U.S. patent application number 17/453324 was filed with the patent office on 2022-02-17 for methods and system for a battery.
The applicant listed for this patent is ESS Tech, Inc.. Invention is credited to Evan Doremus, Yang Song.
Application Number | 20220052363 17/453324 |
Document ID | / |
Family ID | 1000005940193 |
Filed Date | 2022-02-17 |
United States Patent
Application |
20220052363 |
Kind Code |
A1 |
Song; Yang ; et al. |
February 17, 2022 |
METHODS AND SYSTEM FOR A BATTERY
Abstract
Systems and methods for operating a redox flow battery system
may include switching the redox flow battery system to an idle
mode, wherein the idle mode includes operation of the redox flow
battery system outside of a charging mode and outside of a
discharge mode; in response to switching to the idle mode,
repeatedly cycling operation of an electrolyte pump between an
idling threshold flow rate less than a charging threshold flow rate
and a deactivation threshold flow rate; and in response to
switching to the charging mode, maintaining operation of the
electrolyte pump at the charging threshold flow rate greater than
the idling threshold flow rate. In this way, a responsiveness of
the redox flow battery system to charging and discharging commands
can be maintained while in idle, while reducing parasitic pumping
losses due to pumping and heating, and reducing shunt current
losses.
Inventors: |
Song; Yang; (West Linn,
OR) ; Doremus; Evan; (Portland, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ESS Tech, Inc. |
Wilsonville |
OR |
US |
|
|
Family ID: |
1000005940193 |
Appl. No.: |
17/453324 |
Filed: |
November 2, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15965673 |
Apr 27, 2018 |
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17453324 |
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62491954 |
Apr 28, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/04753 20130101;
H01M 8/188 20130101; H01M 8/18 20130101; H01M 8/043 20160201; H01M
8/04201 20130101; H01M 8/2455 20130101; Y02E 60/50 20130101 |
International
Class: |
H01M 8/04746 20060101
H01M008/04746; H01M 8/18 20060101 H01M008/18; H01M 8/04082 20060101
H01M008/04082; H01M 8/043 20060101 H01M008/043; H01M 8/2455
20060101 H01M008/2455 |
Goverment Interests
ACKNOWLEDGMENT OF GOVERNMENT SUPPORT
[0002] This invention was made with government support under
contract no. DEAR0000261 awarded by the DOE, Office of ARPA-E. The
government has certain rights in the invention.
Claims
1. A method of operating a redox flow battery system, the method
comprising: operating the redox flow battery system in an idle
mode, wherein the idle mode includes operation of the redox flow
battery system outside of a charging mode and outside of a
discharge mode; and while operating the redox flow battery system
in the idle mode, repeatedly cycling operation of an electrolyte
pump between an active state and an inactive state, wherein the
active state comprises pumping electrolyte via the electrolyte pump
at an idling threshold flow rate that is less than a charging
threshold flow rate, and wherein the inactive state comprises
operating the electrolyte pump at a deactivation threshold flow
rate that is less than the idling threshold flow rate.
2. The method of claim 1, wherein the electrolyte pump is ON in the
active state.
3. The method of claim 1, wherein the redox flow battery system is
switched from a different operational mode to being operated in the
idle mode.
4. The method of claim 3, wherein the different operational mode is
the charging mode or the discharging mode.
5. The method of claim 1, further comprising: switching operation
of the redox flow battery system to the discharge mode; in response
to switching to the discharge mode, increasing the flow rate to be
greater than the idling threshold flow rate; and maintaining the
flow rate at greater than the idling threshold flow rate while
operating the electrolyte pump in the discharge mode.
6. The method of claim 1, wherein operation of the electrolyte pump
at the deactivation threshold flow rate is maintained for at least
a first threshold duration, wherein operation of the electrolyte
pump at the idling threshold flow rate is maintained for at least a
second threshold duration, and wherein the first threshold duration
is greater than the second threshold duration.
7. The method of claim 1, wherein the electrolyte pump is OFF in
the inactive state.
8. A method of operating a redox flow battery system, the method
comprising: operating the redox flow battery system in an idle mode
during a condition when the redox flow battery system is operating
outside of a charging mode and outside of a discharge mode while a
DC current remains zero; during operation in the idle mode,
repeatedly cycling operation of at least one electrolyte pump
between an active state and an inactive state, wherein the
electrolyte pump is ON in the active state, wherein the active
state comprises pumping electrolyte at an idling threshold flow
rate less than a charging threshold flow rate, and wherein the
inactive state comprises pumping electrolyte at a deactivation
threshold flow rate that is less than the idling threshold flow
rate; switching to operation in the discharge mode; and in response
to switching to the discharge mode, maintaining operation of the at
least one electrolyte pump at a discharge threshold flow rate.
9. The method of claim 8, wherein cycling operation of the
electrolyte pump between the active state and the inactive state
includes deactivating the at least one electrolyte pump when
switching to operation in the inactive state.
10. The method of claim 9, wherein the electrolyte pump is OFF when
in the inactive state.
11. The method of claim 8, wherein the discharge threshold flow
rate is greater than the idling threshold flow rate.
12. The method of claim 8, wherein the idling threshold flow rate
is adjusted based on an anticipated load demand of the redox flow
battery system.
13. The method of claim 12, wherein the idling threshold flow rate
is adjusted higher in response to the anticipated load demand of
the redox flow battery system being higher, and wherein the idling
threshold flow rate is adjusted lower in response to the
anticipated load demand of the redox flow battery system being
lower.
14. The method of claim 8, wherein the DC current is positive
during the charging mode, and wherein the DC current is negative
during the discharge mode.
15. A redox flow battery system, comprising: a power module,
including a plurality of redox flow battery cell stacks, each of
the plurality of redox flow battery cell stacks including a
respective redox flow battery cell having a positive electrolyte
chamber and a negative electrolyte chamber; an electrolyte pump
capable of delivering electrolyte from an electrolyte tank to the
power module; and a power control system with a controller
including instructions thereon, the instructions executable to:
operate the redox flow battery system in an idle mode, wherein the
idle mode includes operation of the redox flow battery system
outside of a charging mode and outside of a discharge mode; and
while operating the redox flow battery system in the idle mode,
repeatedly cycle operation of an electrolyte pump between an active
state and an inactive state, wherein the active state comprises
pumping electrolyte via the electrolyte pump at an idling threshold
flow rate that is less than a charging threshold flow rate, and
wherein the inactive state comprises operating the electrolyte pump
at a deactivation threshold flow rate that is less than the idling
threshold flow rate.
16. The system of claim 15, further comprising a heater thermally
coupled to the electrolyte, wherein the instructions are further
executable to reduce an electrolyte temperature to an idling
threshold temperature in response to switching to the idle
mode.
17. The system of claim 16, wherein the idling threshold
temperature is increased in response to an anticipated load demand
of the redox flow battery system being higher, and the idling
threshold temperature is decreased in response to the anticipated
load demand of the redox flow battery system being lower.
18. The system of claim 17, wherein power electronics are
deactivated in response to switching to the idle mode.
19. The system of claim 18, wherein the idling threshold
temperature corresponds to a temperature below which electrolyte
precipitation occurs.
20. The system of claim 15, wherein the electrolyte pump is ON in
the active state.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S.
application Ser. No. 15/965,673, entitled "METHODS AND SYSTEM FOR A
BATTERY", and filed on Apr. 27, 2018. U.S. patent application Ser.
No. 15/965,673 claims priority to U.S. Provisional Application No.
62/491,954, entitled "METHODS AND SYSTEM FOR A BATTERY", and filed
on Apr. 28, 2017. The entire contents of the above-listed
applications are hereby incorporated by reference for all
purposes.
FIELD
[0003] The present description relates generally to a redox flow
battery system and methods of operating redox flow battery
systems.
BACKGROUND AND SUMMARY
[0004] Redox flow batteries are suitable for grid scale storage
applications due to their capabilities of scaling power and
capacity independently, and charging and discharging for thousands
of cycles with minimal performance losses. While idle and not
actively charging or discharging, redox flow battery systems
typically maintain electrolyte temperatures at charging/discharging
levels, and continue pumping of electrolytes at
charging/discharging flow rates in order to sustain a readiness of
the system to supply power in response to a charge or discharge
command.
[0005] However, the inventors herein have recognized potential
issues with such systems. Namely, a flow battery in a charged state
can lose its energy storage capacity much faster than a traditional
battery in the same charged state, while sustaining idle mode. In
particular, flow battery systems can lose capacity by way of shunt
current losses through the conductive electrolytes and from ionic
movement through the membrane. Continuously cycling fresh
electrolyte to the battery cells, such as during idle operation of
the flow battery, can maintain these shunting losses at higher
levels. Furthermore, the redox flow battery system may suffer from
parasitic power losses due to continuous pumping and heating of the
electrolyte at charging/discharging levels during the idle state,
including pumping parasitic loss and heating parasitic loss.
[0006] In one embodiment, the issues described above may be at
least partially addressed by a method of operating a redox flow
battery system, comprising switching the redox flow battery system
to an idle mode, wherein the idle mode includes operation of the
redox flow battery system outside of a charging mode and outside of
a discharge mode. Furthermore, the method may include in response
to switching to the idle mode, repeatedly cycling operation of an
electrolyte pump between an idling threshold flow rate less than a
charging threshold flow rate and a deactivation threshold flow
rate, and in response to switching to the charging mode,
maintaining operation of the electrolyte pump at the charging
threshold flow rate.
[0007] In another embodiment, a method of operating a redox flow
battery system may comprise, operating the redox flow battery
system in an idle mode during a condition when the redox flow
battery system is operating outside of a charging mode and outside
of a discharge mode. Furthermore, during operation in the idle
mode, the method may include repeatedly cycling operation of an
electrolyte pump between an active state and an inactive state,
wherein the active state comprises pumping electrolyte at an idling
threshold flow rate less than a charging threshold flow rate, and
the inactive state comprises deactivating the electrolyte pump and
decreasing a heater set point. Further still, in response to
switching to the discharge mode, the method may include maintaining
operation of the electrolyte pump at the discharge threshold flow
rate.
[0008] In another embodiment a redox flow battery system may
comprise a power module, including a plurality of redox flow
battery cell stacks each of the redox flow battery cell stacks
including a redox flow battery cell, an electrolyte pump capable of
delivering electrolyte from an electrolyte tank to the power
module, and a power control system with a controller. The
controller may include executable instructions thereon to, switch
the redox flow battery system to an idle mode, wherein the idle
mode includes operation of the redox flow battery system outside of
a charging mode and outside of a discharge mode; in response to
switching to the idle mode, repeatedly cycling operation of the
electrolyte pump between an idling threshold flow rate less than a
charging threshold flow rate and a deactivation threshold flow
rate; and in response to switching to the charging mode,
maintaining operation of the electrolyte pump at the charging
threshold flow rate.
[0009] In this way, the technical effect can be achieved of
maintaining a responsiveness of the redox flow battery system to
charging and discharging commands while in idle, while reducing
parasitic power losses due to pumping and heating, and reducing
shunt current losses.
[0010] It should be understood that the summary above is provided
to introduce in simplified form a selection of concepts that are
further described in the detailed description. It is not meant to
identify key or essential features of the claimed subject matter,
the scope of which is defined uniquely by the claims that follow
the detailed description. Furthermore, the claimed subject matter
is not limited to implementations that solve any disadvantages
noted above or in any part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows a schematic of an example redox flow battery
system.
[0012] FIG. 2 shows a side view of an example layout for the redox
flow battery system of FIG. 1.
[0013] FIG. 3 shows a high level flow chart of an example method
for operating the redox flow battery system of FIG. 1.
[0014] FIGS. 4-5 show flow charts of example methods for operating
the redox flow battery system of FIG. 1 in an idle mode.
