Most travel fridges (Engel, Waeco, etc) use a small 12v compressor for cooling, running at ~2-5 amps, and are optimised for low power consumption to extend battery life when the vehicle engine is shut down. Unfortunately, the fridge needs to be plugged in to the car's 12v supply, a generator or be 3-way (propane/kerosene-ammonia cycle). And generally aren't that powerful running from battery alone.
There exists another type of travel fridge using an eutectic type of coolant effect, (with a high thermal mass) but I have heard firsthand reports of these systems being awfuly heavy, expensive and unreliable.
I propose a backup system in the fridge using a small high ratio compressor to fill a vessel with compressed air to liquidus point. This compressor will only run when the engine is on. When the fridge is unplugged, and the thermostat indicates the temperature is rising above the set point, the compressed gas is slowly discharged through a secondary coil system to produce a sustaining refrigerant effect. This could possibly extend the duty cycle of the fridge out to more than a day, and different capacity reserviors could be purchased for different applications. If gas mixing/fractional distillation of the refrigerant gas is a problem, possibly a membrane or osmosis filter to restrict it to only compressed nitrogen would work. This system could also be used for a "snap freeze" function on the fridge.-- Custardguts, Apr 04 2006 all round Ireland, apparently. http://www.tony-haw...ireland_gallery.php [po, Apr 04 2006] (?) Coolchips http:// www.coolchips.gi/about.shtml[molecat]'s link as a link [BunsenHoneydew, Feb 08 2009] I confess to reading only the title and first paragraph before my concentration began to wane, but [+] anyway because it seemed feasible.-- Mr Phase, Apr 04 2006 3-way fridges generally run quite long on gas and when driving they can be switched on 12 Volts operation to save gas. 12V operation is not very effective as power consumption on electricity is high with 3-way fridges. Compressor fridge takes much less power and can be used with big enough solar panel and auxiliary battery separated from main vehicle battery. Then there are those 12V coolers which take most power and are not fridges as such anyway. Don't leave them running when you stop your engine, you get a flat battery very soon.-- Pellepeloton, Sep 14 2006 Hi Cus, There is a highly efficient (85% if you can believe the numbers) solid state refrigeration device soon to be available. I't based upon electron tunneling, and could make this and many other refrigeration applications extremely portable. http:// www.coolchips.gi/ about.shtml-- molecat, Jan 08 2007 [Custardguts] You cannot liquify nitrogen or oxygen at room temperature, regardless of the pressure. At best, then, this will be a compression heat pump, which is inherently less efficient than a phase change heat pump, and would not give you the large store of cooling potential you lust for.
Assuming you did have a mobile liquid air plant, you would get woefully poor efficiency because of the large difference between the temperature of the fridge and that of the boiling air - like driving to the top of a mountain, then coasting ٠if the way down, in order to arrive Πof the way up.
The best solution I can think of is to make plenty of ice in the freezer compartment while you have power. In fact, I've done that in a small domestic fridge during hot weather, so that I could run it at night (when it's cooler, and the house is being ventilated) and leave it off all day (when it would have to work harder, and the waste heat would end up in the house).-- spidermother, Jan 26 2012 random, halfbakery