This page collects some material on designing efficient solar air heating collectors.
There is a commonly held believe that solar air heating collectors are less efficient than solar water heating collectors. I guess this is due to the difficulty of getting good heat transfer between the absorber and the air stream. The fact that the few air collectors that the SRCC has tested have not be stellar performers would seem to support this.
But, the material below indicates that with careful design of fins and baffles, air collectors can achieve quite high efficiencies, and that while careful design is needed, the cost and build difficulty are not greatly increased. The two papers below measure efficiencies as high as 80% for carefully baffled and finned air collectors.
I hope to do some work on this in the Spring. I would like to come up with a simple air collector design with with verified high efficiency that is easy to build and has a good long life.
If you have any ideas on this, please send them in -- Gary...
Experimental Investigation of Solar Air Heater with Free and Fixed Fins:
Efficiency and Exergy Loss
International Journal of Science & Technology Volume 1, No 1, 75-82, 2006,
Download here: http://web.firat.edu.tr/IJST/1_1/11.pdf
The paper describes some testing on solar air heating collectors. They measure the efficiency of an air collector with a plain metal absorber, and the same metal absorber with several different fin configurations (one in picture below). Measurements were made for several flow rates and sun intensities.
The plain metal absorber air collector has a mid day efficiency of about 45%. The best of the finned absorber collectors shows around 80% efficiency under the same conditions. So, good fins appear to nearly double the efficiency.
Picture showing absorber plate with one of the fin arrangements.
The air solar collectors:
Comparative study, introduction of baffles to favor the heat transfer,
Ben Slama Romdhane, June 2004
Institut Superieur des Sciences Applique'es et de Technologie, 6029 Gabes, Tunisia
This paper looks at several different baffle designs for air collectors. Smoke flow is used to visulize flow patterns in the collector, and to find dead spots that are not receiving sufficient airflow for good heat transfer.
While the end baffle/fin configurations are different than for the paper above, the results are similar -- careful design of baffling can result in efficiencies in the 80% area.
The paper is available on one of the document servers at a fairly stiff price, but you may be able to get it through your university library.
Gary Feb 16, 2008