ZCell home solar battery storage system reviewed


Energy storage has become a growing consideration for Australian householders keen to cut costs while also equipping their residence with a back-up power option, with local firm Redflow one of a number of Aussie companies having made moves in the emerging market this year.

The recent blackout in South Australia brought the issue of back-up power options into focus, with home-owners running home energy storage systems, capable of harvesting and storing energy from solar panels, such as Redflow’s ZCell home battery storage system, having the potential to see out power outages without interruption.

Research firm IHS Technology has forecast that the Australian energy storage market is on track to grow from less than 500 installations at the end of 2015 to more than 5,000 systems this year. This would put Australia in the top five markets for distributed energy storage in 2016, behind the US, Japan, Germany and the UK. Looking further ahead, IHS expects that approximately 30,000 Australian households will have solar photovoltaic energy storage systems by 2018.

The wider energy market is evolving and battery technology is poised to play an increasing role moving forward. As noted via Redflow’s website: “The nature of electricity consumption is changing, creating the need for innovative energy storage solutions.”

The first ZCell systems are now finding their way to the local market. So, what should consumers considering moving to a home battery storage system know about the ZCell?

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How does the Redflow ZCell work?

zcell unit

In the emerging home battery market, amid the various battery technologies available, the Redflow ZCell’s flow technology is a key differentiator. As described via Redflow’s ZCell website, the “ZCell is a storage system built around a unique zinc-bromine flow battery” (the Redflow ZBM2), with it capable of delivering 10 kilowatt hours (kWh) of stored energy daily. It’s not bad to look at, pictured courtesy of the Redflow website.

The battery technology employed by the ZCell is different to that employed by many other products in the home battery market, with a number of lithium-ion battery products, in particular, currently available. Redflow describes the ZCell as “a new sort of battery without the overhead or degradation problems that can impact the performance of lithium-based or lead-acid batteries”.

“Each ZCell contains about 100 litres of a water-based zinc bromide salt solution circulating in two separate hydraulic circuits,” Redflow advises. “When charging, zinc is extracted from the zinc bromide solution and stored on a plastic membrane. During discharge, zinc is released back to the solution. In short, energy is stored by moving zinc around inside the battery.

“The key to the long life and 100 per cent depth-of-discharge capabilities of ZCell is that this process is completely reversible, and that there is no damage to the battery when the zinc is completely removed from the plastic membrane at the end of each discharge cycle.”

Redflow states that the benefits of the ZCell when compared to other batteries include:

  • 100 per cent cycle depth, supporting daily delivery of 10 kWh of energy at a 100 per cent depth of discharge with no risk of battery damage
  • Expectation of retaining the full output capacity over the battery’s useful life, compared to large loss of output capacity with age being normal for most other battery types
  • No fire risk, with the bromide-based electrolyte a natural fire retardant, while mechanical damage does not risk explosion or “thermal runaway”
  • Robust high-temperature operation without active cooling
  • Essentially unlimited shelf life, with it capable of being switched off at any state of charge and hibernated for storage
  • Made with major components that are easy to recycle or reuse

The ZCell system

Aside from the ZBM2 flow battery, the ZCell system incorporates a number of additional components. A battery controller box is designed to run key aspects of the battery, from operation through to monitoring and maintenance, with it also maintaining statistical data.

The controller also watches for a variety of potential system faults and responds to them, with it taking automatic action to protect the battery, if required, automatically resuming normal operation later, if appropriate. Additionally, it communicates with ZCell’s Battery Management System (BMS), which Redflow describes as acting “as the glue between one or more underlying batteries, and an energy system using those batteries”.

“The BMS is a small device that connects in between one or more ZCell batteries and the inverter/charger that your installer provides for your installation,” Redflow advises.

“It provides a way to set up, monitor and control your ZCell battery using a web browser. It also provides battery status and operational information to your AC inverter/charger so that it can optimise its use of the electrical energy stored in your ZCell.”

BMS features include:

  • Commissioning of one or more ZCells via web browser (Wi-Fi or Ethernet connection, smartphone or PC)
  • Fully manages and schedules internal maintenance processes (configurable strategies)
  • Presents a “virtual battery” interface to inverter/charger from one or more ZCells on site
  • Multiple physical layer interfaces, designed for easy inverter/charger integration
  • System performance status display and performance graphs
  • Remote data logging and diagnostic functions (cloud-based, with internet access required)
  • In-field software updates

Should the BMS be unavailable or disconnected, the battery controller will continue to internally manage and protect the underlying battery module autonomously.

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The ZCell is designed for outdoors installation, against a wall and on a solid flat surface, with the associated AC inverter/charger mounted nearby on the wall, interfaced into home power cabling.

Its custom-designed outdoor enclosure measures in at around 1,000 mm long, 500 mm wide and 1,150 mm high, with it weighing around 290 kg. It is also possible to install the ZCell indoors, such as in a garage, with it requiring a reasonably well-ventilated spot that isn’t prone to getting unreasonably hot.

A cooling fan pulls air in from one end of the container, exhausting it out of the other end, when the ZCell gets warm, Redflow advises.

“When placed indoors, our recommendation is to attach a duct from the outflow side of the container to the outside, much as you might do for a clothes dryer, so that warm/exhaust air can leave the room during fan-driven cooling periods,” Redflow states.

“There are no gas emissions from the unit under normal circumstances. However, there are rare circumstances where the unit could emit a ‘whif’ of bromine gas for a short period. This is the reason why we specify (for indoor installations) that the outflow air path is vented to the outside.”

A ZCell installer will assist in choosing an appropriate AC inverter/charger brand and specification, and will also include any additional items, such as solar panels and energy flow monitoring devices, for additional installation.

“A full system includes your ZCell (including outdoor enclosure and BMS), a suitable AC battery inverter/charger, other control and measurement gear, plus mounting and wiring, commissioning, and so on,” Redflow advises.

It should also be noted that it is possible to add more than one ZCell, either during the initial installation or at a later date.

How much does the Redflow ZCell cost?

As advised by Redflow, cost will vary depending on householder needs and requirements, with the total system cost to be determined by the installer. Redflow has previously advised that the expected fully installed cost of the system will start from $17,500 – $19,500, including GST.

The ZCell product warranty against defective materials or workmanship is for 10 years or 36,500 kWh of delivered energy (whichever comes first) for the ZBM2 electrode stack, and three years on the balance of system components (including electrical and electronic components and enclosure).

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