During the past years, lithium ion (Li-ion) batteries have emerged as the most widely deployed technology for stationary energy storage projects. These batteries are typically selected due to the attractive combination of low cost, high energy density and highly reputable integrators such as Powin, Fluence, FlexGen and Wartsila using first in class battery cells such as LG, Samsung and CATL.

Driven by increases in manufacturing capacity and sales volumes from the complementary EV industry, Li-ion battery pack costs have decreased dramatically. Yet, despite their popularity, Li-ion batteries may not always be the best option for certain stationary storage applications.

Flow Batteries

As discussed on our previous blogpost on Utility-Scale Battery Energy Storage Systems, flow batteries may play a key role in the future as they are a more environmentally sustainable alternative to the current lithium-ion technologies.

Figure 1. Flow Battery Scheme. Image sourced from Wikipedia

A flow battery is a type of rechargeable battery where the battery stacks circulate two chemical components dissolved in liquid electrolytes contained within the system. The fundamental difference between conventional batteries and flow cells is that energy is stored in the electrode material in conventional batteries, while in flow cells it is stored in the electrolyte. Due to the energy being stored as electrolyte liquid it is easy to increase capacity through adding more fluid to the tank or ultimately increasing the tank’s capacity. Larger tanks lead to longer duration systems.

While the volume of liquid electrolyte determines the battery energy capacity, the surface area of the electrodes or number of stacks in the power unit determine the battery power.

Quite a number of different materials have been used to develop flow batteries. The most common types are:

Vanadium Redox Flow Batteries (VRFB)

Vanadium is a globally-produce metal. Vanadium Redox Flow Batteries are the most common type of flow batteries. In VRFBs, the electrolyte is composed of vanadium dissolved in a stable, non-flammable, water-based solution being safer to transport and operate. This is a great advantage against Li-ion batteries which suffer from thermal runaway and fire/explosions.

Besides, this technology gives the batteries a lifetime of around 25 years with unlimited cycle life as there is no chemical degradation. Also, depth of discharge (DoD) is of 100%.

Figure 2. Vanadium Flow Battery. Image sourced from ResearchGate

Invinity is one of the main VRFB providers today. The company develops and supplies VRFB especially for commercial, industrial and utility-scale applications in grid-connected, weak-grid, and off-grid scenarios. With their modular, easily scalable solution there is no overcharging on scaling batteries and help reduce battery waste as they are almost 100% recyclable.

The VRFB are generally utilized for power systems ranging from 100kW to 10MW in capacity, meaning that it is primarily used for large scale commercial projects.

While Invinity is indeed focused on large scale battery solutions, start-up Storen Technologies, focuses on small sized batteries for residential applications. Their 5kW/30kWh is their smallest self-contained battery. Based on a sweet spot sizing, it is able to fulfill several market applications, from residential to several industrial and commercial installations, such as telecom tower back-ups, smart grids and microgrids integration, both connected to the grid and/or renewables, and to Virtual Power Plants.

Storen has an interesting business model approach. They want to manufacture the stack, because it requires a level of automation but the rest of the battery can be sourced and assembled locally (providing jobs locally too and reducing costs). They want to partner with companies in different countries that finish the construction and assembling in each country.

V-Flow Tech’s also offers solutions from small size batteries ideal for solar tracking application in solar farms and for telco towers and at the same time batteries scalable to MWh for grid and utility applications. Their products are design to resist harsh environments and can be installed in remote locations.

Zinc Bromine Flow Batteries (ZnBr FB)

The Zinc-bromine flow battery is the most common hybrid flow battery variation. The hybrid flow battery uses one or more electroactive components deposited as a solid layer. In the zinc-bromine, the cathode terminal contains bromine in a solution whilst the anode terminal is water-based and Zinc metal is plated on it.

As a chemistry, zinc-bromide is one of the cheapest abundant materials in the world. So, you would obtain the same benefits as with VRFB but at a lower cost. Among the companies that provide this solution we can find Redflow, Primus Power, Zinc 8. All of these targeting behind the meter, C&I buildings and utilities.

Iron-Salt Flow Batteries

The Iron-Salt Flow Batteries are another type of hybrid flow batteries. The technology utilizes abundant and non-toxic iron-chloride as electrolyte as a low-cost solution.

