Sunday, September 27, 2020

Design - Solar Collector - Problem-Solving

This is fifth post on the solar collector but, undoubtedly, not the last since I will reporting on its performance over time. The purpose of this post is to report that it has not lived up to expectations and to suggest changes that will make it work.  

Lack of Performance
The glass for the collector was installed by the middle of July just in time for lots of clear skies and 90+ degree temperatures that should have maximized airflow from the collector into the conduits.  I gave the system a few days to rev up, thinking that it may take a while for the hot air pushing up the conduits to overcome the cold air dropping down.  During the early afternoon on a bright and hot day, I checked the air flow from the north ends of the conduits and found that, if there was any movement at all, it was so minimal that I could not detect it.  I waited for another sunny day but one without any wind in order to dangle thin strips of paper from the conduit outlets that would detect any air movement.  Still no detectable convection.

At our latitude, the temperature of the soil where the house sits would be in the mid-60s by mid-summer at the depth of the conduits.  However, since the house temperatures during the past couple of winters have stayed above 40 degrees (despite no insulation), it is reasonable to think that the temperature of the earth under the house surrounding the conduits is higher than it would be if not shielded by the house.  If so, the temperature of the air falling out of the conduits might be as high as the 70s but apparently still too cold for the heated air from the collector to overcome. 

The performance was disappointing but not totally unexpected given the dearth of practical information available in print and on the web, which means that our design had to be largely original.  I look at the situation as just another problem that needs to be solved and reported on just like many other surprises and challenges that we have encountered with such a unique build.

Are the Conduits the Problem?
In a previous post, I listed some of the unknowns that come into play.  "Assuming the design of the collector is adequate, its function is still at the mercy of many unknowns about passive air flow through the conduits.  Will 4" diameter conduits be the optimal size for sufficient airflow?  Are conduits that are nearly 90' long from collector to daylight behind the house too long to expect passive flow?  Do they angle upward enough from 10' below floor level when they leave the collector to a depth of 3 or 4' below floor level at the back wall of the house and then make 45 degree turns to daylight?  Will using the corrugated (rather than smooth) piping under the house (the intent for which is to cause turbulence in the air flow and thereby improve heat transfer to the soil) slow the flow too much?  Will the cooler soil during the first winter and, to a lessor extent, after each succeeding winter, cause cool air to flow backwards towards the collector to the extent that the warm air from the collector cannot passively reverse the flow?"   Of all of the items on the list, only the one in italics can be addressed at this point -- the rest are what they are.  At this juncture, I would add one other possibility.  The conduits terminate with two 90 degree fittings in order to keep rain out.  Perhaps if the conduits pointed straight up, there would be less resistance to passive flow.

Is the Solar Collector the Problem?
The problem could also be in the design of the collector rather than in the conduits.  Maybe one layer of galvanized roofing is not enough; maybe multi-layers are necessary.  Maybe there is so much space between the metal and the glass that the volume of heated air is insufficient for spontaneous escape up the conduits.  Perhaps the collector is not large enough to supply nine conduits passively.  Maybe it will be necessary to add to the system one or two what might loosely be called "solar chimneys" whereby the conduits would be brought together and exit to daylight through a common chimney, with or without the assistance of a fan.

At first I assumed the problem lies with the
Termini of the nine conduits.  The one in the middle was
modified to accept a vacuum hose for testing.

conduit portion of the system instead of the collector which means there is only 
one factor I can test -- the one in italics above. I manipulated the airflow in one conduit to see if it could be jump-started to overcome the effect of the cool soil by cutting away the double 90s in order to fit the end of the conduit with an end-cap having a hole the same size as a vacuum hose.  I pulled air through the conduit with a vacuum for a couple hours hoping that, when the cap was removed, I would feel warm air, or any air for that matter, coming out of the conduit.  Such was not the case. 

The next probable cause for under-performance that could easily be investigated was to measure the amount of heat the collector was producing.  Having assumed that the temperatures would be too high for plant growth, 
Thermometer resting on the metal is maxed out.

