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

 

[mmurphy]

GaryGary, are you Gary Reysa from "Build it Solar"?

 

[GaryGary]

GaryGary, are you Gary Reysa from "Build it Solar"?

That's me

Gary

 

[forgeblast]

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.

**should I then use the soffit along with 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?

I like this idea, It would be an easier sell, I think, to my wife.

, this level of airflow will give you about a 50 to 60F temp rise with good sun.

***Is this from the outside air temp? or what is already inside?


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


With the fans pulling the air out, would i be able to then get away with using a heavier dampner, ie dryer vent as opposed to the plastic that you have used?


Thank you so much for all your info already. I was talking to the wife and she said "make sure you post that im skeptical and I want to see pictures of what it will look like on the inside".
I went to build it solar and she has a much better idea, ty again. I am also going to try to find wooden cover for the fans.

The fans should I be looking at bathroom vent fans or a different type of fan?

ty once again.

**when you use the screening do you also use baffles to control the direction of the air flow. how do you control the flow with the al soffit?

these are two fan styles that i found.

http://compare.ebay.com/like/230511...ff16e93&itemid=230511560762&ff4=263602_304662

http://www.hvacquick.com/products/r...-Register-Boosters/Suncourt-Room-to-Room-Fans

 

[hr3]

Thank you so much for all your info already. I was talking to the wife and she said "make sure you post that im skeptical and I want to see pictures of what it will look like on the inside".

The inside of mine looks like any other warm air grill you see in the floor or ceiling.. Do you have a cellar window near where you want to put your panel...If so look at the pic I posted,, Supply and return both through the window opening ..

 

[forgeblast]

I did see your pic hr3, i wish i had a basment window there, it would have helped, but we have no windows on that side (basement wise).
So you ran yours through the basement window and then up through the floor?

 

[hr3]

I did see your pic hr3, i wish i had a basement window there, it would have helped, but we have no windows on that side (basement wise).
So you ran yours through the basement window and then up through the floor?

Yes ,, The basement on this side of the house is finished off and there are three rooms all part of the master bedroom above .. The cold air supply comes from the exercise room out to the panels then back into the hot tub room as soon as this area is warm enough a damper closes and supply's heat to the master bedroom .. It does very well,, Also in the summer I will run it manually to cool the bedroom at night.. Mine are vertical and do not get hot in the summer and in the winter the sun is so low they work fine,, even better with snow on the ground..

I used a fantech fg series very quiet and fully speed adjustable..

Even if you don't have windows on the side you want to put it you should still have rim joist space enough to exit and enter..

 

[GaryGary]

Hi,
Some comments embedded below at the --> marks:


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.

**should I then use the soffit along with the screen?

--> I'd say probably not.
I think you would be better off with dividing the collector in two.
If the top of the collector was not interrupted by the window, I'd say an alternative would be to have plenums either along the top and bottom or vertical ones at the right and left edges, but the window makes either of those questionable.
You might think about adding a third layer of screen this might improve the thermal efficiency a bit, and would give a little more pressure drop that would help spread the air better.

Bear in mind that there is just not a lot of material out there on how air really behaves in these collectors -- about the best you can do is make an educated guess.




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?

I like this idea, It would be an easier sell, I think, to my wife.

, this level of airflow will give you about a 50 to 60F temp rise with good sun.

***Is this from the outside air temp? or what is already inside?

--> That's along the lines of room air enters the inlet at about 65F, and leaves the collector at about 120F in good sun conditions. You won't get that kind of temp rise in part sun, or in the early morning or late afternoon, and you don't want the temp rise to get so low that the air feels cool because its moving. But you also don't want too much temperature rise, as this means the absorber is running hot, and is losing a lot of heat out the glazing -- this is important as this heat loss out the glazing is most of the reason that collectors run at 50% or so efficiency rather than 90%.



With the fans pulling the air out, would i be able to then get away with using a heavier dampner, ie dryer vent as opposed to the plastic that you have used?

--> Yes, I think so.
Either pushing or pulling, fan forced collectors can use dampers that require more pressure to open than thermosyphon collectors.
You just want to make sure the inlet and outlet vents and the damper opening are not too small as this will cause pressure drop and less flow.



Thank you so much for all your info already. I was talking to the wife and she said "make sure you post that im skeptical and I want to see pictures of what it will look like on the inside".
I went to build it solar and she has a much better idea, ty again. I am also going to try to find wooden cover for the fans.


--> On the making the collector look better front:
- If you have clear glazing like SunTuf or glass, you will see the screens, and to some degree the color of the back wall of the collector.
The screens actually look pretty good -- kind of a nice soft look.
The screen collectors (I think) look better if you don't paint the back wall black, but choose some medium dark color that matches the house.
On my barn screen collector, you can in the pictures that it does not have the "big black square" look because you see the siding color a bit through the screen. The collector would likely be just a bit more efficient with the black back wall, but (for me) the aesthetic gain outweighs the small efficiency increase.
- Another option is to use twinwall glazing. I think it has a really nice look, and you don't see the internals of the collector through it. If you are in a cold climate it also adds a bit of efficiency. If you can find a good local source, it does not add all that much to the cost -- a good price is about $1.80 a sqft. I'd use 10mm thick or thicker.
greenhouse cover glazing materials at reasonable cost
Shipping is a killer on this stuff, so local is best if you can.

