Summary:
Redox flow battery systems have been under investigation for use in energy storage applications for some time. The early systems had certain limitations such as the electrolyte cross-contamination, however, the vanadium redox flow battery system does not suffer from such limitations due to the common element in both positive and negative half-cell electrolytes. Various components of the vanadium battery have been developed by methods suitable for large scale commercialisation. A 5kW/13kWh system has been installed in a solar demonstration house and its suitability for application in energy self sufficient housing is currently being evaluated. A 4kW/4kWh vanadium battery system has also been installed in a golf cart and initial vehicle road trials have shown its viability in traction applications. Presently a 4kW vanadium battery system is under development for evaluation as a back-up battery in submarines.
The vanadium battery is a redox flow battery system which was
pioneered by Skyllas-Kazacos et. al. (1) and is currently under
development at the Vanadium Battery Development Laboratory, in the
School of Chemical Engineering and Industrial Chemistry at the
University of New South Wales.
Redox flow battery systems have been investigated by a number of
world government and independent research organisations over the
last two decades. An increase in interest in recent years has seen
certain systems reaching the demonstration stage. The primary
components of the redox flow system are illustrated in Figure 1.
picture:vanads1.gif
Figure 1: The redox flow battery system.
A redox flow cell consists of two half-cells, one positive and one
negative that are separated by a membrane. To enable electric charge
transfer in and out of the system each half-cell contains an iert
electrode. The energy is stored in the positive and negative
half-cell electrolytes which are pumped around the system. The flow
through the redox cell stack is parallel ant the half-cell
electrolytes are stored in separate storage tanks. An overview on
redox flow batteries has benn presented by Ritchie and Sira (2) and
a historical bibliography an their development by Bartolozzi (3).
Energy in batteries is produced when electrons flow between the
positive and negative species. In flow batteries this corresponds to
the two redox species which have different electrochemical
potentials.
In conventional energy storage systems solid state electrode
reactions are employed as in the lead-acid battery. In redox flow
cells the redox couples are all soluble solution species.
For practical application high currents and voltages are generally
required. Redox cell can be stacked in series to increase the
voltage and the cells can be electrically connencted in parallel for
high currents. In connecting individual cells in series to form a
battery stack, bipolar electrodes are employed and flow to the cells
is hydraulically in parallel performed by the use of manifolds.
In most redox flow batteries different metal species are employed in
the positive and negative half-cell electrolytes as in the case of
the iron-chromium (Fe-Cr) redox cell. Results on the research and
development of a 10kW class Iron-Chromium redox flow battery were
presented byHamamoto et. al. (4). Cross-contamination of the
electrolyte can occur by the ions crossing through the membrane
separator resulting in a decrease in battery capacity.
The vanadium redox battery employs vanadium ions in both half-cell
electrolytes. The V(II)/V(II) redox couple which was investigated by
Sum and Skyllas-Kazacos (5) is employed in the negative electrolyte.
The positive electrolyte employs the V(IV)/V(V) redox couple which
was investigated by Sum et. al. (6). If solution cross-over occurs
the vanadium half-cell solutions can be remixed and the system
brought back to its original state.
The following half-cell reactions are involved in the vanadium redox
cell:
V(V) - e ---¯ V(IV) (discharge)
V(V) - e ®--- V(IV) (charge)
V(III) - e ---¯ V(II) (charge)
V(III) - e ®--- V(II) (discharge)
At the concentration of 1 mole per litre for each vanadium species
and 25degC the standard cell potential is 1.26 Volts. Under
operating conditions the actual open-circuit cell voltage obtained
at 50% state of charge is 1.4 Volts.
The relatively fast kinetics of the vanadium redox couples allow
high coulombic and voltage efficiencies to be obtained but the value
of these efficiencies also depends on the internal resistance of the
cell.
One of the most important features of redox flow batteries is that
by using solutions to store the energy the system power and the
energy storage capacity are independent. The vanadium redox battery
can therefore be tailored to specific storage applications.
