MODELING, SIMULATON AND SIZING OF PHOTOVOLTAIC/WIND/FUEL

CELL HYBRID GENERATION SYSTEM

M.MAHALAKSHMI

Research Scholar, Department of EEE, Thiagarajar College of Engineering, Madurai, Tamilnadu, India

maha_parvathy@yahoo.co.in

Dr.S.LATHA,

Associate Professor, Department of EEE, Thiagarajar College of Engineering, Madurai, Tamilnadu, India

sleee@tce.edu

Abstract

The depleting fossil fuel reserves and increasing concern towards global warming have created the need to surge for the alternative power generation options. Renewable energy sources like Wind, Solar-PV, Biomass and fuel cells are gaining prominence nowadays, as they are more energy efficient, reduce pollution and also they serve as a promising solution to the toughest energy crisis faced during the recent years. This paper focuses on the modeling and simulation of solar – photovoltaic, wind and fuel cell hybrid energy systems using MATLAB/Simulink software. The intermittent nature of solar and wind energy sources make them unreliable. Hence Maximum Power Point Tracking (MPPT) is used to extract maximum power from the wind and sun when it is available. The standard perturb and observe method of MPPT is used for the PV system and for the wind generation system. The simulation results of the PV/Wind /Fuel cell hybrid system are presented in graph showing the effectiveness of the proposed system model. Also, hardware implementation of microcontroller based MPPT for solar-PV alone and unit sizing of the hybrid system for the PG simulation lab in EEE Dept. of Thiagarajar College of Engineering is depicted in the paper.

Keywords- Maximum power point tracking, photovoltaic array, wind energy conversion system, permanent magnet synchronous generator, fuel cell, boost dc-dc converter, dc-ac inverter

1. Introduction A Hybrid renewable energy system combines two or more energy sources, usually solar and wind power. The main advantage of hybrid system is the enhancement of reliability of the hybrid generation system used. Also, the battery size can be reduced as the Solar and Wind energy sources are complementary in nature .The surge for suitable alternative energy sources is growing more intense than ever in order to reduce the heavy dependence on fossil fuels[1]. Fuel cells are another rapidly developing generation technology. Fuel cells have high efficiency, low carbon emissions, high reliability due to the limited number of moving parts and longer life than batteries. In this proposed hybrid model three renewable energy sources are depicted. First one is permanent magnet synchronous wind generator and it is connected to diode rectifier, buck converter and common DC-link. On the other hand, second and third source is Photovoltaic (PV) array and Proton Exchange Membrane Fuel cell (PEMFC) respectively. These dc power sources are connected to the common DC link via DC/DC boost converter . The maximum point power tracking of the wind generator and PV array is performed by using MPPT algorithms. MPPT is an algorithm that is used for extracting maximum available power from PV module and wind energy system under varying operating conditions. In solar photovoltaic system, MPPT is achieved with the DC to DC converter which operates PV module at its maximum power point by using the simple Perturb and Observe (P&O) Algorithm . In case of Wind energy system, the tracking of windmill power around the maximum power operating point is achieved by the proposed P&O algorithm[2]. PIC microcontroller based MPPT design for the 80W UPV module

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available at EEE Dept. of Thiagarajar College of Engg is realized in hardware. Finally, sizing of the proposed hybrid system and payback calculation of solar-wind hybrid system is also discussed in this paper. 2. Hybrid System Modeling

The block diagram of the hybrid generation system is shown in Fig.1. As shown in the figure, the hybrid power system includes PV array, Wind energy conversion system employing permanent magnet synchronous generator, Fuel cell stack and power electronic converters to supply the load demand. The subsystems for solar and wind are described as follows:

Fig.1. Block Diagram of the hybrid generation system

2.1. Solar –Photovoltaic Energy System

2.1.1. PV Array

A solar cell is basically a p-n semiconductor junction, that generates current when exposed to the light. The equivalent circuit of a solar cell consists of a current source parallel with a diode as shown in Fig 2.