[0015] FIGS. 6 and 7 show time line plots illustrating operation of
the redox flow battery system of FIG. 1 an idle mode.
[0016] FIG. 8 shows a plot comparing operation of the redox flow
battery system of FIG. 1 using the methods of FIGS. 3-5 to a
conventionally operated system.
DETAILED DESCRIPTION
[0017] The following description relates to systems and method for
a redox flow battery.
[0018] Hybrid redox flow batteries are redox flow batteries that
are characterized by the deposit of one or more of the
electro-active materials as a solid layer on an electrode. Hybrid
redox flow batteries may, for instance, include a chemical that
plates via an electrochemical reaction as a solid on a substrate
throughout the battery charge process. During battery discharge,
the plated species may ionize via an electrochemical reaction,
becoming soluble in the electrolyte. In hybrid battery systems, the
charge capacity (e.g., amount of energy stored) of the redox
battery may be limited by the amount of metal plated during battery
charge and may accordingly depend on the efficiency of the plating
system as well as the available volume and surface area available
for plating.
[0019] In a redox flow battery system the negative electrode 26 may
be referred to as the plating electrode and the positive electrode
28 may be referred to as the redox electrode. The negative
electrolyte within the plating side (e.g., negative electrode
compartment 20) of the battery may be referred to as the plating
electrolyte and the positive electrolyte on the redox side (e.g.
positive electrode compartment 22) of the battery may be referred
to as the redox electrolyte.
[0020] Anode refers to the electrode where electro-active material
loses electrons and cathode refers to the electrode where
electro-active material gains electrons. During battery charge, the
positive electrolyte gains electrons at the negative electrode 26;
therefore the negative electrode 26 is the cathode of the
electrochemical reaction. During discharge, the positive
electrolyte loses electrons; therefore the negative electrode 26 is
the anode of the reaction. Accordingly, during charge, the negative
electrolyte and negative electrode may be respectively referred to
as the catholyte and cathode of the electrochemical reaction, while
the positive electrolyte and the positive electrode may be
respectively referred to as the anolyte and anode of the
electrochemical reaction. Alternatively, during discharge, the
negative electrolyte and negative electrode may be respectively
referred to as the anolyte and anode of the electrochemical
reaction, while the positive electrolyte and the positive electrode
may be respectively referred to as the catholyte and cathode of the
electrochemical reaction. For simplicity, the terms positive and
negative are used herein to refer to the electrodes, electrolytes,
and electrode compartments in redox battery flow systems.
[0021] One example of a hybrid redox flow battery is an all iron
redox flow battery (IFB), in which the electrolyte comprises iron
ions in the form of iron salts (e.g., FeCl.sub.2, FeCl.sub.3, and
the like), wherein the negative electrode comprises metal iron. For
example, at the negative electrode, ferrous ion, Fe.sup.2+,
receives two electrons and plates as iron metal on to the negative
electrode 26 during battery charge, and iron metal, Fe.sup.0, loses
two electrons and re-dissolves as Fe.sup.2+ during battery
discharge. At the positive electrode, Fe.sup.2+ loses an electron
to form ferric ion, Fe.sup.3+, during charge, and during discharge
Fe.sup.3+ gains an electron to form Fe.sup.2+: The electrochemical
reaction is summarized in equations (1) and (2), wherein the
forward reactions (left to right) indicate electrochemical
reactions during battery charge, while the reverse reactions (right
to left) indicate electrochemical reactions during battery
discharge:
Fe.sup.2++2e-Fe.sup.0 -0.44V(Negative Electrode) (1)
2Fe.sup.2+2Fe.sup.3+2e- +0.77V(Positive Electrode) (2)
[0022] As discussed above, the negative electrolyte used in the all
iron redox flow battery (IFB) may provide a sufficient amount of
Fe.sup.2+ so that, during charge, Fe.sup.2+ can accept two
electrons from the negative electrode to form Fe.sup.0 and plate
onto a substrate. During discharge, the plated Fe.sup.0 may then
lose two electrons, ionizing into Fe.sup.2+ and be dissolved back
into the electrolyte. The equilibrium potential of the above
reaction is -0.44V and thus this reaction provides a negative
terminal for the desired system. On the positive side of the IFB,
the electrolyte may provide Fe.sup.2+ during charge which loses
electron and oxidizes to Fe.sup.3+. During discharge, Fe.sup.3+
provided by the electrolyte becomes Fe.sup.2+ by absorbing an
electron provided by the electrode. The equilibrium potential of
this reaction is +0.77V, creating a positive terminal for the
desired system.
[0023] The IFB provides the ability to charge and recharge its
electrolytes in contrast to other battery types utilizing
non-regenerating electrolytes. Charge is achieved by applying a
current across the electrodes via terminals 40 and 42. The negative
electrode may be coupled via terminal 40 to the negative side of a
voltage source so that electrons may be delivered to the negative
electrolyte via the positive electrode (e.g., as Fe.sup.2+ is
oxidized to Fe.sup.3+ in the positive electrolyte in the positive
electrode compartment 22). The electrons provided to the negative
electrode 26 (e.g., plating electrode) can reduce the Fe.sup.2+ in
the negative electrolyte to form Fe.sup.0 at the plating substrate
causing it to plate onto the negative electrode.
[0024] Discharge can be sustained while Fe.sup.0 remains available
to the negative electrolyte for oxidation and while Fe.sup.3+
remains available in the positive electrolyte for reduction. As an
example, Fe.sup.3+ availability can be maintained by increasing the
concentration or the volume of the positive electrolyte to the
positive electrode compartment 22 side of cell 18 to provide
additional Fe.sup.3+ ions via an external source, such as positive
electrolyte chamber 52 or an external positive electrolyte storage
tank. More commonly, availability of Fe.sup.0 during discharge may
be an issue in IFB systems, wherein the Fe.sup.0 available for
discharge may be proportional to the surface area and volume of the
negative electrode substrate as well as the plating efficiency.
Charge capacity may be dependent on the availability of Fe.sup.2+
in the negative electrode compartment 20. As an example, Fe.sup.2+
availability can be maintained by providing additional Fe.sup.2+
ions via an external source, such as negative electrolyte chamber
50 or an external negative electrolyte storage tank to increase the
concentration or the volume of the negative electrolyte to the
negative electrode compartment 20 side of cell 18.
[0025] In an IFB, the positive electrolyte comprises ferrous ion,
ferric ion, ferric complexes, or any combination thereof, while the
negative electrolyte comprises ferrous ion or ferrous complexes,
depending on the state of charge of the IFB system. As previously
mentioned, utilization of iron ions in both the negative
electrolyte and the positive electrolyte allows for utilization of
the same electrolytic species on both sides of the battery cell,
which can reduce electrolyte cross-contamination and can increase
the efficiency of the IFB system, resulting in less electrolyte
replacement as compared to other redox flow battery systems.
[0026] Efficiency losses in an IFB may result from electrolyte
crossover through the separator 24 (e.g., ion-exchange membrane
barrier, micro-porous membrane, and the like). For example, ferric
ions in the positive electrolyte may be driven toward the negative
electrolyte by a ferric ion concentration gradient and an
electrophoretic force across the separator. Subsequently, ferric
ions penetrating the membrane barrier and crossing over to the
negative electrode compartment 20 may result in coulombic
efficiency losses. Ferric ions crossing over from the low pH redox
side (e.g., more acidic positive electrode compartment 22) to high
pH plating side (e.g., less acidic negative electrode compartment
20) can result in precipitation of Fe(OH).sub.3. Precipitation of
Fe(OH).sub.3 can damage the separator 24 and cause permanent
battery performance and efficiency losses. For example,
Fe(OH).sub.3 precipitate may chemically foul the organic functional
group of an ion-exchange membrane or physically clog the small
micro-pores of an ion-exchange membrane. In either case, due to the
Fe(OH).sub.3 precipitate, membrane ohmic resistance may rise over
time and battery performance may degrade. Precipitate may be
removed by washing the battery with acid, but the constant
maintenance and downtime may be disadvantageous for commercial
battery applications. Furthermore, washing may be dependent on
regular preparation of electrolyte, adding to process cost and
complexity. Adding specific organic acids to the positive
electrolyte and the negative electrolyte in response to electrolyte
pH changes may also mitigate precipitate formation during battery
charge and discharge cycling.
[0027] Additional coulombic efficiency losses may be caused by
reduction of H+ (e.g., protons) and subsequent formation of H2
(e.g., hydrogen gas), and the reaction of protons in the negative
electrode compartment 20 with electrons supplied at the plated iron
metal electrode to form hydrogen gas.
[0028] The IFB electrolyte (e.g., FeCl.sub.2, FeCl.sub.3,
FeSO.sub.4, Fe2(SO.sub.4).sub.3, and the like) is readily available
and can be produced at low costs. The IFB electrolyte offers higher
reclamation value because the same electrolyte can be used for the
negative electrolyte and the positive electrolyte, consequently
reducing cross contamination issues as compared to other systems.
Furthermore, owing to its electron configuration, iron may solidify
into a generally uniform solid structure during plating thereof on
the negative electrode substrate. For zinc and other metals
commonly used in hybrid redox batteries, solid dendritic structures
may form during plating. The stable electrode morphology of the IFB
system may increase the efficiency of the battery in comparison to
other redox flow batteries. Further still, iron redox flow
batteries reduce the use of toxic raw materials and can operate at
a relatively neutral pH as compared to other redox flow battery
electrolytes. Accordingly, IFB systems reduce environmental hazards
as compared with all other current advanced redox flow battery
systems in production.
[0029] FIG. 1 provides a schematic illustration of a redox flow
battery system 10. The redox flow battery system 10 may comprise a
redox flow battery cell 18 fluidly connected to a multi-chambered
electrolyte storage tank 110. The redox flow battery cell 18 may
generally include a negative electrode compartment 20, separator
24, and positive electrode compartment 22. The separator 24 may
comprise an electrically insulating ionic conducting barrier which
prevents bulk mixing of the positive electrolyte and the negative
electrolyte while allowing conductance of specific ions
therethrough. For example, the separator 24 may comprise an
ion-exchange membrane and/or a microporous membrane. The negative
electrode compartment 20 may comprise a negative electrode 26, and
a negative electrolyte comprising electroactive materials. The
positive electrode compartment 22 may comprise a positive electrode
28, and a positive electrolyte comprising electroactive materials.
In some examples, multiple redox flow battery cells 18 may be
combined in series or parallel to generate a higher voltage or
current in a redox flow battery system. Further illustrated in FIG.
1 are pumps 30 and 32, both used to pump electrolyte solution
through the flow battery system 10. Electrolytes are stored in one
or more tanks external to the cell, and are pumped via pumps 30 and
32 through the negative electrode compartment 20 side and the
positive electrode compartment 22 side of the battery,
respectively.
[0030] As illustrated in FIG. 1, the redox flow battery cell 18 may
further include negative battery terminal 40, and positive battery
terminal 42. When a charge current is applied to the battery
terminals 40 and 42, the positive electrolyte is oxidized (lose one
or more electrons) at the positive electrode 28, and the negative
electrolyte is reduced (gain one or more electrons) at the negative
electrode 26. During battery discharge, reverse redox reactions
occur on the electrodes. In other words, the positive electrolyte
is reduced (gain one or more electrons) at the positive electrode
28, and the negative electrolyte is oxidized (lose one or more
electrons) at the negative electrode 26. The electrical potential
difference across the battery is maintained by the electrochemical
redox reactions in the positive electrode compartment 22 and the
negative electrode compartment 20, and can induce a current through
a conductor while the reactions are sustained. The amount of energy
stored by a redox battery is limited by the amount of
electro-active material available in electrolytes for discharge,
depending on the total volume of electrolytes and the solubility of
the electro-active materials.
