Chapter 4. Impact of Dust on Solar PV Module: Experimental Analysis

Transcription

1 Chapter 4 Impact of Dust on Solar PV Module: Experimental Analysis 53

2 CHAPTER 4 IMPACT OF DUST ON SOLAR PV MODULE: EXPERIMENTAL ANALYSIS 4.1 INTRODUCTION: On a bright, sunny day the sun shines approximately 1,000 watts of energy per square meter of our planet's surface. The devices that used to convert sunlight directly into electricity are known as photovoltaic (PV) cells. PV cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This absorbed energy allows some electrons to flow freely. PV cells also have one or more electric fields which act to force those free electrons to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power that the solar cell can produce. In this chapter, the main physical principles of the photovoltaic cells is discussed. Also about the p-n junction, the I-V characteristics and the efficiency of the solar cell are discussed. Photovoltaic is an important energy technology because it makes use of the abundant and free energy in the sun, also it has little impact on our environment. This technology can be used in a wide range of applications. But the single PV cell is unbeneficial; it must be added to some other components within a system known as the Photovoltaic System, before it can be used in any application Finally, the limitations which get in the way of the solar energy applications will be considered. Dust deposition rate and its effect on cell performance have also to be estimated. Also, dust has a significant effect on cell current but no variation on cell voltage. Studies related to dust accumulation is critical as a further decrease in system efficiency due to the phenomenon of environmental and weather condition which may be varied from time to time, place to place and season to season However, consequent upon dust settlement on the modules will be carried out into account. Degradation in power and efficiency of solar panel by day to day dust deposition (gm/m2). The absorption and reflection of solar radiation by the dust deposition on solar panel surface is to be quantified 54

3 4.2 PHOTOVOLTAIC EFFECT The photovoltaic (or PV) effect is the basic physical process through which sunlight converts into electricity. The word photovoltaic is a joining of two words Photo, meaning light, and voltaic, meaning electricity. And this is the phenomenon by which certain materials, properly processed and fabricated into suitable devices; generate a voltage when they are exposed to light. The photovoltaic cell or self generating photocell is one that generates an output voltage in proportion to the intensity of incident light. Solar photovoltaic is a semiconductor device. Power generated by illuminated PV junction in forward bias condition. The I-V characteristic of solar as shown in figure below. This is basically a diode characteristic. The voltage current characteristic curve for the p n junction diode is described by the following Schottky diode equation (4.1) Power is consumed in a device if its operation is either in 1st or 3rd quadrant, So P = V * I = + V e Power is delivered if operation is in either 2nd or 4th quadrant, So P = V * I = - V e (4.3) 55

4 What happens in the vicinity of a p n junction when it is exposed to sunlight. As photons are absorbed, hole-electron pairs may be formed. If these mobile charge carriers reach the vicinity of the junction, the electric field in the depletion region will push the holes into the p- side and push the electrons into the n-side. The p-side accumulates holes and the n-side accumulates electrons, which creates a voltage that can be used to deliver current. A simple equivalent circuit model for a photovoltaic cell consists of a real diode in parallel with an ideal current source as shown in F 4.1(B). The ideal current source delivers current in proportion to the solar flux to which it is exposed. Under illumination solar cell can be operated in the fourth quadrant so corresponding to delivering power to the external circuit. Current in the illuminated solar cell is negative, as flows against the conventional direction of a forward diode as shown in Fig. 4.1(A) Solar PV current expression we can get from continuity equation as (4.4) 4.3 DIFFERENT PARAMETERS OF SOLAR CELL Solar cell can take a place of a battery in a simple electric circuit. In the dark, the cell in the circuit does nothing. When it is switched on by light it develops a voltage, or e.m.f., analogous to the e.m.f. of the battery in circuit. The voltage developed when the terminals are isolated with infinite resistance is called open circuit voltage (Voc). The currents drawn when the terminal are connected together is known as short circuit current (Isc). Solar cells are characterized and compared with each other with four parameters: short circuit current, open circuit voltage, fill factor and efficiency. In real cell power is dissipated through the resistance of the contacts and through leakage currents around the sides of the devices. These effect are equivalent electrically to parasitic resistance in series (R s ) and parallel (R sh ) with the cells SHORT CIRCUIT CURRENT (I sc ) The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). The short-circuit current is due to the generation and collection of light-generated carriers. The short-circuit current is the largest current which may be drawn from the solar cell. In Eqn. 4.5 at V=0 if current represented by Isc Then 56

