CHAPTER-2 Photo Voltaic System - An Overview

Transcription

1 CHAPTER-2 Photo Voltaic System - An Overview 15

2 CHAPTER-2 PHOTO VOLTAIC SYSTEM -AN OVERVIEW 2.1 Introduction With the depletion of traditional energies and the increase in pollution and greenhouse gases emissions, renewable energy will contribute more and more to the energy sources globally. The different renewable energy sources are hydro power, solar power and wind power. Among these, solar power is one of the most important parts of renewable energy systems and may be accepted by more countries and companies. However, the electric power generated with photo voltaic cell is inconsistent, since the solar Energy produced mainly depends on the solar irradiation conditions [19]. Solar cells are made from semiconducting materials that behaves as conductors when supplied with light or heat, but they become insulators at low temperature. At low temperature, the conduction band of semiconductor is empty while the valence band is fully occupied. Thus, the material cannot conduct electric current. If the materials are exposed to light, photons from the light enter the materials [4]. The photon energy (E) of light depends on the frequency of light as follows E = hf or (2.1) Where h is Plank s constant light and is the wavelength of light., f is frequency of light, c is the speed of As particles such as photons and electrons have small amount of energy, the most common unit for photon energy is electron-volt (ev). An electron-volt is defined as the required energy to raise an electron through a voltage of 1V. Hence, 1eV= J [5]. For energy in ev and light wavelength in αm, a commonly used expression for the photon energy is expressed as follows, E=1.24/ λ αm (2.2) 16

3 The most common semiconductor material used to produce solar cells is silicon (Si). Before producing a solar cell, the semiconductors are contaminated or doped with certain chemical elements. Depending on the dopants used, semiconductors can be either n-type or p- type. Figure 2.1: Electron-hole generation (Source: Solid state Physics Laboratory Manual, California State Polytechnic State University) The Photovoltaic energy conversion in solar cells has two important steps: generation of electron-hole pairs and separation of electron hole pairs. First, the electron-hole pairs are generated by means of light absorbed by the materials [Fig 2.1]. The absorption of photons generates both majority and minority carriers. Generally, the number of majority carriers in an illuminated solar cell does not change significantly because the doped materials are already rich in majority carriers. However, the number of photo-generated minority carriers dominantly surpasses the number of minority carriers present in the solar cell in dark. Therefore, the number of light generated carriers can be approximated by the number of minority carriers [79]. Second, the generated electron-hole pairs are separated by the produced electric field in the junction; electrons to the negative terminal and holes to the positive terminal. The minority carriers have a meta-stable state and will recombine after a time interval equal to minority carrier life time. To prevent minority carrier recombination, the p-n junction collects these carriers and the electric field produced at the junction sweeps the carriers to the other sides. Consequently, the carriers become majority carriers at their respective sides. The separation of charges, electrons on the n-side and holes on the p-side of the p-n junction, creates an electric field at the junction that opposes the existing electric field at the junction. Therefore, the net electric field decreases. The decreasing of the electric field increases the diffusion current. The semiconductor is in a new equilibrium in which a voltage 17

4 exists across the p-n junction. The diffusion current is a photo-generated current. A solar cell also has another component of current, the drift current, which is caused by an external electric field. The drift current is also called the forward bias current. The total current is the difference between the photo-generated current and the forward bias current expressed as follows [79]. (2.3) where I is the photo current, I 0 is the dark saturation current, I L is the photo-generated current, q is absolute value of electron charge (C), n is ideality factor, V is the diode voltage, k is Boltzmann s constant and T is the absolute temperature (K). Above equation can be used to illustrate the I-V characteristics of a solar cell for dark and illuminated conditions [Fig 2.2]. Related to the I-V characteristic, solar cell outputs can be characterized by using three parameters: short circuit current I sc, open circuit voltage V oc and fill factor FF. The short circuit current I sc is equal to the light-generated current. The second parameter, the open circuit voltage V oc, can be derived from above equation by setting I to zero. The result is, (2.4) Figure 2.2: I-V characteristics of solar cells at dark and illuminated conditions (Source: Solar Energy by Robert Foster, Majid Ghassemi, Alma Cota /fig 5.13, page no. 126) The third parameter is the fill factor (FF) that defines how square the output characteristics are [45]. 18