[0031] The flow battery system 10 may further comprise an
integrated multi-chambered electrolyte storage tank 110. The
multi-chambered electrolyte storage tank 110 may be divided by a
bulkhead 98. The bulkhead 98 may create multiple chambers within
the storage tank so that both the positive and negative electrolyte
may be included within a single tank. The negative electrolyte
chamber 50 holds negative electrolyte comprising electroactive
materials, and the positive electrolyte chamber 52 holds positive
electrolyte comprising electroactive materials. The bulkhead 98 may
be positioned within the multi-chambered electrolyte storage tank
110 to yield a desired volume ratio between the negative
electrolyte chamber 50 and the positive electrolyte chamber 52. In
one example, the bulkhead 98 may be positioned to set the volume
ratio of the negative and positive electrolyte chambers according
to the stoichiometric ratio between the negative and positive redox
reactions. The figure further illustrates the fill height 112 of
storage tank 110, which may indicate the liquid level in each tank
compartment. The figure also shows gas head space 90 located above
the fill height 112 of negative electrolyte chamber 50, and gas
head space 92 located above the fill height 112 of positive
electrolyte chamber 52. The gas head space 92 may be utilized to
store hydrogen gas generated through operation of the redox flow
battery (e.g., due to proton reduction and corrosion side
reactions) and conveyed to the multi-chambered electrolyte storage
tank 110 with returning electrolyte from the redox flow battery
cell 18. The hydrogen gas may be separated spontaneously at the
gas-liquid interface (e.g., fill height 112) within the
multi-chambered electrolyte storage tank 110, thereby precluding
having additional gas-liquid separators as part of the redox flow
battery system. Once separated from the electrolyte, the hydrogen
gas may fill the gas head spaces 90 and 92. As such, the stored
hydrogen gas can aid in purging other gases from the
multi-chambered electrolyte storage tank 110, thereby acting as an
inert gas blanket for reducing oxidation of electrolyte species,
which can help to reduce redox flow battery capacity losses. In
this way, utilizing the integrated multi-chambered electrolyte
storage tank 110 may forego having separate negative and positive
electrolyte storage tanks, hydrogen storage tanks, and gas-liquid
separators common to conventional redox flow battery systems,
thereby simplifying the system design, reducing the physical
footprint of the system, and reducing system costs.
[0032] FIG. 1 also shows the spill over-hole 96, which creates an
opening in the bulkhead 98 between gas head spaces 90 and 92, and
provides a means of equalizing gas pressure between the two
chambers. The spill over hole 96 may be positioned at a threshold
height above the fill height 112. The spill over hole further
enables a capability to self-balance the electrolytes in each of
the positive and negative electrolyte chambers 52 and 50 in the
event of a battery crossover. In the case of an all iron redox flow
battery system, the same electrolyte (Fe.sup.2+) is used in both
negative and positive electrode compartments 20 and 22, so spilling
over of electrolyte between the negative and positive electrolyte
chambers 50 and 52 may reduce overall system efficiency, but the
overall electrolyte composition, battery module performance, and
battery module capacity are maintained. Flange fittings may be
utilized for all piping connections for inlets and outlets to and
from the multi-chambered electrolyte storage tank 110 to maintain a
continuously pressurized state without leaks. The multi-chambered
electrolyte storage tank 110 can include at least one outlet from
each of the negative and positive electrolyte chambers, and at
least one inlet to each of the negative and positive electrolyte
chambers. Furthermore, one or more outlet connections may be
provided from the gas head spaces 90 and 92 for directing hydrogen
gas to rebalancing reactors 80 and 82.
[0033] Although not shown in FIG. 1, integrated multi-chambered
electrolyte storage tank 110 may further include one or more
heaters thermally coupled to each of the negative electrolyte
chamber 50 and the positive electrolyte chamber 52. In alternate
examples, only one of the negative and positive electrolyte
chambers may include one or more heaters. In the case where only
the positive electrolyte chamber includes one or more heaters, the
negative electrolyte may be heated by transferring heat generated
at the battery cells of the power module to the negative
electrolyte. In this way, the battery cells of the power module may
heat and facilitate temperature regulation of the negative
electrolyte. The one or more heaters may be actuated by the
controller 88 to regulate a temperature of the negative electrolyte
chamber 50 and the positive electrolyte chamber independently or
together. For example, in response to an electrolyte temperature
decreasing below a threshold temperature, the controller may
increase a power supplied to one or more heaters so that a heat
flux to the electrolyte is increased. The electrolyte temperature
may be indicated by one or more temperature sensors mounted at the
multi-chambered electrolyte storage tank 110, including sensors 60
and 62. As examples the one or more heaters may include coil type
heaters or other immersion heaters immersed in the electrolyte
fluid, or surface mantle type heaters that transfer heat
conductively through the walls of the negative and positive
electrolyte chambers to heat the fluid therein. Other known types
of tank heaters may be employed without departing from the scope of
the present disclosure. Furthermore, controller 88 may deactivate
one or more heaters in the negative and positive electrolyte
chambers in response to a liquid level decreasing below a solids
fill threshold level. Said in another way, controller 88 may
activate the one or more heaters in the negative and positive
electrolyte chambers only in response to a liquid level increasing
above the solids fill threshold level. In this way, activating the
one or more heaters without sufficient liquid in the positive
and/or negative electrolyte chambers can be averted, thereby
reducing a risk of overheating or burning out the heaters.
[0034] Further illustrated in FIG. 1, electrolyte solutions
typically stored in the multi-chambered electrolyte storage tank
110 are pumped via pumps 30 and 32 throughout the flow battery
system 10. Electrolyte stored in negative electrolyte chamber 50 is
pumped via negative electrolyte pump 30 through the negative
electrode compartment 20 side, and electrolyte stored in positive
electrolyte chamber 52 is pumped via positive electrolyte pump 32
through the positive electrode compartment 22 side of the
battery.
[0035] Two electrolyte rebalancing reactors 80 and 82, may be
connected in-line or in parallel with the recirculating flow paths
of the electrolyte at the negative and positive sides of the
battery, respectively, in the redox flow battery system 10. One or
more rebalancing reactors may be connected in-line with the
recirculating flow paths of the electrolyte at the negative and
positive sides of the battery, and other rebalancing reactors may
be connected in parallel, for redundancy (e.g., a rebalancing
reactor may be serviced without disrupting battery and rebalancing
operations) and for increased rebalancing capacity. In one example,
the electrolyte rebalancing reactors 80 and 82 may be placed in the
return flow path from the positive and negative electrode
compartments 20 and 22 to the positive and negative electrolyte
chambers 50 and 52, respectively. Electrolyte rebalancing reactors
80 and 82 may serve to rebalance electrolyte charge imbalances in
the redox flow battery system occurring due to side reactions, ion
crossover, and the like, as described herein. In one example,
electrolyte rebalancing reactors 80 and 82 may include trickle bed
reactors, where the hydrogen gas and electrolyte are contacted at
catalyst surfaces in a packed bed for carrying out the electrolyte
rebalancing reaction. In other examples the rebalancing reactors 80
and 82 may include flow-through type reactors that are capable of
contacting the hydrogen gas and the electrolyte liquid and carrying
out the rebalancing reactions in the absence a packed catalyst
bed.
[0036] During operation of a redox flow battery system, sensors and
probes may monitor and control chemical properties of the
electrolyte such as electrolyte pH, concentration, state of charge,
and the like. For example, as illustrated in FIG. 1, sensors 62 and
60 maybe be positioned to monitor positive electrolyte and negative
electrolyte conditions at the positive electrolyte chamber 52 and
the negative electrolyte chamber 50, respectively. As another
example, sensors 72 and 70, also illustrated in FIG. 1, may monitor
positive electrolyte and negative electrolyte conditions at the
positive electrode compartment 22 and the negative electrode
compartment 20, respectively. Sensors may be positioned at other
locations throughout the redox flow battery system to monitor
electrolyte chemical properties and other properties. For example a
sensor may be positioned in an external acid tank (not shown) to
monitor acid volume or pH of the external acid tank, wherein acid
from the external acid tank is supplied via an external pump (not
shown) to the redox flow battery system in order to reduce
precipitate formation in the electrolytes. Additional external
tanks and sensors may be installed for supplying other additives to
the redox flow battery system 10. Sensor information may be
transmitted to a controller 88 which may in turn actuate pumps 30
and 32 to control electrolyte flow through the cell 18, or to
perform other control functions, as an example. In this manner, the
controller 88 may be responsive to, one or a combination of sensors
and probes.
[0037] Redox flow battery system 10 may further comprise a source
of hydrogen gas. In one example the source of hydrogen gas may
comprise a separate dedicated hydrogen gas storage tank. In the
example of FIG. 1, hydrogen gas may be stored in and supplied from
the integrated multi-chambered electrolyte storage tank 110.
Integrated multi-chambered electrolyte storage tank 110 may supply
additional hydrogen gas to the positive electrolyte chamber 52 and
the negative electrolyte chamber 50. Integrated multi-chambered
electrolyte storage tank 110 may alternately supply additional
hydrogen gas to the inlet of electrolyte rebalancing reactors 80
and 82. As an example, a mass flow meter or other flow controlling
device (which may be controlled by controller 88) may regulate the
flow of the hydrogen gas from integrated multi-chambered
electrolyte storage tank 110.
[0038] The integrated multi-chambered electrolyte storage tank 110
may supplement the hydrogen gas generated in redox flow battery
system 10. For example, when gas leaks are detected in redox flow
battery system 10 or when the reduction reaction rate is too low at
low hydrogen partial pressure, hydrogen gas may be supplied from
the integrated multi-chambered electrolyte storage tank 110 in
order to rebalance the state of charge of the electro-active
species in the positive electrolyte and negative electrolyte. As an
example, controller 88 may supply hydrogen gas from integrated
multi-chambered electrolyte storage tank 110 in response to a
measured change in pH or in response to a measured change in state
of charge of an electrolyte or an electro-active species. For
example an increase in pH of the negative electrolyte chamber 50,
or the negative electrode compartment 20, may indicate that
hydrogen is leaking from the redox flow battery system 10 and/or
that the reaction rate is too slow with the available hydrogen
partial pressure. In response to the pH increase, controller 88 may
increase a supply of hydrogen gas from integrated multi-chambered
electrolyte storage tank 110 to the redox flow battery system 10.
As a further example, controller 88 may supply hydrogen gas from
integrated multi-chambered electrolyte storage tank 110 in response
to a pH change, wherein the pH increases beyond a first threshold
pH or decreases beyond a second threshold pH. In the case of an
IFB, controller 88 may supply additional hydrogen to increase the
rate of reduction of ferric ions and the rate of production of
protons, thereby reducing the pH of the positive electrolyte.
Furthermore, the negative electrolyte pH may be lowered by hydrogen
reduction of ferric ions crossing over from the positive
electrolyte to the negative electrolyte or by proton generated at
the positive side crossing over to the negative electrolyte due to
a proton concentration gradient and electrophoretic forces. In this
manner, the pH of the negative electrolyte may be maintained within
a stable region, while reducing the risk of precipitation of ferric
ions (crossing over from the positive electrode compartment) as
Fe(OH).sub.3.
[0039] Other control schemes for controlling the supply rate of
hydrogen gas from integrated multi-chambered electrolyte storage
tank 110 responsive to a change in an electrolyte pH or to a change
in an electrolyte state of charge, detected by other sensors such
as an oxygen-reduction potential (ORP) meter or an optical sensor,
may be implemented. Further still, the change in pH or state of
charge triggering the action of controller 88 may be based on a
rate of change or a change measured over a time period. The time
period for the rate of change may be predetermined or adjusted
based on the time constants for the redox flow battery system. For
example the time period may be reduced if the recirculation rate is
high, and local changes in concentration (e.g., due to side
reactions or gas leaks) may quickly be measured since the time
constants may be small.
