Comparison of control strategies for waste heat recovery Organic Rankine Cycle systems

This paper investigates the control of waste heat recovery Organic Rankine Cycle systems. A dynamic simulation model of the system was developed under Modelica language. This model associates the sub-models of the different components of the system: the evaporator, a scroll expander, the condenser, the liquid receiver and the pump. Both the speeds of the expander and of the pump are controlled in order to adjust the evaporating pressure and maintain the superheat at the outlet of the evaporator. Two types of controllers were implemented: Proportional Integral (PI) and Model Predictive Control (MPC) controllers. The PI controller on the expander speed is able to adjust the evaporating pressure with a high accuracy. It is also shown that the MPC controller on the pump speed maintains the superheat close to the set point value with a much better performance than the PI controller. Hence, the use of a MPC controller allows decreasing the set point value on the superheat without any risk of feeding the expansion machine with liquid droplets.