5 (2.5) Where V mp is the voltage that results in maximum power and I mp is the current that results in maximum power. Ideally, the fill factor is only a function of the open circuit voltage. This function can be expressed by the following empirical equation [45] (2.6) Where, and in which q, k and T have been defined in eq. (2.4). From these three parameters, the energy conversion efficiency η, is defined as follows. (2.7) Where P in is the total power in the light incident on the cell. Since the cell generates power, Eq. (2.4) is rearranged as (2.8) The I-V characteristic of solar cell for illuminated condition is plotted in fig Figure 2.3: I-V characteristics of solar cells for generating power 2.2 Characteristics of PV or Solar Cells The solar panels are mainly made of semiconductor materials such as silicon which is most widely used. To increase the output voltage solar cells are connected in series. Similarly to increase the current output, the solar cells are connected in parallel. Series connected cells are called as PV modules and the interconnection of solar cells in series and parallel combination is called an array. Usually, the cells operate in reverse direction so as to obtain desired current drift. When the PN junction is exposed to light, photons having energy greater than the threshold energy are absorbed, causing the production of electron-hole pairs. These charge 19

6 carriers are separated under the influence of existing electric fields at the junction, creating a current which is proportional to the incidence of solar irradiation. Solar cells naturally exhibit a nonlinear I-V and P-V characteristics which vary with the solar irradiation and cell temperature [126]. The typical I-V and P-V characteristics of solar cell are shown in figure 2.4. Figure 2.4: Characteristics of Solar Cell. The fundamental parameters related to solar cell are as follows: Short Circuit (I sc ) is the current flow when the output impedance is low and the voltage equals to zero. I sc = I (V = 0) I sc occurs at the beginning of the forward-bias sweep and is the maximum value of the current in the power quadrant. For an ideal cell, this maximum value is the total current produced in the solar cell by photon excitation. Open Circuit (V oc ) is the output voltage when the output impedance is high and there is no current passing through the cell. The Open circuit voltage is calculated when the output current equals to zero. V oc = V (I=0) V oc is also the maximum voltage across the cell for a forward-bias sweep in the power quadrant. V oc = V m for forward-bias power quadrant. The open circuit voltage can be expressed as: (2.9) Where : 20

7 I PV : Light generated current I S : Saturation or leakage current of the diode q: Electron charge ( C) K: Boltzmann constant ( J/K) T c : Cell temperature in Kelvin (K) A: Ideality constant Both k and T c should have the same temperature unit, either Kelvin or Celsius. The ideality constant is different for different PV technology. Ideality constant of different PV technology is given in Table 2.1. Table 2.1: Ideality Constant for different PV technology Type of PV Technology Ideality Factor Si-monocrystalline 1.2 Si-polycrystalline 1.3 a-si:h 1.8 a-si:h tandem 3.3 a-si:h triple 5 CdTe 1.5 CIS 1.5 AsGa 1.3 Maximum Power Point is the operating point P-V curve at which the power is maximum across the load. P m = V m I m (2.10) where, V m is the maximum voltage and I m is the maximum current. Efficiency of solar cell is given by the ratio of the maximum power to the incident light power. Efficiency = (P m ) (P in ) 100 (2.11) P in is taken as the product of the solar irradiation of the incident light (G=λ/1000), measured in W/m 2 with the surface area (A c ) of the solar cell in m 2. P in = G A c (2.12) Fill Factor (FF) is calculated by comparing the maximum power to the theoretical power (P m ) that would be output power with both the open circuit voltage and short circuit current taken together. Typical fill factors range from 0.5 to The fill factor reduces along with the increase in cell temperature. FF = (P m ) (V oc I sc ) (2.13) 21

