Redox flow batteries (RFBs) are rechargeable batteries, which are generally based on two liquid electrolytes. These electrolytes contain the redox species in the form of dissolved salts, which are carrying the electrical charge. A RFB is composed of a central electrochemical cell (or stack) and of two storage tanks, each containing one of the electrolytes. During the operation of the battery, the electrolytes are directed from the storage tanks to the electrochemical cell, where the exchanges of electrons occur and then return to the storage tank. As such, the battery is progressively charged or discharged while the electrolytes are being converted.
Different salts can be used as redox active species in the electrolytes, which nature may vary greatly ranging from metal-based to organic-based. Moreover, it is theoretically possible to assemble any of the positive redox couple to any of the negative one and this results in a large variety of RFB chemistries. One of the very first developed ones was the Fe/Cr RFB, studied at the NASA in the 1970s. However, the most common and most advanced RFB currently is the so-called “all-vanadium” RFB (or VRFB). In this battery, the same metal, vanadium, is used in each electrolyte. Its working principle is well explained in the Youtube movie below.
One of the main advantages of these particular batteries, compared to others, is that they have an energy capacity that is independent of their output power. This significant difference is a direct consequence of their liquid nature. Indeed, while the size and the number of cells dictate the power output, the size of electrolyte tanks –i.e. volume of the electrolytes– determines the energy capacity of the battery. This decoupling allows for a wider range of applications. Moreover, they are intrinsically safer than other batteries and have a long lifetime, without requiring much maintenance. Their main disadvantage is their relatively low energy density, meaning that they occupy a rather large volume.
RFBs are now commercially available, for instance by the following suppliers:
- Gildemeister – Cellstrom (all-vanadium)
- Redflow (zinc-bromide)
- ViZn (zinc-iron)
- Imergy (all-vanadium)
- UniEnergy Technologies (all-vanadium)
- Rongke Power (all-vanadium)
Due to their versatility, RFBs can be used for a wide range of applications. In particular, they can be implemented in complementarity to a photovoltaic farm or a wind turbine. Indeed, they are well suited for buffering renewable energy fluctuations as an intermittent charge or discharge does not affect significantly the lifetime of the battery and the response time of RFBs is short (typically less than 1s). RFBs can also be integrated in microgrid network, where they play the same role of storing the excess production and supplying power when the consumption is higher than the production. RFBs have also been installed as backup (or UPS) systems, or in remote areas for telecommunications stations, for instance.
Redox flow batteries in numbers:
|Range of power [kW]||10 – 10,000|
|Range of capacity [kWh]||10 – 50,000|
|Lifetime [years / # of cycles]||10 – 15 / 10,000 – 15,000|
|Volumetric energy density [Wh/L]||35|
|Cell voltage [V]||1 – 2.5|
|Time of charge/discharge [h]||1 – 10|
|Energy efficiency [%]||70 – 80|