Simon Hackett, Redflow System Integration Architect
introducing Pod Z
Redflow is in the midst of building a new “Pod” based energy storage architecture, with the first customer for that deployment also being its largest customer to date.
The story below includes a link to a short video about the new architecture Redflow has designed to support entry into the Grid-Scale energy storage market:
When we made that video, we hadn’t formally announced our partnership with TRUMPF Hüttinger, the company we’re working with to deliver this scalable architecture. A few days later, we were able to make that announcement:
How Does Pod Z Work?
Now we’ve made that announcement, I can explain how this nifty stuff works.
Here’s a photo of a Redflow Pod Z with some of the covers taken off:
On the left is the battery cabinet with one of four covers removed. This cube-shaped cabinet holds 16 x ZBM2 storage modules, 8 to a side.
Ventilation paths run inward at the base of each pod. Air flow then rises via a ‘cold aisle’ in the middle of the pod, proceeds through each ZBM2, and exits as warmer air via fans installed behind each cover door.
While the Redflow modules you can see here are the existing Redflow ‘Gen2.5’ devices, the upcoming Redflow Gen3 modules will also be a perfect fit, in both physical and electronic terms (and they are designed to be).
To the right is the electronics cabinet for the pod. Inside that cabinet, at the lower right, is a cluster of six Trumpf DC1010 units with a Trumpf System Controller.
The DC1010 module achieves something quite special. It is a bidirectional DC/DC converter that can shift voltage up to a very high ratio (around 15:1).
These modules are clustered together and driven in a unified manner via the the system controller.
On the the low-voltage side, these modules interface to a standard 48V telco standard voltage bus (compatible with Redflow ZBM2 modules).
In fact, beyond merely being ‘compatible’ with Redflow ZBM2’s – the Trumpf product has been specifically designed for flow batteries!
This product line has been created by Trumpf in response to the rising demand for the use of flow batteries in large energy systems.
Flow batteries are long duration, 100% depth-of-discharge, durable and high-temperature-tolerant workhorses. They compliment, rather than conflict, with the use of shorter-duration/higher-peak-output capability Lithium batteries.
Trumpf have developed this product line at a time and in a manner that is an ideal complement to Redflow energy storage modules, and that paves the way to create grid-scale hybrid deployments that include flow batteries.
Flow battery support in the DC1010 includes a capability to have the low voltage DC bus operating all the way down to zero volts. The units then support smoothly raising the voltage of a flow battery back up to its normal operating voltage range, smoothly ramping a current-limited/current-controlled energy supply to the modules, as part of commencing the next charge cycle.
The DC1010 modules are each rated at 200 Amps continuous on the low voltage bus, meaning that this cluster of six units can support up to 1200A on that bus.
In the first Pod Z configuration, Redflow has rated each pod at a nominal continuous energy throughput of 50kW.
The low voltage bus comes together on the left hand wall of the control cabinet. On that wall is a pair of DC bus-bars sandwiched around insulation layers, with built in ‘comb’ connectors that allow the battery circuit breakers to bolt straight on to the bus-bar.
A set of 48V bus cabling (visible on the left hand wall) fans out from the bus-bar to all 16 batteries on the left, and additional cables run down to the DC1010 cluster at the lower right.
On the high voltage side, the DC1010 units support a (software selectable) interface voltage in the 765-900 Volt range.
On the lower part of the rear wall, you can see the output side cables for the high voltage side. Those (small!!) wires run at circa 800 volts and come together to the high voltage interface terminals. At a 50kW power level, across six DC1010 units, at (say) 800V, the current being carried from each of the cluster modules is a mere 10 amps (50,000 / 6 / 800). This is why those wires are so small – because they really don’t need to be larger.
This is one half of the key to grid scale battery deployments. That high DC voltage means the total cable size – even for a DC bus visiting many Pods in parallel in a single site – really isn’t all that large.
The other half of how this architecture supports high scale deployments is the way that voltage management happens on that high voltage DC bus. We are operating these modules in a mode that Trumpf call ‘Voltage Static Mode’.
