Redflow Zcell battery review

Redflow has launched it’s ZCell battery in Australia. With impressively little fanfare, the product that started at the University of Qld in Brisbane, launched on the ASX in the end of March. A month later, at the Australian Solar Council’s ‘Solar Expo’ Redflow was near the top of my to-do list. Unfortunately, I wasn’t the only solar nerd that was keen to check out what would have been an uninspiring display, had it not been for a weird looking plastic tank sitting on the ground and an informative bloke called Andrew Kempster.

I soon realised that understanding this new method of energy storage and the company behind it wouldn’t happen by throwing a bunch of questions at a bloke while he was mobbed by lads hungry for more. I swapped cards with Andrew, and when he was in Brisbane last week, he invited me out to Redflow’s headquarters in Seventeen Mile Rocks.

It’s a stone’s throw from UQ’s “grow your ideas” iLab, the scenic and inspiring location where Redflow began – but the feel couldn’t be more different. At dusk, you’d more likely hear a bogan’s burnout than a kookaburra’s call. It’s got that good ol’ run down industrial estate vibe happening. The dodgy car wrecker across the road compliments the drums of zinc bromide scattered around the driveway.  This place is not about show: it’s about getting things done. And that’s what Redflow have been doing.

So what’s all the fuss about Redflow?

Why did every salesman and his sparkie flock to the Redflow stand? It’s not because of marketing hype – I’ll explain later how they have a lot of work to do on marketing. It’s not because Redflow has a huge and trusted brand like LG. It’s not because they have a charismatic chairman like Elon Musk. It’s much more exciting than that. Redflow has redeveloped home energy storage. They called it the ZCell.

The ZCell battery works by pumping a liquid electrolyte (water and zinc bromide) between a storage tank and a reaction chamber, where the electrodes are stacked. The charge cycle entails coating the electrodes with zinc bromide, and the discharge cycle entails removing the zinc bromide from the electrodes. Flow batteries are a well-understood class of energy storage, but it’s new in the domestic market.  Redflow’s plan is disruptive.

This post will start by explaining the “wow” factor of Redflow. I’ll then balance the praise with red flags that have been raised in my mind, and question marks that still hang over Redflow’s ZCell battery. 

The Redflow wow factor

Redflow is solving problems. The foundations of what they demeaningly call “legacy battery technology” was thrown out the window along with the serious downfalls of lead acid and lithium batteries. There are two massive problems that Redflow has solved for the energy storage market. The first is battery degradation, and the second is battery safety.

Degradation

The problem with batteries has always been that they work great out of the box, but discharge and charge them a bunch of times and they will gradually lose their capacity to store power. For example, let’s look at the battery that I am trialing in my office; the LG Chem. Don’t get me wrong; the LG Chem is just about as good as a lithium battery gets. But the warranty states:

LGC warrants and represents that the Subject of the Warranty retains at least 80% of Nominal Energy 6.4kWh for 7 years after the date of invoice and at least 60% of Nominal Energy 6.4kWh for 10 years after under proper conditions of the use during the Term of Performance Warranty.

So by the time the 10-year warranty is up, your 6.4kWh LG Chem battery is only a 3.8kWh battery – that’s if you don’t work it too hard. The faster you charge and discharge a battery, the depth of charge that you go down to, and the more often you cycle the battery, the quicker it will degrade.

The Redflow ZCell battery allows a 100 percent discharge. It can be cycled 3000 times at its full capacity of 10kWh. It will take a healthy 3kW of charge and discharge and peak at 5kW. On it’s 30 000th cycle (after 8.2 years if it’s once a day) it will still have a 10kWh discharge capacity. Unlike lead acid and lithium batteries, the ZCell battery capacity is apparently not affected by degradation.

Safety

Neither lithium nor lead acid are an intrinsically safe form of energy storage. Lithium battery manufacturers are all too aware of the potential risks. Just last week I was talking with Andy from Sungrow about our beta testing on their up and coming SH5K EPS inverter, I was pushing Andy of the release date of a Sungrow battery.  “Sungrow does not want to be Growatt, Mark” Andy advised me, referring to the Growwatt home storage battery that caught on fire in a residential solar storage setup in Victoria. Responsible manufacturers like Sungrow will go to extreme lengths to make their lithium-ion home storage solutions as safe as possible, but only so much can be done. A lithium-ion battery will always be a hazard in a house fire. Lithium ion batteries will always be at risk of explosion if a truck runs over it. However, no doubt, when the price war begins with home storage solutions, unscrupulous manufacturers will be selling lithium batteries that could otherwise be described as time bombs.

The Redflow ZCell Battery, on the other hand, is inherently safe. The battery can shut down in a few seconds by isolating the power supply to the flow pumps. Since no reactants are supplied to the electrodes when the pumps are shut down, electrical energy cannot be transferred. In just a few seconds, the Redflow battery becomes no more than a tank of coloured water. The colouring, Zinc Bromide, is itself a natural fire retardant. If the battery was to be run over by a truck, it wouldn’t explode. Unlike lithium batteries, there will be no dangerously high short circuit currents or thermal runaway. If fire is your concern, then Redflow ZCell is your energy storage friend.

