Paolo Pezzini

Control Impacts of Cold-Air Bypass on Pressurized Fuel Cell Turbine Hybrids

A pressure drop analysis for a direct-fired fuel cell turbine hybrid power system was evaluated using a hardware-based simulation of an integrated gasifier/fuel cell/turbine hybrid cycle (IGFC), implemented through the Hybrid Performance (Hyper) project at the National Energy Technology Laboratory, U.S. Department of Energy (NETL). The Hyper facility is designed to explore dynamic operation of hybrid systems and quantitatively characterize such transient behavior. It is possible to model, test and evaluate the effects of different parameters on the design and operation of a gasifier/fuel cell/gas turbine hybrid system and provide means of evaluating risk mitigation strategies.The cold air bypass in the Hyper facility directs compressor discharge flow to the turbine inlet duct, bypassing the fuel cell and exhaust gas recuperators in the system. This valve reduces turbine inlet temperature while reducing cathode airflow, but significantly improves compressor surge margin. Regardless of the reduced turbine inlet temperature as the valve opens, a peak in turbine efficiency is observed during characterization of the valve at the middle of the operating range. A detailed experimental analysis shows the unusual behavior during steady state and transient operation, which is considered a key point for future control strategies in terms of turbine efficiency optimization and cathode airflow control.Copyright

Equivalence Ratio Startup Control of a Fuel Cell Turbine Hybrid System

Initial startup of a direct-fired fuel cell turbine power system with equivalence ratio control using cathode air bypass valves to minimize thermal shock to the fuel cell was evaluated using the Hybrid Performance (Hyper) project hardware-based simulation facility at the U.S. Department of Energy, National Energy Technology Laboratory. The turbine in the system was started with the minimum possible airflow through the fuel cell cathode from a cold condition using two bypass valves to mitigate thermal shock failure in the fuel cell. The limitation of bypass flow was set by air requirements to maintain a combustor equivalence ratio below 0.6 during turbine windup. A 1D distributed fuel cell model operating in real time was used to produce individual cell transient temperature profiles during the course of the turbine start. The results provide insight into the procedural requirements of starting a fully coupled hybrid system.Copyright

Multi-Coordination of Actuators in Advanced Power Systems

Multi-coordination of actuators for a highly integrated, tightly coupled advanced power system was evaluated using the Hybrid Performance (Hyper) project facility at the U.S. Department of Energy’s National Energy Technology Laboratory (NETL). A two-by-two scenario in a fuel cell, turbine hybrid power system was utilized as a representative problem in terms of system component coupling during transients and setpoint changes. In this system, the gas turbine electric load is used to control the turbine speed, and the cold air bypass valve regulated fuel cell cathode mass flow.Perturbations in the turbine speed caused by variations in the waste heat from the fuel cell affect the cathode airflow, and the cold-air bypass control action required for constant cathode airflow strongly affects the turbine speed. Previous implementation of two single-input, single-output (SISO) controllers failed to provide acceptable disturbance rejection and setpoint tracking under these highly coupled conditions. A multiple-input, multiple-output (MIMO) controller based on the classic internal model control (IMC) concept was implemented and experimentally tested for the first time using the Hyper project facility.The state-space design of the MIMO configuration, the control law integration into the digital control platform, and the experimental comparison with the SISO case are presented.Copyright

Research and Educational Opportunities in Hardware-in-the-Loop Simulation of Advanced Power Systems: An International Perspective

Hardware-in-the-loop simulation (HiLS) is a specific technique designed in the experimental environment for studying the coupling between different technologies, where simulated and hardware components interact to each other. Two different HiLS facilities used for educational and research purposes are examined in the paper: the Hybrid Performance (Hyper) project facility at the U.S. Department of Energy, National Energy Technology Laboratory (NETL), and the Hybrid system emulator at the Thermochemical Power Group (TPG) facility, run by the University of Genoa in Italy. Since one facility is at a national laboratory and the other one in a university environment, both facilities dedicate considerable resources to the education of students with a different perspective: industrial and experimental approach.A description of the two configurations, the unique and overlapping attributes of each facility and the experimental results are reported and discussed to show different possibilities for students and researchers. Undergraduates, Postgraduates and Ph.D. students have the opportunity to learn innovative configuration of energy power systems, innovative control strategies applied to hybrid configurations, how to design real hardware components, and how to implementation real-time simulation models. The strong impact of these two laboratories is to show to students the applicability about their knowledge studied during lectures.Copyright

