Vanadium Redox Flow Batteries for Solar PV Systems

Several energy storage systems exist for solar PV applications, and it can be difficult to do the research and figure out exactly what battery to buy.

Are vanadium redox flow batteries one of the solar power storage systems you have in mind?

Throughout this article, we will explain everything you need to know about these batteries so that you can make a more informed solar storage investment decision.

By the end of this article, you should know:

• What vanadium redox flow batteries are and how they work
• The possible applications in a solar PV system
• Characteristics of vanadium flow batteries and how to choose the best, and
• The pros and cons of these batteries

Let's get started.

What is a Vanadium Redox Flow Battery?

Vanadium redox flow batteries can be defined by describing two crucial terms: flow batteries and redox reactions.

Flow batteries are energy storage systems that use liquid electrolytes to produce electricity in cells utilizing electrochemical reactions.

Redox reactions are essentially reduction-oxidation reactions that, with regards to vanadium batteries, are for various vanadium forms.

Vanadium redox flow batteries are, therefore, storage systems that use vanadium-based electrolytes to store electrical energy in the form of chemical energy through redox reactions involving multiple vanadium oxidation states.

How does a Vanadium Redox Flow Battery Work?

The operation of vanadium flow batteries is initiated at the electrolyte. For vanadium flow batteries, the electrolyte is stored in sealed tanks and pumped to the cell stacks of the battery on demand.

If the cell stacks already contain the electrolyte, power can still be drawn from the batteries but for shorter durations.

The commonly used electrolyte in Vanadium redox flow batteries is vanadyl sulphate (VOSO4) in sulphuric acid solution. The electrolyte is converted into various oxidation states at the positive and negative electrodes of the battery.

At the positive electrode, the oxidation states are V4 + and V5 +, while at the negative electrode, there're V2 + and V3 + ions. Each of these oxidation states is referred to as a redox couple and happens in each half cell of a Vanadium flow battery.

On passing current from a solar PV system, the charging process occurs. It involves converting V3+ ions at the negative electrode to V2+ and the V4+ at the positive electrode to V5+. The electrodes used in vanadium redox flow batteries are carbon felt.

Reaction at the positive electrode (cathode)

VO2+ + 2H+ + e- <=> VO2+ + H2O

Reaction at the negative electrode (anode)

V2+ <=> V3+ + e-

Overall reaction

VO2+ + 2H+ + V2+ <=> VO2+ + H2O + V3+

During discharge, the above reactions are reversed, converting the chemical energy stored in the batteries to electrical energy.

The discharge process can be represented by the left to the right reaction of the overall reaction above.

Hydrogen ions pass through a hydrogen-permeable membrane to maintain the current produced by the cells. A power conditioning system is also used to control the charging and discharging process of vanadium flow batteries.

The two vanadium redox couple reactions happen at high-speed kinetics and have very high reversibility.

Due to this, vanadium redox flow batteries can be easily overcharged and over-discharged, thus eliminating the gassing issue, which is prone to occur during rapid charging cycles.

Additionally, the fast chemical kinetics in the vanadium flow batteries lead to high voltage efficiency at large current density.

Still on the reactions, it’s evident that a single active element (vanadium) is used in both electrodes. This eliminates the issue of cross-contamination, which is common in batteries that use distinct active elements at each electrode.

To reduce electrical resistance, the electrodes are connected with a bipolar plate to each other.

The standard voltage of a single cell in a Vanadium flow battery is 1.26V at standard temperature.

This standard voltage varies with temperature changes. If more voltage is needed from the battery, a stack of several cells can be used in a single battery.

Vanadium Redox Flow Battery Applications

Vanadium redox flow batteries can either be used for small, short-duration applications or large, arbitrary applications.

Vanadium redox flow batteries can fulfill these two applications simply by altering the electrolyte volume that is fed into the battery.

For solar renewable energy integrations, vanadium flow batteries are great for stationary large power applications and remote/ off-grid applications.

