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

     A couple things which could cause difficulties include:

  1. 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.

  2. 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.

  3. 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.

  4. 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 granted.
  5.      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