In this mode, the DC1010 cluster converts the variable ‘48V rail’ DC voltage into a fixed voltage outcome on the high voltage side. All the pods are programmed to have the same HVDC voltage when idle, and they are just wired together in parallel.
To connect the DC pods to an AC energy grid, third party AC inverter/chargers are used.
When those inverter/chargers wish to discharge or charge from the overall Pod array, they simply ‘pull’ or ‘push’ against that DC voltage. If the inverters try to discharge, they naturally draw the DC voltage down in the process. The external draw-down on the DC rail acts as an automatic signal to the DC1010’s to start to deliver output energy. The amount of energy they deliver is proportional to the voltage shift that the inverter/charger initiates on the DC bus.
In the reverse direction (array charging), the inverter/charger drives the DC voltage up, and the DC1010’s respond to this by moving energy from the DC rail into the Pods. Again, the amount of current that flows is controlled via the voltage shift that the inverter/charger initiates.
Here is a diagram showing this result, just to make it clear (taken from the Trumpf system documentation):
The deep point of this operating mode is that each pod acts like an ‘ideal’ battery on the HVDC side, that:
- always sits at the ‘perfect’ voltage, waiting for work to do
- can be wired to an arbitrary number of similar pods
- can be wired to, and commanded by, inverter/chargers
… all using the chosen DC link voltage – and shifting of that voltage – as a command mechanism that requires no software interface and no real-time cluster synchronisation for it to work.
In the initial deployment site there will be twelve pods sitting in rows on concrete plinths, delivering 600kW throughput via 12×16 module pods, for a total of 192 Redflow ZBM2 modules on site:
Meantime, back inside each Pod Z, the Redflow ZBM2 modules are coordinated and controlled by Redflow’s purpose designed BMS, operating in a cluster-friendly ‘Slave’ operating mode.
Each Pod Z’s BMS:
- Drives the Redflow ZBM2 module operating cycle internally in each pod, including coordinated maintenance cycles for batteries at appropriate times
- Actively controls the operation of the Trumpf equipment cluster using a newly developed Trumpf operating module in the BMS. This code sends continuous updates via MODBUS-TCP to the Trumpf cluster, keeping it informed about the present operating limits of that particular cluster of ZBM2 modules. The key parameters sent are the maximum charge capability, maximum discharge capability, and the target charge voltage for the ZBM2 cluster.
- The BMS provides secure remote management access to the Trumpf system controller and manages the high level configuration of the pod (including setting the HVDC operating voltage and current limits in Voltage Static Mode).
- The BMS also watches over the Trumpf cluster, monitoring and logging operating parameters including voltage, current, and three temperature measurement points inside each DC1010 cluster member
With each Pod Z being fully managed by its internal BMS, all that remains is to aggregate the status of all Pods together, for the benefit of, and the coordination of, the on-site inverter/chargers and on-site Microgrid controller.
A Redflow BMS operating in in ‘Master’ mode is interfaced over ethernet to all the downstream Slave BMS in each Pod Z that it watches over.
The Master BMS passes overall status data to the on-site microgrid controller. This includes System State of Charge, overall site charging and current discharging capacity, temperature, and state of health. This information is provided via industry standard CANBus ‘Smart Battery’ protocol, MODBUS-TCP, and/or JSON queries to the Master BMS.
The Redflow Pod Z architecture has been designed with, and around, the Trumpf ‘TruConvert’ DCDC power system architecture.
The combination creates a powerful, integrated high voltage energy storage system that can be scaled to an essentially unlimited extent.
The interface mechanisms being used allow the high energy, high voltage Pods to be parallel-wired without complex or difficult real-time synchronisation or balancing mechanisms. Instead, the software-mediated Trumpf cluster creates a ‘perfect’ battery voltage for every Pod, driving the simplest possible integration path for high scale site designers.
The Redflow BMS acts as a per-Pod orchestrator for the Redflow ZBM2 modules downstream, and as a coordination and control point for the Trumpf DCDC converters in each Pod, and passes aggregated energy storage status data upstream to the on-site master Microgrid controller.
This architecture plays to the strengths of all of the components concerned. It is is designed for reliability, redundancy, and scale.