On the Performance Analysis of Using Non-azeotopic Mixtures in Solar Rankine Cycle System with an Evacuated Tube Collector

In this study, we use a xenon arc lamp as the artificial sunlight to construct the Rankine cycle system, and investigate the effect of the operating pressure and the working fluid on the theoretical power output, the heat transfer efficiency of the evacuated tube, and the thermal efficiency of the Rankine cycle. In addition, we employ the thermal resistance analysis to evaluate the heat dissipation of the evacuated tube. The system pressure varies from 200 kPa to 700 kPa, and three different types of working fluids are tested: pure water, ethanol/water, and ethylene glycol/water mixtures. The mass fraction of the ethanol in water are w = 0.2, 0.4 and 0.6. For ethylene glycol/water mixtures, the mass fraction are w = 0.1, 0.2, 0.3 and 0.4. The experimental results show that pure water yeilds the maximum theoretical power output and the highest heat transfer efficiency of the evacuated tube. This is because the lower latent heats and the lower boiling temperatures associated with the alcohol-water mixtures lead to higher superheat at the exit of the evacuated tube, which results in greater heat loss from the evacuated tube. Consequently, the ethanol/water of w = 0.6 mixture produces the lowest theoretical power output and the poorest thermal effciency. For the ethylene glycol/water mixtures at higher operating pressure, the superheated state is not reached due to their higher latent heat. So that the theoretical output power are higher than the ethanol/water mixtures. To obtain higher theoretical power output, fluid with larger latent heat and higher operating pressure are prefered. Theoretically, during the process of power producing, the working fluid should be maintained at superheated state, and the superheat at outlet of evacuated tube should not be too high. Due to the heat dissipation is as growing as the temperature of working fluid increasing, so that the ethanol/water system is suitable than pure water for lower irradiance, and ethylene glycol/water system is most not recommended for using as Rankine cycle working fluid. Because of its saturation temperature is higher than the pure water and the theoretical power output is the lowest among three working fluid type.