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- 1. Fill Factor Based Maximum Power Point Tracking (MPPT) for Standalone Solar PV System 4th International Conference on Advances in Energy Research (ICAER 2013) at Indian Institute of Technology Bombay (IITB) by D.K.Sharma, G.Purohit Sir Padampat Singhania University, Udaipur
- 2. Introduction • • • • • • • A fill factor based new approach for maximization of efficiency of solar PV system is investigated using modified MPPT technique. On the I-V characteristic curve of solar PV module, fill factor is one of the most important parameter for determining and tracking the maximum power points (MPPs) throughout the day. In the proposed technique, the area under the I-V curve is to be optimized .for a fill factor value of 1. The fill factor having a value 1 represents the maximum possible efficiency of a solar PV module yields . The present technique may be considered as the modified version of the constant voltage method for MPPT. Apart from the mathematical description of the proposed technique, the simulated curves of I-V and P-V characteristics (with MPP) of solar PV module are presented. The proposed MPPT technique has many features over the existing MPPTs like reduced sensor requirement, direct determination of MPP, reduced tracking time, maximized efficiency and cost effectiveness.
- 3. Introduction (Contd.) • In recent times, the overwhelming consumption of fossil fuels for various energy needs has reflected a big danger to our environment. • Electricity generation from fossil fuels (like coal, gas and other petroleum products) in various thermal power plants across the world is the most hazardous sector for our environmentally safe society. • Different greenhouse gases (GHGs) like CO2, CO, SO2, NOx and other pollutants are the uncontrolled byproducts from these thermal power plants. These GHGs pollute the environment at a very severe level and causes many dangerous and fatal diseases in humans, animals and plants in our society. The massive production of these poisonous gases not only leads to many health problems but it also contributes to the biggest climate change challenge called global warming. • Global warming is the biggest challenge and a frontier to work upon for a sustainable and ecologically balanced society. Due to global warming, the various environment related problems like flood and drought may arise in different part of the world. • Apart from all these ecological problems, one more factual point is important that the fossil fuels will not be available for a long time.
- 4. • Fossil fuels are depleting day by day due to their uncontrolled and extensive consumption in power plants, automobile and other energy applications. • Due to all these facts, the solar energy has drawn attention of energy professionals for electrical power generation for many years. The efficiency of solar power generating system using photovoltaic (PV) technology is the most important parameter in utilizing the solar PV systems. • The efficiency represents the maximum power output from solar PV power system. The maximum power output can be achieved by an electronic tracking of maximum power points (MPPs) on the characteristic curve of solar PV module. • The MPPs can be tracked by a power electronic circuitry called maximum power point tracker (MPPT). • In a MPPT, a DC-DC converter is used to adjust the output voltage from PV module to the load in such a way that it indicates the maximum possible power on the characteristic curve. • The operation of DC-DC converter is carried out by its electronic switch (usually MOSFET). The switching is controlled by means of a control algorithm. • Many MPPTs have been discussed and developed till date. Here Perturb and Observe (P&O) and Incremental Conductance method are discussed and a new fill factor based MPPT technique has been proposed for maximized efficiency of solar PV system.
- 5. STANDALONE SOLAR PV SYSTEM • A complete standalone solar PV system comprises of the following components, solar PV module(s), maximum power point tracker (MPPT), electrical load, charge controller, battery, inverter (for ac loads). Figure 1 represents a typical solar PV power system. Fig 1 Typical Standalone Solar Photovoltaic (SPV) System
- 6. PV MODULE CHARACTERIZATION Electrical circuit and Characteristic equations of solar cell The characteristics of PV module are the basic requirement for tracking the maximum power points (MPPs) using any MPPT technique. For characterizing the solar PV module, it is required to model the characteristic equation from an electrical equivalent of solar cell (module) as in following figure 2 Fig 2 Electrical Equivalent model of solar PV cell
- 7. The current produced by the solar cell is given by: I = IL − ID − ISH where, I = output current (amperes) IL = photo generated current (amperes) ID = diode current (amperes) ISH = shunt current (amperes). The current through these elements is governed by the voltage across them: Vj = V + IRS where, Vj = voltage across both diode and resistor RSH (volts) V = voltage across the output terminals (volts) I = output current (amperes) RS = series resistance (Ω).
