1.Fundamentals oF Photovoltaicsolar technology For BatteryPowered aPPlications

2. Fundamentals of Photovoltaic Solar Technology for Battery Powered ApplicationsSolar is a natural energy source for many battery powered applications. With energy harvested from the sun, the size of batteries can bereduced, or the battery life of a system can be extended. All thats needed is a solar module, right? Is it really that simple? Lets take a lookat whats possible in photovoltaic (PV) solar powered applications.In its simplest form, a solar cell is essentially a specialized p-n junction, a type of diode. When direct sunlight or indirect light is applied to thecell, photons strike the cell and collide with electrons. When electrons are impacted with enough energy, they can be elevated from the valenceband to the conduction band, and when swept across the semiconductor junction, an electric current is produced. Multiple PV technologiesare used to create solar cells crystalline silicon cells have been used for decades. Thin film solar materials are amorphous silicon, cadmiumtelluride (CdTe) and copper indium gallium diselenide (CIGS). Each technology has different efficiency, performance characteristics and cost.One unique feature of solar modules is that they are sensitive to temperature. Generally, the higher the temperature, the lower the poweroutput. In addition, solar cells degrade over time, on the order of 20 percent over the course of 20 years.Table 1. Comparing PV Technologies Capacitor Type Conversion EfficiencyDiffused/Low Light FlexibleCostCrystalline Silicon 14-18% Not as goodNoLowAmorphous Silicon 8%GoodYes MediumCIGS11% GoodYes MediumCdTe10% FairNo LowestHow to maximize energy from solar modules.For off-grid applications, the most commonly used cells are crystalline and amorphous silicon. One of the features of crystalline siliconis that its spectral response curve shows that it is most responsive to the infrared spectrum of light. Direct sunlight is needed forcrystalline silicon solar modules to be most effective. Amorphous silicon cells are less efficient than crystalline silicon (a larger area isneeded to produce the same amount of energy). However, amorphous silicon has a spectral response very similar to that of the humaneye. It produces the most energy from light in the visible spectrum. This means that amorphous silicon solar cells will produce energyfrom indirect sunlight, and even from indoor light, under conditions that will not produce much usable energy from crystalline solar cells.Figure 1. Spectral Sensitivity of PV Technologies[ 2 ] 3. Fundamentals of Photovoltaic Solar Technology for Battery Powered ApplicationsSolar panels have a characteristic I-V curve which varies depending upon irradianceand temperature. Short circuit current (Isc) and open circuit voltage (Voc) are keyoperating points shown on the chart below. As can be seen in Figure 2, there isa point on the IV curve where the panel will be generating maximum power, thisis called maximum power point voltage (VMP) or maximum power point current (IMP).In many solar applications, the system is designed to operate the panel at this point,generating maximum power. This is called maximum power point tracking (MPPT).Because of the maximum power point feature of solar modules, if the impedance ofthe load differs significantly from the impedance of the module, much of the energygenerated can be lost. Consider a system to charge a 12 V lead acid battery with atypical 5 W solar module. In this case, Voc is usually around 21 V, and VMP is around17 V. If the 12 V lead acid battery is attached directly to the solar module, the outputvoltage of the module will drop to 12 V. In Figure 2, if Voc is 21 V, and VMP is 17 V, youcan see that dropping the output of the solar module to 12 V will drop the power ofthe module by roughly 30 percent (shown as Vcharge in the figure to the left). WhatFigure 2. Solar Panel Voltage/Current andhappens to the rest of the power? It is dissipated as heat in the module, and is Power/Voltage Characteristicsnot used at all. How can all of the power available from the solar modules be used?Answer: by making sure that the module operates at the VMP using MPPT.MPPT can be performed in a variety of ways, usually through an active electronic circuit. One method is called fixed voltage. The circuit removesthe solar panel from the load, and measures Voc and Isc. The VMP is then calculated based on the datasheet of the solar module being used.This fixed voltage should be adjusted based on temperature. Another common method is to use a specialized DC-DC converter. This methodfrequently combines both MPPT and battery charging, using feedback loops both at the input to the converter from the solar module, and at theoutput to the battery. One of the most common MPPT algorithms is called Perturb and Observe. The controller measures the output power of thesolar