State-of-the-Art Measurements for PV - Energetics Meetings and ...

State-of-the-Art Measurements for PVGrand Challenges forAdvanced PhotovoltaicTechnologies andMeasurementsKeith Emeryemail:keith.emery@nrel.govph: 303-880-2913May 11-12, 2010NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

OutlinePV Theory– What is the true value?Primary Reference Cell Methods– Absolute Total Irradiance, Absolute Spectral Irradiance versusAbsolute Spectral ResponsivityConcentrator PV– Linearity• Isc vs. total irradiance 1 to 1000 suns from spectral responsivity• Reference detector for use 1 to 1000 suns pulsed or continuous– Flux mapping up to 2000 suns continuous up to 2 by 2 cm.Broadband detector– GaInAs cell with thin InP window responds form 300 to 2200 nmsuitable for use in spectral radiometers and for quantumefficiency.National Renewable Energy Laboratory Innovation for Our Energy Future

What is the True Value? = P maxE totA 100 1 = I R,RI R,S MM = 2 1 2 1E RefE Ref( )S Ref ( )d( )S T ( )dWhere:P max is the maximum electrical power produced when illuminated understandard reference conditions– 25 °C junction temperature, 1000 Wm -2 total irradiance,– IEC 60904-3 ed. 2 / ASTM G173 global reference spectrum,other reference spectrumA is the total area of the device including contacts andperipheral bus barsE tot is the total irradiance at standard reference conditions, 1000 Wm -2National Renewable Energy Laboratory Innovation for Our Energy Future 2 1 2 1E SE SReference cell methoduses PV cell or thermaldetector for indoor oroutdoor calibration.( )S T ( )d( )S Ref ( )d

Uncertainty – Primary reference cellThe PV community expects a lower uncertaintythan what the calibration labs can deliver assecondary standards (

Module UncertaintyThe PV community expects a lower uncertainty thanwhat the calibration labs can deliver ! (< 1% error in P max )Module P maxNational Renewable Energy Laboratory Innovation for Our Energy Future

OutlinePV Theory – what is the true value?Primary Reference Cell Methods– Theory• A cell whose I sc under standard reference conditions is known– US• Absolute Total Irradiance using Cavity Radiometer• Dominant error sources• NIST / WRR contribution to dominant error– Germany• Absolute QE• NIST issues– Japan• Absolute Spectral irradiance• NIST issues– Space programsNational Renewable Energy Laboratory Innovation for Our Energy Future

NREL U.S. – Absolute Total IrradianceNational Renewable Energy Laboratory Innovation for Our Energy Future

ISO GUM Uncertainty Analysis4000CV = I M E refsc T 3004000E t300( ) S r ( )dE ref( )d4000E s3004000E s300( )d( ) S r ( )d= CV u(25°C) k = CV u M t kExpress all errors as percentage of value and all formulas as products of terms toeliminate the sensitivity factor including partial derivatives and pages of algebraU = 2 U Isc32+ U CV U (25°C ) n 2+ U CV Un2 0.021U = 23 + 0.272 35 + 0.0832 85 + 0.342 3 + 0.142 3 + 0.802 2 Spectral correction factor k uncertainty (0.80%) determined through Monte-CarloAnalysis using estimated uncertainty of spectral irradiance and QEmeasurements.U CV u are statistical error sources (standard deviation), n’ is the total number ofpoints averaged over many days, n is the number of measurements averagedto get one point.U E m is dominated by the uncertainty in the World Radiometric Reference (0.34%)National Renewable Energy Laboratory Innovation for Our Energy Future2+ U E M32+ U M T32+ U k220.50.5

Absolute Spectral ResponsivityNational Renewable Energy Laboratory Innovation for Our Energy Future

AIST Japan – Absolute Spectral IrradianceAbsolute Spectral Irradiance, Relative Spectral ResponsivityUse a solar simulator that has a minimumSpatial nonuniformity – Hard to correct for this errorTemporal stability – Spectral radiometer measurementassumes no change in intensity during measurment(s)Deviation from the reference spectrum – Minimizes error incorrection factorI sc= I sc E totbE r()S r()dStandard lamp and corresponding spectral radiometercalibration dominates uncertaintybab E r()d E M()S r()daaNational Renewable Energy Laboratory Innovation for Our Energy Future

Space Based CalibrationsPrimary AM0 (Balloon)National Renewable Energy Laboratory Innovation for Our Energy Future

Uncertainty• What is today’s cutting edge?• Cell I sc U 95 uncertainty in ±1% (primarystandard, small area)• Module P max U 95 ±2 to 5% depending on sizeand technology• What will be considered “cutting edge” in 15-20 yrs?• Primary cell I sc U 99 uncertainty at ±0.2%• Production cell P max uncertainty ±0.5%• Production Module P max uncertainty ±1%National Renewable Energy Laboratory Innovation for Our Energy Future

Uncertainty• What barriers are in the way of this future, and how do weovercome them?• Build a better more accurate Cavity radiometer that can beaccurately calibrated using a cryogenic radiometers with uncertaintybelow ±0.1%• Improve absolute spectral responsivity measurements to reducethe uncertainty in I sc below ±0.5%• Calibration of spectral radiometers to better than 0.1% from 300 to3000 nm.• The spatial nonuniformity of module simulators at ±2% for a ClassA spatial nonuniformity rating is a dominant error source. Small areanonuniformities can exceed ±5% for module simulators where thedistance to the bulb is small such as in Spire’s Spi-Line. Spectral andtemporal issues can be corrected for for single junction technologies.Module simulators with good spatial nonuniformity (±1%) can beachieved by a long distance 5 to 10 m from bulb but these simulatorshave bias rate artifacts that impact thin-films and high lifetime Si.National Renewable Energy Laboratory Innovation for Our Energy Future

