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Energyplus, Design day : date and solar radiation

asked 2019-09-26 02:00:01 -0500

Alexis's avatar

updated 2020-01-15 11:26:38 -0500

Hello everyone,

I am working currently on a hotel project with rooms faced North and rooms faced South. I used the summer design day to size my AC.
According to the ddy file, the summer design day happens the 11th November.
I got my first results and the cooling capacity was below my expectation.
I started to dig in the different outputs and I noticed that the solar radiation for the 11th November is not the maximum of the year. The maximum happens the 13th June (my project is in Asia). Then I changed the date of my summer design to the 13th June, simulated my file and then my cooling capacity meet my expectation.
My question : Is it a relevant method ? I guess that the date of the design is chosen according to the maximum outdoor temperature, what about the solar radiation ?

Thank you for your help

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answered 2019-09-27 01:47:11 -0500

Gio's avatar

updated 2019-09-27 04:21:26 -0500

The solar radiation is not influenced by the weather file: of course it is right to take a look into the weather data to realize the highest values.

However, E+ usually uses a solar model to simulate the Summer Design Day (if you take a look in the solar profile during the design day you will get a perfectrly symmetrical profile): the standard is the AshraeClearSky method but you could choose other models already built it E+ like the AshraeTau (you should check the coefficients in the Ashrae reports here "") or the Zhang Huang.

The solar model is selected in the object "SizingPeriod:DesignDay" in the field "Solar Model Indicator". Alternatively, you could use your own solar radiation profile selecting the option "Scheduled".

Therefore, the day you chose only influence the position of the sun (and the radiation power given by the model) during the design simulation and do not have any relation with the radiation in the corresponding weather file day.

Please note that this is really relevant since E+ can not foresee the shape and the solar overhangs of your building: the maximum solar radiation may not correspond to your building cooling peaks or maximum solar gain. In this case I advise adding more than one design day.

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Thank you very much for your answer, a lot of information. However, I didnt undertand everything. So in summary, do you suggest to use the summer design day in the ddy file ? In my case, the 11th november.
Or do you recommand to change the date with the highest solar radiation rate ?

Alexis's avatar Alexis  ( 2019-09-28 01:30:00 -0500 )edit

If you have no idea and if it does not take too long, add 12 "SizingPeriod:DesignDay" objects, one for each month

Gio's avatar Gio  ( 2019-10-01 12:56:18 -0500 )edit

I also interested in this case. I already change the month of design day according to the sun position which has the highest contribution to the fenestration, it produces higher cooling capacity, but it is worthy to just change the month without change to DB temperature? Coz I've looked into the weather file, in June the DB temperature not as high as in the design day.

Eru's avatar Eru  ( 2019-10-03 20:05:08 -0500 )edit

@Eru You are right, if you don't change the air temperatures you could get an oversized result. To solve it, you can use the ashrae report data (example here, at the table named "Monthly Design Dry Bulb and Mean Coincident Wet Bulb Temperatures" you'll find a value for each percentile level (rows) and for each month (columns).

Gio's avatar Gio  ( 2019-10-04 09:13:27 -0500 )edit

@Gio Really helpful, thank you

Eru's avatar Eru  ( 2019-10-06 19:52:02 -0500 )edit

answered 2019-09-26 02:50:12 -0500

Telis's avatar

For the desing days you open the weater data in IDF Editor and in SizingPeriod:DesignDay you can see a list of the potentional SummerDesingDays and WinterDesingDays. Usually use the ones with the maximum/minimum (or the second ones) to size your heating and cooling capacity. Now for the solar radiation you can run your simulation and select the summary report from the output section. There you can see the values per month about solar radiation in the sections about outdoor enviroment.

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Thank you for your answer but my question is more about the solar radiation and the design day. Because here in my case, it seems that the parameter "solar radiation" is important for the AC sizing. I understand that the sizing day is chosen according to the temperature but what about the solar radiation ? How is it taken into account for the choice of the design day ?

Alexis's avatar Alexis  ( 2019-09-26 19:52:37 -0500 )edit

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Asked: 2019-09-26 02:00:01 -0500

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Last updated: Sep 27 '19