Extended capacity travel fridge

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.