I first learned of how EnergyPlus interpolates the solar
radiation for smaller time-steps from a paper given by Tim
McDowell at the SimBuild 2015 Conference in Salt Lake City a month
ago. I was frankly surprised that EnergyPlus takes the hourly
value at the middle of the time step and then interpolates
linearly to the next hourly value set to the middle of the next
time step. This bothers me because the solar radiation on the
weather file is an aggregate over the time step, and not an
instantaneous rate as are temperature, wind speed, etc. (Well,
actually simulation programs like EnergyPlus, DOE-2, etc., treat
all parameters as the average rate over the previous time step but
that's a slightly different topic).
When such a linear interpolation is done, the area under the
curve, i.e., the total amount of solar radiation over the hour, no
longer matches what's shown on the weather file. I've heard
anecdotal stories from people running EnergyPlus at progressively
smaller time steps, and seeing the numbers change monotonically,
and now wonder if this could be an artifact of this linear
interpolation?
Since the solar radiation on the weather file is an aggregate rather than a rate, its value should be maintained when going to finer resolution. I would think it would be better to keep the solar radiation constant throughout the hour, with the only variation those that can attributed to differences in sun angles.
I'm responding on this topic mostly because several of us who
listened to Tim's presentation have been discussing it via
e-mail for the past month.
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 205A Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx http://weather.whiteboxtechnologies.com for simulation-ready weather data (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
Thank you
On Friday, September 2, 2016 12:55 PM, "Linda Lawrie linda@xxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
First,
G_global = G_horzdirect_Solar (i.e. G_direct_normal *SIN (solar_height)) + G_Diffuse_Horz
I expect your differences are due to the interpolation that EnergyPlus does.
At 02:10 AM 9/2/2016, theisped@xxxxxxxxx [EnergyPlus_Support] wrote:
How do you extract the Global radiation from Eplus? I've been looking into the weatherfiles and the data entries for Global radiation does not seem to correspond with those reported by EnergyPlus under following two outputs:
- Site Diffuse Solar Radiation Rate per Area
Diffuse solar is the amount of solar radiation in W/m2 received from the sky (excluding the solar disk) on a horizontal surface.
- Site Direct Solar Radiation Rate per Area
Site Direct Solar Radiation Rate per Area is amount of solar radiation in W/m2 received within a 5.7° field of view centered on the sun. This is also known as Beam Solar.
I'm aware that you have to adjust the direct solar with respect to solar angle, but even then i do not seem to get the same results. I've been using the following formula for the conversion:
G_global = G_direct*cos(90-SolarAltitude)+G_Diffuse.
Thanks in advance for any assistance.
EDIT: By the way, the specific values i see in the weatherfile and from EnergyPlus output is as follows:
At 12:00, the weather file reads: Global radiation: 103 W/m2 Diffuse radiation: 63 W/m2 Direct radiation: 264 W/m2
Energy plus outputs (also 12:00) read: Solar Altitude: 11.2 degrees Site diffuse: 50.5 W/m2 Site direct: 188 W/m2
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Posted by: Joe Huang <yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx>
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