Fig.2: Equivalent circuit of solar cell Fig.3. Flow chart for the P&O method

The V-I characteristics of the PV cell can be determined by following equations. The diode current is given by:

1/exp KTIRVqII SOD (1)

and output current of the solar cell is given by

SHDL IIII (2)

SHSSOL RIRVKTIRVqIII /1/exp (3)

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where I : Solar cell current (A); IL :Light generated current (A) ; IO :Diode saturation current(A); q : Electron charge (1.6×10-19 C); K : Boltzman constant (1.38×10-23 J/K); T : Cell temperature in Kelvin(K); V : Solar cell output voltage (V); RS : Solar cell series resistance (Ω); RSH: Solar cell shunt resistance (Ω)

2.1.2. MPPT Algorithm for PV Array

Many algorithms have been found to improve the MPPT based on specific use. Perturb and observe is one of the most commonly used [4-5]. This algorithm measures the voltage and current inputs and arbitrarily increments or decrements the voltage. Then the new data is compared to the previous readings. If the power increases the voltage is moved in the same direction as the last adjustment. This continues until the new value shows less power than the previous one. The direction is then changed to try and reach the peak power output. The flow chart is given in Fig 3.

2.2. Wind Energy Conversion System 2.2.1. Wind Turbine Wind energy systems harness the kinetic energy of wind and convert it into electrical energy or used it to do intended work. Like the other renewable energy sources, wind energy is clean and safe. The aerodynamic power at the rotor of the turbine is given by the following equation [3]:

),(2

1 3 pCAvP (4)

where ρ (kg.m-3) is the air density, Rt (m) is the turbine radius, v (m.s-1) is the wind speed and Cp(λ,β) is the power coefficient which represents the aerodynamic efficiency of the turbine and also depends on speed ratio λ and the pitch angle β. The speed ratio λ, is given by

v

R (5)

where R is the rotor blade radius and is the rotor angular speed.

2.2.2. Wind Generator The permanent magnet synchronous generator is modeled in dq reference frame and the modeling equations can then be written as:

][/1/ qrqdddd ipLRiVLdtdi

][/1/ rdrqqqq pipLqRiVLdtdid

])([5.1 qdqdqe iiLLipT

where Ld , Lq :d and q axis inductances; R: Resistance of the stator windings ; id, iq: d and q axis currents;

Vd , Vq : d and q axis voltages ; ωr :Angular velocity of the rotor:

λ : Amplitude of the flux induced by the permanent magnets in the stator phases; P: number of pole pairs;

Te : Electromagnetic torque

2.2.3. MPPT algorithm for wind The wind generator consists of a wind turbine coupled with a permanent magnet synchronous generator (PMSG). The three phase diode rectifier is used for converting AC voltage to DC voltage and then fed to the buck converter in that the voltage input-output ratio is controlled by a PWM (Pulse width modulation) signal from the MPPT controller. The MPPT controller read the voltage and current values of the wind generator output to determine the PWM signal [6]. The schematic of the Wind Energy Conversion System (WECS) is shown in Fig.4 and the flowchart for P&O algorithm is illustrated in Fig.5.

(6)

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Fig.4. Schematic of WECS Fig.5. Flow Chart for P&O algorithm

The P&O algorithm operates by varying the duty cycle of the buck converter, thus varying the output voltage of the wind generator and observe the resulting power to increase or decrease the duty cycle in the next cycle. If the increase of duty cycle produces an increase of the power, then the direction of the perturbation signal (duty cycle) is the same as the previous cycle. In Contradiction, if the perturbation duty cycle produces a decrease of the power, then the direction of perturbation signal is reversed. 2.3. Fuel Cell System A fuel cell system mainly consists of a fuel cell stack and power conditioning unit. A simple representation of a fuel cell system is given in Fig. 6. The fuel cell uses hydrogen as input fuel and produces DC power at the output of the stack. The ideal open-circuit voltage is a potential difference between anode and cathode when there is no current flowing through the external circuit. The performance of PEMFC is based from the voltage produced by PEMFC as the fuel cell current increases. Theoretically, the ideal voltage is defined as Nernst potential, Enernst or the reversible voltage of cell. When the current flowing through the external circuit, there are three voltage losses involved in fuel cell output voltage that are activation voltage drop, ohmic voltage drop and concentration voltage drop.

