Solar powered fan with temperature control

[colebrookman]

Finally built a three season room, all glass, adjacent to our living room. We have a small window on the common wall. Is there a solar powered fan that could be set to operate when that room reaches 75 degrees or so pumping the free heat into the living room when we are at work. I know the solar fans are available but don't know how to control them. Thanks in advance for your ideas. Be safe!
Ed

 

[fishingpol]

Check out the builditsolar.com website. Gary who runs the site is also a Hearth.com member. You would probably have to get a decent size fan that runs on dc and pick up a solar panel to match it. When the sun hits the panel, it should go. Cloudy days may limit the time the fan runs. I am in the final stages of building a solar heat box out of a basement window opening on a south-facing wall. Trial runs have been good, and I'll update that post as I finish. The dc fans are quiet too.


Edit: You would probably need a limit switch so that lets the current flow when the temp in the room are warm enough before letting the fan off.

 

[colebrookman]

Thanks fishingpol, know the site well. Yes I figured that the solar will only work when the sun's out and also heating the room. I would like an ajustable switch which would only come on once the room is up to temp. Just a matter of finding the correct one.
Ed

 

[begreen]

Yes, there are solar powered greenhouse fans available. One thing I would try that may seem counter intuitive at first is to set the fan low in the adjacent area and have it blow the cool room air toward the warm stove room. That will work with natural convection. The displaced cool air will be replaced with warm air from the stove room.

You could make one of these with a small 12 v panel and a 5 or 6" 12v computer fan. Add a battery and it will run even when cloudy.

 

[colebrookman]

Thanks BeGreen. That's exactly what we do in our bathroom. We have a small, noiseless fan on the floor blowing cool air toward the stove room and bringing in hot air by convection. Works much better than when we had it attached to the top of door frame. I'll check the greenhouse fans. May be the way to go.
Ed

 

[GaryGary]

Hi,
There may be better solutions out there, but here is one that might work for you:
http://www.costco.com/Browse/Product.aspx?prodid=11609956&whse=BC&topnav;=&cm_sp=RichRelevance-_-itempageVerticalRight-_-BrandTopSellers&cm_vb=itempageVerticalRight|BrandTopSellers

This is meant to be a solar powered attic ventilation fan, but I think it could be mounted in an opening between your sunspace and the house.
The PV panel is separate from the fan, so you could mount the PV panel where it will get good sun.

It comes with a thermal snap switch that turns the fan on around 85F(?) -- it then turns off when the air reaches about 75F(?).
I can look up the exact switch temps if you need them.

I saw one of these at our local Costco just a couple days ago, and they were selling off the last few (I guess), because the price was only $80.
I could not resist and bought one with the idea of using it for an upcoming solar attic collector. You might check your local Costco and see if they have it.
It appears to be well made.

The thermal snap switch could be used as is to control it, or you could bypass there thermal switch and put in your own. If you put your own in, it could be located up in the peak of the sunspace to sense the hottest temperatures. Home Depot sells a replacement thermal snap switch that is intended for attic fans that should work fine -- the nice thing about it is that its adjustable in temperature. Or, PEXsupply.com has a wide selection of fixed ones at less than $10 each.

If you have any questions about the unit, just let me know as its sitting in my garage right now.

Gary

 

[colebrookman]

Thanks for the heads up Gary. I saw the Pex site but the Home depot idea is even better. I checked out attic and greenhouse fans but they would be overkill and too noisy for my purpose. Because it will use the window between rooms I can't use a rush of air and one of the many joys of woodburning is the quiet so I'm leaning towards a small, quiet fan. I do have a few doorway fans, electric, that I can try to narrow down which works best before buying the DC fan. I'll be checking out HD to see what's available. Many thanks for your comments. Be safe.
Ed

 

[daveswoodhauler]

Interesting thread. I'm thinking that your setup will want to include some type of damper/baffle to keep the cool/cold air during the night time from your 3 season room to escape into your heated area. (Not sure how it would work, but was thinking of one of the dampers/baffles that you would find on the exhause port of a bathroom vent fan...basically, something that would close shut while the fan is not in operation.

