2-1 OverviewOne way of utilizing the energy of the sun isto generate electricity directly from sun light by the PVs. The photovoltaiceffect is defined as the generation of the electromotive force as a result ofthe absorption of ionizing radiation 4. Groups of PV cells are electricallyconfigured into modules and arrays, which can be used to charge batteries,operate motors, and to power any number of electrical loads. With appropriatepower conversion equipment, PVs can produce alternating current compatible withany conventional appliances, and can be operated in parallel with the utilitygrid.2-2 History of Photovoltaic:The first conventional PV cells were producedin the late 1950s, and throughout the 1960s were principally used to provideelectrical power for earth-orbiting satellites. In the 1970s, improvements inmanufacturing, performance and quality of PV modules helped to reduce costs andopened up a number of opportunities for powering remote terrestrialapplications, including battery charging for navigational aids, signals,telecommunications equipment and low power needs. In the 1980s, PV became apopular power source for consumer electronic devices, including calculators,watches, radios, lanterns and other small battery charging applications.
Following the energy crises of the 1970s, significant efforts also began todevelop PV power systems for residential and commercial uses both forstand-alone, remote power as well as for utility-connected applications. Duringthe same period, international applications for PVs to power rural healthclinics, refrigeration, water pumping, telecommunications, and off-gridhouseholds increased dramatically, and remain a major portion of the presentworld market for PV products 5,6.2-3 Principles of PV- Cells Operation:PV cells are made of semiconductor materialsthat can generate electricity electromagnetically when exposed to sunlight. Ifa minority electron-hole pair generated by absorption of photons in thesemiconductor material (the holes in n-regions, and the electrons in p-region)diffuses into a boundary region in which there is an electric filed, theelectron will be accelerated into the n-region, and the hole into the p-region.This causes the n-region to accumulate a negative charge and the p-regionbuilds up a positive charge, resulting in a photovoltage. If there is a closedexternal circuit, a photocurrent and photovoltage can be measured by theexternal resistance. The work of PV cells as shown in Fig.2.
1 (a) and (b). Thebalance of electrons and holes can be shifted in a silicon crystal lattice bydoping it with other atoms. Atoms with one more valance electron than siliconare used to produce n-type semiconductor materials.
Atoms with one less valenceelectron result in p-type material. Once the n- and p-type semiconductormaterials are reached, the process will produce potovoltage. The most commonmaterials used informing n- and p-type semiconductor materials are phosphorusand boron1. A p-n junction can be formed either through a high temperaturediffusion process or an ion implantation process. Diffusion can be made eitherfrom a vapor phase or a solid phase.
Crystalline silicon and amorphous siliconare the most dominant semiconductor materials for commercial PV cells 1.Fig.2.1(a) and (b): PV cell operation diagram;3.
2-3-1 Equivalent Circuit of SolarCell: Theequivalent circuit of a solar cell consists of a constant current source Isc, anonlinear function (diode) impedance Dj, a shunt resistance Rsh due to leakage nearthe edge and corner of the cell, a series resistance Rs due to the resistanceof the cell material, the resistance encountered when electrons travel alongthe thin top sheet of n- or p-type doped material, the contact resistance, andfinally a resistive load RL 7. Fig.2.2 shows the equivalent circuit.Fig.2-2 Equivalent circuit of a solar cell including a resistive, load;72-3-2 Current–Voltage Characteristic:When thesolar cell exposed to light a constant current Isc is generated which, causes acurrent IL to flow in the load RL.
Fig.2.3 shows an I-V characteristic togetherwith the power curve. At zero voltage, the current flow Ij through the junctionis zero and IL = Isc (short circuit current). For small increase in voltage Ijremains effectively zero and the slope of I-V curve depends only on the cellshunt resistance.
If Rsh infinite, the curve would be horizontal in this region1.Fig.2.3Current-voltage characteristic together with the power curve of solar cell; 1At a certain potential however the junctionbegins to conduct current, increase exponentially with voltage, causing IL todecrease rapidly. At Voc (open circuit voltage), Ij effectively equals Isc andno current flows through the load.
In the region of knee of the curve to Voc,the slope of the I-V curve is governed by Rs, high values of Rs leads to steepslopes. The power delivered to the load at any point on the I-V curve is IxV.- Efficiency and fill factor: Efficiency of the solar cell is defined as:Where: Pout = the electrical output powerof the cell.
Pin = the input power of the cell.Ps = the solar radiation level perunit area. As = the active cell area. The maximum cell efficiency can be defined as:Where: Imp = Current at maximum powerpoint. Vmp = Voltage at maximum powerpoint.
To optimize the cell efficiency, one hasoptimized Imp and Vmp. The maximum voltage and current achievable are Voc andIsc. – Definethe fill factor (FF): The fill factor is a practical quantity to usewhen one wishes to compare the different solar cells under the same conditions.
2-4 Solar irradiation:The solar radiation of PVs depends critically on the spectraldistribution of the radiation coming from the sun 14. To good approximation,the sun acts as a perfect emitter of radiation (black body) at a temperatureclose to 5800 k. In general, the total power from a radiant source falling on aunit area is called irradiance 1.2-4-1 Calculation of Average Powerfor One PV Module: The electrical power generated and terminalvoltage of PV module depends on solar irradiance and ambient temperature.
Theequivalent electrical circuit describing the solar cells module used in the analysisis shown in Fig.2.2.15 The circuit consists of a light dependent currentsource and a group of resistances, including internal shunt resistance, Rsh,and series resistance, Rs. The series resistance should be as low as possible,but the shunt resistance should be very high, so that most of the availablecurrent can be delivered to the load. The mathematical equation describing theI-V characteristics of a PV solar cells module are given by 16:Where: I(t) : The hourly output current,Amp. V(t) : The hourly output voltage,Volt.
