Dynamic Model of an Organic Rankine Cycle System. Part I – Mathematical Description of Main Components.

A set of flexible, fast and scalable models, specifically developed for the dynamic simulation of an ORC (Organic Rankine Cycle) system, is presented. A basic ORC features a pump, an evaporator, a turbine and a condenser. The paper provides an insight on the way these components have been mathematically modeled and the corresponding Simulink® models created. Each model features a dialog window where the type of fluid and geometrical dimensions can be set, allowing full scalability and generalization. Evaporator and condenser are the most complex components to model and they are designed as counterflow pipe in pipe heat exchangers with phase change in the inner pipe. The exchangers have been split into nx longitudinal volumes where the conservation equations are applied in differential form, according to a 2D grey-box, state determined approach. Four state parameters can be defined for each axial node leading to a 4×nx system of partial differential equations, solved in time according to a finite difference approach through a discrete state S-Function written in Matlab®. Turbine and pump have been modeled as black-box, not state determined components since it is assumed that energy mass and momentum storage is negligible for them. The modeling approach followed for these components is based upon characteristics. The applications of these components to create a dynamic ORC model in the Simulink® environment is presented in ‘Part II’ of this paper.