Overview of various methods for MPPT of a pv cell method
Transcription
Overview of various methods for MPPT of a pv cell method
National Conference on Recent Trends in Engineering & Technology Overview of various methods for MPPT of a pv cell and implementation of incremental conductance method J. B. Borad#1, A. R. Patel*2 #1 Electrical Dept. ,#1Gujrat Technological University B.V.M. College, V .V. Nagar India [email protected] *2 Asst. Prof. Electrical Dept., B.V.M. College, V. V. Nagar India [email protected] Abstract—Due to the limitations in the energy available from conventional sources, worldwide attentation is being focused on renewable sources of energy. Especially, the energy obtained from solar arrays and the fuel cells, becomes more and more important. Solar power plant became a peak research topic in India as well as over seas. Basic parts of a solar power plant are PV cell series, Dc-to-Dc converter and grid connection (to load). How to achieve high step-up and high efficiency DC/DC converters is the major consideration in the renewable power applications due to the low voltage of PV arrays and fuel cells. Paper is all about various method of MPPT of a PV cell. And implementation of incremental conductance method for close boost converter for solar installation. Circuit models for close loop systems are developed using the blocks of simulink in PSIM. The simulation results are compared with the theoretical results. This paper represents that by using incremental conductance method maximum power is obtained in any condition. parts. The power produced by solar panel depends on two factors which are irradiation and temperature. As irradiation and temperature level changes rapidly, the voltage produced fluctuates and becomes inconstant. Fig. 1 shows the block diagram of solar system for PV grid connected. Keywords— Boost converter, solar panel, photovoltaic cell, Modelling and control II. MPPT OF A SOLAR ENERGY I. INTRODUCTION Using renewable energy is no longer only recommended from the environmental point of view to tackle climate change and air pollution but also from economic point of view, because renewable energy is becoming huge business on global scale as the global clean energy race is on. There are also several other reasons why developing countries should focus on renewable energy as one of the best available energy options. For instance, developing countries could especially use renewable energy in distant, rural areas because in these areas producing renewable energy locally is economically more viable energy option compared to generating energy from fossil fuels (because of high transmission and distribution costs). Photovoltaic (PV) sources are used today in many applications such as satellite power systems, battery charging, home appliances and many more. PV is becoming more famous in the world of power generation because they have the advantages of free pollution, low maintenance, and no noise and wear due to the absence of moving 13-14 May 2011 Fig 1: Block diagram of solar system for PV grid connected [Ref 2] The output characteristics of photovoltaic arrays are nonlinear and change with the cell‘s temperature and solar irradiance. For a given conditions there is a unique point in which the array produces maximum output power. This point is called maximum power point (MPP) which varies depending of cell temperature and present irradiation level. To obtain the maximum power from a photovoltaic array, a maximum power point tracker (MPPT) is used. The Perturbation and observation is one of the most commonly used MPPT methods for its simplicity and ease of implementation. The P&O works well when the irradiance change slowly but it presents drawbacks such as slow response speed, oscillation around the MPP in steady state, and even tracking in wrong way under rapidly changing atmospheric conditions. There are various methods for obtaining maximum power tracking for pv cell discussed as below: 1) PERTURB AND OBSERVE METHOD The P&O method is based on an adaptive algorithm which automatically adjusts the reference voltage step size to achieve B.V.M. Engineering College, V.V.Nagar,Gujarat,India National Conference on Recent Trends in Engineering & Technology dynamic response and search MPP under rapidly changing conditions by exploiting networks capabilities. The perturb and observe (P&O) best operation conditions are investigated in order to identify the edge efficiency performances of this most popular maximum power point tracking (MPPT) technique for photovoltaic (PV) applications. P&O may guarantee top-level efficiency, provided that a proper predictive (by means of a parabolic interpolation of the last three operating points) and adaptive (based on the measure of the actual power) hill climbing strategy is adopted. The characteristic slop during a perturbation cycle provide the best information concerning how much the operating point is far from the MPP in steady state, but when a variation occur suddenly , this information will alter the behaviour of the algorithm and cause it divergence by moving the operating point far from the MPPT. This problem can be solved if the algorithm acquires a skill which allows the detection of the working conditions variations and its extents also. It is well known that any atmospheric condition variation induce a proportional PV array output power variation. connected in parallel with the individual solar cells, connecting the cells in parallel will increase the effective capacitance seen by the MPPT. From this, the difference in MPPT efficiency between the parasitic capacitance and incremental conductance algorithms should be at a maximum in a high-power solar array with many parallel modules. 