Theoretical And Experimental Investigation On The Working Fluids For Low Temperature Solar Rankine Cycle System

A low-temperature solar powered Rankine cycle system which covers the temperature range from around 25oC to about 100oC can utilize common flat plate collectors or vacuum tube collectors. In this study, we describe the basic frame of the intending low-temperature solar powered Rankine cycle system. And we mainly investigate the thermal performance of the low-temperature solar Rankine cycle in theory and by experiments.Four typical working fluids R600, R134a, R245fa and R236fa are chosen to be the working fluids of the low-temperature solar Rankine cycle system. Under the proposed working conditions, four working fluids are investigated and compared from aspects of pressure, cycle efficiency, power output, thermodynamics perfection and environment security. Investigation shows that in the considered temperature range from 25oC to 80oC, utilizing R245fa as the working fluids, the pressure level is relatively low and the efficiency of low-temperature solar Rankine cycle can achieve about 13%. Results presented in this paper points out that R245fa is an available and effective working fluid for low-temperature solar Rankine cycle.Different mass fractions of R245fa/R152a are chosen to propose three typical zeotropic mixtures Ca, Cb and Cc. In the proposed temperature range, the zeotropic mixtures are investigated as the working fluids of the low-temperature solar Rankine cycle. An internal heat exchanger (IHE) is introduced to the system because of the obvious temperature glide during phase change for zeotropic mixtures. Investigation shows that utilizing zeotropic mixtures can extend the range of choosing working fluids for low-temperature solar Rankine cycles. Among the proposed zeotropic mixtures, the Rankine cycle efficiency of isentropic working fluid is not the highest, while the expansion volume ratio is lowest. For zeotropic mixtures a significant increase of thermal efficiencies can be gained when superheating is combined with IHE.Utilizing R245fa and a flat plate collector with internal diameter of 8mm, the experimental investigation is carried out for the low-temperature solar powered Rankine cycle system. And the results show that the efficiencies of the solar collector in the Rankine cycle system would fluctuate sharply around a certain value of the solar radiation. With the condition that the working fluids entirely accomplish the evaporation in the collector tubes, the efficiency of the solar collector will follow the flow rate of the working fluid, and will deviate the solar radiation.By analyzing the experimental data, the best-efficiency-curve of the collector is gained in this thesis.