OutlinePV Theory– What is the true value?Primary Reference Cell Methods– Absolute total irradiance, Absolute Spectral Irradiance orAbsolute Spectral ResponsivityConcentrator PV– Linearity• Isc vs. total irradiance 1 to 1000 suns from spectral responsivity• Reference detector for use 1 to 1000 suns pulsed or continuous– Flux mapping up to 2000 suns continuous up to 2 by 2 cm.Broadband detector– GaInAs cell with thin InP window responds form 300 to 2200 nmsuitable for use in spectral radiometers and for quantumefficiency.National Renewable Energy Laboratory Innovation for Our Energy Future

Isc Linearity with Total IrradianceMeasurement Issues Nonlinearity Wavelength dependent Method giving the true value in absence of measurement error is spectralresponsivity vs. bias light level. I sc vs. E total assumes source spectrum represents reference spectrumotherwise may appear linear when nonlinear. Summation method least sensitive to spectrum and uniformity changing withlight level Temperature gradient between plate and cell increases withconcentration. Signal to noise ratio decreases with increasing light Little work >10 suns in this area since early 1990’sNational Renewable Energy Laboratory Innovation for Our Energy Future

OutlinePV Theory– What is the true value?Primary Reference Cell Methods– Absolute total irradiance, Absolute Spectral Irradiance orAbsolute Spectral ResponsivityConcentrator PV– Linearity• Isc vs. total irradiance 1 to 1000 suns from spectral responsivity• Reference detector for use 1 to 1000 suns pulsed or continuous– Flux mapping up to 2000 suns continuous light up to 2 by 2 cm.Broadband Detector– GaInAs cell with thin InP window responds form 300 to 2200 nmsuitable for use in spectral radiometers and for quantumefficiency.National Renewable Energy Laboratory Innovation for Our Energy Future

Detectors that respond from 300 to 2500 nm– Multiple reference detectors arecurrently needed for spectralresponsivity and spectralirradiance measurements.– No technical barrier to cell design.No commercial source. NREL hasmade numerous such devices butthey are not well packaged orcommercialized. CommercialGaInAs detectors limit theirresponse below the energy gap ofInP to achieve high speeds andlow capacitance. With a thin InPwindow the response of GaInAscan be extended below 300 nm.National Renewable Energy Laboratory Innovation for Our Energy Future

Total irradiance - Traceability to SI units• The Wm -2 for solar applications is currently traceable toSI units through the World Radiometric Reference (WRR)scale maintained by a set of 6 self calibrating Absolutecavity radiometers by the World MeteorologicalOrganization in Davos Switzerland.• This scale is transferred to the world via intercomparisonevery 5 years.• The estimated U 95 uncertainty estimate is ±0.33 %• 0.3% from WRR itself, standard deviation 0.1%.• This has been transferred to my cavity every year at anNREL hosted intercomparison with a U 95 of 0.34%National Renewable Energy Laboratory Innovation for Our Energy Future

Total irradiance - Traceability to SI unitsNational Renewable Energy Laboratory Innovation for Our Energy Future

Total irradiance - Traceability to SI units• In 2009 our ISO 17025 auditors from A2LA foundthat the cavity calibration is not traceable to SI !• If NIST calibrated it this would not be an issue.• The cavity U 95 uncertainty should be less than0.1% to not be a significant error source in primarycalibrations• An indoor calibration procedure validated byintercomparison with the WRR at < 0.1% U 95uncertainty would be desirable.National Renewable Energy Laboratory Innovation for Our Energy Future

Total irradiancePyranometer -180° field of view, ±2.5 to 5% depending onthe uncertainty analysis and how it is calibrated and used.– cosine and temperature corrections?– cleaning schedule– horizontal versus tiltedAbsolute Cavity Radiometer – 0.34% uncertainty but unithas no window so dust and debris can change calibration.Addition of window increases uncertainty above 0.5%.Cavity radiometer for high flux levels. Absolute Cavityradiometer 0.1 to 1 –sun light levels. Marginal irradianceoverlap with cryogenic radiometerRadiometer that matches the field of view of the sun minusthe circumsolar for concentration above 20 times. ±1% orless uncertainty.National Renewable Energy Laboratory Innovation for Our Energy Future

Recommendations for NIST related activities– More accurate calibration of standard lamps or sources oralternative procedures for spectral radiometer calibration at±1% from 300 to 2500 nm.– Spatial nonuniformity of Module solar simulators is adominant error source. Super uniform simulators betterthan ±0.5% spatial nonuniformity over a 2 by 3 m region.This capability should be available for continuous andpulsed light sources (0.1 sec pulse length). Pulsed lightsources remove temperature as an error source.Continuous light sources remove bias rate as an errorsource.National Renewable Energy Laboratory Innovation for Our Energy Future

Recommendations for NIST related activities–Ability to perform PTB style PV calibrations.• Spectral responsivity or QE vs. temperature, andirradiance 0.001 to 2 sun• Cell package fully illuminated up to 200 cm 2–NIST calibrations assume detector uniform with noreflection artifacts• I sc from integrated QE giving I sc at 1-sun, 25 °C witha U 95

Recommendations for NIST related activities–Ability to determine QE vs. irradiance from 1 to1000 suns at selected wavelengths. Correct wayto measure nonlinearity in I sc for PV sincenonlinear behavior is wavelength dependent–Ability to determine linearity of thermal detectorsfrom 0.1 to 1000 suns.National Renewable Energy Laboratory Innovation for Our Energy Future

Recommendations for NIST related activities•More accurate, broader irradiance range forbroadband radiometers.180° field of view (better cosine response, lowersensitivity to air temperature) U 95