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1226 X.R. Zhang, H. Yamaguchi / Applied Thermal Engineering 28 (2008) 1225–1233 Nomenclature A effective area of solar collector, m2 h1 specific enthalpy value at the collector outlet, J/kg h4 specific enthalpy value at the collector inlet, J/kg I solar radiation, kW/m2 It total solar radiation during the test time period per day, Eq. (1) J mCO2 CO2 mass flow rate in the CO2 loop, kg/s mCO2 time–weighted average mass flow rate of CO2 fluid per day, Eq. (2) kg/s qct total heat quantity collected in the collector dur- ing the test time period per day, J td test time period per day, s Greek symbols gcollector solar collector efficiency gcollector the time-weighted daily average collector effi- ciency, Eq. (6) technology based on ecologically safe ‘natural’ working fluids. Carbon dioxide (R-744) is a non-flammable and non-toxic fluid and friendly to environment. CO2 has a very low Ozone Depletion Potential (the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11) and Global Warming Potential (the ratio of the warming caused by a substance to the warming caused by a similar mass of carbon dioxide) [13]. The ther- modynamic and transport properties of CO2 seem to be favorable in terms of heat transfer and pressure drop, com- pared to other typical fluids [13]. The critical pressure and temperature of CO2 are 7.38 MPa and 31.1 °C, respec- tively. This critical temperature is much lower than those of other working fluids. Therefore, the objective of this paper is to propose a novel solar collector – a collector using supercritical carbon dioxide as working fluid. The present study is a logical extension of the recent work of the authors [14–17], in which a CO2-based solar thermody- namic cycle is investigated. A prototype machine [16] was designed and tested to give the most basic experimental data for the proposed idea of using CO2 as working fluid in solar Rankine cycle to generate power and collect heat. A mathematical model [14,18] was set up in order to investigate the performances and characteristics of the CO2-based thermodynamic cycle powered by solar energy. Furthermore, an experimental work [19] was carried out during one year and its result was presented to show the annual performance of the CO2 thermodynamic cycle. And a numerical model and code was developed to study flow behavior and heat transfer characteristics of the super- critical CO2 in a horizontal circular tube [20], in which the CO2 flow is found never being fully developed not only in flow field, but also in temperature field in the tube. Based on the above investigations, it is found that the CO2-based thermodynamic cycle gives a high efficiency of combined power generation and thermal energy supply. As the first step of a fundamental understanding and estimation of the proposed solar collector, in this paper, experimental investigations were conducted in order to study its basic characteristics, including CO2 temperature, pressure, mass flow rate in the collector and collector efficiency and so on. 2. Experimental set-up In order to study the CO2-based collector characteristics well, a closed CO2 loop including the collector is necessary. The CO2 loop is designed and it consists of a solar collector array, flow regulating valve (throttling valve), heat exchanging system, and feed pump. The details of the experimental set-up are shown in Fig. 1. The solar collector is used to heat CO2 fluid contained in heating channels and increase CO2 temperature. The supercritical CO2 flows through the valve, which can be used to adjust the CO2 flow rate for the present study. The CO2 flowing out of the valve is cooled in the heat exchanging system. After that, it is pumped by the feed pump, back into the higher pressure condition in the solar collector. As shown in Fig. 1 the experimental set-up is a closed cycle of CO2 fluid, which is mainly comprised of evacuated solar collector arrays, a throttling valve, heat exchangers 1 and 2 (CO2/ water heat exchanger), liquid CO2 feed pump, and mea- surement and data acquisition system. In the experimental set-up, the valve (WHITEY needle valve) can provide various extents of opening for the cycle loop. To effectively heat CO2 to a higher-temperature supercritical state in the experimental set-up, all-glass evac- uated solar collectors with a U-tube heat removal system are used, shown in Fig. 2a and b. These collectors consist of a glass envelope (38 mm in diameter) over an inner glass tube (27 mm in diameter) coated with a selective solar absorber coating. This coating with a high solar absorptiv- ity 0.927 and a low emissivity 0.193 is applied on the vac- uum side of the inner glass tube. The transmissivity of the glass envelope is 0.930. The absorbed heat is conducted through the inner glass tube wall and then removed by heat removal fluid in a stainless U-tube (3.6 m in length and 0.005 m in internal diameter) inserted in the inner tube with an aluminum fin (1.7 m in length) connecting the outlet arm of the U-tube to the inner glass tube. The thickness of the stainless tube in the collector and all the CO2 loops in the system is 1.0 mm. In the present study, evacuated solar collector of efficient area 9.6 m2 is used. As the first step of the study, the collector used is commercial productPDF Image | study on evacuated tube solar collector using supercritical CO2
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