🔋Going With The Flow

Are flow batteries the solution to long duration storage?

Mar 30, 2022

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Germany’s Energy Fumble

In an effort to reduce greenhouse gas emissions, energy generation sources like wind and solar are promising alternatives to burning fossil fuels. Since nuclear power seems to be unjustly thrown under the bus these days, and hydroelectric power is geographically constrained, wind/solar it is. The Biden administration is putting wind/solar first in their plans to reduce emissions.

However, without sufficient generation and storage capacity of renewables, you get what we are seeing with Germany now (a whole story in itself, I’ll keep it brief). Germany doubled down on shutting down their nuclear power even though they were very aggressive in building out wind/solar. Roughly half of their energy comes from wind/solar and they are finding out that these renewables are not reliable enough and it is projected their emissions will actually increase. Many questionable policy decisions have led them to be bound to Russia for much of their energy and do things like re-open coal plants to secure energy. This is less than ideal because coal remains the worse than liquid hydrocarbons and natural gas in terms of carbon dioxide emissions. Refer to Doomberg’s fossil fuel hierarchy.

The problem is that these energy generation sources are intermittent and cannot operate when it is calm and dark. This is a huge issue because winds are seasonal and it is dark half of the day. In addition, the closer you get to the poles the more variability in day length and sun intensity there is, and certain locations are not very sunny. The way to get around this is to store energy in batteries for later use. You can see the principle in this diagram that shows storage filling in the gaps of energy demand.

This puts into question the feasibility of wind/solar as energy sources today as the price of Li-ion batteries (LIBs) remains high. You can get residential solar and battery packs through Tesla Powerwall for example, but these options remain an expensive investment for most homeowners. We have seen Australia introduce Tesla megapacks at a large scale, but this project was heavily subsidized by the government. Better transmission infrastructure is required for renewables to reach their potential which adds further costs. For these reasons, I remain skeptical of the following graph when I see it floating around twitter because it does not paint the full picture. The reality is that there are other major cost drivers including batteries for the reliable adoption of renewable wind/solar sources.

Flow Batteries

Can another energy storage solution be a cheap and functional alternative to LIBs? They are a proven power/energy dense energy storage solution, which is why I frame it in that question and the reason I will mention LIBs so much throughout this piece. Flow batteries are a promising energy storage technology for grid-storage applications. Grid storage means that it will be used to store the energy generated through sources such as hydro, wind, or solar at a large scale. They definitely will not be used for small devices, electric vehicles, and possibly even homes as the energy density is too low. They excel though when the storage durations increase past 4, 6, 8+ hours. Flow batteries are very promising because scaling lithium-ion batteries for grid storage is expensive and less robust over the long term. Flow batteries could excel over longer lifetimes and scale to very large applications more economically.

Instead of the energy being stored in the electrodes as LIBs do, the energy is stored in the electrolytes in a flow battery. In the diagram below of a vanadium flow battery, the tanks are where the electrolyte is stored. When the electrolytes are pumped through the center where the electrodes and membrane are, an ion-exchange occurs that produces electricity. Energy from an external source (wind/solar, etc.) must be supplied in order to separate back out the ions to “recharge” the flow battery. Energy is stored in these large tanks which means it is scalable and all you need to do is get larger tanks. The power density depends on the electrode/membrane area or power generating component, which means that power and stored capacity are completely independent.

Pros

Cost to scale is not linear
Better for grid applications
Better safety
Better lifespan/efficiency
Less supply chain constraints

Since the stored energy is decoupled from the system power the cost of scaling is not linear. This is probably the single biggest draw for flow batteries. Need more energy, just get larger tanks. Meanwhile in a LIB system, more of the identical packs are required, meaning the cost of scaling is linear. David Schroeder used the example of the engine vs gas tank,

Doubling the size of your gas tank wouldn’t be very expensive, doubling the power output of your engine is expensive.

This means that the cost of flow batteries for grid applications may become cheaper than LIBs. The use cases would become utility, commercial, or industrial scale energy storage. A large apartment building or large business with rooftop solar could store energy in a large flow battery system in the basement or outside the building. Another example would be wind from the grid could be stored with flow battery systems by the utility companies.

