Notes from Windward: #69
Parabolic Solar Trough
putting it all together
As the spring wound in to summer, it is more and more rewarding to
work on the parabolic solar trough. I have already built the "T-rex
ribs", that is, all the framework necessary to support the reflective
panels. I had a little trouble at first since the satellite-dish arms
are made from aluminum, which means that they cannot be welded to the
horizontal bars that keep the ribs together. So instead, I inserted
short pieces of steel into the the hollow aluminum arms. These steel
pieces were welded to the horizontal supporting bar, also steel, and
then bolted into the arms.
Oana, Pip and Patrick give scale to the t-rex frame
with Cleo looking on
Lastly, I attached the "spine" to the "ribs" using some more 1/4"
bolts. The tough part about this was getting the holes to fit just
right. I measured everything very carefully and used a jig--a small
piece of metal with holes drilled into it to act as a pattern for all
the other holes--but still had to give some leeway by drilling bigger
holes in some places. The compromise was not enough to jeopardise the
strength and flexibility of the structure.
Oana prepares to cut the galvanized sheet metal to length
Finally, here's the best part: putting up the shiny stuff! The
reflective components of the solar trough are large steel sheets which
I have screwed onto the "ribs" with self-tapping screws.
Screwing the sheet metal onto the ribs
things which could cause difficulties include:
- Creasing the metal: Any creases created, for example while
carrying the sheets, will reduce the amount of solar energy collecting
onto the boiler tube, which is at the focus of the assembly. This
happens because the crease will reflect the light and heat into some
other direction instead of towards the focus as a parabola should.
Sure, one little crease is not going to make a big difference, but the
more energy that is focused on the boiler tube, the better. The best
way to prevent creases is to carry the sheets carefully with another
person and to communicate ahead of time in which direction the sheet
should be flipped.
- Keeping the metal flush to the ribs: The "ribs" are
parabolic to begin with, but if the steel sheeting is not screwed down
correctly, the reflection may not arrive at the focus. The best way to
prevent this warping is to use lots of wood clamps (they have more
surface area than metal clamps so they prevent creasing and
scratching) and to get someone to help hold the metal down while the
holes are drilled and the screws are ratcheted in.
- The wind: If it's windy, clamp the sheet down so it doesn't
fly away and go pet a goat. Working to screw down the metal in the
wind can cause all sorts of unprecedented problems relating to offsets
caused by a shifting of the steel.
- The sun: If it's very sunny and the parabola is reflecting
the sun directly in your eyes, sunglasses help to a degree. Past that,
either move and work on a different part or take the goats for a walk.
By the time you come back, the sun will have shifted enough for you to
continue (and the goats will be happy). Don't take your vision for
The steel sheets will be cleaned before mounting, and boy will they
shine! The intention is to try using the steel sheets as-is, and if
more reflectivity is desired, perhaps we will add some aluminum foil.
It will be a tricky job, as aluminum foil doesn't like to cooperate
much in the way of reducing creases.
The next step is to mount the parabolic troughs onto the boiler tube
which is suspended above the shipping container. Once they are
mounted, it's testing time!
the first of the T-Rex panels ready to mount
Notes From Windward - Index - Vol. 69