I began to suspect that heat generation was insufficient when a couple of plants sprouted along one edge of the collector.  I pulled the plants and placed a thermometer of the common type with a scale to 120 degrees inside the collector.  It recorded temperatures approaching 100 in the early morning when only the west half of the collector was sunlit and the thermometer was shaded by the east wall of the collector.  As the sun reached the glass fully, the temperature readings quickly rose and stopped at the maximum capability of the thermometer somewhat above 120.  And further plant growth has been non-existent.  So, pending the purchase of a thermometer with a higher range, the initial readings are encouraging enough to look elsewhere for ways to make hot air flow through the conduits. 

Reconfiguring the Terminal Ends of the Conduits
It is becoming obvious that the conduits will require redesigning at their terminal ends.  Instead of nine conduits exiting to daylight independently, I am now convinced that they need to converge into one or two solar chimneys fitted with a solar fan(s).  Since each conduit is nearly 90 ft long, I think two chimneys with four or five conduits each would be more efficient than one chimney located 40-50 feet from the termini of the most outlying conduits.  At the time of this writing, it is mid-September and completion of exterior trim for the house is the highest priority.  Reconfiguring the conduits will have to wait until spring.

Tuesday, September 22, 2020

Construction - Solar Collector - Safety Fence

The previous post covered the functioning part of the collector.  This, the fourth post on the collector, describes the safety fence surrounding it, the design for which needed to meet
The framework for the safety fence around the collector shell.
several criteria.  It needed to be tall enough to meet code for handrails, it needed to be as attractive as possible considering its prominence in front of the house and it should not block the view from the house any more than necessary.

(Reminder: click on any picture to enlarge it for better viewing.)

Upgrading the Top of the Wall
When the collector shell was built five years ago, the top of the walls were covered with 2 x 12 pressure treated planks that were anchored with bolts embedded in concrete.  While they protected the top of the walls, they left much to be desired aesthetically and begged to be replaced.  It took only a few minutes at the landscape supply store to decide in favor of concrete pavers instead of pricey capstones.  I dry-fitted them to determine where the posts for the fence should be located in order to minimize laborious notching of the pavers.  I then installed the posts and notched the pavers using a diamond blade in the radial arm saw.  I also custom crosscut a few pavers as necessary to filled the gaps that would not accommodate full sized units. 

Post Placement
Like the solar collector framework, the pressure treated lumber for the fencing was stickered and air-dried for several months so that it remained straight and would accept and hold paint.  Then, before assembly, it was undercoated on all six surfaces and final coated on at least three surfaces.  The 4 x 4 posts extend below the ground to the depth of 12 - 16" so they needed to be ground-contact rated; the pressure treated lumber for the rest of the railing system needed only to be rated for above-ground use.  The corner posts were "V"-shaped at the bottom so as to rest on the top of the wall and straddle both surfaces of the wall below.  Two long and robust Tapcon concrete screws through each surface was more than adequate to anchor them firmly and, with the sometimes help of composite shims, hold them plumb.  Instead of "V-ing" the bottoms of the intervening posts (south and north sides), they were reduced in thickness by a half so as to rest on top of the wall and extend downward where they were anchored by four screws each.  Backfilling and tamping the soil around all of the posts will help to support them as well.

With the posts in place I could lay the pavers in mortar.  The weather was hot and I got in a hurry to get done so I laid them without the benefit of a mason's line.  The result, I am sorry to say, looked pretty amateurish.    

East and West Sides 
The wood framework for the enclosure supports a wire grid fashioned from cattle fencing that I cut from panels that come 4' high and 12' long.

The fencing for the east and west sides of the enclosure presented a challenge in that it had to be stepped to follow the contour of the stepped walls and its top had to slant to follow
Temporary layout board.  A few of the cap stones
 are yet to be mortared in.
the slope of the stepping.  Since the cattle panels were 4' high, I installed a temporary board 4' above the lowest step and leveled it.  Off of that I could measure the amount that the fence panel would have to be reduced over each step uphill from the lowest step. These measurements, along with the length of each step and the slope at the top, were laid out on a cardboard pattern.  After dry-fitting and tweaking the pattern, I laid it on a section of fencing that had been cut to length and marked the wires to follow its edges.  An angle grinder with a metal-cutting diamond blade easily handled the 4 gauge wire.  
The cardboard was accurate enough to be used as a guide for cutting and fitting the support board at the top of the fence.  I used 2-by blocks under the wire panel to hold it off the wall slightly as I attached the board at the appropriate height to catch enough of the top of the fence board for secure fastening.

