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Not long ago the ASHRAE Technical Committee TC 4.1, Load Calculation, had a "bake off" of sorts between different peak air-conditioning load calculation schemes and programs. One of the outcomes of this exercise was the realization that practitioners and software developers make largely different assumptions about how solar energy absorbed by window glass and by window shades contributes to the room solar heat gain. For unshaded glass windows there is general agreement (ASHRAE 2005). For a shade, however, there were two extremes in the models—one assumes that the shade rejects most of the solar energy that it does not transmit, which is logical if the shade is highly reflective and the glass is highly transmissive, and the other assumes that all radiation absorbed by the shade is immediately convected into the room.

Suffice it to say, we are not the first to derive and present the fundamental equations of this heat transfer problem. What we have done is to avoid any simplifying assumptions in formulating the problem while allowing that some physical constants, convection coefficients in particular, are not well known and need to be parameterized. We whet the readers’ appetite by revealing that for a glass/shade system where the glass was 22% transmissive and the shade 52% transmissive, the total heat gain to the room from this window assembly was nearly half of the incident radiation. Of course, "it all depends!"

Units: SI


Citation: ASHRAE Transactions, vol. 114, pt. 2, Salt Lake City 2008