Abstract
Efforts to reduce the contribution of refrigeration to global warming have resulted in a phase-down of the use of fluorinated refrigerants. The alternative, natural refrigerants, used in vapor compression refrigeration machines (VCR), such as propane (R290) have significantly lower GlobalWarming Potential and are less harmful to the environment. However, these refrigerants are flammable, which does not make them the preferred choice for some applications like electronics cooling. An alternative refrigeration cycle, the so called Reverse Brayton Cycle (RBC), does not require a refrigerant that undergoes a phase change and can work with air as a refrigerant (R729). Air has the advantages that it is harmless and easily accessible. It can be used for direct cooling, which has the potential to allow more effective and uniform cooling, faster cooldown, and a less complex cooling cycle. Despite its significant efficiency disadvantage at cooling temperatures above -50°C, this can make R729 an attractive refrigerant for innovative and simple refrigeration systems in applications where efficiency is not the most critical parameter. In this paper, the design considerations of compact, electrically driven RBC cooling machines (about 2 kWe) are discussed. Preliminary system design is presented for two different potential applications, one at high cooling temperatures (25 °C), the other at low cooling temperatures (-90 °C). The design process of the turbine stage of a RBC cooling machine including a method for matching compressor and turbine are presented. A demonstrator was built and tested achieving a cool down of about 28 °C.
