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

Recovering waste heat from the exhaust gases of heavy-duty diesel truck engines by using the organic Rankine cycle can reduce both fuel consumption and greenhouse gas emissions. The design of such systems is constrained by the space available for installation; thus, an optimization procedure is required. In this research work, a two-objective optimization, based on the maximization of power output and minimization of the total heat exchanger surface area, is implemented in the Engineering Equation Solver software package. Application of the developed procedure is illustrated in a case with an exhaust gas temperature of 394.2 degrees C and mass flow rate of 0.306 kg/s. Configurations both with and without a recuperator are analyzed, and four working fluids (water, ethanol, toluene, and hexamethyldisiloxane) are compared. Of these fluids, ethanol could be recommended as the best when considering the organic Rankine cycle power output and the total area/volume of heat exchangers. For a total surface area of 10 m(2) in the case without the recuperator, the power output values for ethanol, water, and toluene are 17.6 kW, 17.2 kW, and 15.8 kW, respectively; with the recuperator, the power output is slightly higher, but the total area of heat exchangers is considerably larger. Again, the best working fluid is ethanol: for a total surface area of heat exchangers of 24 m(2), the power output values for ethanol, toluene, and hexamethyldisiloxane are 18 kW, 17.1 kW, and 15.8 kW, respectively. The presented procedure is useful both for the conceptual design of a complete waste heat recovery unit and for the optimization of all heat exchangers.