8 2.3 Principle of Operation of Solar Cell The ideal photovoltaic cell is as shown in figure 2.5. Mathematically, current of an ideal PV cell is given by [2.14]: I = I pv - I d (2.14) Where, I d is the Shockley s equation and it can be expressed as, (2.15) The equivalent circuit of solar cell consists of a current source connected in anti-parallel with a diode, a resistance (R s ) in series and a resistance (R sh ) connected in parallel is shown in figure 2.5. Figure 2.5: Equivalent Circuit of Solar Cell. Based on above, the output current of the solar cell can be calculated as, (2.16) (2.17) Where, N s is number of cells connected in series in a PV module. The light generated current which depends on the solar irradiation and its working temperature, is expressed as, (2.18) Where, I sc is the short-circuit current of cell at STC. K i is the short-circuit current temperature coefficient of the cell. T c and T r are working temperature of cell and reference temperature respectively in o K. G is the solar irradiation of the incident light. Now, the diode saturation current of the cell which also varies with the cell temperature, is expressed as, 22

9 (2.19) where, I rs is the reverse saturation current of the cell at a reference temperature and a solar irradiation. E g is the band gap energy of the semiconductor used in the cell (E g =1.12 ev for the polycrystalline Si) at 25 o C. The reverse saturation current of a cell I rs is (2.20) According to S. Sheik Mohammed [32], R s and R sh are unknowns, which can be found by making the maximum power calculated from I-V curve of the model (P m,m ) equal to the maximum power obtained from the experiment i.e., given in the data sheet (P m,e ) i.e., (P m,m =P m,e ). Now solve the resulting equations for R s, using iteration method by slowly increasing the value of R s from zero i.e., the iteration starts with R s = 0 till condition satisfies. (2.21) or (2.22) 2.4 Modeling and Characteristics of Solar Cells Solar cell can be model as physical model and behavioral model. Details of models are as follows Physical Model The basic solar cell model consists of a diode and a current source connected in parallel as shown in fig The current source is directly proportional to the solar radiation while the diode represents the p-n junction of a solar cell. Equations representing the ideal solar cell model are (2.23) (2.24) 23

10 Where J is the photo current density (A/m 2 ), J sc is the short circuit current density (A/m 2 ), I is the photo current, I 0 is the reverse saturation current, V is the diode voltage, V T is the thermal voltage ( C). The currents I sc and I 0 relate to their current densities J sc and J 0 as follows, Where A is the total area of the devices excluding the metal covered area. (2.25) The more accurate model of a solar cell, the general model, consists of a current source, two diodes connected in parallel, one shunt resistance and one series resistance as shown in fig The relationship between current and voltage for the general model is given by, (2.26) Where I L is the photo-generated current, I 01 is the reverse saturation current of the first diode, I 02 is the reverse saturation current of the second diode, n 1 is the quality factor of the first diode, n 2 is the quality factor of the second diode, R s is the series resistance (Ω), and R sh is the shunt resistance (Ω). Figure 2.6: Simple physical model of solar cells Figure 2.7: General physical model of solar cells Solar cells are represented by models in Figs. 2.6 and 2.7. The I-V Characteristics of solar cells are also shown in Fig

11 Fig. 2.8(a) relates irradiance to the short circuit current and open circuit voltage of the solar cell. The increasing of irradiance leads to the increasing of the open circuit voltage logarithmically and the increasing of the short circuit current linearly. The arrow direction shows the increasing of irradiance. The influence of the cell temperature on the I-V characteristics is illustrated in Fig. 2.8(b). The increasing of the cell temperature significantly reduces the open circuit voltage while slightly increases the short circuit current Behavioural Model Figure 2.8: I-V Characteristics of solar cells Behaviour model gives solar cell modeling depending upon irradiance and temperatures Single Solar Cell Model Simulations of the solar cell behaviour for changing temperature and irradiance conditions are required for the purpose of modeling photovoltaic systems. The best way to simulate this behaviour is by using a behavioural model[46], In this model the short circuit current is assumed to have two main components. The first component relates to irradiance, G. while the second component is the function of cell temperature, T cell. The equation for short circuit current is given by, (2.27) Where I sc is the short circuit current, J scr is the reference short circuit current density (A/m 2 ), dj sc is the temperature coefficient of short circuit current (A/ o C), dt and T r is the reference temperature (taken as 25 o C). The diode component is represented by, (2.28) 25

