There’s no shortage of potential solutions to the world’s critical need for renewable energy storage. But there is a shortage of accessible and cheap resources to use for those solutions. With the limitations of materials like lithium and vanadium in mind, it’s logical to look for alternatives that are basically limitless. How about regular, run-of-the-mill salt?
Redox flow batteries, or RFBs, can exploit the abundance of elements like sodium and iron. One U.S. company already has saltwater batteries ready to go, with at least two others developing iron-flow variations built to effectively run on rust. They promise to last longer and be far cheaper than the competition.
So, what happens if we go with the flow?
Before we get into why regular old salt may be a winning ingredient to our energy storage needs, it’s important to understand what it’s up against. Two major problems hold back renewable energy: intermittency and curtailment. You could sum it up as it being an issue of feast or famine. We can’t rely on the forces of sunshine and wind 24/7. At the same time, without proper infrastructure, we lose out on a lot of power when supply overtakes demand.1 To complete the renewable energy transition and cheapen energy for everyone, we need long-term storage for excess power.
That’s where grid storage systems come into play. Lithium-ion batteries are a popular, powerful option, but they can be unstable, both in the cost of materials and on a chemical level. It’s just not feasible to rely on lithium-ion as our primary storage option. By comparison, flow batteries are a lot cheaper, and some chemistries can lower or even eliminate the risk of fire. Ultimately, each approach serves its own purpose. Generally speaking, lithium-ion batteries are light and compact. Flow batteries tend to be bulky. That said, they’re inherently scalable in ways that li-ion isn’t.
We’ve covered flow batteries before, both vanadium and bromine. But if “RFB” sounds more like a music genre than a battery to you, here’s a quick rundown of how they work. A redox flow battery, or RFB, takes the form of two tanks: a catholyte and an anolyte. These tanks surround a chamber that’s split down the middle, and the liquid electrolytes flow from the tanks into either side. The process of reduction-oxidation, AKA redox, sends electrons from the catholyte to the anolyte when the battery is charged. The reverse happens when the battery is discharged.
When it comes to design, a major edge that RFBs offer is the direct control over their energy and storage capacities. As long as you have enough electrolyte, you can theoretically increase the storage as much as you want by increasing the size of the tanks. And if you increase the number of electrode cell stacks, or the middle bit between the tanks, that means more power.2
There’s an overwhelming number of variations in RFB electrolyte, but vanadium is currently the gold (or maybe the rainbow?) standard. It has the capacity to exist in multiple oxidation states, which allows vanadium RFBs to function with fewer electroactive components. As a result of how the redox reaction works out, VRFBs last longer relative to other chemistries. For a more detailed look at the science behind this, you can check out our previous video.
Here’s where vanadium, like most things, runs into trouble: costs. The element is notorious for its price volatility.3 It’s one of the most used materials in the industrial sector, like alloying steel, so vanadium’s beauty and flexibility does not come cheap. According to a 2016 study, the chemicals needed for VRFBs can represent nearly 60% of the overall cost of the system.45 Newer research shows that prices have only gone up. A 2021 study by the University of California, Irvine found that electrolyte using vanadium pentoxide accounted for a whopping 80% of a VRFB’s total cost.6
Here’s where things get salty. U.S.-based company Infinity Turbine LLC has proposed its Salgenx flow battery as an answer to this problem. It functions the same way any other RFB does, except one of its tanks contains chlorine gas dissolved into a proprietary electrolyte, and the other is full of nothing but good ol’ NaCl and H2O.7 In the company’s words, it’s “just add water”…but with salt.8 The source could be the ocean, geothermal brine, power plant cooling ponds, or even a home air conditioner.910
As you might expect, taking advantage of the ubiquity of saltwater means saving a lot of money. A 2022 U.S. Department of Energy study estimates that depending on the acid and grade of vanadium used, VFRB electrolyte can cost between $105 and $180 per kWh.11 Meanwhile, Salgenx claims that the cost of its batteries’ electrolytes is less than $5 per kWh.12
As for how the Salgenx battery stacks up to existing products on the market, Infinity Turbine claims that its “less expensive to acquire and faster to deploy” than the Tesla Megapack, but notes that the Megapack involves less moving parts.8
