Dynamic study of ORC evaporator operating under fluctuating thermal power from waste heat sources

Waste heat recovery systems such as the Organic Rankine Cycle (ORC) face the challenge of being subordinated to the main process, and therefore they are not allowed to control the heat source as conventional power systems. Because of the conceivable fluctuating characteristic of the heat source the dynamic response of the system has to be analyzed. The component of major importance for the dynamics of the system is the evaporator, because it links the heat source to the ORC. Although different dynamic models of heat exchangers for ORC are available in literature, researchers have hardly focused their attention on a thorough analysis of the key factors that affect the evaporator dynamics. In particular, the influence of the evaporator geometry and materials is analyzed in this work. A characterization approach based on the main heat exchanger construction parameters is carried out in order to predict the evaporator response time and how it is affected by those design factors. With the help of dynamic simulations, the approach is applied to a real kettle boiler geometry. Possible improvements in the heat exchanger geometry that can lead to a dynamically more robust system and buffering some of the intermittency of the heat source are foreseen.