Grid-tied Micro Inverters and battery storage

Tontoe · November 2021

Greetings from the UK

Ten years ago I fitted an on-roof array of 12 Hitachi 60W panels, each with its own Enecsys micro-inverter [to eliminate the effects of shading and get the best from the array].

Yes, contrary to popular belief, the sun does occasionally shine enough here in the UK for Solar PV to be a decent proposition

The output from the micro inverters [230V as we are in the UK] is summed and then goes through a generation meter [which records power generated to calculate the payment to be received for the power we export to the grid] and thence to a spare way in a consumer unit [or fuse box], which is protected by a 16A RCBO.

I would now like to find a way of storing some of that power in some kind of battery array.

Enecsys went bust a few years ago, so I can't ask them.

I wondered if anybody might be able to offer some advice about a charge controller or charge controller/inverter that could "fool" the microinverters into seeing a grid-type connection when the grid goes down.

Hope this ain't a stupid question

BB. · November 2021

Welcome to the forum Tontoe,

There is a "simple" answer which is yes. A basic PSW/TSW (pure/true sine wave AC inverter--possibly just the low frequency conversion type?) can do what you ask...

More or less, you use a transfer switch to connect the Micro Inverters to the 230 VAC 50 Hz output of your AC inverter... The inverter (must have relatively accurate 50 Hz output and stable voltage)--And after ~5 minutes, the micro inverters will "qualify" the 230 VAC from the inverter and start generating current from the solar array.

And the Off Grid PSW inverter will take that energy and use it to charge the battery bank.

Issues:

Older OG inverters were not designed for this function. Some (few, many, most?) may not actually charge the battery bank and possibly fault instead.
Older OG inverters that did function and charge the battery bank--They had no method of stopping charging when the battery bank was full... Could damage your batteries (or worst case even cause a fire). Or "fault" the inverter (AC over voltage, DC over voltage).

Newer OG inverters--Some are designed expressly for this type of operation. They will monitor battery voltage and (typically) vary the output frequency by +/- 1-2 Hz (52/48Hz). That will cause the micro inverters to shut down (out of frequency range) and stop operating (for 5 minutes after the OG inverter is back at 50 Hz).

There were workarounds done in the past. One is to put a "dump controller" on the battery bank. When bank is full, the dump controller turns on and passes excess energy to a resistor bank (wasted as heat). Another was to put a relay between GT and OG inverters to turn off the GT inverter AC power if the battery is full.

And last is to find a OG inverter that supports the frequency shift function.

Got to go... But this is a start to your answer. The details matter--So don't just buying hardware yet.

-Bill

mike95490 · November 2021

your older Enecsys micros are not likely to support frequency shift power management, but if the new inverter shifts enough, they will start to fault and disconnect when the batteries are full.
But to enable this mix of equipment will take a lot of detail reading of manuals and spec sheets, to see if indeed, brand X will work with brand Y.

Grid-tied Micro Inverters and battery storage

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