C H A P T E R 3

Photovoltaic System ConversionLana El Chaar Electrical Engineering Department, The Petroleum Institute, Abu Dhabi,

UAE

Abstract

This chapter discusses the conversion of solar energy into electricity usingphotovoltaic system. There are two types of PV systems: the grid‑connected PV system and the stand‑alone PV system. Solar cells arecomposed of various semiconductor materials that become electricallyconductive when supplied with heat or light. The voltage generated bythe array depends primarily on the design and materials of the cell,whereas the electric current depends primarily on the incident solarirradiance and the cell area. Photovoltaic technology is used to produceelectricity in areas where power lines do not reach. In developingcountries, it helps improving living conditions in rural areas, especially inhealth care, education, and agriculture. Many controllers have the abilityto sense the excess of electricity drawn from batteries to the load and stopthe flow until sufficient charge is restored to the batteries. The latter cangreatly extend the battery’s lifetime. The choice of the PCU has a greatimpact on the performance and economics of the system. It depends onthe type of waveform produced, which in turn depends on the methodused for conversion, as well as the filtering techniques of unwantedfrequencies.

Keywords

Photovoltaic system

Solar cell

Maximum power point tracker

Perturb and observe

Incremental conductance technique

Battery for PV system

Standalone PV system

Grid‑connected PV system

CH A P T E R O U T L I N E

3.1 Introduction 155

3.2 Solar Cell Characteristics 156

3.3 Photovoltaic Technology Operation 160

3.4 Maximum Power Point Tracking Components 161

3.4.1 Voltage Feedback Control 162

3.4.2 Power Feedback Control 162

3.5 MPPT Controlling Algorithms 162

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3.5.1 Perturb and Observe (PAO) 162

3.5.2 Incremental Conductance Technique (ICT) 163

3.5.3 Constant Reference 164

3.5.4 Current‑Based Maximum Power Point Tracker 164

3.5.5 Voltage‑Based Maximum Power Point Tracker 165

3.5.6 Other Methods 165

3.6 Photovoltaic Systemsʹ Components 166

3.6.1 Grid‑Connected Photovoltaic System 166

3.6.2 Stand‑Alone Photovoltaic Systems 169

3.7 Factors Affecting PV Output 170

3.7.1 Temperature 171

3.7.2 Dirt and Dust 171

3.7.3 DC–AC Conversion 171

3.8 PV System Design 171

3.8.1 Criteria for a Quality PV System 171

3.8.2 Design Procedures 171

3.8.3 Power‑Conditioning Unit 172

3.8.4 Battery Sizing 172

Summary 172

References 172

3.1 IntroductionFor many years, fossil fuels have been the primary source of energy.However, due to the limited supply, the rate of deployment of fossil fuelsis more rapid than their rate of production, and hence, fossil fuels willeventually run out. Moreover, the threat of global climate change causedby carbon dioxide (CO ) emissions from fossil fuels is one of the mainreasons for the increasing consensus to reduce the consumption of suchfuels. This reduction can be achieved by switching to renewable energy formany energy­requiring applications, since it is “clean” and “green.”Today, the global trend is to use nondepletable clean source of energy fora healthier and greener environment to save the future generation. Themost efficient and harmless energy source is probably solar energy, whichis so technically straightforward to use in many applications. Almost, allrenewable energy sources, except nuclear and geothermal, are the energyforms originating from the solar energy.

Solar energy is considered one of the most promising energy sources dueto its infinite power. Thus, modern solar technologies have beenpenetrating the market at faster rates, and photovoltaic (PV) technologythat has the greatest impact, not because of the amount of electricity itproduces but because photovoltaic cells – working silently, not polluting –can generate electricity wherever sun shines, even in places where noother form of electricity can be obtained [1]. PV is a combination of theGreek word for light and the name of the physicist Alessandro Volta [2].PV is the direct conversion of sunlight into electricity by means of solarcells.

This chapter will highlight in brief how solar cells produce electricity andwill discuss in detail the various techniques available to track the sun inorder to maximize the output power generated by the PV array. Moreover,the various components required to operate PV systems efficiently will bedescribed.

