Construction - AGS System for Passive Solar Heating and Air Conditioning - Cont'd

This is the second of three posts on the actual design and installation of the AGS system. The first post dealt with the conduits that distribute solar energy through the thermal mass under and surrounding the house.  This post focuses on the second important element of the system -- the solar collector.  Another element is the insulation/watershed umbrella.  I will describe it in detail once it is installed.  Its installation will fall sometime between wrapping up the concrete work and beginning wood construction -- possibly as soon as a couple of months out, weather permitting.

(Reminder: click on any photo below to enlarge it for closer inspection.)

Depth of the Solar Collector
My initial concept of the solar collector was not well thought through.  I simply failed to realize that the 15 degree slope in front of the house was too shallow for the floor of the collector to be 9' below the floor level of the house and still be even close to the height of the surrounding grade.  Fortunately, having to bury the collector does not significantly limit solar gain because the summer sun orbits so high in the sky that, even when the glass is 4 - 6' below the surrounding grade, it will shine on the collector a sufficient number of hours each day. This is in contrast to a typical winter passive solar system for which the glazing would have to be closer to the surface in order to catch the low-angle winter sun,.

Size of the Solar Collector
Several years ago, we visited near Spokane an AGS-conditioned straw bale house having 3' x 20' glazing for its collector in a climate with a third more heating degree days and considerably less available summer sun than we have here.  Therefore, I was confident that the tempered glass panes we found for free on Craigslist -- enough for a 4' x 18' collector -- would do the job.  I could easily conceptualize the 18' dimension but not the 4' dimension. Once I determined the angle that put the glass perpendicular to the summer sun, I realized how flat the glass would be and how big the shell for the collector would have to be.  Unfortunately, I didn't do the math before the excavation for the collector was done and had to have the contractor dig some more and, even then, the pit was too small in the N-S direction for easy wall construction of the south wall of the collector.  In the second photo below, notice the proximity of the excavation to the south (right) wall of the shell for the collector.

Excavation Between the House and the Collector
As detailed in the first post , the nine corrugated conduits begin at the future front wall of the house, fan out under the house to connect with smooth pipes in the backfill behind the

View showing the extent of the excavation for the smooth
pipes; a few of the corrugated pipes can be seen at the
base of the drop-off; the rest have been buried by gravel
and soil washing out of the conduit trenches; the
tripod at the right supported a rope and pulley for swinging
heavy items into the pit while working alone

house that run to daylight.  At the front wall, they are bunched together so as to be lined up for a run to the collector in parallel using smooth PVC pipes. Rather than trenching for the smooth pipes individually, the run to the collector was opened up completely with enough depth that the smooth pipes will increase slightly the 3 degree inclination that was already built into the corrugated pipes. (The inclination is critical for the heated air from the collector to pass passively through the conduits and exit some 70' later behind the house).

The excavation between the house and the collector was not inconsequential, being over 20' square and sloping from 5' deep at the corrugated pipes to 6' at the collector.  It also undermined the front wall of the house to such an extent that five piers on individual footings resting on virgin soil in the floor of the excavation were installed before backfilling the excavation. The footing for the foundation will be doubled in thickness (height) since it spans the excavation as a beam supported jointly by the piers and rock backfill.

Excavation for the Collector

The excavation for the collector was not inconsequential as well. The pit for the collector was dug 22' in the E-W direction and 10' in the N-S direction.  As already mentioned, it would have been better had it been 12' north to south and 24 feet E-W to give more room for dry-stacking the block walls. The original depth for the excavation at the collector was approximately 9' below the floor level of the house. The conduits will eventually penetrate the wall of  the collector about 3' above the floor of the excavation and the north wall of the collector will need to be at least 6' higher than the conduits as a retaining wall for restoration of the original slope of the ground in front of the house. The the other three walls will need to be more like 3' higher than the collector.  The disparity in wall heights will be handled with natural rock retaining walls extending outward from both ends of the north wall.

Unfortunately, I did not photograph the pit before installing the first three courses of blocks. So this picture, taken after the first bond beam course was partially filled with concrete, is a little premature for this post but is included to give perspective on the size of the excavation for the collector.  The rigid AGS conduits (smooth pipes) will connect with the collector on the north side (left in the photo).

Glazing
The link "Duration of Sunlight for 2015" for Collinsville, IL, shows almost identical "solar isolation" (maximum amount of available sunlight) for the weeks leading up to June 21 versus the weeks following. The website does not account for cloudy weather so I am betting there are more clouds interfering with solar collection during the rainy weeks preceding June 21 than during the dry summer weeks following it. Accordingly, I decided to use the sun angle for July 10 instead of June 21, which was easy to do by going online to Solar Position Calculator.  Our sun angle for July 10 is a steep 74 degrees off horizontal. The most efficient angle for the glass is one that is perpendicular to 74 degrees  or 16 degrees off horizontal.  

 "April showers bring May flowers" seems be an oxymoron anymore.  Our Spring rains seem to come later and later to the extent that this year our rainfall for June was three times normal and the rain during July is running above normal as well. Maybe, in the future as the weather changes in response to global warming, a July 10 target will prove to be too early for capturing the most sunshine .  Although we are not doing it, it might make sense to consider making the tilt of the glass adjustable.

The width of the four panes of glass is 4' and their total length is 18'. The perpendicular angle to the sun angle is close enough to horizontal that the glazing for the collector will cover 80% of the total area inside the concrete shell.  There will be only 18" of access space along the south wall for maintenance, such as cleaning the glass and clearing out leaves, and to allow an unobstructed flow of air into the front of the collector.  The glazing for the collector is quarter inch tempered glass which may be not be strong enough, at such a flat angle, to withstand the kind of hail storms we see frequently here in the Midwest.  If not, it may have to be replaced with 3/8 inch tempered or a plastic/fiberglass material of some kind.

Clear Glass or Translucent?
When sunlight passing through glass strikes various objects and surfaces behind the glass, the short wavelength energy is converted into long wavelength energy that is incapable of passing back through the glass -- hence, the "greenhouse effect". Darker objects tend to absorb the energy and lighter objects tend to reflect the energy.  For our AGS system, we want as much energy as possible not to be absorbed and bound up in the collector but to be reflected and encouraged to leave through the conduits.  With our clear glass panes, a white gravel floor for the collector that reflects rather than absorbs will be best even though some of the energy will not be diffused and will be lost back through the glass.

However, the best of all worlds would be translucent glass instead of clear glass.  To paraphrase one authority, "translucent glass is transparent to the incoming wavelength and opaque to the outgoing wavelength".  The glass itself diffuses the incoming energy and traps it all rather than some going back through the glass.  I suppose this is why old-fashioned greenhouses with real glass roofs usually have what looks like whitewash on the glass and why untinted translucent fiberglass is recommended over clear materials for homeowner greenhouses.  We will be using this principal for the second story windows of the house as will be discussed in a future post.

Update - Summer 2016
When this post was written, I thought I had a grasp on how the solar collector should work. Since then, a scientist friend has helped me understand the design from a thermodynamic perspective which is much different than what I wrote above.  In the near future, I will be devoting an entire post to the design of the collector.  Stay turned.