposite polarized electrode forming the Helmholtz double layers, thus constituting a so-called supercapacitor.

[0020] In a sub-specie of supercapacitors, often referred to as pseudo-electrochemical capacitors, most of the electrical energy is stored in the form of reversible faradaic redox reactions occurring on the surface of suitable electrodes, as depicted in FIG. 3. Such a pseudo-capacitance results from an electron charge-transfer between electrolyte and electrode, from a de-solvated and adsorbed ion. whereby only one electron per charge unit is participating. This faradaic charge transfer originates by a very fast sequence of reversible redox, intercalation or electro-sorption processes. In this case, contrary to what happens in a redox battery, the adsorbed ion has no chemical reaction with the atoms of the electrode (no chemical bonds arise) since only a charge-transfer takes place.

[0021] The electrons involved in the faradaic processes are transferred to or from valence electron states (orbitals) of the redox electrode reagent. They enter the negati ve electrode and flow through the external circuit to the positive electrode where a second double-layer with an equal number of anions has formed. The electrons reaching the positive electrode are not transferred to the anions forming the double-layer, instead they remain strongly ionized and "electron hungry" transition- metal ions of the electrode's surface. As such, the storage capacity of faradaic pseudo-capacitance is limited to the finite quantity of reagents onto the available electrode surface.

[0022] A faradaic pseudo-capacitance only occurs together with a static double-layer capacitance, and its magnitude may exceed the value of double-layer capacitance, for the same surface area, by a factor in the order of hundreds, depending on the nature and the structure of the electrode (or much higher on graphene electrodes that has a surface active area of more than 2600 square meters per gram), because all the pseudo-capacitance reactions take place only with de-solvated ions, which are much smaller than solvated ions with their solvation shells.

[0023] The amount of electric charge stored in a pseudo-eapacitanee, expressed in Farad, is linearly proportional to tire applied voltage.

[0024] The system of this invention implies the use of such pseudo-supercapacitors and redox flow batteries that, differently from rechargeable batteries, have an open circuit cell voltage of similarly linear' charging and discharging voltage-charge characteristics, as depicted in FIG. 3. Indeed, the internal resistance of the battery cells will determine a temporary higher storage of electricity in the capacitors as long the charging current remains higher than zero, but the excess of energy stored in the capacitors is released to the battery cells as the charging current becomes zero, with the cells recovering to their open circuit voltage.

[0025] In the ensuing description, the shorter word capacitor(s) shall be used intending with It the above identified type of capacitor, namely the pseudo-supercapacitor. Another relevant aspect are the substantially linear characteristics of the cell voltage of a redox flow battery, in the example an al- vanadium battery, versus current density over the cell electrodes, during charging and discharging phases, as depicted in FIG. 4.

[0026] A diagram of a power distribution grid comprising an energy storage system for voltage stabilization of the type and capabilities of this disclosure is shown, in a self-explanatory manner, in FIG. 5.

[0027] According to this invention the practical limits of serially connecting a relatively large number of supercapacitors in series for high voltage energy storage and the costs of equipping each supercapacitor with active or passive voltage limiting means to prevent overvoltage damaging, are eliminated by implementing electrical connections of selected plate electrodes of an associated multicell redox flow battery stack to respective selected intermediate nodes of the string of supercapacitors connected in series, according to a “ratio” dependent on the overvoltage limit of the supercapacitor employed and open circuit cell voltage characteristic of the type of redox flow battery employed.

[0028] FIG. 6 exemplifies electrical connections of every second intermediate node of the string Sc of supercapacitors 1 to plate electrodes 2 of the batteries Bl, B2, B3 that define between them three battery cells, that is a ratio of two supercapacitors of the string in parallel to three battery cells. Considering tire use of commercial supercapacitors having a limit operating voltage generally comprised between 2.7 V and 3.0 V, and of an all-vanadium redox flow batteiy, the ratio guaranties that the limit voltage of the supercapacitors be never surpassed as the cell voltage of a batteiy cell is generally between 0.9 V and 1.75 V and, under any condition, remains well below 2.0 V.

