Study of organic Rankine cycle with zeotropic mixtures and optimization of mixture composition

There are two main parts of this study. In the first part of the study, the aim is to develop an accurate ORC model which is validated using reliable steady-state data. For this purpose, tests are conducted at four different heat source temperatures ranging from 80 to 110 ℃ to obtain data in a repeatable manner. The proposed model considers pressure drop and heat transfer correlations to improve the accuracy of model predictions. The results show that the steady-state ORC model is significantly accurate, with ±1% deviations in pressure and ±1 ℃ in temperature, and ±5% in mass flow rate, heat transfer rate and refrigerant pump power predictions. In the second part, the performance of ten different R134a blends with varying mole fractions is evaluated by the modified version of the ORC model developed in the first part. The fluid screening is made for various criteria including low flammability and toxicity indexes, low ODP values, and molecular weight of the pure refrigerant candidates. The parametric performance evaluation of the selected R134a blends with the variable mass fraction is made with respect to two performance indicators including net power output, and the cycle thermal efficiency. Expander outlet pressure and hot fluid inlet temperature are selected as decisive parameters. For the parametric optimization, firstly, all the parameters except the hot fluid inlet temperature are fixed, and R134a blends with varying mass fractions are searched for hot fluid inlet temperature ranging between 363-403 K. Then, the expander outlet pressure is changed between 550-700 kPa for a hot fluid inlet temperature field of 363-393 K to seek the best performing R134a blends with variable compositions in terms of the selected performance indicators. Results show that zeotropic blends with more volatile second components than the pure R134a including R134a/R32, R134a/R143a, and R134a/R125 blends show the best performance in terms of selected performance indicators for all applicable temperatures and the expander outlet pressure range, although they have limited mass fraction of the second components. Besides, azeotropic blends that have closer boiling temperatures to R134a can also provide a better performance than pure R134a.