# fan for passive solar?



## joe&katie (Jun 14, 2010)

I have an idea for using passive solar to help heat my workshop. I got the idea, called thermosiphoning, from the Mother Earth news article here:

http://www.motherearthnews.com/Renewable-Energy/2006-12-01/Build-a-Simple-Solar-Heater.aspx 

In my application, I won't be able to simply have the air moving into the room directly from the back side of the collector wall, I will need to have a couple of flex duct vents at the top, about 10-12 ft long, into the room. And, a single, larger air return at the bottom to feed the cool air back into the collector. 

I don't think the convection will be powerful enough without help because of the length the air must move. To help move the air, I'm thinking of using a solar powered fan. I want to just have the fan run while the collector is heating up, so I thought a solar powered fan would be perfect. I don't really want to get complicated on this, as it will be my first attempt with solar.

I found one online, here:

http://www.amazon.com/gp/product/B0...PTI_9?ie=UTF8&m=A1U6M4R2T7APTI&n=&s=&v=glance

It has good reviews, and I think it will move enough air when the collector is getting sun. I think my question is, should I put the fan to blow the cool air back into the air return, or get 2 fans and place them to pull the hot air at the top? I really have no idea if 2 would be too much, and can't figure out how to size the amount of air movement I need.


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## Bearfootfarm (Jul 13, 2006)

I'd put them up high, and if possible angle them to blow warm air down towards the floor level where you need the heat


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## NW Rancher (Mar 8, 2008)

I would start with one, and see how it works. If you move too much air too fast, you won't see much heating. If the air exiting the system is still really hot with one fan, you may be able to increase airflow with a second fan and still get adequate heat.

I second Bearfootfarm's suggestion of blowing the air down from the exhaust.


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## SolarGary (Sep 8, 2005)

Hi,
That's actually my collector 

There is more on it here:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
I just added a big update on it after 9 years of operation.
http://www.builditsolarblog.com/2012/01/large-thermosyphon-collector-updates.html

If you do a large collector like the one I did (160 sf) then its a lot of air to transfer. The flowrate is when with full sun good sized vents is nearly 4 cfm per sqft of collector area, or 640 cfm for the whole collector. That's really more than that 6 inch fan is likely to be able to handle.


I'm currently using what was made to be a solar attic fan mounted on the ceiling near the collector exit vents to take the hot air up by the ceiling and mix it around. Its probably about 13 inches in diameter -- it works pretty well and move quite a bit of air. Something like that hooked to a large duct might work.
Another option might be to use a couple of those 6 inch fans with each serving half of the collector?


If you want to provide more info on your setup, I'd be happy to help with getting a good duct and fan setup designed. 

Gary


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## baldylocks (Aug 15, 2007)

I monkeyed around with a solar fan to push air through my system. You can see the details on my blog here:
My solar furnace.

There are no commercial links, ads, etc on my site so this isn't a sales pitch


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## Harry Chickpea (Dec 19, 2008)

FWIW, putting a fan on the COLD side (intake) will make it run cooler and last a little longer.


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## joe&katie (Jun 14, 2010)

Everyone, thanks for all the help! 

SolarGary, nice to meet you! I looked at a lot of articles about easy to build solar heating, and yours was the one I liked best, made the most logical sense to me. I noticed on the update site you posted that you talked about making smaller openings at the top and the resultant decrease in efficiency. I guess I didn't realize until I saw the picture from the inside of the building, that you had a lot of fairly large openings all the way across the collector. I was thinking I could use just 2 standard furnace vent openings along the top, and the fans would pull the hot air out of the collector and into the room and would be fairly efficient. Do I need more vents than that? The collector would be 8 ft high by 10 ft long, with 2 upper vents, and a larger air return vent at the bottom. 

My building started out as just a 48 foot cargo trailer. Then I put a roof over it, and added 16 feet alongside it. Then I enclosed the whole thing. The long wall faces directly south, with no windows, and the trailer is in the southern half of the building (oh, how I wish I had planned ahead and put the trailer on the north side). So, the building is about 26X51, and the workshop is about 16X51. I want to put up a wall on the inside between the trailer and the workshop so that the workshop is sealed and insulated, but the trailer's "bay" isn't going to be heated. So, I need the flex duct to move the hot air over the top of the trailer, into the open room. Otherwise, the basic design would have been perfect for this.