Dry-fitting the cardboard pattern to the east end of the 
The cardboard pattern in place over the cattle fencing
for the east end of the collector.

collector showed that, with one minor adjustment, it could be used for cutting the east fencing panel. I added the top board on the east at a height that matched the board on the west.  Confident that metal panels were installation-ready, I set them aside so that they would not interfere with laying the pavers with mortar.  

North and South Sides
Compared to the east and west sides, the north and south sides were easy.  For the north side, I installed the top boards at a height that matched the northern ends of the side boards and the bottom boards at 1 1/2" above the pavers.  I added 2x2 nailers to the sides of the corner posts to receive the vertical edges of the
Fencing completed except for caprails.  The ladder
used during construction is still in the collector.

fencing panels. 
 As an cosmetic touch after the panels were fastened to the top and bottom boards and to the posts, I added  1-bys on the collector side of the entire framework.

I decided to delay the fence for the south side until the glass was installed on the collector in order to leave plenty of access with the heavy glass panes.  I entertained the idea of building an access gate into the south fence but decided against it based on looks, which means that we will have to drop an extension ladder into the shell when access is necessary for maintenance.  The design of the south fence was then identical to the north fence.

Post Caps and Cap Rail
Finally, I added store-bought pressure treated post caps to the tops of the posts -- for aesthetics and to protect the end-grain of the
In lieu of a gate in the fence, a ladder will have to
be used for servicing the collector

posts from deterioration.  The caps for the top rails will be 2 x 6s that I customize with bi-sloped tops to shed moisture eventually.  Their addition is being postponed while more urgent projects on the house are handled.

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!

Sunday, May 3, 2020

Design - Solar Collector - Maximizing Solar Gain

The previous post on the solar collector for the AGS system dealt with the mechanisms for trapping solar energy and converting it to usable heat.  This post discusses the factors that go into maximizing the amount of solar energy collected.

Original Assumptions
My early thinking, that appears in at least one prior post, was that the best tilt for the glass and steel would be perpendicular to the sun angle for St Louis shortly after the summer solstice (June 21), say July 21.  Since the choice was empirical, I wanted to flesh it out with data if possible; hence, the following analysis based upon three factors -- optimal sun angle, warm weather collection period and available daylight.

Optimal Sun Angle
NOAA Solar Position Calculator is seemingly a useful tool for knowing the elevation of the sun angle from horizontal. However, Gary, my mathematician brother-in-law, calculated the angles and found the NOAA data to be incorrect. Following are his sun angle calculations (rounded up or down) for St Louis.  The optimal tilt for the glass and galvanized steel roofing in the collector would be 90 degrees from the sun angle during the warm season but the question is what date would be best to use as the default.  For the sake of discussion, the  figures for five scenarios are listed below :  

     June 21:    Sun elevation from horizontal = 75 degrees; collector glass angle = 15                       degrees from horizontal

     July 21:  Sun elevation = 72 degrees; glass angle = 18 degrees

     August 21:  Sun elevation = 64 degrees; glass angle = 26 degrees

     September 21:   Sun elevation = 52 degrees; glass angle =  38 degrees

     January 21:  Sun elevation = 32 degrees; glass angle = 58 degrees

A default date of July 21 with a sun angle of 72 degrees is only 3 degrees less than June 21 and, by observing the play of the sun in the collector shell for a few summers now, I think that the difference between the two is moot, i.e., the additional amount of sun entering the collector on June 21 vs. July 21 is negligible.  

The sun angle for January 21 was included above to contrast the difference between the typical passive solar design that uses the energy from the low-angle winter sun versus the high-angle summer sun that energizes our AGS system.  For solar gain in winter, the optimal tilt for our 38 degree St Louis latitude of, say, greenhouse windows, would be tilted 58 degrees off horizontal to be at 90 degrees to the sun angle compared to the 18 degrees off horizontal that optimizes the output of our solar collector on July 21 -- a difference of 40 degrees.

Parenthetically, the south facing windows of our house are 90 degrees from horizontal rather than the 58 degrees that is optimal for January 21. The 32 degree difference might be important for a classic winter-centric passive solar build but, as detailed in a prior post, maximizing solar gain in winter is not very important for our Annualized GeoSolar System, especially after the first couple of years.

