Voltage and Current Based MPPT of Fuel Cells for fuel Consumption Optimization andmismatching Compensation

Transcription

1 Technical Journal of Engineering and Applied Sciences Available online at TJEAS Journal / SSN TJEAS oltage and Current Based MPPT of Fuel Cells for fuel Consuption Optiization andmisatching Copensation Mohaad Sarvi 1*, an Soltani 1. Assistant Professor, Faculty of Engineering and Technology, a Khoeini nternational University Qazvin, ran. MSc Student, Faculty of Engineering and Technology, a Khoeini nternational University Qazvin, ran Corresponding author: Mohaad Sarvi ABSTRACT: A fuel cell (FC) is a device that converts the cheical energy into electricity energy. Fuel cell is a clean and efficient source of electricity. n order to bring up the syste efficiency, this paper oltage and Current Based ethod for Maxiu Power Point Tracking of PEMFC s Syste. n this work a coplete study of PEMFC s syste integrating an MPPT controller is studied, also siulation work dealing with MPPT controller, a DC/DC boost converter feeding a resistive load is achieved. The siulation result of designed proposed ethod. oltage and Current Based MPPT has better response tie, less oscillation and uch ore accurate tracking. Significant extracted results are given to prove the validity of the proposed PEMFC s syste control schee. n this paper, siulation odel of PEMFC syste s axiu power point tracking based on the oltage and Current BasedMPPT control ethod is developed in the MATLAB software. Keywords: PEM Fuel Cell, Maxiu Power Point Tracking (MPPT),MPPT, CMPPT. NTRODUCTON Fuel Cells are one of the ore proising generation systes of electric power at present and are particularly interesting as vehicle propulsion coponents, because they are not constrained of Carnot efficiency. Soe of the present-day challenges are the production and optiization of the fuel use in Fuel Cells based systes. n this paper, an algorith for tracking the axiu power point of a Fuel Cell Syste is presented(nehrir.h et al., 006). The topology proposed in this paper does not work to axiu fuel flow. This topology has a DC/DC Boost converter that adjusts it s input current at the axiu power generation point of the stack, this for the iniu fuel consuption that can produce the desired electric power(masou et al., 00). Fuel cell output power depends on the applied current or voltage and fuel cell output voltage is dependent on operating conditions, including cell teperature, air pressure, oxygen partial pressure, and ebrane water content (Masou et al., 1998). Fuel cells have nonlinear voltage-current characteristic, and there is only one unique operating point for a fuel cell syste with a axiu output under a particular condition. However, the axiu power point (MPP) varies with teperature and ebrane water content. Therefore, the axiu power point tracking (MPPT) at all operating conditions is a challenging proble. n fact, in the MPPT algorith, the stack current and fuel flow are controlled under various operating conditions to optiize fuel consuption and the extract axial power of the fuel cell (Masou et al., 1998). So it is necessary to force the syste to operate in conditions which atch up with fuel cell axiu power point (MPP). A MPP tracking (MPPT) which utilizes a MPPT algorith can undertake this duty. When fuel cell operates in this condition, since MPP is a unique and fixed point of operation, load power requireents are not taken into account and output voltage will vary with variations of load resistance. Different axiu power point tracking (MPPT) techniques for photovoltaic systes have been reported (Masou et al., 00), (Masou et al., 1998), (Sarvi et al., 00), (Masou et al., 005), (Karlis et al.,

2 Tech J Engin & App Sci., 3 (1): , ), (Hussein et al., 1995), (Noguchi et al., 00), (Enslin et al., 1997). Aong the Perturbation and Observe (P&O) is the ost coonly used ethod because of its siple algorith. Also oltage-based (MPPT) and current-based (CMPPT) axiu power point tracking are siple and practical approaches for iproving the efficiency of photovoltaic systes(masou et al., 00), (Masou et al., 1998). This paper investigates MPPT and CMPPT ethods for FC. Also the ipact of varying teperatures on the perforance of MPPT and CMPPT techniques is analyzed. The siulated and theoretical results are copared. Siulations are carried in MATLAB facilities. Fuel Cell Characteristic Fuel cell voltage as a function of current density in a steady state can be represented by a polarization curve, which is influenced by such paraeters as the cell teperature, oxygen partial pressure, hydrogen partial pressure and ebrane water content. When current is drawn fro a fuel cell, the cell voltage decreases fro its equilibriu therodynaic potential E nernst (open circuit voltage)[10]. This voltage drop consists of activation loss ( act ), ohic loss ( ohic) and concentration loss ( con ). The basic expression for the cell voltage is (Zhi-dan et al., 008) cell (1) cell E nernst act ohic con Reversible therodynaic potential 008) 4 Enernst (T )... () T(lnP 0.5lnP ) H O E nernst is described by the Nernst equation. This voltage is (Zhi-dan et al., Activation overvoltage ( η act )is described by the Tafel equation as the following (Zhi-dan et al., 008) act 1 T 3 O 4 T lnc T ln (3) whereξ (i= 1:4) are paraetric coefficients for each cell odel. CO is the concentration of dissolved oxygen at 3 the gas/liquid interface ( ol.c ), which can be calculated as (Zhi-dan et al., 008) C O PO (4) 6 ( ) exp( 498 / T ) Ohic overvoltage ohic results fro the resistance of the polyer ebrane in electron and proton transfers. t can be written as (Zhi-dan et al., 008) ohic R (5) The resistance R is given by: rt R A (6) where r is ebrane resistivity ( c ) to proton conductivity, t ebrane thickness ( c ), A cell active 846