[0040] Turning now to FIG. 2, it illustrates a side view of an
example redox flow battery system layout 200 for the redox flow
battery system 10. Redox flow battery system layout may be housed
within a housing 202 that facilitates long-distance transport and
delivery of the redox flow battery system. In some examples, the
housing 202 can include a standard steel freight container or a
freight trailer that can be transported via rail, truck or ship.
The system layout 200 can include the integrated multi-chambered
electrolyte storage tank 110 and rebalancing reactors 80 positioned
at a first side of the housing 202, and a power module 210, and
power control system (PCS) 288 at a second side of the housing 202.
Auxiliary components such as supports 206, as well as various
piping 204, pumps 230, valves (not shown), and the like may be
included within the housing 202 (as further described with
reference to FIG. 1) for stabilizing and fluidly connecting the
various components positioned therein. For example, one or more
pumps 230 may be utilized to convey electrolyte from the integrated
multi-chambered electrolyte storage tank 110 to one or more battery
cell stacks 214 within the power module 210. Furthermore additional
pumps 230 may be utilized to return electrolyte from the power
module 210 to the negative electrolyte chamber 50 or the positive
electrolyte chamber 52 of the integrated multi-chambered
electrolyte storage tank 110.
[0041] Power module 210 may comprise one or more redox flow battery
cell stacks 214 electrically connected in parallel and/or in
series. Each of the redox flow battery cell stacks 214 may further
comprise a plurality of redox flow battery cells 18 connected in
parallel and/or series. In this way, power module 210 may be able
to supply a range of current and/or voltages to external loads. The
PCS 288 includes controller 88, as well as other electronics, for
controlling and monitoring operation of the redox flow battery
system 10. Furthermore, PCS 288 may regulate and monitor voltage
supplied to external loads, as well as supplying current and/or
voltage from external sources for charging of the power module 210.
The PCS 288 may further regulate and control operation of the redox
flow battery system during an idle state. The redox flow battery
system being in an idle state may include when the power module 210
is not in charge mode or discharge mode. As an example, the power
module 210 may be in charge mode when an external voltage or
current is supplied to one or more redox flow battery cells of the
power module 210 resulting in reduction of electrolyte and plating
of the reduced electrolyte at the negative electrode of the one or
more redox flow battery cells. For the case of an IFB, ferrous ion
may be reduced at the plating electrode of one or more redox flow
battery cells, thereby plating iron thereat during charging of the
power module. As another example, the power module 210 may be in a
discharge mode when voltage or current is supplied from one or more
redox flow battery cells of the power module 210 resulting in
oxidation of plated metal at the negative electrode resulting in
deplating (e.g., loss of metal) and solubilizing of the oxidize
metal ions. For the case of an IFB, iron may be oxidized at the
plating electrode of one or more redox flow battery cells, thereby
solubilizing ferrous ion thereat during discharging of the power
module. Further details regarding conditions for entering and
exiting the charge and discharge modes of the redox flow battery
system are described with reference to FIGS. 3-5 below.
[0042] Turning now to FIGS. 3-5, they show flow charts for method
300, 400, and 500, respectively, for operating the redox flow
battery system of FIGS. 1 and 2. Instructions for carrying out
methods 300, 400, and 500 may reside on board and be executed by a
controller, such as controller 88 of FIG. 1. For example, the
executable instructions may be stored in non-transitory memory on
board the controller and may be executed in conjunction with
signals received from sensors of redox flow battery system, such as
the sensors described above with reference to FIG. 1. The
controller may further employ actuators including pumps, valves,
heaters, and the like, as described above with reference to FIG. 1,
of redox flow battery system to adjust battery operation, according
to the methods described below.
[0043] Method 300 includes a method for determining when a redox
flow battery system may be in a charging, discharging, or idle
mode. The method 300 may begin at 302, where the method includes
determining, estimating, and/or measuring current battery operating
parameters. Current battery operating parameters may include, but
are not limited to, one or more of battery state of charge (SOC),
power module voltage, DC current, pump activity (e.g., electrolyte
pump ON/OFF statuses, electrolyte pump flow rates, pump timers, and
the like), electrolyte temperatures, power (including current and
voltage) supplied to the power module, power (including current and
voltage) supplied by the power module, internal power demand set
points, and external power demand set points, and the like.
[0044] At 304, the method 300 includes determining if the redox
flow battery system is currently being charged or in a charging
mode. The redox flow battery system being in a charging mode may
include one or more redox flow battery cells of the redox flow
battery system being in a charging mode. The charging mode may be
indicated by a set point or desired SOC for one or more redox flow
battery cells being greater than an actual SOC for the one or more
redox flow battery cells. In another example, charging mode may be
indicated by the desired SOC being greater than the actual SOC by
more than a charging threshold SOC difference. Additionally or
alternatively, the redox flow battery system may be charging when a
DC current from the power module is positive. In one example, DC
current may be positive when current is flowing into the power
module from an external power source. The DC current magnitude and
direction may be measured by determining the voltage drop across a
shunt resistor electrically connected with the power module.
[0045] In an alternate example, a redox flow battery system being
in a charging mode may be indicated by a supply of power (including
a supply of current and/or voltage) to the power module being
greater than a charging threshold supply of power. The charging
threshold supply of power may refer to a rate of power supply to
the redox flow battery system greater than an incidental or
auxiliary rate of power supply to the power module used for
powering sensors, lighting, and other auxiliary devices related to
the power module. In this way, supplying power greater than the
threshold supply of power indicates that current is being supplied
directly to one or more plating electrodes, resulting in reduction
of metal ion at the plating electrode surface and plating of the
reduced metal thereat. Similarly the charging mode may be indicated
by SOC of one or more redox flow battery cells increasing at a rate
greater than a threshold rate of SOC increase as a result of
supplying power to the redox flow battery system during
charging.
[0046] In another example, determination of the redox flow battery
system being in a charging mode may be based on a flow rate of
electrolyte being supplied to one or both of the negative and
positive electrolyte chambers of a redox flow battery cell being
greater than a charging threshold flow rate. The first threshold
(negative or positive electrolyte) flow rate may refer to a flow
rate that is greater than a flow rate of a pump used for
electrolyte recirculation during an idle mode Pumping the
electrolyte at a flow rate greater than the charging threshold flow
rate may enable the flow rate of electrolyte being supplied to the
negative and positive electrolyte chambers to be high enough to
sustain a desired charging rate. The rate of supply of electrolytes
may thus be related to the stoichiometry of the redox reactions
occurring at the redox flow battery cell electrodes. As an
idealized simple example, in the case of an IFB, for every two
electrons supplied during charging at the negative electrode, one
ferrous ion are supplied to the negative electrolyte chamber for
reduction thereat, and two ferric ions are supplied at the positive
electrolyte chamber for oxidation thereat. In this way the
electrolyte flow rates and the charging threshold electrolyte flow
rates to each of the positive and negative electrolyte chambers
corresponding to operation in a charging mode may be unequal.
Furthermore, electrolyte flow rates in excess of the idealized
stoichiometric flow rates may be supplied to the redox flow battery
cell to support a desired charging rate to account for non-ideal
mixing and losses within the system.
[0047] As a further example, while charging, the ionic species in
the positive and negative electrode compartments 22 and 20 may be
changing at rates characteristic of being in a charging mode or may
attain steady-state values (relative to an electrolyte pumping
rate) associated with charging mode. For example, during charging,
plating of ferrous ion may result in a characteristic decrease (or
characteristic rate of decrease) in ferrous ion concentration in
the negative electrolyte compartment. Similarly the concentration
(or rate of change in concentration) of other ionic species such as
ferric ion, chloride ion, hydrogen protons (e.g. pH), and other
species may be characteristic of operating the redox flow battery
cell in a charging mode. Furthermore, other electrolyte properties
such as ionic strength, pH, and the like may have characteristic
stead-state values or rates of change that can be used to indicate
the redox flow battery system being in a charging mode. In other
words, the controller may estimate and/or measure one or more
species concentrations (or rate of change thereof), including
measuring a pH and/or ionic strength, and determine, based on one
or a combination of those measurements being beyond a charging
threshold value characteristic to operation in charging mode, if
the redox flow battery system is in a charging mode. Furthermore,
the controller may determine the charging mode based on a rate of
power supply to the power module, a measured DC current by way of a
voltage drop across a shunt resistor electrically connected to the
power module, a rate of increase in SOC, a difference between a
desired and actual SOC, and/or a flow rate of electrolytes to one
or more redox flow battery cells, as described above.
[0048] In further examples, the controller may enter a charging
mode when a SOC of one or more of the redox flow battery cells has
discharged below a lower threshold SOC. The lower threshold state
of charge may include when the redox flow battery cell has been
fully depleted of charge. In another example, the lower threshold
state of charge may correspond to a SOC below which a risk of
degradation of the redox flow battery cell may be increased. Other
conditions for entering or beginning a charging mode may include
when a desired power from an external load is greater than the
available power from the redox flow battery system by more than a
threshold power difference. Thus, the controller 88 may also
determine that the redox flow battery system is in a charging mode
when a condition for entering or beginning a charging mode is
met.
[0049] Upon determining that the redox flow battery system is in
charging mode, method 300 continues to 306 where the controller may
initiate or resume/continue charging mode of the redox flow battery
system. As described above, charging the redox flow battery system
may include operating an electrolyte pump (e.g., one or more of
negative positive electrolyte pumps 30 and 32 of FIG. 1) to flow
electrolytes to redox flow battery negative and positive
electrolyte chambers at a charging negative and positive threshold
flow rates, respectively. In another example, the controller may
supply power to the power module greater than a charging threshold
supply of power in order to raise an actual SOC of one or more of
the redox flow battery cells to a desired SOC. Raising the SOC of
one or more of the redox flow battery cells to a desired SOC may
include increasing the SOC by a rate of SOC increase greater than
the charging threshold rate of SOC increase. Furthermore, the
controller may operate one or more actuators in order to maintain
one or more of a combination of electrolyte species concentrations,
pH, ionic strength, and other electrolyte characteristics at a
desired value that may correspond to the redox flow battery system
being in a charging mode. In one example the desired values may
include being beyond a threshold value characteristic to operation
of the redox flow battery system in the charging mode.
[0050] If a redox flow battery is not being charged, then the
method 300 proceeds from 304 to 308 to determine if redox flow
battery is being discharged. The redox flow battery system being in
a discharge mode may include one or more redox flow battery cells
of the redox flow battery system being in a discharge mode. The
discharge mode may be indicated by a set point or desired SOC for
one or more redox flow battery cells being less than an actual SOC
for the one or more redox flow battery cells. In another example, a
discharge mode may be indicated by the desired SOC being less than
the actual SOC by more than a threshold difference.
[0051] In one example, a redox flow battery system being in a
discharge mode may be indicated by a supply of power (including a
supply of current and/or voltage) from the power module to an
external load being greater than a charging threshold supply of
power. The charging threshold supply of power may refer to a rate
of power supply from the redox flow battery system to an external
load being greater than an incidental or auxiliary rate of power
supply to the power module used for powering sensors, lighting, and
other auxiliary devices related to the power module. In this way,
supplying power from the power module greater than the charging
threshold supply of power indicates that current is being supplied
directly to the external load, resulting in oxidation of metal
plated at the plating electrode surface to metal ion and
solubilizing the metal ion into the negative electrolyte
compartment. Similarly the discharge mode may be indicated by SOC
of one or more redox flow battery cells decreasing at a rate
greater than a threshold rate of SOC decrease as a result of
supplying power from the redox flow battery system during
discharge.
[0052] Additionally or alternatively, the redox flow battery system
may be in a discharge mode when a DC current from the power module
is negative. In one example, DC current may be negative when
current is flowing out of the power module to an external load. As
described above, the DC current magnitude and direction may be
determined by measuring the voltage drop across a shunt resistor
electrically connected to the power module.
