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solar air heater question


By gameman, Section Heat
Posted on Wed Oct 07, 2009 at 08:20:19 AM MST
which is best  take the heat from the bottom of the absorber material or top.....

i have seen both types of solar air heaters  where the air path is on top of the absorber material  and where it's under the absorber material  like in the pop can heaters  but which would give the most heat ??
or is both ways about the same ?
i took some black foil put it in a sunny window and checked it with a no contact thermometer and both side seem to be about the same themp
 thoughts or comments anyone ?
solar air heater question | 11 comments (11 topical)

Re: solar air heater question (5.00 / 1) (#1)
by GaryGary on Wed Oct 07, 2009 at 08:56:34 AM MST

Hi,
Most of the collectors that use a sheet metal absorber use flow along the back of the absorber only.  The logic behind this is that having fast moving air flowing between the front of the absorber and the glazing will result in much more heat loss out the glazing by convection.

The pop can collectors I know of have the airflow through the pop cans, so they are really neither front or back flow collectors.

As you say, the absorber will be just as hot on the back side as the front, so it makes no difference from that point of view -- its just the heat loss out the glazing that argues for the back side only airflow.

There is an  argument for having airflow on both sides of the absorber -- it goes like this: its hard to get enough airflow over the absorber surface to efficiently absorb all the heat.  Getting enough airflow over the absorber is really the key to good performance.  If you don't get enough airflow over the absorber, then the absorber has to run hotter to get rid of its heat, and, the hotter absorber loses more heat heat out the glazing by radiation.  If you run air both behind and in front of the absorber, you get more airflow over the absorber.  So, there is an argument for flow on both sides of the absorber.  But, most designs out there come down on the side of airflow on the backside only.

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com



Re: solar air heater question (3.00 / 0) (#2)
by gameman on Wed Oct 07, 2009 at 01:18:49 PM MST

thank you thats good info..
now whats your thoughts on the thinkness of the sheet metal or Aluminum sheets (or what ever one can get his hands on )that is to be used for the heat absorber ... like the thinner stuff (0.002 black Aluminum foil)will heat up fast ( about 120 F in around 10 secounds ) and i'm guessing one would need more fan CFM's to keep it cool  and the thicker stuff ( like 0.010) i'm guessing one would need less CFM's .. what are some of the pros and cons  ( if any ) for the lets say 0.002 and the 0.010  ??
thanks

[ Parent ]


Re: solar air heater question (5.00 / 1) (#3)
by GaryGary on Thu Oct 08, 2009 at 08:16:46 AM MST

Hi,
For an air collector, the absorber thickness does not make much difference.
Once its warmed up, the air will be taking heat out of it at the same rate the sun is adding heat heat in.  As you say, a thicker one will take a little longer to come to equilibrium with the solar input, but once its there it will behave just like the thin one.  I think the thickness depends more on just being able to work with it, and holding itself in position.

Some people use non-metalized, flow through absorbers in air collectors, and they work fine as long as the material can stand the temperature.

Water heating collectors are a different story because the absorber fins have to transfer the heat along the absorber to the tube, and it needs to be thick enough to do this efficiently.

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]



Re: solar air heater question (3.00 / 0) (#4)
by gameman on Fri Oct 09, 2009 at 07:07:30 AM MST

thanks for the great info..
gary  my plan is to make about 2 or 3 solar air heaters to heat my house on sunny days  about 700 sq ft area ( i think 3 solar air heaters should be more then enough) then build a set up ( like your Solar Shed House Heater ) so about what size Collectors would i need and about how much water Storage to heat about 700 sq ft just at night  with an outside temp ( at the coldest in the teens ) so i can heat with air heaters of a day and the water system  at night  then when i have cloudy days my heat pumps would take care of things .
i'm new at the water heating stuff so i may not have enough info for you to go on
thanks

[ Parent ]


Re: solar air heater question (3.00 / 0) (#5)
by GaryGary on Fri Oct 09, 2009 at 12:28:04 PM MST

Hi,
That's not easy to answer.

It depends a lot on your climate and probably even more on how well your home is insulated and sealed.

You can start to get a rough idea by running this home heat loss calculator:
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

Have a look at what your outside temps are for a typical winter day (don't use the worst case).  Weather.com has this kind of data.  Put this average overnight temp in as the "Design Outdoor Temperature" input.  Fill in all the insulation levels etc.
When you do the calculate, the "Design Loss" output column will give you an idea what the heat loss per hour is at that temperature.
To get the heat loss for the full night, just multiply it by the time overnight -- maybe 12 hours.  This is the amount of solar heat you would have to store to heat the house overnight.  I used 12 hours instead of something like 16 hours because you can get the house a bit on the warm side during a sunny day with the air collectors (hopefully), and coast a while on this as there is some stored heat in the house thermal mass, and you probably will setback the night temp a bit.