12 It gives the relationship between diode current and voltage. From above equation, it is obvious the diode current depend on the short circuit current and open circuit voltage. The temperature dependence of the diode current is given by V T. The open circuit voltage is the function of the cell temperature and short circuit current as follows (2.29) Where V oc is the open circuit voltage, V ocr is the reference open circuit voltage, is the temperature coefficient of the open circuit voltage (V/ 0 C) and I scr is the reference short circuit current. The cell temperature is derived from the nominal operating conditions temperature (NOCT) which is the temperature of the cell at 800 W/m 2 of irradiance and at 20 o C of ambient temperature, (T a ) [45]. That is, (2.30) The series resistance is expressed as follows: (2.31) In which and, where P max is the maximum power (W), FF o is the fill factor under ideal conditions, v oc is the normalized value of the open circuit voltage.the current and voltage for the maximum power point (MPP) is given by (2.32) (2.33) Where I m is the MPP current, I mr is the reference MPP current, G r is the reference irradiance (W / m 2 ), V m is the MPP voltage PV Module Model A photovoltaic module consists of combination of solar cells in series and in parallel. The behavioural model of a photovoltaic module is similar to the model for a solar cell. The voltage of the PV module can be increased by connecting appropriate number of solar cells in series, while the current can be increased by connecting appropriate number of solar cells in parallel. The equations below are derived from the equations for a solar cell. 26

13 (2.34) (2.35) (2.36) (2.37) (2.38) Where N sm is the number of cells connected in series in a module, N pm is the number of cells connected in parallel in a module, I scm is the short circuit current of a PV module, V mm is the MPP voltage of a PV module and R sm is the series resistance of a PV module (Ω) PV Array Model A photovoltaic array consists of photovoltaic modules connected in series and parallel. Again PV modules are connected in series to increase the array voltage and PV modules are connected in parallel to increase the array voltage. The relationships between array and module currents and voltages are given below. (2.39) (2.40) (2.41) (2.42) (2.43) 2.5 PV Systems PV system can work as standalone PV system or Grid connected PV system Stand-Alone PV Systems (SAPV) Stand-alone PV (SAPV) systems are the most common type of PV systems. The SAPV systems are used for rural and residential off-grid applications. The basic SAPV system has the following components: PV arrays, a charge controller, a battery, an inverter, and loads as illustrated in figure 2.9. PV arrays is the only power source of the system. The charge controller controls the charging process of the battery and is needed because the power supplied by PV arrays depends on solar irradiance. The battery stores the energy from 27

14 the PV array while also provides power for the loads through the inverter that converts a dc voltage to the ac voltage required by the loads. A maximum power point tracker is incorporated into the charge controller to keep the PV arrays providing maximum power [84]. To increase the reliability of the SAPV system, another power source may be connected to the system. The additional power source is usually a diesel generator. Because the power sources are mixed, the SAPV system is called hybrid SAPV system. The PV arrays could be sized to supply the common loads while irregular maximum loads can be supplied by the additional power source. Figure 2.9: Block diagram of a stand-alone PV system. Figure 2.10, the ac generator will operate when the PV arrays do not have enough power to supply the loads. The ac generator will also charge the battery when the PV arrays do not have enough power to charge the battery. Figure 2.10: Block diagram of a hybrid PV system The transfer switch will switch the generator on when it is needed and switch it off when PV arrays have enough power to supply the loads. In this topology, the ac generator has specific loads that do not operate frequently but require high power that cannot be supplied 28

15 entirely by the PV arrays Grid-Connected PV Systems The applications of PV systems are not only for off-grid loads. In developed countries, large-mw grid connected PV systems are becoming more popular [75]. A common block diagram of grid connected PV systems is shown in figure The residential loads are connected to the grid through a kwh meter where the power flow through kwh meter is bidirectional. When the PV power is not enough to supply the residential loads, the grid will give additional power. Figure 2.11: Block diagram of a grid-connected PV system When the PV power exceeds the residential load demands, the excess power will be supplied to the grid. This system does not require a battery to store extra energy. The calculation of the energy used by the PV system owner is based on net metering principle. 2.6 Environmental Effect Assessment on Solar PV System The objective of this section is to study the effect of different environmental factors on the performance of solar photovoltaic panel. The photovoltaic solar power represents one of the most promising energy in the world. It is also the cleanest form of energy. But the implementation of a PV system has shown that their reliability and efficiency depend upon many factors. The output of solar PV system is mainly affected by different environmental factors like dust, colour, irradiance, shading, etc. Because in all the cases, the output power and efficiency are more rather than affected by changing environmental condition. Here the number of experiment has been conducted to verify the change in I-V and P-V characteristics of the system with the change in environmental conditions. The result shows that the output power and efficiency are affected by environmental factors. All the experiments were 29

