Various types of flow batteries exist in the market today and one of them is the redox reduction-oxidation flow battery which, due to its nature and characteristics, is a highly flexible technology for large scale stationary energy storage. The redox flow battery has a very rapid response compared to other battery types, minimised environmental effect and considerable potential for reducing costs.
Most redox flow batteries consist of two separate electrolytes; one stores the electro-active components for the negative electrode reactions and the other for the positive electrode reactions.
Both electrolytes can be circulated and stored in a separate storage tanks as shown in Fig. 1 , which is taken from [], to control the concentrations of the electro-active species. An ion selective membrane is often used to prevent mixing or cross-over of the electro-active species between the tanks. Only the ion carrier can access through the membrane.
For example in the vanadium redox flow system, the electrolytes circulate in a sulphuric acid solution. Vanadium ions as V+2 is oxidized to V+3 at the anode and V+5 is reduced to V+4 at the cathode, Hydronium ions are transported across a proton conducting membrane from the anode to the cathode.
Figure 1 Principles of a redox flow battery
Redox flow batteries have the important feature of flexible layout because the power and energy components are separate within the cell. They also have a quick response time because its operation does not require solid-to-liquid or other phase-to-phase transitions, environment friendly, and require low maintenance. On the other hand, their setup is quite complicated compared to other battery types.
(due to separation of the power and energy components), long cycle life (because there are no solid-solid phase transitions), quick response times, no need for "equalisation" charging (the over charging of a battery to ensure all cells have an equal charge) and no harmful emissions. Some types also offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/ overdischarge.
On the negative side, flow batteries are rather complicated in comparison with standard batteries as they may require pumps, sensors, control units and secondary containment vessels. The energy densities vary considerably but are, in general, rather low compared to portable batteries, such as the Li-ion.
The basic configuration of a vanadium redox flow battery, a very pop is shown
in Fig. 1. A sulfuric acid solution containing vanadium
ions is used as the positive and negative electrolytes,
which are stored in respective tanks and circulated to
the battery cell.
RFB is, by its very nature, a modular and highly flexible technology with very rapid response, little environmental impact and considerable potential for cutting costs. This is the reason why Redox Flow Batteries are emerging as a very promising option for stationary storage in general and for renewable applications in particular.
Various classes of flow batteries exist including the redox (reduction-oxidation) flow battery, a reversible fuel cell in which all electroactive components are dissolved in the electrolyte. If one or more electroactive components are deposited as a solid layer the system is known as a hybrid flow battery;[2] that is, the electrochemical cell contains one battery electrode and one fuel cell electrode. The main difference between these two types of flow batteries is that the energy of the redox flow battery, as with other fuel cells, is fully decoupled from the power, because the energy is related to the electrolyte volume (tank size) and the power to the electrode area (reactor size).
A flowbattery is a rechargeable fuel cell in which electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell that reversibly converts chemical energy directly to electricity. Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also known.[1] Flow batteries can be rapidly "recharged" by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines) while simultaneously recovering the spent material for re-energization.
Redox
flow cells are designed to convert and store electrical energy into chemical energy and release it in a controlled fashion when required.
Classes of flow batteries
The hybrid flow battery, similar to typical batteries, is limited in energy by the size of the battery electrode (reactor size).
Energy producing electrochemical cells are generally divided into two categories. Cells that can be discharged only, with irreversible electrochemical reactions, are termed primary cells, while rechargeable cells with reversible reactions are termed secondary cells[3] (see also primary and secondary batteries). Using this historical convention, a redox flow battery is better described as a secondary fuel cell or regenerative fuel cell, with the fundamental difference between batteries and fuel cells being whether energy is stored in a solid state electrode material (batteries) or in the electrolyte (fuel cells). This difference leads to the decoupling of energy and +power in a fuel cell described above.
The misnomer of "redox flow battery" rather than "reversible fuel cell" has led to a great deal of confusion in understanding and terminology. For example, the processes of solid-state diffusion and intercalation in a Lithium Ion Battery do not apply to redox flow batteries, but the heterogeneous electron transfer in a fuel cell does. In industrial practice, fuel cells are usually, and unnecessarily, considered to be primary cells, such as the H2 / O2 system, with limited examples of reversible systems (i.e., the unitized regenerative fuel cell on NASA's Helios Prototype). The European Patent Organisation classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).
Examples of redox flow batteries are the vanadium redox flow battery, polysulfide bromide battery (Regenesys), and uranium redox flow battery.[4] Hybrid flow batteries include the zinc-bromine, zinc-cerium [5] and lead-acid flow batteries. Redox fuel cells are less common commercially although many systems have been proposed
Advantages and disadvantages
Applications
Flow batteries are normally considered for relatively large (1 kW·h - 10 MW·h) stationary applications. These are for
Load balancing - where the battery is connected to an electrical grid to store excess electrical power during off-peak hours and release electrical power during peak demand periods.
Storing energy from renewable sources such as wind or solar for discharge during periods of peak demand.
