Thursday, August 25, 2005

frame construction

The hard work of building a solid frame for up to 12 reflectors is pretty much finished now. I'm watching the concrete footings dry in the cool early morning.

I took apart the original structure to build this in it's place. In the last couple of weeks, the night-time temperature has definitely had a chill and without even the two reflector system for heat, the pool temperature has fallen to about 72F. The days are hot, but no-one wants to swim. We haven't used the propane heater at all this year and aren't going to start it up now.

I've built this structure to hold the next set of collectors, up to twelve in all.

It's made of pressure treated 4x4's. They are held together by galvanized metal joining plates from the hardware store using deck screws. The vertical ones are four feet in length and are sunk about 18 inches into concrete in ten inch sonotubes.

The structure forms a couple of straight parallel beams, about four feet off the ground and eight foot six inches apart. The reflectors will hang from this structure, with the pipe spanning the gap. The pipes will be installed with their reflectors at two foot intervals between the beams.

In spite of some difficulties like twisted beams and rocky ground, the final top beams ended up very straight, probably much straighter than they need to be, about a quarter inch over the each 24 foot length and parallel by the same tolerance. Hopefully they will be still be straight and parallel after the frost heaves the ground a few winters!

As I push against the these beams, set in concrete to test their satisfying stiffness I wonder if I haven't gone a bit overboard with the structure? After all, this frame will support the weight of several hundred pounds of water and copper pipe and must also withstand whatever wind load that nature will provide. So I'm happy that this support is adequate for my solar heater.

While it involved ripping out the previous prototype, I've gone to this type of structure primarily for simplicity. The previous method, that of passing the collector pipes through a holes in beams, was far too complicated to line up correctly and required one set of posts for each collector. With the new parallel beams, the collectors will just "drop into" the frame and be screwed into place with two pipe clamps, one at each end, into the beam. Couldn't be stronger or simpler, could it?

It's close to the ground to make it easy to work on and to cut down the wind load. I plan to surround it with a fence of the same, four foot height, to reduce the wind load.

Also considered, but rejected this time around, was to drop the southern facing beam about a foot or so to better match the sun's track, but I decided to make them parallel in the horizontal plane for simplicity of the control mechanism and because a thrust bearing would be required at the lower end. Perhaps I'll tackle that on another iteration.

It's important to store the 4x4 beams for a while in a dry place, like the garage for a month. Turn them over once in a while and check for any beginning warpage. I've been doing a few outdoor structures this year and have of pile of wood in storage in this manner to weed out the troublesome ones that are going to warp. There is no guarantee with wood however and the placement and final orientation to the sun may determine whether a beam is going to warp or not. Incidentally, the warped ones (there were several) were chop sawed into the four foot lengths for the vertical supports. The fact that they were twisted slightly over their length down into a concrete footing didn't make a lot of difference and used up the troublesome timbers.

One final word on the mirrors. We dropped one of the collector pipes into a reflector when removing it and the mirror did break, although not into thousands of little pieces like glass. Here's what it looks like.

Fortunately, the reflector material is easy to change if necessary and with the plastic mirror the cost of each is about $40.

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