Dynamic Modeling, Simulation and Control of a Small Wind-Fuel Cell Hybrid Energy System for Stand-Alone Applications

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Dynamic Modeling, Simulation and Control of a
Small Wind-Fuel Cell Hybrid Energy System for
Stand-Alone Applications
Graduate Student Seminar Master of Engineering
June 29, 2004

• Mohammad Jahangir Khan
• mjakhan_at_engr.mun.ca
• Faculty of Engineering Applied Science
• Electrical Engineering

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Outline

• Introduction
• Renewable Energy, Hybrid Stand-alone Power
  Sources
• Emerging Technologies, Scope of Research
• Pre-feasibility Study
• Load, Resource, Technology Options
• Sensitivity Optimization Results
• Model Formulation
• Wind Energy Conversion System, Fuel Cell System,
  Electrolyzer, Power Converter
• System Integration
• Simulation
• Results
• Random Wind Variation
• Step Response
• Conclusion

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Canada and the Global Energy Scenario

• At present, proportion of renewable energy in the
  global energy mix is about 14 only.
• Various environmental regulations and protocols
  aim at increasing this ratio towards 50 by 2050.

Source German Advisory Council on Global Change
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• In Canada, utilization of renewable resources is
  less than 1 (excluding hydroelectricity)
• Vast wind energy potential is mostly unexplored.

Source The Conference Board of Canada
Source Natural Resources Canada
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Emerging Technologies in Energy Engineering

• Wind and Solar energy technologies are the
  forerunners
• Hydrogen based energy conversion bears good
  potential

Source Worldwatch Institute
Source Plug Power Inc., NY

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Hybrid Energy Systems in Stand-alone Applications

• Energy from a renewable source depends on
  environmental conditions
• In a Hybrid Energy System, a renewable source is
  combined with energy storage and secondary power
  source(s).
• Mostly used in off-grid/remote applications
• Could be tied with a distributed power generation
  network.

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Wind-Fuel Cell Hybrid Energy System

• A wind turbine works as a primary power source
• Availability of wind energy is of intermittent
  nature
• Excess energy could be used for hydrogen
  production by an electrolyzer
• During low winds, a fuel-cell delivers the
  electrical energy using the stored hydrogen
• Radiated heat could be used for space heating
• Power converters and controllers are required to
  integrate the system

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Scope of Research

• Q1. Is a wind-fuel cell hybrid energy system
  feasible for a given set of conditions?
• Pre-feasibility Study
• Site St. Johns, Newfoundland.
• Q2. What are the alternatives for building and
  testing a HES, provided component cost is very
  high and technology risk is substantial?
• Computer aided modeling
• System integration and performance analysis
  through simulation

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Pre-feasibility Study

• Investigation of technology options,
  configurations and economics using
• Electrical load profile
• Availability of renewable resources
• Cost of components (capital, OM)
• Technology alternatives
• Economics constraints
• HOMER (optimization software)

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• HOMER Implementation
• St. Johns, Newfoundland
• Renewable (wind/solar) non-renewable (Diesel
  generator) sources
• Conventional (Battery) non-conventional
  (Hydrogen) energy storage
• Sensitivity analysis with wind data, solar
  irradiation, fuel cell cost diesel price.

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Electrical Load

• A typical grid connected home may consume around
  50 kWh/d (peak 15 kW)
• A HES is not suitable for such a large load
• Off-grid/remote homes should be designed with
  energy conservation measures
• A house with 25 kWh/d (4.73 kW peak) is
  considered
• Actual data is scaled down

Source Newfoundland Hydro
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Renewable Resources

• Hourly wind data for one year at St. Johns
  Airport.
• Average wind speed in St. Johns is around 6.64
  m/s.

• Hourly solar data for one year at St. Johns
  Airport.
• Average solar irradiation in St. Johns is around
  3.15 kWh/d/m2.

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Components

• Wind turbine
• Solar array
• Fuel cell
• Diesel generator
• Electrolyzer
• Battery
• Power converter

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Sensitivity Results

• At present, a wind/diesel/battery system is the
  most economic solution
• Solar energy in Newfoundland is not promising

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• A wind/fuel cell/diesel/battery system would be
  feasible if the fuel cell cost drops around 65.
• A wind/fuel cell HES would be cost-effective if
  the fuel cell cost decreases to 15 of its
  present value

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Optimization Results

• Considering
• wind speed 6.64 m/s
• solar irradiation 3.15 kWh/m2/d
• Diesel price 0.35 /L
• The optimum solutions are

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Wind-Fuel Cell System Optimization
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Model Formulation

• Models Developed for
• Wind Turbine (7.5 kW) Bergey Excel-R
• PEM Fuel Cell (3.5 kW) Ballard MK5-E type
• Electrolyzer (7.5 kW) PHOEUBS type
• Power Converters (3.5 kW)
• Approach
• Empirical physical relationships used
• Components are integrated into a complete system
  through control and power electronic interfaces
• Simulation done in MATLAB-Simulink

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Wind Energy Conversion System (WECS)

• Small wind turbine BWC Excel-R type
• Wind field
• Rotor aerodynamics
• Spatial Filter
• Induction Lag
• PM DC generator
• Controller
• Reference speed generator
• Fuzzy logic controller

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Small WECS
Power in the wind Captured power
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Small WECS Model Formulation
Wind Field
Spatial Filter Induction Lag
PM DC Generator
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Controller Design

• Control Problem
• Below rated wind speed Extract maximum available
  power
• Near-rated wind speedMaintain constant rated
  power
• Over-rated wind speed Decrease rotor speed
  (shut-down)

