Hi Carl,
Bill Kreamer's collector design uses polyester felt for the absorber. The plans for his collector can be downloaded here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Active
Just search down the page for "Kreamer".
A number of these have been built, and I belive that the design is efficient. One thing that you need to be careful about is that you don't overheat the felt to the point where it melts. I believe that Bill does not recommend using the felt absorber on tilted collectors -- only on vertical ones where the summer stagnation temperatures are lower.
On my air collector, I used 2 layers of black alum bug screen. I tried 1, 2, and 3 layers, the result being that 2 and 3 layers performed very nearly the same, and both better than one layer. This is a thermosyphon collector (no fan), so the flow resistance of the absorber must be low. So, a fan with less pressure head capability could be used with this type of absorber. I've also heard of using expanded metal lath, and furnace filter media (painted black). Unfortunately, no one (to my knowledge) has done side by side tests to see which actually performs best, or (as is my prejudice) that there is little difference between them. This is my collector:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
Its going to be on the cover of Mother Earth News next month -- yahooo!
You, of course, want to organize the flow such that the air has to flow through the absorber, and so that the air starts on the glazing side, and exits on the "back" (north) side of the collector -- this keeps the hottest air away from the glazing and reduces heat loss out the glazing.
It sounds like your current collector is working well, but I think that it might do even better with more airflow to reduce the output temperature a bit. 140F is kind of high, and will result in higher losses. You can get a pretty good idea how well its working with this procedure:
http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
This is not so important if you are using an established design, but it sounds like you will be experimenting some, and this procedure will tell whether a change you made hurt or helped performance. Getting a gain in output temperature does not necessarily (or even usually) mean that the collector is performing better -- its the product of airflow time temperature rise that says how well a collector is doing. To use it, you will need a way to measure outlet vent air velocity -- I use a little Kestrel wind meter, which is fairly cheap, and works well.
Good luck on your project -- please post the rsults.
Gary
