Slam Valve Motor, supplying
efficient steam engines for homeowners and professional solar installers
The Tiltable Fresnel, the best solar powered steam generator you ever made yourself
The purpose of a spar is to reflect sunlight onto an
evacuated tube suspended normally one foot above the array. The tube is
almost six feet long, 1.8 inches wide and will be between 1 foot and
1.4 feet away from the mirror. The normal mirror to use is 1.5
inches wide by six feet long and flat. To save cost, one can use
fewer and wider mirrors if the mirrors can be bent. The optimal
bend would be a circular bend in the short axis with diameter of 4.8
feet. This would have a focal length of 1.2 feet. Putting a
single triangular bend into a mirror is a good compromise and also
improves the rigidity of the spar. The spar must be rigid to be
able to focus and must be suspended by the ends only. Fewer and
larger spars save cutting and assembly costs, but increase the cross
section to wind. It is not required that the mirrors be a single
six foot long piece if its easier to cut shorter pieces, see the photo
To test a spar, hold it by
the ends in sunlight and reflect the light onto a wall. If the
light is in a narrow strip that will be inside 1.8 inches or less when
focused on the tube, its a good spar. If the light is snake
shaped or smears, its not a good spar. Bends on the spar will
tend to bend the strip as well.
reflectors, there are a few materials to choose from. Stainless
steel is weather resistant, lightweight, not too expensive, and can be
ordered cut to size. Number 8 finish is a true mirror, this makes
a sharp reflection in both axis. However number 8 is normally 20
gauge and is heavy. Also the best stainless steel mirror looses
about 1/3 of the sunlight from absorbtion. When dirty, the mirror
looses more. On the postitive side dirt does not like to stick to
stainless, it will never rust, shatter or scratch in normal use, and
its easy to work with. Cutting number 8 makes it twist a bit,
bend it flat by hand before mounting.
Some stainless finishes other than number 8 can be
used as well, these will be lighter and cost less. The trick is the
grain (lines visible on the surface) must go the short way across the
spar not the long way. Normally metal suppliers like to cut with
the grain, you need them to cut against the grain creating lines that
are 1.5 inches long. If you cut it this way, the light will be
blurred in the long axis which is less important but sharper in the
short axis, so most of the light will end up on the tube. Some
finishes are too course, test a sample by reflecting light onto a
wall from a rectangular sample. Two sides of the light spot
should have a sharp border, the other two will be soft and blurry.
If the sharp border is sharp enough, its a suitable material to
Other popular reflectors include
mylar. The heavy mylar from Mcmaster Carr can be bonded directly
to the support and hang over the edge, but it wont focus in any kind of
wind. Mylar bonded to a stiffener material like aluminum sheet
stock will remain flat in wind. Use a permanent spray on
adhesive, not the repositionable which allows the mylar to creep and
wrinkle over time. Mylar is very reflective, over 90% when clean.
As an insulator it also has a static charge and tends to attract
dirt. Mylar is the cheapest of all reflector materials.
Aluminum is also very reflective. When
polished, aluminum is the most reflective of all mirrors. Spray
on a clear coat after polishing to prevent oxidation, Krylon
makes a good product for this. After treatment, it is delicate so
treat with care. Puncturing the krylon will allow the entire
surface to oxidize, and polishing is very labor intensive especially in
1.5 inch strips. Clear coat must go all the way around and be
applied after cutting.
is the best performing of all mirrors. Glass is durable, cheap,
very reflective, not very static generating. However it is heavy
and brittle. Thin mirrors in short segments may be an option.
Do not cut existing mirrors, the coating on the back will degrade
from the edge inwards over time. Mirrors must be coated in their
steel is another option. Chrome plate is reasonably cost
effective in large batches. However it also absorbs about 1/3 of
the sunlight. Chrome plating is a small local business available
in many cities, bring your spars in for coating. Chrome is a bit
delicate depending on the coating thickness, never cut after plating.
Under the reflector is a structure to hold it rigid.
This is a 3/4 inch aluminum angle stock 1/16 inch thick.
There is no better choice. In high wind areas
consider 1/8 inch thick stock. To evaluate a material for a
stiffener, hold it by one end, put the other end on the ground, look
down its length to see how much it bends and twists. The
structure is held by the ends using an element sometimes called a
coupling nut or sometimes called a standoff. It is a rod with a
threaded hole drilled down the center, in this case for a #6-32 thread.
Use a high strength epoxy to fasten the standoff to each end of
the angle stock. Use glue or pop rivits to fasten the mirror to
the angle stock.
The spar is
held on the top end rigidly by the tilt mechanism, but must still
pivot. The screw on the top side will be tightened to hold it
against the tilt mechanism. An unthreaded standoff forms the
bearing which allows the tight rigid screw to pivot against its array
mount. On the bottom side, the screw is loose and held by a
liquid lockwasher in the standoff so it wont back out completely.
The screw itself forms the bearing which pivots at the bottom