Integrated SOFC/GT Systems with Improved Dynamic Capabilities for Mobile Applications.

Abstract

This work is focused on developing control and system integration solutions to achieve rapid and reliable load following operation of solid oxide fuel cell/gas turbine (SOFC/GT) systems for mobile applications. Both the traditional recuperating-SOFC/GT system and the newly proposed sprinter-SOFC/GT system are studied through model-based methodologies. It is shown that solutions developed in this research could enhance system performance and meet operating objectives. For the recuperating system, the generator/motor (G/M) dual mode operation and its implications are investigated. Active shaft load control is used to manage transients by: (a) pre-conditioning of G/M power for load step-up transients; and (b) absorbing the excessive power through motoring operation for load step-down transients. Feedback and optimization algorithms are developed. By taking advantage of the dual operating G/M, better trade-offs between power tracking and thermal safety can be achieved, the battery requirements can be reduced and system performance can be enhanced. The sprinter-SOFC/GT system, which has far superior load following capability than traditional systems, is proposed in this research. In the system, the SOFC operated at constant temperature provides only the baseline power with high efficiency while the GT-generator’s transient capability will be fully explored for fast dynamic load following. System design and control framework suited for the proposed system are investigated. An SOFC operational strategy is derived to keep fairly constant SOFC power and temperature over the entire load range. A design procedure is also developed to determine various component sizes. The “actual” operational envelope is determined by integrating the SOFC power/temperature constraints with safety factors. An optimization problem is proposed to determine the optimal feed-forward operation map. Control analysis and feedback design are presented for the sprinter system. The stability of steady-state operation is studied through numerical simulations and linearized analysis of a simplified “2-state” model. Open-loop instability was identified for the low and medium airflow regions. Open-loop analysis and relative gain array (RGA) technique are used to gain insights on system operation and input-output interactions. Feedback control design is performed to address transient issues. The sprinter system achieves far superior performance than its recuperating counterpart for fast and safe load following.