The Australian company Redflow is accepting pre-orders for its new home energy storage system, the ZCell battery. It has a 10 kilowatt-hour usable storage capacity, can provide 3 kilowatts of continuous power, and is suitable for on-grid and off-grid use. Deliveries will begin in August 2016.
In this Z-Cell battery review I go deep into the zinc bromide technology’s pros and cons compared to its main challenger: lithium ion batteries.
While there are many lithium-ion storage systems on the market, the Redflow ZCell stands out as the only zinc bromide flow battery. Its unique chemistry gives it several advantages over lithium-ion systems. One is it can lie dormant for long periods at any level of charge without suffering from deterioration. And another is its storage capacity will never decline over its over its entire lifespan.
Multiple ZCell batteries can be installed to provide additional power and storage capacity. It is designed for outdoor installation and weighs 240 kilograms. It will come with a comprehensive product warranty of 10 years or 30,000 kilowatt-hours, whichever comes first. If fully cycled once each day it will take 8.2 years to reach the end of its warranty. As most households are unlikely to use it at full capacity, many people could buy a ZCell and be confident they won’t be out of pocket if a fault develops for 10 years.
The ZCell’s Interesting Characteristics
|Nominal Storage Capacity||10 kilowatt-hours|
|Usable Storage Capacity||10 kilowatt-hours|
|Continuous Power||3.0 kilowatts|
|Installation Location||Preferably Outside|
|Electrolyte Operating Temperature||15 to 50 Celsius|
|Warranty||30000 kilowatt-hours or 10 years|
|Battery Chemistry||Zinc Bromide Flow Battery|
|Compatible Inverters||To Be Announced|
Is A Zinc Bromide Battery As Good As You Zinc, Bro?
The chemistry of the ZCell battery is very interesting. It is, at its heart, a reversible electroplating machine. When the ZCell is charged, electrical current is sent through the battery which causes zinc to be removed from zinc bromide and electroplated onto plastic electrodes. When the battery is discharged, the opposite happens and zinc is removed from the electrodes and attached to bromine to make zinc bromide and an electrical current is produced.
So, yes, I’ve pretty much said you can run a battery off cheap jewellery and galvanized iron.
The chemical process is completely reversible, which means the ZCell battery’s energy storage capacity never declines. Unfortunately, it will stop working at some point, but, fingers crossed, not for a long time.
Are Zinc Bromide Batteries Safe?
Is zinc bromide safe to handle? Hell no! It’s toxic. If you drink that stuff, you’ll die. However, you will be safe if you remember to follow this one simple safety procedure: Don’t drink zinc bromide. If you can manage to do that, you should be fine.
There are about 125 liters of reddish hued zinc bromide solution, at 20% concentration, that weighs around 150 kilograms in the ZCell battery. And if I remember my high school chemistry correctly, and yes, I am so lazy I would rather try to remember something I learned a zillion years ago than actually bother to type something into Google, the more dilute it is, the less dangerous it is. And so the fact that it is mostly water is great news if you decide to splash it on like some sort of toxic aftershave. It should really reduce the irritation.
But unless your hobby is shooting rifles at random objects in your backyard, you should never come into contact with the zinc bromide solution. The bromine that is formed when the battery is charged is stored as an oil inside a thick polyethylene tank inside another thick polyethylene tank. The zinc bromide is kept in the outer tank. And the entire battery sits in a metal enclosure which has another tank designed to catch any fluid in the unlikely event that leakage does occur.
Zinc bromide solution doesn’t evaporate easily, so if it is spilled it will just tend to sit there. But it is likely to give off a small amount of bromine gas and this gas could also be potentially released directly from the battery. However, this gas comes with its own built in safety system. Bromine comes from the Greek word brómos, which means stench. So if there is leaking bromine gas you will know about it and you won’t want to hang around.
Zinc bromide is not a teratogen. This means two things. Firstly, it is not a monster from Doctor Who. And secondly, it does not cause birth defects. In other cheerful news, it does not cause cancer. Or at least it doesn’t cause more cancer than stuff we normally don’t consider to cause cancer such as vegemite or wood.
I could go on about its health effects and exactly how much zinc bromide solution you can drink before you need to call an undertaker, but I don’t think that would really be fair on the flow battery. After all, I don’t tell people not to break open their lithium-ion batteries and eat the insides. And I also don’t warn people not to drink petrol or shower in it. So basically, what you need to know is, the ZCell battery is not bloody likely to emit bromine gas. If it does emit bromine gas you’ll bloody well know about from the stench. If it somehow leaks zinc bromide out of both the battery and its enclosure, you’ll know about it from the literally bloody colour. And I’ll assume you’re not bloody stupid enough to try to lick it up.
Every time something new comes out people have a bad habit of focusing on any potential negatives, while ignoring the fact that everyday activities such as driving a car are far more dangerous than installing a zinc bromide or lithium-ion battery.