Flow batteries are fundamentally different from LIBs. One advantage is that they do not have the flammable electrolyte, or have to worry about solid phase transitions that cause detrimental reactions to occur that could release oxygen. The tanks of liquid are not flammable and are not prone to catching fire as we have all seen with LIBs. No battery/thermal management system would be required as well, which is a cost inhibitor for LIB systems.

LIBs are state-of-charge constrained, meaning the lifespan of the battery is hurt if the battery is charged to 100% many times. Flow batteries don’t have this issue as the tanks can be fully reacted without detrimental reactions or consequences to lifespan.

There are a lot of materials that go into LIBs, and this can add cost or supply chain concerns. Flow batteries can use metals dissolved in water and have much fewer supply chain considerations. They do not have to worry about the lithium, nickel, manganese, and cobalt markets for example.

Cons

Less energy/power density
Only grid applications
Material selection

The low energy density means that they cannot pack enough energy into a small enough space to work in phones or cars. There are large tanks required which are bulky and take up more space than LIBs. Could some companies even be willing to pay more for a LIB storage system because it will take up less space? Another issue is that if the electrode/membrane is small then the power output does not scale with the increasing tank size. A larger/different electrode area would be needed to get better power density since it is independent of the energy storage capacity. The energy/power densities of various flow chemistries can be seen in comparison to LIB below. Energy density is between 4-10x better, while power density is between 2-7x better for LIBs.

Mainly for the aforementioned reasons, flow batteries are constrained to large scale, grid, or other long duration applications by their nature. Furthermore, from Nguyen and Savinell,

The main disadvantage of flow batteries is their more complicated system requirements of pumps, sensors, flow and power management, and secondary containment vessels, thus making them more suitable for large scale storage applications.

The successful and most extensively studied version is the vanadium flow battery. Vanadium is expensive and toxic in a similar way that cobalt is for LIBs so it is not likely to succeed. Certain configurations require expensive, toxic, or corrosive materials however a successful flow battery will look to avoid these problem areas. Material selection and cost will be crucial in the applicability of a flow battery on the grid considering cost is the biggest factor.

Outlook

There are a few companies working on flow batteries with varying chemistries. Invinity is a vanadium based flow battery company which services multiple continents around the world. The UK based company raised further funding at the end of the year, but has the possible disadvantage of using vanadium instead of other more friendly elements. ESS is a US based company that has an iron flow battery, with advertised iron, salt, and water components that are cheaper than LIBs for the grid. They recently made a deal with SB Energy to deploy their technology in CA. There are various types of organic flow batteries, one of which has been developed by Lockheed Martin. It was recently announced they are launching a grid storage project in Canada. Other companies like Primus Power (Zinc-Bromine), ViZn (Zinc-Iron), Jena Batteries (organic), Honeywell, and Redflow that are working on flow batteries as well.

Some of these companies have deployed large scale storage systems around the world, but flow batteries have yet to really take off. Li-ion is very energy dense, so any application that is looking for performance or has a volume constraint is not likely to choose a flow battery. Furthermore, it remains to be seen whether the cost savings of flow batteries for long duration applications beat out the current LIB.

Li-ion is king when it comes to battery storage right now. Will any of these systems be able to scale and defeat LIBs at the grid? They are safer, better cost scaling for long durations, no nickel/cobalt/lithium supply chain issues, and last a long time.

Unfortunately the total price of wind/solar will also depend on the cost of these storage technologies and any transmission infrastructure needed. More energy is better than less. I don’t think wind/solar will ever eliminate the need for fossil fuels nor do we truly* want them too, but flow batteries are an exciting technology that could make them cheaper and more reliable for the grid. That is the ultimate goal, as wind/solar are dependent on a successful (or combination of) grid storage technology to be reliable energy production methods.

-Grayson

*Fossil fuels are required for many vital industries including food, plastics, energy, graphite production, and more. Fossil fuels have made 21st century possible. More on this in the future.