Integrated working fluid-thermodynamic cycle design of organic Rankine cycle power systems for waste heat recovery

Today, some established working fluids are being phased out due to new international regulations on theuse of environmentally harmful substances. With an ever-increasing cost to resources, industry wants toconverge on improved sustainability through resource recovery, and in particular waste heat recovery.

Inthis paper, an organic Rankine cycle process and its pure working fluid are designed simultaneously forwaste heat recovery of the exhaust gas from a marine diesel engine. This approach can overcome designissues caused by the high sensitivity between the fluid and cycle design variables and otherwise highresource demands, which through conventional methods cannot be addressed.

The global optimal designwas a 1.2MW cycle with 2,2,3,3,4,4,5,5-octafluorohexane as the new fluid. The fluid has no ozone depletionpotential and a global warming potential under the regulatory limit. By using the simultaneousdesign approach the optimum solution was found in 5.04 s, while a decomposed approach found thesame solution in 5.77 h.

However, the decomposed approach provided insights on the correlationbetween the fluid and cycle design variables by analyzing all possible solutions. It was shown that thehigh sensitivity between the fluid and cycle design variables was overcome by using the simultaneousapproach. Correlation between net power output and the product of the overall heat transfer coefficientand the heat transfer area could further be addressed by employing a new solution strategy includingmaximum constraints for this product.

The use of such constraints resulted in the design of a new fluid(5-chloro-4,5,5-trifluoro-2,3-dimethylpent-2-ene) with a 1.25 MW net power output. Finally, a comparisonwith conventional fluids was shown where 2,2,3,3,4,4,5,5-octafluorohexane offered an improvementon net power output and economic and environmental metrics.