How to Choose a Vanadium Redox Flow Battery/ Characteristics?

Battery capacity (Size)

The battery capacity or size of a battery is the amount of energy that the battery can store and supply on demand.

Vanadium redox flow batteries are rated differently in terms of capacity (5KWh, 10KWh, 20KWh, 40KWh, etc.), and the choice here depends on your specific energy needs.

If you have solar appliances such as a solar refrigerator or a solar roof pump that consume a lot of power, you need huge capacity vanadium redox flow batteries.

If you want a conservative estimate, go with the average American electricity power consumption per household per day that’s 30KWh. So let’s say you’re looking to buy 5KWh batteries, you’d need 6 of those to power your home.

If you want better energy per footprint (when you don't want the batteries to occupy a large space in your home), then three 10KWh batteries should be sufficient.

Cycle life

If you go entirely off-grid or rely on your PV system to power most of your residential processes, you’ll definitely be cycling your solar batteries daily.

In essence, the longer the cycle life of a vanadium redox flow battery, the more you’ll use it to store and provide power on demand.

So, if you can go for the premium vanadium redox flow batteries with a cycle life as high as 20,000 cycles (30 years), the better.

Depth of discharge

Vanadium redox flow batteries can be discharged up to 100% hence are perfect for stationary appliances such as a solar refrigerator that needs extended powering.

Vanadium redox flow batteries offer a high depth of discharge that's hard to find in conventional batteries, making them an excellent choice for solar PV systems.

Please note: Depth of discharge does not affect the battery’s cycle life and overall lifetime, so no matter the extent to which you discharge your Vanadium flow batteries, you still need to take proper care of them.

Rate of self-discharge

Vanadium flow batteries have low self-discharge since the electrolyte is stored in sealed tanks that are kept separate from the main battery unit.

With this, no internal reactions can occur and cause power loss when the battery is not connected to a load.

Warranty period

Vanadium redox flow batteries are warrantied for the battery capacity that’s sustained up to the guaranteed cycle life.

So, ensure that the warranty period covers the cycle life of the Vanadium flow battery.

Therefore, you can always get a replacement unit if the first one loses its expected capacity before the service life is up.

To know the warranty period, you should get for your vanadium redox flow batteries, refer to the following table.

Round trip efficiency

Round trip efficiency is a crucial consideration since it determines the amount of power you can draw from a fully charged vanadium redox flow battery.

The round trip efficiency of vanadium redox flow batteries is around 75% – 85%. This means that if you have a 5KWh Vanadium flow battery that’s fully charged, the maximum you can draw from it is 4.25KWh.

The round-trip efficiency is not in any way related to the cycling process of the battery. Essentially, it's caused by 3 main factors:

• The extent of the electrochemical reactions
• Shunt current caused by manufacturing defects on the battery unit
• Pumping losses that occur when the electrolyte is delivered to the cell stacks

Manufacturers

No brand of Vanadium redox flow battery is made like the other, and you have to do your research to determine the best deal.

A good strategy is to read customer reviews, then conduct a local survey to evaluate how well the Vanadium battery performs in your area.

Choosing between a startup company or an established one has to do with what part of the buying equation you're comfortable sacrificing.

Startups will generally produce top-tier Vanadium batteries, but the problem is that there’s no track record to reference.

On the other hand, an established company may not have the best batteries, but there’ll always be a track record to check on.

Below are a few examples of leading companies for vanadium redox flow batteries:

• VRB Energy
• Big Pawer
• VFlow Tech
• VisBlue
• UniEnergy Technologies
• Enerox
• Ashlawn Energy
• Largo Energy
• VoltStorage
• NEXTracker

Advantages of Vanadium Redox Flow Batteries

Suitable for large scale applications

The capacity of vanadium flow batteries can be increased by changing the electrolyte volume and concentration. As well, a larger number of cells in a battery stack can increase its power capacity.

Because of their flexible capacity increases, these batteries can be used for large, stationary applications.

