Theoretical Analysis of the Impact of an Energy Recovery Expansion Device in a CO_2 Refrigeration System

摘要

Carbon dioxide (CO_2) is being widely used as a refrigerant in HVAC&R applications due to its low Global Warming Potential (GWP). There are many aspects of CO_2 systems that make it unique to other traditional refrigerants in that it has higher pressure levels and typically operates at transcritical levels. Up to 40% of the losses in a transcritical CO_2 cycle can be due to throttling across the expansion valve. These significant losses make CO_2 systems ideal for installing an energy recovery expansion device that consists of a nozzle, micro-turbine and a generator. The expander has been developed by Regal Beloit Corporation and will be referred to as the Viper Expander. The device functions by using a nozzle to convert the high pressure of the refrigerant into a high velocity jet that is directed into the impeller of the micro-turbine. The turbine impeller then spins a shaft that is coupled with a generator to generator electrical energy. The Viper Expander is to replace the standard passive expansion device. Experimental testing of this device with R410A indicates that the device performs better for refrigerant with low viscosity and high pressure. For these reasons, the implementation of this energy recovery device in a CO_2 refrigeration system has been investigated. The results of this paper quantify the potential impact that this device could have in the system by predicting theoretical recoverable power and improvements to the coefficient of performance (COP). The analysis predicts that the Viper Expander can improve the COP from 1 - 11% and indicates that further experimental investigation should be conducted to verify the results.