3 ) VOLTAGE BASED PEAK POWER TRACKING METHOD The basis for the constant voltage (CV) algorithm is the observation from I–V curves that the ratio of the array‘s maximum power voltage as shown in fig. 2, VMPP, to its open-circuit voltage, VOC, is approximately constant. Fig 2: Constant voltage algorithm 2) PARASITIC CAPACITANCE METHOD The parasitic capacitance algorithm is similar to incremental conductance, except that the effect of the solar cells‘ parasitic junction capacitance CP, which models charge storage in the p–n junctions of the solar cells, is included. By adding this capacitance to the lighted diode equation, Equation, and representing the capacitance using dV i (t ) C dt Equation is obtained. dV p I I L I O exp Vp RsI / a 1 C P dt I F (V p ) C P C P dV p dt On the far right of Equation, the equation is rewritten to show the two components of I, a function of voltage F(V P) and the current in the parasitic capacitance. Using this notation, the incremental conductance of the array gp can be defined as dF(Vp)/dVp and the instantaneous conductance of the array, gL can be defined as –F(Vp) =Vp. The MPP is located at the point where dP/dVp= 0. Multiplying Equation by the array voltage Vp to obtain array power and differentiating the result, the equation for the array power at the MPP is obtained. dF (V p ) dV d 2V F VP C 0 P dV V V V P P The three terms in Equation represent the instantaneous conductance, the incremental conductance, and the induced ripple from the parasitic capacitance. The first and second derivatives of the array voltage take into account the AC ripple components generated by the converter. The reader will note that if CP is equal to zero, this equation simplifies to that used for the incremental conductance algorithm. Since the parasitic capacitance is modelled as a capacitor 13-14 May 2011 In other words: VMPP 1 VOC The constant voltage algorithm can be implemented using the flowchart shown in Figure The solar array is temporarily isolated from the MPPT, and a VOC measurement is taken. Next, the MPPT calculates the correct operating point using Equation and the preset value of K, and adjusts the array‘s voltage until the calculated VMPP is reached. This operation is repeated periodically to track the position of the MPP. Constant voltage control can be easily implemented with analog hardware. However, its MPPT tracking efficiency is low relative to those of other algorithms. Reasons for this include the aforementioned error in the value of K, and the fact that measuring the open-circuit voltage requires a momentary interruption of PV power. It is possible to dynamically adjust the value of K, but that requires a search algorithm and essentially ends up being the same as P&O. 4) INCREMENTAL CONDUCTANCE METHOD The incremental conductance method consists in using the slope of the derivative of the current with respect to the voltage in order to reach the maximum power point. To obtain this point, dV/ dI must be equal to –I/V as shown in Figure [5] & [7]. For any solar panel, the output power is function of the temperature and the sunshine values of the site where the panel is placed. This power can decrease or increase as result of any temperature and/or shining variations. In Solar panels, output power is not constant. To maximize this power and maintain it constant at high values, it is necessary to define the Maximum Power Point Tracking (MPPT) methods, and apply these methods to controlled dc-dc converters (choppers). Now, let see the incremental conductance & solar panel modulation Modelling of a solar panel In the obscurity, a semiconductor presents a high resistance. When it is strongly illuminated, its resistance decreases. If the energy of the photons that constitutes the luminous ray is sufficient, these photons will be able to excite the electrons blocked in the valence layer to jump to the conduction layer. It is the phenomenon of photo conductibility. The expression of the diode current is described by the following equation: B.V.M. Engineering College, V.V.Nagar,Gujarat,India National Conference on Recent Trends in Engineering & Technology eV I P I CC I d I CC I S Exp P 1 kT with: Ip and Vp are the current and voltage of this photovoltaic cell, Is is the saturation current, ICC and