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# Thickness of internal mass material

Hi everyone

I have a rather simple question, but I haven't been able find the right answer yet. I am doing some research on internal thermal mass and would be using the "InternalMass" section in EnergyPlus.

In the 'engineering reference' I found that for internal mass object that "if both faces are within the zone, this situation can be modeled either by creating another internal mass surface with the reverse construction, or, for symmetric constructions, simply doubling the area of the surface." (3.7 Adiabatic Boundary Conditions)

My question now is, which thickness do I use when such an object has its both faces within the zone? When doubling the area of the surface, do I use the full thickness or only half of the thickness?

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Half. Since you are doubling the area, if you want the correct total mass for the internal mass material, the thickness should be divided in half.

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Yes, I would think that too, but in search of the answer I performed some simulations on a very basic model. A space of 3m x 3m x 3m, no insulation, only subjected to varying outside boundary conditions.

Out of these simulations I calculated the average diurnal temperature range and how increasing the amount of internal mass decreases the average diurnal temperature range within the test cell. I did these simulations using two different methods for entering the internal mass.

( 2018-05-06 03:53:38 -0500 )edit

The first method used the 'ZoneCapacitanceMultiplier:ResearchSpecial' object in which I increased the 'temperature capacity multiplier' in according with the amount of internal mass. The second method used the 'InternalMass' object in which I performed simulations using the full thickness and half of the thickness.

When analysing these results I found that the results when using the full thickness approximates the results of the capacitance multiplier method better than when using only half of the thickness.

( 2018-05-06 03:58:40 -0500 )edit