16 conducted in an indoor laboratory and are executed by measuring output voltage and current produced by solar PV panel. Power output = V oc I sc FF Where V oc represents the open circuit voltage, I sc represents the short circuit current and FF represents the fill factor. Efficiency of solar panel = [(V oc I sc FF) (P in )] 100 Where, P in represents the input power. The different environmental factors are: a) Effect of dust b) Effect of colour c) Effect of shading d) Effect of irradiance Specification of solar panel: Model ELDORA 40 (Vikram Solar Pvt. Ltd.) Power Output: 36 W (1000W p /m 2, 25 o C) Open Circuit : V oc = 21V Short Circuit : I sc = 2.4A Rated Max. : V m = 17.2V Rated Max. : I m = 2.2A Figure 2.12 shows the experimental set up in the laboratory to conduct the experiment on the effect of different climatic conditions on the output of the PV system. The Fill Factor calculated from the data sheet of the PV cell =

17 Effect of Dust Figure 2.12: Experimental set up The objective of this section is to study the effect of different types of dust on the performance of solar photovoltaic panel. Accumulation of dust on the modules of solar photovoltaic system is a natural process. This dust decreases the radiation reaching the solar cell and produces losses in generated power. In this case the degradation of PV performance by using different types of dust has been investigated. Experiments concerning the effect of dust (talcum powder) on power generated are conducted and analyzed. Here the effect of dust deposition is considered. Photovoltaic modules are contaminated to other means of contamination like e.g bird dropping, fallen leafs, growth of moss, etc, which were not 31

18 studied here. The result shows that accumulation of dust on solar panel can reduce the system efficiency.it also reduces the total system power output Experimental set up Basically the system comprised of a solar photovoltaic panel, a halogen light and the electrical circuit system. The solar module is made up of silicon mono-crystal cells. The system was installed in an indoor lab and the radiation energy was delivered by halogen light system of rating 1000 watt. The experiment was conducted by using one 36 watt solar panel mounted on a stand. The electrical parameters like voltage and current were measured to study the effect of dust. At first the corresponding voltage and current were measured during clean condition, that means the panel was exposed to white light. Then voltage and current were measured when solar panel exposed to different densities of dust. The use of natural dust is avoided because it might not be well distributed on the surface of solar photovoltaic panel. Figure 2.12 shows the experimental setup of solar panel under different condition. 2.7 Result and analysis As the experiment was conducted in laboratory, one halogen lights was used instead of white sunlight For clean condition- At first case the panel was exposed to white light, which means at that time no dust was covered on solar panel. The experiment was conducted by keeping the temperature at C and the intensity of light 200Wp/m 2. The variation of current and voltage is given in table 2.2 Table 2.2: Parameters in clean conditions

19 Under this case the maximum voltage in the circuit was 19 volt at no load condition, (current was 0.06 A) and the maximum load current was 0.22 A, which is called short circuit current (when the voltage is 0.4V). The variation of voltage and current was tabulated by varying the voltage. Assuming the FF to be 0.75, input power at 200 W P /m 2 = = 50W. Efficiency at clean condition = For talcum powder- Under this condition the solar panel was exposed to talcum powder with light dusting. The output of the module was connected to the electrical circuit. The experiment was conducted by keeping the temperature at C and the intensity of light 200 W p /m 2. The variation of current and voltage is given in table 2.3 by varying the voltage. Table 2.3: Parameters with Talcum powder (light dust) At this time the maximum voltage in the electrical circuit was 17.5 Volt and the short circuit current was 0.19 A with respect to voltage 0.6V. Efficiency at light dust condition = [( ) / 50] 100 = 4.99% For talcum powder (Heavy Dusting)- Again the solar panel was exposed to talcum powder with heavy dust. The experiment was conducted by keeping the temperature at C and the intensity of light 200 W p /m 2. The variation of current and voltage is tabulated in table 2.4 by varying the voltage. 33

