A bit OT to thread, of protecting against power surges.. but..
150Ltr of ICE is a massive amount, and containing it, and effectively using it, would be hard to implement.
Ice expands on freezing that causes major issues.
I can't see how having (what I think was) a "Tucker Box" type freezer full of ice, is in any way useful.
I can't see how practical it would be , to take a domestic Frost Free refrigerator, and cut holes in the freezer floor.. It would never work normally, when on daylight power, and would FREEZE to goods in the main PC compartment
Firstly, we have to be practical.. I use PET bottles, part filled, with Sodium Nitrate solution.. These are in the top of the freezer.. If something happens whilst away, they will give a small amount of help, but it still won't be automatic..
Were I setting out to devise a "Cold Backup" refrigerator, I would modify a standard domestic (top mount) frost free Fridge/Freezer..
[I am assuming that you have (effectively) an off grid power system for lighting etc., but not good enough to run the fridge overnight..]
SO - changes - The fridge's MECHANICAL thermostat directly operated the freezer-fan and a small 240V relay
The relay operates the compressor via a time clock or a light sensor.. ie.. the compressor would only run when solar power was available.
I would stand, PART filled PET bottles with Salt Solution in the freezer.
The FAN would move air around these bottles, cooling the air, just as the compressor would normally do
These bottles would freeze during the day and then the Thermostat/Fan would demand circulate the cold to the PC compartment..
That would have to work efficiently, with minimal "OBVIOUS" changes to the basic refrigerator..
((If you don't have 240Vac at night and just battery power, then a DC fan would be needed eg. 12V from a Fisher Paykel freezer..))
The real challenge is with the frozen stuff.. There I think the chest freezer has to be the answer.. easily will cope with an overnight sleep.. a special shallow basket tray in the top with those salt-water bottles could help.. but a pain to have to lift them out every time you want to get to the peas..
I think that I would be assuming that there was a daily cycle and just cover that day.
Bad Weather would need to be covered by alternate means - ie generator..
Thanks for taking an interest, you are the first in Australia to do so.
PS - the topic has been covered MANY times, in various ways and in various threads..
My philosophy - KISS - Leave the device as original as possible.. ( Keep the bride happy )
I noted your comments about what seemed to be adding a small freeze on top of the existing fridge..
what, did you seal them together and cut holes between...
Be sure of where the existing condenser coils are, if thinking this way.. MANY modern fridges have the condenser pipes under the outside skin..
Two compressors to run..