module, and if it is higher than the previous reading, it bumps the output voltage in the same direction as the previous adjustment. Ifthe output power has decreased, it bumps the output voltage in the opposite direction. This is commonly called a hill climbing algorithm. Manyother MPPT algorithms exist, but the goal of all of them is the same to operate the panel as close as possible to the maximum power point ofthe power curve.The potential for off-grid solar applications has grown to the point where semiconductor manufacturers have designed and brought tomarket specialized devices designed to maximize energy transferred from solar panels, often in combination with a battery charger circuit.Table 2. PV MPPT and MPPT/Battery Charging ICs:PartPanelBattery BatteryMax Charge Integrated Supplier Number(s) Voltage (V) Voltage (V) ChemistryCurrent FETs?MPPT Type TopologyCommentsLi-Ion / Thin MaximMAX17710 0.8 - 5.54.125 20 mA YesNoneBoost Additional 1.8, 2.3 and 3.3 V LDO output Film BatteriesSM72442, Proprietary Buck, National Solar Magic Programmed9 - 100Programmed ProgrammedNo Programmable chipsetSM72295Algorithm Boost Provides the MPPT algorithm only NXP Proprietary Buck, MPT612 Programmed5 - 50 Programmed ProgrammedNo using an ARM Cortex-M3 core, SemiconductorsAlgorithm Boosteverything else is up to the userPerturb / Boost mode, panel voltage must STMicroelectronics SPV1040 0.3 - 51-5 Trickle Charge1A YesBoostObserve Algorithm always be lower than battery voltage Li-Ion /Single cell Li-Ion charger, can be used Texas InstrumentsBQ24210 3.5 - 74.2 800 mA YesNoneBuck Single Cellwith solar cell input TemperatureLi-Ion / Lead Acid, Buck mode, panel voltage must be Texas InstrumentsBQ24650 5 - 28 2.6 - 2610 A No Compensated BuckLi-Ion Phosphate higher than battery voltage to charge VoltageLi-Ion / ThinBoost mode, panel voltage must Texas InstrumentsBQ255040.13 - 3 2.5 - 5.25Film Batteries / 200 mA Yes Constant Voltage Boostalways be lower than battery voltageSupercapacitors [3] 4. Fundamentals of Photovoltaic Solar Technology for Battery Powered ApplicationsThere are a number of factors that have an obvious impact on the productivity of solar modules. When light strikes a solar cell, the smallerthe angle of incidence (from perpendicular), the more efficient the cell will be. If the solar module doesnt have a motorized solar tracker tostay pointed directly at the sun, then the designer must take into account that the solar module wont be producing maximum energy.For example, assume that the module is placed flat on the ground or on another surface parallel to the ground. Not only that, but itis placed in St. Louis, Missouri, in the winter, at noon, and the sun is at 37 degrees (or 53 degrees from perpendicular). Best-casescenario, the solar module will produce around 55 percent of the total amount of energy it would be capable of producing if the modulewere positioned perpendicular to the sun.Example power productionLets look at some crystalline silicon and amorphous silicon solar modules and get a feel for their sizes, and how much energy they willproduce under various conditions. Solar modules are rated for a certain power output this is called their DC nameplate rating.Nameplate ratings mean irradiance of 1000 W/m2 at 25 C at specific atmospheric conditions. Unless the module sees 1 KW/m2 at25C, it will not produce the amount of power rated on the panel.Table 3. Example PV module power and size comparisons Power Ratings Supplier Part Number Full Sun Indoor SizeApplicationsPowerfilmMPT6-150 6 V 100 mA (600 mW)1.67 mWCD case Portable electronicsPowerfilm SP3-373V 22 mA (66 MW) 140 uW2/3 Credit cardLow power sensorsSolartech Power SPM001P-12.75 V 360 mA (1 W) 0.99 mWSmart PhoneWireless sensor nodeSolartech Power SPM005P-R17.1V 290 mA (5 W)3.08 mW 8.5 x 14Solar powered lantern Figure 3. PV ModulesHow much energy can the solar module be expected to produce from a given area? Solar module efficiencies for crystalline siliconare in the range of 13 percent to 18 percent efficient. So, a 1m2 solar module will produce around 150 W. But, again, this is under testconditions of direct sunlight at 1 KW/m2.Size vs. Output The amorphous silicon MPT6-150 (600 mW) is 4.9 x 5.9, about the size of a CD case. The amorphous silicon SP3-37 (66 mW) module is about the size of a box of Tic-Tacs or, about 2/3 the size of a credit card. The 1 W crystalline silicon module is a little bigger than a large smart phone. The 5 W solar module is about the size of an 8.5 x 14 sheet of paper. A 230 W module is almost the size of a large door or the top of a large desk.Indoor applicationsWhat about indoor lighting? How much energy can be harvested indoors vs. outdoors? Office lighting seems bright enough and indoorlight does, obviously, produce a fair amount of visible light. So, the energy output should be about the same, right?The human eye is magnificent. What the human eye perceives as bright and certainly more than enough light to perform daily tasks,actually may have a very low energy level. Sunlight ranges from 50,000 Lux to 150,000 Lux. Typical office fluorescent lighting rangesfrom 200 to 400 Lux more than two orders of magnitude lower than direct sunlight. Logically then, the energy produced should be oneto two orders of magnitude less than in full sunlight.