Warm Weather Collection Period
In order to keep cold air out of the AGS system, the north ends of the conduits will be capped during the cold months -- roughly from the end of September until the beginning of April.  Consequently, the question becomes, "Shouldn't the default date for the sun angle fall in the middle of the six-month April through September collection period?"  If so, it would be sometime in June.  However, it is more likely that lingering cold weather in the spring will delay opening the conduits than early cold weather will cause them to be capped prematurely in the fall, thereby skewing the midpoint of the collection period backward to, say, sometime in July.  So, again, any advantage of a June default date over a July date is questionable, particularly in view of the amount of cloudy weather in the spring as discussed below and recurring warmer autumns due to global warming.

Available Daylight
In addition to optimal sun angle and which months are included in the collection period, it is useful to consider the available daylight during the collection period. NOAA's Sunrise-Sunset Calculator is helpful in this regard.  Here are some representative values spanning the seven months that would be in play as the optimal collection period.

     April 1:  12 hours, 40 minutes

     May 15:  14 hours, 18 minutes

     June 15:  14 hours, 51 minutes

     July 15:  14 hours, 35 minutes

     August 15:  13 hours, 39 minutes

     September 30:  11 hours, 48 minutes

     October 30:  10 hours, 36 minutes

These data seem to indicate that a June 21 date in the middle of a six-month collection period -- April through September -- would provide more available daylight than would a mid-July date with a six-month collection period -- May through October.  
However, according to Climate for St Louis, there is much more rain in April through June than in August through October.  More rain means less available sunlight.  Less available sunlight in spring and early summer therefore makes May through October a more attractive collection period than an April through September period despite the latter's shorter days and makes July 21 a better midpoint than June 21.

Three factors influence the choice for the best date on which to base the angle for the glass and steel panels:  (a) sun angle from horizontal, (b) the timing of the collection period and (c) the available daylight during the collection period.  A close look at sun angles and collection periods results in essentially a wash between June 21 and July 21. But the third factor, the available daylight/sunlight during the collection period, seems to tip the scales in favor of July 21 which validates my original hunch.

Tuesday, April 14, 2020

Design -- Solar Collector - What Is It and How Will It Work?

This is the first of five posts on the solar collector.  The first two parse its design; the second two detail its construction and the fifth reports on its actual performance.  In preparation for writing these posts, 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.  At this stage, I cannot not be sure that it will work as planned so I am reconciled to the possibility that, even after completely installed, it will have to be re-configured.  If there are problems, I will report them in the fifth post and describe how we handled with them.

What Exactly is a Solar Collector?
Solar collectors capture the sun's heat, as opposed to photovoltaic panels that use the sun's light.  Solar collectors can take many forms, some more complicated than others.  By using a flat-plate solar collector, we benefiting from the simplest of forms -- entirely passive, having no moving parts.  As is typical of flat-plate designs, ours is a rectangular box with a glass cover and a heat-absorbent bottom.  Sunlight passes through the glass, warming the air inside the box and pushing the warm air through a series of pipes to heat the thermal mass under and behind the house.

Finally It's Time to Build Out the Collector
Two 2015 posts detailed the construction of the dry-stacked concrete block shell for the solar collector located in front of the house (as opposed to the photovoltaic array behind the house). The two posts were Construction of the Solar Collector and More on the Construction of the Collector.

Shell for the solar collector showing the AGS conduits
exiting the back wall of the collector 6' below the floor
 level of the house (line of holes near the bottom)the
glass top over the working part of the collector will be
 situated just above the conduits; notice the white conduits
 running to daylight at the back of the excavation for
 the house (beyond the blue tarps) at a distance of about
65 ft from the collector.  (Click any picture to enlarge it for
 better viewing.)
It is time now to convert the shell into a functioning part of our Annualized GeoSolar system -- something to which, until recently, I had not given much thought beyond realizing that it would not be as simple as covering it with glass as if it were a greenhouse roof.  Fortunately, our friend, Ben, a retired metallurgical engineer, has been sharing his knowledge.
The red arrows bracket the solar collector, also identifiable
by the ladder protruding from it.  Although it looks as if it
is attached to the house, it is actually 18-20' away from it.
Parenthetically, notice the berm (yellow arrows) that directs
 runoff to a rain garden (magenta arrow).  The garden is the
 third in an interconnected series of four with the first one
 situated beside the garage. 