3.2 Solar cell characteristicsSolar cells are composed of various semiconductor materials that becomeelectrically conductive when supplied with heat or light. The majority ofthe first­generation solar cells produced are composed of silicon (Si),which exists in sufficient quantities. However, more than 95% of thesecells have power conversion efficiency about 17% [4], whereas solar cellsdeveloped over the last decade in laboratory environment have efficiencyas high as 31% [5]. All technologies related to capturing solar energy to beused as direct electricity generator are described as photovoltaictechnology, which is subdivided into crystalline, thin film, andnanotechnology.

Doping technique is used to obtain excess of positive charge carriers (p­type) or a surplus of negative charge carriers (n­type). When two layers ofdifferent doping are in contact, then a p­n junction is formed on theboundary.

An internal electric field is built up causing the separation of chargecarriers released by light, freeing electrons within the electric field

2

(3.2)

(3.3)

(3.1)

(3.4)

proximity, which then pull the electrons from the p­side to the n­side (Fig.3.1). The primary solar cell equivalent circuit (Fig. 3.2) contains a currentsource with a parallel diode, in addition to parasitic series (R ; normallysmall) [6] and shunt (R ) resistances (relatively large) [7]. R is mainlyaffected by the factors such as the bulk resistance of the semiconductormaterial, metallic contacts, and interconnections, whereas R is affectedmainly by the p­n junction nonidealities and impurities near the junction[8].

FIGURE 3.1 Effect of the Electric Field in a PVCell [3].

FIGURE 3.2 Solar cell equivalent circuit [8].

A simplified equivalent circuit is shown in Fig. 3.3.

FIGURE 3.3 Model for a PV cell [9].

The diode current is given by the Shockley equation:

where I is the reverse saturation current, q is the charge carrier, k is theBoltzman constant, T is the cell temperature, and n is the ideality factor.

The PV module has two limiting components (Fig. 3.3): open­circuitvoltage (V ) and short­circuit current (I ). To determine I , set V = 0 andI = I Eq. (3.4), and this value changes proportionally to the cellirradiance. To determine V , set the cell current I = 0, hence Eq. (3.3)leads to

The PV module can also be characterized by the maximum point when theproduct (V (voltage, where power is maximum) × I (current, wherepower is maximum)) is at its maximum value. The maximum poweroutput is derived by

and

s

sh s

sh

0

oc sc sc

sc ph

oc L

mp mp

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(3.5)

(3.6)

(3.7)

(3.8)

(3.9)

(3.10)

A PV module is normally rated using its W , which isnormally 1 kW/m under standard test conditions (STC), which definesthe PV performance at an incident sunlight of 1000 W/m , a celltemperature of 25°C (77°F), and an air mass (AM) of 1.5. The product (V× I ) is related to the product generated by (V × I ) by a fill factor (FF)that is a measure of the junction quality and series resistance, and it isgiven by

The closer the FF is to unity, the higher the quality of the PV module.

Finally, the last and most important factor of merit for a PV module is itsefficiency (η), which is defined as

P represents the incident power depending on the light spectrumincident on the PV cell.

To achieve the desired voltage and current levels, solar cells are connectedin series (N ) and parallel (N ) combinations forming a PV module. ThePV parameters are then affected as shown below [9]:

This model is shown in Figure 3.4.

FIGURE 3.4 PV module circuit model.

In order to obtain the appropriate voltages and outputs for differentapplications, single solar cells are interconnected in series (for largervoltage) and in parallel (for larger current) to form the photovoltaicmodule. Then, several of these modules are connected to each other toform the photovoltaic array. This array is then fitted with aluminum orstainless steel frame and covered with transparent glass on the front side(Fig. 3.5).

FIGURE 3.5 Photovoltaic cells, modules,panels, and array [10].