Peak shaving, where spikes of demand are met by the battery.[10]
UPS, where the battery is used if the main power fails to provide an uninterrupted supply.
power conversion - because all cells share the same electrolyte/s. Therefore, the electrolyte/s may be charged using a given number of cells and discharged with a different number. Because the voltage of the battery is proportional to the number of cells used the battery can therefore act as a very powerful DC/DC converter. In addition, if the number of cells is continuously changed (on the input and/or output side) power conversion can also be AC/DC, AC/AC, or DC/AC with the frequency limited by that of the switching gear.[11]
Electric vehicles - Because flow batteries can be rapidly "recharged" by replacing the electrolyte, they can be used for applications where the vehicle needs to take on energy as fast as a combustion engined vehicle.
Stand-alone power system - An example of this is the telecomms industry for use in cellphone base stations where there is no mains power available. The battery can be used alongside a solar or a wind power to compensate for their fluctuating power levels and alongside a generator to make the most efficient use of it to save fuel
3-1 Operating Principles of Redox Flow Batteries
Features
Redox flow batteries offer the following features,
and are suitable for high-capacity systems that differ
from conventional power storage batteries.
(1) Simple operating principle and long service life
The battery reaction only involves a change in the
valence of vanadium ions in the electrolyte. There are
none of the factors which reduce battery service life
seen in other batteries that use a solid active substance,
such as loss or electrodeposition of the active substance.
Furthermore, operation at normal temperatures ensures
less deterioration of the battery materials due to temperature.
Pumps and piping that are widely used in facilities
such as chemical plants are usable as established technologies.
(2) Simple installation layout
The system configuration is such that battery output
(cell section) and battery capacity (tank section) can be
separated, therefore layout of sections can be altered
according to the place of installation. For example, the
tank can be placed underground. Design can be easily
modified according to required output and capacity.
For example, if the capacity is to be doubled while leaving
output the same, then one has only to double the
size of the tank.
(3) No standby loss and quick start-up
The charged electrolyte is stored in separate positive
and negative tanks when the battery has been
charged, therefore no self-discharge occurs during prolonged
stoppage, nor is auxiliary power required during
stoppage. Furthermore, start-up after prolonged stoppage
requires only starting of the pump, thus making
start-up possible in only a few minutes.
(4) Easy maintenance
The electrolyte (i.e., the active substance) is sent to
each battery cell from the same tank, therefore the
charging state of each battery cell is the same, eliminating
the need for special operations such as uniform
Power plant
AC/DC converter
Customers
Charge
Discharge
+.
V5+/V4+
Electrolyte
tank
V2+/V3+
Electrolyte
tank
e - e -
V5+
H+
V2+
V4+ V3+
Pump Pump
Electrode Cell Membrane
charging. Furthermore, maintenance is also easy
because the electrolyte is relatively safe and the operations
are at normal temperature.
(5) Superior environmental safety
The electrolyte is relatively safe and assures superior
environmental safety.
(6) Superior recyclability
Waste vanadium from power generating stations
can be used. Furthermore, the vanadium in the electrolyte
can be used semi-permanently.
5.
Component Technology of Redox Flow
Batteries
The single cell battery, as shown in Fig. 2, is composed
of a positive and a negative electrode, which are
separated by a membrane. In order to obtain a high
voltage, a bipolar plate is used to stack the components
in a serial connection. This is called a battery cell stack.
MembraneFrameElectrode(Ion exchange Bipolar plate(PVC)(Carbon felt)
membrane)(Carbon plastic)
Components of a cell
Battery cell stack
Fig. 1. Principles of a redox flow battery
Fig. 2. Construction of a cell stack
SEI TECHNICAL REVIEW · NUMBER 50 · JUNE 2000 · 89
The cell stack converts electrical energy to chemical
energy, therefore its performance is an important determinant
of the battery's basic performance, including its
charging and discharging efficiency. In order to
increase the efficiency of the battery cell stack, it is necessary
to reduce internal resistance, which makes the
development of materials like electrodes, membranes,
and bipolar plates critical. The required specifications
of each component are described below:
(1) Electrodes: In consideration of resistance to acid,
low pressure loss during electric flow and electric
conductivity, carbon felt with an activated electrode
surface is used.
(2) Membrane: In consideration of electrical insulation,
proton (H+ ion) conductivity, and vanadium
ion insulation, a negative ion exchange membrane
is used.
(3) Bipolar plate: This component must allow only electricity
to pass while keeping the electrolytic fluid
inside. For this reason, a conductive plastic made by
embedding carbon (which is electrically conductive)
into plastic (which is fluid impermeable) is
used.
(4) Frame: In consideration of resistance to acid and
cost, hard vinyl chloride is used.
Other battery cell stack requirements, besides lowered
internal resistance, include the following:
q
Pressure loss during pumping of the electrolyte
should be minimal.
w
Shunt current loss, which is self-discharge loss
through the electrolyte supply pipe, should be mini
mal.
Keeping pressure loss during electrolyte pumping
to a minimum helps raise system efficiency by reducing
pumping power. Furthermore, although shunt current
loss can be reduced using a narrower fluid route to raise
electric resistance, this also increases pressure loss during
fluid pumping and thereby increases the required
pump power. Thus, it is important to achieve a design
that minimizes the sum of pump power and shunt current
loss.
