Research projects at ILK Dresden are investigating how the natural substance carbon dioxide can be used as a working fluid for specialized refrigeration applications in the temperature range below -50°C.
For carbon dioxide, sublimation (that is, the transition from the solid to the gaseous phase) is necessary, since its triple point is at -56.6°C.
The basic idea for achieving low temperatures is to utilize the heat of sublimation from dry ice in a manner similar to evaporation in a conventional compression refrigeration process.

Development of a Sublimation Heat Exchanger

A specific area of application in refrigeration technology is the provision of cooling capacity in the low-temperature range below -50°C, which is required, for example, for laboratory operations, freeze-drying, or cryomedicine. Currently, synthetic refrigerants with very high GWP values are mostly used for such applications; these are still permitted under an exemption in the EU F-Gas Regulation 517/2014. This is because, for safety reasons, no flammable substances are accepted as alternatives in this specific segment of refrigeration technology. Furthermore, other technologies, such as cold gas processes, are generally not technically or economically viable alternatives in this temperature and application range.

For this reason, ILK Dresden worked on a research project to develop an environmentally friendly alternative for refrigeration below -50 °C that is also acceptable to industry. The natural, non-flammable substance carbon dioxide is used as the working fluid. The triple point of carbon dioxide is -56.6°C. Therefore, evaporation (i.e., the phase transition from liquid to gas) cannot be used to generate cold, as in the conventional cold vapor process; instead, sublimation (i.e., solid to gas) must be utilized below the triple point. This requires new technologies, including a sublimation heat exchanger that can be integrated into the CO₂ refrigeration cycle as a standard component, just like a conventional evaporator. Such a sublimation heat exchanger was developed as part of the research project. Numerous tests and studies were conducted using it, demonstrating the practical feasibility of the newly developed sublimation heat exchanger.