Practically off the grid!

My philosophy with solar air heating is to go big or don’t bother.  One reason for this is that solar air heaters are so cheap to build that increasing the collector size really does not add a huge amount to the cost of construction.  Also, since the materials you have available for a DIY heater may not give you the most efficient heater, it won’t really pay off to build an undersized heater anyway.  If you are going to buy into the same philosophy with your solar air heater project then you will want to build it right the first time.  To aid you in your design process, I have compiled a list of research experiments on the subject of solar air heating.  I am not sure if a comprehensive resource exists on the subject but if you piece together the bits of knowledge presented by each of these papers and incorporate the best techniques in your design you will be bound for success.  If you know of some more valuable information out there that I have yet to find please send me a link to it and I will add it to the list.

Solar Air Heaters – An Application Guide

This paper offers a simple comparison of unglazed, transpired and glazed collectors.  It includes some basic descriptions to help you understand the basic differences between each design strategy but does not have a lot of experimental data to share.

Analysis of High Efficiency Solar Air Heater for Cold Climates

This paper is a well documented study of a collector using porous material for the absorber.  There are a lot of equations and graphs presented that back up the theories presented.  

Experimental Investigation of Solar Air Heater with Free and Fixed Fins

This paper may change some of your perceptions about collector efficiency by offering a well recorded comparison of a flat plate, fixed finned and free finned solar air heater.  You might think it would be best to keep all of the air channels open to keep the air flowing through quickly however this does not actually increase the collector efficiency.  

Numerical Simulation Study on Transpired Solar Air Collector

This is a neat mathematical simulation of a transpired collector.  This type of design may be the best fit for you if you are planning on using your solar air heater to preheat outdoor air on it’s way to your air exchanger.

Solar Air Systems:  A Design Handbook

This is perhaps the most comprehensive resource on solar air heaters that fully integrate their air loops into the building structure through walls and floors.  Enjoy this free Google Books preview while it is still available.

Thermal Performance of Wire-Mesh Roughened Solar Air Heater

If your collector uses a flat absorber plate you will need to create turbulence behind the absorber to transfer as much heat as possible from that hot absorber and increase your collector efficiency.  One method to do this is to fasten wire-mesh behind the absorber plate.  This is a simple experiment comparing the effectiveness of different sizes of wire mesh.

Comparative Analysis of Active and Passive Solar Heating Systems With Transparent Insulation

This paper compares data from six different collectors in France that use either a passive or active design.  Transparent insulation was used in some cases and was shown to increase the collector efficiencies.

Simulation of Solar Heating at a Constant Temperature

This report describes an air heating system that incorporates a large heat storage mass and variable speed fan to manage the temperature swings that come with solar heating and deliver heated air at more consistent temperatures.

Air-Type Solar Collectors for Agricultural and Residential Use

This is a comparison between collectors with air flow on top of the absorber, both sides of the absorber and behind the absorber.  Which design do you think came out on top?

Thermal Performance of Solar Air Heater by Using Shot Peened Absorber Plate

This paper shows how an increase in roughness of the absorber plate can lead to a significant increase in collector efficiency.

How to Build a High-Efficiency Air-Type Solar Space Heating Collector

This document offers detailed plans for a collector that uses black polyester felt for the absorber.  Air is forced from one side of the felt to the other while it passes through the collector so all of the air must make contact with the warm surface.  In theory, this should lead to higher efficiencies and according to Bill Kreamer it operates with over 72% efficiency.

If you are considering building a solar air heater yourself you may be interested in the tools needed and the approximate budget for the air heater I have built for my house.  I have described the construction of the heater in earlier posts which you can find here: Trial 1, Trial 2, Completed

solar air heater in winter

My research told me that a collector should be about 1/10th of the floor area you wish to heat and my main floor is 440 square feet so that is how I determined the size of the heater area.  I also wanted to be sensitive to the integration of the heater into the house facade and that limited its size.  The next time I build a solar air heater it will be bigger.  Why not really?  You can always get rid of excess heat but you can get more heat from an undersized collector. Anyway, I ended up building a 43 square foot collector and this post is about the tools and expenses needed to build a collector of this size.  

I have listed the equipment and materials that I used for the project.   You could easily complete the project without all of the equipment in the list but the materials are all pretty essential.  The one area of variation in the materials section might be the absorber.  Some studies I have seen of felt and screen absorbers show potential.  Other than that element though you should really have everything on the list.  You especially don’t want to skimp on the fan and solar panel because they will be driving the whole system.  The upfront cost of this solar panel/ DC fan combo is high but will save energy and headaches in the future.  The alternative would be to hook up a grid tied AC fan with a thermostat but an AC fan will use a lot more power and you’ll be paying for it.  I guess you could also use an inverter to hook up a grid tied DC fan but again that is just more equipment, wiring and power you will be paying for.  The simplicity and long term economics of the solar panel and DC fan are just too attractive to ignore I think.

Equipment:

tin snips

drill

skill saw

paint brush

staple gun

hand saw

(a jig saw or reciprocating saw would have been nice for the holes in the wall)

Materials:

bubble foil radiant barrier:  $30

4 salvaged double pane windows: $80

aluminum flashing: $60

flat black high temp. spray paint:  $20

aluminum tape: $10

12 inch DC fan: $150

25 watt solar panel: $200

Other: silicon, screws, stain, wood for frame, staples, wire: $50

Total:  $540

That total is trivial compared to the cost of a well known commercial model like the Cansolair Solar Max 240 which retails for $2695.  That purchase would also require taxes and shipping so the total is really more like $3000 to get the unit to your door.  That collector only has an area of 28 square feet and mine covers an area of 43 square feet so a cost per square foot would be even more telling of the economy of building your own collector.  A simple calculation reveals that my collector has a cost per square foot of $ 12.50 and the cost for the Solar Max 240 is around $100 per square foot!   In an field where size is king and low tech possibilities are plentiful why not build a collector yourself for a tenth of the cost.  Depending on your access to materials you may sacrifice a bit in terms of efficiency but that is a difference you can make up easily in collector size.  In addition you will enjoy the pride that comes with doing the project yourself and can integrated the collector more appropriately into the design of your home.

The recent modifications to my solar air heater required some changes to the vents in the front entry.  While I am always delighted to see the solar powered fan spinning away I wanted something to dampen the sound a bit, prevent any backdrafting and make some use of all of that wall space.  

new vents

The solution I decided on was to build a coat rack with enclosed inlet and outlet vents for the air heater.  I learned from some trial and error that the vents on the top of the shelves had to be positioned horizontally so that cool air did not flow out of the vents when the heater was not operating.  With the vents positioned like so, the cool more dense air just sits below the vent waiting to be pushed by the fan when the Sun comes out. 

air heater vent through coat rack

The large square openings on the left and right sides allow for convenient access to the fan and duct space behind the unit in case I ever need to make any small changes.  The photo below shows the finished coat rack with the square openings now covered with cork board.  I’ll find something fun to stick on there soon.

completed coat rack

The rack has been installed for a few days now and I am pretty happy with everything so far.  It allows me to make better use of the space in the front entry and it also works to dampen the fan noise and prevent backdrafting as required.  One downside may be that it does impede air flow.  I kept the openings as large as possible to minimize the additional air flow resistance but I think the unit has slowed things down slightly.  With a rating of 500CFM my fan for this solar air heater is still a bit oversized so I am not worried about sacrificing much of the performance.  I’ll continue to make observations and make any changes if needed in the future but as for now I am excited to be finished the project and plan to enjoy the solar air heater in its completed state for some time.

solar air heater