두 개의 다른 증발온도를 가지는 이단압축 이산화탄소 사이클의 해석적 연구

Abstract

HCFC, which is widely used as a cooling agent in air conditioning systems is subject to international regulation regarding environmental pollution, global warming and ozone depletion. Among the alternatives, environment-friendly carbon dioxide generates a lot of interest because of its good compatibility with used materials and media and high production capacity. However, the refrigerant has many other thermodynamic characteristics compared with the existing subcritical cycle, which are cooling in supercritical region and high pressure operation of the system. Therefore, supercritical carbon dioxide system must improve the system performance, safety and reliability in order to compete with the existing subcritical system or replace previous used HVAC systems. Generally, performance capacity of a heat pump using carbon dioxide is lower than that of subcritical refrigeration system because of large irreversibility during compression and gas cooling. Moreover, system reliability is rather low because of strong performance variation for variable outdoor temperature. For using in commercial refrigeration systems, system performance reliability must be improved. For CO2 heat pump the use of two-stage compression cycle offers the best means to this end. In earlier studies, performance research of the CO2 cycle has been mainly performed for a single-stage compression cycle using analytical methods. Only few studies have considered CO2 systems with two-stage compression cycle. Therefore, it is very important to analyze performance characteristics of the two-stage CO2 compression cycle with differential temperature evaporators and find out optimal operating pressure and mass distribution. In this study, the performance characteristics of a two-stage CO2 system with two-different evaporator temperature have been analyzed according to outdoor air temperature, outdoor air velocity, and 2nd-stage EEV opening in order to increase the better understanding of the thermodynamic characteristics of the CO2 cycle. Through the verification of evaporator model with frost growth, the frost growth with time shows a similar trends under same frost surface temperature condition. Beside, the variation of cooling capacity of the present model has the similar one of other studies. As a result of simulation on performance of two-stage compression cycle, the cooling COP decreases by 22.9%, 16.1%, 18.9%, and 19.4% for 0%, -10%, -20%, and -30% of non-frost cooling capacity, respectively, when outdoor temperature increases. When the air velocity increases, the cooling COP improves by 9%, 9.2%, 7%, and 7.1% for 0%, -10%, -20%, and -30% of non-frost cooling capacity. Besides, the system has the maximum cooling COP for –10%, -20% of non-frost cooling capacity when the 2nd-stage EEV opening is 56%. Maximum cooling COP for –30% of non-frost cooling capacity has 1.003 when the 2nd-stage EEV opening is 52%.