A new report from the DOE compares lithium-ion batteries, lead–acid batteries, pumped-storage hydropower, compressed-air energy storage, redox flow batteries, hydrogen, building thermal energy storage, and select long-duration energy storage technologies.
The report is missing some of the recent developments but overall has lots of information.
One of the main factors against RFB has been the cost of the material (mainly for Vanadium). This gets solved if you produce/own the Vanadium. The report mentions the deal between Bushveld and Invinity. IMHO Largo has a better solution since it owns both the Vanadium production as well as the RFB company.
From th report:
"A large determinant of RFB viability is system cost, which is strongly coupled to the active material choice. Historically, vanadium has been preferred due to several attractive electrochemical properties, including relatively long life. However, as Li-ion prices have dropped, vanadium RFBs can begin to compete economically. Because the RFB’s design fundamentally creates a cost advantage in the 4–6hour duration segment (which is increasingly important), strategies are evolving to improve active material economics. Several investors and startup companies are demonstrating new RFB technologies, which may impact future deployments, if successful. For example:
• A vanadium producer and an RFB company formed a joint venture to rent vanadium electrolyte to redox flow battery developers [35].
• Iron is an extremely low-cost active material and is seeing some early RFB commercial successes. For example, a young iron RFB company whose device can provide up to 10-hour duration is now scaling to >5 MW and expanding manufacturing capacity. It is receiving support from both investors and its nascent future supply chain [36].
• A zinc-bromide RFB has been designed as a standard 5-hour duration product and is manufactured as a commercial 25-kW design. The California Energy Commission recently awarded this startup a contract for further demonstrations [37]. Aqueous-flow cells based on organic active materials, instead of inorganic actives, are potentially cost-effective and viable for widespread adoption as they are not limited by natural earth abundance. Several organic chemistries have shown promise and are receiving much interest as next-generation redox flow chemistries."
Long-Duration Energy Storage: "As duration increases, the marginal value of storage decreases and, therefore, so does the affordable total capital. The competitiveness of a technology will thus depend on the required hours of duration. Total capital is defined here as the fully installed cost, including both the device and complete balance of plant."
"RFBs are projected to be cost-effective for durations greater than 6–8 hours, depending on the particular chemistry. Vanadium-based RFB costs depend substantially on the cost of vanadium, which is historically variable and often high. Other chemistries using earth-abundant materials may be more competitive for LDES [53]. For example, as noted earlier, a company with an iron-based RFB designed for up to 10-hour duration is now scaling to >5 MW and expanding manufacturing capacity"
Links: DOE Energy Storage Report PV Magazine article