Multi Disciplinary Modeling , Control, and Optimization of a Solid Oxide Fuel Cell/Gas Turbine Hybrid Power System

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ABSTRACT

This thesis describes a systematical study, including multidisciplinary modeling, simulation, control, and optimization, of a fuel cell– gas turbine hybrid power system that aims to  increase the system efficiency and decrease the energy costs by combining two power sources.

The fuel cell-gas turbine hybrid power systems can utilize exhaust fuel and waste heat from fuel cells in the gas turbines to increase system efficiency. This research considers a hybrid power system consisting of an internally reforming solid oxide fuel cell and a gas turbine. In the hybrid power system, the anode exhaust, which contains the remainder of the fuel, is mixed with the cathode exhaust as well as an additional supply of fuel and  compressed air and then burned in a catalytic oxidizer.

The hot oxidizer exhaust is expanded through the turbine section, driving an electric generator. After leaving the gas turbine, the oxidizer exhaust passes through a heat recovery unit in which it preheats the compressed air that is to be supplied to the fuel cell and the oxidizer.

This research concentrates on multidisciplinary modeling and simulation of the fuel cell-gas turbine hybrid power system. Different control strategies for the power sharing  between the subsystems are investigated. Also, the power electronics interfaces and controls  for the hybrid power system are discussed. Two different power sharing strategies are studied  and compared.

Simulation results are presented and analyzed. Transient  response of the  hybrid energy system is studied through time-domain simulation. In addition, in this effort,  Particle Swarm Optimization (PSO) is used to optimize the power sharing for the hybrid power  system to increase the efficiency and decrease the fuel consumption.

HYBRID POWER SYSTEM STRUCTURE AND MODELING

To determine the operating conditions and constraints of the physical system, a system simulation is usually helpful. Therefore, to study the hybrid power system performance it is easier to model and analyze it as a test bed consisting of its components.

Structure of Fuel Cell/Gas Turbine Hybrid Power System:

The hybrid power system in this study is a combination of a Gas Turbine (GT) with a Solid  Oxide Fuel Cell (SFOC). Several significant advantages of SOFC lead us to consider it as a  part of the hybrid power system. First of all, an SOFC operates at high temperatures. Also,  the energy conversion efficiency of an SOFC stack can reach up to 65%; and its overall  efficiency, when used in combined heat and power (CHP) applications, i.e., as an integrated SOFC combustion turbine system, can even reach up to 70%.

Schematic of a Hybrid Power System

Schematic of a Hybrid Power System.

Fuel Cell Description:

Fuel cells are electrochemical devices which directly convert the chemical energy of a reaction into electrical energy. The basic physical structure of a fuel cell consists of an electrolyte layer in contact with an absorbent anode and a cathode on either side. A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell.

Schematic of a Fuel Cell.

Schematic of a Fuel Cell.

Generator:

An electrical generator is a device which converts mechanical energy to electrical energy. The operation of a generator depends on electromagnetic induction, which is described as the production of an electromotive force in an electric circuit by a change of magnetic flux passing through a circuit.

HYBRID POWER SYSTEM CONTROL STRATEGIES

The Hybrid Power Control System (HPCS) can be considered a newly recognized branch in the power system industry. Power generation systems, including the SOFC-GT hybrid power system,  have their best performance at a specific design condition. However considering the change in the demand power, they must be able to operate well under conditions that vary from the  specific design condition. In the case of the SOFC-GT hybrid power system, the efficiency is  significantly influenced by the change in power.

Control System Design:

Fuel Controllers for Fuel Cell and Burner

There are four valves in the hybrid power system, which control the input fuel and air flow rates for the fuel cell and the burner. The proportional-integral (PI) algorithms are used to control these vales. Different power sharing strategies can be used to manage the valve  controllers. The control system design for each valve may change based on different control  strategies.

Power Sharing Strategies:

In the SOFC/GT hybrid power system, the efficiency is significantly influenced by load power changes. Since the dynamic response of the SOFC to these changes is quite slow, it is very  important to select an efficient control strategy to achieve the optimal performance from the system. In this section, two different control strategies are studied to control the presented SOFC/GT hybrid power system.

PARTICLE SWARM OPTIMIZATION

Power sharing is an important challenge in hybrid power systems. The demand output power is produced by the generators which make up the power system. Therefore to operate the system efficiently, the power sharing ratio should be determined appropriately. In this respect, the  design of hybrid power systems can be regarded as an optimization problem. The feasible  solution set of this problem must be obtained while the hybrid system should meet certain  power and voltage requirements. Thus, determining the optimum power sharing in increasing the efficiency of the system is one of the focuses of this study.

PSO Mechanism:

As mentioned earlier, the PSO technique is used to optimize the hybrid power system. First, an initial population of particles is generated with preliminary speeds and positions. By operating the best positions encountered by itself and its neighbors, each particle updates its position according to its own flight experience and that of its companions (motion role).

Graphical Illustration of the Mechanism of Velocity Update

Graphical Illustration of the Mechanism of Velocity Update

PSO Algorithm to Maximize the Efficiency:

As mentioned, one of the important challenges in the hybrid power system is to reach a high efficiency of the system. Therefore the issue of finding the amount of the power generated by each generator in the hybrid power system to maximize the efficiency is formulated as an  optimization problem. In this research, two types of configurations based on different assumptions have been studied to determine their optimum efficiency.

SIMULATION RESULTS

In Chapter two, the modeling of the SOFC-G T hybrid power system was studied. The mathematical model for each component was introduced there. Also, those models had the capability to investigate the transient response of the system. Chapter three has presented different  strategies that can be used to control the hybrid power system. In addition, the control  design for each strategy has been explained. In Chapter four PSO has been introduced as the optimization method to improve the efficiency of the system. This method has been implemented for tow different problems.

Simulation Results and Discussions:

The hybrid power system is tested on a standalone load supplied by a voltage source inverter. The SOFC consists of 600 cells and provides nominal output power 150 KW. Gas turbine consists of compressor with pressure ratio  (PRc) 3,  heat  exchanger  and  a  turbine with pressure ratio (PRT) 3.

The nominal power for gas turbine considered as 50 KW. The compressor input temperature in this simulation is ambient temperature. The generator  is a two-pole generator working at 60 Hz by 50kW nominal power and 200V nominal voltage.  During the simulation, SOFC provides 90kW power while the rest of the demanded power is  supplied by gas turbine generator.

CONCLUSION

The fuel cell-gas turbine hybrid power system can utilize exhaust fuel and waste heat from fuel cells to increase the system efficiency. This research considers an internally  reforming solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system, where the anode exhaust,  which contains the remainder of the fuel, is mixed with the cathode exhaust as well as an  additional supply of fuel and compressed air and then burned in a catalytic oxidizer. The hot  oxidizer exhaust is expanded through the turbine section, driving an electric generator.  After leaving the gas turbine, the oxidizer exhaust passes through a heat recovery unit in which it preheats the compressed air that is to be supplied to the fuel cell and the  oxidizer.

FUTURE WORK

Hybrid power systems have been introduced to the power industry to combine different property  of various energy resources to achieve a global goal such as high power density, fast transient response, maximum efficiency, minimum emission, and minimum coast. Although the  presented model can be a good reference for the study of the hybrid power system it is not complete and there would be more work to improve the system. Also, putting more effort to study and optimize the system can help to achieve an economic and efficient system.

Source: University of Miami
Author: Atid Abbassi Baharanchi

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