Comparison Between Uncontrolled and Controlled Solid Oxide Fuel Cell Hybrid Systems

Because of the high complexity of Solid Oxide Fuel Cell hybrid systems, a transient analysis is mandatory to implement a control system able to maintain safe operation during disturbances or regular operational load variations. In fact, several parameters, such as the turbine rotational speed, the surge margin, the temperatures within the fuel cell, the turbine inlet temperature, the differential pressure between the anodic and the cathodic side and the Steam-To-Carbon Ratio need to be monitored and kept within safe limits. On the other hand, the system response to load variations is required to be as quick as possible in order to meet the energy demand. To develop a control strategy for these cycles, the work starts from the implementation of a transient model necessary to simulate a hybrid system based on the tubular SOFC technology. In fact, the first goal of this work is the analysis of the response to a step decrease in the fuel mass flow rate of the uncontrolled system, focusing the attention on the time scales of the transient phenomena and discussing the results from electrochemical, fluid dynamic and thermal point of view. The simulation shows that while the cathodic side is driven only by the temperature variation, because of the rotational speed is assumed to be constant, the anodic side is characterized by three different time-scale phenomena. In fact, all the plant properties show a negligible fluid dynamic delay, a depressurization time delay and a thermal long time-scale effect mainly due to the high thermal inertia of the cell. The considerations, carried out with the uncontrolled system, are used in the second part of the work to develop a control strategy to follow the power demand over time avoiding malfunctions or risk situations. The paper focuses the attention on a detailed presentation of the control system layout based on the by-pass valve between the compressor outlet and the turbine inlet, necessary to overcome the difficulty due to the difference between the small mechanical inertia of the microturbine shaft and the very high thermal inertia of the fuel cell stack. The simulations, carried out with a load step decrease, show the transient behaviour of the controlled SOFC hybrid system, presenting, over time, the values of the main critical parameters. Finally, the paper presents the results obtained with a power step increase to investigate the limitations of this control strategy focusing the attention on the fuel cell average temperature and the fuel utilization factor.