Thermosyphon Solar Air Space Heater (with 1 year payback)
These articles describe the simple and inexpensive passive solar collector that I use to heat my shop/barn. The total cost for the 150 sqft collector is about $350, and all the materials are available at your local hardware/lumber store. No fans or electronics -- its driven by natural thermosyphoning. The design can be adapted to suit a wide variety of spaces, including a living space. At our current propane prices ($2.00 per gallon), this project has a one year payback.
I would suggest reading both the Mother Earth and Home Power articles (below), as they each have some unique material that will be helpful if you are building the collector. Also review the 2012 updates section for some potential refinements.
NEW -- A video by Peter Brown on our thermosphon collector...
NEW -- 2012 Updates...
Mother Earth News article on the collector.
+ The December/January 2006 issue of Mother Earth News --
Read the online version of the Mother Earth News article
+ The October/November 2005 issue of HomePower Magazine
Download the HomePower Article (500K pdf) -- Thanks to HomePower for providing this!
The collector was built in the winter of 2003-2004 and has been faithfully pumping out heat ever since. This is an update on how things have gone with it, some recent performance testing, and a new "commercial" version of the collector:
+ Potential changes and refinements
+ Recent Testing:
+ Effect of reducing vent size on performance
+ Effect of dust filters on performance
+ Efficiency testing
+ Flow rate testing
+ Back of collector as large radiant heating panel
+ Commercial version
Note: I noticed that the design guidelines for thermosyphon collectors did not make it into the document above, so here they are:
The buoyancy forces that drive the air flow through a Thermosyphon collector are
The collector must be designed to minimize flow resistance. High flow resistance will
result in higher collector temperatures, higher losses, and lower efficiency. For low flow
resistance, flow passages must be large, and the absorber must transfer heat to the air
efficiently and with little flow resistance.
The depth of the collector should be about 1/15th of the height (more is better).
The top vent and bottom vent areas
should each be at least 50% of the collector cross
sectional area (more is better).
The air flow path through the
collector should be as shown in the “How It Works”
sketch. This path keeps the cooler air near the glazing to reduce losses to the outside,
and also provides good heat transfer from the absorber to the air as it passes through
The absorber must 1) have low air
flow resistance, 2) absorb a large fraction of the
incident solar energy, and 3) transfer heat to the air well. The window screen absorber
seems to do this well, but other materials may also work.
Streamlining the entrance and exit
vent areas may also help reduce airflow resistance
and increase collector efficiency.
Changes that improve collector flow
path will normally result in a decrease collector
output temperature along with a more than compensating increase in flow rate. For best
efficiency you want a lot of air flow with a modest temperature rise.
Gary 5/12/2006, 11/19/06, Nov 18, 2007