Development of Multi-Stage Two-Evaporator Transcritical Carbon Dioxide Cycle for Experimental Comparisons of Expansion Work Recovery Technologies

摘要

As environmental concerns and accompanying regulations grow around the world, the Heating, Ventilation, Air Conditioning, and Refrigeration (HVAC&R) industry is working to develop technologies that utilitze low-global warming potential (GWP) refrigerants and remain within competitive coefficient of performance (COP) values of previous hydrofluorocarbon (HFC) systems. Carbon Dioxide (CO_2) has been investigated extensively in the past 25 years as a potential substitute for HFCs in refrigeration application in mild ambient climates. In efforts to increase the efficiency of CO_2 systems, researchers and industry have identified cycle modifications that are particularly beneficial in transcritical CO_2 cycle application, such as expansion work recovery and economization. This paper presents an overview of the design and operation of a multi-stage transcritical CO_2 cycle with two independently-controlled evaporators. The cycle utilizes three stages of compression with intercooling between the second and third stages, as well as flash tank economization at each evaporator, including evaporator bypass lines for vapor from the flash tank. Furthermore, the cycle was designed to allow transition between methods of expansion work recovery without the need to stop the compressors. The cycle can switch between electronic expansion valve (EXV) mode, ejector mode, and a turbomachine expander mode with the actuation of several ball valves mid-operation. The purpose of this work is to describe the theory and rationale behind system design, discuss how the test stand will be controlled, and provide instruction on the transition between expansion methods without shutting down the compressors.