5 (4.5) Also ISc can be represented by photo current (4.6) Where, A = The area of the solar cell. G = The number of charge carrier generated, W= depletion region width, Lp &Ln= minority carrier length. Solar Irradiance: ISc is proportional to solar irradiance(w/m2) a power law having exponent α (4.7) OPEN CIRCUIT VOLTAGE (VOC) The open-circuit voltage, Voc, is the maximum voltage available from a solar cell, and this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell junction due to illumination. By setting Itotal = 0 (4.8) The above equation shows that Voc depends on the saturation current of the solar cell and the light-generated current. While ISc typically has a small variation, the key effect is the saturation current, since this may vary by orders of magnitude. The saturation current, I0 depends on recombination in the solar cell. Open-circuit voltage is then a measure of the amount of recombination in the device. The VOC can also be determined from the carrier concentration (4.9) The relationship between open circuit voltage and solar irradiance is to follow a logarithmic function based of an ideal diode equation. Relationship between temperature and 57

6 open circuit voltage, It goes down with increases with temperature. Can be explained by below formula. (410) β and γ can be explained by following formula. (4.11) SHUNT RESISTANCE (RSH) Significant power losses caused by the presence of a shunt resistance, RSh, are typically due to manufacturing defects, rather than poor solar cell design. Low shunt resistance causes power losses in solar cells by providing an alternate current path for the light-generated current. Such a diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell. The effect of a shunt resistance is particularly severe at low light levels, since there will be less light-generated current. The loss of this current to the shunt therefore has a larger impact. In addition, at lower voltages where the effective resistance of the solar cell is high, the impact of a resistance in parallel is large. (4.12) SERIES RESISTANCE(R s ) An even better equivalent circuit will include series resistance as well as parallel resistance. Series resistance in a solar cell has three causes: firstly, the movement of current through the emitter and base of the solar cell; secondly, the contact resistance between the metal contact and the silicon; and finally the resistance of the top and rear metal contacts. The main 58

7 impact of series resistance is to reduce the fill factor, although excessively high values may also reduce the short-circuit current. Finally, let us generalize the PV equivalent circuit by including both series and parallel resistances as shown in F 4.2. We can write the following equation for current and voltage: (4.13) FILL FACTOR (FF) The FF is defined as the ratio of the maximum power from the actual solar cell to the maximum power from an ideal solar cell. (4.14) EFFICIENCY (η) Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. (4.15) The efficiency is the most commonly used parameter to compare the performance of one solar cell to another. Efficiency of a cell also depends on the solar spectrum, intensity of sunlight and the temperature of the solar cell. 4.4PHOTO-VOLTAIC CELL, MODULE AND SYSTEM GENERATION Usually, solar cell is a thin slice of semiconductor material which converts light energy into electrical energy. The surface is treated to reflect as little visible light as possible and appears dark blue or black. When it is charged by the sun, this basic unit generates a dc photovoltage of 0.5 to 1.0 volt and, in short circuit possesses maximum current and reasonable voltage. To produce useful dc voltages, the cells are connected together in series and encapsulated into modules. A module typically contains 28 to36 cells in series to generate dc voltage of 12 volt in standard test condition. Modules within arrays are similarly protected. 59