Fig.6 Dynamic model of the Fuel cell System

The overall output voltage of the fuel cell stack can be obtained as

(7) The Nernst equation can be simplified to [7]:

(8) The different voltage drops are estimated as follows: 1) Activation Voltage Drop The Tafel equation is used to calculate activation voltage drop [2].

(9) The parametric coefficients for ξ1, ξ2, ξ3 and ξ4 are: 1=−0.948 ; 2= 286+20 +4.3 2 ×10−5; 3=7.6×10−5; 4=−1.93×10−4 ; where the concentration of Oxygen, 2 and Hydrogen, 2 are obtained by using Henry’s Law. 2) Ohmic voltage drop

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The equation to determine the ohmic voltage drop is [8]:

(10) where: i is the fuel cell current (A), RC is the constant resistance to transfer of protons through the membrane (Ω). RM is the equivalent resistance of the membrane (Ω) 3) Concentration Voltage drop This voltage drop can be described as:

(11) where: J is the current density of fuel cell and Jmax is the maximum current density of fuel cell. B is a constant equal to 0.016.

A PEMFC fuel cell subsystem is modeled in Matlab/Simulink using the equations [7-11] to calculate the voltage produced by a single fuel cell . The fuel cells are stacked and the output of the stack is boosted up using the boost converter to obtain the required sytem voltage.

3. Matlab/Simulink Implementation of the hybrid system

Mathematical model of Wind and Solar Energy Systems are used as Subsystems in the Hybrid generation Systems and interfaced with power electronics components and battery available in the Simpowersystems library of MATLAB/Simulink[5-9]. The overall hybrid system model is shown in Fig.7.

Fig. 7. The Overall Solar/Wind/Fuel cell Hybrid System

The Wind Subsystem consists of the wind turbine, 1 KW permanent magnet synchronous generator (PMSG), diode rectifier and buck converter as shown below in Fig.7.1. The wind turbine and the generator are mathematically modeled using the equations [4-6] .

Fig 7.1 Wind Subsystem model

The Solar Photovoltaic subsystem consists of 85 Watts PV Module, DC –Dc Boost Converter with MPPT control [9,10] as depicted in Fig 7.2

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Fig 7.2 Model of the Photovoltaic subsystem

The dynamic model of the fuel cell system consists of Fuel cell stack, boost converter and dc-ac inverter. The PEMFC fuel cell subsystem model and boost converter model in Matlab/Simulink is shown in Fig 7.3.

Fig.7.3 Model of the Fuel cell System in Matlab/Simulink

4. Simulation Results 4.1. Photovoltaic subsystem The PV module characteristics for insolation values of 200,400, 600,800 and 1000W/m2 is shown in Fig 8(a) &8(b).

Fig 8(a) I-Vcurves for different insolaion values Fig 8(b) P-V curves for different insolation values

Fig.8(c). Boost Converter output voltage

The PV module output voltage is boosted to 50V and fed to the DC-AC inverter. The output voltage waveforms of the boost converter is shown in Fig.8(c). 4.2 Wind Energy Subsystem The output power of 1 KW Permanent Magnet Synchronous Generator(PMSG) is shown in Fig.9 below.

Voltage (V)

Power(W)

C u r r e n t (A)

Voltage (V)

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Fig 9. PMSG output power

The three phase currents Iab ,Ibc and Ica and voltages Vab ,Vbc and Vca of the wind turbine generator is depicted in Fig 9.1 and 9.2 respectively.