We shouldn’t fear the new. Except for babies. Those things are terrifying.
The ZCell Never Declines In Storage Capacity
The ZCell battery can be discharged 100% every single day without suffering any harm at all, which is very different from other battery chemistries on the market. In addition it does not decline in storage capacity over time. It starts off with 10 kilowatt-hours on the day its first turned on, and it will have 10 kilowatt-hours on the day it dies. And according to Redflow, for a family that uses it at 80% capacity, that’s likely to be around 14 years later.
In comparison, the Tesla Powerwall is a 6.4 kilowatt-hour battery and only 5.44 kilowatt-hours of that is actually warranted to be usable. But after the first 2 years that drops to 4.6 kilowatt-hours. And 3 years later it drops down to only 3.8 kilowatt-hours of warranted storage. So after 5 years it would take 3 Tesla Powerwalls to have the same warranted energy capacity as the ZCell.
While the Tesla Powerwall is the poster child of not being able to keep its kilowatt-hours up, every battery that isn’t a flow battery like the Redflow ZCell will decrease in capacity over time.
The ZCell Can Rise From The Dead
At any point when it is charging or discharging the ZCell battery can be put into hibernation mode and be left as it is for as long as you like without suffering any harm. And it will maintain whatever charge it had at that point for potentially decades. This is very different from other battery chemistries which will gradually lose charge if alone and deteriorate once they’re completely drained.
However, the Redflow ZCell can raise from the dead 3 days, 3 months, or 3 years or more later and continue operating as if nothing had happened. Some external power will need to be supplied to get them going, but not much. For an off-grid system a hand crank should be sufficient.
This ability to completely shut down at any time without harm and without losing stored energy makes them ideal for applications where they might only be used occasionally, such as holiday homes or seasonal businesses.
The ZCell Is Very Unlikely To Start A Fire
The good news is bromides are a flame retardant, so unlike lithium-ion batteries, the electrolyte is not flammable. The bad news is if there is a serious fault it is still potentially possible for the battery to give off hydrogen, which is explosive and doesn’t give a damn about flame retardants. However, the chances of the ZCell giving off a dangerous amount of hydrogen when installed in an outside location appears extremely small to me.
If you are wondering where hydrogen can come from in a zinc bromide battery, the electrolyte solution is mostly water which will produce hydrogen when current is passed through it.
The ZCell battery operates with an electrolyte temperature from 15 to 50 degrees Celsius, which is not the same as ambient air temperature. If the temperature of the electrolyte falls out of this range the battery will shut down, but won’t be harmed.
Because the ZCell has a lot of thermal mass, generates heat when charging and discharging, and has a fan to blow off excess heat, I wasn’t sure just what this meant for its operation in different Australian climates, so I called Andrew Kempster, the Sales Director of Redflow, and he assured me that provided the installation was appropriate for the location, that is, not exposed to direct sunlight in the middle of Cunnamulla, then the ZCell was expected to function without problem in places that are hot in summer, such as central Australia, and cold in winter, such as Canberra.
Of course, if the ZCell ever does shut down because of low temperatures, just because I said bromides are a flame retardant does not mean it is okay to set it on fire to get it started again.
The Redflow ZCell Is 100% Recyclable
All components of the ZCell are completely recyclable. One could be pulled apart right now and all the plastic and metal could be fully recycled using currently existing methods. The zinc bromide solution can be cleaned and reused in another ZCell.
Where Is The ZCell Manufactured?
Redflow is an Australian company and they designed, prototyped, and tested their zinc bromide battery technology here. However, because Australia’s mining and agricultural industries export so much, it does make it difficult to manufacture things here. As a result, the ZCell is made overseas, under license, by a Singapore based company called Flex, which carries out production in Mexico.
Flex, or Flextronics, as it is also known, made my original xbox and it still works, so that’s a good sign. But oddly enough, the Redflow website doesn’t say the ZCell battery is made in Mexico. They just say they are made in North America.
For the benefit of those who don’t speak marketing, I will explain that when companies talk about manufacturing locations, Canada means Canada, the United States means the United States, and North America means Mexico. Why they don’t just say Mexico, I don’t know. After all, I think we as a country have moved past the horrors of the Great Cane Toad/Mexican War of ’87. And we’re probably not dumb enough to assume the ZCell is full of tequila just because it comes from Mexico.
The ZCell’s Warranty Is Excellent
The ZCell will come with a comprehensive 30,000 kilowatt-hour or 10 year warranty which will cover any faults it may develop in that time. And the warranty covers 100% of the battery’s capacity for that period, unlike lithium-ion batteries which typically only cover 80% of their original capacity, or 56% in the case of the Tesla Powerwall.