8 4.4.1 SOLAR CELL Current sizes of Si solar cells are of 12.5 x 12.5 cm 2 and 15 x 15 cm 2. Shape of the cell can be like Pseudo square mono-crystalline cells (or circular), truly square multi-crystalline cells. Current generating capacity of Si cells is about 30 ma/cm 2 at 1000 W/m 2.A cell of 15x15 cm 2 will generate about 6.75 A current SOLAR MODULE PV module array of several solar cells connected in series and parallel for getting larger power output. Various procedures are there in industry to interconnecting cells. For Thin film technology cells are connected while process of manufacturing of solar cell. On the other hand Wafer based technology Solar cells are manufactured first and then interconnected. Usually cell in a module exhibits identical characteristics. Shape of the I-V curve of the module is same as that of cells with change in scale of axis. Wattage of the modules depends on the current generating capacity & voltage capacity of the module. The current from a module is linearly proportional to its size (but voltage is independent of size) ARRAY A photovoltaic array is a linked collection of solar module. The power that one module can produce is around 70 W to 300 W [19] most of the time that is insufficient for domestic or commercial requirement. So, the modules are linked together to form an array. Most PV arrays are interfaced with the load via power electronic devices. Those devices work in higher voltage level for reducing loss or operate in higher current level (like power MOSFET cannot withstand very high voltage, but handle high current) to meet their power demand. The modules in a PV array are usually first connected in series to obtain the desired voltage; the individual strings are then connected in parallel to allow the system to produce more current. 60

9 4.5 PROBLEM EXISTS Whatever may be the system, the solar charging arrangement has to be kept outside and module surface is covered with dust particle. Due to high temperature effect module output degrades. Due to dust accumulation, the sunlight could not enter into the module, so power generation detreoriate. All these issues require to be addressed before Solar Power come to the commercial market. Power reduction by high temperature or dust accumulation making a complicating phenomena. One has to calculate how much power is reduced due to temperature and due to dust accumulation. 4.6 METHODOLOGY The module is tested in city areas and measured the density of dust accumulation along with the measurement of figure of merit of PV cells and module It has been observed that in urban areas 5% of intensity get reduced in winter and2.5% reduced in summer where as less than 1% reduction in July and August The above studies indicated two fold impact on the cell, dust accumulation as well as scattering of radiation to the upper part of cell. This indicates that winter time,as there is no sun, density of dust particle increases. The accumulated dust on the module and due to floating particle of dust, a largest part of solar radiation get scattered which ultimately reduced the level of intensity of particular colour to the cell surface. For power 61

10 prediction of a solar system, a numerical model is developed for the solar cell/solar module by MATLAB SIMULINK, and the validation of the model is performed with the experimental 4.7data. The effect of natural dust on solar panel on test may be observed EXPERIMENTAL SETUP AND EQUIPMENTS Three type of setup we have used here for the measurement process First setup is for analysis the performance of solar cell at different sun irradiance, different temperature. Second setup is for analysis the performance of solar module at different rate of dust deposition. Third setup is for the measurement how much solar irradiance absorbed and reflected due to dust deposition on glass plate surface and correlate the data with effect on solar panel power output and efficiency loss SETUP FOR FIRST EXPERIMENT (F A. Appendix - 3) A closed box with roof is made of glass plate, A GaAs solar cell area about 4 cm² which is tested before. Surya mapi for intensity measurement, A digital display meter attach with inside the box where solar cell was taken, this display meter display humidity and temperature, Light source which consist of a 1000 watt halogen bulb and a computer is connected with the solar cell output via software (Interactive Characterization Software ), for I-V curve tracking at various condition of environmental parameter SETUP FOR SECOND EXPERIMENT (F B, Appendix - 3) Two Identical Solar module (35 Wp), With same efficiency tested before, A variable Load resistor, Two millimeters, Digital Power Meter for measuring Open circuit voltage and Short Circuit Current at an instant SETUP FOR THIRD EXPERIMENT (Fig. C, Appendix - 3) A glass plate with 81 cm2 area, Artificial light source., A scale to measure the weight of glass plate with dust and without dust, leading to the measurement of dust deposition. 62

