FAQ

What is a flow battery?

A flow battery is a device used to reversibly store energy for stationary applications. They typically have two tanks which each store a liquid electrolyte, a reactor that allows redox reactions with the liquid electrolytes called a “stack”, and two pumps with piping to continuously circulate the electrolyte between the tanks and the stack.

The stack typically has internal fluid manifolds that distribute the two electrolytes through several electrochemical cells in parallel, each cell usually consisting of two porous conductive electrodes (like graphite felt) separated by a membrane that conducts ions. The membrane can be a simple porous separator like paper, or a more specialized ion-exchange membrane that is designed to allow only positively or negatively charged species through. Redox reactions occur on the surface of the porous conductive electrodes, alternately reducing and oxidizing the chemical active species present in each electrolyte. Solid, impermeable bipolar plates are in contact with the porous conductive electrodes to allow electrons into and out of the system. These bipolar plates also allow for “stacking” the cells in series, electrically, to achieve a higher voltage for the whole stack.

The cells in the stack are therefore connected electrically in series with bipolar plates, and hydraulically in parallel with internal fluid manifolds. There are typically two electrical connections to the stack, positive and negative terminals, and four fluid connections: positive electrolyte inlet and outlet, and negative electrolyte inlet and outlet.

What’s it for?

Storing energy, usually renewable, for stationary applications. In a residential setting, this could include:

  • providing backup power during power outages
  • increasing self-consumption of rooftop PV panels
  • participating in Virtual Power Plant (VPP) programs

Why are you doing this?

Flow batteries are a promising technology but aren’t yet widespread or available. We hope to accelerate their development using an open-source approach, encouraging global collaboration to iterate quickly and efficiently towards standardized electrolyte formulations and stack designs that could be manufactured for real-world use.

Some possible advantages of flow batteries compared to lead-acid or lithium-ion include:

  • Longer lifetimes, due to fewer and slower possible degradation modes

  • Lower costs of energy stored per cycle

  • Safer: nonflammable water-based electrolytes

  • More sustainable: possible to manufacture electrolytes with globally abundant minerals/chemicals

  • More recyclable: comparatively easy recycling at end-of-life

Disadvantages include:

  • Lower energy density (kWh/m3), limits to stationary applications: no EVs or portable electronics

  • Possibly lower round-trip energy efficiencies vs. lithium-ion (but not much real-world data on this yet)

What’s the point, when lithium-ion cells are “cheap and available”?

Lithium-ion batteries are great at what they are designed to do, storing a lot of energy in a small space reversibly for reasonably long lifetimes, and are excelling in stationary storage applications now because they are currently the cheapest available technology. That doesn’t mean, however, that they are the best technology for the application. Lithium-ion benefits from massive economies of scale which drive down their costs. Flow batteries have yet to benefit from a similar reduction in price, since comparatively few have been manufactured. Flow batteries seem to have unrealized benefits to offer society for applications in stationary, long-duration energy storage.

How much energy does it store / how efficient is it?

It depends! We are currently working on our benchtop flow battery kit for R&D and educational use, which will be able to test a variety of electrolytes, each with different performances. When that kit is complete this data will be front and center.

After development of the kit, FBRC will begin work on a larger cell format that will form part of an actual flow battery stack, to target power/energy demands that would be relevant for use in residential energy storage.

What about iron-based chemistries?

We plan to test these! Iron-based chemistries that rely on iron plating at the negative electrode (a “hybrid” flow battery configuration) typically suffer from severe hydrogen evolution, however, and require additional systems to recombine the produced hydrogen with the iron (III) cations in the positive electrolyte. This lowers the energy efficiency of the system, and also makes them not practical for FBRC’s first electrolyte, because of the additional system requirements.

Have you heard of “Company X”?

Probably! We’ve been watching the space for a while—but ask anyway! We plan to eventually introduce a database of flow battery companies.