[0053] In another example, determination of the redox flow battery
system being in a discharge mode may be based on a flow rate of
electrolyte being supplied to one or both of the negative and
positive electrolyte chambers of a redox flow battery cell being
greater than a discharge threshold flow rate. The discharge
threshold (negative or positive electrolyte) flow rate may refer to
a flow rate that is greater than a flow rate of a pump used for
electrolyte recirculation during an idle mode. Pumping the
electrolyte at a flow rate greater than the discharge threshold
flow rate may enable the flow rate of electrolyte being supplied to
the negative or positive electrolyte chambers to be high enough to
sustain a desired redox flow battery system discharge rate. The
rate of supply of electrolytes may thus be related to the
stoichiometry of the redox reactions occurring at the redox flow
battery cell electrodes. As an idealized simple example, in the
case of an IFB, for every two electrons supplied from the redox
flow battery system during discharge at the negative electrode, one
ferrous ion is oxidized, and two ferrous ions are supplied at the
positive electrolyte chamber for reduction thereat. In this way the
electrolyte flow rates and the discharge threshold electrolyte flow
rates to each of the positive and negative electrolyte chambers
corresponding to operation in a charging mode may be unequal.
Furthermore, electrolyte flow rates in excess of the idealized
stoichiometric flow rates may be supplied to the redox flow battery
cell to support a desired discharge rate to account for non-ideal
mixing and losses within the system.
[0054] As a further example, while in discharge mode, the ionic
species in the positive and negative electrode compartments 22 and
20 may be changing at rates characteristic of being in a discharge
mode or may attain steady-state values (relative to an electrolyte
pumping rate) associated with discharge mode. For example, during
discharge, plating of ferrous ion may result in a characteristic
decrease (or characteristic rate of decrease) in ferrous ion
concentration in the negative electrolyte compartment. Similarly
the concentration (or rate of change in concentration) of other
ionic species such as ferric ion, chloride ion, hydrogen protons
(e.g. pH), and other species may be characteristic of operating the
redox flow battery cell in a discharge mode. Furthermore, other
electrolyte properties such as ionic strength, pH, and the like may
have characteristic values or rates of change that can be used to
indicate the redox flow battery system being in a discharge mode.
In other words, the controller may estimate and/or measure one or
more species concentrations (or rate of change thereof), including
measuring a pH and/or ionic strength, and determine, based on one
or a combination of those measurements being beyond a threshold
value characteristic to operation in discharge mode, if the redox
flow battery system is in a discharge mode. Furthermore, the
controller may determine the discharge mode based on a rate of
power supply to the power module, a rate of increase in SOC, a
difference between a desired and actual SOC, and/or a flow rate of
electrolytes to one or more redox flow battery cells, as described
above.
[0055] In further examples, the controller may enter a discharge
mode when a SOC of one or more of the redox flow battery cells has
charged above a higher threshold SOC. The higher threshold state of
charge may include when the redox flow battery cell has been fully
charged to capacity. In another example, the higher threshold state
of charge may correspond to a SOC above which a risk of
overcharging and degradation of the redox flow battery cell may be
increased. Other conditions for entering or beginning a discharge
mode may include when an actual power supplied from the redox flow
battery system to an external load is less than the desired power
by more than a discharge threshold power difference. Thus, the
controller 88 may also determine that the redox flow battery system
is in a discharge mode when a condition for entering or beginning a
discharging mode is met.
[0056] Upon determining that the redox flow battery system is in
discharge mode, method 300 continues from 308 to 310 where the
controller may initiate or resume/continue discharge mode of the
redox flow battery system. As described above, discharge of the
redox flow battery system may include operating an electrolyte pump
(e.g., one or more of negative and positive electrolyte pumps 30
and 32 of FIG. 1) to flow electrolytes to redox flow battery
negative and positive electrolyte chambers at discharge negative
and positive threshold flow rates, respectively. In another
example, the controller may supply power from the power module to
an external load greater than a discharge threshold supply of power
in order to lower an actual SOC of one or more of the redox flow
battery cells to a desired SOC. Lowering the SOC of one or more of
the redox flow battery cells to a desired SOC may include lowering
the SOC by a rate of SOC increase greater than the threshold rate
of SOC decrease. Furthermore, the controller may operate one or
more actuators in order to maintain one or more of a combination of
electrolyte species concentrations, pH, ionic strength, and other
electrolyte characteristics at a desired value that may correspond
to the redox flow battery system being in a discharge mode. In one
example the desired values may include being beyond a threshold
value characteristic to operation of the redox flow battery system
in the discharge mode.
[0057] Returning to steps 304 and 308, for the case where the redox
flow battery system is not operating in either a charging mode or a
discharge mode, method 300 continues at 312 where the controller
places the redox flow battery system in an idle operating mode. In
one example, the redox flow battery may be in the idle mode when a
DC current from the power module is less than or substantially
equal to an idle threshold current. In one example, the idle
threshold current may be zero. Method 400 and 500 of FIGS. 4 and 5,
respectively, illustrate two embodiments of idling a redox flow
battery system that can aid in lowering system capacity losses. The
idle mode operation described in both the methods 400 and 500
includes cycling activity of the pump to maintain redox flow
battery voltage and/or SOC within a threshold voltage and/or SOC
range such that redox flow battery is promptly ready to provide a
desired amount of power during a subsequent charge. In this way, a
lag time and/or warm-up phase for a redox flow battery system may
be reduced.
[0058] Turning now to FIG. 4, it shows a first method 400 for
idling a redox flow battery system that can aid in lowering system
capacity losses. Method 400 includes adjusting a pump ON/OFF status
based on a time elapsed between pump activation cycles during redox
flow battery idle mode. Method 400 may begin following 312 of
method 300 of FIG. 3, when the redox flow battery system enters
idle mode.
[0059] The method 400 may begin at 402, where the controller 88 may
estimate and/or measure operating parameters of the redox flow
battery system. As described above at 302 of FIG. 3, the controller
88 may determine one or more of battery state of charge (SOC),
power module voltage, pump activity (e.g., electrolyte pump ON/OFF
statuses, electrolyte pump flow rates, pump timers, and the like),
electrolyte temperatures, power (including current and voltage)
supplied to the power module, power (including current and voltage)
supplied by the power module, internal power demand set points, and
external power demand set points, and the like. Various operating
parameters may be indicated by one or more sensors of the redox
flow battery system.
[0060] At 404, in response to the redox flow battery system being
in idle mode, the method 400 includes deactivating power
electronics. Power electronics may include one or more of a DC/DC
converter, DC/AC inverter, and a power module contactor.
Deactivating power electronics may aid in reducing power
consumption of the redox flow battery system while in idle mode.
Deactivating the power electronics may include a controller
signaling to one or more actuators of redox flow battery to power
OFF, which may inhibit an ability of redox flow battery to
discharge and/or charge. Deactivating the power electronics may
include a mechanical switch that user may set in idle mode. In
other words, Additionally, deactivating power electronics may
include display of a message at a human machine interface (HMI) to
alert a user that the redox flow battery is in (or initiating) the
battery idle mode. Furthermore, the display of the HMI may be
dimmed or put in sleep mode, thereby reducing an illumination
emitted therefrom.
[0061] Next, at 406, in response to the redox flow battery system
being in idle mode, the controller 88 may reduce the electrolyte
temperature in order to further reduce power consumption while
operating in idle mode. Reducing electrolyte temperature also may
aid in reducing overall heat losses to the environment during idle
mode due to lower temperature gradients between the redox flow
battery system and the surrounding ambient atmosphere. In one
example, reducing the electrolyte temperature may include adjusting
a heater set point based on the redox flow battery system being in
idle mode. For example, the controller 88 may send a control signal
to one or more heater actuators to reduce an electrolyte
temperature below an idle threshold temperature. The one or more
heaters may be thermally coupled to the negative and positive
electrolyte chambers 50 and 52, and/or the negative and positive
electrolyte chambers of the multi-chambered electrolyte storage
tank 110. Adjusting a heater set point may further include reducing
a heater output power set point to reduce heater output power in
order to reduce the electrolyte temperature below the idle
threshold temperature. The idle threshold temperature may be based
on a solubility or stability of the electrolytes during idle mode.
For example, below the idle threshold temperature, a risk of
destabilization of the electrolyte may be increased;
destabilization of the electrolyte may include precipitation of
electrolyte salts, which reduces the redox flow battery system
capacity and performance. In contrast, above the threshold
temperature, a risk of destabilization of the electrolyte is
reduced and electrolyte stability can be maintained without
precipitation of any electrolyte salts. The relationship between
electrolyte solubility, the idle threshold temperature, and the
control signal (e.g., heater output power) for the heater may be
pre-determined or may be empirically determined for a particular
redox flow battery system.
[0062] In another example, reducing the electrolyte temperature may
include the controller 88 adjusting a control signal to one or more
heaters to decrease a heater set point in order to decrease the
electrolyte temperature during idle mode relative to an electrolyte
temperature during battery charge and discharge modes. In one
example, lowering a heater output power during redox flow battery
idle mode may cool or lower an amount of heat transferred from the
heater to the redox flow battery electrolyte relative to the amount
of heat transferred from the heater to the electrolyte during redox
flow battery charge and discharge modes. A temperature of a redox
flow battery during battery charge and discharge modes may be
substantially equal to 60.degree. C., in one example. However,
during redox flow battery idle mode, the heater setting may be
decreased to heat redox flow battery to an idle threshold
temperature equivalent to an ambient or room temperature range
between 25-30.degree. C.
[0063] At 408, in response to the redox flow battery system being
in idle mode, the controller 88 begins cycling of the electrolyte
pumps, including deactivating the electrolyte pump and initiating a
first timer, timer 1. Timer 1 may be used to indicate an elapsed
time since one or more electrolyte pumps have been deactivated.
Deactivating the electrolyte pump may include deactivating the
electrolyte pump, wherein the pump may remain dormant while the
redox flow battery system may maintained in a state (e.g., SOC
greater than a threshold SOC) where the redox flow battery system
can readily provide a desired power output promptly upon receiving
a power request. In one example deactivating the one or more
electrolyte pumps may include deactivating pumps 30 and/or 32. In
other examples, deactivating the one or more electrolyte pumps may
include deactivating a sufficient number of pumps such that
circulation of electrolyte to and from the redox flow battery cells
is stopped. In further examples, deactivating the one or more
electrolyte pumps may include deactivating a sufficient number of
pumps such that circulation of electrolyte to and from the redox
flow battery cells is reduced below a deactivation threshold flow
rate. In this way, an electrolyte flow rate and pumping of
electrolytes from the multi-chambered electrolyte storage tank 110
to the negative and positive electrolyte chambers 50 and 52 of or
more redox flow battery cells 18 may be stopped or maintained at
the deactivation threshold flow rate. In one example, the
deactivation threshold flow rate may correspond to a flow rate
below which shunting losses are substantially reduced since the
supply of fresh electrolyte to the redox flow battery cells is
reduced. In another example, the deactivation threshold flow rate
may correspond to a zero flow rate, and shutting off the
electrolyte pump. In some cases, having a non-zero deactivation
flow rate may help to preserve a life of the electrolyte pump, by
avoiding completely shutting the pump off. Stopping the electrolyte
flow and/or reducing the flow of electrolyte to the deactivation
threshold flow rate during idle mode can aid in reducing shunt
losses conducted through the flowing electrolytes since the amount
of fresh electrolyte supplied to the redox flow battery cells is
reduced. Furthermore, shunt current losses may be confined to the
existing volume of electrolyte within the power module when the
pumps are deactivated, including operating below the deactivation
threshold flow rate. Having a lower concentration of fresh
electrolyte in the redox flow battery cells during idle mode can
reduce a driving force for current shunt losses through the
electrolyte. Furthermore, as described previously cycling the
electrolyte pumps, including deactivating the electrolyte pumps at
408 can aid in reducing parasitic pump power losses.
