[Bldg-sim] Answers re: Test your knowledge of simulation weather file formats Part 2: the EnergyPlus *.epw format

Joe Huang yjhuang at whiteboxtechnologies.com
Tue Dec 19 10:19:46 PST 2017

It's been pointed out to me that the GHI and DNI on the Legnica weather file were not switched, the appearance of which happened during
my postprocessing of the epw file.


Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

On 12/19/2017 9:44 AM, Joe Huang via Bldg-sim wrote:
> I only got two responses this time, which I attribute to either the close proximity of 
> the holidays or perhaps reader fatigue about weather data
> However,  the answers I did receive, especially the second one, were so much in line 
> with what I saw on the weather file that it looks copied which I must  affirm was not 
> the case (see the answers at the bottom of this post).
> So, the three things I found unusual about this weather file are, in order of ascending 
> importance:
>   1. The year is given as 2005 throughout, but the comment says that it's a "typical 
> year" file made from the time period 1970-2000.
>   2.  Line 8 says that the first day of the weather file, i.e., Jan. 1, is set as a 
> Sunday, but January 1 2005 was a Saturday.
>   3. The solar radiation appears to be shifted a half-hour ahead, probably because the 
> original file was created in Europe, which tends to report the solar around the time 
> step, e.g., -0:30 to +0.30, whereas in North America it's reported for the preceding 
> time step, i.e., -1:00 to 0:00.  Although that was the only criteria for why I chose 
> this weather file (Legnica, Poland), I later found other more troubling aspects to the 
> solar radiation that made me expand the discussion to  QC'ing the solar on weather files 
> in general.
> I first became aware of the different conventions of reporting solar back in 1993 when I 
> was involved in an IEA project on Low-Energy Cooling where one of the first items of 
> business was to compare the climate conditions in the participant countries. When I 
> tried to  work with weather data from European colleagues, I was getting impossibly 
> large spikes of Direct Normal Irradiance (DNI) for many sunrise hours due to this 
> difference in convention, since I was using a US simulation program that follows the US 
> convention and calculates the sun position at the midpoint of the preceding time step.
> Although it might seem hard to tell which convention is being used in a weather file, 
> it's actually quite noticeable if one were to compare the hourly profiles to those 
> calculated by a Clear Sky Model.  For example,  this plot shows July 1 - 4, where the 
> forward shifting of the solar on the weather file can be seen, particularly if you look 
> for the sunrise and sunset hours. What really surprised me, though, was that the Global 
> Horizontal Irradiance (GHI) and DNI seem switched on the weather file. The GHI profile 
> is generally more smooth and the DNI more spiky, but here it's the reverse.  Also,  the 
> GHI greatly exceeds the Clear Sky GHI on the morning of Day 3, which simply cannot happen.
> The data for Jan. 1 -4 reveals more indications that the GHI and DNI on the weather file 
> may be switched., with the GHI (?) again more spiky and on Jan. 2  almost three times as 
> much as the Clear Sky GHI.  The 30-minute shift is less visible owing to the small 
> values, but can still be detected by looking at the sunrise and sunset hours.
> So what does this say about QC'ing weather data?  It seems that more emphasis has gone 
> into QC'ing excursions in temperature than solar radiation. However, since the solar in 
> almost all weather files is not measured but calculated, there should be all the more 
> reason to regard it more carefully.  Luckily, there are several simple, if not 
> simple-minded,  facts that can be used as reality checks, e.g., (1) the GHI should 
> always be non-zero when the sun is above the horizon, making it possible to determine 
> the hours of sunrise and sunset,  (2)  the GHI and DNI could never be greater than their 
> Clear Sky values (don't worry about ground reflectance or atmospheric phenomena like 
> cloud lensing since we're only dealing with calculated  data),  and (3) the Direct 
> Horizontal ( DHI = DNI * arcsin of the solar angle) cannot be greater than the GHI.
> I hope this answer has provided some insight into the contents of weather files. As in 
> the previous contest, both respondents, Samuel  Letellier-Duchesne and Michael Kummert, 
> are declared winners and entitled to one free weather file of their choice from the WBT 
> archive.  Just let me know. (Full disclosure: Samuel appears to be Michael's graduate 
> student :-))
> Holiday greetings to all!
> Joe

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