All elements attract electrons with a different force. Any two metals in a solution of their salts will fight to attract the electrons off the other. Each pair of metals, or elements have their own unique voltage.
In the example on the right, copper ions in solution will attract electrons off the zinc metal. When the electrons leave the zinc, it will become an ion and go into solution. The reverse happens on the other side. When the electrons arrive, they join the copper ion, converting it to copper metal. So the copper gains mass, than the zinc loses mass into solution.
When the solution has no more ions on one side, then the cell is either fully charged, or fully discharged, ie flat.
If a higher voltage is applied in the reverse direction, the reactions reverse and the cell recharges.
In an electro chemical cell the solution are stored in the cell, usually as a paste.
In a flow battery, the solutions are stored in external tanks and are pumped past the electrodes. This has several advantages.
Flow batteries can be divided into three types: Redox, hybrid, and membraneless.
Redox has all the reactants in solution and is regarded at a rechargeable fuel cell. Examples are: vanadium redox flow battery, polysulfide bromide battery, and uranium redox flow battery.
Hybrid has one side in solution, and the other as the element. Examples are: zinc-bromine, zinc-cerium, and lead-acid flow batteries.
Membraneless separates the two liquid by using laminar flow. This type of flow is in non mixing layers rather than tumbling as in turbulent flow.
Uses of flow batteries
to even out fluctuations due to peak hour demand, sudden power station failures, wind farms, sudden loads and shutdowns, etc.
Power conversion - quite remarkable!
DC-DC, AC-DC, DC-AC, AC-AC.
Because cells can be used in series or in parallel, all in the same solution, then several remarkable conversions are possible: If say 120 cells at 2 volts each were set up, in the same solution, then:
In parallel they can be charged at 2 volts, then in series, discharged at 240 volts. Or vice versa. 240 could be converted to 120, 60, 30, 20,12, 8, 6, 4, 2.
With a zinc bromine battery the cell voltage is 1.67 V. so 144 cells would be needed to produce 240V. It could discharge at any voltage that is a multiple of the cell voltage and divides into 144V. There are complications and opportunities here, you'll need to think it through yourself.
With switching gear, an AC current can be fed into the cell as DC, then retrieved as DC, or through switching gear, as AC. Again at various voltages.
Quick charging for electric vehicles
Flow batteries can be rapidly recharged by replacing the electrolyte.
Zinc Bromine flow battery
A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged the solutions (electrolytes) are pumped through a reactor stack and back into the tanks. One tank is used to store the electrolyte for the positive electrode reactions and the other for the negative. Zinc bromine batteries from different manufacturers have energy densities ranging from 34.4–54 W·h/kg.
The predominantly aqueous electrolyte is composed of zinc bromide salt dissolved in water. During charge, metallic zinc is plated from the electrolyte solution onto the negative electrode surfaces in the cell stacks. Bromide is converted to bromine at the positive electrode surface of the cell stack and is immediately stored as safe, chemically complexed organic phase in the electrolyte tank. Each fully recyclable high-density polyethylene (HDPE) cell stack has up to 60 bipolar, plastic electrodes between a pair of anode and cathode end blocks.
The zinc–bromine battery can be regarded as an electroplating machine. During charging zinc is electroplated onto conductive electrodes, while at the same time bromine is formed. On discharge the reverse process occurs, the metallic zinc plated on the negative electrodes dissolves in the electrolyte and is available to be plated again at the next charge cycle. It can be left fully discharged indefinitely without damage.
RedFlow ZBM module: 5 kW and 10 kW·h.
These battery systems have the potential to provide energy storage solutions at a lower overall cost than other energy storage systems such as lead-acid, vanadium redox, sodium–sulfur, lithium-ion and others.
Vanadium redox flow battery
The vanadium redox (and redox flow) battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy. The present form (with sulfuric acid electrolytes) was patented by the University of New South Wales in Australia in 1986  An earlier German Patent on a titanium chloride flow battery was registered and granted in July 1954 to Dr. Walter Kangro, but most of the development of flow batteries was carried out by NASA researchers in the 1970s. Although the use of vanadium in batteries had been suggested earlier by Pissoort, by NASA researchers and by Pellegri and Spaziante in 1978, the first known successful demonstration and commercial development of the all-vanadium redox flow battery employing vanadium in a solution of sulfuric acid in each half was by Maria Skyllas-Kazacos and co-workers at the University of New South Wales in the 1980s.
