solar thermal heating

[dmccarty]

I saw these heaters in Northern Tool a while ago. Figured if we ever built our garage we would have to be able to integrate this type of solar heating into the building. Too bad there was no way to do this in our house since we have brick and we are not going to be cutting through the wall...

Then I started thinking.

We do have a six inch PVC pipe that runs under the slab to bring combustion air to the back of the wood stove. The outside ending of the pipe is on the south side of the house. The house takes advantage of passive solar and we have a gravel walkway on the south side of the house.

Sooo, we could build a collector and use the six inch pipe to pump hot air into the house but we have no way to get cool inside air to the box. And we would have to use a fan to push the air. The house has a roof over hang that lets in the winter sun but keeps out the summer sun. The gravel does get warm even during the winter. Some of the performance data I have seen on these things seems to be showing a 30-40 degree temperature rise. Seems like worst case using outside air we would be pushing 60-70 degree air into the house instead of 20-30 degree air being sucked in by the fire.

Has anyone ever done something like this?

The only other way we could use this technology would be to have the box run up to the eaves and then duct the air into our ceiling. The problem is return air. We would still be stuck using either outside make up air or have to suck in warm air from the ceiling. I suppose we could run ducts to the north side of the house....

Does anyone know of any limits on the duct runs and duct size?

Later,
Dan

 

[GT2]

Hi Tim,
I do think that a dual purpose collector will work. Here is one example:
DIY Solar Air Heating Collector with Water Heating

You would, of course, be splitting the heat between the two -- no free lunch

I've not looked in any detail at the multi pass air heating collectors, so I can't really say on that.

Gary

Thanks for the link Gary.
I was thinking of making collectors like on your solar shed, except make them 3 pass thermal also.
I'm thinking the 3 pass may allow more heat absorption for the thermal hot air, because cant really put a screen or sofit across all the copper tubes and fins?
How is your solar shed working out?
Thanks, Tim

 

[Luremaker]

Hi Gary,

I am sure most of the guys here really appreciate all the work you have done testing solar panels. Your website is a real wealth of information.

I am looking forward to your test results on the soffit vs screen solar panels. These panels look as though they are much easier to build than the popcan or aluminum downspout panels. With the screen/soffit type of solar panels do you find they have to be opened and cleaned every few years or due you filter the air before it enters the solar panel?

Have you done any testing of the popcan style solar panels? Last year I collect hundreds of popcans and started to assemble the columns of cans but thought I'd hate to cut several holes in my shop's steel siding.

 

[hr3]

I built this about 3 years ago and it heats 3 lower room nicely on sunny days ...

 

[GaryGary]

Thanks for the link Gary.
I was thinking of making collectors like on your solar shed, except make them 3 pass thermal also.
I'm thinking the 3 pass may allow more heat absorption for the thermal hot air, because cant really put a screen or sofit across all the copper tubes and fins?
How is your solar shed working out?
Thanks, Tim

Hi Tim,
How would the 3 pass collector work? That is, what are the 3 passes the air makes?
I've never really looked at the multi-pass collector designs, so can't say much about how they compare.

The Solar Shed is doing fine. For the last two seasons, all I've had to do was turn it on in the fall and turn it off in the summer.

As mentioned in the writeup on it, it can overheat in the fall when the sun is lower and shines more directly on it, and the days are still warm so no heat is needed. The vents take care of this, but I still have not worked out a way to automate this. If the collector were more vertical, or not double glazed, the vents would not really be needed.

Gary

 

[GaryGary]

Hi Gary,

I am sure most of the guys here really appreciate all the work you have done testing solar panels. Your website is a real wealth of information.

I am looking forward to your test results on the soffit vs screen solar panels. These panels look as though they are much easier to build than the popcan or aluminum downspout panels. With the screen/soffit type of solar panels do you find they have to be opened and cleaned every few years or due you filter the air before it enters the solar panel?

Have you done any testing of the popcan style solar panels? Last year I collect hundreds of popcans and started to assemble the columns of cans but thought I'd hate to cut several holes in my shop's steel siding.

Hi,
We have not tested a pop can collector. I think we will eventually, or if not a pop can, then probably a gutter downspout collector which should be similar. My guess is that it will do well --it has good air distribution and lots of absorber area for good heat transfer to the air. But, I'd guess (emphasis on guess) that it won't be any better than the good flow through absorber collectors.