 

[colebrookman]

Sounds like a good suggestion Dave. I was thinking dryer vent but the small fan wouldn't have enough volume to open it fully. For now I can open and close the window and the door between rooms but that won't work when we're not home. Greenhouses use a temp. controlled arm to ventilate the heat, I'll look into it. I like simple and frugal so it's a work in progress. Love this forum because it's a way to brainstorm with some great people. Many thanks. Be safe.
Ed

 

[woodgeek]

I'm going to be my usual wet blanket. For the fan to move much heat, the sun space will need to be a lot hotter than the house, the fan will need a lot of cfm, or both. IF you use a too small fan, then your sun space will get really hot, and the stream of air from the fan will be really hot, and you will think 'hot damn, I am getting a lot of solar energy', but in reality the cfm is so low as to not move much heat, and the high temp in the sunspace will mean you are losing a lot more heat to parasitics out the windows AND it will be difficult at times to use the room as a ROOM. If alternatively you get a big fan, the room and the stream are tepid, perhaps not even noticeably warm, you will be getting tons of heat and not be able to tell (although the fan will be loud!).

So, as an engineer, I think you should experiment and crunch some numbers. Close up the room on a clear, cool day and see how hot it gets (stagnation temp). IF the stagnation temp is not more than 40 degrees over the outdoor temp, then skip the fan. You have a three season room that partially heats itself in the winter, like most people, and be happy with that. If it gets to 60-70 degrees over outdoor ambient on a sunny day, then you should first make sure it gets reliably shaded (overhangs, trees?) in the summer so you don't fry the room contents, small children or pets trapped out there etc. IOW, its like a closed car. Then you can think about getting a fan to harvest some of the heat. But note, that if the stagnation temp is 70 degrees over ambient (pretty darned good), you are still losing half your heat to parasitics when it is near freezing out, and almost all of it when it is 10 out.

I like the sunspace idea a lot, but they are generally big and inefficent--so lots of heat (maybe) in the shoulder seasons, and not a lot or none in the dead of MA winter. Still a BTU is a BTU.

As for the fan, they move ~1 BTU/hr per cfm per degF. So a 200 cfm fan and a +10 deg room moves 2000 BTU/hr, about 600W. IF your room is 'on' 12% of the time (typical solar, 6 hours every other day), say 4 shoulder months per year, then you get ~300 hours of run time/year tops. In the associated fan example, 300*2000 = 600 kBTU/yr, offsetting 5 gals of oil, maybe $20/yr. In terms of payback, I would skip the solar panel and just get a regular AC fan. The panel will be several times more expensive than the fan it drives, and if you economize on the fan size, you will lose more heat to parasitics, and come out behind a properly sized utility fan.

OF course, a nice feature of the fan is that you can use it to preheat the room off your central/wood source on cloudy cool days you want to use the room.

Personally, for noise reasons alone, I would try to use natural convection, not a fan. When I was a kid in MA, we opened the sliders to our 3-season, and the room stayed warm from the house--the reverse should be true. Rig up a couple big motorized dampers, or buy automatic doors from a defunct grocery store or something....now that would be cool.

 

[colebrookman]

Oh sure Woodgeek, pop my bubble with logic. And bumblebees really can't fly. Seriously I agree with you. It will probably be just a slight help on sunny days in the shoulder season. Right now we do as you mentioned, just open the door and window between rooms and hope that convection will warm the living room a little, which it does. And a BTU is a BTU. I was just hoping to gain a little more, especially when we aren't home during the day. Cost and quiet are also limiting factors. But tinkering is fun and much easier on the back then splitting wood. Thanks for adding a nice touch of reality and no you're not a "wet blanket" Be safe
Ed

 

[GaryGary]

Hi,
Some more numbers to add to the confusion:

Not sure what the glazing is like on your sunspace, but just for the sake of argument, lets say it has the equivalent of 200 sqft of south facing vertical glazing and that you are a 40 deg latitude, and that its double glazed with clear glass having a transmission of 78%. These may be way off for your sunspace, but you can adjust the calcs below to your situation.

