Compared to pure fluids, zeotropic mixtures have the potential to lower the irreversibilities in low temperature Rankine cycles by better temperature profile matching of the working fluid with the heat source/sink. However, having a gliding temperature does not guarantee performance boost over pure fluids, as many factors influence the exergy efficiency of the cycle. In this study, 25 pure fluids and 104 binary mixtures of natural working fluids are analyzed in subcritical ORCs with heat source temperature range of 125-300 degrees C and different condensing conditions and the results are investigated within two frameworks: (1) comparing the mixtures to their pure constituents, (2) comparing the mixtures to the best performing pure fluid. In one behavior type, the performance of the mixture falls between the performance of its pure constituents for all evaporator pressure range, and the mixture provides no benefit. However, some mixtures could provide performance boost in a specific evaporator range. Therefore, the maximum allowable evaporator pressure plays an important role in the performance comparison of zeotropic mixtures to their pure constituents. Mixtures which outperform their pure constituents in the first perspective, are further analyzed in the second perspective. Finally, a screening method is presented to map the binary mixtures with performance boost compared to their pure constituents and high absolute exergy efficiency. This method is based on the key thermophysical properties of the fluids including critical temperature and normal boiling point, as well as working conditions such as heat source and heat sink temperature and PPTD in the evaporator and the condenser.

Extending the heat transfer area is a simple technique to improve the thermal performance of phase change materials with low thermal conductivity. However, as the governing mechanisms differ in solidification and melting, fins can affect the processes in different ways. This demands assessment of fin enhancement in a combined analysis on both solidification and melting, often neglected in literature. This paper presents visual-izations of solidification and melting of n-eicosane in a rectangular cavity and experimentally investigates the enhancing effect of vertical fins with varying number and length. Experiments were conducted at water inlet temperature ranges of 15-25 degrees C and 50-60 degrees C for the solidification and melting processes, respectively. The results show that the vertical fins can be more influential in solidification rather than in melting with similar losses in the storage capacity. In the solidification process, as natural convection is absent, the mean power is enhanced by a maximum of 395% with a 10% loss in the storage capacity, as compared to the benchmark. In the melting case, the mean power is increased by a maximum of 90% with a 9% loss in the storage capacity. Although increasing the surface area with vertical fins contributes to development of convective structures, it makes a modest enhancement. In overall, increasing the fin volume fraction, in exchange for the loss in the storage capacity, enhances the solidification significantly while it has relatively low enhancement effect in melting. At the end, the performed experiments could be helpful for validation of future simulation tools with complex features, particularly solidification models lacking in literature.

Low temperature Rankine cycle is a prominent solution for power generation in Waste Heat Recovery (WHR) application. The performance of this cycle is affected by various parameters including characteristics of the heat source, working fluid and constraints in the system. In cases where the heat source has a limited mass flux and therefore variable temperature, the amount of extracted heat affects the performance of the cycle including net power and efficiency which is also related to the working fluid. This is in the paper expressed in terms of a heat utilization factor, ?, which shows the ratio of extracted heat to the maximum possible extraction rate in the specific case. This factor affects the performance of the cycle by moving the pinch point location in the evaporator. Results indicate that this factor has great impact on the performance of the cycle and the effect varies for different working fluids. 

Working fluid selection determines various characteristics of low temperature Rankine cycles. Among other factors, the selected working fluid affects thermal performance, apparatus size and economic feasibility of the cycle. Beyond only affecting characteristics of the system, unrealistic preconditions for the working fluid of the system may force the designers in using environmentally harmful mixtures and force the outcome beyond boundaries of environmental regulations. There has been numerous research and scrutiny on various working fluids, but due to the unstructured and unorganized orientation of previous studies, there is no comprehensive insight on relationship of different characteristics of the working fluid and overall performance of the system. This work intends to develop a numerical evaluation approach, using a modified stochastic optimization algorithm as a search engine. The paper further explores and questions the existing criteria for optimization of working fluids in Rankine cycle. Rather than just finding the optimum fluids for different cases, this study aims to investigate the behavior of different fluids around optimum points and see the bigger picture to find trends in different fluid behaviors. Analysis of results show two main behaviors among the fluids in subcritical cycles. In the first type behavior, the optimum points for output work, thermal efficiency and exergy efficiency lie very close to each other, while in second type, these optimum points are not close. There is a transition from first type behavior to second type for a ratio of critical temperature around 0.9 of heat source inlet temperature. These results also show the importance of key performance parameter determination.