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## SolarGary (Sep 8, 2005)

joe&katie said:


> Everyone, thanks for all the help!
> 
> SolarGary, nice to meet you! I looked at a lot of articles about easy to build solar heating, and yours was the one I liked best, made the most logical sense to me. I noticed on the update site you posted that you talked about making smaller openings at the top and the resultant decrease in efficiency. I guess I didn't realize until I saw the picture from the inside of the building, that you had a lot of fairly large openings all the way across the collector. I was thinking I could use just 2 standard furnace vent openings along the top, and the fans would pull the hot air out of the collector and into the room and would be fairly efficient. Do I need more vents than that? The collector would be 8 ft high by 10 ft long, with 2 upper vents, and a larger air return vent at the bottom.
> 
> My building started out as just a 48 foot cargo trailer. Then I put a roof over it, and added 16 feet alongside it. Then I enclosed the whole thing. The long wall faces directly south, with no windows, and the trailer is in the southern half of the building (oh, how I wish I had planned ahead and put the trailer on the north side). So, the building is about 26X51, and the workshop is about 16X51. I want to put up a wall on the inside between the trailer and the workshop so that the workshop is sealed and insulated, but the trailer's "bay" isn't going to be heated. So, I need the flex duct to move the hot air over the top of the trailer, into the open room. Otherwise, the basic design would have been perfect for this.


Hi,
The collector as I did it is a pure thermosyphon -- there is a lot of upside to that, but the one downside is that it relies on just natural convection to move the air, and if its going to efficient, that requires good sized vents distributed along the top and bottom. 

For a 10 ft wide collector, you could think about it as 2 bays of 5 ft each, and each bay should have its own vents. So, you have two exit vents about 6 ft apart at the top and 2 inlet vents about 6 ft apart at the bottom. You could box those in on the inside with one box at the top for the exit vents and another at the bottom for the inlet vents. Then , connect a duct to the middle of the box, and use a blower in the duct to move the air over to where you need it. That's some extra work, but I don't see why it would not work.

Another option would be to go to something like this collector:
http://www.mofga.org/Default.aspx?tabid=488

Which is based on this collector:
http://www.builditsolar.com/Projects/SpaceHeating/TEACollector/TEAConstruction.htm

These are designed to be fan forced collectors and have manifolds along the edges to distribute the air evenly.
They are good designs -- the only thing I don't care for is that they use the backpass solid absorber design, and I think that the flow through design I used on mine is better.
You might find other options you like on this page:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
Look in the "Active Air" section.

Don't know what kind of climate you are in, but if you have all that room on the south wall, I'd think about making the collector wider. It only costs about $2 a sqft to build and the amount of work involved in a 20 ft wide one is not a lot more than a 10 ft wide one.

There


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## Jim Bunton (Mar 16, 2004)

Hi Gary I was looking at your glazed door arrangement, and was wondering what type and amounts of insulation you are using in that shop. The system looks like it would fit my needs very well.

Jim


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## SolarGary (Sep 8, 2005)

Jim Bunton said:


> Hi Gary I was looking at your glazed door arrangement, and was wondering what type and amounts of insulation you are using in that shop. The system looks like it would fit my needs very well.
> 
> Jim


Hi Jim,
Two of the walls are house walls, so no heat loss there.

The area above it is a "bonus" room that we do not heat -- I'm not sure what the effective R value is -- maybe R50ish?

The West wall is 2 by 6 studs insulated with R19 FG -- that wall has two double glazed windows and a door (not well weatherstripped). I'm guessing the west wall is 30 ft long.

The south wall that the collector is on is mostly the garage door, which is insulated to (maybe) R5ish -- plus the R2ish glazed outer doors ( say R2 on the doors, because the lower glazed panels are now twinwall rather than the single pane of acyrlic that's in the pictures).
None of the south wall elements are wheatherstripped as well as they should be -- on the todo list 
I'm guessing the south wall is 25 ft wide.

The concrete slab floor is not insulated underneath at all. I've thought about getting some old carpet and putting it down on the floor where the sun shines in in order to keep the suns heat from being absorbed by the floor.

Its hard to overstate how nice it is in there on a sunny day -- warm and nice and bright. My wife will walk through and comment that its the most comfortable place in the house -- she's not entirely happy about that 

I believe the performance of the south glazed doors as a collector is about the same as a well designed active solar collector of the same size, and the insulated garage door limits the heat loss at night (which would be large).

As I think I mention in the description, its my all time favorite solar project -- if you have a shop with a large door facing the right direction I can definitely recommend it.

http://www.builditsolar.com/Projects/SpaceHeating/SolarGarageCollector/garcol.htm
http://www.builditsolar.com/Projects/SpaceHeating/SolarGarageCollector/GarCol20F.htm

Gary


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