Control Strategy for a Direct-Fired Fuel Cell Turbine Hybrid Power System

A hardware-in-the-loop-simulation (HiLS) procedure for a direct-fired fuel cell turbine hybrid power system was evaluated for an integrated gasifier/fuel cell/turbine hybrid cycle (IGFC), implemented through the Hybrid Performance (Hyper) project at the National Energy Technology Laboratory, U.S. Department of Energy (NETL). The Hyper facility is designed to explore dynamic operation of hybrid systems and quantitatively characterize such transient behaviour. It is possible to model, test and evaluate the effects of different parameters on the design and operation of a gasifier/fuel cell/gas turbine hybrid system and quantify risk mitigation strategies.The previous implementation of emergency shut-down control strategies resulted in turbomachinery hardware failure. The primary linking event in these cases was compressor stall and surge resulting from the sudden loss of fuel during implementation of the standard double block and bleed strategy used during emergency failure. A new mitigation strategy involving automated ramps is proposed and described in detail to control the system from start-up to forced emergency shut-down. The control architecture shows how the virtual fuel cell model can be coupled to the real gas turbine safely, in all of stage of operations. The paper includes improvements to the emergency shutdown procedure, failure analyses, and the comparison of experimental data with previous results.Copyright

Multicoordination Control Strategy Performance in Hybrid Power Systems

This paper evaluates a state-space methodology of a multi-input multi-output (MIMO) control strategy using a two-by-two tightly coupled scenario applied to a physical gas turbine hybrid power system. A centralized MIMO controller was preferred compared to a decentralized control approach because previous simulation studies showed that the coupling effect identified during the simultaneous control of the turbine speed and cathode airflow was better minimized. The MIMO controller was developed using a state-space dynamic model of the system that was derived using first-order transfer functions empirically implemented through experimental tests. The controller performance was evaluated in terms of disturbance rejection through perturbations in the gas turbine operation, and set-point tracking maneuver through turbine speed and cathode airflow steps. The experimental results illustrate that a multi-coordination control strategy was able to mitigate the coupling of each actuator to each output during the simultaneous control of the system, and improved the overall system performance during transient conditions. On the other hand, the controller showed different performance during validation in simulation environment compared to validation in the physical facility, which will require a better dynamic modeling of the system for the implementation of future multivariable control strategies.

Multiobjective Neuroevolutionary Control for a Fuel Cell Turbine Hybrid Energy System

Increased energy demands are driving the development of new power generation technologies with high efficient. Direct fired fuel cell turbine hybrid systems are one such development, which have the potential to dramatically increase power generation efficiency, quickly respond to transient loads (and are generally flexible), and offer fast start up times. However, traditional control techniques are often inadequate in these systems because of extremely high nonlinearities and coupling between system parameters. In this work, we develop multi-objective neural network controller via neuroevolution and the Pareto Concavity Elimination Transformation (PaCcET). In order for the training process to be computationally tractable, we develop a computationally efficient plant simulator based on physical plant data, allowing for rapid fitness assignment. Results demonstrate that the multi-objective algorithm is able to develop a Pareto front of control policies which represent tradeoffs between tracking desired turbine speed profiles and minimizing transient operation of the fuel cell.

Avoiding Compressor Surge During Emergency Shut-Down Hybrid Turbine Systems

A new emergency shutdown procedure for a direct-fired fuel cell turbine hybrid power system was evaluated using a hardware-based simulation of an integrated gasifier/fuel cell/turbine hybrid cycle (IGFC), implemented through the Hybrid Performance (Hyper) project at the National Energy Technology Laboratory, U.S. Department of Energy (NETL). The Hyper facility is designed to explore dynamic operation of hybrid systems and quantitatively characterize such transient behavior. It is possible to model, test and evaluate the effects of different parameters on the design and operation of a gasifier/fuel cell/gas turbine hybrid system and provide means of quantifying risk mitigation strategies.An open-loop system analysis regarding the dynamic effect of bleed air, cold air by-pass and load bank is presented in order to evaluate the combination of these three main actuators during emergency shut-down. In the previous Hybrid control system architecture, catastrophic compressor failures were observed when the fuel and load bank were cut-off during emergency shut-down strategy. Improvements were achieved using a non-linear fuel valve ramp down when load bank was not operating. Experiments in load bank operation show compressor surge and stall after emergency shut-down activation. The difficulties in finding an optimal compressor and cathode mass flow for mitigation of surge and stall using these actuators are illustrated.Copyright

Real-Time Simulation of an Experimental Rig With Pressurized SOFC

The availability of reliable simulation models can reduce the time required for commissioning test rigs as well as preventing components from suffering serious damage during testing. The aim of this study is to set up and validate, against experimental data, a real-time model referring to the Rolls-Royce Fuel Cell System Limited (RRFCS) hybrid system concept, based on SOFCs. The dynamic model of the SOFC “block” has been developed, run in real-time, and successfully validated against experiments. Initially, the dynamic evolution of the model is considered under constant inputs at steady-state and is compared against experimental data; secondly, transient behaviour is also considered. Step variations of the main air flow, main fuel flow, syngas flow and electrical current were performed. The model can now be employed to carry out the following: performance analysis, design verification, development of control strategies, on-line analysis and integration with Human Machine Interface.Copyright