The Design Of Working Fluids Of Organic Rankine Cycles (ORCs) For Low Grade Heat Recovery

A shortage of fossil energy sources boosts the utilization of renewable energy.Among numerous novel techniques, recovering energy from low-grade heat sourcesincluding industrial waste heat, geothermal energy and solar energy through powergeneration via organic Rankine cycles (ORCs) is one of the focuses.Properties of working fluids are crucial for the ORC’s performance. Manystudies have been done to select proper working fluids or to design new workingfluids. However, no researcher has systematically investigated the relationshipbetween molecular structures and thermal efficiencies of various working fluids for anideal ORC. This paper has investigated the interrelations of molecular structures,molecular entropies, and thermal efficiencies of various working fluids for an idealORC. By calculating thermal efficiencies and molecular entropies, we find that themolecular entropy is the most appropriate thermophysical property of a working fluidto determine how much energy can be converted into work and how much cannot in asystem. Generally speaking, working fluids with low entropies will generally havehigh thermal efficiency for an ideal ORC. Based on this understanding, the directinterrelations of molecular structures and entropies provide an explicit interrelationbetween molecular structures and thermal efficiencies, and thus provide an insightfuldirection for molecular design of novel working fluids for ORCs.Finally, the paper puts forward some important molecular structures andmolecular groups as well as an approach to evaluate the newly-designed workingfluids, and presents some selected high-performance working fluids.