solar air heater question

[xboxman]

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 ?

[GaryGary]

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

[xboxman]

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

[GaryGary]

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

[xboxman]

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

[GaryGary]

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

[ghurd]

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-

[Ungrounded Lightning Rod]

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.

[GaryGary]

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

[xboxman]

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

[spinningmagnets]

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...

[GaryGary]

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

[sbotsford]

This one reason that there is merit in making your absorber plates folded like an accordian.  The folds give you more surface per square foot to run the air past.

The glazing in the front will convect, hot air will collect in the top of the collector in front of the absorber plate.  Some merit in running your air from bottom to top so that the hottest part of the collector plate is doing the final heating of the air.

In cold climates it may be worth double glazing the collectors.

[vawtsup]

A little late with this reply, but you might want to have a look at this video.

http://www.youtube.com/watch?v=2Cm-cbOWvSs&NR=1

This design reaches 220°F output in just over an hour.

[wdyasq]

This design reaches 220°F output in just over an hour.

Similar to quoting instant amps and instant volts - The honest way is energy collected over a time period. Anything less is closer to manure than truth.

Ron

[GaryGary]

A little late with this reply, but you might want to have a look at this video.

http://www.youtube.com/watch?v=2Cm-cbOWvSs&NR=1

This design reaches 220°F output in just over an hour.

I don' see any advantage in pop cans inside of downspouts over just the pop cans alone or just the downspouts alone. 
Also don't see any advantage at all to the aluminum sheet behind the pop cans.

There is no advantage to adding thermal mass inside a collector -- don't know where this notion that mass in collectors improves performance comes from -- its really just the opposite:
http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

For a good solar air collector, you want an absorber that absorbs the incoming sun -- this is the easy part, as just about any black painted material will absorb about 95% of the incoming sunlight.  And, you want good heat transfer between the air and the absorber -- this is the hard part because air has a low density and a low heat capacity, so you have to run a lot of air over the absorber to transfer the absorber heat to the air.   So, the absorber in an air heating collector wants to provide a lot of surface area for heat transfer, and you want even flow of the air over the full area of the absorber.  While I've not seen any real measurements, it seems like pop cans or downspouts should make good absorbers since they provide a lot of surface area, and they encourage even airflow.  But, I don't see any reason why putting one inside the other helps -- it may actually hurt by reducing the airflow over the surface that is actually absorbing the heat (the downspout in this case).

The test he did does not (it seems to me) give you much of any idea how well the collector will perform.  He measured stagnation (no air flow) temperature.  If he painted the back of the box black and left the pop cans etc out, he would likely have gotten very near the same temperature.   
The useful heat output of the collector is (Flow Rate)(Temperature rise) (specific heat of air) -- for efficient operation, you want the minimum temperature rise that results in useful heat to the room -- usually this is about 50F to 60F.  If you are getting a lot  more temperature rise than this, the airflow should be increased to bring the temperature rise down.  Running the collector with a high internal temperature just results in large heat losses out the glazing and low efficiency.


Gary

[Hicky]

Hello everyone, I just built a 4'x8' pop can collector and it really gets hot. inlet temperature 71 deg. and outlet 217 deg. with a 35 to 50 cfm computer fan. The temperature does not drop at all. I was thinking of purchasing a 6" inline fan of 424 cfm with a variable  speed reducer.Is there any thoughts on that? I've never used a speed reducer before and don't know how much it can slow it down. I think it would be nice if I could get the temperature down to about 90 to 110 deg. It's not connected to the house yet. Still tinkering with it so far and still not sure where I want to put it. I know it sounds silly but I,m thinking of putting it on the east side of the house. Usually in the spring and fall the house is chilly in the morning and bringing that temperature up would probably stay up the rest of the day. Anyway I'm building another one on the south side and it will be 3'x 24'. Hopefully I can get 25,000 btus out of it.

[Shadow]

I don' see any advantage in pop cans inside of downspouts over just the pop cans alone or just the downspouts alone. 
Also don't see any advantage at all to the aluminum sheet behind the pop cans


I wonder though if it was just downspout with no cans if the air would flow too fast, or with just pop cans not enough mass . But the combination of the two is just right. The pop cans are slowing the air movement just right.
Wouldnt the aluminum sheet help heat the underside by distributing the heat better?

[GaryGary]

I don' see any advantage in pop cans inside of downspouts over just the pop cans alone or just the downspouts alone. 
Also don't see any advantage at all to the aluminum sheet behind the pop cans


I wonder though if it was just downspout with no cans if the air would flow too fast, or with just pop cans not enough mass . But the combination of the two is just right. The pop cans are slowing the air movement just right.
Wouldnt the aluminum sheet help heat the underside by distributing the heat better?