If you have a steel sided shop and the steel is (or could be) dark colored, you could use the steel itself for the absorber. If you don't want to cut holes in the steel and there is no way to route the air to the collector without going through the steel, then here is an idea you might think about:
http://www.builditsolar.com/Projects/LowTechCol/LowTechCollectorR2.pdf

In this scheme, you glazed over all or part of the steel. The glazing can be quite close to the steel, and I think that if you do it carefully, the glazing will be nearly invisible -- you will just see the steel. The steel does not have to be black -- and dark shade is OK.

On the inside, you have to make any insulation behind the steel removable (as in doors or sliding panels) -- the bare steel needs to face the shop.

So, the way this works is that the sun heats the outside of the steel, which gets quite warm with the glazing to reduce heat loss. On the inside the steel acts like a big radiator -- it radiates heat into the shop and also starts a convective flow of air up the warm steel.

At night, if you don't care about the building going cold, you can just leave the bare steel -- it will lose heat kind of like a double glazed window would. If you want to shop to retain more heat at night, then some form of insulating panels, thermal shades, ... could be used.

To my knowledge no one has tired this solution in a real full scale application -- you could be a pioneer! If you do try it, take lots of pictures.
From the little test I did, I think it would work well.

Gary

 

[GT2]

Hi Tim,
How would the 3 pass collector work? That is, what are the 3 passes the air makes?
I've never really looked at the multi-pass collector designs, so can't say much about how they compare.

Gary

I'm not sure if the 3 pass is more of a siphon break, or if it helps with absorption?
See side view attch.

 

[GaryGary]

I'm not sure if the 3 pass is more of a siphon break, or if it helps with absorption?
See side view attch.

Hi tim,
I don't see how the extra up and then down passage in the inlet would help with absorption at all since the absorber is up at the front of the collector.

I would guess that its aimed at stopping reverse flow at night, which would cool the room the collector is attached to. I guess the idea is that the cool air made by the collector at night settles to the bottom of the collector and can't get over the 1/4 inch plywood vertical divider. This might work to prevent backflow.

My concern would be that the two extra passages add quite a bit of flow resistance, and I think it would have trouble circulating much air. Thermosyphon collectors are very sensitive to added flow resistance -- the buoyancy forces that push the air through the collector are very weak. The rule of thumb for thermosyphon air collectors is that the depth of the flow passage should be 1/15th the height of the collector, so about 6 inches for an 8 ft high collector. Adding two extra passages the air has to go through that are only 1 inch deep is a very long way from meeting this design rule.
Is there a working model of this collector to verify that the airflow is descent?


I think that a regular backdraft damper might make more sense?

On mine, I use poly film sheets on the upper vents that close against some hardware cloth across the vents:
$350 Solar Heating Thermosyphon Collector

These are quite effective -- if not too pretty.
They would work a little better if placed in the bottom vents instead of the top vent, but you would have to work out a way to let the poly swing into the vent to open -- this is a bit more work, but no doubt doable.

Rob just uses manually operated vent doors on his:
Solar Thermosyphon Space Heater

Here is a scheme for building a backdraft damper into a duct:
http://www.builditsolar.com/Projects/SpaceHeating/TEACollector/Appendix A3 Backdraft Damper.pdf

A bit more in this doc:
http://www.builditsolar.com/Projects/SpaceHeating/SolAirHtSysBook/Chap02.pdf

This is a little collector I made with the back draft damper at the bottom:
ColShade
Works fine.





Gary

 

[forgeblast]

Does anyone use these systems in reverse in the summer time to cool? Ie opening up the vents at night to cool the house?

Attached is the measurements of the south facing wall of our house. It does face due south. we are at about -12 declination

I want to do as few holes in the wall as possible, i would like to wire it up using a snap disk switch that would come on when its hot enough and use a fan to pull the air out.

first link is the measurements of our wall that we can put it on the second the plan for the solar collector.

I am thinking about making one that uses 3/8 plywood on the back, with 5/8 on the sides, IKO Enerfoil foam as the insulation with grey aluminum screening 2 layers inside, was thinking of adding the flashing/soffit in the middle, not sure though) with 1 - snap disk switch (fan control switch) open at 90 deg. F, close at 110 deg. F, and 1 - 110/220 electric baseboard thermostat,  1 - fan, 100-120 cfm, 1 - dryer vent cover with four flaps, and a register collector. The problem is this would be installed in our tvroom which is all knotty pine, so i would have to find register collectors that match/or can be stained.