This is extremely impressive. While the ZCell battery only has three moving parts, which are two pumps and a fan, pumps do have a bad reputation for breaking down. However, with proper engineering they can last a long time. My parent’s pool pump has lasted for 15 years. I know this because they keep bringing it up. “What’s that, Ronald? You’re out of work again? You know our pool pump has worked reliably for 15 years and we didn’t even pay to put it through university.” Personally, I’ve never had a pump last that long. All my ex-wives used to complain to me about a lack of reliable pumping.
Expected Operational Life
Redflow expects the typical ZCell to exceed its warranty by one third and provide 40,000 kilowatt-hours of stored electricity in its lifetime. If used at full capacity it would take 11 years to reach that point, and if used at 80% of capacity it would last for reasonably close to 14 years.
Disadvantages Of The ZCell Battery
There are a few disadvantages to the ZCell and zinc bromide batteries in general:
- Their efficiency is lower than lithium-ion batteries.
- Their power output is lower per kilowatt-hour of storage than most but not all lithium-ion batteries.
- A single ZCell is large and heavy.
- They are not well suited for indoor installation.
- They need to fully discharge and shut down for around two hours to purge their electrodes of zinc every week or so.
- Just like lithium-ion battery systems, they are very unlikely to save you money at current prices.
The ZCell’s Efficiency Is Low
The DC to DC roundtrip efficiency of the ZCell battery is around 80%. This means for every kilowatt-hour of electrical energy put into the battery, only 0.8 kilowatt-hours can be taken out. For a zinc bromine battery this is amazingly high. The Redflow company has clearly done a fantastic job of maximizing the efficiency of their ZCell battery. Unfortunately, it is still worse than the efficiency of lithium-ion battery systems. For example, the Tesla Powerwall claims to have an efficiency of 92.5% and the LG Chem RESU 95%.
The 80% figure applies if the ZCell is charged with DC from solar panels. This is called DC coupling and is the most efficient method. However, if it is charged with AC, a situation called AC coupling, the efficiency of the ZCell, or any other battery system, takes an additional hit and the overall efficiency of the ZCell battery can easily drop to 70% or less.
Low Efficiency Is Not A Major Problem
A low round trip efficiency of 80%, or even 70%, is not as bad as it may seem for the economics of a battery system. This is because the feed-in tariffs for solar electricity exported into the grid are now far lower than the cost of grid electricity. If a household only receives a 6 cent feed-in tariff, then with 80% efficient energy storage it will effectively cost them 7.5 cents to store one kilowatt-hour. But if this saves them having to pay 30 cents for one kilowatt-hour of grid electricity in the evening, then it is worthwhile – if the cost of storing that kilowatt-hour comes to less than 22.5 cents. The ZCell won’t store a kilowatt-hour of electricity for less than 22.5 cents, but if it did, it would pay for itself despite having low efficiency.
Its Efficiency Is Likely To Decline Over Time
While the capacity of zinc bromide batteries doesn’t decline over time, their efficiency does, as is the case with all batteries. I don’t know if the decrease in efficiency will be better or worse than a typical lithium-ion battery system, but I have been told that the decline in its efficiency percentage should be a single digit over the course of its lifetime. So probably less than a 1% decline a year.
The ZCell May Be Worse For The Environment Than The Competition
Under the right circumstances it should be possible for home energy storage to reduce greenhouse gas emissions. However, at the moment, when used on grid it is likely to increase greenhouse gas emissions. This is because one kilowatt-hour of solar electricity exported to the grid will reduce fossil fuel generation by around one kilowatt-hour, but storing one kilowatt-hour of solar electricity at 80% efficiency for later use will only reduce fossil fuel generation by around 0.8 kilowatt-hours. And since the efficiency of the ZCell battery is lower than that of lithium-ion systems, it may be worse for global warming.
In the future when our renewable generating capacity has expanded, energy storage will allow us to kick fossil fuels off the grid. But if you are thinking of installing battery storage on-grid, be aware that at this point in time it does not help the environment. As far as keeping the icecaps intact is concerned, you are far better off spending your money on buying your sister a rooftop solar system. Or if you don’t have a sister, buying my sister a rooftop solar system. Alternatively, you could expand your own rooftop solar or invest in energy efficiency measures such as insulation, heat pumps, or a bicycle.
The ZCell’s Power Output Is Low
The ZCell can store 10 kilowatt-hours of electrical energy, but can only release it gradually, so its power is lower than that of most, but not all, lithium-ion battery systems. It can supply power at a continuous rate of 3.0 kilowatts which is less per kilowatt-hour of storage than all lithium-ion systems I am aware of, except for the Enphase AC Battery.
However, it can supply up to 5 kilowatts for a limited period of time. When fully charged and at an optimal temperature, it is capable of providing that much power for over 30 minutes.
