Effect of Air Filter on Thermosyphon Collector Air Flow

This solar air heating collector that I use to heat my shop has no obstruction in the inlet vent.  Mike is thinking about doing one of these collectors, but would want to have an air filter in the inlet to keep dust out of the collector, and is wondering if the air resistance of the filter will have an impact on air flow because of the extra resistance of the filter.

This seems like a good question, and I did this test to get some idea how much effect adding an air filter in the inlet vent has on flow and on heat output.



I used some filter material that I had around, and just cut it out a bit bigger than the vent opening, and stapled it to the sheathing that the vent is cut into.




Click on pictures for full size

The filter material used.

Filter material stapled over the
inlet vents for one bay.

Air enters bottom vents (with filters)
rises up through collector, and
exits the top vents.  Two identical bays, one
one with vent filters and one without were


I measured the exit vent airflow velocity on the collector bay with the filter material over the inlets and compared this to the measurements for the adjacent (identical) collector bay without the filter.

The average of several readings over about an hour ending around solar noon:

  With Inlet Air Filters With inlet vent open
Vent velocity 60 fpm 90 fpm
Temperature rise 32F 22F

Ambient conditions were full sun with air temperature around 82F, July 6, 2009

So, the inlet filter cuts the air velocity by about 1/3 under these conditions.  But, there is some compensating increase in temperature rise through the collector.

The heat output of the collector is proportional to (Vent Velocity)*(Temperature Rise).  If you multiply these out, the with and without filter actually come out nearly tied.   So, under these conditions, the filters decrease vent velocity, but don't decrease heat output.

I think that in the winter the results may be somewhat worse for the collector bay with filters because the ambient temperature would be mush lower, and the heat loss from the collector bay out through the glazing would be larger -- this would reduce the temperature rise for the filtered collector.

These were not ideal conditions to do the testing.  In mid summer, the sun is quite high, and this reduces the gain of the collectors a lot -- mid-winter air velocities are in the 140 fpm area.  I did not have my Dwyer Vaneometer to measure air velocity and had to use the Kestral wind meter -- it only measures to the nearest 10 fpm, and 60 fpm is about at the bottom of its range.  So, while I think the numbers give a rough idea what's going on, take them with a grain of salt.  I left the filters stapled in place, and will repeat the test under mid winter conditions.

While I hate to do anything to increase flow resistance through a thermosyphon collector, it appears that a low resistance air filter might be used in the inlet without impacting performance too much.  The filter should have as low a resistance as possible, and it would be a good idea to make the inlet vents larger to increase the flow area.

I should mention that there is very little dust in the collector bays on mine after 5 years, but I do not normally do dusty activities in this shop.


Gary July 6, 2009