There are a number of useful things to know about Off-Grid deployments using Victron energy handling hardware with Redflow batteries.
It is worth considering each of these at both the design and deployment stages of an off-grid project using this combination.
In no particular order:
Off-grid systems (in general) are such that - without the grid to support them - energy demands of a 'surge' nature (e.g. electric motors) can generate energy peak demands far in excess of their continuous rating.
This guide page contains more information and advice on how to manage these situations.
Off-Grid AC PV must be shut down when the battery array charge rate falls below a chosen threshold level
In an off-grid site, the production of AC PV is regulated dynamically (via frequency shifting) to balance the total AC PV production to meet the total system demand (on site load plus battery charging rate).
Any sudden change in the balance between AC solar generation and on-site loads (e.g. a sudden rise in solar output or a sudden drop in on-site loads) results in a sudden surplus of energy in the system that has to 'go somewhere'. The 'somewhere' is into the battery array.
This is the origin of the Victron Energy 'Factor 1.0' rule - which aims to ensure that there is enough battery charging capacity in the Multi/Quattro installation to accept that energy surge and push it into the battery array while the AC PV adjusts (via frequency shifting) to the changed circumstances on the site.
However, where Redflow batteries are used, their unique technical characteristics mean that the 'Factor 1.0' rule is not a workable approach to use. There are circumstances where the Redflow array is simply unable to absorb the necessary level of surge charge current. Even if there is enough charge capacity in the Multi/Quattro part of the system, there may be insufficient charge capacity left in the Redflow battery array to absorb the surge. This is because each battery in the Redflow array, as it reaches 100% full, physically blocks charge with a diode. Hence the total physical capacity of the array to accept further charge energy at all falls away progressively as each battery reaches 100% and 'locks off' ... all the way to the system being incapable of accepting even a single amp of charging current when all batteries are full and/or in the process of discharging back down to the point where they can accept further charge.
AC PV handling using Redflow batteries
The way to handle this situation with Redflow batteries, Instead of relying on the 'Factor 1.0' system sizing rule at all, is to configure the Redflow BMS to proactively shut down the AC PV array during periods where the remaining charge rate available in the battery array is below an appropriately chosen threshold level.
One interesting benefit of taking this approach is that it allows for deployments where the size of the AC PV array can be far larger than would be otherwise 'allowed' via the Victron Energy 'Factor 1.0' rule!
When the BMS commands the AC PV on the site to shut down, a 100% AC PV based system will then be powered entirely by the batteries (just as if it is night time), until enough charge capacity opens up in the battery array, at which point the AC PV will be re-enabled by the BMS.
Note: Sites can include additional solar capacity delivered using DC MPPT's, with that DC MPPT solar capacity not being subject to being turned off and on. Hence, by adding at least some DC MPPT to a site in addition to the AC PV array, it also becomes possible to complete the remaining charging of the Redflow array (if desired) all the way to '100% full'. The other cool benefit of DC MPPT solar is that it is capable of 'black starting' a site in case of long term grid failure and/or generator failure.
AC PV Control (shutdown/restart) approach
The usual control mechanism for this is a MODBUS-TCP register in the Victron GX called "AC PV Disable".
If you write a '1' to that register, it forces the output frequency to 53Hz as a shutdown command to any AC PV on the site. Once set back to 0, the AC PV can restart and can dynamically re-balance against site energy demands.
Note that for this to work you require two things:
- To have the Multi/Quattro system has got either the "ESS Assistant" or the standalone "PV Assistant" installed; and
- To have no upstream grid or generator feeding AC energy into the system. Frequency-shifting is disabled any time there is is an 'upstream' AC feed present in the system
This control is effected from the BMS by creating a MODBUS-TCP 'endpoint' in the BMS to allow the BMS to send values to that GX control register.
Here's an example of the endpoint to create (under Configuration->Digital I/O->Endpoints):
IMPORTANT: The unit number for the AC PV Disable endpoint varies, depending on the hardware type of the GX unit in use. Use a unit number of 227 for a Cerbo GX, use a unit number of 246 for a CCGX.
(More information about how to define Victron MODBUS-TCP endpoints is available here)
Obviously you need to use the IP address of the GX system on your site, in the endpoint that you create. The IP address assigned to the GX also needs to remain stable (either by manually configuring it, or by setting up a static mapping entry in the DHCP server function of your Internet router).
With the endpoint created, you can test it by writing 0 or 1 to that endpoint via the Tools->Digital IO menu item.
Writing a 1 should set the output frequency to 53Hz and shut down your AC PV inverters. Writing a 0 should restore normal dynamic frequency-shifting control.
Note that in some cases it may be preferable to disable the AC PV using a dry contact relay input directly into the AC PV inverter(s) on the site.
If you prefer to do that, then a 'relay' "AC PV Disable" endpoint can be created instead, either using the built in BMS relay (endpoint name: BMS_Relay), or by using a USB or MODBUS-TCP attached relay module and setting up an endpoint to control that 'off board' relay.
Implementing the AC PV shutdown based on the battery array charging capacity
A pair of 'Rules' entries need to be created in the Digital I/O Configuration page, to send control signals via your endpoint, at the appropriate times.
Here is an example of the pair of rules required:
In this example:
The first rule entry disables AC PV when the "Permitted Charge Current" (the maximum current the battery array can accept right now) falls below 400 amps.
The second rule entry re-enables the AC PV when the "Permitted Charge Current" rises back above 450 amps.
The gap between the two values is intended to ensure there is a reasonable change in the available charge margin before the AC PV solar is re-engaged.
(More information on driving the DIO Configuration engine is available here)
Calculating the appropriate charge current threshold
To determine the appropriate cutoff point (in terms of Permitted Charge Current) for your site, just determine the largest single energy load on the site (the device with the largest on/off change in energy - e.g. the largest air conditioner, or motor, or whatever). Remember that what matters is the drop in load when the device stops, not the surge demand when the device starts.
If that turned out to be, say, 10kW (as the largest 'sudden' drop in load expected on the site), then this can be translated into a 'Charge Limit' figure in amps by dividing it by the typical charging voltage (57V).
e.g. Using that 10kW example:
This might be up-rated to say 12000 VA to allow for overheads/power factor differentials (and to allow for some margin), then divided by 57V to yield 12000 / 57 = 210 Amps as the cutoff level (in amps of charging rate available), to then program in to the Digital I/O rules for the site concerned.
Preference for DC MPPT's
Pure off-grid sides are simpler to manage if at least some (or ideally all) of the solar on the site is deployed using DC MPPT units.
The benefits are:
- The site can self-black-start if required, in the absence of a generator, booting up from the DC MPPT's alone. The BMS has special software support to make this automatic, including to signal upstream systems not to discharge from the battery array until there is a minimum amount of energy charged up into the array.
- There is no need to control the PV (to turn it on and off) as noted above for AC PV
The downside of pure DC MPPT solar capacity is that the entire site energy output capacity on the AC side is limited to the total inverter capacity of the battery inverters alone.
By contrast, the use of AC PV where (when the sun is shining) allows the total energy output to the site to be the sum of both battery and solar inverter capacities.
Hybrid DC / AC solar systems
Hybrid systems (some DC MPPT Solar and some AC PV Solar) can be built, and the Victron GX system controls such hybrid systems automatically.
This can allow for, for instance, some DC MPPT for 'black start' and dedicated battery charging, in addition to some AC PV for handling daytime high energy demands.