A few reference spectra, based on SMARTS
version 2.9.2, have already been standardized for various
applications. Most useful to the solar energy community in general
should be standard ASTM G173 "Standard
Tables for Reference Solar Spectral Irradiances: Direct Normal and
Hemispherical on 37° Tilted Surface", which was first approved in 2003, and is available from http://www.astm.org.
This standard replaces the older G159 standard, which itself replaced both
E891 and E892, first issued some 25 years ago. Standard G173
offers a lot more accuracy and spectral resolution than the
spectra it replaces. Although the new and old global spectra on a
37° sun-facing tilt are closely comparable in magnitude, the new
direct normal spectrum is noticeably enhanced compared to the
older versions. This result conforms to the request of the
concentrating solar power industry, which is targeting very clear
areas of the world for the siting of solar power plants. An edited
version of the global tilted spectrum in G173 is also used by
the International Electrotechnic Commission as standard IEC
Principles for Terrestrial Photovoltaic (PV) Solar Devices with
Reference Spectral Irradiance Data" for flat-plate PV applications.
For applications related to the
degradation of materials, ASTM has also adopted standard G177-03
"Standard Tables for
Reference Solar Ultraviolet Spectral Distributions: Hemispherical
on 37° Tilted Surface", which is limited to the
280-400 nm waveband, with a considerably increased magnitude
compared to the UV part of the G173 global spectrum.
Finally, in 2008,
ASTM relased standard G197 "Standard
Tables for Reference Solar Spectral Distributions: Direct and
Diffuse on 20° Tilted and Vertical urface", which is intended to be used in building applications, such as spectrally-selective fenestration, daylighting, and building-integrated PV (BIPV).
For illustration purposes, the
two reference spectra of G173 appear in Fig.1. The UV
spectra from G173 and G177 are compared in Fig. 2, and the spectra from G173 and G197 are compared in Fig. 3.
Comparison of direct normal irradiance and global
irradiance on a 37°-tilted sun-facing surface per ASTM G173
Comparison of global irradiance on a 37°-tilted sun-facing
surface per ASTM G173 and G177 standards.
Comparison of direct, diffuse and global irradiance on various tilted sun-facing
surfaces per ASTM G173 and G197 standards.
Some Typical Applications
PV cells have widely
different spectral responses depending on their technology.
Because the rating of PV cells is obtained by reference to a
single standard spectrum, their performance under a very different
spectrum cannot be known a priori. It is therefore possible that,
under real conditions, PV system A performs better than system B
at clean-dry site X, but worse at tropical site Y. This can be
analyzed beforehand by comparing typical spectra (that would be
generated by SMARTS) for sites X and Y to that of the standard
spectrum, and convoluting them with the spectral response of
systems A and B. Wavelength-by-wavelength ratios can be easily
obtained, from which a broadband mismatch factor can be
calculated. This mismatch factor relates the performance of a
solar cell under specific conditions to that under standard
conditions. In other words, it relates the PV cell performance
under any real-world condition to its standardized rating.
A similar procedure may be
applied to the analysis of solar heat gains through
spectrally-selective glazings in the built environment. Most
fenestration devices (e.g., windows, glazed doors or skylights)
sold in North America display a performance sticker approved by
the National Fenestration Rating Council (NFRC, http://www.nfrc.org/).
This sticker contains various pieces of information, including
Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT).
These two coefficients are now obtained using a sophisticated
methodology involving spectral calculations, and a single
weighting function, which is currently the ASTM G159 direct
spectrum. More than a decade ago, it has been shown that the
variability in the solar spectrum caused non-negligible
perturbations in SHGC. This in turn interferes with cooling load
calculations, and finally in the design and cost of the air
conditioning systems needed to counterbalance the cooling load in
To help engineers better
design these air-conditioning systems, ASHRAE (https://www.ashrae.org/)
has funded a research project (RP-1143) to make spectral radiation
information accessible to them in an easily assimilated way.
SMARTS is the pivotal calculation tool in this project, one
essential outcome of which being that scientists and practicing
engineers are now able to select a major North American city and
transparently obtain the necessary inputs to SMARTS that
correspond to summer design conditions (with respect to
solar radiation) for the selected area. In support of this
project, a series of field measurements on tilted and vertical
surfaces using portable spectroradiometers have been specially
undertaken at NREL to validate the SMARTS irradiance predictions.