II
III
I

• Control method
• A PD-type fuzzy logic controller (FLC) is employ
• Reference rotor speed is estimated from rotor
  torque
• Difference in actual ref. Speed is used to
  control the dump load

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Determination of Ref. Rotor Speed

• Rotor torque is assumed available
• Below rated reference rotor speed
• Near-rated conditions
• Over-rated reference rotor speed

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Design of Fuzzy Logic Controller
A PD type FLC is used for the whole range of wind
variation Variable Identification Error Rate
of change of error Fuzzification Five Gaussian
membership functions for all variables Rules of
inference Fuzzy Associative Memory Defuzzificatio
n Centroid method (Mamdani)
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Summary

• Dynamic model of a Small wind turbine (BWC
  Excel-R type)
• Wind field, Rotor aerodynamics, PM DC generator
• Controller (Reference speed generator, Fuzzy
  logic controller)
• Mechanical sensorless control (rotor torque
  assumed estimable)

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Fuel Cell System

• PEM fuel cell Ballard MK5-E type
• Empirical physical expressions
• Electrochemistry
• Dynamic energy balance
• Reactant flow
• Air flow controller

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PEM Fuel Cells

• Polymer membrane is sandwiched between two
  electrodes, containing a gas diffusion layer
  (GDL) and a thin catalyst layer.
• The membrane-electrode assembly (MEA) is pressed
  by two conductive plates containing channels to
  allow reactant flow.

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Fuel Cell Model Formulation

• Electrochemical Model
• Cell voltage Stack voltage
• Open circuit voltage
• Activation overvoltage
• Ohmic overvoltage

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• Reactant Flow Model
• Performance depends on oxygen, hydrogen vapor
  pressure
• Anode Cathode flow models determine reactant
  pressures
• Ideal gas law equations and principles of mole
  conservation are employed

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• Thermal Model
• Fuel cell voltage depends on stack temperature
• Stack temperature depends on load current,
  cooling, etc.
• Total power (from hydrogen)
• Electrical output Cooling Surface Loss
  Stack Heating
• A first order model based on stack heat capacity
  is used

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Summary

• Dynamic model of a PEM fuel cell (Ballard MK5-E
  type)
• Electrochemical, thermal and reactant flow
  dynamics included
• Model shows good match with test results

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Electrolyzer

• Alkaline Electrolyzer PHOEBUS type
• Empirical physical expressions
• Electrochemistry
• Dynamic energy balance

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Alkaline Electrolyzer

• Aqueous KOH is used as electrolyte
• Construction similar to fuel cell

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Electrolyzer Model Formulation

• Electrochemical Model
• Cell voltage
• Faraday efficiency
• Hydrogen production
• Thermal Model

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Power Electronic Converters

• Variable DC output of the Wind turbine/Fuel cell
  is interfaced with a 200 V DC bus
• Load voltage 120 V, 60Hz
• Steady state modeling of DC-DC converters
• Simplified inverter model coupled with LC filter
• PID controllers used

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Power Converter Models

• WECS Buck-Boost Converter
• Inverter, Filter R-L Load

• Fuel Cell Boost Converter

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System Integration
Power flow control
Wind-fuel cell system interconnection
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MATLAB-Simulink Simulation
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Simulation

• Simulation time 15 seconds
• Constant temperature in fuel cell electrolyzer
  assumed
• Step changes in
• Wind speed
• Load resistance
• Hydrogen pressure

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Results
System response with random wind
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WECS performance (step response)
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Power balance (step response)
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Fuel cell performance (step response)
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Electrolyzer performance (step response)
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Power converter performance (step response)
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• Summary
• Highest settling time for the wind turbine
• Controlled operation of the wind turbine, fuel
  cell, electrolyzer and power converter found to
  be satisfactory
• Coordination of power flow within the system
  achieved

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Contributions

• For a stand-alone residential load in St. Johns,
  consuming 25 kWh/d (4.73 kW peak) a
  pre-feasibility study is carried out.
• A mathematical model of wind-fuel cell energy
  system is developed, simulated and presented. The
  wind turbine model employs a concept of
  mechanical sensorless FLC.
• The PEM fuel cell model unifies the
  electrochemical, thermal and reactant flow
  dynamics.
• A number of papers generated through this work.
  Explored fields include
• Wind resource assessment
• Fuel cell modeling
• Grid connected fuel cell systems
• Small wind turbine modeling

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Conclusions

• A wind-fuel cell hybrid energy system would be
  cost effective if the fuel cell cost reduces to
  15 of its current price. Cost of energy for such
  a system would be around 0.427/kWh.
• Performance of the system components and control
  methods were found to be satisfactory.
• Improvement in relevant technologies and
  reduction in component cost are the key to
  success of alternative energy solutions.

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Further Work

• Development of a faster model for investigating
  variations in system temperature and observing
  long term performance (daily-yearly).
• Inclusion of various auxiliary devices into the
  fuel cell and electrolyzer system.
• Use of stand-by batteries
• Research into newer technologies such as, low
  speed wind turbines, reversible fuel cell etc.
• Comprehensive study of relevant power electronics
  and controls

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Acknowledgement

• Faculty of Engineering Applied Science, MUN.
• School of Graduate Studies, MUN.
• NSERC
• Environment Canada
• Dr. M. T. Iqbal.
• Drs. Quaicoe, Jeyasurya, Masek, and Rahman.

Thank You
For your attention presence
Questions/Comments