In the Organic Rankine Cycle (ORC), zeotropic mixtures are more thermally efficient than pure fluids. Due to their higher boiling points, zeotropic working fluids have higher net outputs of energy at the low temperatures of the Rankine Cycle than pure substances. Zeotropic working fluids condense across a range of temperatures, allowing external heat exchangers to recover the heat of condensation as a heat source for the Rankine Cycle. The changing temperature of the zeotropic working fluid can be matched to that of the fluid being heated or cooled to save waste heat because the mixture's evaporation process occurs at a temperature glide (see Pinch Analysis).
R21/R245fa and R152a/R245fa are two examples of zeotropic working fluids that can absorb more heat than pure R245fa due to their increased boiling points. The power output increases with the proportion of R152a in R152a/R245fa. R21/R245fa uses less heat and energy than R245fa. Overall, zeotropic mixture R21/R245fa has better thermodynamic properties than pure R245fa and R152a/R245fa as a working fluid in the ORC.