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Re: [EnergyPlus_Support] Photovoltaics - shading calculations?
Justin:
There is no easy way to account for shading on PV surfaces.
A reply from the TRNSYS link expert . . .
-----------------
In trying to keep the photovoltaics as self contained as possible, it
is actually TRNSYS that reads the E+ weather file, picks out what it
needs and then passes the data to the PV model (a TRNSYS Type).
Justin is quite correct that the beam and diffuse components of the
radiation are handled separately. TRNSYS is taking the total and
direct normal radiation from the weather file, passing it through a
radiation processor that tells me what the beam, diffuse,
circumsolar, ground reflected etc. radiation is on the surface
defined in the Photovoltaics IDD entry, and then passing that on to
the photovoltaics. If Justin is familiar with TRNSYS input files and
knew at what time the sun rises and sets over the parapets, he could
modify the file eplus.def to cut out beam radiation before and after
those times, leaving the diffuse radiation alone. That may be well
deeper into the internal workings of the connection than he wants to
get though.
--------------------
Some additional thoughts from me . . .
With some effort, you could achieve an approximate answer by doing
several runs:
1. Run the PV modules with the normal weather (no shading on the PV).
2. Run the PV modules with diffuse only. You could do this by
modifying the eplus.def file for TNSYS, or by making a copy of the
weather file and zeroing out all of the beam solar (converting to csv
simplifies the editing task, then convert back to a new epw file).
3 and 4. Add a dummy zone to your building with a heat transfer
surface(s) representing the PV panel(s). For the dummy PV surface,
report Surface Ext Solar Beam Incident[W/m2] and Surface Ext Solar
Sky Diffuse Incident[W/m2]. Run this once without the parapet
shading surfaces and once with them.
With the solar values from runs 3 and 4, you should be able to take
the PV outputs from runs 1 and 2 and estimate the actual shaded PV
power output. I'm not sure if I've thought this through correctly,
but you should be able to come up with a similar approach to get a
good approximation. Assuming the answer is worth this much effort.
Mike
On 23 Apr 2003, at 16:26, Justin Harrell wrote:
> Hello,
>
> I am trying to use E+ to simulate a building with a substantial grid-
> tied PV system on the roof. Unfortunately, there are 42" high parapet
> walls on the east & west sides of the roof. These parapets will shade
> portions of the array from direct irradiance in the morning and
> evening when the solar altitude is low. These shaded modules will
> still receive diffuse irradiance in proportion with their view factor
> to the sky.
>
> My understanding is that the full version of TRNSYS can account for
> the influence of these direct and diffuse components separately. Does
> the TRNSYS module of E+ also have this capability? The weather file
> info states that only the global horizontal irradiance is used in
> photovoltaics calcs. Does the program decompose this data into direct
> and diffuse irradiance?
>
> I couldn't find a way to specify the location of the array relative to
> any building surfaces. Is there a way to take advantage of the "full
> external shading" capability of E+ to calculate partial array shading
> from building surfaces or external structures such as nearby tall
> trees or neighboring buildings?
>
> I would appreciate any help on these topics. Thanks,
> Justin
>
>
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