The voltage generated by the array depends primarily on the design andmaterials of the cell, whereas the electric current depends primarily onthe incident solar irradiance and the cell area. This current fluctuatessince the path of the sun varies dramatically over the year, with winterand summer seasons being the two extreme excursions. The elevation

p

mp

mp OC SC

in

s p

2

2

(3.11)

angle of the sun ( ) is expressed in degrees above the

horizon. Azimuth angle ( ) of the sun is expressed in degrees from

true north. Zenith angle ( ) of the sun equals 90 degrees less thanthe elevation angle of the sun, or

Azimuth, zenith, and elevation angles are illustrated inFig. 3.6

FIGURE 3.6 Azimuth, zenith, and elevationangles of a vector pointed toward the sun [11].

The output from a typical solar cell that is exposed to the sun, therefore,increases from zero at sunrise to a maximum at midday, and then fallsagain to zero at dusk. The radiation of the sun varies when reaching thesurface of the earth due to absorption and scattering effect in the earth'satmosphere. PV system designers require the estimate of the insolationexpected to fall on a randomly tilted surface, hence need a goodevaluation of global radiation on a horizontal surface, horizontal directand diffuse components, in order to estimate the amount of irradiationstriking a tilted plane.

3.3 Photovoltaic technology operationPhotovoltaic technology is used to produce electricity in areas wherepower lines do not reach. In developing countries, it helps improvingliving conditions in rural areas, especially in health care, education, andagriculture. In industrialized countries, such technology has been usedextensively and integrated with the utility grid.

Photovoltaic arrays are usually mounted in a fixed position and tiltedtoward the south to optimize the noontime and the daily energyproduction. The orientation of fixed panel should be carefully chosen tocapture the most energy for the season, or for a year. Photovoltaic arrayshave an optimum operating point called the maximum power point(MPP) as shown in Fig. 3.7 [12].

FIGURE 3.7 (a) I–V characteristic of a solar cellshowing maximum power point (MPP); (b) P–Vcharacteristic of a solar cell showing MPP.

It is noted that power increases as voltage increases, reaching a peak valueand decreases as the resistance increases to a point where current dropsoff. According to the maximum power transfer theory, this is the pointwhere the load is matched to the solar panel's resistance at a certain levelof temperature and insolation. The I–V curve changes as the temperatureand insolation levels change as shown in Fig. 3.8 and thus the MPP willvary accordingly [13].

FIGURE 3.8 (a) PV panel insolationcharacteristics; (b) PV panel temperaturecharacteristics [13].

It is shown that the open­circuit voltage increases logarithmically whilethe short­circuit current increases linearly as the insolation level increases[14]. Moreover, increasing the temperature of the cell decreases the open­circuit voltage and increases slightly the short­circuit current, causingreduction in the efficiency of the cell.

The PV panels, usually mounted on the roof or at a near open area, are

fixed to face the sun at an angle matching the country's latitude. Ifpossible, seasonal adjustment of the module's direction toward the sun isdone manually. Since solar power technology is relatively expensive, it isimportant to operate panels at their maximum power conditions.However, to collect as much solar radiation as possible, it is moreconvenient and efficient to use a sun tracking mechanism causing themodule's surface to track the sun throughout the day.

The tracking can be along either one axis or two axes, whereby doubleaxes tracking provides higher power output. The energy yield can be thusincreased by about 20% to 30% depending on the seasonal climate andgeographical location [15–17]. Although some claim that a fixed systemcosts less and requires almost no maintenance [18], different trackingmechanisms utilized to control the orientation of the PV panels haveproved their superiority over fixed systems in terms of converted powerefficiency.

To get maximum power from the PV panel at the prevailing temperatureand insolation conditions, either the operating voltage or current shouldbe controlled by a maximum power point tracker (MPPT) that shouldmeet the following conditions [19]:

• Operate the PV system close to the MPP irrespective of the atmosphericchanges.

• Have low cost and high conversion efficiency.

• Provide an output interface compatible with the battery­chargingrequirement.

3.4 Maximum power point tracking componentsThe MPPT increases the energy that can be transferred from the array toan electrical system. The main function is to adjust the panel's outputvoltage to supply the maximum energy to the load. Most current designsconsist of three basic components: switch­mode dc–dc converter, controlsystem, and tracking component.

