Comparative energy and exergy analysis of R744, R404A and R290 refrigeration cycles

Comparative energy and exergy analysis of R744, R404A and R290 refrigeration cycles .............................................................................................................................................................. J.A. Shilliday1,2,*, S.A. Tassou2 and N. Shilliday1 1 Shilliday Refrigeration and Air Conditioning, 12 Ballinahonemore Road, Armagh BT60 1ED, UK 2 School of Engineering and Design, Brunel University, Uxbridge, Middlesex UB8 3PU, UK ............................................................................................................................................. Abstract A detailed energy and exergy analysis of the low global warming potential refrigerants R744 and R290 was preformed and compared against the commercial refrigerant R404A in a single-stage vapour compression cycle and R744 in a two-stage vapour compression cycle with an internal heat exchanger. Keywords: R744; exergy; refrigeration; CO2; irreversibility *Corresponding author: Received 23 March 2009; revised 11 May 2009; accepted 11 May 2009 ................................................................................................................................................................................ 1 INTRODUCTION The refrigeration industry has changed immensely since chlorofluorocarbons (CFCs) such as R12 were identified as a source of ozone layer destruction and were replaced by hydrochlorofluorocarbons (HCFCs) and more recently by hydrofluorocarbons (HFCs) such as R404A. Most HFCs although have zero ozone depletion potential (ODP), possess a high global warming potential (GWP) and contribute significantly to the total greenhouse gas emissions from refrigeration systems. For this reason, natural refrigerants are now being considered as a potential solution for the future. Natural refrigerants possess environmental-friendly properties such as zero ODP and low GWP when compared with traditional CFCs and HCFCs. Of the number of natural refrigerants available, R744 (carbon dioxide) has gained considerable attention since Lorentzen and Pettersen [1] published the first experimental results of a prototype transcritical R744 refrigeration system with internal heat exchanger. In a subsequent publication [2], the energy performance of the system was demonstrated by exergy flow charts indicating higher exergy destruction for the gas cooling and throttling processes but lower exergy destruction for the compression and evaporation processes when compared with a R-12 system. First law energy analysis determines thermodynamic efficiency and can be used for comparative analysis of alternative cycles; the coefficient of performance (COP) is calculated and used for this purpose. Irreversibilities within these cycles reduce the cycle’s COP and must be minimized in order to increase a cycle’s efficiency. First law energy analysis does not determine where these irreversibilities occur, in which component or process, or the magnitude. Second law exergy analysis can be used to determine both the location and magnitude of the irreversibilities relative to the other processes or components in the cycle. This makes exergy analysis a powerful tool in the design, optimization, and performance evaluation of refrigeration cycles [3]. There have been a number of studies on the exergy analysis of refrigeration cycles. The exergy analysis of an ammonia vapour compression refrigeration system was carried out by Yumrutas et al. [3]. They reported that evaporating and condensing temperatures had a strong effect on the exergy losses in the evaporator and condenser but little effect on the other components of the cycle. A paper by Stegou-Sagia and Paignigiannia [4] compared the performance of refrigerants R-404A, R-410A, R-410B and R-507, as alternatives to R-22. They concluded that the exergy behaviour of these refrigerants was generally inferior to that of R22. Aprea et al. [5] carried out exergy analysis of R-22, R-407C and R-507 for a variable speed compressor refrigeration system. R-22 provided the best performance overall, with R-407C being the best from the alternatives. Yang et al. [6] analysed thermodynamically a transcritical carbon dioxide cycle with and without an expander. In the expander cycle, the main exergy losses occurred in the gas cooling and compression processes. A transcritical heat pump system for simultaneous heating and cooling operation was analysed by Sarkar et al. [7]. They found that the temperature difference between the heat exchangers (evaporator/condenser/ gas coolers) contributed to more than 90% of the irreversibilities. Li and Groll [8] preformed an exergy analysis on a carbon dioxide/ammonia cascade refrigeration system and found the optimal cascade condenser temperature to be 2158C based on the minimization of exergy destruction. International Journal of Low-Carbon Technologies 2009, 4, 104– 111 # The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please email: doi:10.1093/ijlct/ctp014 Advance Access Publication 16 June 2009 104 Comparative energy and exergy analysis In another paper by Yang et al. [9] the exergy efficiency of a two-stage transcritical carbon dioxide cycle with an expander was found to be 9.1% lower than that of a single-stage cycle with an expander. A recent study by Cavallini and Zilio [10] found that the process that mostly penalizes the carbon dioxide transcritical cycle compared with a R-22 cycle was throttling. This paper also discussed that the introduction of two-stage compression process with a gas intercooler between compression stages would decrease the work of the compressors and reduce both compression and heat rejection irreversibilities. A paper by Girotto et al. [11] considered the in-field performance of an all R744 system compared with a traditional R404A system. The R744 system operated transcritically when the ambient temperature rose above 158C and was estimated to have an annual energy consumption of 10% higher than that of a R-404A system. This was attributed to the increased energy consumption of the medium temperature refrigeration equipment during the summer months when the ambient temperature rose above 158C. To date it is generally accepted that a transcritical R744 system has a lower COP compared with a R-404A system. In this paper a comparative exergy and energy analysis is preformed on transcritical R744 cycles for commercial refrigeration use and compared with R-404A and the Hydrocarbon R-290. This study aims to identify the worst performing components in a transcritical R744 cycle and indicate how the exergy destruction in the cycle can be decreased by introducing variations to the basic cycle. The EES software was used as the basis for the thermodynamic analysis of the cycles and system components [12]. Figure 1. Schematic diagram of a single-stage refrigeration cycle. 2 EXERGY AND ENERGY ANALYSIS 2.1 Vapour compression cycles (...truncated)