The internalmass object uses a user defined area for calculations. Geometrically, it's not really there in the zone. So I think e+ uses the area for an approximated convection and radiation heat exchange. Convection is not so important, as it exchanges with the zone air node. Radiation is a bit more complicated. With which surfaces should it exchange radiation and to what extent? How much surface should receive direct beam? Internal mass usually represents things standing on the floor and infalling direct beam mostly lands on up-facing surfaces. So I assume e+ looks at the user entered surface area, makes some approximations. I don't know what those are, but if I were to guess, I would guess that it receives all direct beam except for the portion that is smaller than the floor. I would guess that the reflected fraction is equally proportioned to all the other zone surfaces except the floor. And that long wave infrared radiation exchange is also equally proportioned to all surfaces except the floor.
If these aspects are important for your model, then I would not rely on my guessing. Check the documentation or if it's not there, the code. Or ask the helpdesk. The forum you've asked.
The "caution" should be more apparent. Why bother simulating detailed internal reflections and radiation exchange when it's all getting proportioned anyway. So basically you loose this accuracy.
Not a very helpful answer, I guess.
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Jean Marais
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On 16.01.2015, at 23:43, "Holly Samuelson hollywas@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
Dear EnergyPlus_Support,Please excuse the accidental cross-posting.I am trying to understand how the internal thermal mass (aka "InternalMass") is being handled in my EnergyPlus models. On page 222 of the EnergyPlus Input-Output Reference it states, "simplifying calculations using internal mass must be used with caution when the "FullInteriorAndExterior" Solar Distribution model is chosen.Could anyone kindly help me understand what "use caution" means? Does that mean that my internal thermal mass does not "see" the solar radiation potentially hitting it (which might realistically be the case for certain interior elements if they are always in shade)? However, would you assume that this internal mass is still exposed to the indoor air, and thus could still be absorbing or emitting heat in that way?Many thanks for any guidance.Best,HollyHolly W. Samuelson, DDes, LEED, AIAAssistant Professor Harvard Graduate School of Design
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