The switch­mode converter is the core of the entire supply because theenergy drawn, stored as magnetic energy, is released at different potentiallevels. By setting up the switch­mode section in various topologies such asbuck or boost converter, voltage converters are designed providing a fixedinput voltage or current, which correspond to the maximum power point,allowing the output resistance to match the battery. To achieve the above­stated mechanism, a controller is essential to continuously monitor thePV system and ensure its operation at the PV maximum power point bytracking this MPP. The controller's aim is to continuously measure thevoltage and current values generated from the PV, and compare them tocertain treshhold values in order to apply either voltage controlledmethod or power feedback control [20].

3.4.1 Voltage feedback control

With the PV array terminal voltage being the controlled variable, voltagefeedback controller forces the PV array to operate at its MPP by changingthe array terminal voltage and neglecting the variation in the temperatureand insolation level [20,21].

3.4.2 Power feedback control

In this method, power delivered to the load is the controlled variable. Toachieve maximum power, dp/dv should be zero. This control scheme isnot affected by the characteristics of the PV array, yet it increases powerto the load and not power from the PV array [20,21]. Factors such as fastshadows may cause trackers to lose the MPP momentarily. It is verycritical to ensure that the time lost in seeking MPP again, which equatesthe energy lost while the array is off power point, is very short. On theother hand, if lighting conditions do change, the tracker needs to respondwithin a short amount of time to the change avoiding energy loss.Therefore, the controller's most important feature is its capability toquickly adjust the system to operate back at the MPPT.

3.5 MPPT Controlling algorithmsSeveral proposed algorithms to accomplish MPPT are described in thefollowing sections.

3.5.1 Perturb and observe (PAO)

The PAO method has a simple feedback structure and few measuredparameters. It operates by periodically perturbing (i.e. incrementing ordecrementing) the duty cycle while controlling the array current as shownin Fig. 3.9 and comparing the PV output power with that of the previousperturbation cycle. It measures the derivative of power Δp and thederivative of voltage Δv to determine the movement of the operatingpoint. If the perturbation leads to an increase (or decrease) in arraypower, the subsequent perturbation is made in the same (or opposite)direction. This cost­effective technique can be easily implemented and ischaracterized by continuously tracking and very efficiently extracting themaximum power from PV. However, such method may fail under rapidlychanging atmospheric conditions due to its slow tracking speed.

(3.12)

FIGURE 3.9 PAO technique [19].

3.5.2 Incremental conductance technique (ICT)

The ICT process based on the fact that the derivative of the power withrespect to the voltage (dp/dv) vanishes at the MPP because it is themaximum point on the curve as shown in Fig. 3.10.

FIGURE 3.10 The slope “conductance” of theP–V curve [22].

The ICT algorithm detects the MPP by comparing di/dv against tillit attains the voltage operating point at which the incrementalconductance is equal to the source conductance [23,24]. The Reference[23] describes in detail the ICT algorithm used for maximum power pointtracking. The algorithm starts by measuring the present values of the Iand V, then uses the corresponding stored value (I and V ) measuredduring the preceding cycle to calculate the incremental changes as: dI = I− I and dV = V − V . Based on the result obtained, the control referencesignal V will be adjusted in order to move the array voltage toward the

MPP voltage. At the MPP, , no control action is needed;therefore, the adjustment stage will be bypassed and the algorithm willupdate the stored parameters at the end of the cycle. In order to detectany changes in weather conditions, the algorithm detects whether acontrol action took place when the array was operating at the previouscycle MPP (dv = 0). This technique is accurate and well suited for rapidchanges in atmospheric conditions; however, because the increment sizeapproach is used to determine how fast the system is responding, ICTrequires precise calculations of both instantaneous and increasingconductance.

3.5.3 Constant reference

One very common MPPT technique is to compare the PV array voltage (orcurrent) with a constant reference voltage (or current), which correspondsto the PV voltage (or current) at the maximum power point, under specificatmospheric conditions. The resulting difference signal (error signal) isused to drive a power conditioner, which interfaces the PV array to theload. Although the implementation of this method is simple, the methoditself is not very accurate because it does not consider the effects oftemperature and irradiation variations in addition to the difficulty inchoosing the optimum point [19].

