Dynamic Behavior and Management Strategy of Hybrid Wind/Fuel Cell System

: hybrid generation system is considered as a solution for the uncontrolled energy production from such dispersed sources as wind generation. In this paper, modeling and control of wind/FC system is proposed. Dynamics models for the main system components, namely, wind energy conversion system (WECS), fuel cell, electrolyses, power electronic interfacing circuits, hydrogen storage tank and ultra-capacitor are developed. Also, a variable speed wind generation maximum power point tracking (MPPT) based on Adaptative Neuro-Fuzzy Inference system (ANFIS) is presented. Based on the dynamic component model, a simulation model for the proposed hybrid energy system has been developed using Matlab/Simulink and the power flows management strategy are proposed. The result shows that this system can tolerate the rapid changes in wind speed and/or power demand. This results shows also that, the overall power management strategy is effective and the power flows among the different energy sources and the load demand is balanced successfully .


I. INTRODUCTION
Wind energy is one of world's fastest growing energy technologies. The disadvantage of wind turbine is that naturally variable wind speed causes voltage and power fluctuation problems at the bad side. This problem can be solved by using hybrid generation systems. The significant problem is to store the energy generated by wind turbine for future use when the power demand is greater than the power variable, a conventional batteries or diesel generator have been used as second energy source. These solutions have several disadvantages. The new trend is to use fuel cell based energy sources. This solution provides significant advantages in terms of energy density and long-term storage [1,2].
Various modeling techniques are developed by researchers to model components of HRES (hybrid renewable energy system). Performance of individual component is either modeled by deterministic or probabilistic approaches [3]. In this paper, a detailed dynamic model, simulation and management of a wind/fuel cell /ultra-capacitor hybrid power generation is developed. The system incorporates a water electrolyzer and storage tank to supply the fuel cell stack with hydrogen.
*Address correspondence to this author at the Tissemsilt University Center, Sciences and Technology Department, Tissemsilt, Algeria; Tel: +213552054623; Fax: +21346417377; E-mail: meharrar_aoued@yahoo.fr The dc power required for hydrogen generation is supplied through the dc bus during surplus /wind power. The generated hydrogen is stored in tanks to be utilized by the fuel cells when the wind energy source fails to supply the load demand [4]. The UC bank serves as a short duration power source to meet the excess power that cannot be satisfied by the FC or the wind system.
The wind/FC/ UC hybrid system modeling and simulation are performed using MATALB/SIMULINK. Simulation results are presented to examine the dynamic performance of the system and its potentiality in autonomous operation.

II. HYBRID WIND/FUEL CELL/ULTRA-CAPACITOR SYSTEM DESCRIPTION AND MODELING
In this section, the dynamic simulation model is described for the wind /FC/UC hybrid generation system. The block diagram of the integrated overall system is shown in Figure 1. The developed system consists of a horizontal axis wind turbine compiled to a permanent magnet synchronous generator (PMSG), an AC/DC controlled rectifier and FC/UC system with a boost type DC/DC.
The common DC bus collects the total energy from the wind and fuel cell subsystems and uses it partly to charge the UC and partly to generate hydrogen from the electrolyzer.

III. WIND ENERGY CONVERSION SUBSYSTEM (WECS)
The block diagram of the wind energy system adopted in this paper is shown in Figure 2. It consists of a horizontal axis wind turbine compiled to a permanent magnet synchronous generator (PMSG). A detailed description of the wind model can be found in [5]. The system is designed to achieve maximum power tracking (MPT) and output voltage regulation within a wide range of wind speed variation by means of MPPT block.

IV. WIND TURBINE MODEL
The power extracted from the wind is given by [6]: Where: : is the air density in (Kg/m 3 ).
A: is the area swept by the rotor blades in (m 2 ).
: is the wind velocity in (m/s).
Cp: is called the power coefficient or the rotor efficiency and is function of tip speed ration and pitch angle.
The tip speed ration is defined as: Where: : is the rotational speed of the wind turbine in (rad/s).
R: is the blade radius in (m).
Manufactures usually give an experimental relationship between C p and parameters, for several values of the rotation speed . In order to evaluate the C p coefficient, interpolation functions are used to approximate this experimental relationship, within each range of instantaneous values of . From this process, the following expressions result: The mechanical system is represented by the following equation: Where: J: is the total inertia which appears on the shaft of the generator in (Kg.m 2 ).
T m : is the mechanical torque in (N.m).
T em : is the electromagnetic torque in (N.m).
: is the rotational speed of the wind turbine in (rad/s).

