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Flow batteries for BESS - approaching commercialisation after 140 years

17 October 2023

The need for BESS (Battery Energy Storage Systems)

Grid scale electricity generation is transitioning towards renewable energy sources. But renewable sources (e.g., solar, wind) are intermittent in nature and need to be coupled with storage systems such as BESS to allow for long-duration energy storage. This in turn helps to ensure a reliable supply of electricity to the grid, enhancing grid resilience.

BESS has become increasingly popular over the last 5 years. BloombergNEF's 2023 Energy Storage Market Outlook [1] indicates that the growth trend for the BESS market is anticipated to remain strong, being driven by affordability, flexibility, evolving battery technology, second-life batteries, and virtual power plants [2].

Harmony Energy's 196MWh Pillswood BESS with 78 Tesla Megapackunits [3]

The technology options

There are several existing battery technologies which could be utilised for a grid-scale, long-duration BESS system. However, the best battery choice for a particular application will depend on its specific requirements. Currently, the state-of-the-art battery type used is lithium iron phosphate (LFP, short for LiFePO4, the material used for the battery’s cathode) as they are commercially proven and offer high energy density at a lower Levelised Cost of Storage (LCOS) compared to alternatives such as lead-acid or sodium sulphur. However, for applications where longer discharge duration, greater cycle life, scalability and ease of maintenance are important selection criteria, flow batteries are now emerging as a promising option.    

LFP batteries

LFP at a glance [4]

LFP batteries are considered to be the-state-of-the-art battery option for BESS applications. Vendors such as Wärtsilä, Contemporary Amperex Technology Co., Limited (CATL) and LG Energy Solutions are key players in the market offering advanced LFP technology that is commercially proven in a range of different applications. The Moss Landing battery storage project (300MW/1,200MWh) located in California is the largest battery storage project in the world and was developed by Vistra Energy using LG Energy Solutions’ LFP technology.

LFP batteries deliver a high current rating (the maximum current that the battery can provide without being damaged), high efficiency, and a moderate-long life span. As a result, they are particularly well-suited for applications requiring high load currents and endurance (e.g., BESS) but for shorter discharge durations < 8 hours where benefits such as peak shaving are desired. Additionally, the good thermal stability of LFP makes it one of the safest and most abuse-tolerant cathode material options available to manufacturers.

For long-duration applications, an attractive alternative option to LFP is the flow battery. Flow batteries are not new; the first flow battery was patented in 1880 [5] (see the figure below), a zinc-bromine variant which had multiple refillable cells. However, despite its long history, the flow battery has been searching for suitable and scalable applications where successful commercialisation can be achieved.

Drawing of the first flow battery patented by John Doyle in 1880 [6]

Flow batteries are batteries which transform the electron flow from an activated electrolyte into an electric current. Within flow batteries, charge and discharge are achieved by pumping a liquid anolyte (negative electrolyte) and catholyte (positive electrolyte) adjacent to each other across a membrane. The electrolytes contain compounds in different oxidation states.

During pumping an electrochemical oxidation-reduction (Redox) process occurs, the compounds within the anolyte are oxidised releasing their extra electron(s) which are then captured by compounds in the catholyte which are considered reduced. This exchange of electrons generates an electrical current which is how power is generated. This is a significant difference to how ‘traditional’ LFP batteries work, in an LFP battery power is generated when lithium ions move from the anode to the cathode through an electrolyte which generates an electrical current.

A schematic diagram of a redox flow battery with electron transport in the circuit, ion transport in the electrolyte and across the membrane, active species crossover, and mass transport in the electrolyte [7]

Additionally, in contrast to LFP batteries, within certain RFB’s (e.g., VRFBs) the power and energy capacity are determined independently. The power is determined by the size of the power cell (the area within an RFB where the electrochemical reactions take place) as the size of the power cell dictates the amount of current that can be generated by the battery. The energy capacity is determined by the quantity of chemicals in the electrolyte tanks (the part of the battery where the chemicals are stored within electrolytes) as the quantity of chemicals in the electrolyte tanks dictates the amount of energy that can be stored in the battery.