20 With this condition the maximum open circuit voltage measured to be 16.1 Volt whereas the short circuit current was 0.18 A with respect to voltage 0.1V. Table 2.4: Parameters with talcum powder (Heavy Dusting) To understand the performance of solar panels the tabulated results (table 2.2, 2.3 and 2.4) are presented in graphical form which is given in figure Efficiency at heavy dusting condition = [( ) / 50] 100 = 4.35% Figure 2.13: I-V characteristics of PV system for different dust condition The graph shows that the output power produced during clean condition is higher in comparison to other cases. As the output power is high it implies that the efficiency also high in the clean condition. In order to ensure high performance it is very essential to provide auto cleaning mechanism to remove the dust particle in the solar panel. 34

21 2.8 Effect of Colour Here an attempt was made to evaluate the effect of colours of light on the performance of solar photovoltaic module. A case study was conducted to experimentally verify the effect of various colour filters on the performance of Solar panel. The sun emits energy in the form of electromagnetic waves. White light from the Sun includes different colours of the visible spectrum and ranges in wavelength from about 400 nano-meters (nm) to about 780 nm. Red spectrum has the least energy, whereas blue spectrum has the most energy. The energy of different spectrum is determined by their frequency (E = hf). Where E is the energy of the photon, f is the frequency in Hz, and h is Planck's constant (h = Js). It is becoming increasingly apparent that wavelength of light have a significant influence on the performance of photovoltaic modules Experimental set up for different colours and Result Analysis: The experiment was conducted in an indoor laboratory by using different colours of light. For a crystalline solar cell, the electrical output voltage is a function of the temperature, intensity and colour of the incident light. The experiment was conducted by keeping the temperature at C and the intensity of light 235 W p /m 2 when the solar panel was covered by green colour filter. The variation of current and voltage measured are tabulated in table For green colour- Table2.5 Colour Spectrum with Green Colour

22 With this condition the maximum open circuit voltage measured to be 19.6V, whereas the short circuit current was 0.19 A. Assuming the FF to be 0.75, input power at 235W P /m 2 = = 58.75W Efficiency with green colour spectrum = [( ) / 58.75] 100 = 4.75% For Red colour- For this the experiment was conducted by covering the solar panel by red colour filter, by keeping the temperature at C and the intensity of light was 235 W p /m 2. The variation of current and voltage measured are tabulated in table 2.6. Table 2.6: Parameters with red colour In this case the maximum open circuit voltage measured to be 20.3V, whereas the short circuit current was 0.43 A. Efficiency with red colour spectrum = [( ) / 58.75] 100 = 11.14% For Violet colour- For this case a violet colour filter was covered on solar photovoltaic panel. The intensity was kept 235 w p /m 2 whereas the temperature was at C. The variation of current and voltage measured are tabulated in table

23 Table 2.7: Parameters with Violet colour With this condition the maximum open circuit voltage measured to be 19.4V, whereas the short circuit current was 0.19 A. Efficiency with violet colour spectrum = [( ) / 58.75] 100 = 4.71% The purpose of this study is to determine the effect of wavelength and colour on the performance of Solar panel. After analyzing the results, it was concluded that the wavelengths of light do affect the performance of solar cell. Red colour light generates more electricity than other colours. The efficiency of solar panels can be improved with an exposure to red light. To understand the performance of solar panels with the effect of light, the tabulated results (table 2.5, 2.6 and 2.7) are presented in graphical form which is given in figure

24 Figure 2.14: I-V characteristics of PV system for different colour 2.9 Effect of Shading Silicon solar panels are commonly used in photovoltaic applications. The maximum voltage produced by each cell is 1V. Hence, in order to obtain the desired voltage and current solar cells are connected in series or parallel. Therefore, even a very small amount of shading on solar cells or panels affects the overall performance of the module to a great extent. Shading is mainly caused due to dust settlement in panels, passing clouds and shadows of neighbouring mounting structures Result and Analysis: To study the variation in I-V characteristics and power, an experiment was conducted in the laboratory. The corresponding voltage and current were measured separately by shadowing the solar module by 25%, 50% and 75% respectively. During the experiment the intensity was kept constant 165 W p /m 2 for all cases For 25%shading of solar panel- In this case the panel was kept in 25% shading condition whereas the intensity was kept 539w/m 2 and the temperature at C. The variation of current and voltage measured are tabulated in table