But the Salgenx battery does have a simplified configuration relative to other flow batteries. Unlike most types of RFBs, the Salgenx battery has no membrane separating the electrode. Instead they take advantage of the natural immiscibility of the electrolyte fluids to keep the components separated. Think oil and water. On its website, Infinity Turbine argues that the Salgenx’s lack of a membrane “saves huge upfront purchase costs, maintenance, and consumable expenses.”8 The idea does hold water. That’s because flow batteries typically rely on ion-permeable membranes to limit what’s known as “crossover”, which can tank the battery’s capacity. These membranes also play a big role in the voltage and energy efficiency of the RFB. Striking this delicate balance is pricey.13
To put these costs into perspective, a 2014 study found that these ion-exchange membranes were undoubtedly an expensive component of flow batteries, with their calculated price coming out to several times more than the batteries’ plates and electrodes. They estimated that the future cost of the membranes would drop significantly, but the cost range was still much higher relative to the rest of the battery parts.14
Economics aren’t the only thing the Salgenx battery has going for it. Infinity Turbine’s website lists saltwater flow batteries’ round-trip efficiency at 91%, which is impressive and on par with many lithium ion battery chemistries.1115
The company also advertises an energy density of 125.7 Wh/L.7 It’s important to note, though, how that’s markedly less than the average energy density of lithium-based batteries, regardless of what kind. Figures from the U.S. National Renewable Energy Laboratory (NREL), between lithium ion phosphate, lithium nickel cobalt aluminum, and lithium nickel manganese cobalt, you’re looking at energy densities from at least 210 Wh/L to as high as 600 Wh/L.16
However, Infinity Turbine has unique applications in store. In March, the company revealed its ongoing development of a desalination system that uses the Salgenx battery. With this tech, the same movement of ions between the electrodes that generates power would also produce freshwater at the same time. This has big implications for places like cruise liners, cargo ships, and military bases.177 And just this month, the company announced that the Salgenx redox process can also be adapted to produce graphene. The fiberglass and carbon fiber fabrication industry, for example, could essentially use a Salgenx battery as both a source of power and a graphene-making machine. Infinity Turbine claims the savings would be serious, cutting the cost of graphene from an average of $100 to $400 a gram to $1.25 a gram.18
Another American company is also experimenting with saltwater RFBs ー and if you’ve seen our previous video on floating solar, it’s doing so in a very familiar place. In February, ESS Inc., an iron salt battery manufacturer, announced its collaboration with the Turlock Irrigation District, a California-based utility. As part of Project Nexus, the District’s initiative to install solar panels over the state’s irrigation canals, ESS’ Energy Warehouse batteries will provide long-duration energy storage. The plan is to finish construction in 2024.19 ESS will also be installing iron flow battery facilities in Sacramento, California, as part of a collaboration with the Sacramento Municipal Utility District starting this year.20
What makes ESS’ Energy Warehouse so special? Well, like Salgenx’s battery, it operates with a salt water electrolyte, but it’s using iron salts in water instead. This is particularly advantageous because iron is at least two orders of magnitude more bountiful than vanadium. We’re talking a crustal abundance of about 52,157 ppm compared with…138 ppm. On top of that, vanadium is more difficult to substitute … and remember: We really like it for our steel.2122
In contrast, the ferrous chloride (FeCl2) present in the Energy Warehouse electrolyte can actually be produced from steelmaking waste.23 The chemical cocktail in iron flow batteries has the_potential_ to make use of the dregs left behind in what steelmakers call “pickle liquor”2423 … which sounds like something you drink while playing pickle ball. So, ironically, what makes vanadium expensive and difficult to obtain is exactly why iron can be so easy to use in RFBs. Where vanadium electrolyte can represent as much as 80% of a flow battery’s cost, iron electrolyte makes up only about 4%.6 Iron flow batteries are also non-toxic, which is obviously helpful in light of the high toxicity of vanadium oxides.6
ESS claims that its Energy Warehouse “reduces or eliminates the need for hazmat permits for transport, HVAC, fire suppression and end of life disposal planning.”25 This is consistent with the conveniences of other flow batteries that use water-based electrolytes, like zinc-bromine. Just like other RFBs, saltwater batteries allow for a higher degree of safety in comparison with li-ion. You don’t have to worry about thermal runaway when a battery’s juice isn’t flammable in the first place.