- The solutions have an indefinite life so only the mechanical
components need replacement at the end of their life.
- Instant recharge is possible by replacing the spent electrolytes
which makes the system ideal for electric vehicle applications.
- The system capacity can be increased by increasing the volume of
solution.
- The vanadium battery can be fully discharged without any
detrimental effects.
- The cost per kWh decreases as the energy capacity increases,
making large scale applications cost effective.
- The system can operate between restricted voltage ranges by the
use of trim cells.
- The capacity of the whole system can be monitored in line by
monitoring the state of charge of the electrolytes.
- The vanadium battery system is environmentally friendly since no
waste products are produced.
A vanadium battery bipolar stack illustrated in Figure 2 showing the
individual call stack components.
picture:vanads2.gif
Figure 2: Vanadium battery bipolar stack
The individual cell components have all undergone development at
UNSW in their own right. Optimisation and manufacturing techniques
for large scale commercialisation applications have also been
considered for the electrodes and flowframes.
There are two types of electrodes used in the vanadium battery
stack. The end electrodes are used as the first and last electrodes
in the stack while the remainder are bipolar electrodes. Studies on
fabrication and activation of conducting plastic electrodes were
undertaken and presented by Zhong et. al. (7).
The bipolar electrodes consist of an electrically conductive
graphite impregnated polymer sheet approximately 0.7 mm in
thinckness. To each side of the polymer sheet graphite felt is heat
bonded to allow the electrode formed to be a flow-through electrode
with a very large contact area.
The end electrodes are similar to the bipolar electrodes except that
felt in only bonded to one side and the other side is copper plated
to allow unrestricted electrical transport to the copper current
collectors.
The main problem with conducting plastic electrodes is to find a
material that is nor only an excellent electrical conductore but is
also resistant and impermeable to the electrolyte. the electrode
must also be of sufficient mechanical strength to be able to
tolerate any pressure changes in the stack.
The development of these electrodes has reached the stage where
polyethylene base bipolar electrodes and end electrodes with area
resistivity as low as 0.6 and 0.8Ohm/square cm respectively have
been obtained.
A polypropylene base material has also been sheet extruded and
resistivities in the range of 0.5-0.6 Ohm/square cm have been
obtained for bipolar electrodes by Haddadi-Asl et. al. (8). Sheet
extrusion for electrode preparation provides a uniform thickness of
conductive polymer sheet and large quantities can be produced in
single production runs at costs as low as $1 per square metre.
The electrolyte in the vanadium redox battery is introduced into and
out of each half-cell by the flowframe. The flowframe determines the
cell thickness and also supports the cell structure.
Up until recently the latest vanadium battery stacks were of an
external fed design. This simply relates to each cell being fed
individually from a common external manifold. The flow frames were
made of 5mm polypropylene and 0.8mm neoprene rubber was used for the
gaskets.
The polypropylene flow frames were machined out of full sheets of
polypropylene and a primary inlet and outlet manifold were heat
welded to each flowframe.
In the latest design of the vanadium battery stack an internal flow
distribution system has been developed which leads to a more compact
and robust stack assembly. The flowframes are injection moulded
resulting in a high level of quality control and a much lower cost
per flowframe. the initial capital outlay of the flowframe mould
will be quickly recovered when commercial production commences.
The flowframe construction material is santoprene and the hardness
of this material has been optimised resulting in the elimination of
gaskets in the battery stack.
A battery stack assembled using santoprene flowframes has been
extensively leak tested. Latest results show that this material
provides a means of obtaining a leakproof stack which had previously
been a concern.
A solar demonstration house has been built on the grounds of tthe
Tha Gypsum factory in Thailand. the house has been designed with
total energy self sufficiency in mind. This house employs solar cell
arrays on the roof for the collection and conversion of energy and a
vanadium battery system for storing the electrical energy collected.
The solar demonstration house is totally energy self sufficient and
the vanadium battery is housed in the battery room. The vanadium
battery has been used to power the lighting and airconditioning as
well as other general appliances.
