I suspect there is another issue going
on. What E+ is reporting is for a horizontal surface, not directly facing the
sun. I think you need another Sin(beta) factor in the hand calc.
From:
Sent: Thursday, July 23, 2009 3:02
PM
To:
Subject: 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,
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@
Sent: Thursday, July 23, 2009 1:41
PM
Subject: Re:
[EnergyPlus_
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,
What is the reporting
frequency? Maybe you are comparing results averaged over an hour to an
instantaneous hand calculation.
From: EnergyPlus_Support@
Sent: Thursday, July 23, 2009
11:40 AM
To: EnergyPlus_Support@
Cc: Feng Kelvin
Subject: [EnergyPlus_
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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