Fig 9.1 Current waveforms of PMSG

Fig 9.2 Voltage waveforms of PMSG

4.3. PEMFC Fuel Cell Subsystem Fig 10.1 shows the output voltage characteristic of a 3 KW PEMFC stack at different operating temperatures. The output voltage of the fuel cell stack is boosted up to 50 V using DC-DC boost converter and then fed to the DC-AC inverter. The output voltage waveform of the inverter is shown in Fig.10.2.

Fig 10.1 PEMFC stack output voltage Fig.10.2 Inverter output voltage

5. Microcontroller based MPPT tracking system In this paper, microcontroller based MPPT design for the 80W UPV module available at EEE Dept. of Thiagarajar College of Engineering is realized in hardware. The block diagram used for hardware realization is given below:

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Fig 11. Block diagram of the solar MPPT system

The Solar array output voltage is always not constant since the temperature of the solar panel and its insolation varies with time. This voltage is given as the input to the DC-DC Converter (Buck / Boost converter). The gate signal of the switch in the converter is fed from the PIC 16F877A. By P&O coding embedded in the PIC Micro controller, the output of the DC-DC converter is maintained constant around 12-13V. Thus the battery is constantly charged with 12-13V.Then the battery is connected to the single phase inverter. The 12 volts DC is converted to 12 volts AC and then with the help of the transformer 230/12 V, it is stepped up to 230 V AC. Finally a small lamp load of 10 watts is connected across it. 5.1 Hardware implementation PIC based MPPT design circuit for 80W solar panel and the hardware model is shown in Fig12 & 12.1respectively

Fig12. Hardware prototype of PIC based MPPT of 80 watts solar-PV module

Fig 12.1 Hardware model Fig 12.2 Gate signal output

The open circuit voltage of the 80W solar panel is found to be 21.6V as provided by the manufacturer’s datasheet. Hence the input voltage is varied till 21V to indicate the changes in output. When the input voltage is greater than 13V then buck mechanism is employed and output voltage is maintained. Similarly when the input voltage is less than 13V then boost mechanism is employed and output is maintained constant at 13V. Hence this voltage is used to charge battery and ultimately supply a load through inverter circuits. But when input voltage is brought down below 5V then the output is just as the input and no buck or boost mechanism occurs as seen in the table 1. The Fig 12.2 shows the gate signal given to the converter circuit in which the duty cycle is varied automatically as the mechanism switches over from buck to boost and vice versa. The output results got from the hardware circuit is tabulated as follows:

S.No Input Voltage (V) Mechanism employed Output voltage (V) 1. 18 Buck mechanism 13 2. 9 Boost mechanism 13 3. 3 - 3

Table 1. Hardware result table

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6. Unit Sizing of the Solar-Wind Hybrid System For an electrical load of 20 CFL lamps, 8 fans and 18 computers making out 2.62 KW in PG simulation lab of EEE department in TCE, the number of PV modules each of 80W that will be required to meet the demand for 8 hours using solar generation alone is calculated as follows. Peak sunshine of 3 hours is taken into account to calcuate the sizing of the PV array.

