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study on evacuated tube solar collector using supercritical CO2

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study on evacuated tube solar collector using supercritical CO2 ( study-evacuated-tube-solar-collector-using-supercritical-co2 )

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Available online at www.sciencedirect.com Applied Thermal Engineering 28 (2008) 1225–1233 An experimental study on evacuated tube solar collector using supercritical CO2 X.R. Zhang a,b,*, H. Yamaguchi a a Department of Mechanical Engineering, Doshisha University, Kyoto 630-0321, Japan b Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, China Received 22 June 2006; accepted 19 July 2007 Available online 27 July 2007 www.elsevier.com/locate/apthermeng Abstract A solar collector using supercritical CO2 as working fluid is proposed in this paper. In order to investigate and estimate the CO2-based solar collector, an experimental set-up was constructed. Of particular interest of this paper are the basic collector characteristics, such as CO2 temperature and pressure in the collector, CO2 flow rate, and collector performances. The collector has been tested under various weather conditions. The results show that the CO2 temperature, CO2 pressure and mass flow rate increase with the solar radiation, which is different from those of traditional solar collector using liquid as working fluid. The solar radiation has influence on the CO2 states, being liquid, liquid-gas or supercritical state in the test, furthermore, affects the CO2 mass flow rate. The annually-averaged collector efficiency is found to be above 60.0% in the case of supercritical CO2 as working fluid, which is much higher than that of water-based solar collector. This study shows the potential of the supercritical CO2-based solar collector in the field of solar thermal utilization. Ó 2007 Published by Elsevier Ltd. Keywords: Solar collector; Supercritical CO2; Solar energy; Solar thermal utilization 1. Introduction The environmental pollution and the energy crisis have brought serious problems to the world environment and sustainable development. The applications of solar energy to electricity generation and heat collection/refrigeration become important, and have received considerable atten- tion [1–8]. The solar collector is the heart of these solar energy utilization systems. During the last two decades a number of researchers have worked on developing new and more efficient solar collector or improving existing ones [9–11]. For example, the performance of a water-in- glass evacuated tube solar heater is investigated and factors * Corresponding author. Address: Department of Mechanical Engineer- ing, Doshisha University, Kyoto 630-0321, Japan. E-mail address: scho@mail.doshisha.ac.jp (X.R. Zhang). 1359-4311/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.applthermaleng.2007.07.013 influencing the operation of water-in-glass collector tubes are discussed. The results show the existence of inactive region near the sealed end of the tube which might influ- ence the performance of the collector [12]. However, almost all the previous studies to improve col- lector performances are based on the methods of changing solar collector structures, improving the absorptivity of the coating or reducing heat loss of the collector. And as far as the authors are aware, few studies on the influence of work- ing fluids on collector performances appear to exist in the open literature. Solar collectors can drive machines only if the working fluid temperature is greatly in excess of the normal boiling point of water. Among the working fluids, water can only be used above 0 °C and also water has to be used in a high working pressure. Air thermodynamic cycle efficiency is very low. Ammonia is toxic and silicon oil has high viscosity and difficult to handle. In addition, from the viewpoint of protecting the ozone layer and pre- venting global warming, there is now strong demand for

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