V. PERMANENT MAGNET SYNCHRONOUS GENERATOR (PMSG)
The space vector theory of the space vector gives the dynamic equation of the stator currents as follows: Where: R s : is the phase resistance of the stator wind in ( ). The electromagnetic torque is given by:

VI. ANFIS CONTROL OF THE WECS
The WECS includes the wind turbine; the PMSG and the rectifier (Figure 3). The rectifier makes it possible to control the PMSG flux and consequently the speed of generator. The Block MPPT wind provides the value I sqref corresponding to the value of the reference electromagnetic torque. In this study, the vector control strategy applied to the PMSG, which consists in imposing a reference of the forward current I sdref to zero, is applied [6]. From the variation of the wind speed v, a Neurofuzzy model made up of radial basis functions computes the optimal speed rotation and thus the aerodynamic torque T mref . This computation is based on the mechanical characteristics of the wind turbine. The optimal torque gives the q-axis reference current I sqref . The d-and q-axis reference currents applied to two PI regulators and two decoupling stages give the dand q-axis reference voltages V sdref and V sqref . These voltages applied to two modulators provide the switching functions of the rectifier which gives the modulated current I l . The control strategy of the WECS, previously described, is illustrated in Figure 4.
The simulation model of the PMSG is illustrated in Figure 5.

VII. FUEL CELL SUBSYSTEM
The fuel cell, as a renewable energy source, is considered one of the most promising sources of electric power. Fuel cells are not only characterized by higher efficiency than conventional power plants, but they are also environmentally clean, have extremely low emission of oxides of nitrogen and sulfur, and have very low noise [7].
Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. They have a potential to achieve a level of efficiency beyond 70% when used in a cogeneration facility [8]. Fuel cells are classified by the type of electrolyte used in the cells and include: (1) proton exchange membrane (polymer) electrolyte fuel cell (PEMFC), (2) alkaline fuel cell (AFC), (3) phosphoric acid fuel cell (PAFC), (4) molten carbonate fuel cell (MCFC), and (5) solid oxide fuel cell (SOFC). These fuel cells are listed in the order of approximate operating temperature, ranging from 80°C for PEMFC to1000°C for SOFC [9].
Conventional FC design and the potential to use many fuels including gasoline and diesel without expensive external reformer that create more volatile chemicals [10].

VIII. SOFC MODEL
Many models have been proposed to simulate fuel cells in the literature [11]. The dynamic model of the fuel cell is the electrochemical model with the component material balance equations. The model is also based upon the voltage activation, concentration and Ohmic losses (the Nernst voltage equation) [10].
According to the mathematical model the stack voltage is given as [12,13]: Where: N 0 : is the number of cells.  P O 2 : is the oxygen partial pressure. P H 2 : is the water partial pressure.
Tacking the Laplace transforms, the expression for the hydrogen partial pressure is described as [14,15]: Where: Similarly, water partial pressure and oxygen partial pressure can be obtained as: Where: The fuel cell system consumes hydrogen according to the power demand. The hydrogen is obtained from high-pressure hydrogen tank for stack operation. During operational conditions, to control hydrogen flow rate according to the FC power output, a feedback control strategy is utilized. To achieve this feedback control, FC current from the output is taken back to the input while converting the hydrogen into molar form [16,17]. The amount of hydrogen available from the hydrogen tank is given by The block diagram of the SOFC model based on the above equations is shown in Figure 6.

IX. DC/DC BOOST CONVERTER
The main advantages of the boost converter are higher efficiency and reduced component count and it converts the unregulated voltage into desired regulated voltage by varying the duty cycle at high switching frequency lowering the size and cost of energy storage components [19]. The selection of components like boost inductor value and capacitor value is very important to reduce the ripple generation for a given switching frequency. However large inductance tends to increase the start-up time slightly while small inductance allow the coil current to ramp up to higher levels before switch turns off [20]. Figure 7 shows the circuit diagram of a boost DC/DC converter. The output voltage regulation feedback is also given in the figure. At steady-state, the average value of the output voltage is given as: Where is the duty ratio of the switching pulse.
The controller for the converter is to regulate the DC bus voltage within a desirable range. The output voltage is measured and compared with the reference value. The error signal is processed through the Pulse Width Modulation (PWM) controller, wich can be a simple PI controller, and the output of the controller is used to generate a PWM pulse with the right duty ratio so that the output voltage follows the reference value [21].