The specifications of the vanadium battery in this application is
set out in table 1.
Table 1: Specifications of the Solar Demonstration House Vanadium
Battery:
*These values are theoretical values based on a cell resistance of 2
Ohm per square centimetre and electrolytes with a 2M vanadium
concentration. The peak power was calculated using a discharge
current density of 67 mA per square centimetre corresponding to 100A.
This system has shown promising results for the application of the
vanadium redox battery in energy self sufficient housing. If the
energy needs of the house increases at a later date the capacity of
the system can simpy be increased by adding extra electrolyte. The
benefits of such a system in South East Asia are that it will
provide reliable power for individual dwellings and commercial
buildings.
A commercially available golf cart powered by lead-acid batteries
was obtained for the development of a vanadium battery powered golf
cart from Deep Down Distributors P/L. The golf cart was originally
powered by six 6 Volt lead-acid batteries that were stored under the
seat.
The specifications of the vanadium battery designed for the golf
cart are shown in Table 2. The actual electrode area of the battery
was determined by the mould already develped for santoprene
flowframes that are prepared by injection moulding as described
above in Section 3.2.
Due to severe budget restrictions for this small demonstration
project the electrodes and flowframes had to be fabricated using the
same moulds as manufactured for the Solar House battery project.
The size of the flowframes used in the golf cart vanadium battery
are for electrodes with an area of 1500 square centimetres. The
resulting battery is thus oversizes for the golf cart. The optimum
size for a vanadium battery specifically designed for the golf cart
would thus be approximately one quarter that used for this initial
trial.
*These values are theoretical values based on a cell resistance of 2
Ohm per square centimetre and electrolytes with a 2M vanadium
concentration. The peak power was calculated using a discharge
current density of 67 mA per square centimetre corresponding to 100A.
The 2 pumps to pump the electrolyte around the vanadium battery
system were 240V AC and a battery monitor-inverter was developed and
used to power the pumps of the battery system. The pumps through the
inverter were found to sonsume a current of 7.5A total for both
pumps at an operating pressure of 45kPa each.
Preliminary road trials of the vanadium battery powered golf cart
have already been undertaken. The golf cart was found to perform
exeptionally well carrying two passengers with ease and a total
vehicle weight including passengers in excess of 400 kgs.
In the first preliminary road trials the battery voltage for the
stationary vehicle with the pumps off was 41.4V. the battery voltage
for the stationary vehicle with the pumps on was 38.9V and the
battery voltage for the moving vehicle on a flat road was 37.6V.
The vehicle can also be run with the pumps off, running simply of
the charge available in the battery stack. This will obviously limit
the distance that can be travelled however, in this case the battery
voltage only decreased from 41.4V for the stationary vehicle to 40.
7V when the vehicle was moving.
The pumps can therefore be run intermittently to conserve power with
the preferred option being the employment of DC pumps. The AC pumps
were employed in the current trials due to suitable DC pumps so far
proving difficult to acquire.
The vanadium battery powered golf cart will soon undergo endurance
testing as well as acceleration and maximum speed trials.
Submarine back-up batteries in the present design consist of NiCad
cells. There are 2 identical niCad banks each consisting of 20 cells
providing 24V.
A vanadium battery system is currently being developed for this
application. There are certain major requirements stipulated by the
Department of Defence for the vanadium battery system. These
requirements are that the battery has the ability to be charged and
discharged between 5% and 95% of the rated capacity for a current
range of 0-160A while remaining in the voltage range of 22-28V.
The preliminary design of the vanadium battery system comprised of 2
identical banks each formed from two, 20 cell stacks. The two, 20
cell stacks would be electrically connected in parallel. This
connection is neccessary to permit the voltage to be in the desired
range. Each stack only needs to support a maximum current of 80A
when considering that the total maximum current the two stacks need
to provide is 160A.
