Solar Cells at converts sunlight energy into ele srical conversion steps. fany solar cells jevice tha Hnrough any intermediate otovoltaic (PV) technolog’ jolar cell is a semiconductor d Aen directly without going @ Tis a fundamental block of solar phi ¢ Boe fre connected together to form solar PV modules. Several solar PV modules are ‘connected together to make PV arra ‘er applications as well as in big power plant applications. Therefore, it is important 10 understand how does a solar Fell work, how to identify a solar cells, what are its paramncicis, how much power a solar call can generate, how the generated power depends on sunlight falling on it, tte. This chapier focuses on providing the fundamental understanding of solar cells, its parameters and how the variation in parameter and ambient conditions affect the performance of solar cell yy in small pow Is of different technologies “The electricity is conventionally generated by using coal energy hydro energy or fuclear energy. One of the most common w nerating electricity is using coal are Better energy. In India, about 55% of electricity is generated using coal energy. A typical toal power plant, shown in Figure 3.1, involves several steps before the energy of coal gets converted into useful energy form, the electricity. The power plant process starts from the buming of coal and ends with the generation of electricity by generators. In the whole process, only fraction of coal energy is ¢ventually used in running our appliances. Most of the coal energy is wasted in the conversion process and transmission of electricity from power plant to our homes. Other than the waste of energy, there is also environmental pollution caused of coal based ie plants. pi. coal as a source of energy is not available in infinite quantity wi a a id ‘means that sooner or later we will run in the shortage of coal. Considering these facts, one must look for alternate source of energy. ‘One of the mod ee lem ways of generating electricity is using solar cells or solar sovoltaics (PV); a technology that converts sunlight i ‘ci ee tare arson 1 sunlight into electricity. Solar cell = y wn lots of attention of engineers, researchers, industries governments in recent times. Therefore, in thi ill foct ; , in this chapter, we will focus on solar cells. So, let us see what is Se ecacet is a solar cell? How is current generated by it and what ays of32 ‘side nd back side of s typical .s, Trainers and Engineers ‘and Systems: A Manual for Technician team turbine Boiler st y converts sunlight into el ees ia ty by photovoltaic effect. Hence, ak 7 ymmercial silicon solar cell is vice which directl Solar cell is a sen sunlig Solar cell converts ee ‘iso called photovoltaic cell. A typi - t Full back metal contact ‘The solar cell generates current and voltage at its terminals when sunlight fall it. The amount of electricity generated by a solar cell depends on the sunlight incident on it. The electricity generated by solar cell depends upon intensity (amount) of light, the area of a cell and the angle at which light falls ‘The higher is the intensity of sunlight, the more is the electricity generated by ell. If area of a sola sed, the current generated by it increases, power generated by the solar cell is optimum when sunlight falling is perpend {o the front side of solar cell In common, all solar cells, irrespective of the technology and material used only two terminals (positive and negative terminals) as output. Typically solar Have front contact at the top, emitte-base junction or p-n junction in the i 4nd the back contact at the bottom. At the emitter-base junction, the of negative and positive charge takes place. Electricity is supplied to a load Connecting its terminals to the front and back contacts of a sol solar ‘0F solar panel as shown in Figure 3.3, ae iChapter 3: Solar Cells | a the earth is basically the bundles of photons or bundles ‘photon in a bundle has finite ‘amount of energy. In solar ‘many photons of different energy. For generation of electricity, “be absorbed by solar cell, The absorption of photon depends upon ‘and the band-gap energy of semiconductor material of a solar - photon energy and the band-gap energy ‘of semiconductor is expressed in Blectron-volt (eV). The eV is @ unit of energy. ‘So, the working of a solar cell can be explained as follows: 1. Photons in the sunlight falling on the solar cell's front face ‘semiconducting materials. 2. Free electron-hole pairs are generate charge and holes are considered as pos ‘connected to a Joad, electron and holes ‘each other. The holes are collected at positive st negative terminal (cathode). Electric potential is built atthe terminals dus f the separation of negative and positive charges. Due to the difference petween the electric potentials at the terminals we get voltage seross the terminals. 