Design Challenges with Redflow technology

There are always swings that go with the roundabouts.

This article discusses some challenges that need to be understood when designing a system using ZCell, compared to other battery technologies. 

Not solid-state

A flow battery moves electrolyte fluid around using pumps.

There are moving parts (albeit only two of them - two pumps) and there is fluid in tanks that creates a technical risk of electrolyte spill.

The electrolyte is a zinc-bromide fluid that is far less challenging to handle than, say, battery acid from a lead-acid battery, but nonetheless (and as is appropriate), the ZCell enclosure includes internal 'secondary fluid containment' (the industry term is 'bunding') to catch and hold the fluid in the unlikely event of a leak in the primary HDPE plastic electrolyte tanks. 

Suitable only for stationary energy applications

The use of moving fluid, and the manner in which the battery uses that store and recover energy (by depositing zinc onto plastic electrolyte layers to charge, and recovering the zinc into solution to discharge) depends upon the battery being stationary and level while it is operating.

Hence this battery can not be used in moving vehicles or in situations where the battery will be subjected to changes in level or changes in g-force during when actively charging or discharging energy. 

Not totally silent

As the (very quiet) pumps move fluid through the electrode stack during normal operation, the battery makes a small sound. Its a low volume, low intensity sound - a 'burbling' noise not unlike a garden water feature. Its arguably quite a relaxing sound - and it is also attenuated by the ZCell enclosure.

There is also a fan included in the system that can start up during high temperature / high workload operations, to cool the fluid electrolyte with ambient air drawn in from the outside. This fan has a rated maximum noise level of 55dB when operating at the highest operating speed normally expected in this application. It is, however, variable speed (runs slower when it has less work to do), and it only runs when needed.

The maximum operating speed of the fan can also be limited (as a software-adjustable setting) in scenarios where very low operating noise is a requirement, with the tradeoff being the potential for the battery to be less capable of remaining within its normal thermal operating limits during severe weather. 

An 'energy' battery (not a 'power' battery)

Our battery is a marathon runner, not a sprinter.

The maximum charge/discharge rate of this battery (in kW) is below its nameplate output capacity in kWh (the industry term here is that "C" is less than 1).

In other words it typically takes up to circa 3-4 hours to drain the battery completely from full, depending on the discharge rate being used. 

We actually consider this an advantage, but it is also a key technical differerence compared to lithium batteries.

Lithium chemistry can support the output of peak energy levels in kW (for short periods) at significant multiples of their 'nameplate' storage capacity figure in kWh). 

That said, when Lithium is deployed in stationary applications it typically is configured to be output-rate-limited (in effect, to have a 'C' value held at levels quite similar to ZCell) to try to maximise its operating lifetime in the stationary energy/deep cycling environment. 

Periodic self-maintenance cycles

The battery needs to periodically undertake a 100% discharge and to spend a couple of hours offline doing self-maintenance. This process is the secret to the long life and 100% discharge capability that we have - each time this happens, the battery is returned to service pretty much 'like new!'.

In typical home solar self-consumption scenarios this occurs around once a week (depending on the operating duty cycle of the battery). The worst case (if the battery is operating at 100% duty cycle - meaning that it is charging or discharging at a significant rate on a 24x7 basis) the maintenence cycle occurs once per four days.

Where multiple batteries are installed, these maintenance cycles can be automatically controlled to maximise overall system availability by sequencing maintenance cycles in an appropriate manner (and in a timing regime configured and controlled by the BMS). Even with a single battery, in most applications, the BMS makes the maintenance cycle practically invisible by causing it to trigger at the most appropriate point in the daily usage cycle of the energy system concerned (e.g. at a point where the battery is at or near a state of complete discharge anyway). 

This self-maintenance cycle, its implications, and its management (via the ZCell BMS) is explained and discussed in detail in the training courses that are held for Redflow system integrators. The Redflow BMS provides for simple handling of this cycle via adjustable system configuration settings. These are chosen to advantage (and synchronise in with) the nature and charge/discharge timing of the customer application cycle concerned.

Low temperature operating limit

The battery (currently) has a minimum electrolyte operating temperature of 10-15 degrees Celcius (10 during charge cycles and 15 during discharge cycles). It has a maximum electrolyte operating temperature of 50 Celcius. Outside of this range the battery will self-protect (disconnect) until the environmental conditions change.

The electrolyte fluid has a high thermal mass (takes a long time to change temperature in either direction).

In terms of the lower temperature limit - it is very important to appreciate that this is an internal electrolyte operating temperature limit and that outside ambient temperature can be substantially outside of this operating range for an extended period before this is an issue in practice.

The battery, when operating, can (if needed) use energy within the battery to warm itself up through control of its internal operating components. In addition, the enclosure also acts as an insulation device during low temperature operations. 

For most domestic locations in Australia we expect that no special actions will need to be taken with our battery in this regard. 

Note that if the electrolyte temperature does reach an operating thermal limit then the battery will self-protect, meaning that it will disconnect from the system load to protect itself and then reconnect when conditions improve. Hence while your energy system may lose access to the battery in this scenario (as a worst case), the battery will come back online automatically, without damage, once the temperature has shifted.

When external ambient is beneficial in terms of adjusting internal electrolyte temperature (up or down), the battery uses its internal software-speed-controlled fan and pumps automatically to adjust its electrolyte temperature. This function operations even if it is not connected to the system load at the time.

In some sites where batteries are to be delivered during cold periods, or where batteries will be turned off and kept in standby for extended periods, the installation of a conventional cabinet temperature heater (easily obtained and installed by your integration partner) may be required. These devices are routinely used in electronic cabinets in cold environments for various reasons. Note that actual cabinet heating is only needed until the battery is running. When running it has been demonstrated to stay at optimal internal temperature 'year round' even in very cold and elevated remote site situations.

Lower round trip efficiency than conventional batteries

The battery has a round trip DC-DC efficiency quoted around 80%. Typical Lithium packs can run at a round trip DC-DC efficiency in the mid 90's.

This means it takes more energy to fill up our battery than to fill up an equivalent Lithium battery, before that energy is then delivered into a customer application load.

The offset here is that generally, finding more input energy isn't the problem in an energy system - if you have a solar array, its often feasible to make it (more than) large enough to get the job done. 

In addition, as our battery ages, the expectation is that degredation will be expressed as a gradual lowering of that DC-DC round trip efficiency, but not as a loss of the capacity to output a full 10kWh of energy into your application load. This means the battery shouldn't need to be over-sized up front.