Optimisation of an organic Rankine cycle constrained by the application of compact heat exchangers

In this paper, a low-power organic Rankine cycle (ORC) for waste heat recovery from a middle temperature heat source, i.e. internal combustion engine exhaust gases, is analysed. The ORC is suitable for application to mobile units, especially to heavy duty long haul trucks. A method for the Rankine cycle optimisation has been developed ; the method includes a thermodynamic calculation of cross- flow tube-fin and plate-fin heat exchangers (HE) based on experimentally determined heat transfer coefficients and friction factors. In the developed multi-objective optimization, the objective function searches for a compromise between requirements for the maximum power output and the minimum total HE surface area or the minimum HE volume. The result of optimisation is the Pareto frontier between the obtained ORC power output and the total HE area or the total volume of heat exchangers. Configurations with a regenerator and without it are analysed and four working fluids, i.e. water, ethanol, toluene, and hexamethyldisiloxan (MM), are compared. Among the considered fluids, ethanol could be recommended as the best one, regarding the ORC power output and the total HE volume.