Flow battery

Flow battery

Discussion
Ask a question about 'Flow battery'
Start a new discussion about 'Flow battery'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia
A flow battery is a form of rechargeable battery
Rechargeable battery
A rechargeable battery or storage battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging anything from a button cell to...

 in which electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

 containing one or more dissolved electroactive species flows through an electrochemical cell
Electrochemical cell
An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...

 that converts chemical energy
Chemical energy
Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction or, to transform other chemical substances...

 directly to electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

. 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. Flow batteries can be rapidly "recharged" by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

s) while simultaneously recovering the spent material for re-energization.

Classes of flow batteries


Various classes of flow batteries exist including the redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 (reduction-oxidation) flow battery
, in which all electroactive components are dissolved in the electrolyte. If one or more electroactive component is deposited as a solid layer the system is known as a hybrid flow battery. The main difference between these two types of flow battery is that the energy of the redox flow battery can be determined fully independently of the battery power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

, because the energy is related to the electrolyte volume (tank size) and the power to the reactor size. The hybrid flow battery, similar to a conventional battery, is limited in energy to the amount of solid material that can be accommodated within the reactor. In practical terms this means that the discharge time of a redox flow battery at full power can be varied, as required, from several minutes to many days, whereas a hybrid flow battery may be typically varied from several minutes to a few hours.

Another type of flow battery is the redox fuel cell. This has a conventional flow battery reactor, which only operates to produce electricity (i.e., it is not electrically recharged). Recharge occurs by reduction of the negative electrolyte using a fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 (e.g. hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

) and oxidation of the positive electrolyte using an oxidant (typically oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 or air).

Examples of redox flow batteries are the vanadium redox flow battery, polysulfide bromide battery
Polysulfide bromide battery
The PolySulfide Bromide battery is a regenerative fuel cell involving a reversible electrochemical reaction between two salt-solution electrolytes: sodium bromide and sodium polysulfide.-External links:*...

 (Regenesys), and uranium redox flow battery. Hybrid flow batteries include the zinc-bromine
Zinc-bromine flow battery
The zinc–bromine flow battery is a type of hybrid flow battery. A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged the solutions are pumped through a reactor stack and back into the tanks. One tank is used to store the electrolyte for the positive...

, zinc-cerium
Zinc cerium redox flow battery
Zinc–cerium batteries are a type of redox flow battery first developed by Plurion Inc. during the 2000s. Negative zinc electrolyte and positive cerium electrolyte are stored in two separated reservoirs and are circulated during the operation...

  and lead-acid
Lead-acid battery
Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large...

 flow batteries. Redox fuel cells are less common commercially although many systems have been proposed.

Chemistries

Couple Max. cell voltage (V) Average electrode power density (W/m2) Average fluid energy density (W·h/kg)
Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-tin
0.62 <200
Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 
0.43 <200
Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-chrome
1.07 <200
Vanadium-vanadium (sulphate)
Vanadium redox battery
The vanadium redox battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy...

 
1.4 ~800 25
Vanadium-vanadium (bromide)
Vanadium redox battery
The vanadium redox battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy...

 
50
Sodium/bromine polysulfide
Polysulfide bromide battery
The PolySulfide Bromide battery is a regenerative fuel cell involving a reversible electrochemical reaction between two salt-solution electrolytes: sodium bromide and sodium polysulfide.-External links:*...

 
1.54 ~800
Zinc-bromine
Zinc-bromine flow battery
The zinc–bromine flow battery is a type of hybrid flow battery. A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged the solutions are pumped through a reactor stack and back into the tanks. One tank is used to store the electrolyte for the positive...

 
1.85 ~1,000 75
Lead-acid (methanesulfonate)
Lead-acid battery
Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large...

 
1.82 ~1,000
Zinc-cerium (methanesulfonate)
Zinc cerium redox flow battery
Zinc–cerium batteries are a type of redox flow battery first developed by Plurion Inc. during the 2000s. Negative zinc electrolyte and positive cerium electrolyte are stored in two separated reservoirs and are circulated during the operation...

 
2.43 <1,200–2,500

Advantages and disadvantages


Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of flexible layout (due to separation of the power and energy components), long cycle life (because there are no solid-solid phase changes), 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
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...

 vary considerably but are, in general, rather low compared to portable batteries, such as the Li-ion.