3.5.4 Current-based maximum power point tracker

Current­based maximum power point tracker (CMPPT) is another MPPTtechnique that exists [22]. Employed numerical methods show a lineardependence between the “cell currents corresponding to maximumpower” and the “cell short­circuit currents.” The current I operating atthe MPP is calculated using the following equation:

where M is the “current factor” that differs from one panel to anotherand is affected by the panel surface conditions, especially if partialshading covers the panel [25]. Although this method is easy to implement,additional switch is added to the power converter to periodically short thePV array, increase the cost, and reduce the output power. This methodalso suffers from a major drawback due to periodic tuning requirement.

3.5.5 Voltage-based maximum power point tracker

Similar to the above­mentioned method, voltage­based maximum powertracking (VMPPT) technique can also be applied [22]. The MPP operatingvoltage is calculated directly from V

b b

b b

ref

MPP

C

OC

(3.13)

where M is the “voltage factor.” The open­circuit voltage V is sampledby an analogue sampler, and then V is calculated by Eq. (3.13). Thisoperating V voltage is the reference voltage for the voltage control loopas shown in Fig. 3.11. This method always “results in a considerable powererror because the output voltage of the PV module only follows theunchanged reference voltage during one sampling period” [9]. Albeit theimplementation of this procedure is simple, it endures severaldisadvantages such as momentarily power converter shutdown causingpower loss. Furthermore, such process depends greatly on the I–Vcharacteristics and requires periodic tuning.

FIGURE 3.11 The conventional MPPTcontroller using open­circuit voltage V [26].

Other researchers argue that these two practices are considered to be“fast, practical, and powerful methods for MPP estimation of PVgenerators under all insolation and temperature conditions” [27].

3.5.6 Other methods

Automated techniques such as Fibonacci line search, ripple correlationcontrol method, neural network, and fuzzy logic have also beenintroduced for MPPT. In order to generate a clear understanding indetermining the advantages and disadvantages of each algorithm, acomprehensive experimental comparison between different MPPTalgorithms was made and run for the same PV setup at South DakotaState University [28], and results showed that the ICT method has thehighest efficiency of 98% in terms of power extracted from the PV array,the PAO technique has the efficiency of 96.5%, and finally, the constantvoltage method has the efficiency of 88%.

The ICT method provided good performance under rapidly changingweather conditions and provided the highest tracking efficiency, althoughfour sensors were required to perform the measurements forcomputations and decision making [23]. If the system required moreconversion time in tracking the MPP, a large amount of power loss wouldoccur [20]. On the contrary, under perturb and observe method, lossesare reduced if the sampling and execution speed were increased. Themain benefit of this procedure is that only two sensors are required, whichresulted in the reduction of hardware requirements and cost.

3.6 Photovoltaic systems' componentsOnce the PV array is controlled to perform efficiently, a number of othercomponents are required to control, convert, distribute, and store theenergy produced by the array. Such components may vary depending onthe functional and operational requirements of the system. They mayrequire battery banks and controller, dc–ac inverters, in addition to othercomponents such as overcurrent, surge protection and, other processingequipment. Fig. 3.12 shows a basic diagram of a photovoltaic system andthe relationship with each component.

FIGURE 3.12 Major photovoltaic systemcomponents [8].

Photovoltaic systems are classified into two major classes: grid­connectedphotovoltaic systems and stand­alone photovoltaic systems.

3.6.1 Grid-connected photovoltaic system

Grid­connected photovoltaic systems are composed of PV arraysconnected to the grid through a power conditioning unit and are designedto operate in parallel with the electric utility grid as shown in Fig. 3.13.The power conditioning unit may include the MPPT, the inverter, the gridinterface as well as the control system needed for efficient systemperformance [29] There are two general types of electrical designs for PVpower systems: systems that interact with the utility power grid as shownin Fig. 3.13a and have no battery backup capability, and systems thatinteract and include battery backup as well as shown in Fig. 3.13b. Thelatter type of system incorporates energy storage in the form of a batteryto keep “critical load” circuits operating during utility outage. When anoutage occurs, the unit disconnects from the utility and powers specificcircuits of the load. If the outage occurs in daylight, the PV array is able toassist the load in supplying the loads.

