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Dear Trnsys,
I hope this message finds you well. I am writing to inquire about a discrepancy I encountered while calculating the U-value of a multi-layered wall assembly. I have used the following materials and their respective thermal conductivities for my calculations:
- 2mm PVC, thermal conductivity 0.177 W/m·K
- 3mm Ethanol gel, thermal conductivity 0.3329 W/m·K
- 50mm Air gap, thermal conductivity 0.025 W/m·K
- 1mm Aluminum oxide ceramic, thermal conductivity 6.207 W/m·K
By manually calculating the total thermal resistance (R-value) for these layers, I arrived at approximately 2.02046 m²·K/W. Consequently, the calculated U-value was approximately 0.495 W/m²·K.
However, when using Windows software for the same calculations, the reported U-value was significantly higher at 2.778 W/m²·K. And when I use this data in Trnbuild, the heat loss is significantly higher than normal.
I would appreciate your insights into understanding this discrepancy. Could it be due to factors such as boundary conditions, convective heat transfer, radiation effects, or interface thermal resistance, which might not have been fully considered in my manual calculation?
I look forward to your guidance and clarification on this matter.
Thank you for your attention to this inquiry.
Best regards,
Xiaohan
@zhangxiaohan I am not very familiar with the methods the LBNL's WINDOW program uses to compute the u-value of glazing systems. I would recommend that you look into their documentation to see if you can find the difference in methodology. A few thoughts, however...
- it is important to include the convection film coefficients for the inside and outside surfaces of the glazing system when calculating it's u-value. In TRNBuild when a u-value for a wall is displayed, the u-value includes h_i=7.7W/m2.K and h_o=25W/m2.K. As a side note, the actual u value may change over the course of the simulation depending on how you have defined the actual inside and outside convection coefficients.
- TRNBuild/Type56 only uses the center of glass information that WINDOW produces. The edge effects and the frame u-value are set within TRNBuild itself.
kind regards,
David
