[GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006 [resolved]

Tue Dec 12 06:59:40 EST 2006

Hi all,

as I promised you (Dylan), here you have a useful reference to derive the
direct beam component from the global radiation.

Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.A, and Kaltsounides N.A.
(2006): ' Comparative study of various correlations in estimating hourly
diffuse fraction of global solar radiation' Renewable Energy, 31, 2492-2504

This paper offers a comparative analysis of different correlations between
diffuse and global radiation. The references [2], [3], [4] and [9] on it, I
think are especially interesting. I mention only this paper because on it
you can find a summary of the most important correlations.

Reference [3] is especially simple and easy to read although it only uses
measurements of Toronto. Also very interesting for you, perhaps, is the
reference [4] since it is based on five USA locations. For example, this
author proposes the correlation

kd = 1.45-1.67kt for 0.3<kt<0.78
kd = 1.02-0.248kt for kt<=0.3
kd = 0.147 for kt>=0.78.

where kt is the ratio of global to extraterrestrial hourly radiation and kd
is the ratio of diffuse to global hourly radiation. Therefore, to calculate
kt only need the measured global radiation and the extraterrestrial
radiation, theoretically calculated for a given location and time (see for
example, Iqbal, Muhammad (1983). An Introduction to Solar Radiation.
Academic Press, New York). Then you get kd from the correlation, and

Direct Beam = Global*(1-kd).

So, if you dispose of global measurements, you can get the direct beam
radiation directly applying the former expression.

By the way, I would like to know what you are trying to do, improving r.sun?
or just working in a personal project? I have used r.sun a little bit and I
would like to collaborate with r.sun. I could work with the model and/or
even developing some snippets of code.

Cheers,

Jose A.