3, Voltage developed at the terminals of a solar cell is used to drive the current in the circuit, The current in the circuit will be direct current or DC current, So, the solar cell with day light falling on it can directly drive DC electrical appliances, But, the amount of electricity generated is proportional to the amount Of light falling. So, the amount of electricity generated throughout the day is not ‘constant. The current generated also depends on several other parameters. In the following section, we will now see why the generated current is not constant? are absorbed by 14, Electrons ate considered as negative itive charge. When solar cell is 'A solar cell converts the sunlight into clectricity. How nicely a solar cell does the conversion of sunlight into electricity is determined the parameters of solar cells. There are several parameters of solar cells that determine the effectiveness of sunlight to electricity conversion. The list of solar cell parameters is following: # Short circuit current (I,), ‘© Open circuit voltage (Voc) and © Maximum power point Current at maximum power point (Ij) Voltage at maximum power point (Vj) Fill factor (FF) Efficiency (7), These parameters can be best understood by Current-Voltage curve (J-V curve) of 44 solar cell. The representation of /-V curve is plotted in Figure 3.4. The Y-axis is normally plotted as current axis and X-axis is plotted as voltage axis.a ineers A Manual for Technicians, Trainers and Engi the Using Figure 3.4, the cell parameters are cohiod ee i sing ers are given by & manufacturer, or scientist At tandard = cl parame aponding 10 1000 Wh? of fut solar radiation — ae 1,): Wis the maximum current a solar cents rirfie the cell. It is measured in roduce. The higher the Ji, better is E re (mA). The value of this maximum current depends oot arpeetl area, amount of solar radiation falling on cell, ere Ntany times, people are given current density rather than The current density is obtained by dividing fe by the area of solar ‘The current density is normally referred by symbol, ‘J’, therefore, d circuit current density, Jie is given by [JA bpen-ciremr-vottage (V,.): It is the maximum voltage that a produce. The higher the V,, the better is the cell. It is measured) (V) or sometimes milli-volts (mV). The value of this maximum open mainly depends on cell technology and operating temp It is the maximum power that voltag Maximum-power-potnt (Py, OF Prax) is sometimes also referred as Wye: oF Wp. A solar cell can operate at many current and voltage combinations. But a solar cell will produce power only when operating at certain current and voltage. This Power point is denoted in Figure 3.4 as Pz, Normally, the maximum Point for a /-V curve of solar cells occurs at the ‘knee’ or ‘bend? bed curve. In terms of expression Py, is given as: 7. Pm Ot Pax = Im X Von 5 Currentat maximum power point (Un) This is the current which will produce when operating at maximum power point. The J, be lower than J. It is given in terms of ampere (A) or milllgy Voltage at maximumepowerpoint (Vn): This is the voltage which. will produce when operating at maximum power point. The V», wil be lower than Vi. It is given in terms of volt (¥) oF mili-vott Lill factom{FPy° As the name su;Capers Setar coms | 43 ‘The efficiency of a solar cell is defined as the maximunt Mg ot Pasa) divided by the input power (Py). The efficiency of given in terms of percentage (%), which means that this percensage “input power is converted into electrical power. Pw for $1 considered as 1000 W/m?, This input power is power density (power divided “ty area), therefor, in order to calculate the efficiency using Pia at STC. we ‘must multiply by solar cell area. Thus, efficiency can be written eta ~ in X Ya FE Pax [Let us pow see what the possible values of solar cell parameters and how the values that depend on the various solar cell technologies 3: Fill below in Table 3.1, the various solar cell parameters and their units by which TABLE 3.1 Solar Cell Parameters and their Units [ISN Wame of parameter Unit of parameter The current density of a solar cell having an area of 100 cm? at Standard Test Condition (STC) is given as 35 mA/cm?, Find out the output current of the solar cell. First, we write the formula for current density of a solar cell given-by i = (mA/em?) Current density (/,.) where, Jgq = Current density (mA/em?) J, = Output current (mA) A= Area (cm) Given that, Je = 35 mA/em* ‘So, the expression for solar cell current can be written as Output current (Isc) = Jyc % A (mA) Now, given that area of solar cell is 100 cm’, then Output current (J,.) = 35 mA/em? x 100 cm? = 3500 mA or 3.5 A Similarly, we calculate output current for different values of solar cell area in the=A Manual for Technicians, Trainer ad EPSIOGSH “4 | Solar Photovoltaic Technology and System™ aula forthe maximum power point of soles ceil Solution we write form Py OF Parax = tm * Von Given that, Im = 0.6 A Vq = 0.5 V ore, the maximum power point, Pm = 0.6 AX 0.5 V = 0.3 W area of 100 cm? gives 3.1 A current at sol 7.5 V at maximum power point at STC. The