ESS is one of the oldest companies in the long duration and flow battery space, having developed a fully commercialized system. The company doesn't publicly state any of its customers, but does claim to have shipped systems across four continents. The company claims its iron flow battery technology offers the lowest LCOS and up to 12 hours of storage for renewable shifting. Its low cost is enabled by the use of abundant, environmentally-safe iron-based electrolyte.

Other Electrochemical Solutions at Rise

Zinc-bromine Gel Battery

The Zinc-bromine gel battery is an evolution of the Zinc-bromine flow battery, as it has replaced the liquid with a gel. The battery is more efficient as the gel enables the ions to transport quicker. This increases the battery life, decreases the charging time, and the gel enables the battery to be portable, unlike typical Zinc-bromine flow batteries.

Gelion developed this technology and the company is focused on entering the market as an innovative battery alternative for primary energy generation. Flow batteries require space and are expensive, impractical for mass production, Gelion reinvented the zinc-bromine chemistry to make the batteries non-flow and facilitate mass production.

Gelion claims it can deliver slow steady discharge or fast capacity discharge depending on the thickness of gel and battery architecture. This allows Gelion to customize the batteries for different solutions.

Figure 3. Zinc Bromine Gel Batteries. Imaged sourced from Gelion

Conducting Polythene

PolyJoule, founded in 2011, has developed a non-lithium form of energy storage that is built purposely for the electricity grid. Over 2020 the company has scaled up and produced 7000 batteries and a full 10kWh system.

PolyJoule is the only vendor using this technology with conducting polythene material, an organic material derived from petrochemicals. By 2021, PolyJoule's batteries are undergoing UL approval for safety certifications and to be allowed indoors and on airplanes.

The company claims their cells can respond to both base loads and peak loads in microseconds, allowing the same battery system to participate in multiple power markets and deployment use cases. By using polythene materials and not involving components like lithium or cobalt in the process. Mining related problems and are avoid and environmental impact reduced.

Antimonium & Calcium

Ambri, founded in 2010, uses calcium and antimonium, a brand-new technology that has not been used in previous solutions. It is classified as long duration technology (4 to 12-hour periods), with little degradation and a with a safe chemistry.

What is interesting about their solution is that don't have any voltage or current until installed in site and energize the electrical heaters. during commissioning. So, they are safe to transport. Besides, they operate at any climatic conditions, from -100°C to 100°C and do not need ventilation.

The company's target market is 10MWh projects or higher. Ambri states that their batteries will cost less at the CAPEX level than Li-ion and then LCOE too due to OPEX savings as well. First pilot systems will be available this year.

Conclusions and Considerations

These new battery chemistries provide solutions that are safer, don’t use toxic materials and have lower environmental impact. The absence of lithium and flammable solvents also means a positive impact on manufacturing and operating costs.

The latter non-flow battery technologies are on development phase. Iron-salt and Zinc-bromine (hybrid flow batteries) are the cheapest alternative chemistry but are not as efficient as VRFB. VRFB use one metal only, so there is no cross contamination of the electrolyte and for that reason, VRFB have unparalleled cycle lives (15,000–20,000 cycles), which in turns results in lower levelized cost of energy (LCOE) and round-trip efficiency. Besides, without cross-crossing contamination the electrolyte can be reused in a new battery.

With Li-ion’s capacity decay, further investments and scaling are needed and the way that cost increases is linear (cost per kWh doesn’t change). This doesn’t happen with flow batteries as they have no degradation; the cost of the stack is a fixed cost, but the more energy duration desired, the more electrolyte you will add. So, as you extend the duration of the VFB the cost per kWh as LCOE goes down.

Hence, when size and volume are not an issue and long-duration storage is required, flow batteries compete against Li-ion.

Still, for any of the presented solutions, energy density is not as high as Li-ion batteries and these are still the economical battery choice regarding CAPEX. If their cost reduction prevails, the cost difference may overwhelm the operational advantages of flow batteries.

Some utility companies are applying to their projects a hybrid solution of Li-ion and flow batteries to benefit from both technologies, using flow batteries for core load and Li-ion batteries for peak. In this way, they are reducing risk of deploying one particular storage solution and will be able to update the system in a few years when Li-ion batteries start decaying and replace the missing storage capacity with a scaling of the system with the technology that results more convenient.

References

Jack Boutchard. Compare Flow Batteries- How do they work? [Online]. Solar Choice. Published: 05/20/2020. Accessed: 03/24/2021. Link.