3 Tech J Engin & App Sci., 3 (1): , 013 area ( c ). Mebrane resistivity depends strongly on ebrane huidity and teperature, and can be described by the following epirical expression (Zhi-dan et al., 008) 181.6[ ( / A) 0.006( T / 303 ) ( / A) ] r [ ( / A)] exp[ 4.18( T 303 / T )].5 (7) where is the ebrane water content. The ebrane water content is a function of the average water activity a ( a 1), { a 39.85a 36a 3, (8) 0 a 1 a 1 3 The average water activity is related to the anode water vapor partial pressure vapor partial pressure P v, ca (Zhi-dan et al., 008) : P v, an and the cathode water a 1 1 pv,an Pv,ca ( aan aca ) (9) P sat The saturation pressure of water 008) : P sat can be calculated with the following epirical expression (Zhi-dan et al., log10 Psat T... (10) T T The value of varies between 0 and 14, which is equivalent to the relative huidity of 0% and 100%. Under supersaturated conditions, however, the axiu possible value of can be as high as 3. n addition, is influenced by the ebrane preparation procedure, the relative huidity of the feed gas and the ebrane age. n this paper, is considered as an adjustable paraeter with a possible value between 0 and 3. Concentration overvoltage con results fro the concentration gradient of reactants as they are consued in the reaction. The equation for concentration overvoltage is shown by (Zhi-dan et al., 008) : RT con ln(1 ) (11) nf i A L 847

4 P(w) (v) Tech J Engin & App Sci., 3 (1): , 013 Figure 1. PEMFC generator syste where i L is the liiting current. t denotes the axiu rate at which a reactant can be supplied to an electrode. Polarization Curves The fuel cell voltage as a function of current density in a steady state can be represented by a polarization curve, which isinfluenced by such paraeters as the cell teperature, oxygenpartial pressure, hydrogen partial pressure and ebrane watercontent. When current is drawn fro a fuel cell, the cell voltagecell decreases fro its equilibriu therodynaic potentialenernst (open circuit voltage).by considering the odel paraeters in (Zhi-dan et al., 008) (A) Figure. Fuel cell output voltage versus current. and using MATLAB facilities, - and P- curves are obtained as Figs. and 3, respectively ncreasing of teperature ncreasing of teperature Figure 3. Fuel cell output power versus current. n the CMPPT technique, the fuel cell current corresponding to axiu power ( p ) is considered proportional to the fuel cell short current ( sc ) as the following K (1) p sc where K is the constant "current factor" (A) 848

5 Tech J Engin & App Sci., 3 (1): , 013 n the MPPT technique, the relation between fuel cell voltage corresponding to axiu power ( p ) and fuel cell open circuit voltage is considered to be linear as the following K (13) p oc where K is the constant "voltage factor". The ipact of teperature on factors K and K is considered and will be investigated in the next section. Analysis ofsiulated Results for MPPT and CMPPT Techniques n order to investigate the ipact of teperature variations on the perforance and accuracy of MPPT and CMPPT techniques, theoretical values of array voltage ( oc, p ), array current ( sc, p )current factor ( K ) and voltage factor ( K ) for different teperature conditions are copared. Siulations are done in MATLABenvironent for two techniques Analysis of the Current-Based MPPT Fuel cell characteristics including the short circuit current ( sc ) and the current corresponding to the axiu power point ( p ) are calculated and then current factor K p / sc are deterined. Fig. 3 shows the current factor ( K ) versus teperature. The stars (*) are the coputed data and the continuous line is curve fitted of the. The voltage factor ( K ) is not constant and depends on array teperature but the changing of K v is approxiately insignificant. Figure4 shows flowchart of CMPPT ethod respectively. Start Calculation of SC p=k i SC Copare p and fc with PD End Figure4. the flowchart of CMPPT ethod Analysis of the oltage-based MPPT n order to investigate the accuracy of MPPT technique under variable teperature conditions, cell open circuit voltage ( oc ) and the voltage corresponding to axiu power ( p ), as well as the voltage factor K / ) are coputed for different teperature conditions. Fig.5 shows flowchart of MPPT ethod ( v p oc respectively.fig. 6 shows the current factor ( K ) versus teperature. Fig. 7 shows the voltage factor ( K ) versus teperature.the stars (*) are the coputed data and the continuous line is curve fitted of the. The voltage factor ( K v ) is not constant and depends on array teperature but the changing of approxiately insignificant. K v is 849

6 Tech J Engin & App Sci., 3 (1): , 013 Start Calculation of oc p=k v oc Copare p and fc with PD End Figure5. the flowchart of MPPT ethod Figure6. Current factor ( K ) versus teperature. Figure 7.oltage factor ( K ) versus teperature. CONCLUSON This paper proposes MPPT and CMPPT techniques for MPP tracking of fuel cell. n MPPT techniques, by increasing the teperature, K is decreasing and the variation of the voltage factor with respect to teperature is linear and slop of variation is approxiately constant. n CMPPT techniques, by increasing the teperature K is increasing and the variation of the current factor with respect to teperature is linear and slop of variation is approxiately constant. n spite of this, the variation of K and K is very insignificant at t can be assued constant. Therefore these two techniques are very siple practical tracking approaches. 850

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