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2054 J. Sarkar et al. / Energy Conversion and Management 46 (2005) 2053ā2067 useful guidelines for system design and its optimisation and help toward energy conservation in heat pump systems based on transcritical CO2 cycles. OĢ 2004 Elsevier Ltd. All rights reserved. Keywords: CO2 heat pump; System simulation; Irreversibility; Exergetic optimization 1. Introduction Discovery of the harmful effects of the synthetic refrigerants on the environment has created a renewed interest in eco-friendly natural refrigerants such as water, carbon dioxide, etc. Its ecolog- ically benign nature, low price, easy availability, non-flammability, non-toxicity, compatibility with various common materials, equipment compactness due to high operating pressures and excellent transport properties are cited as some of the reasons behind the revival of carbon dioxide as a refrigerant. Carbon dioxide based heat pumps offer extensive possibilities in simultaneous heating and cooling applications due to the large temperature glide present in the gas cooler. Exergy based second law analysis of various systems has become a very effective tool to measure system effectiveness and to design the system to maximize energy savings. Some researchers have conducted exergy analyses for conventional vapour compression refrigeration systems and its com- ponents. Bridges et al. [1] have reported second law analysis of a R410a based domestic refrigerator and a split air conditioner with fan coil optimization. Exergy analysis has been reported for an ammonia based vapour compression refrigeration system [2] and also for a conventional refriger- ant based heat pump air conditioning system [3]. Apera et al. [4] conducted out exergy analyses of a compressor speed controlled vapour compression refrigeration system with refrigerant R22 and its substitutes R407c, R417a and R507 and chose R407C as the most suitable substitute. Exergy anal- ysis has also been done for system components such as heat exchangers for optimized performance [5]. Reported exergetic analyses of transcritical CO2 systems [6ā9] have all been based only on the thermodynamic cycle using ideal assumptions. Such analyses for a complete CO2 heat pump sys- tem including the heat exchangers and the secondary fluids and considering entropy generation due to both heat transfer and fluid flow effects have not been reported yet. This paper presents a component level exergy analysis for the carbon dioxide based heat pump to provide heating and cooling services simultaneously. The irreversibilities of all components and the second law efficiency of the system for different values of the operating parameters such as water inlet temperature, ambient temperature, compressor speed and the area ratio of the heat exchangers have been estimated. System optimization based on these results has been presented as well. Finally, techniques to reduce the irreversibility for various components, which leads to improved system exergertic efficiency, have been suggested. 2. Mathematical modelling A simplified sketch of a carbon dioxide based heating and cooling system showing the main components is shown in Fig. 1. Water is supplied as the heat exchanger fluid to both the gas cooler and evaporator through flow control valves. Both these heat exchangers are of the double pipePDF Image | Transcritical CO2 heat pump systems
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