The energy flow method for modeling and optimization of Organic Rankine Cycle (ORC) systems

This contribution constructs the energy flow model for an ORC system, which synthetically considers fluid characteristics and governing laws of heat transfer and thermodynamic. Then, combined the Kirchhoff's law with the thermodynamic constraints, a group of mathematical equations is obtained. It not only reveals both system topological structure and the component characteristics, but also connects the system output to the boundary conditions, the structural parameters and the operating parameters. Finally, operation optimizations for a certain ORC system are implemented via Genetic Algorithm (GA). Two sets of Pareto frontiers with different heat source conditions are obtained. The results show that with a 16 K increment of heat source temperature, the net power output and thermal efficiency will increase by 84% and 25%, respectively. In the other hand, the heat source flow rate shows limited impact on optimizations. Besides, design optimizations with the objective of minimizing required total thermal conductance are also conducted. It is concluded that enlarging net power output by 14.9% leads to an increase in the total thermal conductance by 54.3% and a decrease in the corresponding working fluid flow rate by 8.7%. Moreover, the operating parameters related to condenser other than evaporator remarkably influence the optimizations, and the characteristic temperatures vary with different conditions in each optimization. Therefore, the proposed method without fixed PPTD or condensation temperature is more flexible to both modeling and optimization.