As there have been so many posts lately about A/C systems, does anyone remember the system that was developed several decades ago that used compressed air as the refrigerant?
It came to mind as I was reading these posts.
It was abandoned, probably for a number of reasons, but used no refrigerant charge at all.
I do remember several like that. I flew jet trainers for awhile (until I washed out), and remember the simple system they used. In fact many jet planes today simply bleed off a little of the compressor air, expand it through a heat exchanger, and feed it to the cockpit or cabin.
I have heard that there is an efficiency issue, that air may not give as efficient a cooling, BTUs per hour of cooling per hp per drive, but I have never pulled out my old thermo textbooks to confirm this myself. I do know that a phase change system does make the system a lot smaller, and in those bleed air systems the air does not change phase.
Phase change generally gives a lot more heat capacity, as you say. High pressures can at least partially offset this fault of compressed air systems.
I seem to remember there were noise issues as well...I would expect there to be a goodly whack of ultrasonic energy generated in expanding very high pressure air. Although you may not be able to hear it, it is still there, and some people seem to be sensitive to it. I'm sure that could be insulated.
A couple of us bombarded the university physics lab one afternoon with ultrasonic waves and noted the reaction when students came to class. Some seemed to 'feel' something was wrong, became agitated. (Others never woke up)
The air cycle isn't quite that simple, there are a series of heat bulky exchangers involved before the bleed air is fed to the cooling turbine. The temperature of the cooling air available to aircraft heat exchangers is generally much colder than a car will ever see during the months A/C is needed and the volume of compressed air to feed the system is only available from turbine engines.
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