25 Table 2.8: Parameters with 25% shading Assuming the FF to be 0.75, input power at 165W P /m 2 = = 41.25W Efficiency with 25% shading effect = For 50% shading- For this case the solar panel was kept 50% of shaded with intensity was kept 165 Wp/m 2 and the temperature at C. The variation of current and voltage measured are tabulated in table 2.9. Table 2.9 Parameters with 50% shading Efficiency with 50% shading effect = For 75% shading Here the panel was kept in 75% of shading condition; the intensity was 165 Wp/m 2 and the temperature at C. The variation of current and voltage measured are tabulated in table Table 2.10: Parameters with 75% shading Efficiency with 75% shading effect = 39

26 To understand the performance of solar panels with the effect of shading condition which are tabulated results (table 2.8, 2.9 and 2.10) are presented in graphical form which is given in figure Figure 2.15: I-V characteristics of pv system for different shading condition 2.10 Effect of Irradiance The overall performance of solar cell varies with varying irradiance and temperature. Irradiance is defined as the measure of power density of sunlight received at a location on the earth. As the solar insolation keeps on changing throughout the day I-V and P-V characteristics also varies accordingly. With the increasing solar irradiance both the open circuit voltage and the short circuit current increases and hence the maximum power point shifts. Temperature plays another major factor on which the solar cell efficiency depends. As the temperature increases the rate generation of photon increases thus increasing reverse saturation current rapidly and this reduces the band gap. Hence this leads to marginal changes in current but major changes in voltage. The cell voltage reduces by 2.2mV/degree rise in temperature. Now a day s Solar panels are made of non-silicon cells as they are temperature insensitive. 40

27 The effects of solar irradiance are tested in different conditions which are tabulated in Table 2.11 to Tabale 2.11: Solar irradiance of 165W p /m 2 with temperature C Assuming the FF to be 0.75, input power at 165W P /m 2 at C = = 41.25W Efficiency at 165W P /m 2 at C = Tabale 2.12: Solar irradiance of 200 W p /m 2 with temperature C Assuming the FF to be 0.75, input power at 200W P /m 2 at 27 0 C = = 50W Efficiency at 200W P /m 2 at 27 0 C = 41

28 Tabale2.13: Solar irradiation of 250 W p /m 2 with temperature C Assuming the FF to be 0.75, input power at 250 W P /m 2 at 29 0 C = = 62.5W Efficiency at 250W P /m 2 at 29 0 C = To understand the performance of solar panels with the effect of irradiance which are tabulated in table 2.11, 2.12 and 2.13 are presented in graphical form which is given in figure Figure 2.16: I-V characteristics of pv system for different radiation From the above plotting, it shows that the output as well as the efficiency increases along with the increase in the solar irradiation level. The IV characteristics shown in figure 2.8 is in ideal condition whereas the IV characteristics shown in figure 2.13 to 2.16 are based practical laboratory conditions. The characteristic 42

29 curves differ from those in ideal conditions as the ideal conditions could not be met in laboratory environment Summary: In this chapter, different characteristics of output current (I) vs voltage and output power (P) vs voltage of the solar PV system were studied under environmental conditions e.g. cleanliness, shading, solar irradiance etc. The results were validated with the experimental results. The output of solar cell is maximum i.e. V oc = 19V and I sc = 0.22A in clean condition with an efficiency of 6.27%. For the wavelength of red colour light produces maximum output i.e. V oc = 20.3V and I sc = 0.43A with an efficiency of 11.14% which reduces to 4.71% for violet colour. In partial shading condition the output of solar cell decreases significantly with the increase in percentage of shading that is from 0.093% to 0.01%. Also the output of solar cell increases with increase in irradiance level i.e. V oc = 1.7V and I sc = 0.27A at 800 W p /m 2. The efficiency increases from 3.54% at 165W P /m 2 to 6.38% at 250W P /m 2. Again, energy storage technology used for PV system has been depicted in details. 43