If you have two ZCells you’ll have double the available power. Three will be triple, four quadruple, and I’m sure you can work out the rest on your own.
The moderately low power is not a major problem for on-grid use as extra power is available from the grid when needed. It can fully discharge in around 3 hours, which means it can be reasonably effective at using most, or potentially all, of its stored electricity during a peak price period for those on time of use tariffs.
People living off-grid with a single ZCell would have to be careful not to draw too much current in the evening. However there are many people at the moment living off-grid with quite small lead-acid battery systems, so having a single ZCell should be a step up for them.
The ZCell Has Considerable Weight And Size
Redflow describes the ZCell battery as being, “theft resistant”. That’s one way of putting a positive spin on the fact their battery system weighs 240 kilograms. That’s heavier than I am. Even if a determined thief did try to steal it, after they collapsed you could just call the police. Or perhaps an ambulance. Unless of course, the thief is my first wife. She could make off with it, no problems.
The ZCell is one meter long, 50cm wide, and 115cm high. It requires a forklift, crane, or lifting device to put in position. Note that Australian work health and safety regulations prohibit the use of a man called Hodor as a lifting device.
While a single ZCell is very heavy, which complicates installation, it doesn’t compare too badly to some lithium-ion storage systems. After 5 years of use it would take 3 Tesla Powerwalls to provide as much warranted energy storage as the ZCell battery and, since they weigh 100 kilograms each, that comes to considerably more weight than the ZCell.
The Best Location For The ZCell Is Outside
The ZCell is designed to be installed outside, against a wall. It can be installed inside, but when in operation it will make a burbling sound. It has a fan that will make noise when activated, although not nearly as much as some energy storage systems. It is also possible that it will, on rare occasions, emit a very small, non-dangerous but stinky amount of bromine gas. For this reason it is recommended it be attached to a ventilation duct that leads directly outside and definitely not to your sister’s bedroom.
A Purge Is Necessary To Cleanse The System
In order to continue to function properly, every week or so, the ZCell battery will need to fully discharge and then shut down for 2 hours while it purges its electrodes of all zinc.
For people who are on-grid this is a very minor inconvenience and will pass by unnoticed. For people living off-grid it will be more of a problem, although it can be programmed to occur at the least inconvenient time. For systems using multiple ZCells it is not a real problem, as they will purge themselves at different times.
Purging the electrodes of zinc enables the ZCell battery to continue to operate without a reduction in capacity. It also prevents zinc dendrites from forming. While they sound like another Doctor Who monster, they actually look like tiny feathers and are very pretty, but they have to be purged from the system so they don’t grow large enough to puncture electrodes.
The ZCell’s Cost Means It Won’t Save You Money
Whenever I write about energy storage systems, I always seem to find myself repeating that they won’t save you money. And the ZCell battery is no different. However, when cycled once per day on solar electricity, as most energy storage is likely to be used in practice, then looking at the cost per warranted kilowatt-hour stored, the ZCell definitely appears to be a little ahead of the competition. So while the ZCell won’t save you money, it is likely to get you closer to saving money than other systems.
Redflow estimates the full cost of a ZCell installation will range from $17,500 to $19,500. This includes labour and everything that is required, including a compatible inverter. And I appreciate Redflow taking everything into account. All too often optimistic estimates of the cost of battery storage rely on the universe spontaneously generating this sort of stuff.
If we assume the total cost of installing one ZCell is $17,500 and it works flawlessly until it has provided the 40,000 kilowatt-hours of storage Redflow expects it to be good for, then $17,500 divided by 40,000 kilowatt-hours comes to 43.75 cents a kilowatt-hour. If it is used at full capacity over 11 years, then including a 5% cost of capital, its total cost will come to 62 cents a kilowatt-hour.
This does not currently pay for itself anywhere on-grid in Australia, and this includes in Canberra and the City of Adelaide which have subsidies for battery storage. However, while it can’t pay for itself, for many people it is likely to be the modern energy storage system that is closest to being cost effective.
One factor that brings the ZCell closer to paying for itself is days in which it is not used at its full capacity shouldn’t have any effect on its total lifespan, while lithium-ion batteries are always constantly very slowly decaying. So if a family takes a month long holiday, a lithium-ion battery system would be just a little more dead inside when they get back, while a ZCell will be just the same as it was and can start up again in exactly the same state as when they left.
Who Should Buy The Redflow ZCell?
If you are an early adopter who enjoys having the latest technology, you appreciate an excellent warranty that will give you peace of mind for many years, you aren’t bothered by the fact that home energy storage doesn’t pay for itself yet, and you’d like to support an Australian company and Mexican manufacturing, then the ZCell could definitely be for you.
There are also many people living off-grid who are very interested in the ZCell on account of its excellent warranty, expected reliability, and extremely low maintenance compared to lead-acid batteries.