Dynamic modeling of an organic rankine cycle

Abstract

A numerical model of an ORC (Organic Rankine Cycle) based on MATLAB en vironment is constructed in this study. The main components of the ORC are pump, evaporator, turbine and condenser. The organic working fluid (r134a) with low satu ration temperature is employed. The low saturation temperature enables organic fluid to change it’s phase with low heat input, thus system gains high amount of enthalpy without having excessive heat source. Waste heat and solar collectors are some popular heat sources for ORC’s. System design and component selections are made according to steady state operating condition. However, when a disturbance appears, one can not calculate the response of the system based on the steady state model. Hence, it is important to utilize a dynamic model of the energy system. Apart from disturbances, start-up and shut-down phases of the system have also transient outcomes. To define those transients in a thermodynamic cycle is a developing topic. The transient model is employed for the heat exchangers of the system. The turbine and the pump are modeled in the steady state. The effect of variable pressure on the fluid density is con sidered as instantly. Discretization of the heat exchangers is carried out according to the backward Euler formulation, and implicit scheme of time integration is employed in the model. Between time steps, an iterative solution method is introduced for guessing the spatial distribution of the mass flow rate. Different correlations are presented for the heat transfer in a plate heat exchanger. The examination of the dynamic model is made with constructed steady state heat exchanger model and with the data acquired from the experiment, which is conducted in BURET Laboratory at the Bogazici Uni versity Kilyos Campus. The dynamic model can be considered as a reliable predictor of outcomes, when system has disturbances or when system is in a transition.