[Radiance-daysim] Comparison Daysim with Measured data

Thu Feb 9 08:18:22 PST 2017

Hi Stefan,

can you test the monitored data against other data sources? For many locations, you can get (at least interpolated) irradiance data from other weather stations. If you find e.g. good accordance between your data and that of others until Sep, but not afterwards, you would have to check the instruments rather than the simulation.

One point to remember is that the approach to reconstruct the sky luminance distribution from the two measured irradiance integrals works well for perfectly clear or perfectly covered skys. If you have a sky with distinguishable clouds, the distribution can not be reconstructed. So while you had a highly non-uniform sky distribution in real world, your sky model would average all couds/sky directions other than the sun. With annual simulations, the assumption is that this does not make a big difference. For point-in-time simulations, this may not apply. In such cases, monitoring of cloud coverage makes sense.

Cheers,
Lars.

> Am 09.02.2017 um 16:41 schrieb Koenders, S.J.M. <s.j.m.koenders at student.tue.nl>:
> 
> Dear Jan & Christoph,
> 
> Thank you for your fast response. To answer both of your questions:
> 
> -        I’m using the gen_dc and ds_illum in the DDS option. This was chosen due to the “most accurate” solution. Christoph, can you explain why you recommend the regular mode instead of the DDS option?
> -        I’m hundred percent certain that the measurement data is from the correct year. It was measured last year, January 2016 till December 2016, and I’m still getting the data.
> -        The data I get from the weather station is DNI, DHI, DiffHI and GHI. I built two weather files (DHI file and DNI file) and simulated with both of them. The first file is built from DHI & DiffHI and the second from DNI & DiffHI. One would expect similar results, but there was a strong deviation between the two results. The weather file based on DNI and DiffHI gave the best results.
> -        There is an obstruction in reality, but this object was placed and designed in such a way that 99% of the time there is no obstruction from this object. Several researches before showed that this obstruction could be neglected.
> -        The system is placed on a flat roof, so there is no other obstruction or strange pattern that I know of.
> 
> What I find particularly strange is the sudden change in behavior from September on. It would be strange if this was due to an obstruction or something, because this should also show in March/April (due to similar solar elevation and azimuth).
> 
> With kind regards,
> 
> Stefan Koenders
> 
>  
> 
> From: Christoph Reinhart [mailto:tito_ at mit.edu <mailto:tito_ at mit.edu>] 
> Sent: donderdag 9 februari 2017 15:06
> To: DAYSIM discussion <radiance-daysim at radiance-online.org <mailto:radiance-daysim at radiance-online.org>>
> Subject: Re: [Radiance-daysim] Comparison Daysim with Measured data
>  
> 
> Hi Stefan,
> 
> Do you use gen_dc and ds_illum in regular mode (recommended) or with the “-dds” option?
> 
>  
> 
> Christoph
> 
>  
> 
> From: Jan Wienold [mailto:jan.wienold at epfl.ch <mailto:jan.wienold at epfl.ch>] 
> Sent: Thursday, February 09, 2017 8:44 AM
> To: DAYSIM discussion <radiance-daysim at radiance-online.org <mailto:radiance-daysim at radiance-online.org>>
> Subject: Re: [Radiance-daysim] Comparison Daysim with Measured data
>  
> 
> Hi Stefan,
> 
> are you sure the weather file you got from the weather station is from the same year where your measurement took place ? 
> 
> What kind of data you got from the weather station? (Direct-normal, Direct horiziontal, Diffuse horizontal, Global)?
> Do you have an obstruction in the simulation (and in reality)? If yes, is it continuous (like a block?) or something with a pattern (like stripes or large leaves from a tree)?
> 
> Jan
> 
> 
> On 09/02/17 13:54, Koenders, S.J.M. wrote:
> Dear readers,
> 
> Currently I’m using Daysim (version 3.1b (beta)) for the simulation of irradiance on a new type of BIPV-system. According to what I’ve read in several articles, this should be possible. I am comparing my simulation results to irradiance data measured at a full-scale mockup of the BIPV-system using on-site recorded solar radiation data as input for the simulations. For a large part of the year, Daysim predicts almost the exact same irradiance as measured. However, suddenly from September on, Daysim significantly under-predicts the results. 
> 
> When looking at daily sun profiles and irradiance simulated and measured, it appears that Daysim uses only the diffuse irradiance instead of both direct and diffuse irradiance. Figure 1 shows a typical day, early in the year, where you can see that the agreement between measurement and simulation is quite good. Figure 2 is for 25 September, where you can see that the measured irradiance is much higher than the simulation results from Daysim and that irradiance simulated with Daysim follows almost exactly the diffuse irradiance.
> 
> Figure 1: https://www.dropbox.com/s/11xu2ivuyf866zg/Comparison%20Daysim%20%26%20Simulation%20-%205%20May.png?dl=0 <https://www.dropbox.com/s/11xu2ivuyf866zg/Comparison%20Daysim%20&%20Simulation%20-%205%20May.png?dl=0>
> Figure 2: https://www.dropbox.com/s/7wj8svs5nraex54/Comparison%20Daysim%20%26%20Simulation%20-%2025%20September.png?dl=0 <https://www.dropbox.com/s/7wj8svs5nraex54/Comparison%20Daysim%20&%20Simulation%20-%2025%20September.png?dl=0>
> I’ve checked the following things:
> 
> -        Perhaps there was a flaw in the weather file. I’m using a weather file created from a measurement station (DNI and DIFHI) and validated that it is correctly implemented in Daysim. I simulated the system also with a weather file based on an EPW file. Similar results occur.
> -        Perhaps there is a flaw in the design. I’ve modelled the system in SketchUp and imported this in Daysim. To see if the model was corrupted I have built a simple cube in SketchUp with sensors on the same orientation as the PV-system. Again similar results occur.
> 
> Even though there are some slight changes during the year, one would expect a mirroring of the results around January (January to June show similar results as July to December, but then mirrored). However, the results in January from the simulation in Daysim show again almost perfect results with the measurements.
> 
> I’ve talked to several people about these deviations, but none of them can give a clear explanation why this happens.
> 
> I hope that some of you can give an explanation and help me with this problem.
> 
> With kind regards,
> 
> Stefan Koenders
> 
> 
> 
> 
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>  
> 
> -- 
> Dr.-Ing.  Jan Wienold
> Ecole Polytechnique Fédérale de Lausanne (EPFL)
> EPFL ENAC IA LIPID
>  
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