[Radiance-general] Calculation of solar declination in sun.c

Fri Dec 18 08:28:21 PST 2015

This is fascinating, and in my opinion a cautionary tale for the community, given all the weight being placed on ASE as a daylight metric lately.

On 12/18/15, 8:40 AM, "Martin Gut" <gut at Transsolar.com<mailto:gut at Transsolar.com>> wrote:

For me, the difference of the real picture and the simulation is quite visible. Maybe this discussion topic is one reason for this.

We came across this issue by doing a combined thermal and dynamic daylight analysis.
One calculation with original DAYSIM  and one DAYSIM calculation with sun position calculated from TRNSYS.
There were quite visible differences, especially in Autumn.

https://drive.google.com/file/d/0B_k3K9s5QAlISGdYWFdlMHZFc00/view?usp=sharing

and

https://drive.google.com/file/d/0B_k3K9s5QAlIb0lRN3VrdXVQaFU/view?usp=sharing

TRNSYS uses the Spencer-Algorithm  from 1971 for the calculation of solar declination.
Implementing this Algorithm is easy because it has just a few more sin() and cos() statements .

This graph:

https://drive.google.com/file/d/0B_k3K9s5QAlIbWVXZHhINXhYTDA/view?usp=sharing

shows the differences between this algorithm, the Cooper Algorithm and the Algorithm used in Radiance.
So my first question is answered by this. By using 368 days per year the deviation is smaller than with the original
Cooper Algorithm.

A good starting point for choosing the right algorithm could be in my opinion :
“Computing the solar vector” in  Solar Energy Vol. 70, No. 5, pp. 431–441, 2001 by  MANUEL BLANCO-MURIEL et al.
Here, the differences of common algorithm are described. They use only the Spencer Algorithm as an example of a simple algorithm.
Following table shows, that there are considerable differences:
https://drive.google.com/file/d/0B_k3K9s5QAlIcS1JQmZFSGttbWs/view?usp=sharing
They recommend the Michalsky-Algorithm or their own PSA-Algorithm, which is a little bit better.
The Michalsky Algorithm is also used in the program SolPos Nathaniel Jones recommended:

https://www.nrel.gov/midc/solpos/

The Source Code for the PSA-Algorithm is available under:

http://www.psa.es/sdg/sunpos.htm

It seems to be comparable to the  from  Christian Humann recommended NREL algorithms which is used in the program spa:

https://www.nrel.gov/midc/spa/

>From my point of view as a user it would be desirable, that the main used Simulation Programs uses the same or at least quite similar algorithm for calculating the sun position.
TRNSYS uses the Spencer-Algorithm, which is due to the upper publications also not the best choice.
It would be interesting what algorithm EnergyPlus uses.

Regards

Martin

Von: Christoph Reinhart [mailto:tito_ at mit.edu]
Gesendet: Donnerstag, 17. Dezember 2015 18:29
An: Radiance general discussion
Betreff: Re: [Radiance-general] Calculation of solar declination in sun.c

Using a more accurate formula that is based on a respectable source like a good idea if the magnitude of the correction is as large as Martin describes. Incidentally, we are having our students do “validations” of the formula by comparing the photograph of an objects as a certain time in the year to a Radiance rendering and that analysis tends to come out pretty well: http://web.mit.edu/sustainabledesignlab/projects/DaylightingHandbook/Exercise_DirectSunlight.htm.

Best,

Christoph

From: Greg Ward [mailto:gregoryjward at gmail.com]
Sent: Thursday, December 17, 2015 12:22 PM
To: Radiance general discussion <radiance-general at radiance-online.org<mailto:radiance-general at radiance-online.org>>
Subject: Re: [Radiance-general] Calculation of solar declination in sun.c

Given the tool construction, it would be simple enough to introduce a global variable that switches between the de facto solar angle calculation and a more precise version.  I'd like to hear from others that this is worthwhile before I spend too much time on it, though.  Also, whether it should be exposed as an option in all of the calling programs (i.e., IES calculation or more accurate on).

Is there a general consensus on which solar calculation is best?  I would just end up googling it, unless you or someone else has a strong recommendation.

Cheers,
-Greg

From: Martin Gut <gut at Transsolar.com<mailto:gut at Transsolar.com>>

Subject: Re: [Radiance-general] Calculation of solar declination in sun.c

Date: December 17, 2015 8:55:38 AM PST

Hi Greg,

Thank You for the quick answer.  I don’t know about the history of this debate because I’m quite new in this mailing list.
I think for the CIE standard stuff, sticking on the standard routines is ok.
  But this formula is also used for all the dynamic daylighting stuff like
Gendaymtx
Gendaylit
Daysim
etc.
also for shading studies: I’m wondering that so much computing effort is investigated to render the shading and on the other side the position of this shading is incorrect (especially in Autumn) because of
a very rough calculation of the sun position. The error is not so big, but it could be avoided very easy by using a better formula for solar declination.
Maybe there could be two functions, one for the standard calculations and one for “real”  calculations.

Martin

Von: Greg Ward [mailto:gregoryjward at gmail.com]
Gesendet: Donnerstag, 17. Dezember 2015 17:14
An: Radiance general discussion
Betreff: Re: [Radiance-general] Calculation of solar declination in sun.c

Hi Martin,

This debate comes up every so often with the solar calculations used in Radiance and related tools.  The main question is, do we stick with the established standard, which provides for easy "apples to apples" comparisons, or do we update our formulas to get a more precise answer?  In the case of the original CIE sky models, which this solar calculation is used in primarily, the accuracy is not that great, so getting the sun in exactly the right place is a minor quibble.  If, on the other hand, you need precise sun position for solar shading studies or the like, then it's easy to argue for a better formula.  I don't think computing power was ever the issue with Radiance.  We don't compute the sun position for every ray-traced or anything silly like that.

As Rick points out, the use of 368 is part of the IES standard calculation, but I'm not entirely sure what anomaly it is correcting for.

Cheers,
-Greg

From: Richard Mistrick <RGMARC at engr.psu.edu<mailto:RGMARC at engr.psu.edu>>

Subject: Re: [Radiance-general] Calculation of solar declination in sun.c

Date: December 17, 2015 7:18:53 AM PST

I’m not certain why it is 368, but this equation has been in this format in the IES Lighting Handbook for many years.

Rick

From: Martin Gut [mailto:gut at Transsolar.com]
Sent: Thursday, December 17, 2015 6:05 AM

Dear Radiance Experts,

I have two questions regarding the calculation of solar declination:

1.       Why has the year in function sdec  in  file sun.c  368 days instead of 365 as in the original formula from Cooper ?

return( 0.4093 * sin( (2*PI/368) * (jd - 81) ) )

https://github.com/NREL/Radiance/blob/master/src/gen/sun.c

2.       Why does Radiance not use a better formula, which takes into account, that the Earth orbit is not a circle?

With today computing power, there is no more reason to use this simple formula

Thanks in advance

Martin

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