Multi-objective optimization of organic Rankine cycle systems considering their dynamic performance

The Organic Rankine cycle system is a well-established technology for converting medium/low temperature waste heat into mechanical or electrical power. Inefficiencies in the internal combustion engines for road transportation lead to large amounts of waste heat that are not exploited. Because of the engine load changes during a driving cycle, the mass flow rate and temperature of the heat source fluctuate rapidly over a broad range. This poses high requirements to the control of the organic Rankine cycle unit, in order to prevent the formation of liquid droplets at the turbine inlet and acid gas corrosion in the evaporator if the exhaust gas temperatures are too low, which reduce the system lifetime. In addition, the fluctuations in the heat source degrade the efficiency of the organic Rankine cycle unit, because of part-load operation. Furthermore, the penalty on the transportable vehicle payload caused by the increase in system mass should be considered. This paper presents a novel design method for organic Rankine cycle systems subject to highly fluctuating heat sources, ensuring safe and efficient operation. An integral optimization code developed in MATLAB®/Simulink® combining the design of the thermodynamic cycle, the system evaporator and the control system with a dynamic simulation model is presented. The multi-objective optimization maximizes the organic Rankine cycle net power output over a driving cycle of a heavy-duty truck, while minimizing the mass of the evaporator. The results indicate that, in order to ensure safe operation, the degree of superheating of the working fluid as well as the exhaust gas temperature leaving the evaporator at design conditions should be higher than what classical steady-state thermodynamic analyses suggest. This work provides a unique benchmark for the optimization of organic Rankine cycle systems subject to high fluctuating heat sources that will be of benefit both for academia and industry.