Posted by schooner on April 16, 2009, 6:59 pm
That type of screen design also puts the flowing air in contact with the
cooler glazing, where the tube or plate style doesn't, so there would be
some loss there as well. All have pros and cons.
Posted by Morris Dovey on April 16, 2009, 8:26 pm
Hmm. I've got to get you to look around a bit more. I use metallic
absorbers and (according to customers) they're working very well in
Iowa, Colorado, Montana, Ontario,...
I agree about the vices, tho. :)
DeSoto, Iowa USA
Posted by Robert Scott on April 20, 2009, 11:51 am
Sometimes it is easier to analyze the efficiency by counting up the losses
rather than by counting up the gains. Minimize the losses and that will
automatically maximize the gains, provided you haven't overlooked some major
In your example, the efficiency of heat transfer from a wavy vs. flat collector
may be hard to determine directly from the energy gains. But just ask yourself
this question: If some energy is not collected, where does that energy go? If
it is harder for heat to move from the collector surface to the circulating air,
then that collector surface will get hotter. As it gets hotter, it transfers
more heat to the circulating air (a good thing) and it radiates more heat out
the front of the collector at infrared (a bad thing) and it conducts more heat
out the back side of the collector (maybe a bad thing, depending on where the
collector is mounted).
So there may be some advantage to a wavy collector, based on how much loss you
expect at infrared due to elevated collector temperatures and how much heat
might be lost by conduction out the backside. The backside conduction can be
minimized with insulation, and the frontside IR loss can be reduced by the use
of low-E glazing (possibly expensive).
There there is the energy required to move the air. Whether you use
thermosiphoning or a fan, this energy should be taken into account. If a wavy
collector causes more air resistance, then that will increase the electric bill
for the fan and work against the goal of more efficient heat transfer.
Whether all this balances out as a plus or a minus for wavy collectors is going
to depend, as Morris said, on lots of things. There is no single generic
answer. You have to look at your particular installation and usage to see if
the end result is an increase in energy efficiency.
Posted by gary on April 17, 2009, 3:37 pm
Just about any dark colored absorber surface will absorb nearly 100%
of the incident solar energy -- it does not mater much whether its
flat, wavy or tin cans.
The hard part in air collectors is transferring the heat that gets
absorbed by the absorber to the air flowing through the collector,
rather than losing the heat out the glazing. The better the heat
transfer from absorber to air, the more efficient the collector will
Good air collector designs concentrate on good heat transfer from
absorber to air -- lots of ways to do this: flow through absorbers
(my favorite) offer lots of surface area for heat transfer, and a flow
path that requires the air flow through the absorber to get to the
outlet. Waves increase surface area for transfer, and (maybe)
increase air turbulence. Beer cans offer a lot of surface area, and
provide a well defined path for the air, so you are unlikely to get
dead spots. Good flat plate absorbers have carefully designed flow
channels, and baffle systems to insure that high velocity air scrubs
the full absorber surface.
One problem is that while lots of people have theories on why a
particular design works best, there is very little data out there that
allows you to compare the efficiencies of different designs. It would
be nice if we could do something about that.
Two things I would concentrate on:
1) avoid a bad design -- From the material I've seen, one way to lose
a lot of efficiency is to have airflow dead areas -- that is, sections
of the absorber that don't get enough airflow for good heat transfer.
The paper listed on this page shows some examples:
2) Build big -- air collectors are cheap to build, and more area is
the one thing you can be sure will help the heat output. You can
optimize a 30 sqft collector until the cows come home, it will never
have heat output anywhere near a run of the mill 60 sqft collector.
So, my 2 cents would be, pick any one of the good designs out there,
and build big.