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



Hi Brent,

I copy the results by using 1min time and set shaowcalculations to 1day.

       Time                                    EDN [W/m2]                              Beta [deg]    
 07/21  05:55:00 167.8932  11.17909312
 07/21  05:56:00 174.5266 11.3528604
 07/21  05:57:00 181.16 11.52678234
 07/21  05:58:00 187.7934 11.70085751
 07/21  05:59:00 194.4268 11.8750845
 07/21  06:00:00 201.0602 12.04946189
 07/21  06:01:00 207.6935 12.22398827
 07/21  06:02:00 214.3269 12.39866221
 07/21  06:03:00 220.9603 12.57348232
 07/21  06:04:00 227.5937 12.74844718
 07/21  06:05:00 234.2271 12.92355539

You wrote:
The time stamp associated with output is the end of the timestep, but the data are for the entire preceding timestep
My understanding is, at one specific step, the output Direct Solar radiation value is averaged through that entire step. Am I right? This averaging is performed linearly?

According to 1mins results, I still don't understand how can we average EDN at 6:00am to 201 [W/m2] if my hand calculation is 449 [W/m2] if we are doing averaging for the entire preceding step.


Dr. Li,

that value EDN E+ output at 12:00 is 862 [W/m2] and my hand calculation is 894 [W/m2]. I notice that the difference at noon time is not that big compared with morning time. That make sense if explain it from averaging perspectives. Since at morning time you have EDN change from a number very close 0 to a number 400 in 1 hour. If you average it throught this period (an hour), it could give you some number around 200.

But I just don't know how this average is performed, especially based on 1mins results. Why average based on pervious period resutls rather than put this time step in the middle, and average from last 30min to future 30mins?

I found compared with hand calculation, the consequence is that in the morning time, hand calculation over perdicet heat load which makes the min load value much bigger than E+ results (since hand calculation give us much higher EDN). But since we have not much difference on EDN at noon and afternoon time. The peak load is very close.

--
Best Regards

Kelvin Feng


On Thu, Jul 23, 2009 at 4:20 PM, Griffith, Brent <brent.griffith@xxxxxxxx> wrote:
 

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

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@xxxxxxxx> wrote:

 

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

 

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Feng Kelvin
Sent: Thursday, July 23, 2009 11:40 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
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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