X. ULTRA CAPACITOR BANK
Ultracapacitors or electrochemical double layer capacitors take advantage of the charge stored in their electrochemical double layer and provide high capacities. Thanks to their compacity, ultracapacitors can store an higher amount of energy than conventional capacitors. Moreover, ultracapacitors are currently available on the market with capacitance ranges up to 2700F for a voltage of 2 to 3V; they can release energy at high or low rate [22].
The classical equivalent circuit of the UC unit, shown in Figure 8, consists of a capacitance (C), an equivalent series resistance (ESR) representing the charging and discharging resistance and an equivalent parallel resistance (EPR) representing the selfdischarging losses [23,24].
The EPR models leakage effects and influences long-term energy storage performance of the UC [25,26]. The state of charge of the UC can be described as a percent age of the rated energy capacity, which depends on the terminal output voltage. The energy flowing out from the UC is directly deter mined by the capacitance and the voltage change as expressed in Eq11, [27,28] Where: The real UC bank can be modeled by using multiple UC cells in parallel and series. The total resistance and capacitance of the UC bank are given by [27]: Where: n 1 : number of capacitors in series. n 2 : number of UC branches in parallel.
R ESR : equivalent series resistance.
The state-of-charge (SOC) of the ultracapacitor is defined as:

XI. ELECTROLYZER MODEL
An electrolyzer is advice that produces hydrogen and oxygen from water. The electrochemical reaction of water electrolysis is given by: There are three principal types of water electrolyser:-alkaline (referring to the nature of its liquid electrolyte), proton-exchange membrane (referring to its solid polymeric electrolyte), and solidoxide (referring to its solid ceramic electrolyte). The alkaline and PEM electrolysers are well proven devices with thousands of units in operation, while the solidoxide electrolyser is as yet unproven. The PEM electrolyser is particularly well suited to highly distributed applications. The alkaline electrolyser currently dominates global production of electrolytic hydrogen.
Alkaline water elecrolysis is the dominating technology today According to faraday' law, hydrogen production rate of an electrolyzer can be obtained as: Where: F: Faraday constant.
i e : Electrolyzer current.
n C : The number of electrolyzer cells in series.
n H2 : Produced hydrogen moles per second.

XII. HYDROGEN STORAGE SYSTEM
One of the hydrogen storage techniques is physical hydogen storage, which involves using tanks to store either compressed hydrogen gas or liquid. The hydrogen storage model based on Eq (22) an it directly calculates the tank pressure using the ration of hydrogen flow to the tank. The produced hydrogen is stored in the tank, whose system dynamic can be compressed as follow [29,30]: Where: z: Compressibility factor as a function of pressure.
The hydrogen state-of-storage (SHS) is therefore: Where: P b : Pressure of tank.
P bmax : is the maximum Pressure of tank.

XIII. OVERALL POWER MANAGEMENT STRATEGY
A control system was developed to determine when to produce hydrogen and when to convert it back to electricity. The control algorithm is based on the UC' state-of-charge (SOC) and the hydrogen' state-ofstorage (SHS). Knowing the load demand, the surplus energy is determined. The surplus energy is used to generate hydrogen from the electrolyzer, which is stored in tanks. At the times when the wind source is unable to supply the load demand, the stored hydrogen is supplied to the fuel cells (FC). The UC bank serves as a short duration power source to meet the excess power that cannot be satisfied by the FC or the wind system.
The flow chart describing the management of the power flow in the proposed system is shown in Figure  9.

XIV. SIMULATION RESULTS
Based on the component models given earlier, a simulation system for the proposed hybrid energy system has been developed in Matlab/simulink. The wind speed ranging between 4 and 10m/s with an average value of 7 m/s is presented on the Figure  10. This sequence is obtained by adding a turbulent component to a slowly varying signal. Respectively.   The UC state-of-charge (SOC) is given in the Figure  13.
When the wind generated power from is not sufficient to supply the load demand (for example see Figure 11, for t=80s to 100s). Under this condition, the fuel cell turns on to supply the power shortage and the UC bank serves as a short duration power source to meet the excess power that cannot be satisfied by the FC or the wind system. When, there is the surplus power available for stored (see Figures 11 and 12, for t=45s to 60s). The control algorithm based on the UC' state-of-charge (SOC) and the hydrogen' state-of-storage (SHS) is also used to fix the power stored in each system (UC bank and electrolyser system).
The H 2 storage tank pressure varies as H 2 flows in and out. It is apparent that the storage tank pressure will go up when there is excess power available for H 2 generation and will decrease when the FC supplies power to load and consumes H 2 . Figure 14 shows the tank pressure variations over the simulation period.  Finally, the user power requirement and the power delivered to the load side by the Wind/FC/UC hybrid system are illustrated in Figure 15.

XV. CONCLUSION
In this paper, a hybrid wind/FC/UC System Performance is proposed; the WECS was modeled using d-q rotor reference frame and the variable speed wind generator maximum power point tracking based on an adaptative Neuro-Fuzzy-Inference-System (ANFIS) was presented.
The results show also the excellent performance of the hybrid topology proposed under variable wind speed and load power requirements, so we can used this system in the non ideal wind speed areas.