12V inverter.

[DaveB]

What I also don't know includes: how would different hysteresis (within the constraints of food - and beer - refrigeration) affect energy usage? And what would be the safe power-out time for a freezer used as a fridge? I suspect it would be a lot longer than for either a normal fridge (poor insulation) or a normal freezer (v. low temps).

Nor how much the energy demand would drop in colder/darker seasons: would colder = less demand = darker ok?

Can't help with the electrickery, not enough GAL genes in the family tree, and no, I dont want any more.

However, as a general rule the fuller a fridge or freezer is the more efficient it becomes. Chilled objects in the fridge don't warm as fast as the air inside does when you open the door, so the temperature remains more stable, leading to less energy consumption. The nature of the items in the fridge would have little impact on this as once the desired temeperature has been reached variations in heat retention will be minimal in this context. Keeping it full of anything is more important than keeping it full of beer/meat/vegetables specifically.

Most conventional freezers will happily go 3-4 hours without power before the interior temperature starts to rise above acceptable levels. You can probably expect at least the same for your homebrew unit, probably longer since the temperature gradient between the interior and exterior is shallower so the temperature will rise more slowly.

In a cold room the chiller will be more efficient as it relies on radiation and convection from it's radiator grill to dump the output from the heat exchanger. Ideally you want it to be in a well ventilated room, without heating and out of direct sunlight. Keeping the rear of the unit clear of walls or other objects will allow better airflow and improve efficiency as well.

[Tarquin Farquhar]

I know, that's why normal freezers come with interior doors or front-walled drawers: to minimise loss of cold are when the door is open. I'm going one better by having a chest type, almost no air loss on opening door.

As for safe for 3-4 hours, I'm looking at days. But that I can experiment on. My main need for info now is on inverters: do I really need to delay powering on the load until after the inverter is powered up?

[VectraMan]

Given that a fridge is essentially a thermostat and a motor driving a pump, can you replace the motor with a 12V DC one and not bother with an inverter?

[xchaotic]

Here's my requirement: I want to operate a 240V fridge from 12V. [...]Keeping the power consumption down is the main point,

These two sentences don't make sense together.
Just a get a 12V fridge, for instance a Peltier based one..

[Tarquin Farquhar]

These two sentences don't make sense together.
Just a get a 12V fridge, for instance a Peltier based one..

Good thoughts but no go:

1. 12V fridges are expensive.
2. I haven't seen a chest version (to cut energy demand drastically by not exchanging air every time I open it).
3. Peltier versions are really coolers rather than fridges. OK for beer and picnics, useful for keeping the shopping cool on the way home, but not good as the main food fridge: they are not at all guaranteed to be able to keep food at safe temperatures (<5C).
4. Also, they are energy-hungry.

Also, though I didn't say it, there may be periods either on-grid or on 240V generator, though obviously 12V could be obtained from that.

[DaveB]

These two sentences don't make sense together.
Just a get a 12V fridge, for instance a Peltier based one..

He's going for an off grid solution and from the sound of it one that is a s cheap as possible using existing resources e.g. a 240 chest freezer.

As the GAL said, Peltier fridges are hideously inefficient and generally only work on a small - mini fridge size - scale.

[Tarquin Farquhar]

He's going for an off grid solution and from the sound of it one that is a s cheap as possible using existing resources e.g. a 240 chest freezer.

that's right. It doesn't need to be with stuff I already have, but I'm not up for big projects. I'm thinking of bending it to my will by control rather than construction. I can build a small circuit myself but I'm not about to modify a refrigerant system myself.

[Tarquin Farquhar]

Thanks guys. Interesting points. Like the beer launcher - I remember an early SF story used a matter transporter to refill your glass, I'll wait for that.

Still don't know whether I really need to delay the 240V load on inverter switch-on?

[Paddy]

Thanks guys. Interesting points. Like the beer launcher - I remember an early SF story used a matter transporter to refill your glass, I'll wait for that.

Still don't know whether I really need to delay the 240V load on inverter switch-on?

From my limited experience of running one in the car I would say that you don’t need the delay. Mine gets switch off on the 12v side on car start-up and back on again. Much has to do with the quality of the inverter. Cheap inverters invert to a square wave whereas the more expensive ones will convert to a nice sine wave. You may find that the long term use of a square waves may cause some side affects.

[Halcyon]

Thanks guys. Interesting points. Like the beer launcher - I remember an early SF story used a matter transporter to refill your glass, I'll wait for that.

Still don't know whether I really need to delay the 240V load on inverter switch-on?

I think you'll find that inverters have a "physical" component to convert DC to AC, which will need to get up to speed before you can draw current from the upstream side. Not sure what the delay would be.

Your project sounds very interesting, is this just a taster for a bigger off-grid implementation?

I think you would be better off sizing the components to do the jobs they do in the way they were designed and accepting the losses rather than trying to interrupt their activity to save pennies unless that is CRUCIAL to the project, in other words a slightly bigger PV panel and battery will allow the system to funtion simply and effectively unless there is a good reason to obviate the inherant losses.

R