I've just posted a file called solar path in the files section. I'm
trying to get an idea of how a heliostat/tower would work on the
lunar equator and I'm confused about the layout. Perhaps the wise
people of the group can help me visualize this process.

Here's my confusion. If we are located on the equator, would the
mirrors have to track the sun from due east, directly overhead, then
watch the sun set due west. This would be the top diagram. If this
is correct, how do the mirrors "in front" of the solar tower reflect
the energy to the tower? The angles are wrong, aren't they?

Could a mirror farm on the equator actually have the layout of the
second diagram? Where there is an angle so a reflection can take
place.

Am I confusing something about where the sun would appear on the
equator?

Please help me visualize this.

Here is an experiment you can try at home to simulate sun angles
and how mirror position and sun position can reflect sunlight onto a
particular area.

Materials;
1 flashlight
1 compact mirror (make-up kit, shaving mirror, ect. Any hand held
mirror)
1 tennis ball
1 lenght of string or twine
2 thumb tacks

Thumb tack one end of the string to the tennis ball, thumb tack the
other end of the string to your ceiling or to the top of a door jam.
The string length should be long enough for the tennis ball to hang
just lower than chest high. The tennis ball is to represent the
tower. You could mount the tennis ball on a stick with a base so it
won't fall over but hanging the tennis ball may be easier.
Hold the mirror below and to the side of the tennis ball. Turn on
the flashlight and hold it above the tennis ball pointing down. The
flashlight is to represent the sun. Point the flashlight at the
mirror. Now vary the angle of the mirror untill the reflected light
is aimed at the tennis ball (you may want to do this in a darkened
room to help emphasise the light).
Now vary the position of the mirror and the flashlight forward,
backward, left and right, trying to keep the relative "altitude"
(meaning height above and below the tennis ball) of the flashlight
and mirror the same. Just play around for a while.

You should get the general idea. If not, I can elaborate further.

Hope this helps you visualize things a bit.

king_rodent (putting the eek in geek)

> I've just posted a file called solar path in the files section. I'm
> trying to get an idea of how a heliostat/tower would work on the
> lunar equator and I'm confused about the layout. Perhaps the wise
> people of the group can help me visualize this process.
>
> Here's my confusion. If we are located on the equator, would the
> mirrors have to track the sun from due east, directly overhead,
then
> watch the sun set due west. This would be the top diagram. If this
> is correct, how do the mirrors "in front" of the solar tower
reflect

The problem I'm having is conceptualizing WHY or HOW the multiple
mirrors can be placed so that their is sufficient angle to reflect
the light to the tower. You see, I'm sure I'm confusing something
about the issue of the lunar equator. I'm ASSUMING that I won't have
the angle, like in the bottom of the picture, that on the equator
I'll have NO ANGLES to work with. That the sun would rise due East
and set due West. What I don't understand is how, at the equator, I
can have the sun appear at an angle so that reflection can tajke
place. I'm sure the answer is simple, but right now I can't figure
it out.

The experiment works, of course, because there are angles to work
with.

> Here is an experiment you can try at home to simulate sun angles
> and how mirror position and sun position can reflect sunlight onto
a
> particular area.
>
> Materials;
> 1 flashlight
> 1 compact mirror (make-up kit, shaving mirror, ect. Any hand held
> mirror)
> 1 tennis ball
> 1 lenght of string or twine
> 2 thumb tacks
>
> Thumb tack one end of the string to the tennis ball, thumb tack
the
> other end of the string to your ceiling or to the top of a door
jam.
> The string length should be long enough for the tennis ball to
hang
> just lower than chest high. The tennis ball is to represent the
> tower. You could mount the tennis ball on a stick with a base so
it
> won't fall over but hanging the tennis ball may be easier.
> Hold the mirror below and to the side of the tennis ball. Turn
on
> the flashlight and hold it above the tennis ball pointing down.
The
> flashlight is to represent the sun. Point the flashlight at the
> mirror. Now vary the angle of the mirror untill the reflected
light
> is aimed at the tennis ball (you may want to do this in a darkened
> room to help emphasise the light).
> Now vary the position of the mirror and the flashlight forward,
> backward, left and right, trying to keep the relative "altitude"
> (meaning height above and below the tennis ball) of the flashlight
> and mirror the same. Just play around for a while.
>
> You should get the general idea. If not, I can elaborate further.
>
> Hope this helps you visualize things a bit.
>
> king_rodent (putting the eek in geek)
>
> > I've just posted a file called solar path in the files section.
I'm
> > trying to get an idea of how a heliostat/tower would work on the
> > lunar equator and I'm confused about the layout. Perhaps the
wise
> > people of the group can help me visualize this process.
> >
> > Here's my confusion. If we are located on the equator, would the
> > mirrors have to track the sun from due east, directly overhead,
> then
> > watch the sun set due west. This would be the top diagram. If
this
> > is correct, how do the mirrors "in front" of the solar tower
> reflect
> > the energy to the tower? The angles are wrong, aren't they?
> >
> > Could a mirror farm on the equator actually have the layout of
the

