EELE408 Photovoltaics Lecture 20: Photovoltaic Systems PDF

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EELE408 PhotovoltaicsSeveral types of operating modesLecture 20: Photovoltaic Systems Stand Alone systemsDr. Todd J. [email protected]– No grid connection needed or wanted Distributed Grid tied– Small residential type systemsDepartment of Electrical and Computer EngineeringMontana State University - Bozeman Centralized power plant– Large PV system located in an optimum location, feeding into the grid2Application AreasPhotovoltaic System Basics Photovoltaic Systems– Cell Panel Array– Balance of System (BOS) Mounting Structures Storage Devices Power ConditionersPV Panel– Load / DC ACBatteryDCDC Load3Modularity: Solar Cell to ArrayChargeRegulatorACAC Load4Specifications of PV Modules Type– c:Si, a-Si:H, CdTe Rated Power Max: Pmax (Wp) Rated Current: IMPP (A) Rated Voltage: VMPP (V) Short Circuit Current: ISC (A) OpenOCircuitCi it VVoltage:ltVOC (V) Configuration (V) Cells per Module (#) Dimensions (cm x cm) Warranty (years)CellModule or PanelInverterArray Cell (c-Si 10 10 cm2 η 15% P 1.5Wp V 0.5V I 3A) Solar panel (36 c-Si cells P 54Wp I 3A V 18V ) Solar array561

Storage Devices (Batteries)Power Conditioners (Inverters) Advantages– Back up for night and cloudy days Disadvantages– Decreases the efficiency of PV system– Only 80% of energy stored retainable– Adds to the expense of system– Finite Lifetime 5 - 10 years– Added floor space, maintenance, safety concerns Limit Current and Voltage to Maximize PowerConvert DC Power to AC PowerMatch AC Power to Utilities NetworkProtect Utility Workers during Repairs7Simple DC8Small DC Direct Powering of Load No Energy Storage Home and Recreational UseCharge RegulatorDCDC LoadDCSingle PanelSingle Battery9Large DC10Large AC/DC Both AC and DC loads Home and Recreational Use Industrial UseDCCharge RegulatorCharge RegulatorDC Load / DCInverterDC LoadACAC LoadMultiple PanelsMultiple PanelsMultiple BatteriesMultiple Batteries11122

Utility Grid ConnectedHybrid System No On-Site Energy Storage Supplement GeneratorInverter / ACDCCharge RegulatorDC LoadAC LoadInverter / AC LoadAC Generator(Wind turbine)Multiple PanelsMultiple PanelsACElectric GridMultiple Batteries1314Photovoltaic System DesignBlock DiagramPV System Design Rules 1. Determine the total load current and operational time 2. Add system losses 3. Determine the solar irradiation in daily equivalent sunhours (EHS) 4. Determine total solar array current requirements5 Determine optimum module arrangement for solar 5.array 6. Determine battery size for recommended reserve timeBatteryAC LoadPhotovoltaicPht lt iGeneratorPowerConditioningDC LoadBack‐upGeneratorGridNot all the subsystems will be necessary15Direct PV driven System16Stand Alone DC SystemBatteryPhotovoltaicPht lt iGeneratorPowerConditioningPhotovoltaicPht lt iGeneratorPowerConditioningDC LoadDC LoadExample: Small Consumer DevicesExample: Attic Fans17183

Grid Tied SystemGrid Tied with Battery backupBatteryAC LoadAC LoadPowerConditioningPhotovoltaicPht lt iGeneratorPowerConditioningPhotovoltaicPht lt iGeneratorDC LoadGridGridNot all the subsystems will be necessaryExample: Most Home Systems19Remote Area Power System20Batteries Requirements– Long life– Very low selfdischarge– Long duty cycle– HighHi h chargehstoragetefficiency– Low cost– Low maintenanceBatteryAC LoadPowerConditioningPhotovoltaicPht lt iGeneratorDC LoadBack‐upGenerator Efficiencies– Coulombic or Charge Amount of charge ableto retrieve from thebattery (85%)– Voltage Charge is retrievedfrom battery at a lowervoltage than inputvoltage (85%)– Energy Product of coulombicand voltage efficiencies(72%)Example: remote cabin21Batteries 2 Power rating– Maximum rate of chargeand discharge (Amps) Battery capacity– Maximum amount ofenergy that can beextracted from thebattery without thebattery voltage fallingbelow a prescribed value– Ampere-hrs at aconstant discharge rate22Lead Acid Batteries Depth of Discharge– Percentage of the ratedcapacity withdrawn fromthe battery– Battery life is a functionof the averageg state ofcharge of the battery– Trade-off betweencycling depth ofdischarge and size ofbattery and lifetime23Most common used in home system244

BEV and HomeEnergy Management25Power Conditioning and RegulationNickel-Cadmium BatteriesUsed in household appliances Advantages––––––––Can be overchargedCan be fully dischargedMore ruggedExcellent low temperatureperformanceLow internal resistancesUniform voltage overdischargeLong lifeLow maintenance26 Diodes Disadvantage– Bypass: prevents hot-spotting and module performance reduction– Blocking: protects the battery from short circuits in the array and preventbattery from discharging through the solar cells when not illuminated– More expensive (2-3x)– Lower charge storageefficiencies (60-70%)– Memory,Miinabilitybilit ttodeep discharge– Much lower capacity Battery Voltage Regulators or Charge Controllers27Power Conditioning and Regulation28Charge Controllers Battery Voltage Regulators or Charge ControllersConfigurations– Shuts down the load when the battery reaches a prescribed state ofdischarge– Shuts down the PV array when the battery is fully tLinearWhole ArraySwitching29On‐OffPWMSub‐array Switching305

