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1232 X.R. Zhang, H. Yamaguchi / Applied Thermal Engineering 28 (2008) 1225–1233 At last, the measured data is presented in the spring, summer, autumn and winter seasons in Figs. 9 and 10. The experiments shown in Figs. 9 and 10 were made from August of 2004 to July of 2005. April, July, November and January are, respectively selected as representative spring, summer, autumn and winter season in Kyoto area. Fig. 9 demonstrates the time-averaged CO2 flow rate with the solar radiation under four different seasons. It can be seen that during the tests of the four seasons, the solar radiation changes in the range of 50–210 MJ in Kyoto area. The measured data of the CO2 flow rate varies from 0.007 kg/s to 0.023 kg/s. The obtained heat quantity col- lected in the collector and collector efficiency are shown in Fig. 10. Here, (Tf Ta) represents temperature differ- ence between the average collector inlet fluid temperature and ambient air temperature. It can be seen that under the four seasons, most of the collector efficiencies are above 50% and the cases above gcollector 1⁄4 50% occupy about 70.0% of all the data in Fig. 10. The annually-averaged col- lector efficiency is found at 60.0%. The theoretical efficiency calculated with water as the working fluid (given by dotted line) [22] is also shown in Fig. 10. From the obtained results above, it can be seen that the collector efficiency using supercritical CO2 as working fluid is confirmed to be much higher than that of using water as working fluid. The col- lector efficiency would be expected to be further improved by increasing CO2 mass flow rate or other methods. The result is encouraging, because it means that the supercriti- cal CO2 is a more efficient heat transfer fluid to collect heat in the solar collector than water. In addition, generally speaking, carbon dioxide is inexpensive and exists everywhere in the world. Due to the lower liquid density and smaller system volume, the charged amount would be lower with CO2, reducing the cost. And recycling or recovery of CO2 would not be nec- essary, either for environmental or for economic reasons. CO2 is also thermally stable and behaves inertly, thus elim- inating material problems or chemical reactions in the system. The high pressure used for the operation of the CO2-based Rankine cycle may increase its cost to a certain extent. But all advantages listed above may reduce system costs to the point where mass production becomes feasible [17]. In the present paper, a detailed economic analysis is not intended to be included, mainly because the experimen- tal facilities are much higher in costs than in large produc- tion, at the same time, it is also difficult to consider environmental profits into the cost, if CO2 used, which may reduce the cost greatly. In the future, an exactly eco- nomic analysis needs to be made to further estimate its market feasibility, especially compared to conventional flat plate collector. 4. Conclusions A solar collector using supercritical CO2 as working fluid is proposed in the paper. The basic characteristics of the solar collector have been experimentally studied. Based on the experimental measurements, the following remarks are made. An experimental set-up was constructed and tested. The measured data show that the CO2 temperature and pressure increase with the solar radiation. And the CO2 mass flow rate in the loop also increases with the solar radiation. The variation of the CO2 pressure and mass flow rate with the solar radiation is much different from the col- lector using liquid as working fluid, which makes the ther- mal output control of the CO2-based collector more complicated and difficult than the traditional collector. During the most time of the tests, the time-weighted daily average collector efficiency is found at above 50.0% and annually-averaged collector efficiency is measured at 60.0%, which is higher than the collector using water as working fluid. But further investigation is needed in the future to study the nature of the supercritical CO2 flow and heat transfer in the collector. a 150 100 50 0 b 80 60 40 20 0 0.0 20 40 60 80 100 120 140 160 180 200 220 Summer season Spring season Autumn season Winter season 0.1 0.2 0.3 0.4 0.5 It (MJ) 50% Summer case Spring case Autumn case Winter case Water case (Tf-Ta)/It (K/MJ) Fig. 10. (a) Variations of the total heat quantity collected in the collector with the total solar radiation under different seasons during one year. q 1⁄4Rtd q Adt is the total heat quantity collected in the collector ct 0 collector during the test time period per day(an integral of the heat quantity collected in the collector on a test time period). (b) Variations of the time- weighted average solar collector efficiency with (Tf–Ta)/It under different seasons during one year. (Tf–Ta) is temperature difference between the average collector inlet fluid temperature and ambient air temperature. ηcollector (%) qct (MJ)PDF Image | study on evacuated tube solar collector using supercritical CO2
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