(If you are new to the blog or unfamiliar with the concept of AGS and the role of its solar collector in eliminating the need for conventional heating and air conditioning, click on "Featured Post" in the left column then follow the links to other posts on the subject. Or, for a quick overview of AGS, go to Wikipedia.)

(Click on any picture to enlarge it for better viewing.)

How the Collector Will Work -- in Layman's Language
Our solar collector will heat a stream of air entering the collector on its south side and exiting through nine 4" conduits on the north side.  
North-south cross-section of the collector. (Click on the
drawing to enlarge it.)
The air will flow between transparent glass that traps the sun's heat and corrugated galvanized roofing panels a few inches below the glass that absorbs the heat then releases it to the passing air (see nearby sketch).  

The hot air passing through the conduits will heat the earth under the house and the earth under the insulation/watershed umbrella behind the house before exiting to daylight.  And, since the conduits are slanted upwards as they fan out and run north, the heated air rises through them passively with no mechanical assistance.

How the Collector Will Work -- in Engineer-speak
According to Ben, the sun must raise the temperature of some material in the collector above that of the air in the collector.  The heated air is then forced by convection out of the collector and into the AGS conduits (which, as mentioned above, are tilted slightly up -- leaving the collector at 6' below the floor level of the house and passing under the north wall of the house at 3' before bending abruptly upward to daylight behind the house.)  For maximum efficiency, the heated material in the collector must have high solar absorptivity.

However, it is not enough to be highly absorptive.  It is also important that the material readily transfers its heat to the air, i.e., be highly emissive.  The higher the ratio of absorptivity to emissivity, the more efficient a material is for solar collection. Ben has vetted an interesting Table of Absorptivity and Emissivity of Common Materials and Coatings that lists nearly a hundred materials with regard to the ratio (third column in the table) of absorptivity (first column) to emissivity (second column). There are only five materials in the table, such as metals plated with nickel oxide or plated with black chrome, having a higher ratio than the one Ben recommends and all of them hard to find and beyond our budget.

Ben's Recommendation
Galvanized roofing
New galvanized steel, with an aborptivity of 0.65 and an emissivity of 0.13, has a ratio of 5. "Exposure to weather" (whatever exactly that means) eventually causes the ratio to drop to 2.90 which is still high compared to most of the materials listed.  Perhaps, under the glass of the collector, the steel will "weather" slower than if it were in direct contact with the elements. And steel panels are cheap enough that replacing them from time to time will not be an issue if the need arises.

The galvanized steel panels will have to be supported by something.  Ben recommends using dirt or sand which will double as a heat sink. When the sun heats the panels, most of the heat will be carried away to the conduits by air movement but some will be conducted to the heat sink below.  When the sun is not shining, some of it will reverse-conduct into the cooler space of the collector and find its way into the conduits. 

In order to be sure that most of the heat in the sand under the galvanized roofing is not lost to the ground below, I am considering laying down at least 2" of foam board insulation before adding the final layer of sand that supports the galvanized panels.  The insulation is depicted with dashes and its label with a question mark in the drawing above because, at the time of this writing, the decision to include it was still in limbo.  .

My inclination, though, is to use it since doing so is consistent with the way we insulated under the nine conduits running between the collector and the house.  In the top photo, notice the pink vertical insulation on the east and west sides of the excavation behind the collector shell. The same insulation had already been laid down under the conduits.  The insulation in the overlying  insulation/watershed umbrella insulates the top.  A single layer of foam board surrounding the conduits was deemed sufficiently insulating because the soil on the outside of the foam is already being warmed by the overlying umbrella but perhaps the case could be made for using more than one layer in the collector.

Air Flow
The collector will have to be designed so that air flows passively between the glass cover and the steel panels.  In order to make sure the volume of air entering the collector is more than enough to replace the warm air exiting into the conduits, the square area of the air intake on the south side of the collector will need to exceed slightly the total area of the openings to the nine 4" diameter conduits.  There is walkable space between the collector and the south wall of the shell that will not only provide a patent air intake but will also give access for clearing leaves and plant growth and for cleaning the glass periodically.

