Hybrid system based on offgrid components

[szczepan]

Hello everyone,

I’m planning to install a photovoltaic system in a rural house. Unfortunately, power outages are frequent in this location, and the voltage is unstable due to a large number of nearby PV installations. This leads to the conclusion that I need to go for a hybrid system. Since I don’t intend to sell/export energy to the grid, I want to base the system on much cheaper solutions than certified hybrid inverters like Deye (cheap MPPT charger and a UPS-type inverter like Easun).

Here is concept/schematic of the system:




The idea is that one or more MPPT chargers, the inverter, and an energy storage system based on LiFePO4 cells (with BMS) are all connected to a common DC bus. The AC grid is connected to the 'GRID' input of the inverter. The inverter has the ability to operate in 'bypass' mode, which will be used when the battery voltage is low, or when the AC power consumption is low and it is more advantageous to use power from the AC grid rather than from the running converter. The controller is a separate issue, I know how to build it with a microcontroller.

What do you think about such a system design?

The advantages I see:

• Low cost (I’m using a cheap inverter and charger). I can even have a backup inverter in case of failure, and it’s still a cheaper solution than all-in-one systems like Deye.
• Easy expansion with additional MPPT chargers and inverters connected to the common DC bus.

Now, the downsides - what downsides do you see? What problems do you foresee? What have I not taken into account?

 

[timselectric]

An off-grid AIO is the best option for cost and ease of installation.
While you can build a system from separate pieces. It will end up costing more, for everything needed to connect it all together.

An off-grid AIO is much cheaper than the hybrids you are pricing.

 

[szczepan]

Thanks for your reply.

So which AIO inverter model can you recommend?

My requirements:

• Power about 5 kW AC / 230V
• 1 or 2 PV/MPPT strings
• Support for LiFePO4 48V (16s) battery
• AC grid bypass mode
• Ability to view statistics and control operating parameters (via RS232/485 port, ethernet, etc) - known/open communication protocol

Ideally, it should have the ability to draw additional energy from the AC grid when needed.

What can you recommend that is attractive in terms of price?

 

[timselectric]

Growatt SPF-5000-ES is a 5kw European 230v AIO.
It should be perfect for your needs. And you can add up to 6 in parallel, if you want to expand in the future.
I have been running them (24/7) for a few years, with zero issues.
I haven't found anything comparable for the price. Which is why I chose them.
I monitor and control them remotely with Solar Assistant.

known/open communication protocol

No idea about this, as I don't use BMS communication.

 

[szczepan]

Growatt SPF-5000-ES is a 5kw European 230v AIO.

Thanks for your reply. It's price looks very reasonable.

I have one more question: can the Growatt SPF-5000-ES work without batteries? I mean working in a way where it draws the missing energy from the AC grid at any given moment. Is such a configuration possible?

My question stems from the fact that currently, the most expensive component of the system are batteries - I would gladly split the investment into two stages: first the PV installation and inverter, and then the energy storage later on as funds allow.

I monitor and control them remotely with Solar Assistant

Great - that means that there is some kind of known communcation protocol. Sometimes things are much simpler to implement when you can communicate with the device and reconfigure in on the fly with some script.

 

[timselectric]

I have one more question: can the Growatt SPF-5000-ES work without batteries? I mean working in a way where it draws the missing energy from the AC grid at any given moment. Is such a configuration possible?

It can, but we don't recommend it, unless you have an export agreement with the utility company. Because it will export small amounts from time to time. And the utility meter may see it. Which will draw their attention to you.

 

[szczepan]

I can sign an agreement for exporting energy to the grid, that’s not a problem for me.

Since the inverter sometimes sends small amounts of energy to the grid, it means that it behaves like a typical on-grid inverter performing a zero export function – the external AC network is not separated from the internal one.In other words, the missing energy drawn from the grid is not converted to DC, which, together with the DC coming from the PV panels, is then converted back to AC on the internal network side.

Am I understanding the principle of operation and internal design of this inverter correctly?

 

[timselectric]

I can sign an agreement for exporting energy to the grid, that’s not a problem for me.

Since the inverter sometimes sends small amounts of energy to the grid, it means that it behaves like a typical on-grid inverter performing a zero export function – the external AC network is not separated from the internal one.In other words, the missing energy drawn from the grid is not converted to DC, which, together with the DC coming from the PV panels, is then converted back to AC on the internal network side.

Am I understanding the principle of operation and internal design of this inverter correctly?

Basically, yes.
This is only in reference to SUB mode.
The other modes do operate separately from the grid.

 

[szczepan]

Basically, yes.
This is only in reference to SUB mode.
The other modes do operate separately from the grid.

Thanks, this is very helpfull reply

A follow-up question arises - since in SUB mode the Growatt behaves like an on-grid inverter, should I expect all the problems on the output that originate from the power grid? I mainly mean issues like too low or too high voltage.

 

[timselectric]

Thanks, this is very helpfull reply

A follow-up question arises - since in SUB mode the Growatt behaves like an on-grid inverter, should I expect all the problems on the output that originate from the power grid? I mainly mean issues like too low or too high voltage.

Yes, it will just be matching whatever the grid is doing.

 

[Brucey]

Thanks, this is very helpfull reply

A follow-up question arises - since in SUB mode the Growatt behaves like an on-grid inverter, should I expect all the problems on the output that originate from the power grid? I mainly mean issues like too low or too high voltage.

You may want to consider something like a chargeverter to decouple from your garbage grid. Often used by certain AIOs because of their system design where spurious exports are possible when large loads stop etc. Not sure how well it would work without a battery to charge though.

 

[szczepan]

Thanks for advice. Operating without the battery is only temporary solution - few months max.