Optimal design and operation of an Organic Rankine Cycle (ORC) system driven by solar energy with sensible thermal energy storage

In this study, the optimal design and operation of an Organic Rankine Cycle (ORC) system driven by solar energy is investigated. A two-tank sensible thermal energy storage system is configured to overcome the intermittency of solar energy. A circulating fluid, also termed as heat transfer fluid (HTF) that connects the solar collector and the ORC system plays a critical role in this system.

The mass flowrate of the HTF determines both the temperature of the HTF and the amount of heat absorbed from the solar collector. A simulation-based optimization model is developed in this work. Process simulation of the ORC is performed in Aspen HYSYS, and the mathematical models of the energy storage system and the parabolic trough collector are developed in Matlab.

The optimal design of the system including the hot tank temperature, cold tank temperature, mass flowrate of the HTF, and operating conditions of the ORC are determined simultaneously based on the simulation-based optimization framework. The control strategy of the solar collector can be determined as well.

The system efficiency of the solar energy driven ORC system is maximized with the proposed optimal operation strategy. With the simulation-based optimization framework, the system efficiency of the recuperative ORC power plant with toluene as the working fluid is increased from 17.9% to 24.8% compared with a previous study in the literature.

The recuperative ORC performs much better than the basic ORC. Toluene performs best among all the investigated working fluids ignoring the problem with vacuum condensation. The cycle type (subcritical vs. supercritical) exerts great influence on the system performance. The supercritical ORC can improve the thermal efficiency by 11.3% and the overall system efficiency by 10.8% compared with the subcritical ORC with n-pentane as the working fluid.