On a sunny day at the winter solstice, 1 sqft of vertical glazing will see 1590 BTU, so 200 sqft will see 317K BTU, of that 78% gets through the glazing, or 254K BTU.

If it averages 40F over the collection period (which is the hottest part of the day), and the inside of sunspace temperature averages 80F, then the heat loss through the R2 double glazing is A*(Tin - Tout)/R = (200 sf)(80F - 40F)/R2 = 4000 BTU/hr, or about 28K BTU for a 7 hour collection period.

So, your net potential gain for ONE sunny shortest day of the winter is 317K - 28K = 289K BTU.

I say potential because how much of this heat that makes it into the sunspace gets into the house vs being lost to the outdoors depends on how the sunspace is built and whether you have a fan sufficient to transfer the heat into the house at the rate its being added to the sunspace.
If the sunspace is not insulated well, the surfaces that are not glazed (like the east and west wall) will lose heat to outdoors, but this can be cut to insignificant with insulation. If the floor of the sunspace is (say) a dark stone tile laid on a concrete slab with no insulation under, then a fair fraction of the solar heat will end up warming the slab and the earth under, but if the floor is not to massive and insulated, then little heat will be lost to the floor. If the fan is too small, then then it won't able to transfer the heat out of the sunspace fast enough, and the sunspace will just increase in temperature until it gets hot enough to lose its heat to the outside.

So, sunspaces with well oriented glazing receive a LOT of solar heat, and sunspaces with good insulation and a good system to transfer the heat into the house can produce a LOT of heat for the house.
Your sunspace may not be the ideal design for transferring heat to the house (don't know), but I would not be to quick to write off the potential for significant heat production from it.

My feeling is that sunspaces are under rated as solar heat producers -- they can be very good, and they serve a lot of other useful purposes as well.

One really good example is the Cliff house:
http://www.builditsolar.com/Projects/SolarHomes/shurcliff-saunders-1.pdf
This design by Norman Saunders produces 100% solar heating for a house in a tough climate. The only solar collection is from a large sunspace.
This shows what a good sunspace can do -- true 100% solar heated homes a very rare.

This is a good description the low thrmal mass sunspace as a house heater:
http://www.builditsolar.com/Experimental/ShurcliffPart1/PolySpace.htm

This link just above is a chapter from the best (or at least the most interesting) book ever written on solar (I think:
http://www.builditsolar.com/Experimental/ShurcliffPart1/TOC.htm

The 1590 BTU per sqft of vertical glazing comes from the free program Radiation On Collector:
http://www.builditsolar.com/Tools/RadOnCol/radoncol.htm

Gary

 

[woodgeek]

Gary's numbers look good to me...he's the expert. To get back to fans, his 200 sq ft example would be 40 kBTU/h, at 10°F above the conditioned space, this system would need to move 4000 cfm, about 3x more than my central air system. To scale it, think 20 cfm/sq ft of glazing? Half that probably would be aok...

I am still concerned about solar resource, which CBman doesn't really describe--any branch shading?

Gary, do you think you can use stagnation temp to estimate parasitic losses and output?

 

[colebrookman]

Thanks Gary for your great input. Wouldn't it be great if we could all start from scratch and design accordingly. As your great web site notes, most of us are out here trying to use what we have and improve in small ways, always constricted by cost/ benefits. Upfront cost is usually the biggest problem: if only solar and wind power were subsidized in much the same manner as coal, oil and gas the playing field would be very different. Such is life. Thank god many of us have wood to lessen the burden but it is aggravating to see that beautiful sun and its' solar power going to waste everyday. Again thank you for joining in on the discussion. Be safe.
Ed

 

[Tatnic Corners]

Hey Colebrook man,

I use the builitsolar site too. From that site I got a differential controller from mydtcstore dot com. I like it a lot. It runs the pump to my solar panels.

you could set it up so that when the temp controller hit 75 degrees it turns on a fan, you could use a small dc computer fan even, depending how much air you needed to move. There are many different controllers that youcould use, a couple of good "homebuilt" ones are on the builditsolar site. I just liked that the one from mydtcstore has the ability to run 4 different pumps.

