Flow Batteries
Current state of the technology
Flow batteries are being built by many different companies and being deployed to provide energy storage.
The basic design concept for Flow Batteries dates back to NASA in the 1970s. The concept is that you have large tanks of chemicals which are pumped through a stack of electrochemical cells, where the two chemicals pass either side of a membrane which allows through certain ions in one direction or the other, depending on whether the electricity is being produced by the battery or being stored. This process changes the chemical composition of the liquids, but both chemical processes are reversible.
To increase the total energy storage of the system, you increase the volume of the chemical tanks. To increase the peak power, you increase the number of cells thus increasing the surface area of membrane separating the flowing liquids.
Advantage over Li is that it can operate at 100% depth of discharge with very little degradation over expected lifetime of battery ~20 years. In some chemistries the membrane in the cell can deteriorate over time, though often the active cells can be replaced without replacing the entire plant.
There are two subcategories: “Redox” flow batteries. “Hybrid” flow batteries.
There are more than 20 flow-battery chemistries including: Zinc-Bromine, Zinc Cerium, Magnesium-Vanadium and Vanadium-Cerium, Vanadium-redox
Redox flow batteries
General overview
Negative electrolyte (anolyte) and positive electrolyte (catholyte) both contain metal ions dissolved in the respective liquids. During charge and discharge the electrolytes are pumped through electrode chambers with a membrane between the two sides which allows the electron transfer to happen between the metal ions, changing their oxidation states.
In vanadium redox batteries the metal ion on both sides is vanadium, so the electrolytes and membrane do not get cross contaminated during the process, so the capacity of the battery does not decrease with time. In Redox Flow batteries which have different metal ions on both sides, the membrane and electrolytes do get “poisoned” with the other metal over time and so their capacity does degrade over time.
Vanadium Redox cycle
The positive half cell reaction (cathode) involves V5+ and V4+
The negative half-cell reaction (anode) involves V2+ and V3+
Overall reaction
Cost breakdown is typically 40%-50% electrolyte & 30% for the cell. The majority of the cell cost is the cost of the membrane.
Vanadium redox flow batteries need a lot of purified vanadium – which is produced in China, South Africa and Russia. The main commercial use of vanadium is in the steel industry, so it is already in high demand and is high cost.
One company (Imergy) has tried using vanadium extracted from oil sludge or fly ash. This has reduced purity but is much cheaper – only about 2/3 the cost.
Hybrid flow batteries
General overview
One active material is stored within the electrochemical cell, the other remains in the liquid electrolyte stored externally in a tank. This is called hybrid as it combines features of conventional batteries and flow battery.
An examples of this sort of battery is the Zinc-Bromide battery where zinc is deposited on the electrode during charge, and Zn2+ goes back into solution during discharge.
Hydrogen-Bromide
A Norwegian company, Elestor are producing a Hydrogen-Bromide flow battery
The main advantage of this design is that the materials are very cheap and in bountiful supply. There is absolutely no chance of running out of either hydrogen or bromine, so this design of battery could easily scale to provide whatever storage requirements were needed.
Power range: 50kW to 50MW
Capacity range: 250kWh to 250 MWh
In any combination. A typical configuration is power 200kW, capacity 2MWh
Typical efficiency 70%
Operating temperature 0oC to 45oC. With a “cold pack option” coping with -25oc to 45oC
Elestor’s batteries are delivered in pairs of 40ft cube containers.
The battery behaves as an electrolyser during charge and a fuel-cell during discharge.
Chemistry and Design
The cathode side is liquid hydrogen-bromide and bromine, the anode side is hydrogen.
The membrane between the two sides allows H+ ions to pass through it and is sandwiched between layers of porous carbon.
At the cathode side the reversible reaction is:
At the anode side:
During charging the HBr is electrolysed to hydrogen gas and bromine liquid. During discharge, the hydrogen recombines with the bromine to produce hydrogen bromide, releasing an electron to the circuit in the process. The hydrogen produced is stored in external pressurised tanks up to a maximum pressure of 20 atmospheres. The energy storage in this system depends on the volume of those hydrogen tanks.
References / external websites:
More details of how Flow batteries work:
https://www.sciencedirect.com/topics/engineering/flow-battery
Zinc Bromine Hybrid FLow battery
https://pubs.rsc.org/en/content/articlelanding/2014/ra/c4ra05946h#!divAbstract
Elestor – Norwegian company producing Hydrogen Bromine flow battery
https://www.elestor.nl/scientific-downloads/
Report of Vanadium Redox Flow battery installation