FIGURE 3.13 Grid­Connected PV system.

The major component in both systems is the DC­AC inverter or also calledthe power conditioning unit (PCU). The inverter is the key to thesuccessful operation of the system, but it is also the most complex

V OC

MPP

MPP

oc

hardware. The inverter requirements include operation over a wide rangeof voltages and currents and regulated output voltage and frequency whileproviding AC power with good power quality which includes low totalharmonic distortion and high power factor, in addition to highest possibleefficiency for all solar irradiance levels. Several interconnection circuitshave been described in [30,31]. Inverters can be used in a centralizedconnection (Fig 3.14a for the whole array of PV or each PV module stringis connected to a single inverter (Fig. 3.14b [29]. The second proposedprocedure is more efficient since it minimizes the losses due tovoltage/current mismatching as well as it enhances it modularitycapability. Moreover, the inverter may contain protective devices thatmonitor the grid and islands the grid from the PV system in case of faultoccurrence [32].

FIGURE 3.14 Grid­Connected PV system.

For the last twenty years, researchers have been working on developingdifferent inverter topologies that satisfy the above listed requirements.The evolution of solid state devices such as Metal Oxide semiconductorField Effect Transistors (MOSFETs), Insulated Gate Bipolar Transistors(IGBTs), microprocessors, PWM integrated circuits have allowedimprovements on the inverter. However, more research is being carried toensure quality control, reliability and lower cost since inverters are thekey for a sustainable photovoltaic market.

The main advantage of PV systems is their flexibility to be implemented inremote locations where grid connection is either impossible or veryexpensive to execute. Such systems are called stand­alone PV systems andare described in the following section.

3.6.2 Stand-alone photovoltaic systems

Stand­alone photovoltaic systems are usually a utility power alternate.They generally include solar charging modules, storage batteries, andcontrols or regulators as shown in Fig. 3.15. Ground or roof­mountedsystems will require a mounting structure, and if ac power is desired, aninverter is also required. In many stand­alone PV systems, batteries areused for energy storage as they may account for up to 40% of the overallstand­alone PV system cost over its lifetime [33].

FIGURE 3.15 Diagram of stand­alone PVsystem with battery storage power DC and ACloads [8].

These batteries cause losses in the PV system due to limited availability oftime and energy to recharge the battery in addition to the insufficientbattery maintenance. Hence, a charge controller is then used to controlthe system and prevent the battery from overcharging andoverdischarging. Overcharging shortens the battery life and may causegassing while undercharging may lead to sulphation and stratification,which result in the reduction in battery effectiveness and lifetime [34–37].

Batteries are often used in PV systems for storing energy produced by thePV array during daytime and supplying it to electrical loads as needed(during nighttime or cloudy weather). Moreover, batteries are also neededin the tracker systems to operate at MPP in order to provide electricalloads with stable voltages. Nearly, most of the batteries used in PVsystems are deep cycle lead­acid batteries [38]. These batteries havethicker lead plates that make them tolerate deep discharges. The thickerthe lead plates, the longer the life span of the batteries. The heavier thebattery for a given group size, the thicker the plates and the better thebattery will tolerate deep discharges [39].

All deep cycle batteries are rated in ampere­hour (AH) capacity, aquantity of the amount of usable energy it can store at nominal voltage[40]. A good charge rate is approximately 10% of the total capacity of thebattery per hour. This will reduce the electrolyte losses and the damage tothe plates [38]. A PV system may have to be sized to store a sufficientamount of power in the batteries to meet power demand during severaldays of cloudy weather, known as “days of autonomy.” The Institute ofElectrical and Electronics Engineers (IEEE) has set several guidelines andstandards for sizing lead­acid batteries (IEEE Std 1013–1990) [41], forselecting, charging, and testing in stand­alone PV systems (IEEE Std1361–2003) [42], and for installing and maintaining them (IEEE Std937–2007) [43].