[snikt]
> Could a mirror farm on the equator actually have the layout of the
> second diagram? Where there is an angle so a reflection can take
> place.
>

I think that the second diagram is correct, except that the angle would have
to be different for each mirror. (I am assuming that you just simplified the
drawing.) Also, each line of mirrors would have to be in a different plane.
(Like in an amphitheater. I am assuming that you also simplified that.)

In that way, the system could work perfectly. (Or so I think, but maybe I am
just too sleepy. ;-) Consider the central row of mirrors. When the Sun rises
in the East, then those mirrors will have to be rotated 45 degrees around
the Z axis in order to heat the tower. During the Lunar noon, the mirrors
will have to rotated 90 degrees around Z and 45 degrees around Y. Finally,
when the Sun is setting in the West, the mirrors will have to be rotated 235
degrees around Z. (I think that I should supply the angles in terms of pan
and tilt, but I just forgot what is pan and what is tilt! ;-)

> Am I confusing something about where the sun would appear on the
> equator?
>

No.

[snikt]
Lucio Coelho

Greetings,

I just posted a gif file
little diagram about using a mirror to focus sunlight onto a tower.

Basically it just says you point the mirror in the direction halfway
between the Sun and the tower focus. A field of mirrors on the Moon would
need to rotate (on an axis parallel to the lunar axis) at 1/2 the lunar
rotation rate. Pretty slow - only 90 degrees in two weeks. If the focus
appeared as one degree across (from the mirror), it would take about 4
hours before this flat mirror would be mis-aligned. All sorts of design
options seem possible. Maybe a robot that takes care of all motions of
mirrors on simple (dumb) joints or stands. All terrestrial designs can
apply to the Moon, except the motions are 14 times slower. Also, the
seasonal north-south motion of the Sun would be smaller, only a few degrees
instead of the 46 degrees on Earth.

I took a look around the web for info on heliostats and solar towers.
Interesting stuff out there.. thanks for reminding us to think about solar
power on the Moon, and lunar rotation geometry.

- Eric Dahlstrom

Here are a few sites I stumbled across.

RED ROCK ENERGY Solar Power Heliostat Arrays.
http://www.redrok.com/main.htm
*Lots* of links. Examples, designs, etc.

Sun Day Symposium "From Basic Research to Industry" May 1996, Israel
http://magnet.consortia.org.il/ConSolar/stepp.html
Professional conference in Israel, with a number of papers demonstrating
operating systems, economics, etc. One page from one of the papers is given
below.

High-Concentration Solar Energy Optics (A. Kribus), page 4
http://magnet.consortia.org.il/ConSolar/SunDaySymp/KRIBUS/HiConcSol4.html
There is a good figure here that shows the practical limits for solar
concentration on terrestrial systems. Why 10,000x systems are available,
but not 40,000x. Anyone care to think about what would be easier (or
harder) on the Moon?