Hi,
You will get better heat transfer from the aluminum to the air if the air velocity against the aluminum that is being heated by the sun is high.  Its similar to putting a fan in front of a car radiator to increase the airflow past the core and transfer more heat. 
Its a good thing to have high speed turbulent airflow over the absorber to remove the heat efficiently.

I guess I don't see any need or benefit for the aluminum sheet under the cans.  The way the cans are installed, all the radiation falls on the cans and none on the back sheet.   Each can row should receive the same amount of solar radiation and have the same internal airflow, so it should end up running the same temperature as its neighbor can rows.  So, there should not be any need to distribute heat from can row to can row if that's what you mean? 

Some nice can collectors here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#NewBeerCanCol


Gary

[willib]

I don' see any advantage in pop cans inside of downspouts over just the pop cans alone or just the downspouts alone. 

funny after reading this i was at Lowes and checked out their downspouts
For the aluminum ones they were $8 and change for a 10 foot section.
with a hotwire and some foam i think i could make a nice Header

[Ungrounded Lightning Rod]

This one reason that there is merit in making your absorber plates folded like an accordian.  The folds give you more surface per square foot to run the air past.<p>

Another reason for accordion folding the collector is that the 5% or so of the light that DOESN'T get absorbed on the first hit will tend to be deflected down into the fold for one or more additional chances to be absorbed.  (This is why velvet is VERY deeply colored and black velvet is VERY black.)

The valleys between columns of cans do this, too, though the middles of the cans do bounce the missed energy back out.  If you use downspouts instead of cans you might want to orient them in "diamond" rather than "square" position so you get valleys between them as well.

The glazing in the front will convect, hot air will collect in the top of the collector in front of the absorber plate.  Some merit in running your air from bottom to top so that the hottest part of the collector plate is doing the final heating of the air.<p>

Also the convection will be helping your circulation rather than fighting it.

If you have enough height difference between your inlet and outlet, they and the passages are large enough that they don't excessively restrict the airflow, and you don't mind your hot air coming into the house a bit above the floor (or bring your cool air out to the collector BELOW floor level, i.e. in the crawl space), you may not need to force the circulation.  The heating can do it for you.

So regarding the extreme temperature rise of the initial collector:  Looks to me like the problem is that the inlet/outlet openings and/or the passages at the top and bottom of the can stacks are too small and are restricting the airflow.

[Ungrounded Lightning Rod]

I don' see any advantage in pop cans inside of downspouts over just the pop cans alone or just the downspouts alone. 

funny after reading this i was at Lowes and checked out their downspouts
For the aluminum ones they were $8 and change for a 10 foot section.
with a hotwire and some foam i think i could make a nice Header

I wouldn't put foam anywhere in such a collector.  If the airflow ever becomes restricted on a sunny day the heat would melt it.

[GaryGary]

I don' see any advantage in pop cans inside of downspouts over just the pop cans alone or just the downspouts alone. 

funny after reading this i was at Lowes and checked out their downspouts
For the aluminum ones they were $8 and change for a 10 foot section.
with a hotwire and some foam i think i could make a nice Header

I wouldn't put foam anywhere in such a collector.  If the airflow ever becomes restricted on a sunny day the heat would melt it.

The polyiso rigid foam board is used a lot in collectors, including most commercial collectors -- it can take quite high temperatures.
Most lumber yards have it, although they may not know it by that name -- the sheets are usually tan with aluminum foil cover sheets -- the word Polyisocyanurate will be printed somewhere on the sheet.

Gary

[xboxman]

what about the fan is it better to pull the air Thru the air heater or push the air  Thru the heater??

if you think of a electric furnace the fan is behind the heating elements so most of the air is pushed Thru the vents...... so on a solar air heater if you push the air then the colder air is more dense so you could move more air with a smaller fan vs pulling the air  and the fan could run cooler

[GaryGary]

what about the fan is it better to pull the air Thru the air heater or push the air  Thru the heater??

if you think of a electric furnace the fan is behind the heating elements so most of the air is pushed Thru the vents...... so on a solar air heater if you push the air then the colder air is more dense so you could move more air with a smaller fan vs pulling the air  and the fan could run cooler

One advantage of pushing room air through the collector is that the fan does not have to be able to withstand as high a temperature.  It can be a room air temp fan rather than one that has to be able to take 120+F or so.

Gary

[Ungrounded Lightning Rod]

what about the fan is it better to pull the air Thru the air heater or push the air  Thru the heater??

if you think of a electric furnace the fan is behind the heating elements so most of the air is pushed Thru the vents...... so on a solar air heater if you push the air then the colder air is more dense so you could move more air with a smaller fan vs pulling the air  and the fan could run cooler

One advantage of pushing room air through the collector is that the fan does not have to be able to withstand as high a temperature.  It can be a room air temp fan rather than one that has to be able to take 120+F or so.

Gary

Another is that this puts positive pressure on the collector.  Any leaks will vent small amounts of building air out, rather than sucking moisture-laden air into the collector.  This should help keep it clean and improve its lifetime and maintenance requirements.

