Australian firm Redflow is drawing cautious praise for its attempts to establish flow batteries as an alternative to lithium-ion in the residential energy storage sector.

“Their idea is certainly interesting, as they're the only flow battery company making a concerted push into the residential segment,” said Brett Simon, an analyst at GTM Research.

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“Redflow has stated that its systems can achieve 100 percent depth-of-discharge, meaning on a per-kilowatt-hour basis, they are cost-competitive with other market players," said Simon.

Ron Van Dell, CEO of U.S. flow battery maker ViZn Energy, also said the technology should be able to beat lithium-ion for residential applications. “Flow batteries typically last for 20 years without aggressive degradation,” he said. “We’re seeing lithium-ion warranties that show nearly 50 percent degradation over a five- to 10-year period. Customers want 20-year lifetimes to match theirsolarsystem’s expected useful life.”

Redflow unveiled its 5-kilowatt, 10-kilowatt-hour ZCell home battery in June. The company says the fully installed cost of the ZCell, including inverter and sales tax, should be between AUD $17,500 and $19,500 (USD $13,000 and $14,500).

The product was due to start shipping to Australian homes at the end of June.

In the meantime, Redflow has confirmed the ZCell would work with solar hybrid inverters from Redback Technologies, another Australian equipment manufacturer, as well as those from Dutch firm Victron Energy.

The ZCell uses a zinc-bromide electrolyte and is about 80 percent smaller than a standard flow battery. Redflow marketing manager Sciobhan Leahy said the technology allows for faster charging than lithium-ion because of its linear charge and discharge voltages.

And it’s cheaper, she said: “We emphasize that our flow battery offers compelling advantages over lithium-ion-based batteries, including lower lifetime cost per kilowatt-hour through sustained storage of 10 kilowatt-hours for the 10-year warranted lifetime of the battery.”

According a battery storage comparison by Australian website SolarQuotes, the ZCell beats the Tesla Powerwall (and, indeed, all other residential batteries on the market) on a cost-per-total-warranted-kilowatt-hour basis, at AUD $0.42 compared to AUD $0.50. 

Redflow Executive Chairman Simon Hackett is pushing for greater support for residential energy storage. “We think any proposal to advance the deployment of battery energy storage at this time is a good thing,” he said. 

At the same time, however, Redflow is keen to push a number of other benefits of flow battery technology, such as the ability to stay at a zero state of charge without accelerated aging. This should help extend flow battery life spans, which could be a significant selling point for the technology, Van Dell said.

“Ten-year lithium-ion warranties with aggressive degradation should open up opportunities for alternatives,” he said.

Another potential benefit of flow batteries is safety. Unlike lithium-ion products, which have been known to catch fire, the ZCell contains a fire-retardant electrolyte.  

And although some experts have voiced concerns over Redflow’s use of bromine, a corrosive and toxic substance, as an electrolyte, Leahy said safety issues had been addressed in the ZCell’s design.

“Our batteries utilize a sealed containment system at the base, and built-in detection devices mean the system will shut down its pumps and halt the flow of electrolyte in the unlikely event of a leak,” she said. “In addition, our residential product uses a new outdoor enclosure with an integrated secondary spill containment system.”

The next few months will be crucial in demonstrating whether Redflow’s ZCell can live up to its promise.

How much longer the ZCell can maintain its price competitiveness is open to question. In the U.S., for example, Stem’s Chief Technology Officer Larsh Johnson revealed that battery costs have fallen by 70 percent in the last 18 months.

“It remains to be seen how actual system performance and pricing stack up compared to other market players' products,” cautioned GTM Research’s Simon. “Flow batteries in general still need to prove commercial bankability.”