Here is a photo of a mirror array focused on a terrestrial tower.
http://hep.uchicago.edu/~stacee/nsttf-e.gif
(Now picture the massive tower replaced with a lightweight frame sized for
0.17 g)

Pictures of the new hexagonal design for mini-heliostat arrays
http://www.multimania.com/rossen/solar/hexdesign.html
Computer models of mirrors focused on a tower - a nice set of 3 graphics
that show the geometry. (Erik Rossen, 2000, Switzerland)

________ InternationalSpace.com _________
PO Box 60606 Washington DC 20039-0606 USA

Thanks for the illustration. The links made for hours of interesting
reading, too.

There was a reference in one of the links to solar chemistry.
They're writing about reducing CaCO2 to CaO + CO. Using the CaO for
cement. That'll be useful to me in my studies in a week or so, when
I start looking at building materials.

So, back to the solar question. Is there any geometry for the
heliostats that can make use of ALL of the mirrors during the lunar
day? How would a field be laid out so that the mirrors between the
tower and the sun can be used?

Am I correct in understanding that heliostat fields that completely
encircle the tower can make use of all of their mirrors when the sun
is 45 degrees or greater above the horizon?

Nobody seems to be addressing my queston of solar path at the lunar
equator. Does that create any unique problems? I'm feeling that I'm
conceptualizing something incorrectly here, namely that a sufficient
angle can be created between the heliostat mirrors and the tower.
Does the equator create any problems in creating the necessary
angles?

> Greetings,
>
> I just posted a gif file
with a
> little diagram about using a mirror to focus sunlight onto a tower.
>
> Basically it just says you point the mirror in the direction
halfway
> between the Sun and the tower focus. A field of mirrors on the
Moon would
> need to rotate (on an axis parallel to the lunar axis) at 1/2 the
lunar
> rotation rate. Pretty slow - only 90 degrees in two weeks. If the
focus
> appeared as one degree across (from the mirror), it would take
about 4
> hours before this flat mirror would be mis-aligned. All sorts of
design
> options seem possible. Maybe a robot that takes care of all
motions of
> mirrors on simple (dumb) joints or stands. All terrestrial designs
can
> apply to the Moon, except the motions are 14 times slower. Also,
the
> seasonal north-south motion of the Sun would be smaller, only a
few degrees
> instead of the 46 degrees on Earth.
>
> I took a look around the web for info on heliostats and solar
towers.
> Interesting stuff out there.. thanks for reminding us to think
about solar
> power on the Moon, and lunar rotation geometry.
>
> - Eric Dahlstrom
>
> ------
> Here are a few sites I stumbled across.
>
> RED ROCK ENERGY Solar Power Heliostat Arrays.
> http://www.redrok.com/main.htm
> *Lots* of links. Examples, designs, etc.
>
> Sun Day Symposium "From Basic Research to Industry" May 1996,
Israel
> http://magnet.consortia.org.il/ConSolar/stepp.html
> Professional conference in Israel, with a number of papers
demonstrating
> operating systems, economics, etc. One page from one of the papers
is given
> below.
>
> High-Concentration Solar Energy Optics (A. Kribus), page 4
>
http://magnet.consortia.org.il/ConSolar/SunDaySymp/KRIBUS/HiConcSol4.
html
> There is a good figure here that shows the practical limits for
solar
> concentration on terrestrial systems. Why 10,000x systems are
available,
> but not 40,000x. Anyone care to think about what would be easier
(or
> harder) on the Moon?
>
> Here is a photo of a mirror array focused on a terrestrial tower.
> http://hep.uchicago.edu/~stacee/nsttf-e.gif
> (Now picture the massive tower replaced with a lightweight frame
sized for
> 0.17 g)
>
> Pictures of the new hexagonal design for mini-heliostat arrays
> http://www.multimania.com/rossen/solar/hexdesign.html
> Computer models of mirrors focused on a tower - a nice set of 3
graphics

> angle can be created between the heliostat mirrors and the tower.
> Does the equator create any problems in creating the necessary
> angles?

Now that I have thought about it for a while, yes. Without the use
of secondary mirrors, at sunrise, the mirrors "in front" (between the
tower and the sun) will not have an angle to reflect sunlight. Only
the mirrors "behind" and "to the side" will have sufficient angle in
the morning. You could use secondary mirrors to redirect the sunlight
but I think that would be defeating the purpose.

king_rodent (putting the eek in geek)