http://www.treehugger.com/files/2005/07/teens_invent_fr.php

The best refrigerant gas over all is ammonia. It is even superior to the HFC refrigerant gases that are in use today, and even the CFC refrigerants that have since been banned. It has a enormous heat capacity for its mass, and would make an absolutely devastating greenhouse gas if it weren't for the fact that it is to reactive to linger around in the atmosphere. It has the added benefit of having no impact on the ozone layer.

Ammonia was used back in the days when bulk delivered ice was the only way to keep stuff cool. Why was it phased out? Firstly, it's toxic. Secondly, it eats away copper (the best material for heat exchange piping) if there is any water or water vapor present. Since cooling usually results in condensation, using ammonia as the refrigerant was a very difficult process, which is why people switched to CFC's to begin with.

To the best of my knowledge, all roads lead to a re-introduction to ammonia now, mostly because it is so darned efficient. My suspicion is that the technology will mostly be concentrated on the safe handling of ammonia. Methane also works, but leaked methane is bad because methane is a potent greenhouse gas, and is flamable. Carbon dioxide is also a potential candidate, and is vastly better than HFC's in greenhous gas impact, plus it's non-toxic and cheap. However, it's comparatively inefficient.

"Best" is a subjective and possibly deluding term when applied to a refrigerant choice. The judgment weighs several parameter, including: weight of the requisite on-board compressor system (affects mileage), flammability, heat exchange capacity, cost, chronic toxicity, acute toxicity, global warming potential, ease of use, material compatibility (already discussed in comment above), energy efficiency of the design, compatibility with lubricants and desiccants, maintenance intervals, ability of service businesses to replenish safely indoors, odor and the related one of "public acceptability" which is highly emotion driven, the propensity of frivolous lawsuits to preclude good technologies, and much much more. The one refrigerant solution most commonly overlooked due to irrational critique is the isobutane and cyclopentane blend. Yes it is flammable. But the total charge needed is only several grams, while right nearby is 14 gallons of highly flammable gasoline. In the kitchen, the frig is across the aisle from a near infinite source explosive natural gas called a stove. Yet we can NOT HAVE BUTANE IN OUR CARS say the sheep.

To Berkana:What about alcohol etc.? Also,menthol could work -and be non-toxic!?

Ammonia certainly does come with all the problems John mentioned. Alcohol/MeOH, if used in an automobile, can be exploited ... by those who really want to get a buzz. You may laugh now, but look at what kids are doing now with paint thinner, aerosol cans, etc. The use of alcohols in a car provides easy access to alcohol and desperate/bored WILL find a way to get a hold of that treasure!

Alcohol condenses/boils (a.k.a. the vapor point) that a temperature range that is too high (at atmospheric pressure) to make it a good refrigerant gas. What you want is somethingat condenses/boils at a very low temperature (such as CFCs or even more thermodynamically efficient, ammonia). That way, the liquid absorbs heat while turning into vapor at a low temperature.

Consider why water is a terrible refrigerant. It has a great heat capacity, which is desireable, but it boils at 212 degrees, and has to be very strongly de-pressurized to boil at low enough temperatures to be useful. This takes a lot of energy to do. Freon (a CFC) boils at a very low temperature without having to be de-pressurized so much, so exposing it to a relatively cool temperature and lowering the pressure a bit using a pump is enough for it to suck heat out of whatever you need to chill. The sucked up heat is then expelled out the heat exchanger by compressing it until it condenses and gives off its heat. (That's the compressor thing in your frigerator: it compresses the heat output side of the loop, and decompresses the refrigerating side.)

What I described is a cycle that involves condensation, known as the Rankine cycle. An even better cycle is the Stirling cycle, where the working gas doesn't even condense. The Stirling cycle is far superior to the Rankine cycle, but so far, it is only used in super efficient refrigerant systems that are used to make liquid nitrogen, dry ice, etc.

Ideally, I would like to see the Stirling cyle heat engine used to generate power, and stirling cycle heat-pumps used to refrigerate. However, there has long been a fixation on Rankine cycle for reasons that are not as valid any more. In the old days, stirling cycle systems had lots of disadvantages that have since been made moot points by advances in material science and other fields of engineering. Let's hope the stirling cycle becomes commonplace.

BTW, when I said that ammonia is the "best" refrigerant, I mean that it is the most thermodynamically efficient: it moves the most heat with the least energy required to do so.