[Shadow]

Getting ready to start building  solar air heater. A few questions, What is the ideal space between the glass and absorber?

I have disassembled a couple of patio doors, salvaged 3 pieces of tempered glass out of 4. Should I build one large collector using all 3 sheets or 3 seperate collectors?  and finally any difference between vertical or horizontal when it comes to performance?

Thanks for now..

[GaryGary]

Getting ready to start building  solar air heater. A few questions, What is the ideal space between the glass and absorber?

I have disassembled a couple of patio doors, salvaged 3 pieces of tempered glass out of 4. Should I build one large collector using all 3 sheets or 3 seperate collectors?  and finally any difference between vertical or horizontal when it comes to performance?

Thanks for now..

Hi,
If you are building the pop can collector, I don't think that the spacing between the cans and the glazing makes a lot of difference, but I've never seen numbers on this.  If it were me, I'd use about 1 inch.

Building one big collector saves some materials, and some work.  BUT, it will be very heavy -- you would just about have to build it in place.
Building the collector in place is fine if its a location you can get to easily -- here is a 100 sqft collector built in place:
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/CollectorConstruction.htm
(I'm still putting the pages for this project together, so the page is incomplete, but you can see how the collector is built right on the wall).

Gary

[Shadow]

Thanks Gary,
 
I see  they have mounted them directly on the house . Would it not be more efficent to mount them at the same angle as the solar panels in the background in the first link?

I plan on making mine so the glass is removable to lighten things up to be moved. One inch spacing is about what I had considered.

[GaryGary]

Thanks Gary,
 
I see  they have mounted them directly on the house . Would it not be more efficent to mount them at the same angle as the solar panels in the background in the first link?

I plan on making mine so the glass is removable to lighten things up to be moved. One inch spacing is about what I had considered.

"They" is me .
The one in the first link is mounted at about 70 degrees.   Its a 40 sqft collector that was intended for solar water heating only, and it did a really good job at that tilt angle year round. 

I took that one out and went to the larger collector in order to try combined water and space heating.

For the larger collector, I mounted it flat on the wall because its intended for both space and water heating.  Its much larger -- about 100 sqft. 
So, in the summer its supposed to supply domestic hot water, and in the winter domestic hot water plus space heating.
The idea of the vertical tilt is that this works well in the winter with the low sun -- especially here where we have a reflective snow on the ground all winter.  In the summer, vertical collectors are inefficient because the sun is so high, but this one is oversize for just domestic water heating, so, the big area makes up for the bad summer angle. 
Its been steady in the 130F to 145F range for the last month and a half.
The only time it got a bit marginal was late May and early June where the high sun combined with cloudy weather made for tank temps as low as 100F.

You have to be very careful if trying the vertical collector because it just gets much less sun in the summer.  If you have something like a roof overhang above this kind of collector, the collector will be mostly shaded by the overhang at mid summer, and won't do anything -- that's OK if its only for space heating, but not for domestic water heating.
Gary

[Shadow]

I'm putting the finishing touches on my 'experimental' solar air heater.

I used the foil sided poly something insulation, and I'm using tin 3 inch by 1 1/2 inch high channel. It was used in a ceiling set up in an office building. I got a huge truck load of it for free so trying to find a project for it. I'm not sure of the gauge but cuts easily with snips.

So its all in columns, vertical into headers top and bottom. My question is should I use aluminum tape over each seam? Or leave each piece of channel as individual?  They fit nice and snug together. But I'm wondering if there is any advantage or disadvantage to the sun hitting a flat surface or hitting a slight corrugated surface?

I wasnt going to glue these pieces down, they are more or less held in place by the headers. But perhaps when I hook the fan up they may leak air through the seams?.. Thanks

[Ungrounded Lightning Rod]

... should I use aluminum tape over each seam? Or leave each piece of channel as individual?  They fit nice and snug together. But I'm wondering if there is any advantage or disadvantage to the sun hitting a flat surface or hitting a slight corrugated surface?

There's an advantage to the sun hitting a steep accordion-shaped surface:  Any light that misses being absorbed on the first hit and skips in specular reflection off any slight gloss on the surface, and much that bounces but is scattered diffusely, goes farther down into the fold for another shot at being grabbed.  After, say, four bounces, a paint that is a 90% absorber becomes a 99.99% absorber.

That's why velvet has such strong colors and deep blacks:  Same trick with end-on or side-on threads.

[Fat Charlie]

Thanks for discussing this.  I'm looking at using downspouts (down behind and up through) for my first solar effort.  It seems that creasing the front of the downspouts (the wider side) by laying a 2x4 on them and applying pressure would give a great accordion surface to the whole face by creasing the front and pulling in the sides.   

I just wish the solar space heater were closer to the top of my to-do list.