There are currently a number of suppliers and developers of these battery systems including Ashlawn Energy in the United States, Renewable Energy Dynamics (RED-T) in Ireland, Cellstrom GmbH in Austria, Cellennium in Thailand, and Prudent Energy in China. The vanadium redox battery (VRB) is the product of over 25 years of research, development, testing and evaluation in Australia, Europe, North America and elsewhere.
The vanadium redox battery exploits the ability of vanadium to exist in solution in four different oxidation states, and uses this property to make a battery that has just one electroactive element instead of two.
The main advantages of the vanadium redox battery are that it can offer almost unlimited capacity simply by using larger and larger storage tanks, it can be left completely discharged for long periods with no ill effects, it can be recharged simply by replacing the electrolyte if no power source is available to charge it, and if the electrolytes are accidentally mixed the battery suffers no permanent damage.
The main disadvantages with vanadium redox technology are a relatively poor energy-to-volume ratio, and the system complexity in comparison with standard storage batteries.
Nickel Zinc flow battery
The CUNY Energy Institute has developed a new low-cost, safe, non-toxic battery with fast discharge rates and high energy densities that could save thousands in energy costs each month.
zinc-nickel oxide flow batteries, which cost half as much as nickel-metal hydride batteries and have energy densities that are twice as high.
An operating prototype zinc-anode battery system has been developed and is now housed in the basement of Steinman Hall on The City College of New York campus.
Flow assisted zinc manganese dioxide battery
Less expensive than most other batteries
The CUNY Energy Institute, in partnership with Rechargeable Battery Corporation (RBC) and Ultralife Corporation, is developing and constructing a water-based flow-assisted battery for grid-scale energy storage. This novel battery starts with the same low-cost materials found in disposable consumer-grade alkaline batteries, namely zinc and manganese dioxide, and then transforms the chemistry into a long-lasting, fully-rechargeable system. The result of this effort will be a 25kW rechargeable system that lasts for 5,000 cycles, costs under $100/kWh, and shows strong potential for scaling to megawatt-hour levels in grid-scale electric energy storage applications.
Hyrogen bromine flow battery - HBr
As described by MIT writer Jennifer Chu, a hydrogen-bromine combination is potentially ideal for flow batteries, since both are relatively cheap and abundant. However, hydrobomic acid likes to munch on membranes, which severely curtails the battery’s lifespan.
To get rid of the membrane, the MIT team relied on a form of parallel flow called laminar flow, in which two liquids stay on their respective courses with little mixing, even though no separating membrane is present.
Basically, the new battery was engineered with a slim channel between two electrodes. As Chu explains:
“Through the channel, the group pumped liquid bromine over a graphite cathode and hydrobromic acid under a porous anode. At the same time, the researchers flowed hydrogen gas across the anode. The resulting reactions between hydrogen and bromine produced energy in the form of free electrons that can be discharged or released.”
The membrane-less design enables power densities of 0.795 W cm−2 at room temperature and atmospheric pressure, with a round-trip voltage efficiency of 92% at 25% of peak power.
Two new characteristics of the MIT team’s hydrogen bromine laminar flow battery (HBLFB) enable the high-power-density storage and discharge of energy at high efficiency:
The use of gaseous hydrogen fuel and aqueous bromine oxidant. This allows for high concentrations of both reactants at their respective electrodes, greatly expanding the mass-transfer capacity of the system.
Both reactions have fast, reversible kinetics, with no phase change at the liquid electrode, eliminating bubble formation as a design limitation.
Round-trip voltage efficiency of the HBLFB as a function of power density for a range of reactant concentrations at a Pe of 10,000.
Observed cell voltage during charging as a function of HBr concentration at a Pe of 10,000 and a Br2 concentration of 1 M. Increasing the HBr concentration increases both the conductivity and the limiting current
GE flow battery - (A vague PR claim)
"Scientists at GE Global Research and Lawrence Berkeley National Laboratory are developing a new kind of water-based “flow” battery for electric vehicles. the batteries could be 75 percent cheaper than car batteries available on the market today. GE engineers say that unlike lithium-ion and other battery systems, the new technology will use water-based solutions of inorganic chemicals capable supplying high energy density by ferrying more than one electron at a time." Source: GEreports