Just Thinking, Mental experimenter at heart.

Submitted: Thursday, Jun 02, 2011 at 15:19

ThreadID: 86708 Views:2953 Replies:6 FollowUps:2

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nickoff

Has anyone else thought about adding to a 3 way fridge, a 12 volt compressor pack to keep in cold when driving, and removing the 12 Volt heating element from the absorption circuit?
I was thinking basically of a caravan type unit, where a compressor can be remotely mounted and the evaporated placed inside, and alongside the existing ammonia evaporator in the fridge/freezer area. This would make the fridge more efficient when driving, and could be used to “boost” the fridges cooling capacity when camped off a solar panel and small battery, (with proper power regulation you might even be able to eliminate the battery) but maintain the ability to keep cool on LPG when a 240V mains or 12V sunlight supply was not available, ie, at night when remote camping or when the sun don't shine. (But who goes camping in the rain.)
The power switching for the 12V compressor circuit could be “piggy backed” off the existing 12V side of the fridge, as it would no longer be used in heating the ammonia/water solution. This would now only be done by 240V or LPG. Thus you would only be able to select one power input to the fridge.
The down side is that you would have to remember to select 12v or LPG when the sun went down. (Could a circuit be devised that would overcome this, IE when 12v power is lost for a period of time, the gas circuit automatically kicks in and keeps the fridge going.)
This would eliminate the need to carry an overlarge battery, or any battery at all, and with just a solar panel, but still give you the best of both worlds.
Has anyone else thought this idea through and found faults or other issues with doing this? I figure it might be an expensive exercise, but sometimes you have just got to give it a try.
The idea started forming a few years back when I saw a Danfross Compressor refrigeration unit as a install yourself setup, for making your own fridge/freezer in a space provided. I went, “That would be a good idea to retrofit into a 3 way fridge,” and have been trying to think the issue through for a long time.
I know we now have access to good ‘T” rated 3 ways, but even these don’t work well on the 12v side due to high amperage draw and voltage loss on the supply side.
12V heating is not the most efficient way of doing things. Too many amps required.
Ideas, discussion?

Cheers all, Nickoff

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Reply By: Notso - Thursday, Jun 02, 2011 at 15:33

Thursday, Jun 02, 2011 at 15:33

EVA Kool used to sell a pack just like what you are talking about, it could be mounted to a good quality esky etc, no reason why you couldn't use it like you suggest.

Dunno about the auto switchover to gas though.

AnswerID: 456156

Reply By: AlbyNSW - Thursday, Jun 02, 2011 at 18:49

Thursday, Jun 02, 2011 at 18:49

Maybe have a look around the boating suppliers for cruisers etc I know you can get kits to add to your own fridge compartment, waeco also does them but not sure about the gas side of things

AnswerID: 456166

Reply By: Member - Joe n Mel n kids (FNQ - Thursday, Jun 02, 2011 at 19:13

Thursday, Jun 02, 2011 at 19:13

As in replys above it is possible and has been done before, there are quite a few kits out there that will slot in to a "box", eskys are normally used, some make the box them selfs also ....................
I have been toying with the idea for some time now but i have looked at it from a different angle ............
I rarely remove the fridge from the car when camping and all my driving is done in "hot" places with the a/c on, i have toyed with building a "built in" fridge unit that runs from either the existing a/c system or a seperate compressor driven off the motor and it is not as hard as it seems, using the existing system you would simply re run the return low pressure line through a coil inside the "fridge" unit, you could tap into the high side but to diffucult to do that way.... and you could run it as a "eutectic" system and would not need a 12V backup system for at least 2 days .... (you can look up "eutectic", to hard to explain ..)
Or simply add a very small compressor, evap unit and small condensor and also not as hard as it seems as most is avail off the shelf ...
Now the backup system will be the 12V "kit" that simply cuts in when the engine stops running, simple system to wire with a relay from the ign "on" side ........
Buuuuuuuuuuuuut ..... i probally never will do it now as i live so remote getting the bits sorted is a pain and i dont have time these days.....
Cheers and have fun thinking about it ..
Joe

AnswerID: 456173

Reply By: Eric Experience - Thursday, Jun 02, 2011 at 21:16

Thursday, Jun 02, 2011 at 21:16

Nickoff.
The heat exchanger on the gas fridge that cools the contents also acts to heat the contents very quickly once the gas is turned of. You will have noticed that when you turn the gas of the fridge heats up rapidly while a compressor type will stay cold for hours. for this reason your idea will not work unless you can somehow insulate the 2 cooling system from each other. Eric

AnswerID: 456185

Reply By: Member - Boobook - Friday, Jun 03, 2011 at 06:36

Friday, Jun 03, 2011 at 06:36

Your 3 way doesn't work well with 12v because it wasnt installed properly. No other reason. It can easily and cheaply be fixed as follows.

1)Rewire the cable in the vehicle with 4 AWG wire or better.
2)Do the same with the van.
3)Remove the fridge and reinstall it with a couple of home insulation batts surrounding it, especially the top and sides.
4)make sure there is a "chimney" affect at the back, ie a lower and higher vent with a chinmey in the middle
5)put is a thermo fan drawing air upwards.
6)Use Anderson plugs or 2 pairs on a 12 pin plug for the fridge.

After doing this, my fridge went from sitting on 8 - 12 degrees while driving, to 2 degrees and sometimes freezing.

You MUST get 13V at the fridge while driving. If you get 9 - 10V, which is usual for a lot of installations, you get about half HALF the cooling power. ( Power = Volts squared x Resistance).

At 9V you get about 81 watts of cooling power. At 13V you get 170 Watts!!

If you do this I guarentee you will be happy.

AnswerID: 456205

Follow Up By: nickoff - Friday, Jun 03, 2011 at 08:37

Friday, Jun 03, 2011 at 08:37

have done that with the current caravan I have, and it does maintain cool when traveling, even across QLD outback during summer. I just see that a compressor type refrigeration as more efficient,drawing less amps that a heating element, but battery storage of electricity as not so efficient, even for a low amp draw.
So basically I would like a system that runs of 12V power, when available, but reverts to LPG when not. This way I avoid the weight penalty of the Batteries, and extend the usable amount of LPG carried, for a limited additional weight.
my first thoughts on this hybrid fridge was to get a 3 way esky and add a compressor to use as a camping fridge for long term stays, using solar for the daylight hours, when the ambient temperature was at its highest, and using the LPG side at night when there was no sunlight, and the heat exchange needs wouldn't be as high. I see the limiting factor on solar power as the storage requirement (batteries) for supply during low or no light periods.
As I said in the heading, just a mental experimenter at heart.

FollowupID: 729235

Follow Up By: Member - Oldbaz. NSW. - Friday, Jun 03, 2011 at 09:03

Friday, Jun 03, 2011 at 09:03

Great work, Boobook, you covered it completely. I now prefer to run my 25 year old Finch on 240 while travelling via an inverter & it works just fine off the single starter
battery using 6 awg wire. Some consider this dangerous but with appropriate fusing
of both voltages I dont consider that to be so. This also brings into play the thermostat which doesnt operate on 12 volt.....oldbaz.

FollowupID: 729241

Reply By: Ray - Friday, Jun 03, 2011 at 08:25

Friday, Jun 03, 2011 at 08:25

I have my 3 way wired up to my battery that is mounted on the draw bar of my caravan. I have used additional wiring from the car which is only used to activate a relay(ignition controlled) so that when the ignition is on. It works quite well and because it is only a short run from the battery to the fridge I have used 8 aswg cable.

AnswerID: 456212