A : The ideality factor for p-n junction.T(t) : The hourly temperature, Kelvin.KB : The Boltzman’s constant in Joules perKelvin, 1.38*10-23 J/k.q : The charge of the electron in Coulombs,1.6*10-19 C.
Io(t) : The hourly reverse saturation current,Amp. This current varies with temperature as follows 16:Iph(t) : The hourly generated current of solarcells module. This current varies with temperature according to the followingequation 16:Where: Tr : The reference temperature,oK. Ego : The band-gap energy of thesemiconductor used in solar cells module. KI : The short circuit currenttemperature coefficient. Ior : The saturation current at Tr, Amp.
HT(t) : The average hourlyradiation on the tilted surface, kW/m2. Isc : PV cell short-circuitcurrent at 250 C and 100 mW/cm2. The hourly output of the solar cells modulecan be calculated by the following equation 20:From the above equations, it can be concludedthat the output current and power of a PV module are affected by solarinsolation and operating cell temperature.2-5 PV Cells, Modules and Arrays: PV cells are connected in series and / orparallel to produce the required voltage and current levels to form PV module,the inter connection of modules on a support structure forms what called a PVarray as shown in Fig.2.
7.Fig.2.7 PV Cells,Modules and Arrays. The PV modules represent the basicconstruction unit of the PV generator. The modules are connected in series toform strings where the number of series modules determined by the selected DCbus voltage, as shown Fig.2.8, and the number of parallel strings is given bythe required load current, as shown Fig.
2.9.1Fig.2.8 Series module.Fig.
2.9 Parallelmodule.2-6 Balance Of System equipment:In addition to the PV modules, there isbalance-of-system (BOS) equipment needed to operate the PVs.
This includesbattery charge controllers, batteries, inverters (for loads requiringalternating current), wires, conduit, earthling, fuses, safety disconnects,outlets, metal structures for supporting the modules, and any additionalcomponents that are part of the PVs 8.2-6-1 Charge Controller: The charge controller regulates the flow ofelectricity from the PV modules to the battery as will as the connected load.The controller keeps the battery fully charged without overcharging it. Whenthe controller senses that the battery is fully charged. It stop the flow ofcharge from the modules to the battery. Many controller also sense when thebatteries are over loaded, and automatically disconnects parts of the loaduntil sufficient charge is restored to the batteries. This last feature cangreatly extend the battery’s lifetime. Charge controller costs generallydepending on the ampere capacity at which PVs will operate and the monitoringfeatures required 9.
2-6-2 Battery:The battery stores electricity for use atnight or for meeting loads demand during the day when the modules are notgenerating sufficient power to meet load requirements. To provide electricityover long periods, PVs require deep-cycle batteries. These batteries aredesigned to gradually discharge and recharge 80% of their capacity hundreds oftimes. Automotive batteries are shallow-cycle batteries and should not be usedin PVs because they are designed to discharge only about 20% of their capacity,the climatic conditions in which it will operate, how frequently it willreceive maintenance, and the types of chemicals it uses to store and releaseelectricity.
A PVs may have to be sized to store a sufficient amount of powerin the batteries to meet power demand during several days of cloudy weather,this is known as days of autonomy 8,17.2-6-3 Inverter: To power an AC equipment inverteris required, which changes the DC electricity produced by PV modules and storedin batteries into AC electricity. Different types of inverters produce adifferent quality of electricity. For example, lights, television, and powertools can operate on lower-quality electricity, but computers, laser printers,and other sophisticated electronic equipment require the highest-qualityelectricity. So, matching the power quality required by the loads with thepower quality produced by the inverter is important 9. Inverters cost for most stand-aloneapplications is affected by several factors, including the quality of theelectricity it needs to produce, whether the incoming DC voltage is 12, 24, 36,or 48 volts, the AC power required, the amount of extra surge power the ACloads need for short periods, and whether the inverter has any additionalfeatures such as meters and indicator lights 17.
2-7 Types of PVs: PV power system is generallyclassified according to their functional and operational requirements and howthe equipment is connected to other power source and electrical loads. The twoprinciple classifications are either stand alone or grid connected system 1.2-7-1 Stand Alone PVs: SAS are designed to operateindependent of the electric utility grid in its simple form, it consists of thearray supplying the load directly, as shown in Fig.2.
10 such system can be usedfor battery charging via charge controller or for water pumping, where thestorage medium is a storage tank. Fig. 2-14 stand-aloneAC system with battery and back-up generator 2-8 PVs Market Overview: In 2004 more than 2700 MWp of PVwere installed worldwide and its applications as shown in Fig.
2.15. Japan hasthe highest installed capacity followed by Germany and USA. These threecountries represents about tow third global PV capacity.
Market grow rate inthe last 10 years were between 20% and 40%, and in recent decades, there was aprice reduction of 20% when the market volume doubled. As a result of this,photovoltaic prices dropped by about 50% every decade. It is not sure how longthis price reduction process will continue. However PVs have the potential tobecome comparative even with conventional grid-connected systems, in the fewdecades 11. Fig.
2.15 World PV modulesproduction, consumer and commercial (MW) 11 The cost of a PV module ismeasured in dollars per peak-watt $/Wp, where “peak watt” is definedas the power of full sunlight at sea level on a clear day. Modules are ratedusing standard test conditions which is 1000 W/m2, an air mass of 1.5 at 25 oC.Thus PV module “costreduction” is the result of either a decrease in manufacturing cost or animprovement in module efficiency.
Crystalline silicon PV module process hasdecreased from $51/Wp to approximately $3.50/Wp in 2002 11.