Will this increase the amount of solar energy transferred in a solar air heater?
C.
I'm looking to build a home-made solar air heater like this model:
http://www.atlanticenergy.ca/projects/WayneLangilleSolarAirCollectors/tabid/72/Default.aspx
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
ducting tubes.
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?
Regards,
Chris
Morris Dovey
> 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.
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
aperture.
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
all losses.
> Am I mistaken? Is there possibly a difference between the amount of
> heat generated in a smooth cylindrical chamber versus a wavy one?
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).
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
C.
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?
Regards,
Chris
Morris Dovey
> Is the efficiency of an absorber is affected at all by its surface
> area?
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
same time).
> Given a fixed amount of solar radiation, will a wavy absorber
> help mitigate any losses by the unit?
Maybe (again).
...and even I don't find my answers very satisfying. :(
Ecnerwal
<60c2aff6-0bb7-4867...@e7g2000vbe.googlegroups.com>,
"C." <hard...@hotmail.com> wrote:
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.
http://www.builditsolar.com/Projects/SpaceHeating/SolarBarn.pdf
--
Cats, coffee, chocolate...vices to live by
schooner
"Ecnerwal" <Lawren...@SOuthernVERmont.NyET.invalid> wrote in message
news:LawrenceSMITH-E08...@feeder.motzarella.org...
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.
Morris Dovey
> Metallic absorbers are the sort of 20th century technology that made
> sure that solar didn't work very well outside of Tuscon...
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. :)
ga...@builditsolar.com
>
> 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
> ducting tubes.
>
> 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?
>
> Regards,
>
> Chris
Hi Chris,
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
be.
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:
http://www.builditsolar.com/Experimental/AirCollectors/AirCollectors.htm
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.
Gary
Robert Scott
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
loss.
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.
Robert Scott
Ypsilanti, Michigan