Posted by C. on April 16, 2009, 3:51 pm
I'm looking to build a home-made solar air heater like this model:
My question is regarding the optimal design for the heating chamber.
Often in these solar air heaters the heating chambers are strings of
aluminum cans, or in the case of the air heater above, aluminum
However, some hobbyists on the Internet have suggested that if the
surface of heating chamber is instead a sheet of aluminum that is bent
to create waves, we would increase the efficiency of the unit. The
theory is that by introducing peaks and troughs in the sheet, we
increase the surface area of the heating chamber, and ergo more solar
energy will be captured.
I would have thought that increasing the surface area of the heating
chamber would not effect any increase in solar radiation as the amount
of sunlight being shone on the unit would not change. I believe that
the only way to increase the amount of energy captured would be to
increase the overall size of the solar air heating unit.
Am I mistaken? Is there possibly a difference between the amount of
heat generated in a smooth cylindrical chamber versus a wavy one?
Posted by Morris Dovey on April 16, 2009, 4:37 pm
Let me muddy the water a bit... :)
You're correct in that for a given orientation and glazing material, the
amount of solar radiation INPUT is determined only by the area of the
For a given input, the amount of solar radiation CAPTURED is a function
of the absorber efficiency.
And - for a given amount of captured energy, the amount of heat actually
DELIVERED will be less than the amount of energy captured by the sum of
You're not mistaken - BUT there will be a difference if the two
absorbers behave differently (ie: have different efficiencies or produce
different loss amounts).
DeSoto, Iowa USA
Posted by C. on April 16, 2009, 4:57 pm
Thanks for the response! I'm very much a layman and still trying to
figure this out!
I guess my question now is:
Is the efficiency of an absorber is affected at all by its surface
area? Given a fixed amount of solar radiation, will a wavy absorber
help mitigate any losses by the unit?
Posted by Morris Dovey on April 16, 2009, 5:23 pm
Oooh - you're not gonna like this...
Maybe. :) It depends on a whole handful of variables, and for every
simple rule that I could think of, I could also find exceptions...
Surface area can work for you or against you (and even do both at the
...and even I don't find my answers very satisfying. :(
DeSoto, Iowa USA
Posted by Ecnerwal on April 16, 2009, 6:07 pm
You want efficient collector, go to the flow-though adsorber designs -
cool air to the bottom/outside, absorber of black screen (several
layers) or furnace filter, hot air on the back/top side. Metallic
absorbers are the sort of 20th century technology that made sure that
solar didn't work very well outside of Tuscon...
I don't have time to type out all the reasoning that goes into these -
it's peen posted here in the mists of the past, and probably lurks at
google groups. The fact that no part of the collector gets much hotter
than the exhaust air is part of the efficincy - hotter parts waste more
heat back out of the collector, rather than transferring heat
efficiently to the air. The design you posted has a hot dead air zone
outside the tubes, and heat must transfer through the tube walls. In a
flow-through absorber, the absorber transfers heat directly to the air
to be heated, with it having to flow through anything.
Read this. You don't have to stick to thermosyphon operation - the
principle works the same with fans. Better designs angle the adsorber a
bit so the plenums are larger as the near the duct.
Cats, coffee, chocolate...vices to live by