Organic Rankine Cycles (ORCs) are primarily used in low-temperature/low-grade heat recovery systems, where water cycles are not efficient enough to economically extract work. For this reason, ORC analysis and ORC fluids have focused on low (< 300 °C) temperatures. Because the Carnot efficiency of a Rankine cycle increases with temperature, the high temperature limit of organic fluids is of interest for exploring the boundary between the economic advantages of organic fluids versus water in Rankine cycles. In this study, the high temperature limit of working fluids and the role of fluid properties on cycle efficiency for critical/subcritical ORCs are investigated. The performance of a wide range of organic fluids is evaluated through the development of thermal property calculator coded in MATLAB using the Peng-Robinson Equation of State (PREOS) and the Design Institute for Physical PRoperties (DIPPR) database correlations for heat capacity. This process allows for the development of an efficient property calculator that allows the rapid characterization of the thermodynamic performance of a large number of working fluids. A regenerative Rankine cycle for organic fluids was compared to a reheated Rankine cycle for water, for temperatures between 100 and 800 °C, with different cycle boundary conditions. The role of working fluid properties such as boiling temperature, critical point, molecular weight, and acentric factor on cycle performance are evaluated. It was found that the limits of efficiency of most of the fluids analyzed results from the limits of the high and low temperature of the cycle. Efficiency improvements due to a combined cycle with two ORCs are also examined. Finally, given the limits and performance of the fluids analyzed, desirable fluid properties for efficient high temperature ORCs are discussed.

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