Assuming the design of the collector is adequate, its function is still at the mercy of many unknowns about passive air flow through the conduits.  Will 4" diameter conduits be the optimal size for sufficient airflow?  Are conduits that are nearly 90' long from collector to daylight behind the house too long to expect passive flow?  Do they angle upward enough from 10' below floor level when they leave the collector to a depth of 3 or 4' below floor level at the back wall of the house and then turn abruptly to daylight?  Will using the corrugated (rather than smooth) piping under the house -- that is intended to cause turbulence in the air flow and thereby improve heat transfer to the soil -- slow the flow too much?  Will the cooler soil during the first winter and, to a lessor extent, after each succeeding winter, cause cool air to flow backwards towards the collector to the extent that the warm air from the collector cannot reverse the flow?  We are only weeks away from having answers to these questions which I will report in the last post about the collector.

Sunken Configuration
The glass of the heat exchanger will be situated about 6' below the top of the back wall of the collector and nearly that deep in front due to its cant southward.  At first blush, it might seem that the sunken configuration will reduce the amount of useful sunshine reaching the glass.  While the east and west walls do indeed block some of the suns rays until mid-morning and after mid-afternoon during the majority of the summer collection period, their angle of incidence to the glass would be so low that most would be reflected from the surface of the glass instead of penetrating it.  Even then, the amount of glass that is shaded at 10:00 am and 4:00 pm comprises less than a third of the total.

Another reasonable objection to using a sunken configuration is that it would hold water, which could be a greater problem in the future
The top of the serendipitous French drain in the walkable space.
with intense storms associated with climate change.  It just so happens, however, that the excavation for the collector was deep enough to uncover one of the seven French drains that were installed early on to keep the soil under the house as dry as it has to be for the Annualized GeoSolar system.  The three rock formations seen in the second photo protect some of the French drains as they emerge to daylight.  The middle one passes through the collector shell.  Without it, we would have had to install a separate French drain for the collector.

While excavating for the house and trenching for the French drains and the AGS conduits, we encountered a layer of glacial till, also known as hardpan.  As will be discussed in a post on the actual build-out of the collector, we found that the dirt floor of the collector also comprised hardpan.  All along, rain falling into the collector must have been shunted by the hardpan to the French drain with the drain carrying it away fast enough to eliminate any pooling.  However, as part of the build-out, we removed the hardpan from the walkable space so that the concentrated flow of water from the glass panels of the collector would be carried away quickly -- by finding the French drain or by simply soaking into the newly exposed permeable soil.

Hail Damage?
One of the advantages of our southern-ish latitude is that the sun is more directly overhead, which is good for harvesting solar heat.  But it also bad because it means that the solar collector glass is more horizontal and therefore more susceptible to hail damage. The glass panels will be salvaged 1/4" thick tempered plate glass that will probably hold their own against normal size hail but with larger size maybe not so much.  If damage does occur, one option would be to switch to transparent fiberplass or polymer panels having UV coatings.  However, not only are they not as efficient for solar gain, UV degradation would limit their useful life-expectancy.

The next post will deal with the optimum angles for the glass and the steel panels.

Tuesday, March 10, 2020

Construction - Steel Siding and Soffets; Garage Doors

As early as mid-2015, we weighed several options for cladding and decided that steel siding would be, by far, the most sustainable.  In a previous post, I said.........

Steel siding is ...... "DIY-friendly, it's virtually maintenance-free, it lasts for plus or minus a century and it has a recyclable end-life. If there is a knock against metal siding, it is that it has fairly high embodied energy which,to some degree, is off-set by its recycled content".

In the first of two posts early in 2019, I described our adventures with buying steel roofing from Menards and followed it with a second post on its installation.  Our steel siding also came from Menards so the following discussion covers only the installation although its purchase was not without additional adventure as well.

As an aside, let me point out that installation of the steel roofing can be a one-person job, at least if it is not too windy, because gravity is an ally.  Installation of steel siding is another matter because gravity is the enemy.  Even with a trim piece at the bottom of the wall on which to stand the sheets while aligning and fastening them, tall panels are virtually unmanageable working alone.  And, for a watertight junction between panels, it is absolutely critical that the overlap between panels be fitted precisely before fastening -- something that is a little more difficult to do by one person.

