Tuesday, April 14, 2020

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

This is the first of four posts on the solar collector.  The first two parse its design; the last two detail its actual construction.  In preparation for writing the four 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.

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. 

Insulation
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.  

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 more of a problem in the future
The top of the serendipitous French drain in the walkable space.
with accelerating 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 was 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.