- The trim around the outside of the collector makes a big difference
on how the collector looks. I kind of like the PVC wood or deck wood, as they look pretty good, and they have a good life.

-- Couple of examples of (I think) nice looking collectors:
This is my new solar water heating collector glazed with twinwall:
A Simple DIY Solar Space and Water Heating System
The page is not done yet, but there are a few pictures that show the look.

This is Knick's collector:
A $1K Solar Water Heating System that Looks Great
twinwall with a nice frame.




The fans should I be looking at bathroom vent fans or a different type of fan?

ty once again.

**when you use the screening do you also use baffles to control the direction of the air flow. how do you control the flow with the al soffit?


--> the vented alum soffit and the screen collectors work the same way. The vent holes in the soffit act the same way as the holes in the screen.
I guess that the ones I've seen (not that many large ones) did not use baffles, and I'm not sure how you would use baffles with flow on both sides of the screen/soffit? They tend to rely on the pressure drop across the screen or soffit to distribute the air. Again, I've not seen anyone who has actually done the work to see exactly where the flow does go, but it does make some sense.
I've actually ordered 16 small thermistor temperature sensors that I can glue to the absorber screen on my 4 by 8 test collector to see how much variation there is in absorber temp -- areas not getting much airflow should show up as hot spots on the absorber.

these are two fan styles that i found.

http://compare.ebay.com/like/2305115...=263602_304662

HVACQuick - residential


--> I could not find a fan curve for either of these. The first one looks promising -- I did email them to ask for a fan curve and/or flow at 0.2 inches of water pressure drop.

Someone mentioned the Fantech FG -- this is the brochure:
http://fantech.net/docs-resi/450286-fg-brochure.pdf
There is a fan curve in the brochure -- it looks like the FG6 would have plenty of grunt -- maybe more than is really needed for your collector? I think it was mentioned that its a quiet fan, and that's a big plus.

Gary


Last edited by forgeblast; Today at 10:08 AM.

 

[forgeblast]

Thank you Gary! I know i will need to make an enclosed unit, it will be installed over t-111. The grooves/texture would make it almost impossible to seal out the outside air.
I am thinking too, that If i do make the unit out of 2x6 or even a 2x8 i can mount the fan in the box itself cutting down on the amount you can see in the house.

Building a Simple Solar Air Heating Collector
(this shows a system on t-111 and its insulated which is probably what i will do)

Building an inexpensive solar heating panel | Mobile Home Repair shows an example of the baffles.

The screen collectors (I think) look better if you don't paint the back wall black, but choose some medium dark color that matches the house.

That is a good idea, the house is a dark brown, and i still have the stain i can match the outside, and then use a dark brown to match the grey of the screen.

roll covering - double wall continuous greenhouse cover looks very promising, its already in a 4' width also.
(How would this be attached, would you use a u channel? or just staple and put a flashing over the ends. Or tape the ends with the aluminum tape)

Solar Collector Glazing Materials (i was spending a lot of time here to check on various materials)

 

[GaryGary]

Thank you Gary! I know i will need to make an enclosed unit, it will be installed over t-111. The grooves/texture would make it almost impossible to seal out the outside air.
I am thinking too, that If i do make the unit out of 2x6 or even a 2x8 i can mount the fan in the box itself cutting down on the amount you can see in the house.

Building a Simple Solar Air Heating Collector
(this shows a system on t-111 and its insulated which is probably what i will do)

Building an inexpensive solar heating panel | Mobile Home Repair shows an example of the baffles.

The screen collectors (I think) look better if you don't paint the back wall black, but choose some medium dark color that matches the house.

That is a good idea, the house is a dark brown, and i still have the stain i can match the outside, and then use a dark brown to match the grey of the screen.

roll covering - double wall continuous greenhouse cover looks very promising, its already in a 4' width also.
(How would this be attached, would you use a u channel? or just staple and put a flashing over the ends. Or tape the ends with the aluminum tape)

Solar Collector Glazing Materials (i was spending a lot of time here to check on various materials)

That Solexx material is polyehthylene. Polyethylene is normally only good to about 130 or 140F, so that seems pretty low for a solar collector. I think I'd stick with the plycarbonate (SunTuf or the twinwall polycarbonate) -- its good for about 270F.
When stagnated, my vertical barn collector gets up to 185F -- tilted collectors can go well above this.


The mobile home collector is a backpass collector (all flow on the back side of the absorber), and these typically do use baffles to get the flow to spread over the absorber. Not sure how you would accomplish this on a collector with a flow through absorber.
At the inlet on the flow through screen collector, you do want a deflector or plat that keeps the room air from impinging directly onto the glazing -- this should be about half an inch from the glazing. In that same are near the inlet you might want to try some form of baffle that deflects the air out to the sides a bit rather than letting it short cut straight toward the outlet -- but, I'd not overdo it.

Gary

 

[forgeblast]

I think i know what was confusing me.
The screen collectors will slow the air allowing it to heat up and then with the fans they will pull the air through the unit.

Units such as the can and soffit units, heat up the can/soffit and in return the air that is traveling around them.
(i have heard some people not able to get temps as hot because the air flow is down.)

So on the unit with the screens, does it need a fan? or does the fan give more air flow which in turn gives a better heat return....or am I still confused...lol