Differences from standard fuel cells


Fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

s are electrochemical energy conversion devices that convert chemical energy directly to electrical energy in which a fuel and an oxidant undergo electron transfer reactions at the anode and cathode of an operating electrochemical cell respectively, separated by an ion exchange membrane. These devices are not subject to Carnot's limitations for heat engines
Thermal efficiency
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...

 and can ideally generate electricity as long as they are supplied with fuel and an oxidant. They differ from batteries in that the active chemical species are supplied externally, rather than stored internally as is the case in batteries. So, power and energy specifications can be scaled up independently for a fuel cell, while the energy density of a battery is limited by the amount of active material that can be stored inside it.

Under these definitions a flow battery is a special type of rechargeable battery in which the dissolution of active species in the electrolyte permits external storage of reactants, thereby allowing independent scale up of power and energy density specifications. Also, external storage of reactants avoids self-discharge that is observed in primary and secondary battery systems. Electrolyte in a fuel cell remains at all times within the reactor (in the form of an ion-exchange membrane
Ion exchange resin
An ion-exchange resin or ion-exchange polymer is an insoluble matrix normally in the form of small beads, usually white or yellowish, fabricated from an organic polymer substrate. The material has highly developed structure of pores on the surface of which are sites with easily trapped and...

, for example). What flows into the reactor are only the electroactive chemicals, which are non-conducting (e.g., hydrogen, methanol, oxygen, etc.) This is in contrast to a flow battery in which at least some of the electrolyte (generally the majority in weight and volume terms) flows through the reactor.

Flow batteries are also distinguished from fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

s by the fact that the chemical reaction involved is often reversible; i.e., they are generally of the secondary battery type and so they can be recharged without replacing the electroactive material. Also, an important factor in the redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 flow battery (see below for classes of batteries) is that the power and energy density of redox flow batteries are independent of each other in contrast to rechargeable secondary batteries.

To add to the confusion the European Patent Organisation
European Patent Organisation
The European Patent Organisation is a public international organisation created in 1977 by its contracting states to grant patents in Europe under the European Patent Convention of 1973...

 classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).

Applications


Flow batteries are normally considered for relatively large (1 kW·h – 10 MW·h) stationary applications. These are for
  • Load balancing
    Load balancing (electrical power)
    Load balancing refers to the use of various techniques by electrical power stations to store excess electrical power during low demand periods for release as demand rises....

     - 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
    Wind farm
    A wind farm is a group of wind turbines in the same location used to produce electric power. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other...

     or solar for discharge during periods of peak demand.
  • Peak shaving, where spikes of demand are met by the battery.
  • UPS
    Uninterruptible power supply
    An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails...

    , 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
    Voltage
    Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

     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.
  • Electric vehicle
    Electric vehicle
    An electric vehicle , also referred to as an electric drive vehicle, uses one or more electric motors or traction motors for propulsion...

    s - 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
    Stand-alone power system
    A stand-alone power system , also known as remote area power supply , is an off-the-grid electricity system for locations that are not fitted with an electricity distribution 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.

See also


  • Glossary of fuel cell terms
    Glossary of fuel cell terms
    The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few. –...

  • Hydrogen technologies
    Hydrogen technologies
    Hydrogen technologies are technologies that relate to the production and use of hydrogen. Hydrogen technologies are applicable for many uses....

  • Load balancing
    Load balancing (electrical power)
    Load balancing refers to the use of various techniques by electrical power stations to store excess electrical power during low demand periods for release as demand rises....

  • Redox electrode
  • Vanadium redox flow battery
    Vanadium redox battery
    The vanadium redox battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy...

  • Zinc-cerium hybrid flow battery
    Zinc cerium redox flow battery
    Zinc–cerium batteries are a type of redox flow battery first developed by Plurion Inc. during the 2000s. Negative zinc electrolyte and positive cerium electrolyte are stored in two separated reservoirs and are circulated during the operation...

  • Zinc-bromine hybrid flow battery
    Zinc-bromine flow battery
    The zinc–bromine flow battery is a type of hybrid flow battery. A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged the solutions are pumped through a reactor stack and back into the tanks. One tank is used to store the electrolyte for the positive...


External links