- 8. I-V and P-V characteristic curves of solar PV module I-V and P-V characteristics of solar PV module are the basic requirement for tracking of MPP by using any of the MPPT technique or algorithm. Therefore the following characteristic curves are simulated in the MATLAB environment. The maximum power point (MPP) is also indicated on both of the curves. Fig. 3 I-V and P-V Characteristic Curves
- 9. SOME EXISTING MPPT TECHNIQUES Several MPPT techniques (algorithms) have been developed in past years varying in terms of implementation complexity and tracking speed and stability of MPP. No MPPT provides an optimized efficiency of solar PV system at any instant of time. In the following section, some existing MPPT techniques have been briefly discussed.
- 10. • Perturb and Observe (P&O) based MPPT This is the mostly used and a primary algorithm which is quite simple to implement. In this method, the voltage is perturbed (changed) and output power is measured for various perturbation stages. Subsequently output is compared with the previous values and the voltage is perturbed accordingly to ensure the point of maximum power. Perturb and Observe method has been used by most of the researchers to develop its new variants like voltage based P&O and Duty cycle based P&O. Perturb and observe based MPPT does not depend on PV array , tracking efficiency is good but with unstable operating points, implementation is simple, sensing parameters are voltage and current. The P&O technique can be represented by the following figure.
- 11. Fig. 4.1 Perturb and Observe (P&O) Technique
- 12. • Incremental Conductance based MPPT The incremental conductance method is based upon the fact that the power would be maximum with the condition that its differential with respect to voltage equals to zero. On the P-V characteristic curve, the differential of power with respect to voltage is zero, positive or negative on the peak of the curve (i.e. at MPP), on the left to MPP and on the right to MPP respectively. The following description of this method gives the mechanism of operation. The maximum power point yields the following situation At MPP, dP/dV = 0 Solving the above equation, the following situation persist for MPP and other two point, dP/dV = -I/V at MPP, dP/dV > -I/V on LHS, dP/dV < -I/V on RHS The operation of Incremental conductance method can be represented as in the following figure
- 13. Fig. 4.2 Incremental conductance method
- 14. • Constant Voltage Ratio based MPPT Constant Voltage Ratio based method is one of the cheapest method for MPPT implementation. It does not require any sensor to sense the voltage or current. It works on the principle that for a given characteristic curve, the ratio of maximum power voltage to the open circuit voltage is constant and that will be less than 1. The greater value of this constant indicates the greater efficiency of the PV system at a given instant of time. After calculating the values of Vmp and Voc, the operating point is forced to be at the calculated value of this constant. The precise value of the constant is the primary requirement of this MPPT technique. The biggest advantage of this technique is that it does not require any voltage or current sensor.
- 15. Fig. 4.3 Structure of Constant Voltage method
- 16. Proposed Fill Factor based MPPT • The is also concerned with the constant voltage method. The proposed technique is also a data based technique so the available or calculated data for the various parameters like VOC, ISC,, VMP and IMP are required. • In the case of constant voltage method, the ratio of two voltages (VMP and VOC ) is calculated which is constant. In the case of fill factor based technique, the ratio of two powers (P MP and PTH ) is calculated and the operating point is forced to operate at MPP. • Mathematically we can state that the maximum possible area (product of voltage and current) formed by the intersection of horizontal line (voltage) extended to the I-V curve and vertical line (current) extended to the I-V curve.
- 17. • The maximum power point is always present at the point of intersection of these extended lines from voltage axis (voltage at maximum power-Vmp) and current axis (current at maximum power-Imp). • This MPPT represents the maximum output power (Po (max) = Vmp x Imp) extracted from the solar PV system to the electrical load. The rectangle formed by these two line represents the fill factor of the solar PV cell. • The optimization of this parameter i.e. fill factor in turn ensures the higher efficiency of the overall solar PV system. • The proposed methodology based upon fill factor maximization may be considered as the modified version of the constant voltage method based MPPT technique. The fill factor based mechanism has been illustrated in the following figure.
- 18. Fig. 5 Fill factor representation of proposed MPPT technique
- 19. • Mathematical Description of Fill Factor The fill factor may be defined as ratio of theoretical power to the maximum power which can be mathematically represented as Fill Factor (FF) = VMP.IMP/ VOC.ISC = P where, VOC= open circuit voltage ISC= short circuit current VMP= Voltage at the point of maximum power IMP= Current at the point of maximum power The optimized Value of fill factor leads to the more squareness of the I-V characteristic curve of solar PV module. Consequently it causes to the maximized efficiency of solar PV power generating system.
