Dynamic modeling and control strategy of an organic Rankine cycle with concentration regulation of the zeotropic mixture working fluid

Fluctuations in heat source/sink conditions degrade the performance of the organic Rankine cycle system and hinder its widespread application. The concentration regulation of the mixture working fluid is proposed in this work as a control approach to enhance the adaptability of the system to heat source/sink fluctuation. Two control strategies, constant-evaporation-pressure operation and constant-output-power operation, are investigated through the developed dynamic model of a 4-kW organic Rankine cycle prototype. The results indicate that, compared to the temperature glide of the mixture during phase change, a more significant mechanism of concentration regulation is the variation of the average evaporation and condensation temperatures. The trade-off between the exergy destructions of the evaporator and condenser results in the optimal system performance at a specific mixture concentration. The mixture concentration alongside the mixture mass flow rate enables dual-objective control of the system. The concentration-adjustable organic Rankine cycle exhibits superior performance in both constant-evaporation-pressure and constant-output-power operations compared to the fixed-concentration organic Rankine cycle and the basic organic Rankine cycle with pure working fluid in two aspects: the enlargement of the effective control range and the improvement of system performance. Under the fluctuation of heat source/sink conditions, the control range of the concentration-adjustable system is about 3–3.3 times wider than the fixed-concentration system, while the performance of the concentration-adjustable system is about 9.01 %–14.81 % higher in terms of the thermal efficiency and about 9.72 %–13.64 % higher in terms of the exergy efficiency than the basic organic Rankine cycle.