Redflow Energy Storage Solutions is planning to release a residential flow battery that observers believe could compete with lithium-ion batteries.

The Australian zinc-bromide flow battery maker is due to release full details of its 10-kilowatt-hour plug-and-play residential storage system at the end of March.

According to reports in the Australian press, the system is expected to cost between AUD$10,000 and $15,000 (USD$7,130 and $10,700) and will be competitive with the Tesla Powerwall on a cost-per-kilowatt-hour basis.

Redflow managed to halve the cost per kilowatt-hour of its batteries over the course of 2015, the Australian Financial Review reported.

The company is still finalizing the design of the product, which is being manufactured by the U.S. firm Flextronics. Redflow shareholders will get a discount on the first batteries produced.

Redflow declined to provide any further commercial details ahead of the March launch. One advantage the Redflow product could have over Tesla’s lithium-ion Powerwall is that the flow battery can be completely charged and recharged on a daily basis with no degradation in performance.

At the same time, though, Redflow’s choice of electrolyte is potentially problematic, because bromine is corrosive and toxic at room temperature.  

“They had better be very, very, very sure that no bromine will ever leak out and cause problems,” said Michael Aziz, a professor of materials and energy technologies at Harvard, who is working on safe flow batteries for household use.

“Then again,” he added, “Tesla has to be very, very, very sure that their lithium-ion battery won’t ever cause a house to burn down.”

“All of our batteries are extremely safe,” countered Redflow’s marketing manager, Sciobhan Leahy. “Our residential system will be utilizing this same design.”

To avoid bromide vapor poisoning, “Our batteries have a catch can with carbon inside it, which holds and absorbs any bromine gas so it is not released into the atmosphere. Most recent testing of bromine gases outside the battery showed [none] detected,” she said.

In addition, the bromide electrolyte in Redflow’s zinc-bromide flow batteries is a natural fire retardant and is used in some fire extinguishers, Leahy said.

“Our flow battery, due to the separation of the stack and the tank, has no chance of thermal runaway, hence a very low fire danger,” she said, “unlike lithium, which is a fire risk.”

Furthermore, Leahy said, the product’s enclosure design will feature an Ingress Protection rating appropriate for household use.

Another potential stumbling block for Redflow is the size of its batteries.

Flow batteries have much lower energy and power densities than traditional battery chemistries and so require relatively large tanks of electrolyte, which has so far restricted their use to grid-scale or commercial and industrial storage projects.

Redflow’s product, which can be installed indoors or outdoors, is said to be similar in size to an air conditioning unit.

“Flow batteries are a better fit for grid-scale applications,” confirmed Ravi Manghani, GTM Research’s senior analyst for energy storage. “This is mostly due to their larger footprint.”

However, he said: “The residential markets that Redflow is initially targeting are Africa and Australia. In these markets, I’d expect footprint not to be a constraint.”

Although the Australian media has been quick to pit Redflow against Tesla, which started shipping to Australia in February, the real play for the flow batteries might be in providing a “residential alternative to lead-acid batteries,” Manghani said.

These are still widely used for backup applications in developing markets. Aquion Energy, which makes sodium-ion batteries, “is targeting such applications with its residential products,” Manghani noted.

Ron Van Dell, the chief executive of ViZn Energy Systems, another flow battery maker, sees flow batteries as strong competitors to incumbents. "They can provide very valuable longer-duration energy services for homes," he said. “Homes on time-of-use billing can set their storage system to use the lowest-cost energy, and they can do this for longer than they could with lithium-ion.”