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.

Unit cell

Here is a simplified representation of a single “unit cell” of a flow battery, which is the basis for a stack:

A single unit cell of a flow battery

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.

Here is a simplified representation of a flow battery stack with three unit cells:

A simplified flow battery stack showing cells electrically in series and hydraulically in parallel. A real stack would have more cells in series than shown here.

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)

Here is a non-scientific relative comparison of the tradeoffs between lithium-ion, lead-acid, and flow batteries (assuming all technologies are manufactured at a similar economy of scale):

For a good overview of the inherent tradeoffs in designing batteries, check out Dan Steingart’s excellent blog post on the matter.

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. See the following question for our timeline.

What is your timeline?

Here is our development plan for the cell and stack hardware:

1. Benchtop development kit, cell area 1-10 cm², target release end of 2024
   1. Purpose is testing materials and electrolytes on a small scale: different graphite felts, separators, formulations, etc. Purpose is not storing energy on a practical scale.
   2. Measure intrinsic properties: efficiencies, achievable states-of-charge, current and power densities, etc.
2. Large-format single cell, area ≥600 cm², target release mid-2025
   1. Purpose is testing established electrolytes from the benchtop kit with a practical cell area, and refining large-scale cell design. Purpose is not storing energy on a practical scale.
   2. Measure and maintain achieved performance levels from benchtop kit
   3. Tackle scale-up and integration challenges: balance-of-plant, centrifugal pump control, high-current power electronics, current and flow distribution, etc.
3. Stack, built from large-format single cells, target release end of 2025
   1. Purpose is storing energy on a practical scale. Enough cells for a 48 V stack connection to residential-scale inverters (depends on chemistry). Power and energy metrics targeted for residential use.
   2. Measure full-system performance with parasitic loads and full balance-of-plant
   3. Tackle stack and system design challengs: shunt currents, thermal management, transient operation, system control.

Here is a visual overview of this same plan:

Roadmap of cell and stack hardware

All outputs of this development, including cell and stack designs, electrolyte formulations, etc. will be released as open-source hardware as defined by OSHWA.

This plan will not be carried out in a strictly linear manner; insights gathered in each step will go back and forth to inform and improve the preceding and following scales.

Development of flow battery electrolyte formulations will happen in parallel, with the majority of the electrolyte development and proving happening with the benchtop cell to conserve resources. Right now we are working on both traditional all-liquid and hybrid electrolyte formulations, the cell and stacks will be designed to accommodate either type.

Why open-source?

We believe an open-source approach to developing flow batteries is the most resource-efficient method that will allow the most relevant people to contribute their skills at the lowest expense.

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.