Analysis of organic Rankine cycles using zeotropic mixtures as working fluids under different restrictive conditions

Organic Rankine cycle has been regarded as a reasonable technology to utilize low-temperature geothermal sources. This paper analyzes the internal relation about zeotropic mixture selection under different restrictive conditions. R601a/R600 and R245fa/R600a are selected as typical examples of symmetrical and asymmetric zeotropic mixtures. The fixed condenser bubble temperature, the fixed cooling water temperature rise and the fixed cooling water flow rate are considered as restrictive conditions. Both the basic and the regenerative organic Rankine cycle system are considered as research subjects. The fixed cooling water flow rate is suggested to select the appropriate zeotropic mixture. The results indicate that the performance corves with the fixed condenser bubble temperature of 25 °C are significant in zeotropic mixture selection, and these corves reflect the full potential of zeotropic mixtures to improve the net power. The temperature glide decreases the net power with the fixed condenser bubble temperature. However, the opposite phenomenon is found when the fixed cooling water temperature rise or the fixed cooling water flow rate is applied. The application of intermediate heat exchangers may lead to the larger, the smaller or the same net power with different temperature glides and restrictive conditions. However, the cycle efficiency is increased all the time.