1. do you think having them all connected will be more efficent then having two seperate collectors/and fans.
2. is that fan size enough to keep the heat moving quietly.
3. any suggestions?

house dimentions by kuch - Google 3D Warehouse


solar air heater plan by kuch - Google 3D Warehouse

 

[GaryGary]

Does anyone use these systems in reverse in the summer time to cool? Ie opening up the vents at night to cool the house?

Attached is the measurements of the south facing wall of our house. It does face due south. we are at about -12 declination

I want to do as few holes in the wall as possible, i would like to wire it up using a snap disk switch that would come on when its hot enough and use a fan to pull the air out.

first link is the measurements of our wall that we can put it on the second the plan for the solar collector.

I am thinking about making one that uses 3/8 plywood on the back, with 5/8 on the sides, IKO Enerfoil foam as the insulation with grey aluminum screening 2 layers inside, was thinking of adding the flashing/soffit in the middle, not sure though) with 1 - snap disk switch (fan control switch) open at 90 deg. F, close at 110 deg. F, and 1 - 110/220 electric baseboard thermostat,  1 - fan, 100-120 cfm, 1 - dryer vent cover with four flaps, and a register collector. The problem is this would be installed in our tvroom which is all knotty pine, so i would have to find register collectors that match/or can be stained.

1. do you think having them all connected will be more efficent then having two seperate collectors/and fans.
2. is that fan size enough to keep the heat moving quietly.
3. any suggestions?

house dimentions by kuch - Google 3D Warehouse


solar air heater plan by kuch - Google 3D Warehouse

Hi,
The collector in the Sketchup diagram might have a problem with even air distribution to the whole collector. It looks to me like the upper right corner is kind of a dead end, and may not bet very much circulation?
The window makes that upper right corner particularly bad as the air can't make its way along the top and to the left to get out the exit.

While I'm not certain of this, the collector seems on the large side to me to have only one inlet and out outlet. The screen will tend to spread the inlet air out over the full surface as there is a bit of pressure drop across the screen, but it seems like you are asking a lot of the screen.

I'd think about doing two collectors with the dividing line extending down from the center of the window. The inlets for the two collectors could be a common inlet under the center of the window and down by the floor. The collector outlets could be upper right and upper left. It seems like this might give a better chance of getting good flow to the full collector?

The flow to the collector should be about 2.5 cfm per sqft of collector, so, it yours is about 8 by 17 = 136 sqft -- maybe more like 110 sqft without the window area. This would want about 2.5*110 = 275 cfm total.
You could go down a bit from this, but 2 cfm per sqft is getting on the low side. It the collector is operating reasonably efficiently, this level of airflow will give you about a 50 to 60F temp rise with good sun. This is about what you want if the air is not to feel cool especially with part sun conditions.

The fan has to deliver this airflow at the pressure drop through the collector. A lot of fans give the cfm rating for essentially zero pressure drop -- so in a real collector with pressure drop they won't deliver any where near the name plate rating.
Try to find a fan that the manufacturer publishes the fan curve for. As a guess, I'd say your collector is likely to have a pressure drop between 0.1 and 0.2 inches of water, so you need a fan (or 2 fans) that will deliver the total of 275 cfm with about 0.1 ot 0.2 inches of water pressure drop -- just look this up on the fan curve.

If you want the fan to be quiet, then bigger (larger diameter) is generally better.

If you wanted to avoid the tow outlet vents being up high on the wall, you could build a vertical plenum into each end of the collector that takes air from the upper outer corner down to an outlet vent close to the floor. This may make the vents less noticeable?

You might want to have a look at some of the large collector designs on my solar space heating page to see what people have done in the way of vents and air distribution. While I know its not the kind of collector you are intending to do, the TEA design is very well thought out, and the docs on it are helpful in a lot of ways.
Solar space heating -- active and passive solar projects for space heating
The TEA one is the 2nd one in this section:
Solar space heating -- active and passive solar projects for space heating
The book just above it is also quite good, but deals very little with flow through collectors -- mostly backpass -- but, a wealth of good hands-on construction and design info.


You could consider leaving the insulation out, as the collector is mounted to the wall, so collector heat loss is house heat gain.
I have one collector where the back of the collector is just the barn siding:
$350 Solar Heating Thermosyphon Collector
Works fine, and does not seem to be bothering the siding after 5 years.

Gary