BTW, when I said that ammonia is the "best" refrigerant gas, I mean that it is the most thermodynamically efficient: it moves the most heat with the least energy required to do so (while remaing a gas in the useful temperature range).

FYI, if you're wondering why I advocate stirling cycle engines and refrigerators, and how they can be the same cycle, here's an explanation:

When you raise the temperature on the heat-input part of a Stirling engine, and lower the temperature on the heat sink part, the Stirling engine extracts work out of the temperature difference extremely efficiently so long as it has a good regenerator and a decent compression ratio. (A very good stirling engine actually approaches the theoretical maximum possible efficiency the temperature difference can support.)
In other words, put the stirling engine between a temperature difference, and you get work extracted from the temperature difference.

Because the Stirling cycle is reversable (unlike the Otto and Diesel cycles, which are used in cars) the reverse is also workable for refrigeration: put work into the engine, and you'll get a temperature difference between the two "halves" of the engine. Put the cold side in a box, and the box becomes a refrigerator, with the heat sucked out of it and output on the hot side. And likewise, it is the most efficient way of getting a temperature difference for the amount of energy spent.

Considering the waste heat that we continue to have with in internal combustion car engines (even the hybrids), has anyone investigated the old Servel refrigerator cooling system as an option? Benefits: No compressor under the hood, and no moving parts other than the engine cooling fan and the hvac fan that are already in place. Better fuel economy as a result of the lack of a power robbing compressor. There is already a ready source of heat at hand to run the process.

I wonder if it would be feasable to design an automotive air conditioning system based on continuos absorption (amonia) useing the heat generated by the engine, similar to propane powered refrigerators, by tapping the heat from within the waterjacket at the cylinder wall( should provide AMPLE heat energy ) elimenating any drag from a conventional belt driven compressor.

The most important natural refrigerants that we must urgently develop as alternatives to the Potent Industrial Greenhouse Gases (PIGGs), the HCFCs and HFCs are ammonia, CO2 and hydrocarbons (such as Propane R290, and Iso Butane R600a, but there are others).

In automotive air conditioning applications hydrocarbons (HCs) are by far the most suitable alternatives beacuse they work very well in existing systems, much better in fact than the HFC R134a, as they operate at about 30% lower pressure, thus causing less system wear, are more efficient (using less petrol to run the system, although this is hard to quantify due to the number of variables that need to be considered), and achieve much lower vent temperatures faster, especially in high ambient temperatures. HCs are compatible with all lubricants, valves and seals, and can be used in old R12 or R134a systems without modification. Their Global Warming Potential is 3 (compared to 1400 for R134a), they are non toxic and are only flammable in a 2-9% mixture in air. In practice, any leaks into the passenger cabin are so small that this is never acheived, and in spite of a huge amount of scare-mongering by fluorochemical manufacturers and vehicle manufacturers they have been in widespread use in North America, Australia, the Middle East and Asia for over 10 years. In my part of the world, HCs have recently been awarded the independent Australian "Good Environmental Choice" eco-label, and are enjoying spectacular sales growth, particularly in NSW since a ridiculous regulatory prohibition was removed in September 2005.

There has been not one recorded fire or explosion incident resulting from the use of HCs in Motor Vehicle Air Conditioning Systems (MVACS), in spite of risk asessments predicting around 50 such incidents per year (see the paper from the International Journal of Refrigeration by Dr Ian Maclaine-Cross available from http://wwwhychill.com.au for details).

There are a number of manufacturers of HC refrigerants that have been designed for use in MVACS, and while we are still waiting for a major manufacturer to start using them in new systems, with the increasing realisation of the huge global warming impacts of HFCs, it is to be hoped that this day is not far off. In the meantime, the next time your air-conditioner needs regassing, I suggest you seek out a mechanic who has seen through the lies perpetrated by the fluorolobby that HCs are too dangerous to be used in MVACS, and use your car AC with a clear conscience.

The fluorochemical manufacturers are right to be concerned. They know how well HCs perform in existing equipment, they stand to lose a huge proportion of their near monopoly on the automotive refrigerant market, and hundreds of millions of dollars in annual global sales of their patented HFCs. Yet these gases are among the most rapidly growing source of greenhouse gases, are 'controlled' under Kyoto, and must be rapidly phased out entirely if the rest of us are to leave a planet worth living on to our children.

The solutions are sitting on the shelf, the only question is how long it will take for us all to wake up and demand that they be implemented.