18 May 2017
In light of recent trends towards energy efficiency and environmental consciousness, the heating, ventilation, air conditioning and refrigeration (HVAC&R) industry has been pushing for technological developments to meet both of these needs. As such, several solutions for harnessing the energy released from refrigerants during the free expansion process of a conventional vapor-compression cycle have been developed to increase overall cycle efficiency. Additionally, the implementation of natural refrigerants with reduced Global Warming Potential (GWP) in refrigeration cycles has become increasingly important since current refrigerants are ozone depleting or linked to climate change. This research focuses on the natural refrigerant carbon dioxide (CO2) due to its GWP and its significant potential for energy recovery during the expansion process. The goal is to investigate the potential impact of installing an energy recovery expansion device, known as the Viper Expander, into a transcritical CO2 refrigeration cycle. The Viper Expander operates by using a nozzle to accelerate the high pressure CO2 into a high velocity jet of fluid impinging on a micro-turbine impeller. The impeller is coupled with a generator, which harvests the kinetic energy of the CO2 by converting it into electrical energy that can be fed back into one of the system components, such as a fan or compressor motor. The feasibility of this application was analyzed through modeling performance in the unit’s proposed test stand for concept validation, and resulted in a model prediction of the Viper harvesting up to 7% of the compressor power consumption at certain operating conditions. Initial experimental testing resulted in a Viper isentropic efficiency of 49.3%. Future work will focus on optimizing Viper design with test conditions, as well as relating Viper performance to overall system performance.