To get an idea (very roughly) how much water collector this might require, figure that on a sunny day, 1 sqft of collector might see about 1700 BTU of solar radiation -- if you do a descent collector, about half of that will get into the storage tank -- so, about 800 BTU stored per 1 sqft of collector on a sunny day.

So, if your overnight loss was (say) 80,000 BTU, then you would need about (80000/800) = 100 sqft of collector to store enough heat on a sunny day to heat the house overnight.  

A storage rule of thumb is to use about 1.5 to 2 gallons of water storage for each 1 sqft of collector -- about a 200 gallon tank in this case.  This amount of storage will store the heat you collect over one sunny day.  

If you want to accommodate cloudy days, then you need both more collector and more storage.  Its tough to accommodate a lot of cloudy weather, and most solar heating systems don't try, and just use a backup heater for cloudy periods.

As you will see when you run through the heat loss calcs, good insulation and sealing make it much more feasible to get a good solar heating fraction.

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]



Re: solar air heater question (3.00 / 0) (#6)
by ghurd on Fri Oct 09, 2009 at 01:57:43 PM MST

Hey Gary, that brings up something I never quite understood, possibly because I am in a cloudy area.

Why not include more storage?

The efficiency would go up because cooler water would be running through around~about the same collector temp.

The total cost would not increase much at all?
After the collector, pump(s), controller, heat dispersement equipment, excavation, etc,
It looks like adding an extra "foot or 2 or 3" of storage length is a minor consideration.

My take on those combined is more efficiency,
and more heat when I could use it or more saved for tomorrow.

My perception is certainly skewed.
If it is sunny in the winter months here,
we don't need much heat, or its nearly so cold fire freezes.
It is cloudy the rest of the time.  :(
G-

[ Parent ]



Re: solar air heater question (3.00 / 0) (#8)
by GaryGary on Fri Oct 09, 2009 at 06:58:10 PM MST

Hi Glen,

I've not actually run numbers on it, but my feeling is that you can go to far with the storage.  If you have a few cloudy days, the tank continues to lose temperature, and then when you do get sun, it takes the tank a while just to recover up to the minimum usable temperature.  I suspect its one of those things where there is an optimum amount of storage somewhere, but the drop off on either side of the optimum is slow.  

Its been snowing off and on all day today, and its supposed to get down to 1F tonight -- in October!!

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]



Re: solar air heater question (3.00 / 0) (#10)
by spinningmagnets on Sat Oct 10, 2009 at 02:03:18 PM MST

Gary, I hadn't considered that before. Now, if I ever build one, I think I might look into making a heat store that had two modular sections. Shouldn't be more than slightly more expensive if planned from the beginning...

[ Parent ]


Re: solar air heater question (3.00 / 0) (#9)
by gameman on Sat Oct 10, 2009 at 01:45:18 PM MST

this is great info ... this heat loss calculator will keep me busy for a while .

now on the space heating with water system..........
what if soemone wanted to buy the water collectors already made what would be a good type to get ?
the evacuated tube collectors sound pretty good
thanks

[ Parent ]



Re: solar air heater question (3.00 / 0) (#11)
by GaryGary on Sun Oct 11, 2009 at 07:06:01 AM MST

Hi,
This has been discussed a lot in the past -- opinions vary.

The data from places like the SRCC and Home Power says there is essentially no difference in performance between the two for most applications.

There can be an issue with snow being hard to remove from evac tubes, which might be a consideration for some areas.

Some internet comparisons:
http://cwc-das.com/logger/tabs.php?loggerids[]=485E700680A6C389&tab=0
and
http://www.thermo-dynamics.com/pdfiles/technical/Solar_Performane_VTvsLFP.pdf

Gary

Gary gary@BuildItSolar.com www.BuildItSolar.com
[ Parent ]



Re: solar air heater question (3.00 / 0) (#7)
by Ungrounded Lightning Rod on Fri Oct 09, 2009 at 05:09:27 PM MST

An additional reason to run air "behind" the absorber is to avoid dust build up on the sun side.  You want that to stay black.



solar air heater question | 11 comments (11 topical)
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