RE: [EnergyPlus_Support] Summer Design Day -- Direct Solar Radiation Calculation

Try a 1 minute timestep and ShadowCalculation frequency set to 1.

The time stamp associated with output is the end of the timestep, but the data are for the entire preceding timestep. You probably want to back up the time of day by half a timestep when doing the hand calc.

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Feng Kelvin
Sent: Thursday, July 23, 2009 1:41 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Summer Design Day -- Direct Solar Radiation Calculation

Hi,

The reporting frequency is time step. My simulation time step is hourly.

You mean the reporting value 201 [W/m2] in my case could be averaged over an hour which is different from hand calculation's one time point? If yes, how this averaged been done? Why not use one instantaneous value that matching with solar altitude angle output?

I also did simulation with 15min time step. I copy the results below:

Time    Direct Solar Radiation [W/m2]    beta [deg]     Hand calculation Direction Solar [W/m2]
6:00            201                                    12.049             449
6:15              300                                     14.68
6:30              400                                     17.37
6:45              458                                     20.029
7:00              516                                     22.74      676

However, I am pretty sure that solar altitude angle beta is an instantaneous time point value.

Another thing is, If averaged, I found incidental solar radiation to individual surface is calculated according to the "averaged" value based on the reporting outputs. And I found big difference on building heat load.

--
Best Regards

Kelvin

On Thu, Jul 23, 2009 at 2:11 PM, Griffith, Brent <brent.griffith@nrel.gov> wrote:

What is the reporting frequency? Maybe you are comparing results averaged over an hour to an instantaneous hand calculation.

From: EnergyPlus_Support@yahoogroups.com [mailto:EnergyPlus_Support@yahoogroups.com] On Behalf Of Feng Kelvin
Sent: Thursday, July 23, 2009 11:40 AM
To: EnergyPlus_Support@yahoogroups.com
Cc: Feng Kelvin
Subject: [EnergyPlus_Support] Summer Design Day -- Direct Solar Radiation Calculation

Hi group,

I am using my E+ v2.2 design day to simulate a simple concrete two rooms case. I found E+'s Direct Solar Radiation output result doesn't match with direct normal solar radiation calculation equation: EDN = A / exp ( B / sin(beta) )

I pick up my case at 6:00am simulation results. E+ results give:

solar altitude angle, beta = 12.049 [deg]
direct normal radiation, EDN = 201 [W/m2]

I calculate solar altitude myself by inputting building altitude, declination angle and solar time and found the solar altitude angle, beta = 12.049 is correct. However, when I use

EDN = A / exp ( B / sin(beta) )
where A = 1093 [W/m2]
B = 0.186

I got EDN = 449.98 [W/m2], not 201 [W/m2] calculated by E+.

Am I missing sth in this, or this has been fixed in new E+ version?

--
Best Regards

Kelvin

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