[0029] In case different supercapacitors are used, and / or a different type of redox flow battery is employed, for example a Fe-Cr redox flow batteiy, every single capacitor of the string would be connected in parallel to two battery ceils, given that the maximum cell voltage is always below 1.25 V and therefore the would-be critical cell voltage of 1.35 V will never occur.

[0030] In this example of implementation, three multicell redox flow batteries Bl , B2 and B3 are shown connected in series and thence to the local DC power rails, + and and sharing the same storage and recirculation means of the chargeable and dischargeable electrolytic vanadium solutions, respectively through the negative and the positive electrode compartments of all the cells of the batteries. Additional strings of supercapacitors in series may eventually be added in an identical manner to the one illustrated in FIG. 6, for augmenting the energy storage capacity available for high power release to the DC power line.

[0031] Generally, the supercapacitors, installed in parallel to the battery cells may increase marginally the total amount of storable and stored energy as they have been deployed for the objective of multiplying by several orders, though for a definite and relatively short period of time, the power deliverable from a redox flow battery energy storing installation.

[0032] Alternatively, given the rapidly increasing capacitance of constantly enhanced supercapacitors one or more strings of supercapacitors capacitors may soon become an economically justified primary storing element of electrical energy and a redox flow battery storage, even of greatly reduced power rating ( i.e. dimensionally small in terms of active areas of the battery electrodes and, as such, marginally contributing to the energy storage capacity), would, according to another important aspect of this invention, provide the indispensable overvoltage limitation onto the individual supercapacitors, even in high voltage applications, requiring extremely long strings of supercapacitors in series.

[0033] The fragmentary, exploded, three-dimensional illustration of functional construction elements of a redox flow battery of FIG. 7 shows the bipolar plate electrodes 2, the separators 3 delimiting the negative electrolyte flow compartment and the positive electrolyte flow compartment of each cell of the multicell battery stack terminating with the negative end electrode 4 and the positive end electrode 9, the inlet and outlet manifolds 5 e 6 of the chargeable and dischargeabl e negative electrolyte and the inlet and outlet manifolds 7 and 8 of the chargeable and dischargeable positive electrolyte.

[0034] A practical predisposition to an eventual electrical connection of otherwise common bipolar plate electrodes of a typical multicell stack of a redox flow battery, may be that of including along their outer perimeter a relatively narrow strip appendix 10 adapted to protrude out of the outer perimeter the frame and of eventual gaskets, upon closing the filter-press assembly of the multicell stack of the battery. Tims, all the battery electrodes will be selectively connectable to a given intermediate node of a string of supercapacitors in series, according to the invention. The excursion of voltage of the battery cell is normally higher during the operations of charging and discharging, because of the internal resistance of the cells but, as soon as the charging or discharging operations are discontinued, the voltage of the cell assumes its open circuit value OCV. There will be a higher storage of electricity in the capacitors when the electric current is higher than zero, but the excess energy in the capacitor will be transferred immediately to the cells as soon as the current drops to zero whereby the cell voltage will stabilize to its OCV, The energy stored in the capacitors will transfer to the electrolytes, increasing (by a small value) the OCV value. Therefore, for the purpose of computing the energy stored in the capacitors, only the value of the OCV may be used. The maximum OCV value of the charged electrolytes could be increased by using the ancillary couple Ce+3 / Ce+4, that has a standard reduction potential of 1.44 V, that is 0,44 Volt higher than the standard reduction potential of V+4 to V+5.

[0035] Using the graphene supercapacitors of greatly enhanced capacitance (100,000 F instead of about 3,000 F of common supercapacitors), the storage capacity in the electrolyte solutions flown through the respective compartments of each cell of the battery may more than double itself when a supercapacitor of such capacitance is installed on each cell of a multicell redox flow battery. This allows a far less dissipative pumping of the electrolytes, as the pumps may intervene intermittently, in some situations, for just about 10% of the time.

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