Long lifetime

A typical solar PV lasts 25-30 years. Since vanadium redox batteries can also be cycled for this period, they make a reliable and cost-effective energy storage system.

The long-lasting characteristic of vanadium flow batteries can be attributed to the non-degradability of the electrolyte used in these batteries.

In fact, if the cells stacks stay in their pristine condition, you can always be assured that the batteries will stand the test of time.

The vanadium electrolyte is recyclable

While there’re toxicity issues with the oxides of vanadium, the VOSO4- H2SO4 electrolyte can be recycled in other applications at the end of the battery’s cycle life.

By doing so, any environmental concerns that might arise from disposing of the batteries and electrolytes into the environment are eliminated.

Non-flammable and safe to use

The electrolyte used in vanadium flow batteries is usually diluted with an equal part of water. This eliminates thermal runaway risks and renders these batteries non-flammable.

A fire is unlikely to happen if pressure/temperatures rise or a short-circuit occurs in a vanadium flow battery’s connection.

Disadvantages of Vanadium Redox Flow Batteries

Not suitable for small scale applications

Vanadium flow batteries require electrolyte storage tanks, pumps to deliver the electrolyte to the cell stacks, and serious power management.

These processes can easily be achieved for large-scale projects, but they can be a little expensive and impractical for small-scale projects.

Slow response during sudden demand peaks

Vanadium flow batteries fall short when it comes to responding to sudden energy demand peaks, say after a disturbance. This is not to mean that these batteries are slow they’re just not made for peak demands.

Vanadium flow batteries are made for high but steady and continuous demand.

Low round trip efficiency

As mentioned earlier, the round trip efficiency of vanadium flow batteries is 75%-85%.

This is not so bad considering that there’re pumping losses and intricate reactions happening in these batteries. But, this is way below what other batteries such as lithium-ion offer.

High upfront cost

Vanadium is an expensive metal, so the upfront cost of the electrolyte and the battery unit is relatively high compared to other batteries.

This is, however, countered by the long cycle life, which is also an assurance that you’ll recoup your investment.

The average cost per KWh of vanadium redox flow batteries is $300-$1000. This may seem high at the moment, but with the current interventions centered around this technology, these operating costs are expected to decrease in the near future.

Frequently Asked Questions

Do vanadium redox flow batteries require special protection from extreme temperatures?

If you want your vanadium redox flow batteries to have a long useful life, you need to protect them from extreme temperatures.

Too low temperatures (below 30 °F) call for high voltage charging while extremely high temperatures (above 90 °F) require that you reduce the charging power.

Definitely, you don’t want to keep changing these parameters, so the best solution is to find other ways to protect your vanadium redox flow batteries. A good idea is to keep them in temperature-controlled battery storage containers.

Can a Vanadium redox flow battery be restored?

A Vanadium redox flow battery can be restored by mixing the electrolyte and adding a chemically equivalent reducing agent.

This rebalances the electrolyte’s volume, concentration, and valence, consequently restoring the pristine productivity of the battery.

The positive and negative electrodes can also be swapped to reduce efficiency loss due to electrode degradation.

In general, restoring a Vanadium redox flow battery can help counter the inevitable capacity loss caused by extended cycling.

Are vanadium flow batteries for PV applications worth it?

If you have stationary, high solar power needs in your home, vanadium flow batteries are the energy storage system to couple with your solar PV system.

If you only need small amounts of power, you'd be better off looking for alternative batteries, since vanadium isn't the cheapest energy storage system to invest your money in.

Conclusion

Vanadium redox flow batteries are highly suitable for solar PV applications due to their high capacity, less sensitivity to depth of discharge, low self-discharge, and their ability to provide independent energy and power.

Obviously, energy storage systems are not all perfect, and it's true they’re more expensive than other batteries. Their round trip efficiency is also lower than that of most conventional batteries.

However, considering that they have an excellent cycle life, you're assured that you'll get the most out of the investment.