As for longevity, both ESS’ Energy Warehouse and Salgenx’s boast a life expectancy of at least 25 years.1524 The Energy Warehouse also knocks other batteries out of the park in terms of cycle life. Broadly speaking, the number of cycles a lithium-ion battery can last is usually limited to just over 1,000 to 2,000 cycles.16 Zinc-bromine batteries can last up to about 5,000 cycles.11 ESS holds that the Energy Warehouse can last for over 20,000 cycles.24 That’s truly the battery that keeps on going.
Duration is another leg-up that ESS claims over li-ion, which translates into lower costs. The company estimates that past the four-hour storage duration mark, its iron flow batteries remain cheaper than li-ion all the way up to their maximum of 12 hours.26
When you take all of this into account you can see why a lot of people are excited about the potential for salt and iron in RFBs. Readily available materials that are cheap and safe, which could help to answer the growing demand for energy storage. Not to get salty here, but when someone says to me, “just go (fill in your favorite tech here),” it drives me a little nuts. There’s not one energy storage (or generation) option to rule them all here. We need a wide variety of options to satisfy all the different use cases we have. And this salty one may be a great one to add to the mix … even if it’s just for flavor.
- Reframing Curtailment: Why Too Much of a Good Thing Is Still a Good Thing ↩︎
- 2020 Grid Energy Storage Technology Cost and Performance Assessment ↩︎
- What Retail Investors Need to Know About Vanadium ↩︎
- High-energy and low-cost membrane-free chlorine flow battery ↩︎
- Applications of vanadium in the steel industry ↩︎
- Life Cycle Assessment of Environmental and Human Health Impacts of Flow Battery Energy Storage Production and Use ↩︎
- Salt Water Flow Battery Topics ↩︎
- Salt Water Flow Battery Frequently Asked Questions ↩︎
- Salt Water Flow Battery ↩︎
- Salgenx S3000, the salt water flow battery ↩︎
- 2022 Grid Energy Storage Technology Cost and Performance Assessment ↩︎
- Salgenx Identifies Top 20 Applications for Grid-scale Flow Batteries ↩︎
- Crossover in Membranes for Aqueous Soluble Organic Redox Flow Batteries ↩︎
- Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries ↩︎
- Startup unveils saltwater flow battery for large-scale storage ↩︎
- USAID Grid-Scale Energy Storage Technologies Primer ↩︎
- Revolutionary Desalination System Using Saltwater Flow Battery Cycle Announced ↩︎
- Saltwater flow battery produces graphene while charging ↩︎
- ESS to Deploy Long-Duration Energy Storage Technology with Turlock Irrigation District to Drive Decarbonization and Water Conservation ↩︎
- Accelerating decarbonization, ESS Inc. and SMUD announce agreement for long-duration energy storage solutions ↩︎
- Iron ↩︎
- Vanadium ↩︎
- Process for producing ferrous chloride ↩︎
- Energy Warehouse™: Long-duration energy storage solution for commercial and industrial applications ↩︎
- ESS Energy Warehouse™ ↩︎
- This iron flow battery could power a more renewable grid ↩︎