[EnergyPlus_Support] Re: direct solar irradiance in the workplace as control system indicator - EMS

["samuel.vries.de@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx>]

Hi Steph,

It sounds like you are trying to implement Reinhart's manual blind control model. A student in our group recently stumbled across the same problem. We solved it by using daysim/Radiance to obtain solar Irradiance hitting the work plane throughout the year and inserted that into EnergyPlus as a schedule which we referenced using EMS. We made sure that we only measured direct solar irradiance by setting all the diffuse global Irradiance values in the EPW to 0 and setting the number of ambient bounces to 0 (-ab 0). 

An easier route however would be to just use the built in Reinhart models/blind logic capabilities that are implemented in Daysim and Honeybee (in our case we were trying to do some other very specific things which made this route impractical). You can use these to obtain a blind position schedule and reference this directly in EP. 

As for trying to obtain an input variable for the Reinhart model using only EnergyPlus, we considered the following options (none of them are ideal): 

• Using 'Surface Window Transmitted Beam Solar Radiation Rate' and dividing this by the window's area. Problem is, this is on a vertical surface so you would have to cosine correct it using the horizontal profile angle. Also at the work plane is not really the same at directly behind the window. 
• Using illuminance values at the work plane from EP's daylighting model and converting them back using EP's output Luminous Efficacy. Problems with this are that the split between diffuse and direct illuminance at the daylight sensor point is hard to obtain. Also you have to weigh your output using the spectral properties of the window as the luminous efficacy EP reports is outside whilst the sensed illuminance is inside.

Concerning the two only EP options it's good to note that the 50 W/m2 is a proxy. I understood that Reinhart found that this value corresponded with the moment that the sun becomes visible through the clouds. So I believe the value itself is not empirically derived. I believe rather it's a rationally deducted value based on the empirical knowledge that users pull down the blinds when the sun becomes visible and keep them down for the rest of the day. So in case you want to keep things simple; you could argue that using transmitted beam solar radiation rate', with a threshold that corresponds with a moment when the sun becomes visible, is in line with the original intent of the model. 

I am still interested to know if anyone has alternative suggestions. Maybe there

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Posted by: samuel.vries.de@xxxxxxxxx

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