Design and Testing of a Compact, Reverse Brayton Cycle, Air (R729) Cooling Machine

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, such as propane (R290), have significantly lower global warming potential (GWP) and are less harmful to the environment. R290 is however flammable, which is problematic for applications like electronics cooling. Reverse Brayton Cycle (RBC) refrigeration can work with air as refrigerant (R729). Air is completely harmless; it can be used for direct cooling, allowing more effective 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 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, high cooling temperatures (25 °C) and low cooling temperatures (−90 °C). The design process of the turbine stage of an RBC cooling machine including a method for matching compressor and turbine is presented. A demonstrator was built and tested.