Wednesday, July 8, 2020

Construction - Solar Collector - Functioning Part

The preceding two posts described the science behind the solar collector.  This is the first of two detailing its construction.  

As I was building out the collector, I was designing and redesigning on the fly.  So I think it is worth repeating what I wrote a couple of posts back "....... I did a Google search on using solar collectors for passive solar heating and air conditioning and found nothing that resembled our situation.  Even the sources with which I was already familiar were short on details.  So I am erring on the side of too much detail in case someone out there is contemplating Annualized GeoSolar and could benefit from our experience."   I hope the focus on minutiae is not too off-putting for the majority of readers.

Sand Bed
Couple of years ago, I used the trackloader to dump 4 or 5 bucket-fulls of sand into the solar collector shell while it was still possible to reach it without scuffing up the landscaping with the loader tracks.  As luck would have it, the amount was just about right to serve as a foundation for the functioning part of the collector.  Before building the framework for the collector, though, the sand had to be reconfigured so that it tilted upward from south to north at about the same angle as the glass will take when the collector is finished.  Doing so got it out of the way of building the framework on the low side.  

I think that I almost wore the sand out moving it.  After the framework was done, a lot of sand had to be moved temporarily from the inside of the frame to the walkable space south of the frame in order to position foam insulation board.  Then it had to be shoveled back again and smoothed out before the corrugated roofing could be laid on top of it.  But these details are getting ahead of the story.

Building the Framework That Supports the Glass
As explained in the previous post, the angle for
Birdseye view of framework.  The arrow
points to the 18 degree slant of the frame;
the "X" is over the walkable space

the glass was 18 degrees off horizontal.  So I used a chalkline to lay out that angle on the east and west walls of the shell so that the high north end of the layout would be about the width of a 2 x 4 higher than the tops of the nine 4" PVC pipes 
(conduits) that exit the north wall of the shell, fan out under the house and run to daylight behind the house through which heated air will leave the collector and warm the thermal mass under and behind the house.  Pressure treated 2 x 4s that had been dried by  stickering and storing out of the weather for at least a couple of months were then cut to fit and painted on all six surfaces before installing.  Their length was determined by the length of the salvaged glass panels that were a Craigslist find many years ago -- almost 5" long.  Once the east and west boards were fastened with concrete anchors, pre-painted 2 x 4s were attached to the north wall so that their tops lined up with the east and west boards and their bottoms were at or just above the tops of the conduits.  That then left the more complicated south part of the frame to design and install.

The South Framework
The south framework had to be free-standing
Arrow points to the air intake space 
between the 2 x 6 wrapped in plastic 
and the 2 x 4 top piece that supports 
the glass; encircled is the one of the
footings for the frame
in order to preserve the walkable space between the collector and the south wall of the shell.  In addition, it had to be perforated to allow the intake of air that exceeds the air flowing through the conduits.  And, being free-standing, it needed to be supported in several places.  

The south frame comprised  2 x 4s on the top and  2 x 6s below with an air space in between.  They were dry-fitted in place then suspended at grade level for final assembly.  I used gussets and metal angle iron to fasten them together.  Knowing that the 2 x 6 would be buried partially in sand and knowing that its pressure treatment was not rated for ground contact, I wrapped it in 6 mil black plastic (maximum resistance to UV degradation vs. clear plastic) and taped the seams.  I am hoping that the degree to which it gets wet will be so minimal that rot will not be an issue especially since the buried side will be inside the collector where the environment should be too hot and dry to support mold. 

The finished south frame was lowered to place and supported temporarily at both ends while four concrete piers were fitted into holes in the dirt such that they were resting on undisturbed soil and not quite in contact with the frame.  I then filled the gap between the piers and the 2 x 6 with mortar for a passive fit.  And I stapled galvanized hardware cloth over the air intake to thwart curious creatures. 

Support For the Glass
Rather than try to balance the glass panels directly on the 2-by
Laying in the foam insulation after 
screeding the sand base.
framework, I attached 2 x 2s flush with the lower edge of the framework's 2 x 4s on which to lay the glass.  This arrangement brought the glass more in alignment with the tops of the conduits for what I imagine will improve the passive flow of air between the air intake and the conduits.  I then stapled the thickest and spongiest (non-rubber) weatherstripping I could find to the 2 x 2s to cushion the glass and make up for any discrepancies in the levelness of the 2 x 2s.  The 2 x 4s on top of the south framework were beveled at 18 degrees and held flush with the tops of the 2 x 2s so that the glass resting on them overhangs enough to dump rain water into the walkable space instead of inside of the collector. 

Cautionary Hiatus 
Once the framework was done, I moved on to building the enclosure around the top of the shell which had to be done before the glass was installed in case somethinfell into the shell that was heavy enough to break the glass .  The next post details with the assembly of the enclosure.

I postponed the construction of the south side until the glass was in place for a couple of reasons:  having the south side open made installing the glass easier and the walkable space served as a buffer between the south wall of the shell and the glass below, making it unlikely that the glass would be damaged by falling objects.

Sand Bed, Insulation and Corrugated Roofing 
There was too much sand inside the framework to lay
Sand bed ready to receive corrugated roofing
the insulation at the proper level so I moved the excess to the walkable space then used a 1-by board to "screed" the remaining  sand smooth to support the recycled 2" foam board.  Then I returned most of the sand in the walkable space to the collector to cover the insulation which I then "screeded" smooth to receive the roofing panels.  I nestled the steel panels into the sand in such a way as to maximize contact with the sand.  As it turned out, another couple of inches of sand under the roofing would have been ideal from the standpoint of narrowing the space between the glass and the roofing.  A narrower space, according to Bernoulli's Principle, would probably speed airflow but there was no easy way to add any without using bagged sand and the number of bags that it would have taken to make a difference was easily disincentivizing.

The excessive amount of heat rising off of the
Corrugated roofing in place. 
roofing on sunny days, that could be felt even while standing on the ground above the collector shell, seemed to indicate that the collector would function as intended after the glass was in place. 

Finally the Glass
I built the framework to fit four pieces of glass that I found for free on Craigslist and stored in a safe place many years ago.  The collector was long enough that I was one piece short which was a good thing.  It gave me reason to get a glass company involved who could help in several ways.  First, a professional could tell me whether the Craigslist glass was appropriate for the collector, could supply the missing piece and, most importantly, would have the tools and experience for getting the glass safely into the shell and laid on the framework.

As it turned out, the CL pieces were plate glass when tempered glass would be four times stronger against large hail.  So I bought all new glass and was fortunate to have caught the dealer out of Plexiglas and therefore temporarily unable to work on its backlog of orders for Plexiglas barriers during the COVID-19 epidemic.  Otherwise, the collector glass might have been delayed until so late in the summer that the amount of heat collected before the AGS system had to be mothballed for winter would have been negligible. 
Parenthetically, by the time the pictures above
were taken, the bonding cement used in conjunction with the dry-stacked concrete block walls of the shell had been painted with white Drylock Waterproofing paint.  It served three purposes:  as a preservative, to improve appearance and to reflect sunlight into the collector.

Installing the Glass
The gods must like our project.  Otherwise, go figure how clouds moved in just before the glass installers arrived and cleared as they were leaving.  Otherwise the heat from the roofing on a 95 degree afternoon the second week of July would have been almost unbearable for the workers, shown in the photo installing the last panel of glass.

Getting the glass in place meant that, five years after the shell was built, the collector was finally functional -- a major watershed in our passive solar build!

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