[0064] At 409, the controller 88 may include measuring the first
timer, and determining a duration for which one or more of the
electrolyte pumps was deactivated. At 410, the controller 88 may
determine if the first timer is greater than a first threshold
duration. The first threshold duration may be based on target
amount of time between successive activation (cycling ON) of the
electrolyte pump during redox flow battery idle mode. As describe
above, deactivating the electrolyte pump may include deactivating
the electrolyte pump, wherein the pump may remain dormant while the
redox flow battery system may maintained in a state (e.g., SOC
greater than a threshold SOC) where the redox flow battery system
can readily provide a desired power output promptly upon receiving
a power request. In other words, the first threshold duration may
correspond to a pump OFF interval during idle mode. In one example,
the first threshold duration may be a fixed interval relative to a
pump ON interval corresponding to a second threshold duration. In
one case, the pump OFF interval may be set relative to the pump ON
interval such that an overall pump OFF duration during idle mode is
5/6 of the overall idle time; in other words a ratio of the pump
OFF interval to the pump ON interval would be 5 to 1 and a ratio of
the first threshold duration to the second threshold duration would
5 to 1. For example, the first threshold duration may be equal to
50 minutes and the second threshold duration may be 10 minutes;
thus, during idle mode, the pump would remain OFF 50 minutes for
every hour of idle time.
[0065] Alternatively, the first threshold duration may be adjusted
based on a power module voltage measured prior to the initiation of
the battery idle mode. In one example, the first threshold duration
may be higher corresponding to the power module voltage just prior
to the entering battery idle mode being higher, and the first
threshold duration may be lower for the case where the power module
voltage just prior to entering battery idle mode is lower. In this
way, the first threshold duration may allow for longer cycling
periods from a higher initial voltage prior to entering idling
mode, and may allow for shorter cycling periods from a lower
initial voltage prior to entering idling mode. For the case where
the first timer is less than the first threshold duration, then the
method 400 proceeds from 410 to 412 to continue to monitor the
first timer and maintains the electrolyte pump deactivated.
[0066] If the first timer is greater than the first threshold
duration, then the method proceeds from 410 to 414 to send a
control signal to the actuator of the electrolyte pump to activate
the electrolyte pump at an idle threshold flow rate. The idle
threshold flow rate may correspond to an electrolyte flow rate
below which idling electrolyte within the power module is not
refreshed enough so that a responsiveness of the redox flow battery
system for supplying power on demand to an external load is reduced
below a desirable level. In other words, if the electrolyte flow
rate is below the idle threshold flow rate, the supply of
electrolyte to the redox flow battery cells may not be sufficient
to promptly respond to a command from the controller 88 for
supplying power to an external load. As such there may be an
undesirable extended delay, allowing for enough fresh electrolyte
to reach the redox flow battery cells, before enough current/power
can be supplied to meet the commanded demand. Said in another way,
if the pump is not reactivated after the first threshold duration,
a responsiveness of the redox flow battery system to promptly
supply power to a commanded external load may be reduced. In one
example, the idle threshold flow rate may be less than the first or
discharge threshold flow rates describe above. For example, the
idle threshold flow rate may correspond to 10% of the charge or
discharge threshold flow rate. In some cases the idle threshold
flow rate to a negative electrolyte compartment may be different
from the idle threshold flow rate to the positive electrolyte
compartment. At any rate, it will be appreciated that the pump flow
rate is reduced for redox flow battery idle mode compared to the
pump flow rate during the charge and discharge modes. A second
timer is initiated in conjunction with the activation of the
electrolyte pump, the second timer measuring a pump ON duration
during the idle pump cycling of method 400.
[0067] At 416, the method includes determining the second timer is
greater than a second threshold duration. The second threshold
duration may be based on an amount of time desired to activate the
pump during battery idle mode to maintain a responsiveness of the
redox flow battery for meeting anticipated power demands from an
external load, while also decreasing battery capacity losses
experienced by the redox flow battery and parasitic power losses
due to operation of the pump and heating element. Capacity losses
may include a mitigated power output of redox flow battery. In one
example, the second threshold duration is 20% of the first
threshold duration.
[0068] If the second timer is less than the second threshold
duration, then the electrolyte has not been adequately refreshed to
achieve the desired system responsiveness to an anticipated
external load command, and the method proceeds from 416 to 418 to
continue monitoring the second timer. The electrolyte pump remains
activated at the idling threshold flow rate while the second timer
is less than the second threshold duration.
[0069] If the second timer is greater than the second threshold
duration at 416, indicating that the electrolyte has been refreshed
enough to allow for achieving a desired system responsiveness to an
anticipated external load command, method 400 continues at 420
where it determines if idle mode conditions are continued to be
met. Meeting idle mode conditions may include determining if the
redox flow battery system is not in either a charging mode or a
discharge mode. Thus, determining if idle mode conditions are still
met may be performed as described for 304, 308, and 312 of method
300. For the case where idle conditions are still met, then the
method proceeds back to 408 to continue idle mode operation. In
this way, during the idle state, method 400 repeatedly cycles the
electrolyte pump between the active state and the inactive state.
For the case where idle conditions are not met (e.g., the redox
flow system enters charging or discharge mode), the method 400
returns to method 300 of FIG. 3 after 312, and ends.
[0070] As described above, each of the idle threshold temperature,
deactivation threshold flow rate, idle threshold flow rate, first
threshold duration, and second threshold duration may be may be
adjusted according to the anticipated power demands during a
battery idle mode. For example, when the anticipated power demands
during a battery idle mode are higher, an idle threshold
temperature may be higher, a deactivation threshold flow rate may
be higher, an idle threshold flow rate may be higher, a first
threshold duration may be lower, and a second threshold duration
may be higher in order to increase a responsiveness of the redox
flow battery system. Conversely, when the anticipated power demands
during a battery idle mode are lower, an idle threshold temperature
may be lower, a deactivation threshold flow rate may be lower, an
idle threshold flow rate may be lower, a first threshold duration
may be higher, and a second threshold duration may be lower in
order to decrease a responsiveness of the redox flow battery system
while reducing parasitic power losses due to pumping and heating
and reducing shunt losses through the electrolyte. In this way,
idle mode operation parameters may be adjusted by the controller 88
depending on the anticipated power needs to maintain a redox flow
battery system responsiveness while reducing parasitic and shunting
losses.
[0071] Turning now to FIG. 5, it shows an alternate embodiment for
a method 500 for adjusting a pump based on a voltage of redox flow
battery measured during an idle mode for redox flow battery system.
As such, the method 500 may be executed following 312 of method 300
of FIG. 3. The method 500 may begin at 502, where the method
includes determining, estimating, and/or measuring current
operating parameters as described for steps 302 and 402 of methods
300 and 400, respectively. Next, step 504, deactivating the power
electronics, step 506, reducing electrolyte temperature by
signaling to decrease a heater set point, and step 508,
deactivating the electrolyte pump, maybe performed as described for
steps 404, 406, and 408 of method 400, respectively.
[0072] Following deactivation of the electrolyte pump at 508,
method 500 may continue at 509, where the controller 88 determines
and/or measures the power module voltage. The power module voltage
may refer to the voltage across the redox flow battery cell stacks
within the power module. In one example, battery is fully charged
and each cell open circuit voltage is about 1.2V. As a result, the
power module voltage is the sum of all the cell voltages minus
shunting voltage loss (voltage drop across shunt resistors).
[0073] Next, at 510, the controller 88 may determine if the power
module voltage is less than a first threshold voltage. The power
module first threshold voltage may be determined by the minimal
load it can sustain with the available electrolyte within the power
module during idle mode, with no additional electrolyte pumping. In
another example, the first threshold voltage may refer to a voltage
below which the redox flow battery system may be unable to respond
to an anticipated power demand from an internal or external load.
In this way the first threshold voltage may be higher when the
anticipated power demand may be higher and the first threshold
voltage may be lower when the anticipated power demand is lower. If
the power module voltage is greater than or equal to the first
threshold voltage, then the method proceeds from 510 to 512 to
continue to monitor the voltage and maintains the electrolyte pump
deactivated.
[0074] For the case where the power module voltage decreases below
the first threshold voltage, then the method 500 proceeds from 510
to 514 to switch the pump to an active state, including sending a
control signal to the actuator of the pump to activate the
electrolyte pump at the idle threshold flow rate. As described
above with reference to method 400, the idle threshold flow rate
may correspond to an electrolyte flow rate below which idling
electrolyte within the power module is not refreshed enough so that
a responsiveness of the redox flow battery system for supplying
power on demand to an external load is reduced below a desirable
level. In one example, the idle threshold flow rate may be less
than the charging threshold flow rate or the discharge threshold
flow rate described above with reference to method 300. At any
rate, it will be appreciated that the pump flow rate may be reduced
to a lower flow rate during redox flow battery idle mode as
compared to the pump flow rate during the charge and discharge
modes. In this way, electrolytes in redox flow battery cells may be
replenished and refreshed sufficiently to increase a power module
voltage, while reducing parasitic power and shunting losses.
[0075] At 516, the method includes determining if the power module
voltage is greater than or equal to a second threshold voltage. In
one example, the second threshold voltage may include a voltage
greater than the first threshold voltage. The second threshold
voltage may correspond to a power module voltage above which shunt
current losses increase appreciably since the flow of fresh
electrolyte recirculated to the redox flow battery cells is higher.
The second threshold voltage may also correspond to a power module
open circuit voltage at the given state of charge or when power
module voltage does not change anymore indicating electrolyte may
be fully replenished. If the power module voltage is less than the
second threshold voltage, then the method proceeds from 516 to 518
to continue monitoring the voltage and maintaining the pump active
(e.g., ON) at the idle threshold flow rate.
[0076] If the voltage is greater than or equal to the second
threshold voltage, then the method proceeds to 520 to determine if
idle conditions are still met. If the idle conditions are no longer
met, then the method proceeds to FIG. 3. If the idle conditions are
still met, then the method proceeds back to 508 to switch the pump
to an inactive state in response to the power module voltage being
greater than the second threshold voltage. In this way, during the
idle state, method 4500 repeatedly cycles the electrolyte pump
between the active state and the inactive state. For the case where
idle conditions are not met (e.g., the redox flow system enters
charging or discharge mode), the method 500 returns to method 300
of FIG. 3 after 312, and ends.
[0077] Thus, an example method of operating a redox flow battery
system may include switching the redox flow battery system to an
idle mode, wherein the idle mode includes operation of the redox
flow battery system outside of a charging mode and outside of a
discharge mode. Furthermore, in response to switching to the idle
mode, the example method may include repeatedly cycling operation
of an electrolyte pump between an idling threshold flow rate less
than a charging threshold flow rate and a deactivation threshold
flow rate, and in response to switching to the charging mode,
maintaining operation of the electrolyte pump at the charging
threshold flow rate greater than the idling threshold flow rate. A
second example of the method optionally includes the first example,
and may further include, in response to switching to the discharge
mode, maintaining operation of the electrolyte pump at a discharge
threshold flow rate greater than the idling threshold flow rate. A
third example of the method optionally includes one or more of the
first and second examples, and further includes wherein operation
of the electrolyte pump at the deactivation threshold flow rate is
maintained for a first threshold duration, operation of the
electrolyte pump at the idling threshold flow rate is for a second
threshold duration, and the deactivation threshold duration is
greater than the second threshold duration. A fourth example of the
method optionally includes one or more of the first through third
examples, and further includes wherein the idling threshold
duration is less than 20% of the first threshold duration. A fifth
example of the method optionally includes one or more of the first
through fourth examples, and further includes wherein the idling
threshold flow rate is adjusted higher in response to an
anticipated load demand of the redox flow battery system being
higher, and the idling threshold flow rate is adjusted lower in
response to an anticipated load demand of the redox flow battery
system being lower. A sixth example of the method optionally
includes one or more of the first through fifth examples, and
further includes wherein the first threshold duration is adjusted
lower in response to an anticipated load demand of the redox flow
battery system being higher, and the first threshold duration is
adjusted higher in response to an anticipated load demand of the
redox flow battery system being lower. A seventh example of the
method optionally includes one or more of the first through sixth
examples, and further includes wherein the second threshold
duration is adjusted higher in response to an anticipated load
demand of the redox flow battery system being higher, and the
second threshold duration is adjusted lower in response to an
anticipated load demand of the redox flow battery system being
lower.
