Maria Serra

Control and optimization of the divided wall column

The divided wall column (DWC) is, in terms of capital costs and energy savings, a promising alternative for separating ternary mixtures. Since its design was proposed, almost 50 years ago, many authors have addressed design considerations. Operation and control of the DWC have received much less attention. However, some works have been published recently. Preliminary results reported indicate that feedback diagonal control structures may be used to control the DWC. In this work, the study of feedback diagonal control strategies has been further extended to consider the DWC control design in detail. Different control structures have been systematically analyzed and compared under performance and robustness considerations. In order to study the effect of the energy optimization on the controllability of the DWC, a column at optimal nominal operating conditions is compared to a column under non-optimal operation. Finally, a complete control strategy is proposed. Linear analysis tools are used for the multiple input multiple output (MIMO) feedback control analysis, and simulations using a non-linear model are performed to study the non-linear behavior of the control systems.

Design and Analysis of Fuel-Cell Hybrid Systems Oriented to Automotive Applications

Hybridization with high specific energy-storage devices such as supercapacitors (SCs) has important advantages in fuel-cell (FC)-based systems. This paper presents an approach for the design and analysis of FC hybrid systems (FCHSs) oriented to automotive applications. The FCHS is considered to be the most attractive long-term option for propulsion of passenger cars. The design stage includes the determination of the electrical topology and the determination of the hybridization degree (HD) according to drivability conditions. With the selected design, the optimal hydrogen consumption for different driving cycles and the energy flows in the hybrid vehicle are analyzed. The entire study is performed with a detailed model of the FCHS in the Advanced Vehicle Simulator (ADVISOR): the determination of the HD according to drivability requirements, the analysis of the energy flows, and the computation of the optimal hydrogen consumption. The results show that hybridization allows a significant improvement in the hydrogen economy through the recovered energy from breaking. At the same time, the results suggest a conflict between a design according to drivability conditions and a design for the highest efficiency. The conclusion is that hybridization with SCs in FC-based vehicles is a meaningful procedure that enhances performance.

ANALYSIS OF DIFFERENT CONTROL POSSIBILITIES FOR THE DIVIDED WALL COLUMN: FEEDBACK DIAGONAL AND DYNAMIC MATRIX CONTROL

Abstract This work addresses the control of the divided wall column (DWC) as a basic issue to be considered when this kind of distillation arrangement is used to take profit of its potential to reduce energy consumption. Different control structures of diagonal feedback control are compared using multiple input multiple output (MIMO) linear analysis tools in the frequency domain. A controllability analysis of the process is done for the separation of different mixtures and for different operating conditions, including optimal operation. As a result, it is seen that a trade off appears between energy minimisation and controllability. As an alternative, application of dynamic matrix control (DMC) to the DWC is also evaluated. Through simulation, the ability of DMC for disturbance rejection and setpoint tracking is studied and compared with that of the feedback diagonal control. Important limitations of using DMC for the composition control of the DWC are finally highlighted.

Study of the divided wall column controllability: influence of design and operation

Abstract The main objective of this work is to studh the influence of design and operating conditions on the divided wall column (DWC) controllability. Firstly, two designs for which the total distillation cost is minimised are compared. For each design, different optimal reflux to minimum reflux ratio is assumed. Secondly, three non-optimal designs are compared with an optimal one. Adding trays to different column sections, the distribution of the distillation effort is changed. The effect of this change on the controllability is studied. The DWC has extra operation degrees of freedom and the operation that minimises the boilup is searched. The influence of the operating conditions over the controllability is studied comparing optimal and non-optimal nominal operations.

Distributed parameter model simulation tool for PEM fuel cells

In this work, a simulation tool for proton exchange membrane fuel cells (PEMFC) has been developed, based on a distributed parameter model. The tool is designed to perform studies of time and space variations in the direction of the gas channels. Results for steady-state and dynamic simulations for a single cell of one channel are presented and analyzed. Considered variables are concentrations of reactants, pressures, temperatures, humidication, membrane water content, current, among others that have signicant eects on the performance and durability of PEMFC.

Fault-tolerant MPC control of PEM fuel cells

Abstract In this paper, fault-tolerant MPC control of PEM fuel cells is addressed. MPC is a suitable control methodology to control fuel cell systems because of their multivariable and complex behaviour. Additionally, MPC is one of the control methodologies that can introduce more easily fault-tolerance. However, the problem of including actuator fault-tolerance in the control loops of these systems has not already been addressed in the literature. This work is focused on the air feeding control. A new control structure that not only uses the compressor voltage as a control variable but also the air valve opening area at the cathode output is considered to improve the fault-tolerance of the air feeding subsystem. It is shown that using this additional control permits to introduce fault-tolerance against compressor faults at the same time that allows to improve control performance. Finally, the proposed approach is assessed on a known test bench PEM fuel cell through simulation.

Advances in HOSM Control Design and Implementation for PEM Fuel Cell Systems

Abstract A second order sliding mode strategy to control the air supply and oxygen stoichiometry of a fuel cell based generation system is presented. The control design is accomplished from a complete model of a experimental plant that was previously developed by the authors and specially suited for nonlinear control issues. The resulting controller endows the system with enhanced dynamic characteristics and robustness to model uncertainties and external disturbances. Simulations and experimental results are provided, showing the feasibility and reliability of the approach.

Analyses of energy management strategies for a PEMFC/UC electric vehicle

In this paper, two energy management strategies considering the hydrogen consumption of hybrid power sources using a PEM Fuel Cell (FC) and Ultracapacitors (UC) are described and compared. First, the Hybrid Electric Vehicle (HEV) architecture and the associated models with their control strategies are described. The two energy management strategies are evaluated based on the Energetic Macroscopic Representation (EMR). The comparison focuses on the global efficiency of the power sources energy management. In particular, a proposed strategy is to manage the UC State-Of-Charge while stabilizing the FC around its maximal efficiency point. Finally, some simulations on a Fuel Cell / Ultracapacitors HEV show the differences between the compared control strategies.

Nonlinear predictive control for the concentrations profile regulation in a PEM Fuel Cell anode gas channel

In this work, a nonlinear model predictive control (NMPC) strategy is proposed to regulate the concentrations of the different gas species inside a Proton Exchange Membrane Fuel Cell (PEMFC) anode gas channel. The purpose of the regulation relies on the rejection of the perturbations that affect the system. The model of the anode channel is derived from the discretization of the Partial Differential Equations (PDE) that define the dynamics of the system, taking into account spatial variations along the channel. Forward and backward discretizations of the distributed model are employed to take advantage of the boundary conditions of the problem. Simulation results are presented to show the performance of the proposed control method over a given case study. Different cost functions are compared and the one with minimum error is identified. Suitable dynamic responses are obtained facing the different considered disturbances.

Fast Model Predictive Control for hydrogen outflow regulation in Ethanol Steam Reformers

In the recent years, the presence of alternative power sources, such as solar panels, wind farms, hydro-pumps and hydrogen-based devices, has significantly increased. The reasons of this trend are clear: contributing to a reduction of gas emissions and dependency on fossil fuels. Hydrogen-based devices are of particular interest due to their significant efficiency and reliability. Reforming technologies are among the most economic and efficient ways of producing hydrogen. In this paper we consider the regulation of hydrogen outflow in an ethanol steam reformer (ESR). In particular, a fast model predictive control approach based on a finite step response model of the process is proposed. Simulations performed using a more realistic non-linear model show the effectiveness of the proposed approach in driving the ESR to different operating conditions while fulfilling input and output constraints.