Clear Air Attenuation
Joe Rich
Kreith "Solar Heating and Cooling". I am implementing some of the
equations in this book in a visual basic program - I find if I do this
modeling, I have a much better understanding of what is actually
happening.
The result of their estimate for clear air atmospheric attenuation for
our latitude at local noon on December 21 is 742 watts/m^2, or -2.6
dB; for June 21 it is 962 watts/m^2, or - 1.4 dB (having designed link
budgets for satellites, I think in dB).
When one takes into account the cosine of the normal of the collector
and the sun angle, the losses throughout the day of a fixed collector
become quite high.
The winter attenuation is a little suprising to me at first glance,
probably because of my ignorance of the subject. Does this sound
correct?
Maybe I should be thinking of a cup half full instead of a cup half
empty. However, my training on satellite comm links makes me think of
$/dB in optimizing the budget, both in regards to signal strength and
finances.
Joe
Tony Luck
> The result of their estimate for clear air atmospheric attenuation for
> our latitude at local noon on December 21 is 742 watts/m^2, or -2.6
> dB; for June 21 it is 962 watts/m^2, or - 1.4 dB (having designed link
> budgets for satellites, I think in dB).
>
> When one takes into account the cosine of the normal of the collector
> and the sun angle, the losses throughout the day of a fixed collector
> become quite high.
A Google search for "insolation san francisco" turned up some daily
insolation tables for a few years. Ignoring the bad values near the
solstices (as presumably cloudy days) I saw around 183 around the
winter solstice, and 636 around the summer solstice. So yes, the
low sun angle during the winter is measurably very bad for solar
harvesting.
I've seen repeated assertions on the internet that a tracking collector
isn't cost effective (none with data to back this up ... but I have difficulty
envisioning a cheap, sturdy, sun-tracking panel that is easy to mount
to a typical house).
-Tony
Joe Rich
a cheap drive - use rear differential from cars / trucks.
I have been thinking of this for awhile - should be able to drive
quite a large panel with relatively simple motors. Just a thought.
Joe
On Mar 19, 6:17 pm, "Tony Luck" <tony.l...@gmail.com> wrote:
John Allen
OK, I cheat. I aim my collector to be perpendicular to noon sun, to the shadow covers more area than my collector. That would explain how I can achieve 500 watts/m^2 at about 50% efficiency in February. I really should calculate, or measure, the shadow.
A big factor that is not much talked about is that w/o any seasonal positioning, your orientation can, at best, be ideal at one moment per year and is way out of whack 6 months from that moment.
The rule of thumb to aim at latitude plus 15 is based on a lot of assumptions that may not be true for a particular installation.
>optimizing the budget, both in regards to signal strength and finances.
ja
Joe Rich
and, if the arrays are fixed, point them for optimum collection in
winter. Then, as the sun rises in declination, the decreased
attenuation (along with the increase in ambient temperature) will
compensate for the cosine of the angle between the sun rays and the
normal to the collector.
Maybe I will do some simulation when I get my software finally
completed. All the functions are essentually done and debugged, just
need to build the top level function to do the simulation, and port it
to VBA in excel instead of VB.
Joe
On Mar 19, 9:17 pm, "John Allen" <johnaal...@gmail.com> wrote:
> >December 21 is 742 watts/m^2
>
> OK, I cheat. I aim my collector to be perpendicular to noon sun, to the
> shadow covers more area than my collector. That would explain how I can
> achieve 500 watts/m^2 at about 50% efficiency in February. I really should
> calculate, or measure, the shadow.
>
> A big factor that is not much talked about is that w/o any seasonal
> positioning, your orientation can, at best, be ideal at one moment per year
> and is way out of whack 6 months from that moment.
>
> The rule of thumb to aim at latitude plus 15 is based on a lot of
> assumptions that may not be true for a particular installation.
>
> >optimizing the budget, both in regards to signal strength and finances.
>
> I'm with you on this attitude.
>
> ja
>
> On 3/19/07, Joe Rich <RichMan...@gmail.com> wrote:
>
>
>
>
>
>
>
> > I have a book, which I have had for a very long time, by Kreider and
> > Kreith "Solar Heating and Cooling". I am implementing some of the
> > equations in this book in a visual basic program - I find if I do this
> > modeling, I have a much better understanding of what is actually
> > happening.
>
> > The result of their estimate for clear air atmospheric attenuation for
> > our latitude at local noon on December 21 is 742 watts/m^2, or -2.6
> > dB; for June 21 it is 962 watts/m^2, or - 1.4 dB (having designed link
> > budgets for satellites, I think in dB).
>
> > When one takes into account the cosine of the normal of the collector
> > and the sun angle, the losses throughout the day of a fixed collector
> > become quite high.
>
> > The winter attenuation is a little suprising to me at first glance,
> > probably because of my ignorance of the subject. Does this sound
> > correct?
>
> > Maybe I should be thinking of a cup half full instead of a cup half
> > empty. However, my training on satellite comm links makes me think of
> > $/dB in optimizing the budget, both in regards to signal strength and
> > finances.
>
> > Joe
>
> --
> John A Allen- Hide quoted text -
>
> - Show quoted text -
Joe Rich
More thoughts on trackers - since I am new to this field, some of this
is probably common knowledge.
I am used to AZ/EL tracking for satellites. However, I also am
familiar with telescope tracking for right ascension (RA) and
declination (DECL). I know that the C-Band dishes use some form of
the RA/DECL tracking, as they only track in one direction, which is
essentially RA.
I would believe that one could fairly easily modify a C-Band tracking
system to track the sun. One could periodically have to correct for
the change in solar declination due to the seasons, but this could be
fairly infrequent and done manually. I beleive a C-Band tracking
system should support a fairly big collector, as some of the antennas
were on the order of 10 to 15 feet diameter. Also, there are a fair
number of these systems not being used anymore as most people have
switched over to the Dish systems.
I am not sure about the control of these systems - how do they know
what the RA is? But this has to be fairly simple and could probably
be implemented with a simple controller and a "few lines of code".
Joe
On Mar 19, 6:17 pm, "Tony Luck" <tony.l...@gmail.com> wrote:
John Allen
and, if the arrays are fixed, point them for optimum collection in
winter.
ja
--
John A Allen