cell gives 3.5 A Pn amd 06 ¥ open circuit voltage. What is the maximum power cvtur cell? Also, find out the efficiency of the cell. rst, we write the formula for the maximum power point of a solar cell, Poy OF Pras = Jn % Ven ‘Therefo ‘A solar cell having an Fi Given that, Maximum power point, Py, = 3.1 A x 0.5 V = 1.55 W Now, we write the formula for efficiency of a solar cell given by P, Py, XA n where, n= Efficiency in per cent (%) Pox = Output power in watt (W) Py = Light input power per unit area in watt/s square meter A= Solar cell area in square meter (m?) ous pa 1.55 W and at STC, Pi, = 1000 Wim? ‘we convert the unit of area from i eer ee Square centimetre (em?) to A= 100 em? = 100 x 104 m? = 0.01 m? the number we can calculate the efficiency of the cell. 1.55 watt 2n point Iq) = 0.40 A ‘power point (Vj) = 0.5 V power point, Pa OF Pax = In X Vo = 0.40 X05 = 02 W In XVe p~ lax Ym Fal Factor, FF = 72 - pr=—fm 02 _ 109 = 63.40% : T,X Vq 04507 ‘Note: In order to represent the FF value in ‘percentage’, multiply by 100 A solar cell having an area of 25 cm? gives a current of 0.85 A and voltage 0.55 V at maximum power point. The short circuit current is 0.9 A and open circuit ‘Voltage is 0.65 V. What is the Fill Factor, maximum power point and efficiency of the solar cell? Consider STC. Given, Short circuit current (lic) = 0.9 A Open circuit voltage (Vac) = 0.65 V Current at max power point (Ij) = 0.85 A Voltage at maximum power point (V,) = 0.55 V Light input power (W/m?) = 1000 W/m? Area = A = 25 em Now, Maximum power point, Py, Of Prox Fill Factor, FF P, 0.4675 a 5_ 100 = 79.91% oe Po Tox Te 09% 0.65 Effi = x 100 = 18.7% ciency (M) = BCG = 18.7% “Note: In order to represent the FF and efficiency values in “percentage’, multiply by 100 in both cases.) 7 A solar cell having Fill Factor (FF) 60% gives 2.5 A current at maximum poster point at STC. The cell gives 3 A shor circuit current and 0.5 V open circuit voltage. ‘What is the voltage at maximum power point of the solar cell? Given that, ead[A Manual for Technicians, Trainers and Engineers ‘Solar Photovoltaic Technology and Systems: where, Je = Short circuit current (A) Jq.= Current at maximum power Vg. = Open circuit voltage (V) Vp = Voltage at maximum power FF = Fill Factor (%) We know, FF = 60% First, v2 convert Fill Factor (FF) from per cent to decimal by di point (A) point (V) 100. 60 Therefore, FF Now, we rewrite the formula for Fill F ‘given by expression below. 1 XV, ¥q = FF xa Voltage at maximum power point, Vy Now, putting the value, we can calculate the voltage at maximum power point os Thus, the voltage at maximum power point is 0.36 V. A solar cell having Fill Factor (FF) 68% gives 0.6 V voltage at maximum powe Point at STC. The cell gives 3 A short circuit current and 0.7 V open circuit voltage What is the current at maximum power point of the solar cell? Given that, 3AChapter 3: Solar Cells | a7 ig “Current at maximum power point, I = FF xp the value, we can calculate the current at maximum power point ib tpn ph tala = 96nx2%07=238 A - Thus, current at maximum power point is 2.38 A. ‘A solar cell has maximum power point of 0:3 W. power point at STC is 0.65 V. What is the curr the solar cell? Given that, Py = 03 W I=? Vm = 0.65 V First, we write the formula for maximum power point given by ‘The cell voltage at maximum mnt at maximum. power point of Pp, OF Prax Of @ solar cell ae Ve Maximum power point (Px) where, Pa = Maximum power point (W) Jy = Current at maximum power point (A) Vm = Open circuit voltage (V) Now, we rewrite formula for maximum power point Pp, of a solar cell to get the value of Jp, given by expression below Fn Vn Current at maximum power point, J Putting the value, we can calculate the current at maximum power point, pl .03 <2 =—— = 0.46 Vy 0.65 é ‘Thus, the current at maximum power point is 0.46 A. 3.2: Current and voltage of a solar cell has been measured under STC at various points of values are given in Table 3.2 below. For this solar cell, calculate the maximum power n solar cell.48. | solar Photovoltaic Technology and Systems: A Manual for Technicians, Trainers and Engineers uurrent and yoltage at various operating has been 1c and fill in the parameters of solar cell PS WORKSHEET 3.3: A solar cell's Worksheet 3.2. Using that /-V data, fill in estimat 33 below. 3.3 Probiem to Find Various Solar Cell Parameters Based on Table 3.2 ‘Current value when voltage Is zero Open circuit voltage (Voc) Voltage value when current is zero Maximum power point, Pr Value of maximum power ‘Current at max power point (In) Current value at maximum power point Voltage at max power point (Vn) Voltage value at maximum power point 4 poe The * Vor Fil Factor (FF) Mibeiotte 2 5 Axa oT 3.4: -V characteristic of a solar cell is given below (Table 3.4). Fill in the blank space TABLE 3.4 To Obtain the Missing Quantities TE Gore 1) Vote V0) Power FO i ooo 058 - 2 oo ie s 3 029 e a re — 057 a 5 119 a is 2 re 055 0.88 054Gatun (Ga), Arsenic (AN, Indium (in), Phosphorus (P) Gorman (Ga) ct wchnclogy comes from the materials used in making solar cols, h Available Cell Areas: Bolar Cell Parameters (1, Jyo. Vox and FF) of Commercial Solar Celis wit Coll area (A) — Output voltage (Voc) ‘Output current (ue) ‘Fill factor nent) gy Newer) oe 095-088 V 20-38 ety ws 080-10 15-25 050-07 V 20-30 30-95 1-4 10-25 15-35 70-85 ‘one technology to other technology and trom one manufacturer to other manutacturer: ‘There are five common factors that affect the power generated by solar cells. They are as follows: 1, The conversion efficiency (7), bya 2. The amount of light (P,.). 