Sujit S. Jogwar

Design and control of energy integrated SOFC systems for in situ hydrogen production and power generation

Abstract This paper studies the design and operation of energy integrated solid oxide fuel cell (SOFC) systems for in situ hydrogen production and power generation. Two configurations are considered: one where the hot effluent stream from the fuel cell is used directly to provide heat to the endothermic reforming reaction, and another where the hot effluent streams are mixed and combusted in a catalytic burner before the energy integration. A comparative evaluation of the two configurations is presented in terms of their design, open-loop dynamics and their operation under linear multi-loop controllers.

Tight energy integration: Dynamic impact and control advantages

Process integration is a key enabler to increasing efficiency in the process and energy generation industries. Efficiency improvements are obtained, however, at the cost of an increasingly complex dynamic behavior. As a result, tightly integrated designs continue to be regarded with caution owing to the dynamics and control difficulties that they pose. The present work introduces a generic class of integrated networks where significant energy flows (either arising from energy recycling or of external origin) result in dynamic models with a multi-time-scale structure. Such networks feature a clear distinction between the fast dynamics of individual units and the slow dynamics of the entire network. We draw a connection between specific (steady-state) design features and structural properties that afford the development of a framework for the derivation of low-order, non-stiff, nonlinear models of the core network dynamics. Furthermore, we demonstrate that tight energy integration and the presence of significant energy flows can facilitate, rather than hinder, control structure design and performance, and propose a cadre for hierarchical control predicated on the use of fast, distributed control for the individual units and nonlinear supervisory control for the entire network. The developed concepts are illustrated with examples.

Reduction of complex energy-integrated process networks using graph theory

Abstract This paper focuses on the analysis of complex (multi-loop) energy-integrated process networks. Simple (single-loop) energy-integrated networks (comprising of large energy recycle or throughput) with two-time scale dynamics are the building blocks for such complex networks. The modular structure of these complex networks lends them to a graph theoretic analysis, whereby weak and strong connections between process units arising from time scale separation are identified from structural information. Subsequently, a graph-theoretic framework for network analysis and control is developed, and connecting links are built to an equivalent analysis using singular perturbations. The proposed analysis framework is illustrated via application to a representative complex process network.

Dynamics and control of high duty counter-current heat exchangers

In this paper, we analyze a class of high duty counter-current heat exchangers. A high duty counter-current heat exchanger is modeled by two first order stiff PDEs, which show a potential of multi-time scale dynamics. Using singular perturbations, a non-stiff reduced model capturing the slow time scale dynamics is derived. Input/output linearizing controllers are derived based on both the full model and the reduced model, and the advantages of using the controller based on the reduced model are documented via simulations.

Graph reduction for hierarchical control of energy integrated process networks

In this paper, we propose a graph-theoretic algorithm that can be used to analyze complex chemical processes comprising of multiple energy integration loops. Such networks are known to exhibit dynamics in multiple time scales. The algorithm uses information on the order of magnitude of the different energy flows and determines automatically the time scales where the units evolve, the manipulated inputs acting in the different time scales and the form of the reduced order models in each time scale. The application of the algorithm is illustrated through a case study of a benchmark chemical process.

Multi-Time Scale Dynamics in Energy-Integrated Networks: A Graph Theoretic Analysis

Abstract Energy integrated networks, designed to reduce energy consumption, offer cost savings at the expense of challenging operation. Simple networks with energy integration, involving either a large recycle of energy or a large throughput of energy, have been shown to exhibit dynamic behavior evolving over two time scales. In this paper, the time scale properties in the dynamics of networks involving multiple, interconnected throughputs and recycle loops are investigated. A graph theoretic analysis framework is developed which allows identifying the time scales where each of the process units in the network evolves, based on knowledge of the order of magnitude of the energy flows in the network. An example network is considered to illustrate the application of the proposed framework.

Control of an energy integrated solid oxide fuel cell system

Solid oxide fuel cell (SOFC) energy systems constitute an alternative solution to the conventional combustion systems for power generation. Their high operating temperature provides the potential for energy integration and higher overall system efficiencies. In this study, an integrated SOFC energy system suitable for stationary applications is considered. Dynamic lumped parameter models for each unit are derived. Control objectives are identified and a control strategy for the integrated SOFC energy system is proposed. A nonlinear model based controller is derived for the control of fuel cell temperature. The effectiveness of the proposed control strategy is illustrated via case studies with varying power demand.

Impact of steam reformer on the design and control of an energy integrated solid oxide fuel cell system

Energy integrated fuel cell systems show the potential for highly efficient and enviromentally friendly power generation. In this study, an energy integrated system with a solid oxide fuel cell (SOFC) and a methane steam reformer is considered. The effect of the reformer design parameters (steam-to-carbon (S/C) ratio and operating temperature) on the steady-state performance of the entire SOFC system is studied. Additionally, the effect of the S/C ratio on the open-loop behavior is also analyzed under small and large steps in the current. Lastly, the closed-loop performance of the entire energy system for a decentralized control scheme is evaluated under a large disturbance in load current.

Multi-scale dynamics in counter-current heat exchangers

In this paper, a class of high duty counter-current heat exchangers, which are typically used in energy integrated systems, is considered. The governing dynamic equations for such exchangers are stiff first order hyperbolic PDEs, and hence show a potential towards multi-scale dynamics. Using singular perturbations, a reduced non-stiff PDE model is derived, which captures the dynamics in the slow time scale. A simulation case study is considered to illustrate these results.

Dynamics and control of reactor - feed effluent heat exchanger networks

In this paper, we analyze the energy dynamics of process networks comprising of a chemical reactor and a feed effluent heat exchanger (FEHE). Using singular perturbation analysis, we show that, in the case of tight energy integration, the energy dynamics of the network evolves over two time scales, with the enthalpy of individual units evolving in the fast time scale and the overall network enthalpy evolving in the slow time scale. We describe a model reduction procedure to derive the non-stiff slow model which can be used for controller design. The theoretical results are illustrated via a simulation case study.