[0078] Thus, an example method of operating a redox flow battery
system may include operating the redox flow battery system in an
idle mode during a condition when the redox flow battery system is
operating outside of a charging mode and outside of a discharge
mode, during operation in the idle mode, repeatedly cycling
operation of an electrolyte pump between an active state and an
inactive state, wherein the active state comprises pumping
electrolyte at an idling threshold flow rate less than a charging
threshold flow rate, and the inactive state comprises deactivating
the electrolyte pump, and in response to switching to the discharge
mode, maintaining operation of the electrolyte pump at the
discharge threshold flow rate. A second example of the method may
optionally include the first example, and further includes in
response to switching to the charging mode, maintaining operation
of the electrolyte pump at the charging threshold flow rate. A
third example of the method optionally includes one or more of the
first and second examples, and further includes wherein cycling
operation of the electrolyte pump between the active state and the
inactive state includes, switching from the active state to the
inactive state in response to a power module voltage increasing
above a second threshold voltage, and switching from the inactive
state to the active state in response to a power module voltage
decreasing below a first threshold voltage, the first threshold
voltage being less than the second threshold voltage. A fourth
example of the method optionally includes one or more of the first
through third examples, and further includes wherein the first
threshold voltage is less than the charging threshold voltage. A
fifth example of the method optionally includes one or more of the
first through fourth examples, and further includes wherein the
idling threshold flow rate is adjusted higher in response to an
anticipated load demand of the redox flow battery system being
higher, and the idling threshold flow rate is adjusted lower in
response to the anticipated load demand of the redox flow battery
system being lower. A sixth example of the method optionally
includes one or more of the first through fifth examples, and
further includes wherein the first threshold voltage is adjusted
higher in response to the anticipated load demand of the redox flow
battery system being higher, and the first threshold voltage is
adjusted lower in response to the anticipated load demand of the
redox flow battery system being lower. A seventh example of the
method optionally includes one or more of the first through sixth
examples, and further includes wherein the second threshold voltage
is adjusted higher in response to the anticipated load demand of
the redox flow battery system being higher, and the second
threshold voltage is adjusted lower in response to the anticipated
load demand of the redox flow battery system being lower.
[0079] As described above, each of the idle threshold temperature,
deactivation threshold flow rate, idle threshold flow rate, first
threshold voltage, and second threshold voltage may be may be
adjusted according to the anticipated power demands during a
battery idle mode. For example, when the anticipated power demands
during a battery idle mode are higher, an idle threshold
temperature may be higher, a deactivation threshold flow rate may
be higher, an idle threshold flow rate may be higher, a first
threshold voltage may be higher, and a second threshold voltage may
be higher in order to increase a responsiveness of the redox flow
battery system. Conversely, when the anticipated power demands
during a battery idle mode are lower, an idle threshold temperature
may be lower, a deactivation threshold flow rate may be lower, an
idle threshold flow rate may be lower, a first threshold voltage
may be lower, and a second threshold duration may be lower in order
to decrease a responsiveness of the redox flow battery system while
reducing parasitic power losses due to pumping and heating and
reducing shunt losses through the electrolyte. In this way, idle
mode operation parameters may be adjusted by the controller 88
depending on the anticipated power needs to maintain a redox flow
battery system responsiveness while reducing parasitic and shunting
losses.
[0080] Turning now to FIG. 8, it illustrates an example plot
showing operation of an example redox flow battery system. Trend
line 810 represents the power module voltage during idling mode
while maintaining an electrolyte pump ON to pump electrolyte
continuously at charge/discharge flow rates, maintaining power
electronics ON, and maintaining an electrolyte temperature at
charge/discharge temperatures. Owing to larger shunt current losses
and higher parasitic pumping losses, the power module voltage
during idling begins to sharply decrease after about 40 h. In
contrast, while in idle mode, reducing the electrolyte temperature
to the idle threshold temperature, turning OFF power electronics,
and cycling the electrolyte pump between an idle threshold flow
rate for a second threshold duration and a deactivation threshold
flow rate for a first threshold duration (e.g., operating the redox
flow battery system according to methods 300 and 400) can mitigate
capacity losses, as shown by trend line 820.
[0081] Turning now to FIG. 6, it shows a time plot 600 graphically
illustrating battery conditions during and outside of a battery
idle mode. The time plot 600 illustrates the methods 300, 400, and
500 executed in parallel by battery system of FIGS. 1 and 2. In
this way, each of the methods 300, 400, and 500 may occur
simultaneously to one another. For example, the voltage measured in
the method 500 may be compared to the first threshold voltage
simultaneously to first and second timers being compared to first
and second threshold durations, respectively. Plot 610 illustrates
an electrolyte pump flow rate, plot 620 illustrates if a redox flow
battery idle condition is being met, plot 630 illustrates a
temperature of redox flow battery, plot 640 illustrates a battery
voltage, and plot 650 illustrates a DC current, for example a DC
current flowing through a shunt resistor electrically coupled to
the power module. Dashed line 612 indicates a threshold
charge/discharge electrolyte flow rate and dashed line 614
indicates an idle threshold electrolyte flow rate. Line 632
illustrates a second threshold battery temperature and line 634
illustrates a first threshold battery temperature. As shown, the
second threshold battery temperature is greater than the first
threshold battery temperature. In one example, the second threshold
battery temperature is substantially equal to a battery temperature
outside of redox flow battery idle mode and the first threshold
battery temperature is substantially equal to a desired battery
temperature during redox flow battery idle mode. Line 642 depicts a
first threshold voltage and line 644 depicts a second threshold
voltage. The first and second threshold voltages may be
substantially similar to those described above with respect to FIG.
5. The DC current may have a directionality based on if the redox
flow battery system is in a charging or discharge mode. For
example, positive DC current may correspond to an external device
flowing current to the battery during charging, and negative DC
current may correspond to the battery flowing current to an
external device during discharge. Thus, neutral DC current (e.g.,
zero charge), may correspond to no current flow to and from the
battery. In one example, positive DC current corresponds to a
charging mode, negative DC current corresponds to a discharging
mode, and neutral DC current corresponds to an idle mode. The plot
600 measures time along a horizontal axis, where time increases
from a left to right side of the figure.
[0082] Prior to t1, the electrolyte pump flow rate (plot 610) is
relatively high and substantially equal to the threshold
charge/discharge electrolyte flow rate (line 612). Battery idle
conditions are not met as shown by plot 620 being aligned with
"NO". A redox flow battery temperature (plot 630) is equal to a
temperature greater than the second threshold battery temperature
(line 632). A power module voltage (plot 640) is decreasing from a
relatively high battery voltage toward the second threshold voltage
(line 644). DC current (line 650) aligns with the negative value,
indicating current is flowing away from the battery to an external
device. As such, the redox flow battery may be operating in a
discharge mode.
[0083] At t1, redox flow battery idle conditions are met and redox
flow battery transitions from the discharge mode to the idle mode.
DC current aligns with zero and/or neutral as substantially no
current flows to and/or away from the redox low battery. As
described with respect to FIGS. 4 and 5, the power electronics are
deactivated upon entering redox flow battery idle mode.
Additionally, the heater is adjusted to a lower set point to heat
the redox flow battery to a temperature less than the first
threshold battery temperature (line 634). Furthermore, the
electrolyte pump is deactivated (e.g., switched OFF), as shown by
plot 610 aligning with "0" and the electrolyte pump flow rate
decreases to a flow rate less than the idle threshold electrolyte
flow rate 614. In this way, electrolytes are no longer flowing to
redox flow battery. The first timer may be started to begin
tracking time corresponding to a duration the electrolyte pump is
deactivated.
[0084] After t1 and prior to t2, redox flow battery remains in the
idle mode. The redox flow battery temperature decreases to a
battery temperature substantially equal to the first threshold
battery temperature. The power module voltage continues to decrease
and decreases to a voltage less than the first threshold voltage
after entering redox flow battery idle mode. Specifically, the
voltage decreases to a voltage less than the first threshold
voltage following an amount of time less than the first threshold
duration. Double headed arrow 602 illustrates the first threshold
duration. In response, the electrolyte pump is activated to the
idle threshold electrolyte flow rate 614 less than the threshold
charge/discharge flow rate 612. In one example, the electrolyte
pump is activated to a flow rate substantially equal to 5-10% of
the threshold charge/discharge flow rate 612. The DC current
remains substantially equal to zero during the idle mode, despite
the electrolyte pump being activated. As such, the pump may be
powered by an external source during the redox flow battery idle
mode. Additionally or alternatively, the DC current may move to
slightly positive and slightly negative positions during the idle
mode. Slightly positive and slightly negative positions are
respectively less than the positive and negative positions during
charging and discharging modes. In this way, the power module
voltage begins to increase as fresh electrolytes are delivered to
the redox flow battery.
[0085] At t2, battery idle conditions are still met and redox flow
battery temperature is substantially equal to the first threshold
battery temperature. The power module voltage continues to
increases and increases to a voltage greater than the first
threshold voltage and less than the second threshold voltage. As
such, the electrolyte pump remains active at the idle threshold
flow rate (dashed line 614).
[0086] After t2 and prior to t3, power module voltage continues to
increase toward the second threshold voltage. As such, the
electrolyte pump remains active. At t3, power module voltage is
greater than the second threshold voltage and the electrolyte pump
is deactivated. Double headed arrow 604 represents a second
threshold duration, which is substantially equal to the second
threshold duration described above with respect to FIG. 4. In this
way, the electrolyte pump was active during redox flow battery idle
mode for an amount of time greater than second threshold duration.
The first timer is initiated at t3 following deactivation of the
electrolyte pump.
[0087] Thus, in some embodiments where the power module voltage is
monitored and electrolyte pump cycles are timed, replenishing the
power module voltage may supersede the fixed time cycles.
Specifically, the electrolyte pump is initiated in response to the
power module voltage falling below the first threshold voltage even
if the first timer is less than the first threshold duration.
Additionally, the electrolyte pump may be maintained active if the
power module voltage is less than the second threshold voltage even
if the second timer is greater than the second threshold
duration.
[0088] After t3 and prior to t4, power module voltage decreases to
a voltage less than the second threshold voltage and greater than
the first threshold voltage. As such, the electrolyte pump remains
deactivated. At t4, the first timer is equal to the first threshold
duration. As such, a controller signals to an actuator of the
electrolyte pump to activate the pump to the idle threshold flow
rate 614. As such, the second time is activated. In alternate
examples, the controller may signal to activate the electrolyte
pump to a flow rate less than the idle threshold flow rate. This
may be due to the power module voltage being greater than the first
threshold voltage. As such, less charging may be desired than when
the power module voltage is less than the first threshold voltage.
In some examples, the electrolyte pump may not be activated at t4
due to the power module voltage being greater than the first
threshold voltage, despite the first timer exceeding the first
threshold duration.
[0089] After t4 and prior to t5, the second timer is compared to
the second threshold duration (double headed arrow 604) and the
electrolyte pump remains active since the second timer is less than
the second threshold duration. The power module voltage increases
to a voltage greater than the second threshold voltage. Redox flow
battery temperature remains substantially equal to the first
threshold battery temperature. At t5, the second timer is
substantially equal to the second threshold duration. The power
module voltage is no longer increasing and is equal to a voltage
greater than the second threshold voltage.
