Flow Battery

In off-grid solar technology batteries are the main source of reserved energy after it is been charged by the solar panels.In solar technology there are different types of batteries  available like Lead-acid batteries ,or Lithium-ion batteries or Sodium Nickel Chloride or Flow batteries.In this section we are going to learn  flow batteries.

One of the latest developments in the solar battery storage space are 'flow' batteries (or 'redox flow batteries' if you want to get technical). The fundamental difference between conventional batteries and flow cells is that energy is stored not as the electrode material in conventional batteries but as the electrolyte in flow cells.

A flow battery, or redox flow battery (after reduction–oxidation), is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange (accompanied by flow of electric current) occurs through the membrane while both liquids circulate in their own respective space. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.2 volts.

A flow battery may be used like a fuel cell (where the spent fuel is extracted and new fuel is added to the system) or like a rechargeable battery(where an electric power source drives regeneration of the fuel). While it has technical advantages over conventional rechargeable, such as potentially separable liquid tanks and near unlimited longevity, current implementations are comparatively less powerful and require more sophisticated electronics.


 Types of flow batteries :

Various types of flow cells (batteries) have been developed,including redox, hybrid and membrane less.

•  Redox
•  Hybrid
•  Membrane-less
•  Organic 
•  Metal Hydride
•  Nano-network
•  Semi-solid 

Flow batteries actually describes a number of similar technologies that all fall under the same umbrella, but this page will predominantly discuss the 'ZCell' Zinc-Bromide batteries manufactured in North America by the Australian company RedFlow.

Discussing the science behind flow batteries can get rather technical, so I'll massively simplify it. A flow battery is mostly made up of a water based liquid (Zinc Bromide) that flows between two tanks. When the battery charges the Zinc is extracted from the liquid and stored separately. When discharging, the Zinc is put back into the liquid. The Zinc 'flows' from the big plastic tank at the bottom of the picture to the electrodes at the top.  Hence the name.

One of the core advantages that flow batteries have over lithium-ion and lead-acid batteries is that they have a 100% depth-of-discharge - which means the entire battery can be discharged in a cycle with no negative effects on the lifespan of the battery. For comparison, lead-acid batteries have a 60% depth-of-discharge, and most lithium-ion batteries have an 80-90% depth-of-discharge.

They can also sit on the shelf forever-and-a-day at zero charge without degrading. Again - most other battery technologies need to be kept at a minimum charge level.

There are a number of other benefits that flow batteries offer:

• They can tolerate extreme weather conditions, up to 50 degrees Celsius.
• The zinc-bromine liquid inside the flow batteries is a natural fire retardant. 
• No chance of a thermal runaway (explosion!) due to the physical separation of the different battery components.
• Cheaper to refurbish due to their simple modular construction. For example, you can restore a 'dead' battery by simply swapping the electrode - for half of the original price of the battery.

A disadvantage of flow batteries is their life cycle compared to some higher-end lithium-ion batteries - they have a lifespan of approximately 4000 cycles at 100% depth-of-discharge, which is less than top quality lithium-ion's 5000-6000 cycles at 80% depth-of-discharge.

They also need to regularly be 100% discharged to 'clean' the electrodes. But, the battery management can be programmed to automatically do this regularly.

Applications

Flow batteries are normally considered for relatively large (1 kWh – 10 MWh) 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. The common problem limiting the use of most flow battery chemistries in this application is their low areal power (operating current density) which translates into a high cost of power.
• 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.
• 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.
• 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. A common problem found with most RFB chemistries in the EV applications is their low energy density which translated into a short driving range. Flow batteries based on highly soluble halates are a notable exception.
• Stand-alone power system – An example of this is in cellphone base stations where no grid power is available. The battery can be used alongside solar or wind power sources to compensate for their fluctuating power levels and alongside a generator to make the most efficient use of it to save fuel. Currently, flow batteries are being used in solar micro grid applications throughout the Caribbean.

Note : Advantages of flow batteries means they might start to seriously compete with lithium-ion batteries to become the de facto battery storage technology in the future - but first, flow battery manufacturers need to get costs down to a level that's more reasonable for mass-market adoption, and iron out some challenges in manufacturing them consistently.

^{Hope This post be helpful to you (JajakAllahu Khair)}