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X.R. Zhang, H. Yamaguchi / Applied Thermal Engineering 28 (2008) 1225–1233 1231 ature at the collector outlet is measured at 162.5 °C. At noon, the CO2 temperature reaches up to about 185.0 °C. The supercritical CO2 fluid with such high temperature can be easily used as the useful solar thermal energy source for hot water supply, refrigeration or others. The curve line in the figure is a polynomial fit of the experimental data and the CO2 temperature at the collector outlet is found in the polynomial of order 2 with solar radiation. Similar to Fig. 4, the measured data of the CO2 pressure at the collector outlet (P1) is shown in Fig. 5. From Fig. 5, the CO2 pressure varies largely with the solar radiation in the collector. The CO2 pressure at the collector outlet changes in the range of 5.5 MPa to 9.3 MPa. During the most time of the test, the CO2 fluid in the collector is above 7.38 MPa and in supercritical state. But the CO2 pressure drops with the decrease of the solar radiation and after 16:30, CO2 becomes subcritical. On the other hand, the CO2 pressure varies with the solar radiation, which is much different from water-based solar collector. For the water- based solar collector, the collector pressure is not affected by the solar radiation and the solar radiation only has influence on water temperature. However, for the CO2- based collector, not only the CO2 temperature, but also the CO2 pressure in the collector are influenced obviously by the solar radiation, which makes the collector character- istics more complicated than traditional collectors using liquid as working fluid. And also the polynomial fit curve of the CO2 pressure is shown in Fig. 5 and it is found that the CO2 pressure is in the polynomial of order 2 relation with the solar radiation. The variation of the measured CO2 mass flow rate with the solar radiation is shown in Fig. 6. The result shows that the achieved CO2 flow rate in the loop increases with the solar radiation (also with polynomial of order 2 relation with the solar radiation). The reason to the phenomena may be that the solar radiation has influences on the fluid temperature and pressure, furthermore on how good the Rankine cycle can be obtained. That means CO2 condition, being liquid CO2 or supercritical state or liquid-gas two phase flow, largely depends on the solar radiation. And the solar radiation has an obvious influence on the CO2 flow rate in the collector. The measured CO2 flow rate changes from 0.006 kg/s to 0.014 kg/s. During the test per- iod, the maximum value (0.014 kg/s) of the CO2 flow rate is two times larger than the minimum value (0.006 kg/s). The characteristics regarding the CO2 flow rate is also much dif- ferent from that of traditional collector using liquid as working fluid. For the traditional collector, there is no influence of the solar radiation on the fluid flow rate. The change of the CO2 flow rate with the solar radiation all the time makes the thermal output control of the CO2- based collector more difficult than that of the traditional one, because the flow rate is directly related to the thermal output of the collector. It is also seen that the fluctuation of the CO2 flow rate is large during all the time of the exper- imental test, which may be due to the pumping of the CO2 fluid. The phenomena can be seen clearly from Fig. 6b, in which the CO2 flow rate changes all the time with large fluctuations. The temperature and pressure values are obtained experimentally, also including the CO2 flow rate. Enthalpy values at the different monitoring points can be calculated based the measured temperature and pressure values using a Program Package for Thermophysical Properties of Flu- ids database version 12.1 (PROPATH 12.1) [21]. Based on the equations described above, the characteristic parame- ters, including the heat quantity collected in the collector are obtained. The obtained values of the heat quantity col- lected in the collector are shown in Fig. 7. The results show that the heat quantity collected in the collector increases with the solar radiation. Both the CO2 temperature in the collector and CO2 flow rate increase with the solar radia- tion, which may contribute to the phenomena in Fig. 7. As the discharge pressure of the pump is subcritical, part of the heating process in the collector will take place in the two phase flow region. It leads to larger pressure drop than that when the discharge condition of the pump is supercrit- ical. The operating point of the circulating pump will result from the total pressure drop through the system. The CO2- based P–h diagrams are shown in Fig. 8, in which the con- dition of the completed CO2 cycle is given at 10:00 and 17:00, respectively. It is seen that the transcritical thermo- dynamic cycles are made throughout the operating time. The inlet and outlet conditions of the pump used are in the CO2 liquid condition. The discharge pressure of the pump is supercritical state, which means the heating pro- cess in the solar collector is from liquid to supercritical, not cross the two phase region. It should also be mentioned here that suitable heat sink with sufficiently low-tempera- ture is an important factor to achieve efficient operation of the CO2 cycle. The heat sink with temperature of 0–10 °C should be found for the real applications. 0.025 0.020 0.015 0.010 0.005 0.000 Summer season Spring season Autumn season Winter season 20 40 60 80 100 120 140 160 180 200 220 It (MJ) Fig. 9. Variations in the time-weighted average CO2 mass flow rate per day with the total solar radiation during theRtest period per day under different seasons during one year. mCO 1⁄4 td mCO dt=td Ris the time– 202 weighted average mass flow rate of CO2 fluid per day. It 1⁄4 td IA dt is the 0 total solar radiation during the test time period per day (an integral of the solar radiation on a test time period). mCO (kg/s) 2PDF Image | study on evacuated tube solar collector using supercritical CO2
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