[0090] After t5, redox flow battery idle conditions are met for a
period of time, wherein during the period of time the electrolyte
pump is deactivated, redox flow battery temperature is
substantially equal to the first threshold battery temperature, and
the power module voltage decreases toward the second threshold
voltage. At t6, redox flow battery idle conditions are no longer
met. As such, redox flow battery heater is adjusted to heat redox
flow battery to a temperature greater than or equal to the second
threshold battery temperature. The electrolyte pump is reactivated
and is set to a flow rate substantially equal to the threshold
charging/discharging flow rate. Lastly, the power module voltage
begins to increase to a voltage higher than the second threshold
voltage. This is further indicated by the DC current moving to a
positive position, wherein an external source is flowing current to
the redox flow battery. In this way, redox flow battery is in the
charge mode and has exited the idle mode.
[0091] Turning now to FIG. 7, it shows a time plot 700 for
operating the redox flow battery system of FIG. 1 according to
methods 300, 400, and 500. The time plot 700 illustrates the
methods 300, 400, and 500 executed by the redox flow battery of
FIG. 1. The methods 400 and 500 are illustrated occurring
sequentially to one another. As such, the methods 400 and 500 do
not occur simultaneously in the embodiment of FIG. 7. Specifically,
the method 500 is illustrated from t1 to t3 and the method 400 is
illustrated from t3 to t5. Plot 710 illustrates an electrolyte pump
flow rate, plot 720 illustrates if a redox flow battery idle
condition is being met, plot 730 illustrates a temperature of the
redox flow battery, plot 740 illustrates a power module voltage,
and plot 750 illustrates a DC current (e.g., a DC current flowing
through a shunt resistor electrically coupled to the power module).
Line 732 illustrates a second threshold battery temperature and
line 734 illustrates a first threshold battery temperature. As
shown, the second threshold battery temperature is greater than the
first threshold battery temperature. In one example, the second
threshold battery temperature is substantially equal to a battery
temperature outside of redox flow battery idle mode and the first
threshold battery temperature is substantially equal to a desired
battery temperature during redox flow battery idle mode. Line 742
depicts a first threshold voltage and line 744 depicts a second
threshold voltage. The first and second threshold voltages may be
substantially similar to those described above with respect to FIG.
5. The DC current may have a directionality based on its charge.
For example, positive DC current may correspond to an external
device flowing current to the battery and negative DC current may
correspond to the battery flowing current to an external device.
Thus, neutral DC current (e.g., zero charge), may correspond to no
current flow to and from the battery. In one example, positive DC
current corresponds to a charging mode, negative DC current
corresponds to a discharging mode, and neutral DC current
corresponds to an idle mode. The plot 700 measures time along a
horizontal axis, where time increases from a left side to a right
side of the figure.
[0092] Prior to t1, the electrolyte pump flow rate (plot 710) is
relatively high and substantially equal to a threshold
charge/discharge flow rate (line 712). Battery idle conditions are
not met as shown by plot 720 being aligned with "NO". A redox flow
battery temperature (plot 730) is equal to a temperature greater
than the second threshold battery temperature (line 732). A power
module voltage (plot 740) is decreasing from a relatively high
battery voltage toward the second threshold voltage (line 744). DC
current (line 750) aligns with the negative value, indicating
current is flowing away from the battery to an external device. As
such, the redox flow battery may be operating in a discharge
mode.
[0093] At t1, redox flow battery idle conditions are met and redox
flow battery transitions from the discharge mode to the idle mode.
As described with respect to FIGS. 4 and 5, the power electronics
are deactivated upon entering redox flow battery idle mode.
Additionally, the heater is adjusted to heat the redox flow battery
to a temperature less than the first threshold battery temperature
(line 734). Furthermore, the electrolyte pump is deactivated (e.g.,
switched OFF), as shown by plot 710 aligning with "0" and the
electrolyte pump flow rate decreases. In this way, electrolytes are
no longer flowing to redox flow battery. The first timer is not
activated due to only the power module voltage being monitored.
[0094] After t1 and prior to t2, redox flow battery remains in the
idle mode. The redox flow battery temperature decreases to a
battery temperature substantially equal to the first threshold
battery temperature. The power module voltage continues to decrease
and decreases to a voltage less than the first threshold voltage
after entering redox flow battery idle mode. Specifically, the
voltage decreases to a voltage less than the first threshold
voltage. In response, the electrolyte pump is activated to the idle
threshold electrolyte flow rate 714 less than the threshold
charge/discharge flow rate 712. In one example, the idle threshold
flow rate may be substantially equal to 5-10% of the threshold
charge/discharge flow rate 712. The DC current remains
substantially equal to zero during the idle mode, despite the
electrolyte pump being activated. As such, the pump may be powered
by an external source during the redox flow battery idle mode.
Additionally or alternatively, the DC current may move to slightly
positive and slightly negative positions during the idle mode.
Slightly positive and slightly negative positions are respectively
less than the positive and negative positions during charging and
discharging modes. In this way, the power module voltage begins to
increase as fresh electrolytes are delivered to the redox flow
battery.
[0095] At t2, battery idle conditions are still met and redox flow
battery temperature is substantially equal to the first threshold
battery temperature. The power module voltage continues to
increases and increases to a voltage greater than the first
threshold voltage and less than the second threshold voltage. As
such, the electrolyte pump remains active at the idle threshold
flow rate.
[0096] After t2 and prior to t3, power module voltage continues to
increase toward the second threshold voltage. As such, the
electrolyte pump remains active. At t3, power module voltage is
greater than the second threshold voltage and the electrolyte pump
is deactivated. The first timer is initiated at t3 following
deactivation of the electrolyte pump. As such, the method 500 is
completed. After time t3, plot 700 illustrates operation of the
redox flow battery system according to execution of method 300 in
conjunction with the method 400.
[0097] After t3 and prior to t4, power module voltage decreases to
a voltage less than the second threshold voltage and greater than
the first threshold voltage. As such, the electrolyte pump remains
deactivated. At t4, the first timer is equal to the first threshold
duration (double headed arrow 702). As such, a controller signals
to an actuator of the electrolyte pump to activate the pump to the
idle threshold flow rate. As such, the second timer is activated.
In alternate examples, the controller may signal to activate the
electrolyte pump to a flow rate less than or greater than the
idling threshold flow rate.
[0098] After t4 and prior to t5, the second timer is compared to
the second threshold duration (double headed arrow 704) and the
electrolyte pump remains active since the second timer is less than
the second threshold duration. The power module voltage increases
to a voltage greater than the second threshold voltage. Redox flow
battery temperature remains substantially equal to the first
threshold battery temperature. At t5, the second timer is
substantially equal to the second threshold duration. The power
module voltage is no longer increasing and is equal to a voltage
greater than the second threshold voltage.
[0099] After t5, redox flow battery idle conditions are met for a
period of time, wherein during the period of time the electrolyte
pump is deactivated, redox flow battery temperature is
substantially equal to the first threshold battery temperature, and
the power module voltage decreases toward the second threshold
voltage. At t.sub.6, redox flow battery idle conditions are no
longer met. As such, redox flow battery heater is adjusted to heat
redox flow battery to a temperature greater than or equal to the
second threshold battery temperature. The electrolyte pump is
reactivated and is set to a flow rate substantially equal to the
threshold charging/discharging flow rate. Lastly, the power module
voltage begins to increase to a voltage higher than the second
threshold voltage. This is further indicated by the DC current
moving to a positive position, wherein an external source is
flowing current to the redox flow battery. In this way, redox flow
battery is in the charge mode and has exited the idle mode.
[0100] Thus, an example of a redox flow battery system may include
a power module, including a plurality of redox flow battery cell
stacks, each of the redox flow battery cell stacks including a
redox flow battery cell; an electrolyte pump capable of delivering
electrolyte from an electrolyte tank to the power module; and a
power control system with a controller, including executable
instructions thereon to, switch the redox flow battery system to an
idle mode, wherein the idle mode includes operation of the redox
flow battery system outside of a charging mode and outside of a
discharge mode, in response to switching to the idle mode,
repeatedly cycling operation of the electrolyte pump between an
idling threshold flow rate less than a charging threshold flow rate
and a deactivation threshold flow rate, and in response to
switching to the charging mode, maintaining operation of the
electrolyte pump at the charging threshold flow rate. A second
example of the redox flow battery system may optionally include the
first example, and further includes a heater thermally coupled to
the electrolyte, wherein the executable instructions include
reducing an electrolyte temperature to an idling threshold
temperature in response to switching to the idle mode. A third
example of the redox flow battery system may optionally include one
or more of the first and second examples, and further includes
wherein the idling threshold temperature is increased in response
to an anticipated load demand of the redox flow battery system
being higher, and the idling threshold temperature decreased in
response to the anticipated load demand of the redox flow battery
system being lower. A fourth example of the redox flow battery
system may optionally include one or more of the first through
third examples, and further includes wherein power electronics are
deactivated in response to switching to the idle mode. A fifth
example of the redox flow battery system may optionally include one
or more of the first through fourth examples, and further includes
wherein the idling threshold temperature corresponds to a
temperature below which electrolyte precipitation occurs. A sixth
example of the redox flow battery system may optionally include one
or more of the first through fifth examples, and further includes
wherein the idling threshold temperature is less than an
electrolyte temperature during the charging and discharge
modes.
[0101] In this way, a redox flow battery comprises a routine for
cycling an electrolyte pump between on and off positions based on
one or more of a time elapsed and a power module voltage during a
redox flow battery idle mode. In one example, the electrolyte pump
is activated in response to the power module voltage falling below
a first threshold voltage. Additionally or alternatively, the
electrolyte pump is activated in response to a first timer
exceeding a first threshold duration, where the first timer
measures an amount of time the electrolyte pump is deactivated
during the redox flow battery idle mode. At any rate, the
electrolyte pump is activated to a flow rate less than an
electrolyte pump flow rate outside of the redox flow battery idle
mode. The technical effect of activating the electrolyte pump to a
decreased flow rate and cycling the pump between on and off
positions is to decrease parasitic power losses due to the pump and
to decrease power capacity losses experienced by the redox flow
battery due to shunting.
[0102] Note that the example control and estimation routines
included herein can be used with various battery and/or vehicle
system configurations. The control methods and routines disclosed
herein may be stored as executable instructions in non-transitory
memory and may be carried out by the control system including the
controller in combination with the various sensors, actuators, and
other battery hardware. The specific routines described herein may
represent one or more of any number of processing strategies such
as event-driven, interrupt-driven, multi-tasking, multi-threading,
and the like. As such, various actions, operations, and/or
functions illustrated may be performed in the sequence illustrated,
in parallel, or in some cases omitted. Likewise, the order of
processing is not necessarily required to achieve the features and
advantages of the example embodiments described herein, but is
provided for ease of illustration and description. One or more of
the illustrated actions, operations and/or functions may be
repeatedly performed depending on the particular strategy being
used. Further, the described actions, operations and/or functions
may graphically represent code to be programmed into non-transitory
memory of the computer readable storage medium in the redox flow
battery control system, where the described actions are carried out
by executing the instructions in a system including the various
battery hardware components in combination with the electronic
controller.
[0103] The following claims particularly point out certain
combinations and sub-combinations regarded as novel and
non-obvious. These claims may refer to "an" element or "a first"
element or the equivalent thereof. Such claims should be understood
to include incorporation of one or more such elements, neither
requiring nor excluding two or more such elements. Other
combinations and sub-combinations of the disclosed features,
functions, elements, and/or properties may be claimed through
amendment of the present claims or through presentation of new
claims in this or a related application. Such claims, whether
broader, narrower, equal, or different in scope to the original
claims, also are regarded as included within the subject matter of
the present disclosure.
* * * * *