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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

Mohammad Jahangir Khan

mjakhan@engr.mun.ca

Faculty of Engineering & Applied Science

Electrical Engineering

Graduate Student Seminar : Master of Engineering

June 29, 2004

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

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

Introduction

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

Introduction

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

Introduction

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.

Introduction

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

Introduction

- Q1. Is a wind-fuel cell hybrid energy system feasible for a given set of conditions?
- Pre-feasibility Study
- Site: St. John’s, 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

Introduction

- Investigation of technology options, configurations and economics using:
- Electrical load profile
- Availability of renewable resources
- Cost of components (capital, O&M)
- Technology alternatives
- Economics & constraints
- HOMER (optimization software)

- St. John’s, 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.

Pre-feasibility Study

- 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

Pre-feasibility Study

- Hourly wind data for one year at St. John’s Airport.
- Average wind speed in St. John’s is around 6.64 m/s.

- Hourly solar data for one year at St. John’s Airport.
- Average solar irradiation in St. John’s is around 3.15 kWh/d/m2.

Pre-feasibility Study

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

Pre-feasibility Study

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

Pre-feasibility Study

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

Pre-feasibility Study

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

Pre-feasibility Study

Wind-Fuel Cell System Optimization

Pre-feasibility Study

- 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®

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

Model Formulation

50 W ~ 10 KW

Diameter

1 ~ 7 m

Hub-height

~ 30 m

Control/Regulation

Stall, Yaw, Pitch, Variable speed

Over-speed Protection

Horizontal/Vertical furling

Generator

DC, Permanent Magnet Alternator

Application

Stand-alone, Grid connections

Small WECS

Power in the wind:

Captured power:

Model Formulation

Wind Field

Spatial Filter & Induction Lag

PM DC Generator

Model Formulation

- Control Problem
- Below rated wind speed:Extract maximum available power
- Near-rated wind speed:Maintain 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

Model Formulation

Determination of Ref. Rotor Speed

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

Model Formulation

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

Defuzzification: Centroid method (Mamdani)

Model Formulation

- 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)

Model Formulation

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

Model Formulation

- 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.

Model Formulation

Electrochemical Model

- Cell voltage & Stack voltage:
- Open circuit voltage:
- Activation overvoltage:
- Ohmic overvoltage

Model Formulation

- 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

Model Formulation

- 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

Model Formulation

- 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

Model Formulation

- Alkaline Electrolyzer: PHOEBUS type
- Empirical & physical expressions
- Electrochemistry
- Dynamic energy balance

Model Formulation

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

Model Formulation

Electrolyzer Model Formulation

Electrochemical Model

- Cell voltage:
- Faraday efficiency:
- Hydrogen production:

Thermal Model

Model Formulation

- 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

Model Formulation

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

- Fuel Cell Boost Converter

Model Formulation

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

Simulation

- 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

- For a stand-alone residential load in St. John’s, 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

- 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.

- 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

- 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

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