However, the battery isn’t made of daisies and roses. Zinc bromide can cause some nasty rashes. You wouldn’t want your dog to lap it up if the tub sprung a leak. Andrew Kempster explained that the battery can be installed inside, but you would want the room to be well ventilated. If you walked into a unventilated room that had a ZCell battery running for a few days, you’d get an awful whiff similar to the smell of a lead acid battery charging. While he said that smell would dissipate quickly, I think I’d be installing it outside.

Redflow red flags

Andrew tells me that the 10kWh battery has a 30 000 kWh energy throughput hour warranty. The whole battery – pump, tank and all – is warranted for that. However, the warranty document he sent me before I met with him in Brisbane opens with the statement:

Twelve Months Defects Guarantee

Redflow guarantees to the Customer that the ZBM product which is installed, used and serviced in

accordance with the relevant product information will be free from defects in material and

workmanship for a period of twelve months from the date of delivery to the Customer (Defects Guarantee).

The same warranty document speaks of a performance warranty of 30 000 kWh throughput power. (So long as it hasn’t fallen apart under the “defect guarantee”.) However, the 30 000 figure is contradicted in the other document Andrew emailed me called “Understanding the Redflow Battery Whitepaper” dated 2015, which states:

The roundtrip efficiency of the battery is above 80% and the life is warranted for a minimum of 1000 cycles at 100% depth of discharge (DoD) for a total of approximately 8MWh of energy throughput.

Again Andrew assured me this would be changed. It would make sense that the January 2016 warranty document is most accurate. By the time you go to buy it, you would want to know what the current document says, and not rely on hear-say – or this blog for that matter.

Efficiency

There was much confusion around efficiency. Probably because I couldn’t articulate my question correctly to Andrew. Here it is concisely:

1. The battery runs at 80 percent efficiency, and it stores 10kWh. Does that mean:
   a) We put 10kW in and get 8kW out or
   b) We put 12.5kW in and get 10kW out?
2. Are both the input and output figures guaranteed to be the same after 30 000kWh?

We thrashed through the process of efficiencies of the charge and discharge cycle, but it seemed we just confused each other. I was interpreting what Andrew was saying to mean the efficiency of the inverter was in reality much less than 80 percent, more like 65 percent. After a tour of the testing facility, I drove away a bit jaded thinking the shine had come off the Redflow. But just as I was pondering it  I received a call back from Andrew, who had spoken with his engineers to clear things up.

The clarified answer

In short the answer was: The amount of energy required to charge the ZCell will increase slightly over time. But the storage capacity and the usable energy will always be 10Kwh. So in year 1, if you put 12.5kw into the battery you will get 10kW out of it. By year 10, you’ll need to put more like 14kW into it to get 10Kwh out of it. Call me skeptical, but I’ll need to see that in a warranty document before I’m convinced.

Charge and discharge rates

The 2015 white paper also states:

Without any ageing effect, it can be charged and discharged completely at rates up to 3kW.

The significance of the Red Flags

It’s disappointing. I wish every duck was lined up, and all of Redflow’s marketing material spoke with one united voice. But how realistic is that? GCL is a perfect example of a warranty being absolute trash until I called them out in this post.  And even the industry’s benchmark battery, the Tesla Powerwall has had significant changes to its warranty. The latest Tesla Powerwall document is so much more generous than the first. If we compare the technical and marketing budget of Tesla and Redflow, it’s probably unfair to hold Redflow to a higher standard. However if you plan on buying a Redflow battery, be sure you have the up to date warranty paperwork – and don’t go on hearsay of what the product can or cannot do. I’ll keep updating this post as more reliable information filters it’s way down to me.

Redflow question marks

Noise

I asked Andrew about the noise levels because I couldn’t find it anywhere in the specs. He gave me an indication that it ran a little louder than a fridge, so more than 40 decibels.

Additional costs

The retail cost of the battery itself is competitive. The cost of transportation, however, will have to be taken into account. It is two-hundred-and-forty-freaking-kilograms. And I thought the Tesla Powerwall was heavy. Redflow will offer a service where they transport the ZCell to the customer’s home, safely crane it off the truck, and move it into position. Obviously, this will come at a cost. Being 240kg, you’re obviously not going to be hanging this sucker on the garage wall. You’ll need to arrange a solid slab of concrete in an appropriate location outside.

Scrubbing cycle

This is not a huge one, but should be taken into account. The Redflow battery works by coating the membrane with Zink Bromide on the charge cycle and then removing the zinc bromide on the discharge cycle. However, on the discharge cycle, not every piece of zinc bromide is removed. Every 96 hours of pump run time the battery is programmed to be taken out of service and put through what the call a “scrubbing cycle” to scrub all remaining zinc bromide off the membranes. In this cycle, the battery cannot be charged or discharged.

96 hours of run time does not mean every 96 hours – it is based on the battery operation. If your battery fully charges in 6 hours during the day, then only discharges once at night over say 5 hours, then it will only be used for 11 hours a day. In this case, a scrub cycle would be once every 8.7 days.

Conclusion

The Redflow ZCell battery is a disruptive technology that is capturing the attention of the solar market. The fact that the battery does not degrade is a huge advantage, and the significant safety value is another. However, as I read the ZCell warranty and the Redflow white paper, concerns were raised over the warranty, the efficiency, and the charge and discharge rates of the battery. As long as it has a 12-month warranty, we won’t be installing them. There are other minor concerns that are worth noting like it’s weight, noise, and downtime. However, the Redflow battery is a significant contribution to the energy storage future that the world is madly trying to map out.

Mark Cavanagh