[ 4 ] 5. Fundamentals of Photovoltaic Solar Technology for Battery Powered ApplicationsIndoor solar applications are possible, however, the output voltages of modules are significantly reduced, and the current generated isseveral orders of magnitude reduced. A small, amorphous silicon solar module which produces Voc 4.5 V, Isc 48.5 mA in direct sunlight,would produce Voc 2.1 V, Isc 90 uA in an office environment at 250 Lux fluorescent lighting. So, micro-watts of power are possible, butunless a large enough solar module is used, milliwatts of power are not possible.Solar applications with a very low active duty cycle, and very low power consumption are possible in indoor applications, but mostportable consumer electronics will need direct sunlight to generate enough power for a viable solar charging solution.According to the difference in spectral responses between crystalline silicon and amorphous silicon solar cells shown in Figure 1,crystalline silicon does not work that well in indoor light, because indoor lighting produces very little light skewed to the IR part of thespectrum. For this reason, amorphous silicon modules are often preferred for indoor applications.How to size a solar panel for a given application?There are a number of realities that must be considered. Nameplate ratings are at test conditions, which are ideal in many ways. Unless thesystem is designed to track the sun across the sky, maximum energy will not be harvested. Is the system pointed at the sun at all? Is it shadedat times? Is it far north? Is it possible the solar modules may get dirty frequently? Is it located in a region where it is often cloudy? Is thereaccess to direct sunlight, or indoor light? All of these factors can significantly impact the amount of energy harvested by a solar module.What is really needed is to calculate a budget for the amount of power required by the application, and the amount of power that canrealistically be produced by the solar module given the location and condition of the module.What is really needed is to calculate a budget for the amount of power required by the application, and the amount of power that canrealistically be produced by the solar module given the location and condition of the module. Please see the article on EDN for detailedinformation on how to select solar module and battery sizes for off-grid applications: Design challenges for solar powered HBLED Lighting.Power vs. Voltage vs. BatteriesOne last thing to evaluate when choosing solar modules is the voltage required to charge the batteries. As already shown, there are anumber of ICs designed to maximize power from solar modules, and to charge batteries of various chemistries. These devices typicallyuse either buck (input voltage is higher than output voltage), or boost (input voltage is always lower than output voltage) topologies. Inchoosing a solar module (or a conversion IC), this is an important consideration.Solar is an abundant, free energy source for charging battery powered applications, however solar modules must be carefully selectedand managed in order to harvest the maximum amount of energy possible.For additional articles and design resources, please visit http://em.avnet.com/solarCopyright 2012, Avnet, Inc. All rights reserved. Avnet, Inc. disclaims any proprietary interest or right in any trademarks, service marks, logos, domain names, companynames, brands, product names, or other form of intellectual property other than its own. AVNET and the AV logo are registered trademarks of Avnet, Inc. While every effort ismade to ensure the information given is accurate, Avnet, Inc. disclaims liability for any errors or mistakes that may arise. Specifications subject to change without notice. [5] 6. about the AuthorHeather Robertson is a technology director at AvnetElectronics Marketing, with responsibilities for Solar Powerand Smart Energy. As Technology Director, Heather worksto accelerate customers solar power and smart energydesigns through differentiated supplier solutions, referencedesigns, seminars and customer training classes. Heatherhas held a number of management and design positionsin digital and mixed signal ASIC design in CMOS, BiCMOSand GaAS technologies, completing more than 20 customdesigns. She has managed ASIC design teams in the US,and in Munich, Germany. Heather is a graduate of theUniversity of Illinois at Urbana-Champaign with a BSEE.Heather RobertsonTechnology DirectorAvnet Electronics Marketingheather.robertson@avnet.com[ 6 ]