11 RESULT AND ANALYSIS The photovoltaic cells being open to environment are significantly affected by the environmental parameters. For the best use of its potential and design optimum cell for a given demographic conditions, the effect of the environmental parameters on the performance of solar cell is very important. To achieve this researchers have performed several studies on the role of environment on the solar cell performance. It is found that the solar cell performance parameters strongly depend on various environmental conditions EFFECT OF DUST ON VARIOUS PARAMETERS It has been confirmed that two photovoltaic module have identical I-V characteristics, To within experimental error, when their surface were clean, and same irradiance were falling upon those two module surface. The performances of two modules were tested simultaneously to calculate the effect of dust upon the panel output EFFECT OF DUST ON SOLAR PANEL I-V CHARACTERISTICS It has been confirmed that two photovoltaic module have identical I-V characteristics, to within experimental error, when their surface were clean, and same irradiance were falling upon those two module surface. The performances of two modules were tested simultaneously to calculate the effect of Dust upon the panel output. On below graphs it is shown that, how Solar panel I-V curve falls down by varying different amount of dust. (Fig. 4.1 Fig. 4.8) What happens in the vicinity of a p n junction when it is exposed to sunlight. As photons are absorbed, hole-electron pairs may be formed. If these mobile charge carriers reach the vicinity of the junction, the electric field in the depletion region will push the holes into the p-side and push the electrons into the n-side. The p-side accumulates holes and the n- Following graphs are showing the power output of the solar panel and the effect of 2 gm, 4 gm, 6 gm dust deposition. 63

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20 The most commonly used is the core of polycrystalline silicon photovoltaic cells semiconductor silicon in the polysilicon is very sensitive to light, resulting in the free electron current. However, only silicon can not make solar photovoltaic panels. In order to ensure use Power is calculated by the formula P=VI Power degradation by 2 gm dust accumulation on solar panel surface is 16.26% Power degradation by 4 gm dust accumulation on solar panel surface is 28.85% Power degradation by 6 gm dust accumulation on solar panel surface is 39.98% 4.8.2DEGRADATION OF SOLAR PANEL POWER OUTPUT BY DAY TO DAY DUST DEPOSITION On the very first day of this experiment the glass plate where dust was accumulated were cleans properly and dried. Weight of glass plate was taken by us it is gm area of the glass is 81 cm2. 72

21 EFFECT OF DUST ON SOLAR IRRADIANCE Solar intensity gets absorbed and reflected by natural dust. It cannot reach to the solar cell p-n junction. This study reports on the intensive experiments which were carried out at the rooftop of the laboratory for 15 days continuously how much solar intensity gets degraded. It is found that solar irradiance is degraded by dust deposition about per cent. This is true in the absence of any specific data regarding dust accumulation in this particular location EFFECT OF DUST ON SOLAR PANEL IN SHORT CIRCUIT CURRENT It is well known that short circuit current (ISc) is proportional to solar irradiance in association with the frequency of dust deposition on panel surface. When the panels include in short circuit on dust deposition and distribution it has a serious impact irrespective of wind velocities. Here in experiment it observes that short circuit current (ISc) is degraded by 15 days of dust deposition, 23.68% EFFECT ON POWER BY DUST DEPOSITION ON SOLAR PANEL SURFACE By the definition of power, it is the multiplication of current and voltage, and current allows changing by dust deposition. As a result, the power is also affected by dust deposition. Hence, Solar panel output power during the period of 15 days dust deposition is also degraged by per cent. 73

22 Simulation Analysis of PV Model Matlab Simulink, a general-purpose software package for dynamic systems, has been selected to carry out the modeling task for many reasons. Indeed, Simulink is a platform for multi domain simulation and model-based design for dynamic systems. It provides an interactive graphical environment and a customizable set of block libraries, and can be extended for specialized applications. This makes it the best candidate for accomplishing the objective of fostering interdisciplinary integration (environmental and electrical). Simulink being integrated with Matlab provides immediate access to an extensive range of tools for algorithm development, data visualization, data analysis and access Simulink being integrated with Matlab provides immediate access to an extensive range of tools for algorithm development, data visualization, data analysis and access, and numerical computation. For model simulation in Simulink, solver type variable step ode45 has been used. Maximum step size 1e-3 and tolerance 1e-7 has been taken for satisfactory result. The details about the model have been cal computation. (Fig. A1, A2, A3, Appendix - 3) 4.9 CONCLUSION The solar photovoltaic power can be achieved, one of the substitutes to conventional power generation systems which has higher potential than any other renewable energy source like wind, biomass, and tidal power. India is blessed with an abundance of sunlight at least 300 sunny days in a year. The solar cell I-V characteristic is non linear nature due to its diode property. Experiment shows the module gets soiled within a couple of days. The soiling is very much prominent in winter season rather than the summer or rainy season. As in summer there are 74

23 northwesters which takes out the dust and in Rainy season water washout the soil. So the power production is very good in July and August. 75