heat pump water heater using R744

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International Journal of Thermal Sciences 132 (2018) 1–13 Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts Experimental investigation on a heat pump water heater using R744/R290 mixture for domestic hot water Fujun Jua,b, Xiaowei Fanb, Yaping Chena,∗, Tuanjie Wangb, Xiangyang Tangb, Amin Kuangb, Shengfei Mab a Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China b School of Energy and Environment, Zhongyuan University of Technology, Zhengzhou 450007, China T ARTICLE INFO Keywords: Heat pump water heaters Substitute refrigerants Zeotropic mixtures R744/R290 COP Temperature profile 1. Introduction The building sector accounts for about 40% of the total energy consumption and domestic hot water supply has become the main contributor to residential building energy consumption [1]. Heat pumps, as efficient energy conversion and emission reduction means, can be adopted to supply domestic hot water efficiently instead of electric water heaters and gas water heaters. However, one of the challenges for heat pumps is to find the appropriate substitutes for the conventional refrigerants such as R22, R134a, R410A and R407C, which have been in prohibition or phase-out in conventional heat pumps or refrigerators for their higher ODP (ozone depletion potential) or GWP (global warming potential) effects [2,3]. With the increasing awareness of environment protection, one of attractive solutions to avoid high environmental impacts of the traditional refrigerants is the use of the natural refrigerants, such as hydrocarbons (HCs), R744 (carbon dioxide) and ammonia or their mixtures [4]. Among the natural refrigerants, HCs (hydrocarbons) offer environ- mental friendliness (null ODP and negligible GWP), non-toxicity, availability and high compatibility with mineral or synthetic oil [5,6]. R290 and R600a were used as refrigerants before the invention of CFCs (chlorofluorocarbons). In recent years, R600a has been successfully used in domestic refrigerators in China, Germany and other European countries [7]. R290 is technically valuable for replacement of R22 in ABSTRACT The natural eco-friendly mixture of R744/R290 was investigated to replace R22 in a fully instrumented water- water heat pump test rig. The test results show that the R744/R290 mixture with optimum concentration of 12%/88% (Mopt) is the suitable working medium for heat pump water heaters because of its higher COP and heating capacity than those of R22 under variable conditions as well as the nominal working condition. The heat pump system with Mopt can supply higher temperature hot water and promote compressor lifetime in sub- stitution of R22. The test results also reveal that the heating COPs of heat pump system with Mopt are 4.98–11.00% higher than those of R22 at the heat sink outlet temperature ranging from 45 °C to 65 °C, and the heat sink outlet temperature has obvious effect on the system efficiency but little on the heating capacity. heat pump water heater system, tumble dryers and heat pump type air condition system because of better energy efficiency, higher compat- ibility with material and lubricant oils and lower cost [8–12]. It is safe to use HCs as refrigerants in the allowable quantity, but if the charge quantity exceeds the danger threshold, some safety measures must be taken to reduce the risks in operation because of their high flammability [13]. In recent years, R744 (CO2) has regained attention as a promising alternative for its favorable features such as environmental friendliness, safety (non-toxicity and non-flammability), availability, and good compatibility with lubricants and materials, good thermodynamic and transport properties [14]. Due to the low critical temperature of 31.1°C, the transcritical R744 cycle technology was developed by Lorentzen and has been applied in refrigeration, air-conditioning and heat pump applications, especially the domestic heat pump water heaters [15–17]. However, there exist two main drawbacks for the R744 transcritical cycle: significantly high operating pressure and the low system energy efficiency mainly related to large throttling loss in expansion process. At present, there is no perfect pure refrigerant that meets all the requirements [18]. However, by mixing purposefully two or more pure refrigerants, a desired mixture might be obtained which can eliminate or alleviate the undesired properties of pure refrigerants. For example, the zeotropic blend of R744/R290 has been proposed as the potential ∗ Corresponding author. Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University Nanjing 210096, China. E-mail address: ypgchen@sina.com (Y. Chen). https://doi.org/10.1016/j.ijthermalsci.2018.05.043 Received 14 October 2017; Received in revised form 27 April 2018; Accepted 28 May 2018 1290-0729/ © 2018 Published by Elsevier Masson SAS.

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