The problem with it is that it has no casing, so you have to build that up too. But if you got the 2 controller model($69), the box ($8) a charger($12) and 2 sensors you could do what you want to do. Put a sensor in the sun room and one in the other room and when the sun room is hotter that the regular room the fan could be turned on. And it could be told to turn off automatically. A little expensive, but set it once and you'll not have to hit the switch for the fan ever again

 

[GaryGary]

Gary's numbers look good to me...he's the expert. To get back to fans, his 200 sq ft example would be 40 kBTU/h, at 10°F above the conditioned space, this system would need to move 4000 cfm, about 3x more than my central air system. To scale it, think 20 cfm/sq ft of glazing? Half that probably would be aok...

I am still concerned about solar resource, which CBman doesn't really describe--any branch shading?

Gary, do you think you can use stagnation temp to estimate parasitic losses and output?

Wow -- nothing 4000 cfm streaming into your living room! Did not realize it would come out that high.

Maybe let the delta T go up some, and, of course, most sunspaces are are going to have some losses out their own walls and floor.
I agree that 20 cfm/sf of glazing should be plenty.

Its probably not a bad idea to try it with a fan you already have lying around to get an idea what size will work well.

I do think that measuring the stagnation temperature on a sunny day would be a good thing to do. If its up toward 120F or so, you know you have some potential. If the solar input and the stagnation temperature are stable over a period of time (say from half an hour before solar noon to half an hour after solar noon), then you could assume the the sunspace is in balance and losing the same amount of heat as it is gaining, and estimate the loss (and gain) just using the Heat Out = (Area)(Tstag - Toutside)/R. This would have to be done for each of the major surfaces of the sunspace -- glazing, walls, floor, ... Maybe more trouble than its worth, but just seeing what the stagnation temperature is would be helpful.

On the tree shading, doing a site survey is a good idea: http://www.builditsolar.com/SiteSurvey/site_survey.htm


Gary

 

[billb3]

Sounds like a good suggestion Dave. I was thinking dryer vent but the small fan wouldn't have enough volume to open it fully. For now I can open and close the window and the door between rooms but that won't work when we're not home. Greenhouses use a temp. controlled arm to ventilate the heat, I'll look into it. I like simple and frugal so it's a work in progress. Love this forum because it's a way to brainstorm with some great people. Many thanks. Be safe.
Ed

I have a couple of wax piston filled ones on a greenhouse.
They don't lift terribly heavy windows.

But if you can fit a lightweight window they work pretty good.
My sister uses one for a little cold frame.
lexan or acrylic window with a thin aluminum frame.

 

[colebrookman]

Thanks Bill, I'm going to check them out on the web as soon as I can. At least I have plenty of wood and today is rainy and raw so a little fire feels good. No sunroom heat today. Be safe.
Ed

 

[semipro]

Thanks for the heads up Gary. I saw the Pex site but the Home depot idea is even better. I checked out attic and greenhouse fans but they would be overkill and too noisy for my purpose. Because it will use the window between rooms I can't use a rush of air and one of the many joys of woodburning is the quiet so I'm leaning towards a small, quiet fan. I do have a few doorway fans, electric, that I can try to narrow down which works best before buying the DC fan. I'll be checking out HD to see what's available. Many thanks for your comments. Be safe.
Ed

We use the adjustable attic cooling fan snap switch from Home Depot to control an in-ceiling fan located in the same room as our wood stove to distribute warm air throughout the house. The switch is simple and reliable.

 

[colebrookman]

Now that's a great idea Semipro. I have a fan in the new sunroom and I was wondering how to have it go on only when the temp is up and we aren't home. That should work just great. Many thanks. Be safe.
Ed