Practically off the grid!

This is the story of my experimentation with a solar air heater.  I started this project in the summer of 2008 and have been fiddling with small modifications since then.  I have learned a lot in the process and now have a very functional supplemental source of heat for my home.  I will share some details about the construction process and the improvements that I have made to the system.

my house

Luckily, my house is blessed with north-south orientation.  This makes the front of my south facing house the ideal location for a vertical solar collector of some kind.  In this first picture, you can see a large shadow cast over the collector area from the giant elm tree almost directly in front of my house.  This shadow has put a major limitation on the gains I can obtain but I still get an appreciable about of sun in the winter when the sun is low enough in the sky to shine light on my house underneath the branches of this tree.

My research for this project entailed a lot of reading on solar air and water collectors and I concluded that an air collector would be the best choice for me for a number of reasons.  First, I wanted to build the collector myself and precision and material requirements in air heater construction appeared to be a bit more forgiving for the do-it-yourselfer.  Second,  by reputation solar water collectors are more efficient but I had found evidence to show that with the proper design, air heaters could compete in efficiency.  Third, the material and technological requirements for air heaters are significantly less that for water collectors.  Lastly, my gains would not be so enormous (given my site challenges) that I would have to store excess heat so an active collector with no storage would work well.  A solar air collector fit this bill perfectly.  

The basic strategy for this collector was to draw in air from my cooler basement, pass this air through the solar collector and blow it back into the main level of my house.  I thought by using the sun’s energy to heat the coolest air in my house I would maximize the efficiency of the collector and the transfer of air from my basement to main level would also help with overall air circulation.

In the photo above, you can see a numer of rectangular vent holes.  Air from the basement will flow in the bottom vent after passing through an insulated duct under my entry floor.  It will then pass through the heater and out the two vents on the top.  I chose to use one main vent on the bottom so I could just build one insulated duct under the entry and two vents on the top to give the air reason to distribute all over the collector.  Behind the OSB is a fully insulated 12 inch thick wall.

Next I installed reflective bubble foil by stapling it to the OSB and taping the staples with foil to maintain air tightness.  The air that is being heated will be passing directly in front of this foil.  Its purpose is to slow the transfer of heat from this heated air to the wall mass behind the foil.  I don’t want my wall to be hot…just the air passing by.  The reflective coating on the bubble foil slows heat transfer by radiation and the bubble interior slows heat transfer by conduction.  The foil also serves as an effective air barrier for the back of the heater. The two top vents are still there behind the foil.  I just had not cut them out yet at the time of this photo.

The collector plates for the solar air heater consist of salvaged double pane windows with aluminum flashing siliconed to the back side.  Aluminum flashing was chosen because it is available at most hardware stores and is highly conductive.  It was painted with flat black spray paint designed for high temperature applications.  The back of the aluminum plates were left as is.  

The heater is designed to force air though a 1.5 inch gap between the back of the aluminum and the reflective foil.  From all of my research I consistently found that in climates as cold as ours in Saskatchewan, the heated air should be kept away from direct contact with the window pane.  

outgoing air vent

 One rule of thumb for air heater design is to maintain the same cross sectional area through all of the air flow channels.  For example, if my air flow channel behind the collector plates is 1.5 inches wide and 72 inches long I have a cross sectional area of 96 sq inches and need to have approximately the same total cross sectional area through all of my other vents.  A view of one of my indoor air vents is shown above with a cross sectional area of 80 sq inches.  I have two outgoing vents for the heated air with a total of 160 sq inches in cross sectional area so that is plenty of space to avoid constricting the flow of air out of the heater.  The incoming air duct at the bottom of the heater is also sized larger.  It is 14″ x 14″ to match area needed for the DC fan I purchased for the heater.  I found this website helpful for system sizing: Rules of Thumb for Solar Air Collectors.

coat rack in front of vents

 To conceal the two vents in the entry I built a simple coat rack.  It is actually supported about 6 inches out from the wall surface so the outgoing air has lots of room to pass by.

exterior air heater vent

The solar air heater also needed a vent on the exterior so that unwanted heat in the summer can be released outside instead of inside.  The giant tree shades the front of the house for most of the day in the summer but for the few hours of sunlight exposure exterior venting is a must.  I drilled a series of 3/4 inch holes through the top of the air channel of the heater and these holes are closed with threaded galvanized plugs that I can remove in the summer.  With the plugs removed the air simply flows out the top of the heater through the vent you see in the picture above which is always in place.

12/24V DC Venturi fan

This is the fan I use to move the air through the heater.  It seemed a bit pricy at first but has been well worth the cost.  I power it directly with a small solar panel in front of the collector and the great thing about this direct connection is that the fan blows the fastest when the sun is the hottest.  It is rated at 500CFM so it moves a lot of air and keeps my outgoing air temperature just a couple of degrees warmer than the ingoing air temperature.  I have seen a lot of designs that use small blowers and rave about the high temperature of air produced but that high air temperature means the air heater is running inefficiently.  The object of an air heater is to extract as much heat as possible from the light energy that is absorbed by the collector plate.  The most efficient way to do this is to keep the collector operating at a low temperature.  A collector that is allowed to overheat will just radiate that energy back through the glazing and you will never get it back.  

That about covers the air heater operation while the sun is shining but there are standby losses that must also me controlled.  Whenever the heater is not running, backdrafting can occur.  This is the flow of air though the heater in the reverse direction.  When the sun is down and the panel actually works quite well as a passive radiator by drawing in air from the top vents cooling that air behind the cold collector plates and pushing the cool air through the bottom vent.  I attempted to stop backdrafting by fastening 1/4 inch mesh over each vent with plastic dampers that opened when the air flowed in the desired direction and closed against the mesh when air was pushed in the wrong direction.  This worked pretty well but I found a small backdraft was always present and this really bothered me because I built this collector to save energy.  This was one of the problems that led to some changes in the solar air heater design.  See the latest changes to this system here:  Solar Air Heater (Trial 2)