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- Outdoor vs indoor measurements- Analyzing and modeling outdoor data- Validating or fault finding device performance- Extracting coefficients- Understanding the differences between modules- Checking performance limitatonsSteve Ransome, Associate Consultant, IntertechPira
Citation preview
Outdoor testing, analysis and performance predictions of PV
technologies
Steve Ransome (Owner SRCL)
and associate consultant (Intertechpira UK)www.steveransome.com
How kWh/kWp values are usedby industry sector
Section of PV industry kWh/kWp relevance Manufacturers Claim high performanceIndoor testers Measure relevant parametersSizing programs (simulation models)
Claim accurate predictions from complex models
Customers Expect high valuesFinancial backers Demand guaranteed values
over lifetimeIndependent outdoor comparisons
Different rankings for each technology (often within experimental error for correctly rated and measured modules)
14-May-09 www.steveransome.com Page 2
Typical daily AC performance of a large array, USA
1. Energy Yield YF (kWh/kWp/d) should be approximately proportionalto the daily insolation YR (kWh/m²/d)
2. Points below the line indicate underperformingperiods
3. The total uncorrected energy yield will include these bad points and worsen apparent yield
14-May-09 www.steveransome.com Page 3
Energy yield losses from AC arrays
14-May-09 www.steveransome.com Page 4
How much of these energy losses are due to
– component number and choice
– down time
– inverter loss (efficiency or low light turn on)
– inherent differences between module technologies ?
– other reasons ?
DC module performance must be studied to quantify the losses
View of typical DC setupISET, Kassel Germany
PV modules
“Spectrally sensitive”Irradiance sensors
Pyranometer
South
30°Tilt
Not shown : temperature sensorsDirect:Diffuse, precipitation , wind speed etc.
Typical DC outdoor measuring setup(single devices are better for characterisation)
Ambient Temperature (C)
Windspeed (ms-1)Plane of array Irradiance (kW/m²)
Device Temperature (C)
Data LoggerMeasure
every 1-10 minutes
Device under test
Impp, Vmppor IV scan
Other sensors ? e.g. horizontal irradiance, precipitation, air pressure, spectrum …
Data Analysis
Independent energy yield test :7 technologies Kassel, Germany
1. Most technologies give similar energy yields (<±4% kWh/kWp)
2. Two are much lower (are they faulty or have they degraded?)
3. Cannot identify reasons from kWh/kWp sums alone
(see my paper PVSEC Valencia 2008 for details)
14-May-09 www.steveransome.com Page 7
How do we find the reason for differences in kWh/kWp ?
Possible reasons
• Monitoring errors e.g. Vmax mistracking
• Pmax declaration (measured/nameplate)
• Shading on some panels only
• Degradation/annealing
• Different technology performances at
– low light
– high temperature
– diffuse light
– different spectra …
Detailed studies should reveal reasons for differences14-May-09 www.steveransome.com Page 8
Data validation for outdoor measurements
Normalise measurements to “measured/expected values”• Vdm = Vdc / Vmax.stc• Idn = Idc / Imax.stc / Irradiance
Define simple limits to remove “bad” data points(e.g. 80-110% of expected value )
Perform a sanity check on meteorological dataIrradiance (e.g. 0 to 1.4kW/m²), Clearness Index (e.g. 0.2 to 0.8)Diffuse Fraction (e.g. 0.1 to 0.9)Temperatures (e.g. -20 < Ambient < 40) etc.
14-May-09 www.steveransome.com Page 9
Normalised electrical parameters showing limits used for Imax and Vmax
Weather data (top)
Electrical data (bottom)
• Correct, interpolate or delete data outside sensible limits (shown in coloured bands)
• “Redundant data” : calculate “NOCT” (Tmodule @800W/m², Tambient=20C, 1ms-1 wind) – should be ~47C
14-May-09 www.steveransome.com Page 10
Diffuse sky (left) vs Clear sky (right)affects PV performance
Large attenuation of Beam
High reflection off clouds
Variable spectrum
Little attenuation of Beam
Little reflection off clouds
Spectrum ~ Air Mass
14-May-09 www.steveransome.com Page 11
Understanding Efficiency vs IrradianceImax and Vmax vs. Diffuse:Beam - cSi
Vmax vs Irradiance Imax vs Irradiance
14-May-09 www.steveransome.com Page 12
1. Most points should be within narrow limits, outlier data due to poor tracking, shade or snow on module or sensor. Can temperature correct.
2. Imax differs whether diffuse or clear sky, Vmax doesn’t
Diffuse Clear
(1) Error
(2) Diffuse
(2) AOIEfficiency = Vmax * Imax
Comparing different module technologieshow important are any differences ?
