ICR0100 International Congress of Refrigeration 2003, Washington, D.C. 1 FUNDAMENTAL ASPECTS OF THE APPLICATION OF CARBON DIOXIDE IN COMFORT COOLING J. Steven Brown * , Piotr A. Domanski ** * Catholic University of America, 620 Michigan Avenue, N.E., Washington, DC, USA, Tel: 202-319-4738, e-mail: brownjs@cua.edu, ** National Institute of Standards and Technology, Gaithersburg, MD, USA ABSTRACT This paper presents entropy generation analyses for the evaporator, compressor, gas cooler, expansion device and liquid-line suction-line heat exchanger for a transcritical carbon dioxide cycle for automotive and residential air- conditioning systems, and presents entropy generation analyses for the evaporator, compressor, condenser, and expansion device for a subcritical R134a automotive air-conditioning system and a subcritical R22 residential air- conditioning system. The analyses show that the CO 2 automotive air-conditioning system generates 36 % more entropy than the R134a automotive air-conditioning system, and that the CO 2 residential air-conditioning system generates 63 % more entropy than the R22 residential air-conditioning system. The biggest contributors to the lower CO 2 performance are irreversibilities associated with the heat rejection process and the expansion related processes. INTRODUCTION Over the last two decades, the refrigeration and air-conditioning industry has been undergoing major changes due to an increasing awareness of the impact that the use of refrigeration and air-conditioning equipment can have on the environment. The major environmental concerns have been ozone depletion and global warming. The Montreal Protocol and its amendments have largely settled the ozone depletion problem. To date, most practitioners have chosen HFC refrigerants as replacements for CFC and HCFC refrigerants (Dupont, 2001). However, the continued use of HFC refrigerants is questioned because of their global warming potentials. With this increasing scrutiny on the use of HFC refrigerants, many researchers are investigating so-called natural refrigerants, e.g., hydrocarbons, air, water, ammonia, and carbon dioxide, with carbon dioxide receiving substantial focus beginning in the early 1990’s (e.g., Lorentzen and Pettersen, 1992). Since that time, several research groups have studied carbon dioxide in various applications, including, mobile air conditioning, heat pumping, residential and commercial air conditioning, and commercial refrigeration. Given the intense interest in the transcritical CO 2 cycle for air-conditioning applications, this paper attempts to contribute to the understanding of how the transcritical CO 2 cycle compares to typical subcritical halocarbon cycles. To this end, we use an entropy generation methodology to compare the amounts of entropy generation in each of the primary system components, and to demonstrate the possibilities for improvements to the transcritical CO 2 cycle. 1 SYSTEM ENTROPY GENERATION The purpose of an air-conditioning system is to transfer heat from a low-temperature source to a high-temperature sink while using the least amount of work, i.e. to maximize the Coefficient of Performance (COP) for a given cooling capacity at given source and sink temperatures. We can restate this goal in terms of entropy using the Gouy- Stodola Theorem. That is, the purpose of an air-conditioning system is to transfer entropy from a low-temperature source to a high-temperature sink while generating the least amount of entropy, or stated in another way, the goal is to generate the least amount of entropy for a given cooling capacity. To begin the analysis, consider the generic air-conditioning system shown in Fig. 1 operating in a steady-state cycle. The energy balance is: L H Q Q W & & & − = (1)