Moisture Barrier
Joseph Lstiburek, in his excellent paper on vapor control, recommends using vapor retarders, such as house wrap or 15# felt, rather than vapor barriers such as plastic sheeting, bitumen-coated Kraft paper and, as often recommended by steel cladding manufacturers, 30# felt.  For a summary of Lstburek's paper, check out a previous post on vapor and air barriers.

In keeping with Lstiburek's advice to use a vapor retarder, our best choices were house wrap or 15# felt paper.  I opted for the felt paper due to atypical dimensions of our walls.  The south-facing walls that were closest to typical were riddled with windows which would mean wasting a lot of house wrap if it were installed first then the window openings cut out.  The rest of the walls were less than the height of a roll of house wrap -- some only a few feet high -- which would necessitate pre-cutting the wrap instead of merely rolling it out on the wall and fastening it.  And, because the joints in the sheathing were taped against air infiltration, there was no need to use the vapor retarder as an air barrier so using large pieces of house wrap that minimized the amount of taping was moot.

Consequently, I chose to use 15# felt paper as the vapor retarder.  It was cheaper, easier for two people to handle, was better suited for the short walls on top of the earth sheltered walls and could easily be customized to fit around the windows that were clustered together and also easily adapted to the sloping tops of rake walls.  We overlapped the courses by at least 6" to thwart moisture infiltration.  We taped the seams between courses with Tyvek tape, not so much as an air or moisture barrier, but to keep the wind from having its way with the felt before we could cover it with steel which proved to be only marginally effective.  Despite fastening the felt with roofing nails instead of stapling it, it pulled loose in a few areas, had to be re-nailed and the pull-through holes in the felt healed with Zip tape.

In retrospect, I would have sprung for the extra cost of the wider and stickier Zip tape that I used to seal the roof and wall sheathing instead of the Tyvek tape.  Not only would it have better protected the edges of the felt from the wind, it would have adhered better and prevented vapor penetration through overlapping edges the felt over the long run -- maybe overkill but why-not?

As is standard procedure, we used "J" trim at the left and right vertical edges of each section of wall into which the edges of the first and last panel fit as well as on each side of the window and door openings. Then any moisture circumventing
Steel "J" mold used at the edge of steel panels to divert
water downward that will eventually be overlapped and
covered by painted wood trim
the edges of a panel is diverted downward.  In order to make the house look less like a commercial building or rural implement shed, the "J" trim will eventually be overlapped by and hidden behind wood trim.  The top edges or panels below the windows
though not ending in J trim, will likewise be hidden behind wood trim.    Most of the trim will be fashioned from pressure treated 2 x 6s,\ and 2 x 8s, being mindful that pressure treated stock is typically stored wet at the lumber yard and, if not handled right, shrinks and warps as it drys.  I found early on with the mud sills under the walls that pressure treated lumber can be rendered dimensionally stable by drying it on stickers for several months, exactly like air-drying sawmill lumber.  In fact, at the time of this writing, I was already installing the trim.

The bottoms of most of the second story panels
Friend, Glen, is laying out the location of a row of fasteners
In this view, notice several things:  the rows of overlapping
15# felt paper taped together, the cedar trim board over-
lapping the termite shield with the bottom steel molding
 between it and the lower ends of the steel panels; the cedar
 board and the molding give a cleaner, less commercial
 look than if the steel panels simply terminated over the
 edge of the termite shield (click to enlarge for better detail).
overlapped the dormer flashing at the wall-roof junction.  For a more finished look, the bottoms of most of the first story panels rested on a trim piece called "bottom trim" in lieu of having them merely overlap the termite shield slightly as is probably more typical. On the front of the house, as an aesthetic upgrade, we overlapped the top edge of the termite shield with a 5/4 cedar board then rested the bottom trim on it.

Customizing the Steel
In a previous post, I described a jig for assembling trusses for the exterior walls of the house.  Eventually, I modified it as a jig to support the steel roofing while custom cutting. Then I expanded it for cutting the wider siding steel panels.  The manufacturer warns against cutting the panels with power tools, like circular saws or grinding discs, that produce red hot fragments (sparks) that burn spots in the finish of the metal, opening the way for rust.  Instead, we found that metal blades in a cordless jig saw do cut rapidly without making sparks.  The only caveat is that, in order to control vibration, the panel has to be clamped securely to the jig while cutting .  Fine-toothed blades with 30 teeth per inch minimized vibration and still cut reasonably fast.

