Reply to: 12V inverter.

[Tarquin Farquhar]

How much power did his thermostat take?

Can't have been much, surely.

Not much, less that 2W, but that comes to 40Wh/day, which is a lot in this application. Personally I'm not sure why he didn't use an electro-mechanical thermostat (e.g. a fridge replacement part). It has been observed that he sells his own thermostat for $AU150.

[Tarquin Farquhar]

Just a final note incase anybody is following this with interest, or remembers it later when they think of going off-grid themselves. My main thought was to use a converted chest freezer to get low energy consumption, and power it from 12V. I'm going off the idea.

The Yahoo Refrigerator Alternatives group, as suggested by al_cam, has loads of discussions and ideas. One good idea is to arrange things so you do without the fridge.

The very low energy consmption figure that I had in mind came from one guy's web site - the contributors on the Yahoo group dispute it, since he had a constantly cool climate, ran a rather warm fridge, and didn't count his thermostat's own consumption.

250Wh/day seems more realistic and doesn't need unusual solutions. In the end, if I do need a fridge, I'll probably just buy a marine 12V chest fridge. Meantime, use a good cooler, and a Peltier cooler short-term if needed.

[Flubster]

I can let you have some tarmac cheap if you're a bit short.

That's heightist that is...

[Tarquin Farquhar]

Can't help with the technical stuff, but this kind of stuff has been discussed on the yahoo refrigerator alternatives group, it is quite quiet, but there are guys on there who seem to know what they are talking about and should be able to help.

Why off grid?

Alan.

thanks for the tip. Off grid: because I fancy being able to, not in everyday town life but out in the country. And because I am thinking of a stealth camper. Basically I want to become a caravan twerp. Or a pikey.

[Tarquin Farquhar]

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

Possibly. The current idea is for a DIY stealth camper, but lessons learnt may be useful.

The point about switching off the inverter is that I expect the ON cycle of the fridge to be a tiny fraction of the time.
Estimated energy usage per day:
1. chest fridge: 100Wh.
2. inverter idle: 144Wh.
So yes, the inverter idle current draw is important. Cost of solar panels would go up from £250 to £800, battery from £100 to £250.

But I won't dismiss it, simply upping the capacity is an option. Although apparently wasteful, it would have some advantages: simplicity and therefore reliability, and flexibility in a number of ways.

Possibly switching off the unit at night might halve the inverter idle consumption without harming the refrigeration, if I force an ON-cycle before and after.


As for more in an off-grid implementation, I would tend to minimise the amount of 240V in the design, whether van, boat, or cabin. Most other things can be done with 12V, gas, or solar warming. Or of course by doing differently or doing without!

Apart from anything else, there are safety factors: as at home, you should have earthing AND equipotential bonding. If you run floating rather than earthed, then the equipotential bonding is vital, I personally I still think it could allow your entire van to float at 240V relative to actual earth, which you will encounter when you step in or out. I am tempted to use an inverter only when necessary, when present, and for a short time (as would also be needed for e.g. charging laptop).

[al_cam]

Can't help with the technical stuff, but this kind of stuff has been discussed on the yahoo refrigerator alternatives group, it is quite quiet, but there are guys on there who seem to know what they are talking about and should be able to help.

Why off grid?

Alan.

[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

[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.

[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?

[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.

[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]

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.

[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..

[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?

[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?