Because power and energy capacity are determined independently, RFB’s are considered to exhibit modular flexibility (decoupled power and energy capacity).  This decoupling means that the power and energy capacity of a RFB can be scaled independently without requiring additional electronics or plant equipment; the power capacity can be increased without increasing the energy capacity, and vice versa which is what makes RFB’s very flexible and scalable for a variety of applications. For example, a RFB can be specifically designed to have a low power capacity and a high energy capacity making it a good choice for applications that require a lot of energy for a long period of time (e.g.a BESS system to store energy from renewable sources).

There are currently three main types of flow battery: redox flow batteries (RFBs), hybrid flow batteries and membraneless flow batteries. RFBs are the most common type, they are commercially proven and offer the most advanced technology of the flow batteries. Depending on the cathode, anode and electrolyte composition, RFBs can be further sub-categorised into

  • Vanadium redox flow batteries (VRFB)

  • Zinc-bromine redox flow batteries (ZRFB)

  • Iron redox flow batteries (IRFB)

Table: Battery composition of different RFBs

Of these, VRFB’s are the most widely deployed so far due to technological advancements; there are several vendors who are competing in a quickly diversifying vendor landscape. VRFB vendors include but are not limited to: Invinity Energy Systems, Sumitomo Electric Group, Rongke Power and Australian Vanadium Ltd. There are also established vendors of ZRFBs, although the ZRFB market is still less mature than the VRFB market to date.

Some current VRFB and ZRFB Vendors

Compared to LFP batteries, RFBs have a lower gravimetric energy density (the amount of electricity a battery can provide in relation to the mass of the battery) and as a result a RFB based BESS requires more space than an LFP system. However, some RFB’s have the ability to completely discharge for extended periods (> 8/10 hours) without any significant negative results for their capacity or to the structural integrity of the battery; indeed, they are observed to offer significantly reduced degradation with almost unlimited longevity. This means that a RFB system will generally have a longer operational period before replacement/comprehensive maintenance is necessary compared to incumbent existing technologies. This is in stark contrast to an LFP battery, in which the lithium hexafluorophosphate (LiPF₆) electrolyte used in many cells will convert to toxic hydrogen fluoride gas and corrosive hydrofluoric acid in the presence of moisture which greatly compromises the structural integrity of the battery cell.

Whilst less mature than LFP (LFP: TRL 8, flow batteries: TRL 5-7), conventional RFBs are quickly emerging as a viable option for a BESS system. Their sweet spot is that they are very good at delivering a consistent amount of power over significantly longer periods. This lends them to applications with longer durations, for example grid-scale BESS with a discharge duration over 8 hours which is a common requirement for countries where solar PV is countered by a 24/7 industrial baseload. Indeed, Rongke Power connected a 100MW/400MWh RFB BESS system to the grid in Dalian, China in October 2022. This system will assist with peak shaving and ‘valley-filling grid auxiliary services, to offset the variability of the city’s solar and wind energy supply’ [8].

RFB based BESS systems are quickly gaining momentum and the vendor landscape has a range of different developers offering various systems that are increasingly becoming cost competitive with other incumbent technologies (see figure below). The power generation industry is continuing to develop and reliance on gas generation is becoming reduced; this, in combination with the use of intermittent electricity sources, is driving commercialisation and emergence of RFB based BESS.

Comparison of 2021 LCOS by technology, power capacity and duration for 100 MW capacity and 1000 MW capacity [9]

Final thoughts

LFP BESS is now commercially widespread. Based on commercial familiarity and vendor expertise, many large-scale BESS systems nowadays are LFP based.

However, LFP batteries are not without their disadvantages and are less suited for applications requiring longer discharge durations. The use of Li as a constituent part reduces the energy density of LFP batteries, makes them harder to recycle and increases the cost of production because Li is a rare element. The use of Li is therefore a fundamental driver for the industry to seek alternatives and RFBs offer a particularly attractive alternative from a material perspective.

It is also clear in a side-by-side comparison that some emerging technologies may be better suited for these specific longer discharge applications. 

*TRL: Technology Readiness Level

Table: Comparison of LFP and various RFB chemistries for BESS

As more emphasis will be placed on consistent power generation for significantly longer durations (> 8/10 hours) this will offer a gap that some would claim emerging technologies (e.g., RFB based BESS) can better exploit. In a rapidly developing sector, it is therefore likely that RFB based BESS will soon emerge as a key competitor to LFP if estimated LCOS can remain competitive for longer discharge duration systems.











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