Hi,I am looking for some feedback on thermal mass optimization for passive solar in DesignBuilder/ EnergyPlus.
I attempted a basic sensitivity analysis for thermal mass optimization for a very basic passive solar design (one south facing window, lots of insulation, all assemblies with interior concrete thermal mass). I modelled this concept using DesignBuilder with various concrete thicknesses (5cm to 50cm) but keeping the envelope U value, and all other parameters constant. I used the CTF algorithm. The annual simulation results indicate that increasing thermal mass past 20cm continues to decrease heating energy consumption, in a somewhat linear fashion. Even more unusual, was that the savings from going with 30cm compared to 20, was much greater than that achieved by the change from 10cm to 20cm.
This does not match with the literature I have read, which suggest that this relationship should be a negative exponential decay (continuous diminishing energy savings with each added thickness). Many guidelines suggest that savings plateau at 20cm of thermal mass (as concrete).

Is this a problem with EnergyPlus? Or DesignBuilder? Any suggestions?

Thanks for your time,

Hayes Zirnhelt