Saturday, June 27, 2015

Construction - AGS System for Passive Solar Heating and Air Conditioning

This post is the first of three on the design and installation of the AGS system.  It focuses on the conduits that carry the heat from the summer sun to the thermal mass under and around the house.  The other posts will be forthcoming as soon as the associated installations have been done.  One will cover the design for the solar collector and the excavations necessary to get it installed.  The last will discuss the construction of the shell for the collector, joining the conduits to the collector, insulating and backfilling the excavations.

AGS Design Review
The design of the Annualized GeoSolar system has been detailed in earlier posts (first postsecond post, third post).  In a nutshell, the heat distribution system itself has two components -- a solar collector for harvesting the heat from the summer sun and a series of conduits for conveying the heat from the collector to the soil under and adjacent to the house for storage.  Another important element is the insulation/watershed umbrella extending +/-20' outward from the house about two feet below grade so as to insulate and keep dry a thermal mass larger than the footprint of the house.  The necessity for dry soil is covered in the three posts cited above and more recently in one of the posts on French drains.  The construction of the umbrella will be detailed later after it is installed, which will take place after all of the concrete work is done and before wall construction begins.

Conduit Configuration
The conduits comprise two types of 4" pipe -- rigid smooth-walled Schedule 40 PVC and flexible un-perforated corrugated drain pipe.  The smooth pipes extend from the collector to just under the front foundation where they are joined to the corrugated pipes then again in back of the house between the corrugated pipes and daylight above the north grade which will be nearly two-stories high. The corrugated pipes are limited to the area under the slab floor.  

The heavy-duty smooth pipes serve two functions -- to withstand the weight of 8 - 14' of backfill and their smooth walls should expedite the flow of heated air from the collector. The corrugated pipes maximize heat transfer from the conduits to the soil by creating air turbulence. They can be utilized for this purpose because the weight of backfill is rendered moot by filling the trenches with gravel and pouring a concrete floor over them.

Designing the diameter, number, depth and inclination of the pipes was an educated guess on my part after reading the article describing AGS (Don Stephens paper) and visiting one of the houses he designed near Spokane (the Mica Peak residence mentioned in the paper) and research on earth homes.  In our iteration, nine conduits were installed which means they are about 10' apart after flaring out under the floor. As for depth, they leave the collector 6' below floor level, pass under the front foundation at 5' below floor level, pass under the back wall of the house 3' below floor level and run to daylight behind the house at a 45 degree angle.  Except for the latter, the inclination is about 3 degrees above horizontal which should be enough slope for passive convection of the heated air but not so much that the flow rate is too fast for efficient heat transfer to the soil.

The decision on the diameter of the pipes was based primarily on a good book on earth sheltering that I have since lost track of and on Hiat's book, "Passive Annual Heat Storage".  Before reading them, it seemed reasonable to me that the larger the pipe the more heat transferred to the soil, so my early thinking was to use at least 6" pipes. However, it turns out that a large pipe with a given internal volume has less external surface and therefore, as a conduit, less contact with the earth than several small pipes whose combined internal volume equals that of the large pipe. The situation is analogous to human body types.  Compared to heavier people (endomorphs), skinny people (ectomorphs) typically tolerate summer heat better and tend to chill more in cold weather because they have a high ratio of skin to body mass and therefore more surface area for heat transfer.  I also reasoned that going smaller than 4" might not suck the heated air out of the collector fast enough to keep the collector from overheating unless the number of pipes was increased beyond reason.

Installing the Conduits
Installing the conduits was a cakewalk compared to installing the French drains.  Our
Installation of the corrugated pipes
contractor, Brian Hayes, brought in an industrial-strength trencher.  As soon as he dug a trench we dropped in the corrugated pipe, connected a smooth pipe to it at the north end, backfilled to within a foot or so of floor level with man-made pea gravel then backfilled to grade with soil.  Most of the soil will be removed during the final grading for 4" of rock sub-base and 4" of concrete.

Solar Chimney Discarded
The original design called for bringing all nine conduits to daylight at the north end via a solar chimney.  However, during installation, I decided to dispense with the solar chimney and run the conduits to daylight separately.  The change was driven by several things but mostly by my fear that the system might prove so efficient that the house would overheat. Stephens mentioned this concern in his paper and suggested incorporating thermometers
AGS conduits headed towards daylight; trenches filled
and the site graded for the sub-base and slab
in the soil below and behind the house in order to monitor the heat uptake and make corrections (presumably by closing some of the conduits at the collector). He suggested it might take several years in any case for the thermal mass to reach a stable year-round floating temperature so overheating would not be a problem for at least a couple of years for our project.  I decided that it made more sense to leave the conduits separated so that some could be capped for all or part of a summer following a winter with an overheating problem rather than trying to manage the problem at the collector end.  And it would be possible to reverse engineer a chimney later if proven necessary.

The other reason for keeping the conduits isolated was to eliminate the cost  and complexity of tying the conduits together and leading them into the solar chimney.