Tuesday, April 19, 2011

Heat Transfer in a Solar Air Heating Collector Made with Gutter Downspouts

An interesting solar air heating collector design to come up recently is made from ordinary aluminum gutter downspouts.  The downspouts are laid out side by side to from the collector absorber.  The downspouts are all connected to air supply and return plenums run along the top and bottom of the collector.  Air is forced through the collector and picks up heat from the sun heated gutters.

This is a picture of a downspout collector from Scott's site...
Amoung the advantages of the downspout collector are that 1) there is no forced airflow against the glazing, which tends to reduce heat loss out the glazing, 2) the plenum and downspouts provide for an even distribution of air over the full absorber surface and minimizes hot spots with poor circulation, and 3) the heat transfer area from absorber to air (a bottleneck in many solar air heating collector) is good due to the large heat transfer surface area of the downspouts.

But, the last advantage of lots of surface area to transfer heat from downspouts to air is only true if the heat from the sun side of the downspout gets conducted around the sides and back to warm them up.  This test was done to see if that happens -- that is, is there really good heat transfer around the full circumference of the gutter section so that heat is transferred to the air by the full surface.

The not very ambitious test setup consists of 3 downspout tubes connected to an air supply plenum.  Air is supplied to the plenum by a small fan, and the temperatures of the front ant the back of the gutter tubes is measured with a thermal imaging camera (my new toy instrument).  Here is a picture of the setup.

The insulation panel on the back is to better simulate the actual collector environment -- it is quickly removed before taking the temperatures on the back side.

These are thermal images of the front and back while the collector is in operation.

Front of the collector

Back of the collector
Its pretty clear just from the images that quite a bit of heat does make its way around the sides and back, and while they run a bit cooler than the front, they are still quite a bit hotter than the airflow and that some heat transfer is taking place around the full periphery.  So, this seems like a plus for the downspout collector.

More details and analysis on the test of the downspout collector here...



  1. Hi Gary,

    You may have already seen my comment on simply solar, but it would be interesting to see what effect mounting the downspouts on black painted aluminum flashing has on heat transfer to the back of the downspouts (as in the collector picture above). Is there some optimal spacing between the downspouts, where you can save some money on purchasing downspouts by using the flashing behind?

  2. Hi Scott

    I suppose the downspouts could be separated a bit even without the flashing under them as all of the sun would hit a gutter surface except for a few minutes a day when it shines between gutters.

    I guess the only negative I see to the flashing plus fewer gutters is that it reduces the heat transfer area to the air which can be a problem on air collectors. But, as you say, there may be an optimum.



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