> -----Mensaje original-----
> De: grassuser-bounces at grass.itc.it [mailto:grassuser-bounces at grass.itc.it]
> En nombre de jararias at ujaen.es
> Enviado el: viernes, 08 de diciembre de 2006 13:09
> Para: grassuser at grass.itc.it
> Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006
> [resolved]
> 
> Hi all,
> 
> firstly, I really hope not to interfere in this conversation. I assist
> expectant to the thread cause is very interesting for me.
> 
> It seems you want to get diffuse component from global radiation. If I
> have understood well, you estimate it as the 10%-20% of the global one.
> Just a little suggestion that I hope not to be very simplistic. Have you
> think about a correlation between the clearness-index and the proportion
> of diffuse? There exist several correlations around the world such as the
> studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
> usually use hourly values. You can easily obtain a estimation for the
> hourly diffuse component (and, therefore, for the hourly direct beam one)
> from the hourly global irradiation measured with a pyranometer as the
> LICOR. If you are interesting in this approach just let me know and I will
> be delighted of sending you more references. Now I can't be more explicit
> because I don't have access to my personal data-base cause I am not at the
> office.
> 
> Kind regards,
> 
> José A.
> 
> > On 12/7/06, Hamish <hamish_nospam at yahoo.com> wrote:
> >> Dylan Beaudette wrote:
> >> > > > > Get local Linke values from the SoDa Solar radiation Databases
> >> > > > >   http://www.soda-is.com/
> >> > > >
> >> > > > Tried this, and got unrealistic values for my site.
> >> > >
> >> > > how so? Have you calculated good Linke values from your light
> >> > > meters? (formula for that, please?)
> >> ..
> >> > Attached is a plot of estimated Linke turbidity values, compared to
> >> > the SoDa  derived data. Details on this calculation soon to follow in
> >> > a new thread.
> >
> > Hi Hamish,
> >
> > Looks like I sent an update this morning with an attachment that was too
> > large!
> >
> > I'll Include it at the bottom of my replies to you last message.
> >
> >
> >> I assume the physical basis for the Linke turbidity factor is the
> amount
> >> of scattering dust, ash, water vapour, turbulence etc. in either the
> >> column of air directly above the sampling site or in the ray between
> the
> >> site and the Sun (which will pass through more atm in winter due to the
> >> lower angle). Maybe the paper sheds some light on that. So e.g. LA
> >> county would have very high turbidity numbers.
> >>
> >> I would figure in the winter the air to be crisper/clearer due to less
> >> water vapour, less turbulence, and less upwelling of aerosol laiden
> warm
> >> air from the Earth's surface. So lower turbidity numbers in the winter
> >> as the SoDa data shows. BUT if you live in a place which is prone to
> >> inversion layers in winter and many locals heat their homes with wood
> or
> >> coal, winter air quality may turn out to be much more turbid.
> >>
> >
> > Actually have a couple good papers on the defintion for the Linke
> > turbidity values:
> >
> > Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
> > Solar Energy, 1986, 37, 393-396
> >
> > Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> > climatic parameters Solar Energy, 2000, 69, 99-111
> >
> > Kasten, F. The linke turbidity factor based on improved values of the
> > integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244
> >
> > My site is in a mediterranean, semi-coastal / semi-arid setting where
> > winter time moisture and summer time dryness are the usual weather
> > patterns. Summer through fall are usually cloud free, and quite warm.
> > Also- this site is rather remote, so urban ag-related dust influences
> > are minimal. These are some of the reasons that I was having second
> > thoughts about the SoDa values.
> >
> >>
> >> > I have calculated Linke turbidity for my site with the equation
> >>
> >> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
> >>
> >> > Unfortunately, there are some aspects to this equation which
> >> > I do not understand: namely the two constants present.
> >>
> >> perhaps they are empirically derived?
> >
> > Ok- big change in direction. I posted an update to the use of the
> > above equation earlier today but it was rejected due to a large
> > attachment. Details below....
> >
> >> I take it I_0 is pure-clear air intentensity?; h is latitude?; and is G
> >> measured intensity, or ..?
> >
> > G = global radiance measured on the ground (beam+diffuse)
> > I_o = extraterrestrial radiance
> > h = solar elevation (angle)
> >
> >> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> >> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> >>
> >> I will have to track down a copy of that. Holy cow, for once our
> library
> >> is actually subscribed to the e-journal. Downloading the PDF now..
> >>
> >
> > ... i know how that can be sometimes!
> >
> >> What software did you use to make that plot? Looks nice.
> >>
> >
> > Thanks. It was made in R, a fun command-line based environment for
> > plotting, analysis, etc. I can post the code used after a small
> > cleanup.
> >
> > Ok, here is the text from the message rejected this morning:
> >
> >
> > Some updates on my findings with respect to obtaining optimal Linke
> > turbidity values via local pyranometer measurements.
> >
> > Chatting with a local atmospheric scientist, I learned that it is
> > possible (with a loss of some precision of course) to dissaggregate
> > beam radiance from global radiance (as measured by pyranometer), for
> > use in an equation [1] for the linke turbidity factor. Compared to a
> > previous paper I mentioned on grass-dev [2], the equation presented in
> > [1] is a much more appropriate approach to calculating Linke turbidity
> > values.
> >
> > The key points to using the equation from [1] are:
> >
> > - historical pyranometer data from a weather station, with coordinates!
> > - r.sun mode 1 to calculate solar elevation at this station, for each
> > day at some time. extraterrestrial radiance is also calculated in this
> > step
> > - dissagregation of the beam component from global radiance with the
> > assumption that diffuse radiance is approx 10%-20% that of the beam
> > radiance
> > - conversion of dissaggregated beam radiance to beam radiance on the
> > normal (B/sin alpha)
> >
> > here is a link to a bit of my thesis on this topic: (rather incomplete!)
> > http://169.237.35.250/~dylan/temp/linke-cucumo2000.png
> >
> > Here is a link of linke turbidity values, as calculated by the method
> > described above, along with a comparison to the SoDa-derived values.
> > There is considerable difference between the two sources!
> >
> > http://169.237.35.250/~dylan/temp/cucumo-
> estimated_daily_linke_values.png
> >
> > With the equation from [1] it is possible to estimate the linke
> > turbidity value for each day (using the 12pm calculation as
> > representative for an entire day), and use these estimates in r.sun
> > mode 2. here is a summary of the incorporation of estimated linke
> > values into r.sun - as compared to the weather station data:
> >
> > http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
> >
> >
> > I'll post some updates as they come. If I have anything wrong please
> > don't hesitate to point it out!
> >
> > Cheers,
> >
> > Dylan
> >
> >
> > 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
> > for the estimation of the Linke turbidity factor Renewable Energy,
> > 2000, 19, 249-258
> >
> > 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> >
> > _______________________________________________
> > grassuser mailing list
> > grassuser at grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grassuser
> >
> 
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