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Will a mirror on equatorial mount going at one rev per 48 hours work?

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Posted by brian white on March 16, 2009, 10:31 pm
 
I have an experiment going to test it but I am not getting enough sun
and the next week will be cloudy too.  (I got that as an answer on
wikipedia almost a year ago I couldnt find any info about it online so I
started the little experiment.
Perhaps someone knows the answer and can save me the trouble?
My experiment is at
http://www.instructables.com/id/Heliostat_experiment_and_finding_true_north_with_e/
Thank you
Brian White

Posted by gary on March 21, 2009, 1:03 am
 
Hi
Maybe I'm missing something, but it seems like if think about an
example where you start at solar noon, and the target is due south (so
mirror is also aimed south) -- then you advance 24 hours with the
mirror turning at the rate of 7.5 deg per hour, it will be facing
north (not south) at the next solar noon?

Gary





Posted by brian white on March 23, 2009, 3:50 am
 I am pretty sure they are talking about double sided mirrors. I have
being trying to visualise it and it is hard!  I am still getting cloudy
weather here so it is hard to do an outdoor experiment of value.
I have found some links to olde physics books. It is pretty interesting.
First the 48 hour heliostat. It is a little bit limited (but not so bad
as it is painted! This type has a "mirror mounted directly on the polar
axis" and the"plane of the mirror contains this axis". (I didn't think
of this (when the back side of the mirror is used, after 24 hours, if it
is not in the plane the angle will be different). But if you only used
24 hours and then rewound?  Anyways, this was conceved in the 17th
century, and was revived by the German E.F. August in 1839 but never
became popular. I think this design would allow a home made "power
tower"  but placement of the tower and of the heliostats would need to
be carefully decided and they might have to be moved a few times during
the year.
Most is written  about 24 hour rotation heliostats.
You can find pictures of silbermann's heliostats on the net. These
things would put anyone off trying to make one! They were the high end
delux models.  (Though I have seen reports that Faucault's heliostat was
better than it). The plebes used George Johnstone Stoney's heliostat
that was designed to be inexpensive, simpler  (and use larger mirrors)
and the heliostats from Rudolf Fuess (who had a factory building
scientific instruments). Fuess apparently made the last design advance
in the olde heliostats.
If anyone has details on their designs, please share! There are some
Stoney heliostats in museums in Ireland and surely some from Fuess in
germany. So it should be possible to find out how they worked.  These
had 24 hr rotations and one mirror.  People keep insisting that I use 2
mirrors! I want to use 1 mirror (alu coated plastic) and I think if the
people 150 years ago used one, I can too.

gary@builditsolar.com wrote:

athttp://www.instructables.com/id/Heliostat_experiment_and_finding_true ...


Posted by brian white on March 23, 2009, 4:32 am
 I approach this thing with an open mind, and I hope you do too. I have
attempted to make models for both the 24 hour and the 48 hour rotation.
One problem with the 24 hour rotation is that you need rods or
parallelograms that modify the angle of the mirror and they have to work
a lot more at certain times. I saw complaints about the "jerky movement"
of silbermanns heliostat and that is the reason. My model of a 24 hour
heliostat took a lot more energy to twist it at certain times too.
Clearly, with the 48 hour concept,  we would be only using a part of the
48 hour rotation, (less than half)  and then rewinding the device when
the sun goes down. With the 48 hour rotation, there is very little
modification of the mirror angle needed and perhaps that is an advantage?
Perhaps the image just moves up and down a little?  This might be just
fine (unmodified) for solar drier applications. You might be able to
train 8 or 9 cheap (mylar streched on a frame) heliostats on  a drier
for greatly improved production with not much extra capital cost. And
please consider the clock based dripper tracker to turn your equatorial
mount. I made my first model totally mickey mouse, just as proof of
concept.
You can do a lot better, if you want. I have people foaming at the mouth
because I suggest the 48 hour rotation. (I put it in a few groups) and
it is just weird.
Perhaps people are selling competing products?
My back yard slopes to the north so I do not get anything solar done for
another couple of weeks.
Feel free to experiment, often you get more insight when something
doesn't work than when it does.
Brian

brian white wrote:

http://www.instructables.com/id/Heliostat_experiment_and_finding_true_north_with_e/  


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