The extra effort going into making a jig for
The width of the jig that was used for cutting the roof
panels was modified to accommodate the wider siding
 panels; the crosspiece on the near end was used to press
 downward on the panels in order to control the vibrations
caused by the jig saw.
cutting metal panels is definitely worth the effort.  However, its rectangular shape is not ideal for cutting the angles associated with hip roofs and rake walls because the panel is better supported the short side of the cut, than the long side that overhangs the end of the table more.  We tried modifying the table with an angled extension which helped but was suitable only for angled cuts in one direction while hips and rake walls needed to be diagonalized in both directions.  We finally simplified things by using the square end of the table for all cuts.  We used a specially-designed crosspiece at the end of the table for  downward pressure to control vibration.  Sometimes we supplement it by additional clamping.

We padded the foot on the jig saw to keep it from scratching the finish on the panels but two problems caused us eventually to cut all panels upside down.  The duct tape we used for padding left smudges and occasionally the tape would wear through and scratch before we realized it.  The disadvantages of upside down cutting, though, is that the layout for angled cuts is more un-intuitive and takes more concentration and the table must be thoroughly cleared of metal fragments that might scratch the front of the panels.

Fastening the Panels
The self-threading hex-headed fasteners that matched the color of the panels came with elastomeric grommets under their heads. The challenge was to drive the fasteners just enough to compress the grommets to the proper degree.  Squashing them either too much or not enough could compromise the water-tight seal under the fasteners.

Metal Soffets
The ventilated soffets were also made of steel. 
Intersecting longitudinal perforated steel
soffets framed with recycled lumber (yet
 to be painted); notice the Rain Handler
 System (arrow) in lieu of a conventional 
gutter and downspout system.
(Click on picture to enlarge for details.)
Instead of short multiple pieces running cross-ways between the fascia and the wall as is typical for most soffets today, the steel comes in long lengths that are 16" wide.  Since the soffets are 24" wide on the house and 36" wide on the porch, it was necessary to frame the metal panels with wood -- wood that, in a former life, was fir roof sheathing on a house that I salvaged previously .  By rabbeting one edge of the wood, the edges of the panels could be tucked under the framing and, 
for a better seal against insects, caulked where necessary.

Rain Handler System
In order merely to disperse the runoff from the roof instead of directing it to a few places using a gutter and downspout system, we used the Rain Handler System.  With a 2" overhang of the steel roof panels, the water falls onto the perforations in the Rain Handler and, essentially, becomes rain drops again before falling to the ground.  The dispersal adds moisture to the shallow backfill over the insulation/watershed umbrella that, because of its lack of volume, stores less moisture to nourish plant life and can use the extra water collected by the roof.  The two places where we did use conventional gutters was a short section over the front entry and another on the side of the porch under a roof valley where the amount of water often overshot the Rain Handler and eroded the soil below.

Garage Doors
Installation of the garage doors was a new experience for me but the manufacturer's detailed instructions made it relatively easy.  I will focus here only on the extra carpentry that was necessary to provide support for the ends of the tracks and the garage door openers in the presence of a vaulted ceiling.

We used a couple of long 2 x 4s that were
Boxes appended to the beam for supporting the closures
(green arrows); notice the one large window instead of
smaller windows within the doors. (Click to enlarge)
(salvaged from a pallet on which steel roofing was shipped) nailed together to form a beam spanning the width of the garage and positioned to be +/-10" above the top of the door openers and aligned with the ends of the door tracks.  It is supported in the middle by a nailer dropped from a roof truss.  Boxes were added to it over where the closures would be located so that the closures could be hung with short lengths of perforated angle iron as would be typical with a conventional 8' ceiling.
 After going the extra mile to insulate the garage walls, floor and ceiling and to use insulated doors, it made sense to minimize the number of potential air-leaking penetrations through the ceiling drywall.  Using the beam meant that there would be only three penetrations as opposed to the 15-20 that would be necessary with individual angle iron supports for the rails and closures.  And the aesthetic difference is a nice plus.

We elected to forego windows in the overhead doors and go with a large window above them.  Doing so eliminates the weight of the glass in the doors and provides more privacy.  And the large window, to my eye, is not only more interesting architecturally, but seems to provide more useful solar gain in winter.