The main goal of this work is the transient analysis of Hybrid Systems based on Solid Oxide Fuel Cells.The work is divided into three parts: in the first the fuel cell transient models are presented and discussed, while in the following papers the anodic recirculation system (Part B) and the entire hybrid transient performance (Part C) are investigated.In this paper the transient behavior of a Solid Oxide Fuel Cell is analyzed through the use of two different approaches: macroscopic and detailed SOFC models. Both models are presented in this paper and their simulation results are compared to each other and to available experimental data. As a first step the transient response of the fuel cell was studied using very detailed model in order to completely describe thisphenomenon and to highlight the critical aspects. Subsequently some modifications were made to this model to create an apt simulation tool (Macroscopic Fuel Cell Model) for the whole plant analysis. The reliability of this model was verified comparing several transient responses to the results obtained with the detailed model. In the following papers (Parts B and C) the integration of the macroscopic fuel cell model into the whole plant model will be described and the transient study of the hybrid plant will be presented.
