EMS modeling variable thermal/solar absorptance coating overwrites both interior and exterior surface absorptance

Issue overview

The issue is found when using EMS to validate the MaterialProperty:VariableAbsorptance object which models coatings with changing thermal or solar absorptance driven by surface temperature, received solar radiation, heating/cooling mode, or a schedule. In the validation step, the same variable-thermal-absorptance coating is modeled on one of the walls (Zn001:Wall001) of the 1ZoneUncontrolled.idf with both the MaterialProperty:VariableAbsorptance object and the EMS Surface Property Thermal Absorptance field. The simulation results of the outside surface temperature of the Zn001:Wall001 show slight differences.

design_day_cmp_1ZoneUncontrolled_ems_vs_feature

Among the 48 hours of the two design-days, the statistics of the average surface outside face temperature difference between the two approaches is as follows

 Min.   :0.0002631  
 1st Qu.:0.0098464  
 Median :0.0098573  
 Mean   :0.0159866  
 3rd Qu.:0.0241629  
 Max.   :0.0351419

The surface temperature differences was traced to result from the difference in state.dataHeatBalSurf->SurfAbsThermalInt(1), the thermal absorptivity of Zn001:Wall001 inside face. EMS is observed to overwrite the absorptance of both the inside and the outside face. The following is a snapshot of the debugging print before the first occurrence of this line zone_info.Emissivity(ZoneSurfNum) = state.dataHeatBalSurf->SurfAbsThermalInt(SurfNum). The left panel is the output with coating implemented using MaterialProperty:VariableAbsorptance and the right panel is the output from the EMS-implemented coating. 0.2 is the absorptance value if the coating is applied, and 0.9 is the absorptance of the base material.

The fact that EMS overwrites both inside and outside face absorptance is not clearly documented. In addition, one suggested use case of the three Material Surface Properties is "modeling switchable coatings such as thermochromic paints." These coatings are unlikely to be applied to both the interior and exterior sides of the wall/roof, so it seems incorrect for EMS to overwrite absorptance on both sides.

In the meantime, users could use MaterialProperty:VariableAbsorptance for coatings with changing thermal or solar absorptance

To fix the issue, there could be different options

Allow users to specify which side of the surface (inside, outside, both) is to be adjusted for three Material Surface Properties.
Provide clear documentation on the fact that both inside and outside face absorptance is adjusted and redirect users to use MaterialProperty:VariableAbsorptance if they only want absorptance of the outside face to be adjusted.

Details

Some additional details for this issue (if relevant):

Platform (Mac, Version 10.15.7)
EnergyPlus develop branch #575d952dc3

EMS model

This is the EMS model used in the testing. Zn001:Wall001 is a single-layer wall with coating on the outside. 1ZoneUncontrolled_EMS.idf.zip the SurfAbsThermalInt value is as follows (0.2 indicates overwritten of inside face thermal absorptance)

12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(1)=0.2
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(2)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(3)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(4)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(5)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(6)=0.9

If another layer is added to the inside of the wall, like in the following model, then the SurfAbsThermalInt value is not overwritten 1ZoneUncontrolled_EMS_twoLayerWall.idf.zip

12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(1)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(2)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(3)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(4)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(5)=0.9
12/21 1-1: state.dataHeatBalSurf->SurfAbsThermalInt(6)=0.9

Checklist

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[ ] Defect file added (list location of defect file here)
[ ] Ticket added to Pivotal for defect (development team task)
[ ] Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

NREL / EnergyPlus