Preliminary Investigations of Outdoor Meteorological Broadband and Spectral Conditions for Evaluating Photovoltaic Modules and Systems Page: 3 of 4
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Preliminary Investigations of Outdoor Meteorological Broadband and Spectral Conditions for
Evaluating Photovoltaic Modules and SystemsD.R. Myers, S.R. Kurtz
National Renewable Energy Laboratory
1617 Cole Blvd, Golden CO 80401ABSTRACT
Historically, flat-plate photovoltaic (PV) modules have been
rated at "peak-output" for power generated under Standard
Reporting Conditions (SRC) of 1000 Watts per square meter
W/m2 global irradiance at a standard temperature (25 C)
and reference spectral distribution. We examine the direct-
normal irradiance, spectral distribution, ambient
temperature, and wind speed to be used for evaluating flat-
plate and concentrator module performance. Our study is
based upon the 30-year U.S. National Solar Radiation Data
Base for conditions observed when the global irradiance on
a 2-axis-tracked surface is 1000 W/m2. Results show
commonly-used values for concentrator testing of 850 W/m2
for direct-normal irradiance and 20 C for ambient
temperature are appropriate. Wind speed should be
increased from 1 m/s to a more frequently observed 4 m/s.
Differences between the reference direct-normal spectrum
and spectra measured at three sites when broadband direct-
normal irradiance and global-normal irradiance are near
SRC irradiances suggest revisions to the reference spectra
may be needed.
1. Performance Reporting Conditions
Various existing standards address the performance of PV
devices, as shown in Table 1. Device performance is
commonly reported with respect to a fixed set of conditions
for total irradiance, device temperature, and reference
spectral distribution. Note that only PV for Utility Scale
Applications ( PVUSA) test conditions and this work
address PV concentrator test conditions.
DNI and GNI correspond to Direct Normal Irradiance and
Global Normal Irradiance, respectively. Standard Test
Conditions (STC) or Standard Reporting Conditions (SRC)
are defined only for flat-plate collectors as 1000 W/m2
irradiance on the module at 250 C Cell temperature, under a
reference spectral distribution (American Society for
Testing and Materials, ASTM E891 and ASTM E892). The
1000 W/m2 irradiance is an arbitrary but convenient
achievable "peak" performance condition.
Flat-plate PV devices are often tested indoors, under
simulated sunlight near SRC per ASTM Standard Test
Method 1036. Indoor testing of PV-concentrator modules is
difficult. There are currently no consensus standards for
reporting PV-concentrating collector performance so the
PV-concentrator industry reports performance based on
conditions (PVUSA test conditions, or PTC) developed as
part of technology procurements.C. Whitaker, T. Townsend
Endecon Engineering
2500 Old Crow Canyon Rd. San Ramon CA 94583
Table 1. Summary of Standard PV Test Conditions.
Standard Name Irradiance Temp. Wind
W/m2 speed Comments
STC or SRC 1000 global 25C Not Indoor peak
(Standard AM1.5 cell applic- performance
Test/Reporting Spectrum able (most
Conditions) [1] E892 catalogues)
PTC (PVUSA 1000 global 20 C 1 m/s Outdoor peak
test conditions) 850 DNI ambient at performance
[2,3] 10 m (utilities)
Nominal 800 global 20 C 1 m/s at nominal
operating ambient module operating cell
conditions [1] height temperature
(NOCT) [1]
1000 global 23.70 4.5 f 2.8 Observed
This work 836 f 44 f8.8 C rn/sat when GNI s
DNI ambient 10m 1000 W/m.
2. Technical Approach
This study provides a technical basis for choosing outdoor-
rating conditions, compatible with existing SRC, as
described in references [4] and [5]. Hours from the 30-year
(1961-1990) NREL National Solar Radiation Database
(NSRDB) with the GNI at 1000 W/m2 g 25 W/m2 were
selected. We analyzed the direct-normal irradiance,
turbidity, temperature, total column water vapor, and wind
speed for these hours. NSRDB does not contain GNI. We
modeled GNI using the Perez Anisotropic Model [6] with
an albedo of 0.2. Two years of modeled and measured GNI
showed the model unbiased with root-mean-square error of
2.5%, similar to measurement uncertainty.
We are investigating comparisons of the ASTM E892
reference spectrum to measured spectra extracted from the
Solar Energy Research Institute (SERI) Solar Spectral Data
Base [7] for GNI and DNI within 10 W/m of 1000 W/m
and 850 W/m2, respectively.
3. Results
Table 2 compares SRC, PVUSA, and the mean results of
our analysis for 37 NSRDB sites in the American southwest
with outdoor conditions near SRC. The frequency
distributions of DNI, ambient temperature, wind speed,
atmospheric turbidity, and precipitable water vapor were
found to be non-Gaussian and site dependent. Figure 1
shows the distribution of median DNI for all sites when
outdoor conditions approximate SRC. Individual
distributions are discussed in detail in references [4] and [5].69
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Myers, D. R.; Kurtz, S. R.; Whitaker, C. & Townsend, T. Preliminary Investigations of Outdoor Meteorological Broadband and Spectral Conditions for Evaluating Photovoltaic Modules and Systems, article, January 1, 2000; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc883167/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.