A computer simulation programme was developed to simulate the
performance of the vanadium battery in the submarine back-up battery
application. The main purpose of this simulation was to detect
whether the stipulated voltage range of 22-28V could be met over the
current range of 0-160A. The back-up battery is to be connected to
the submarine instrumentation continuously even in the charge cycle.
The simulation carried out on one 20 cell stack revealed that the
open circuit voltage (OCV) for the 20 cell stack was over 28V and
that this voltage and that this voltage was also exceeded when
charging at the maximum of 80A on one stack.
Further simulations suggested that one way to overcome the exceeding
voltages was to use 19 instead of 20 cells in the stack to bring the
OCV within the prescribed voltage range. During charging a tapping
cell in the stack could be used that would bring the voltages
obtained during charging within the voltage range required. A
tapping cell is to be employed at cell 17 in the 19 cell stack and
Table 3 gives the specifications for one bank. Table 4 illustrates
the stack voltages are in line with the voltage range required over
the conditions to be expected during operation of the vanadium
back-up battery system.
The use of a tapping cell thus enables the difference between the
charging and discharge voltage to be minimised and any variance at
the outer extremities of the voltage range may be overcome by
further refinement.
Number of battery stacks 2
Stack connection parallel
Number of cells (total) 38
Tapping cell (in each stack) Cell No. 17
Volume of electrolyte per half cell 70 l total: 140 l
Peak Power 4.2 kW*
Cell flow distribution design: internal
*This value is a theoretical value based on a cell resistance of 2
Ohm per square centimetre and electrolytes with a 2M vanadium
concentration. The peak power was calculated using a discharge
current density of 53 mA per square centimetre corresponding to 80A.
Table 4 Vanadium back-up battery stack voltages under open circuit,
discharge with 19 cells and charge with 17 cells.
SOC (%) Stack OCV (V, 19 cells)
95% 28.53
50% 25.65
5% 22.77
SOC (%) Discharge voltages at different currents (V) for a
19 cell stack.
80A 40A 5A
95% 26.50 27.51 28.40
50% 23.62 24.64 25.52
5% 20.75 21.76 22.65
SOC (%) Charge voltages at different currents (V) using 17
cells
80A 40A
95% 27.56 26.88
50% 24.99 24.31
5% 22.42 21.74
The vanadium redox flow battery system has undergone optimisation
and the manufacture of various components have been streamlined and
designed to meet the needs for full scale commercial production.
This system has already been used in a domestic load levelling
application and has shown that energy self sufficient housing is not
a future possibility but indeed a reality. An electric vanadium golf
cart has been completed and initial road trials indicate that this
battery system shows great promise for specialised traction
applications although further research to increase energy density is
required before it can be used in commuter vehicles. Back-up power
systems are incorporated in virtually all industries and a vanadium
bach-up battery system is currently under development for use in
submarines.
The vanadium redox flow battery system has demonstrated an ability
to be applied in various energy storage applications. As the
development continues more applications will reveal its full
versatility and potential.
1. Introduction
![[IMAGE]](vanads1.gif)
At the positive electrode:
E(null) = 1.00 V (1)
At the negative electrode:
E(null) = -0.26 V (2)
2. Advantages of the Vanadium Redox Battery
3. State of Development
![[IMAGE]](vanads2.gif)
3.1. Electrodes
3.2. Flowframes
4. Current Applications and Designs
4.1. Energy Storage - Solar Demonstration House
Number of battery stacks 1
Number of cells 36
Volume of electrolyte per half-cell 200 l total: 400 l
Peak Power* 4.9 kW
Capacity* 13.0kWh
Cell flow distribution design: external
4.2. Mobile Application - Electric Golf Cart
Table 2 Specifications of the Electric Golf Cart Vanadium Battery
Number of battery stacks 1
Number of cells 30
Volume of electrolyte per half cell 60 l total: 120 l
Peak Power 4.1 kW*
Capacity 3.9 kWh*
Cell flow distribution design: internal
4.3. Back-up Battery - Submarine
Table 3 Specifications of the submarine back-up battery:
5. Conclusion