Comparison of High and Low Temperature Working Fluids for Automotive Rankine Waste Heat Recovery Systems

Internal combustion engines will remain the major trend for many years. For these engines, only one third of the chemical energy available in the fuel is converted to mechanical energy, the two others thirds are lost as heat. Exergetic quality is better at exhaust thanks to higher temperatures, but the amount of valuable heat is near twice if both exhaust and engine cooling waste are considered. Rankine system is a candidate for waste heat recovery and the working fluid choice is a key question.
“Low” temperature fluids operating at boiling temperatures lower than 100°C (R134a, R1234ze...), value both engine cooling system and exhaust losses. “High” temperature fluids operating at boiling temperatures higher than 150°C (Ethanol, water...), value mainly exhaust losses. Our target is to show, by system simulation, the strengths and weaknesses for each fluid family. Theoretically, low temperature fluids are more sensitive to ambient conditions, because the condensation is an important issue for such a system. However, the conclusion of our investigations is that at 22°C as ambient temperature, the low temperature fluids are well adapted for driving speeds less than 100-110 km/h. For higher velocities, high temperature fluids remain best candidates.
For current driving cycle procedures, low temperature Rankine is a challenging alternative.