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.