Investigation of the Criteria for Fluid Selection in Rankine Cycles for Waste Heat Recovery

The organic Rankine cycle is a promising way for the conversion of low temperature heat to electricity. Different fluids can be used in Rankine cycles for the utilization of waste heat. The suitability of a certain fluid will depend on its thermodynamic properties as well as on the conditions at which the heat is available, thus it is often unclear if an organic fluid has any advantage compared to inorganic fluids like water. Various substances starting from the refrigerants to high boiling organic liquids have been investigated as possible working fluids for the different temperature ranges at which the waste heat is available. The present communication reports the results for three different classes of substances: 1) a hydrocarbon (n-heptane); 2) two refrigerants 1.1.1.3.3-pentafluoropropane (R245fa) and pentafluoro butane mixture (Solkatherm, SES36); and, 3) water in the intermediate temperature range (473 to 773 K) where the exhaust gases of combustion engines may be used as energy source for cogeneration. In this range it turns out that for many conditions, water and heptane are well suited working fluids for cogeneration systems. In the present investigation, the attention was not laid on the cycle efficiency alone, but also on the total exergy usage from an enthalpy stream (e.g. exhausts gas). This is taken into account while discussing the quality of the process. The results for different thermodynamic parameters and the surface area of the heat exchanger have been discussed. T-(H) over dot diagrams were also used for judging the suitability of a fluid. It turns out that water is well suited for many cases in the intermediate temperature regime.