3. The solar cell area (A), 4, The angle at which day light falls (8), and 5. The operating temperature (7) ‘Of the total light energy falling on a solar cell, only some fraction of the light ‘energy gets converted into electrical energy by the solar cells. The ratio of electrical ‘energy generated to the input light energy is referred as conversion efficiency of ‘solar cells. The conversion efficiency of solar cell is fixed, based on material and Once a solar cell of given material is manufactured, itsTrainers and Engineers “A Manual for Technicians, ie Technology and Systems: 50 | Solar Photovolta is directly deten put power, the value of the output power cao wc’ sonversion feny and solr cll ree, The sola higher efficieney values. will always sive be pea ey x cll efficiency is percentage (96), the unit of Pay is no se hamaty Wind or Wren? and unit of cell area is im? ox em, The efficiency is given for standard test condition (STC) and under the Ree Ta he density, Py is taken as 1000 Wim? or 0.1 Wiem?. power tom the same area, of inp a solar cell if its efficiency (in %) E ‘alculate the output power from a aaett [etal | input power density is 1000 W/m’ rst, we write formula for efficiency of a solar cell given by PB 7a tee rey rout Efficiency in per cent (%) ut (W) Light input power per unit area in watt/metre? (Wim?) Solar cell area in metre? (m2) 11 = 30%, 24%, 19%, 16%, 1 = 1000 Wim? First, we convert cell area from cm? to m? Mis given that cell area A = 100 cm? = 100 x 10-* m? Now, we solve for solar cell of efficiency 30% Above equation can be written as: P, "max = 1X Pry XA We put the respective terms values and we get, 30 = (2) x10 «100 «10423 wysowcatot — ony tart cticloney 12% Fagan in?) ese rae "> ponte termina ot terminal of ‘walt meter We should keep in mind that the amount of sunlight (intensity of sunlight) falling on solar cells keeps changing from moming to evening. The current and voltage output of a solar cell depends on the amount of light falling on it. The electric current ‘generated by solar cell is directly proportional to the amount of light falling on it ‘Suppose, a solar cell produce 1 A current under 1000 W/m? input solar radiation, then under 500 W/m? input solar radiation, the cell will only produce ¥2 A current (because input radiation is half). As the amount of sunlight falling on the solar cell increases from morning till afternoon, the current output of @ solar cell also increases from morning till aftemioon, From afternoon, till evening, the amount of sunlight falling on the solar cell decreases, and hence, the current output of a solar cell also decreases from afternoon till evening. The output voltage of a solar cell is not affected strongly by change in the amount of light. If a solar cell produces 1 Vat noon time, its voltage will roughly remain same in the morning as well as in evening hours. ‘The solar cell current output is proportional to the amount of solar radiation and voltage is relatively not affected by the variation in sunlight intensity. Therefore, the amount of power generated (Current x Voltage) by solar cell is. proportional to the amount of light falling on it. The amount of power generated by the solar cells throughout the day keeps changing (i.e., it is not constant). So, @ solar cell ‘gives high power when the intensity of light falling is high. Similarly, less power is generated when the intensity of light falling is low. Calculate the output power for solar cells of efficiencies 16%. When the input power is say, 1000, 800, 600 and 400 W/m? and area of solar cell is 100 em?. First we write formula for efficiency of a solar cell given byfor Technicians, Trainer and Engineers Sl | Solar Photovoltaic Technology and Systems: ‘ Sti mm?) to sq Now, we solv Above equation We put the respective terms values and we Bet ’, 16). 1000 x 100 x 10 = 1.6 W Fos “(100 we calculate output power for other efficiencies in table Si shown in Table 38 ble for Example 3.10 TABLE 3.8 AA om) Am) 01100 Pre = "6 1000 100 001 16 200 100 oot 6 600 100 oot 100 01 The amount of power generated by solar cell de " -d by solar cell depends on the amount of a ae is shown in Figure 3.7. From above table, it clear that when of light falling on a solar cell reduces, the out reduces s, the output power generated also nck Pome: a 16% . an oo ef , Day light iency 16% bey re