(i) For solar PV generation alone:-

The daily electrical output from one module in units of W h per day at 12 V= 140 Wh Daily requirement of appliances = 2.62 K W h Minimum number of solar modules = Daily requirement of appliances Daily O/P of one module x Battery charging efficiency Therefore, minimum number of solar modules needed is = (2.62 K X 100) / (140X 80) = 24.38 ie.,25 modules Solar panel cost= No. of modules x cost of one module = 25 x 10000=Rs.250000 So, power generation using solar alone costs 2.5 lakhs + tower structure + installation= 4 lakhs approx. (ii) For solar-wind hybrid generation:- For 1KW generation of electricity, Wind System Cost=Rs. 1,95,000/- For 1KW generation of electricity, PV System Cost=Rs. 1,04,000/- Total cost of solar and wind hybrid system= Rs. 3,00,000/- approx The cost of the system varies from Rs 3 lacs to Rs 3.50 lacs per kW depending on the ratio of wind and solar components. The approximate cost of installation, including civil works, is about Rs 10,000 per kW. Repair and maintenance cost is about Rs 3000 per kW per annum. Payback Period calculation for solar- wind hybrid system:- Total Cost of Solar and wind hybrid system=Rs. 3,00,000/- Total Cost of utility supply= Approximate Annual bill + initial cost (substation, transformer and transmission line cost)= Rs.1,00,000 + 3,000= Rs. 1,03,000/- So , payback period for hybrid system will be, Payback Period= Total cost of solar and wind hybrid system Total cost of utility supply Payback Period= 3,00,000/1,03,000= 2.91 years = 3 years (approximately) So, Solar and Wind Hybrid System is more efficient for the electrification of the above cited location. (ii) For solar-wind-fuel cell hybrid generation:- For an electrical load of 2.62 KW in PG simulation lab of EEE department in TCE, solar-wind hybrid system is assumed to be already available in the campus making out 2 KW. So, the remaining 62 W is to met with fuel cell system generation. The cost of H-100W fuel cell stack (H-100 PEM Fuel Cell System includes control electronics, connectors, and electronic valves) is about 1342.81$ ie.,Rs.71840 /- and hydrogen cylinders for fuel supply to the stack is around 40-80 $. Therefore, the cost of fuel cell system can be approximated to one lakh rupees. Cost of solar and wind hybrid system= Rs. 2,00,000/- Total cost of solar-wind-fuel cell hybrid system= Rs.3,00,000 /- approximately. Cost per KW of fuel cell seems to be high, but would decrease with higher ratings and in due course of time. 7. Conclusion

A complete model simulating the proposed hybrid generation system including the wind, solar and fuel cell system is done using Matlab/Simulink. The MPPT control system has been developed for both the wind and solar energy sources. The simulation results showed satisfactory performance of the hybrid system. PIC microcontroller based MPPT tracking for solar module has been realized in hardware and unit sizing of the

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hybrid system is also presented. The future work will be to design the proposed hybrid system and implement in hardware. Also, the system has to be extended to higher ratings and solve for the synchronization issues.

8. Acknowledgement

The author would like to thank the Management and Principal of Thiagarajar College of Engineering for their consistent encouragement and providing with necessary facilities to carry out this research work and also like to thank the Department of EEE of Thiagarajar College of Engineering for the facilities rendered for this work. 9. References [1] M. R. Patel, Wind and Solar Power Systems, CRC Press LLC, 1999. [2] Xin Wang Yuvarajan, S. Lingling Fan., “MPPT control for a PMSG-based grid-tied wind generation system”, IEEE Transactions on

Energy Conversion, 2010 [3] Yuvarajan, S. Kaderbhai, M. ,“Hybrid Renewable Energy System with Wind Turbine and PV Panels”, Green Technologies

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Tracking”, IEEE Trans. Ind. Electronics, vol. 54, no. 3,June 2007. [5] Trishan Esram, and Patrick L. Chapman, “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques” IEEE

Trans. Energy Conversion, vol. 22, no. 2, June 2007 [6] Eftichios Koutroulis and Kostas Kalaitzakis , “Design of a Maximum Power Tracking System for Wind-Energy-Conversion

Applications”, IEEE Trans. Ind. Electronics, Vol. 53, NO. 2, April 2006 pg no: 486-494. [7] Wolf Vielstich,Arnold Lamm, Hubert A. Gasteiger “Handbook of Fuel Cells: Fundamentals,

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applications in autonomous underwater vehicles”, Journal of Power Sources, Volume 196, Issue 8, 15 April 2011, Pages 3810-3823. [9] Seul-Ki Kim Eung-Sang Kim Jong-Bo Ahn., “Modeling and Control of a Grid-connected Wind/PV Hybrid Generation System”, IEEE

Power Engg Society, 2006 [10] J. Manuel Carrasco, L.Garcia Franquelo, “Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A

Survey”, IEEE Trans on Indus Electr ,2006, pg: 1002 – 1016.

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