Shunt ControllerSeries Variable resistant element in parallel with the battery usually aMOSFET or BJT As resistance is reduced more current from array is divertedthrough resistor and less through the battery May need to dissipate a large amount of power Regulator placed in series with battery Dissipates much less power– Low voltage across regulator when charging– Low current through regulator when chargedLoad Disconnect SwitchLoad Disconnect SwitchRegulatorBatteryLoadPV ArrayLoadBatteryRegulatorPV Array31Series Pulse Width Modulation Switch set to open at batteryvoltage and close at anotherbattery voltage32Sub-array SwitchingBatteryVoltage Used in large systems Array disconnected as chargingcurrents increase towardsmidday Reconnected as chargingcurrents fall later in the dayControltimeChargeCurrenttimeLoad Disconnect SwitchPulse PV ArrayPV Array3334Sizing ProcedureInverter1PanelInclinationDaily Solar Radiation2DataA4LoadSpecifications5Daily Load Profile Convert power from DC to AC– 12, 24, 48 VDC to 110 VACAvailable Daily Solar Energy36Load Power Demand Self Commutating Fixed Frequency Inverter:– use of transistor switches to reverse polarity of supply under internalcontrolAverage Load Current9 Line Commutated Fixed Frequency Inverter:Energy balance on anaverage day10– uses thyristor switches that require an alternating load voltage from anexternal source (Grid) to turn switches off Inverter failure remains one of the primary causes of PVsystem failureDC Operating Voltage7Numberb offmodules inseriesBArray oversize factor11Winter Charge Deficit1435Add climatic chargedeficit158Daily Charge Deficit13Number of parallelstrings12CStorage ChargeCapacity16Seasonal & climaticenergy storage 17DTotal EnergyStorage186

A.B. Number of Series-connected ModulesInput to the sizing procedure1PanelInclinationDaily Solar Radiation2Data4LoadSpecifications5Daily Load ProfileNumber ofmodules inseriesDC Operating VoltageAvailable Daily Solar Energy376Load Power Demand8 7. The DC operating bus bar voltage VDC of the system isspecified. (For home systems it is usually a multiple of 12V) 8. The number (Ns) of modules connected in series is directlyd tdeterminedi dbby ththe DC operatingti voltagelt 1-3. Determine the energy input– The radiation data for the site , along with the panel orientation are usedt determinetod ti theth incidenti id t solarl radiationdi ti on ththe panell ffor a ttypicali lddayfor every month of the year– Vm is the operating voltage of one module– Usually 36 cells in string for a 12V system 4-6. Determine the load demandNs – The load specification or typical load for a similar system– Allow for battery efficiencies– f: fraction of load stored in battery before useVDCVm battery1 f 1 battery 37C. Number of parallel stringsAverage Load Current9C. Number of parallel stringsEnergy balance on anaverage day10E L Wh / day PSH I PV VDC I PV 24 I LPSH The average load current x number of hours in a day nominal PV current x number of peak solar hours 11,12. The number of modules in parallel is calculated by:EL Wh / day Typical power requirement24VDC 10. Define the nominal current required from the PV generatorwhen irradiated with AM1.5 (1kW/m2) (PSH: Peak SolarHours)I L A EL Wh / day PSH I PV VDC I PV Number of parallel 12stringsArray oversize factor11 The number is determined by the current requirement of theload 9. The equivalent load current is calculatedHours in a day38 Isc is the short circuit current under standard illumination SF is a sizing factor to oversize the arrayN p SF 24 I LPSHI PVI SC39D. Sizing the storage subsystemD. Sizing the storage subsystemDaily Charge Deficit13Winter Charge Deficit14Add climatic chargedeficit15Storage ChargeCapacity16Seasonal & climaticenergy storage 1740Total EnergyStorage18 1313. TheTh dailyd il andd seasonall chargehddeficitsfi it are calculated.l l t dExcess energy generated and not used must be stored. DailyCharge/Discharge percentages of the battery must notexceed safety valueDaily Charge Deficit13Winter Charge Deficit14Add climatic chargedeficit15Storage ChargeCapacity16Seasonal & climaticenergy storage 17Total EnergyStorage18 14. The energygy balance for the yyear is set such that thesummer excess can be stored to cover the energy deficitduring the winter EQ yd VDC 15. Allow for number of days of operation without energy input– Maintenance– Service– Lack of sunshine (Bad storm)41427

Loss MechanismsAnnual Energy BalanceProduce more than you useBilling cycle usually setnear this pointPower to Grid or LoadJulPV Output PowerAprMayAugJunMarSepOctNovDecJanFebUse more than you produce4344Final Lab Report (Wafer Number) Background– Fabrication Sequence– Device Cross Section Measurements–––––N Final Sheet ResistivityP Final Sheet ResistivityFront Al Sheet ResistivityBack Al Sheet ResistivityFront Al Thickness Device Testing– IV curves (4) Resistance Estimation Fill Factor Efficiency Estimations– Dark I-V curves (SDA) Linear Semilog Analysis– 4 solar cell data table– Series resistancecalculations– Annealing impact– Class Data Table– 4 devices Variances– ComparisonCioff ClClass DDatat Summary– Results Maximum VoltageMaximum Current DensityMaximum Fill FactorEfficiency– Course recommendations458

Photovoltaic System Basics • Photovoltaic Systems – Cell Panel Array – Balance of System (BOS) • Mounting Structures • Storage Devices • Power Conditioners – Load • DC ~ PV Panel 4 •AC / = DC AC Charge Regulator Inverter Battery DC Load AC Load Modularity: Solar Cell to Array 5 • Cell (c-Si 10×10 cm2 η=15% P=1.5Wp V=0.5V I=3A)