14-May-09 www.steveransome.com Page 13
Crystalline Silicon #1 and #3 Thin Film #4 and #6
(2) Vmax very similar, TF slightly better voltage thermal coefficients
(1) Imax quite similar, TF slightly worse current variability – spectral mismatch/annealing
Diffuse Clear
Vmax
Imax
(2)
(1)
Comparing module technologiesEfficiency vs Clearness (top) & Beam Fraction (bottom)
14-May-09 www.steveransome.com Page 14
Thin Film #4
Diffuse Clear
Effi
cie
ncy
Clearness
Beam Fraction
Effi
cie
ncyCrystalline Silicon #3
(1) (2)
(3) (4)
Efficiency = Vmax * Imax
Insolation (kWh/m²/y) vs Irradiance, Clearness index and Beam fraction
14-May-09 www.steveransome.com Page 15
More Insolation at :
1. Higher irradiance than lower –most sites
2. Higher clearness index (clear skies)
3. Higher beam fraction (low diffuse)
than at lower valuesIrradiance (kW/m²)
Insolation (kWh/m²/y) vs. Tmodule and Irradiance
Tmodule (C)
Irra
dia
nce
(kW
/m²)
More Irradiance at high light levels than low light even in Germany
More frequent measurements show even more high light level
How all weather parameters are correlatedmaking understanding data more complicated
Indoor (STC) Outdoor
<Worse weather Better weather>
Irradiance 1 kW/m² Lower Higher
Module temperature 25 C Colder Warmer
Spectrum AM 1.5 G Redder Bluer
Angle of incidence 0° normal Away from normal Nearer normal
Direct : Diffuse All Direct Mostly diffuse Mostly direct
14-May-09 www.steveransome.com Page 17
Extracting temperature coefficients from outdoor data Imax and Vmax
Values may differ from internal measurements as weather parameters are correlated (e.g. spectrum and temperature) which will affect multi junction thin film more than c-Si.
1. Vmax more accurate than
2. Imax
Need to filter out low irradiance/temperature data as too variable
14-May-09 www.steveransome.com Page 18
Empirical modelling predicting Tmodule, Vmax and dc Power
• Simple empirical models can predict Tmodule, Vmax and Pmax
Tmod = f(Irrad, Tamb, WS, …)
Vmax = f(Irrad, Tamb, WS, …)
Pmax = f(Irrad, Tamb, WS, …)
• Can characterise measured and predict future PV performance
fits (black dots)measured (coloured dots)
Empirical modellingFlow chart
Empirical formulae and coefficientsInputs
Irradiance (kW/m²)Ambient Temperature (C)Windspeed (ms-1)
Cell Temperature C
MPP Voltage V
MPP Current A
MPP Power W
Validate measurements
learning mode to derive coefficients
Sum (Power) = Energy Yield
Report discrepancies
Empirical modelling –validating Tmodule, Vmax and dc Power
(1) Tmod
(2) Vmax
(3) Pmax
Simulating outdoor performance, extracting indoor parameters
14-May-09 www.steveransome.com Page 22
DC module(IV scan)
DC module(IV scan)
String(Vmp track)
AC array(Inverter)
Efficiency vs
IrradianceTemperature
SpectrumAOI etc.
Pactual/nameplate,
Dirt,Thermal
Annealing, Degradation
Weather Correlation
Module Mismatch
Inverter efficiency,
Partial shading,Wiring loss,
String mismatch
Meas. Stage
indoors outdoors
Parameter extraction Performance modelling
Finding shading - Max. irradiance per hour of the day and month of the year
• Good unshaded sites will give smooth, symmetrical oval shapes as shown
• Shading will show as lower maximum irradiance than expected for certain times and months (e.g. after 14:00 November to January for low horizon in the west)
14-May-09 www.steveransome.com Page 23
• Well performing arrays will give smooth, symmetrical oval shapes as shown
• Thermal problems would be seen by summer afternoon dips (although this array seems good)
• Turn on problems would be seen by low values in the morning
AC Performance : Maximum ac yield per hour of the day and month of the year
14-May-09 www.steveransome.com Page 24
Finding shading : Solar Irradiance and array sum energy vs Solar position
Total insolation vs Solar height and azimuth in 10° bins
Good unshaded sites should have a symmetrical shape like this in Germany
Horizon shading appears as wide low irradiance areas
Tree or pole shading is seen in tall low irradiance areas
14-May-09 www.steveransome.com Page 25
1. Shading would show as poor performance in horizontal shapes
2. “Turn on” problems appear as missing data at beginning of day
3. Missing data all day
Finding “Turn on” problems and Shading Performance/predicted Vs. Date and Time
(2)(3)
(1)
Conclusions
14-May-09 www.steveransome.com Page 27
• Sophisticated outdoor testing has been used to compare dc modules with ac arrays
• Sum kWh/kWp figures alone are not enough to qualify measurements
• A detailed knowledge of dc performance helps understand AC data
• Normalise data for easier error checking V, I etc.
• Max. Irradiance or Power vs. time of day and month can identify shading or thermal problems
• Checking raw data enables faults, limits and weather effects to be analysed.
Thank you for your attention
Thanks to ISET for the DC data
This paper and previous ones are available at
www.steveransome.com