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Text from PDF Page: 001IGSHPA Technical/Research Conference and Expo Denver March 14-16, 2017 Direct expansion ground source heat pump using carbon dioxide as refrigerant: Test facility and theoretical model presentation Parham Eslami Nejad Messaoud Badache Mohamed Ouzzane Zine Aidoun ABSTRACT In an attempt to address recent challenges on using natural refrigerants and to develop further knowledge and expertise in the field of direct expansion ground source heat pump (DX-GSHP), an experimental transcritical carbon dioxide (CO2) test bench was built at CanmetENERGY Research Laboratory. A previously developed theoretical model of the system was modified and validated against a set of experimental results and adopted to investigate the system performance in a wide operating range. A parametric analysis was also performed using the theoretical model for understanding the system and at exploring the performance improvement actions for future installations. Validation results showed that the model predicts the experimentation very well within the uncertainty of the measurement. Furthermore, parametric analysis showed that improper control of some parameters such as gas cooler CO2 outlet temperature and discharge compressor pressure can degrade the system performance by up to 25% and the heat pump heating capacity by 7.5%. INTRODUCTION Although the detrimental environmental impacts of conventional refrigerants have raised global concern, due to the worldwide growing energy demand, high energy efficiency of heat pumps still remains a great incentive for using this technology in residential and commercial buildings. Over the last decade, several studies have been conducted to replace synthetic refrigerants with natural ones. Among the candidates, CO2 has been attracting more attention as it is environmentally benign and safe. This, together with the established energy efficiency advantages of the GSHP, makes the CO2 GSHP a promising, environmentally friendly, and energy efficient alternative to other heating equipment. However, the scarcity of related technical knowledge may slow down its development pace. Many studies have been performed on the air-source and water-source transcritical CO2 heat pump systems that are not the focus here; in general however, modeling of the DX-GSHP has been rarely studied (Ndiaye 2016). Kruse and Russmann (2005) and Bertsch et al. (2005) proposed a ground heat pipe technology with two-phase CO2 as a secondary fluid for extracting heat from the ground and transferring it to the GSHP using the thermosyphon principle. Both studies used pipe-in-pipe configurations. They compared the proposed system with a conventional system using a single phase water/brine solution. Recently Mastrullo et al. (2014) performed modeling of a CO2-filled U-tube ground heat exchanger (borehole) under thermosyphon principle for the secondary loop GSHP systems. Another study by Eslami Nejad et al. (2014) Parham Eslami Nejad (Parham.email@example.com) is a research scientist at Natural Resources Canada, CanmetENERGY. Messaoud Badache, Mohamed Ouzzane and Zine Aidoun are research scientists at Natural Resources Canada.
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