Simultaneous working fluid and expander selection method for reaching low-threshold technology organic Rankine cycle (ORC) design

Maximizing the utilization of an available source serves as the ideal approach, provided that only technical factors are considered. For sources with low heat flux, however, cost-effective solutions are more suitable due to the minimal net power generated, regardless of the effectiveness of the energy conversion. In such cases, utilizing low-threshold technology may be the most fitting solution, the layout of these cycles should be simple and inexpensive. In the case of organic Rankine cycles (ORCs)-based power cycles, this means the omission of superheaters or recuperative heat exchangers and the use of simple expanders and small heat exchangers. Simplifying the design, however, requires additional considerations about the elementary steps of the cycle. This work presents a procedure to select favorable working fluids for ORC while considering the expander's internal efficiency. The criteria for favourability is to have a nonideal expansion process starting and ending in (or very near) saturated vapor states to avoid problems related to wetness/dryness between the given maximal and minimal expansion temperatures. It is demonstrated that the design can be simplified under the simultaneous working fluid and expander selection method presented in this study, regardless of the type and isentropic efficiency of the expander. The resulting methodology applies the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space. The proposed approach is illustrated with a case study finding optimal working fluid for an ORC system fitted to industrial waste heat, a low-temperature geothermal, and a cryogenic heat source.