Here is an interesting article on transcritical CO2 heat pumps. I dislike the terms "organic vs. inorganic as my many hours in organic chem labs make me aware that anything with carbon in it is "organic".

Transcritical CO2 heat pump / Industrial Heat Pumps

These CO2 driven heat pumps are now in production and work incredibly efficiently. Even better than the original ammonia driven systems (incredibly dangerous) were very efficient.


Steve

The thing I don't like about the CO2 systems is the enormously high working pressure. These things have high-side pressures over 1000-1500 PSIG! Not a bad thing if you buy a unit and never mess with the refrigerant circuit, but not a good unit to hack into. The R-410 units have a high-side pressure range that is difficult enough to deal with for me.

Cryogenic refrigeration
 

Take a look at the extreme system kids and their projects. The amount and depth of knowledge is astounding.

Vapor Phase Change Cooling

The main goal of their cryogenic cooling rigs is to present as low a temperature as possible to the heat-producing parts of their computers. That way, they can overclock and overdrive their systems to levels that would normally not be attainable. Most of these cooling systems are not built with efficiency (or simplicity) in mind; evaporator temperature and capacity take priority over COP. Accordingly, the systems of choice for that application are the classical cascade cooler and the autocascade evaporator.

At first glance, the site appears as a couple of gurus directing a massive amount of laymen. But that's not the case at all. In fact, there are many experienced builders who are members, a whole lot of engineers, refrigeration scientists, and other technically astute members. Being inside the realm of computer science and IT helps out in that most technically challenged individuals don't reach the point of needing cryogenic cooling, I'm sure.

In this respect, the site parallels that of this forum. The "average" computer user, homeowner or renovator is satisfied with a certain level of performance at a certain price point, doing a certain amount of damage to the environment or their wallet (directly or indirectly) in the process. An experienced Ecorenovator or Xtremesystems forum member seeks a higher level of performance than is considered "sufficient" to meet their demands.