REFERENCE SPECTRA

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 G173-03 "Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface", 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 will also be used by the International Electrotechnic Commission as standard IEC 60904-3 "Measurement Principles for Terrestrial Photovoltaic (PV) Solar Devices with Reference Spectral Irradiance Data", currently in its final stages of revision.

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, there is preliminary activity at the Commission Internationale de l'Eclairage (CIE) to develop SMARTS-based reference spectra for lighting and daylighting purposes. For illustration purposes, the two reference spectra of G173 appear in Fig.1, whereas the UV spectra from G173 and G177 are compared in Fig. 2.

Figure 1  Comparison of direct normal irradiance and global irradiance on a 37°-tilted sun-facing surface per ASTM G173-03 standard.

Figure 2  Comparison of global irradiance on a 37°-tilted sun-facing surface per ASTM G173-03 and G177-03 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 buildings.

To help engineers better design these air-conditioning systems, ASHRAE (http://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.