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1143-RP 1143-RP

REFERENCE PROCEDURE TO OBTAIN TYPICAL SOLAR SPECTRA FOR ADVANCED
SOLAR HEAT GAIN CALCULATION

BACKGROUND:
The determination of solar gains through windows is an essential part of the cooling load calculations
necessary in designing HVAC systems. For decades, the recommended method to obtain these solar gains
was based on broadband solar irradiance estimates from an empirical and simplistic radiation model
developed for ASHRAE in the `50s, and described in Chapter 29 of the current 1997 edition of the
Handbook of Fundamentals (HOF). The Solar Heat Gain Factor (SHGF) tabulations provided in HOF are
based on these solar radiation estimates, but their accuracy has been recently questioned in cases where the
fenestration solar heat gain properties contain significant levels of spectral selectivity over the solar
spectrum and when the sun is close to the horizon.

TC 4.5 is moving to include a spectrally-based method for determining the solar gain of fenestration
systems, as specified by the ASHRAE standard that is expected to result from the work of Special Project
Committee 142. A spectrally-based method for computing solar gain is described in general in the 1997
HOF, and will be required in the next edition (2001) of HOF, for fenestration systems needing this more
detailed calculation. SPC-142's draft standard currently specifies only a single solar spectral irradiance
distribution. For the methodology to be applicable to a variety of locations, some means must be developed
for using different solar spectra, particularly spectra that are more realistic for HVAC applications. To meet
this need, a solar spectral irradiance calculation methodology must be developed that is sensitive to changes
in atmospheric conditions from place to place and time to time.

JUSTIFICATION:
During the last decade, the optical characteristics of glazings have evolved dramatically. Complex
fenestration systems are now frequently used in all kinds of buildings in many climates, for instance to
minimize heat gains while maintaining a high visible transmittance. Such glazings use thin reflective
coatings and absorbing dyes of varying nature with highly specific spectrally selective properties. Even
more advanced technologies will enter the market soon, such as electrochromic devices which induce
variable optical characteristics depending on the outside temperature or radiation conditions. For the
purpose of solar gain calculations, the main optical characteristics of each glazing are its spectral
transmittance, absorptance and reflectance. These can have large variations throughout the solar spectrum,
in contrast with the relatively flat spectral variations of the ordinary clear or tinted glass of the past.

The only way to calculate accurate solar gains through these spectrally selective devices is by using a
spectral calculation method. To recognize this evolution, a spectrally-based method for computing solar
gain was outlined for the first time in the 1997 edition of HOF, but it was not required. A complete spectral
method is now being standardized by ASHRAE, as described in Standard 142P [1]. In the next edition
(2001) of HOF, this spectrally-based method for determining the solar gain of fenestration systems will be
required where needed for accurate calculations. This method implies that the resulting bulk optical
properties of the fenestration system (e.g., its broadband visible transmittance, or the Solar Heat Gain
Coefficient, SHGC) are obtained, at the last step, by a weighted averaging of their respective spectral
values using a reference solar spectral irradiance distribution.

For these calculations, the method specifies only a single solar spectrum for direct beam irradiance at
normal incidence and for a fixed solar elevation. However, most windows are also subjected to diffuse
irradiance emanating from the sky and from the surrounding environment, so that the total (direct + diffuse)
solar spectrum should be considered. Moreover, it has been shown [2, 3] that the SHGC can be
significantly affected by changes in the solar spectrum, resulting from changes in solar position or
atmospheric conditions. Taking these spectral variations into consideration is certainly a critical factor in
experimental work. Because calculation methods always need to be tested against experimental values, it is
important to compare the predicted SHGC values obtained with ASHRAE's standard method to the measured values obtained for the same fenestration when tested in an outdoor calorimeter. Recent tests by
leading experimenters indicate discrepancies which can be attributed, at least in part, to this spectral effect.

For the spectral solar heat gain methodology to be applicable to a variety of locations and climates in North
America or elsewhere, some means must be developed for using various solar spectra and for helping the
user to correctly select amongst them, depending on the envisioned application. Some spectra are more
realistic than others for a given climatic area or for given HVAC applications. To meet this need, a spectral
irradiance calculation methodology must be developed that is sensitive to changes in gases and other
atmospheric conditions from place to place and time to time. That work must thus include the identification
of an appropriate spectral irradiance model from among those available in the literature, and must
document its accuracy.

Since atmospheric variables are not commonly known to HVAC engineers, the work must also provide the
needed variables to them for use as inputs to the solar spectral irradiance model just selected. Furthermore,
since most HVAC engineers are unfamiliar with the principles of atmospheric optics, some means must be
provided to assist them in an intelligent selection of the needed input parameters. This is a critical aspect of
the work described here.

ASHRAE will eventually seek to adopt a complete procedure for specification in HOF and/or in a new
standard destined to complement Standard 142P discussed above. This will result in far more accurate
determinations of solar heat gains and cooling loads than with the existing approximate broadband method.

OBJECTIVE:
To propose one or more complete calculation procedures that permitting the generation of realistic incident
solar irradiance spectra, for use in the determination of solar heat gain through windows at any North-
American site and under typical clear-sky conditions. To provide a means that is easy-to-use by the HVAC
engineer in choosing appropriate atmospheric parameter and optical property inputs to the model(s) chosen.

SCOPE:
Proposing an accurate cooling load calculation method to HVAC engineers is essential to the mission of the
Society. Because of the current transition from broadband-based radiation and solar heat gain calculations
to a spectrally-based method, it is vital that ASHRAE be ready to implement the new spectrally-based
method by 2001 as already committed in the current edition of the Handbook of Fundamentals. The
proposed research will precisely fill this gap and provide the Society with an essential part of the
methodology which is currently unavailable.

This proposed research involves the selection of one or more appropriate spectral solar radiation model for
cloudless skies. A performance assessment of the chosen model(s) is also required, to determine its
accuracy under a variety of atmospheric conditions. The preparation of datasets of monthly-average values
of the atmospheric variables needed to run the model in North America is required. Production of typical
spectra and resulting solar heat gain predictions are also needed, to demonstrate the application of this
procedure (in combination with proposed standard SPC-142) to spectrally-selective fenestration. The
detailed items of work are listed below numerically.

It is anticipated that this spectral irradiance procedure will eventually be computerized, and linked to the
SHGC procedure to form a single user-friendly "solar" software with different options, but the preparation
of that user-friendly software is not part of the present work statement.

1.
The contractor shall survey the literature on solar spectral models and their dependence upon
atmospheric constituents. Comparative descriptions should be given, listing at least the degree to
which these models consider attenuation by atmospheric gases and particulates, what kind of input data
they use, and in what kind of format the results can be obtained. Three important requirements of the
sought-after model will be i. that it can be used with easily obtainable climatic or atmospheric data, ii.
that it can be easily used and programmed by engineers, and