Id are the short-circuit and the direct currents, k is the Boltzmann constant (equal to 8,62.10- 5eV/°K), T is the absolute temperature, e is the electron charge This equation corresponds to a current generator, which models the sunshine, and a diode in parallel, which represents the PN junction. The equivalent circuit of the ideal photovoltaic is given in figure. Figure 4 Flow Chart of Incremental conductance method [6] Figure 3: Ideal photovoltaic [5] To draw the real model of photovoltaic cell shown in fig. 3, it is necessary to take in account the losses due to the manufacture. Therefore, two resistances should be added to the ideal model, one placed in series and the other in parallel. In fact, the resistance Rs represents the losses dues to the contacts and the connections. The parallel resistance Rsh represents the leakage currents in the diode. The characteristic equation becomes then: V I P I CC I d Rsh In fact, applying a variation on the voltage toward the biggest or the smallest value, its influence appears on the power value. If the power increases, one continues varying the voltage in the same direction, if not, one continues in the inverse direction. The simplified flow chart of this method is discussed in chapter 1. In addition, by using the power formula P=V.I, its derivative by dP = V dI + I.dV The duty cycle (αn) of the used chopper (dc-dc converter) is calculated by the following expression, αn = αn−1 ± ∆α Where ∆α is the duty cycle step 13-14 May 2011 The parasitic capacitance algorithm is similar to incremental conductance shown in fig. 4, except that the effect of the solar cells‘ parasitic junction capacitance CP, which models charge storage in the p–n junctions of the solar cells, is included. By adding this capacitance to the lighted diode equation, Equation, and representing the capacitance using dV i (t ) C dt On the far right of Equation, the equation is rewritten to show the two components of I, a function of voltage F(V P) and the current in the parasitic capacitance. Using this notation, the incremental conductance of the array gp can be defined as dF(Vp)/dVp and the instantaneous conductance of the array, gL can be defined as –F(Vp) =Vp. The MPP is located at the point where dP/dVp= 0. Multiplying Equation by the array voltage Vp to obtain array power and differentiating the result, the equation for the array power at the MPP is obtained. dF (VP ) dV d 2V F (VP ) C 0 P V VP V dVP The three terms in Equation represent the instantaneous conductance, the incremental conductance, and the induced ripple from the parasitic capacitance. The first and second derivatives of the array voltage take into account the AC ripple components generated by the converter. The reader will note that if CP is equal to zero, this equation simplifies to that used for the incremental conductance algorithm. Since the parasitic capacitance is modelled as a capacitor connected in parallel with the individual solar cells, connecting the cells in parallel will increase the effective capacitance seen by the MPPT. From this, the difference in MPPT efficiency between the parasitic capacitance and incremental conductance algorithms should be at a maximum in a high-power solar array with many parallel modules. B.V.M. Engineering College, V.V.Nagar,Gujarat,India National Conference on Recent Trends in Engineering & Technology III. BOOST CONVERTER ANALYSIS III. CHARACTERISTIC OF SOLAR ARRAY Solar cell consists of semiconductor materials which is able to convert solar irradiation into DC current using PV effect. The characteristic equation for solar panel is given by q IRs 1 V I I lg Ios exp Rsh AkT V IRs where Ilg is the light generated current, IOS is the reverse saturation current, q is the electronic charge, A is dimensionless factor, k is Boltzmann constant, T is the temperature in K, Rs is series resistance of the cell, and Rsh is the shunt resistance of the cell. The variation of the output I-V characteristic of a solar panel is shown in Figure 2. The maximum power point is located at the knee of the I-V output characteristic. Figure 3 shows a typical output characteristic of a solar panel under different temperature and irradiation level. A simple boost converter consists of an inductor, a switch, a diode, and a capacitor as shown in Figure. Boost converter circuit can be divided into two modes. Mode 1 begins when the switch SW is turned on at t = Ton as shown in Figure 5. The input current which rises flows through inductor L and Switch SW. During this mode, energy is stored in the inductor. Mode 2 begins when the switch is turned off at t= Toff. The current that was flowing through the switch would now flow through inductor L, diode D, capacitor C, and load Energy stored in the inductor is then transferred to the load. Therefore, the output voltage is greater than the input voltage and is expressed as 1 Vout [ ]Vin 1 k Where Vout is the output voltage, k is duty cycle, and Vin is input voltage which in this case will be the solar