Nickel–cadmium batteries are also used for PV stand­alone systems butare often expensive and “may have voltage compatibility issues withcertain inverters and charge controls” [44]. However, their mainadvantage is they are not affected by temperature as other battery types,hence mostly recommended for industrial or commercial applications incold locations. IEEE has also drafted some guidelines for installation andmaintenance (IEEE Std 1145–1999) [45].

To extend battery's lifetime and for efficient system's operation, a chargecontroller is needed to regulate the flow of electricity from the PVmodules to the battery and the load. The controller keeps the battery fullycharged without overcharging it. Many controllers have the ability tosense the excess of electricity drawn from batteries to the load and stop

the flow until sufficient charge is restored to the batteries. The latter cangreatly extend the battery's lifetime. However, controllers in stand­alonephotovoltaic system are more complex devices that depend on batterystate of charge, which in turn depends on many factors and is difficult tomeasure. The controller must be sized to handle the maximum currentproduced. Several characteristics should be considered before selecting acontroller such as adjustable set­points including high­voltage and low­voltage disconnects, temperature compensation, low­voltage warning, andreverse current protection. Moreover, the controller should ensure that nocurrent flows from the battery to the array at night.

3.7 Factors affecting PV outputPV systems produce power in proportion to the intensity of sunlightstriking the solar array surface. Thus, there are some factors that affectthe overall output of the PV system and are discussed below.

3.7.1 Temperature

Output power of a PV system decreases as the module temperatureincreases. For crystalline modules, a representative temperaturereduction factor suggested by the California Energy Commission (CEC) is89% in the middle of spring or in a fall day, under full­light conditions.

3.7.2 Dirt and dust

Dirt and dust can accumulate on the solar module surface, blocking someof the sunlight and reducing the output. A typical annual dust reductionfactor to use is 93%. Sand and dust can cause erosion of the PV surface,which affects the system's running performance by decreasing the outputpower to more than 10% [46–49].

3.7.3 DC–AC Conversion

Because the power from the PV array is converted back to ac as shownearlier, some power is being lost in the conversion process, in addition tolosses in the wiring. Common inverters used have peak efficiencies ofabout 88–90%.

3.8 PV System designThe goal for a solar direct electricity generation system or photovoltaicsystem is to provide high­quality, reliable, and green electrical power.

3.8.1 Criteria for a quality PV system

The criteria for quality PV system are as follows:

• Be properly sized and oriented to provide electrical power and energy

• Good control circuit to reduce electrical losses, overcurrent protection,switches, and inverters

• Good charge controller and battery management system, should thesystem contain batteries

3.8.2 Design procedures

The first task in designing a PV system is to estimate the system's load.This is achieved by defining the power demand of all loads, the number ofhours used per day, and the operating voltage [50]. From the loadampere­hours and the given operating voltage for each load, the powerdemand is calculated. For a stand­alone system, the system voltage is thepotential required by the largest load. When ac loads dominate, the dcsystem voltage should be chosen to be compatible with the inverter input.

3.8.3 Power-conditioning unit

The choice of the PCU has a great impact on the performance andeconomics of the system. It depends on the type of waveform produced,which in turn depends on the method used for conversion, as well as thefiltering techniques of unwanted frequencies. Several factors must beconsidered when selecting or designing the inverter:

• The power conversion efficiency

• Rated power

• Duty rating, the amount of time the inverter can supply maximum load

• Input voltage

• Voltage regulation

• Voltage protection

• Frequency requirement

• Power factor

3.8.4 Battery sizing

The amount of battery storage needed depends on the load energydemand and on weather patterns at the site. There is always a trade­offbetween keeping cost low and meeting energy demand.

SummaryThis chapter discussed the conversion of solar energy into electricity using

photovoltaic system. There are two types of PV systems: the grid­connected PV system and the stand­alone PV system. All majorcomponents for such systems have been discussed. Maximum powerpoint tracking is the most important factor in PV systems to provide themaximum power. For this reason, several tracking systems have beendescribed and compared. Factors affecting the output of such systemshave been defined and steps for a good and reliable design have beenconsidered.

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