- 20. • The maximum theoretical FF from a solar cell can be determined by differentiating the power from a solar cell with respect to voltage and finding where this is equal to zero. which gives, • The above equation only relates Voc to Vmp, and other equations are required to find Imp and FF. Therefore the expression for Fill Factor (FF) can be determined empirically as: • This whole expression is the function of open circuit voltage (V OC). It means that the optimized value of this function will yield to ensure the extraction of maximum power from solar PV system to the load at any instant of time throughout the day. The above mathematical description of fill factor reflects that at the point of maximum power (at dP/dv = 0), the fill factor will be having a maximized efficiency of solar PV system.
- 21. Proposed Maximum Power Angle Determination Based Maximum Power Point Tracking (MPPT) for Maximizing the Efficiency of Solar PV System •In the proposed MPPT method, the maximum power point (MPP) is tracked using the geometrical and mathematical analysis of the I-V characteristic curve of the solar PV module. •In this method, the angle made from the intersection point of open circuit voltage (VOC) and short circuit current (ISC) with respect to the voltage (V) axis to the point of maximum power (MPP) is determined using the known parameters of the solar PV module. The MPP angle determination for the following characteristic curves is analyzed as follows. •The point of theoretical power (PTH) is the intersection point of VOC and ISC. Therefore a hypotenuse is drawn from the point P TH to the point of origin. Then the angle made by this hypotenuse and the voltage (V) axis is 45 0 (as evident from the figure). It also reflects that the angle made by this hypotenuse and current (I) axis is also 450. Now it is needed to determine the angle made by the line (joining PTH and ISC) and the line (joining PTH and MPP).
- 22. • The following calculation steps are used for determining the MPP angle for the given characteristic curves. The corresponding value of voltage at MPP (VMP) = 17 volts The corresponding value of current at MPP (IMP) = 3.5 A The corresponding value of voltage at VOC = 21 volts The corresponding value of current at ISC = 3.8 A It is also clear (from the figure) that AB = 3.8-3.5= 0.30 A and BC = 3.5-3.05= 0.45 A therefore AB<BC where AB = distance between MPP and ISC – PTH line and BC = distance between MPP and hypotenuse (line joining P TH to origin) Therefore the maximum power point angle will be 00 < ΘMPP (MPP angle) < 22.50
- 23. • In right-angled triangle ∆ ABPTH, BPTH2 = AB2 + APTH2 or BPTH = √AB2 + APTH2 where, AB = 0.30 APTH= 21-17 = 4 Therefore BPTH = √0.302 + 42 = √16.09 = 4.011234 Now the ΘMPP (MPP angle) can be easily determined as the following formula Cos ΘMPP = APTH / BPTH or ΘMPP = Cos-1(APTH / BPTH) = Cos-1(4 /4.011234) = Cos-1(0.9972) = 4.28860 ΘMPP = 4.28860
- 24. • This is the approximate value of MPP angle for the given characteristic curve. • The generalized formula for determining the MPP angle has been derived as ΘMPP = Cos-1(APTH / BPTH) • In this method, the operating point is forced to reach to MPP using proposed MPP angle determination methodology. • The following illustrations show the mechanism for the proposed MPPT control method based on MPP angle determination. • For tracking the MPP using this formula, the information about the characteristic parameters are to be known.
- 25. Mechanism of maximum power point (MPP) angle technique for MPPT
- 26. CONCLUSION The effect of fill factor on the design of an improved control algorithm for a MPPT of standalone solar PV power generating system is investigated. The empirically derived expression of fill factor is proved to be a potent basis for a modified MPPT which ensures the maximum possible efficiency of the solar PV system. It has been also observed that the proposed MPPT technique is having an empirical relationship with constant voltage method for MPPT. The fill factor based technique is a form of data based method. Due to its data based approach, the external circuitry like voltage and current sensors are not needed for MPPT implementation. Apart from fill factor based MPPT a new MPP angle based approach has been also discussed. This proposed technique eliminates the requirement of sensors therefore the cost of the system decreases. The proposed methodology is proved to be an economically sustained alternative to the existing MPPT techniques.

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