panel voltage. Fig 5: Typical solar panel I-V and P-V Characteristic Fig 7: Circuit diagram of Boost-Converter during Ton Fig 6: Typical output characteristic of solar panel based on insulation and temperature changes Fig 8: Circuit diagram of boost converter during Toff Fig 9: Circuit diagram of open loop boost converter 13-14 May 2011 B.V.M. Engineering College, V.V.Nagar,Gujarat,India National Conference on Recent Trends in Engineering & Technology [1 k ]^ 2 k R L min 2 f In order to operate the converter in continuous conduction mode (CCM), the inductance is calculated such that the inductor current IL flows continuously and never falls to zero. Fig 11: Waveform for discontinuous conduction mode Where Lmin is the minimum inductance, k is duty cycle, R is output resistance, and f is the switching frequency of switch SW. The output capacitance to give the desired output voltage ripple is given by k ------------------- (4) C min f R Vr Where Cmin is the minimum capacitance, k is duty cycle, R is output resistance, f is switching frequency of switch SW, and Vr is output voltage ripple factor. Vr can be expressed as Vr Vout Vout ----------------- (5) IV. Control Approach Fig 10: Waveform during continuous conduction mode 13-14 May 2011 In this paper, the incremental conductance method is treated [5]. This method consists in using the slope of the derivative of the current with respect to the voltage in order to reach the maximum power point. To obtain this point, dI/ dV must be equal to –I/V. In fact, applying a variation on the voltage toward the biggest or the smallest value, its influence appears on the power value. If the power increases, one continues varying the voltage in the same direction, if not, one continues in the inverse direction. The simplified flow chart of this method is given in figure 4. B.V.M. Engineering College, V.V.Nagar,Gujarat,India National Conference on Recent Trends in Engineering & Technology Fig 13: Circuit diagram of close loop boost converter V. SIMULATION RESULT The studied system is shown in figure 10. With the ‗Incremental conductance‘ method, which is already explained in the previous section, the curve of the output power versus time is illustrated in figure. This figure shows that the power value remains approximately constant, with small ripples. Also, the time response is not negligible. any disturbance at the input of solar panel. This converter has advantages like reduced hardware and good output voltage regulation as compare to other step-up converter. VII. ACKNOWLEDGEMENTS I take this opportunity to express my sincere thanks and heartful gratitude to my project supervisor Asst. Prof. A. R. Patel, my friend Asst. Prof. N. H. Adroja. It was their repeated encouragement, supervision, and invaluable guidance that helped me in completing this project. I am deeply indebted to him for giving clarity of vision and thought which enabled me to complete the paper. I am deeply grateful to my husband, Jatin, for his patience, understanding, and for being a constant source of motivation throughout. VIII. REFERENCES [1] A & V. Olgyay, ‗‗Solar control and shading devices‘‘ Book, Princeton (1976). [2] V. Salas, E. Olias, A. Barrado & A. Lazaro, ‗‗Review of the Fig 14: Power waveform by using incremental conductance MPPT method [3] VI. CONCLUSION [4] It became apparent that all tests conducted have shown that the Boost converter produced more power than the standard system without the technology. Boost converter with incremental conductance method able to transform unusable power into usable power, which itself is a significant capability improvement to the current technology as compare to previous technology. PSIM models for open loop system of boost converter and close loop boost converter are developed using the blocks of simulink and the same are used for simulation studies. The open loop system is not able to maintain the constant voltage. But using closed loop, boost converter provides constant voltage without affecting by 13-14 May 2011 [5] [6] maximum power point tracking algorithms for stand-alone photovoltaic systems‘‘, Elsevier, 2005. Seoul, A stand alone photovoltaic (AC) scheme for village electricity A.M.Sharaf', SM IEEE, A.R.N.M. Reaz UI Haque, Jan. 2005, ISSN: 0160-8371, ISBN: 0-7803-8707-4 D. P. Hohm and M. E. Ropp, 2003. ―Comparative study of maximum power point tracking algorithms,‖ in Prog. Photovolt: Res. Appl. 2003, pp. 47-62. 10). J. Kouta, A. El-Ali, N. Moubayed and R. Outbib, ―Improving the incremental Conductance control method of a solar energy conversion system‖, Department of Electrical Engineering Lebanese University, IEEE-2005. European Journal of Scientific Research ―Development of microcontroller based boost converter for photovoltaic system‖ ISSN 1450-216X Vol.41 NO. 1 (2010) [7] http://www.renewables-info.com [8] http://www.soton.ac.uk/~solar/intro/tech0.htm [9] http://en.wikipedia.org/wiki/File:Fuel_Cell_Block_Diag ram.svg B.V.M. Engineering College, V.V.Nagar,Gujarat,India