Passive solar air ?

Discussion in 'Solar and Geothermal Water Heating Forum' started by mar3232, Nov 6, 2011.

  1. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    I thought of building a 4 x 8 passive solar air heater, south facing for am Indiana winter. You know, the type with the hot air sucked out of the top and cold air sent to the bottom.

    Painted black, insulated, many designs use gutter downspout.

    BUT -- just wondering, has anyone ever used one?

    Is it worth the effort?

    I already have a couple of decent south facing windows which seem to bring in a lot of heat -- just wondering if I'll gain anything?

    (Or maybe even lose on days where the temps are in teens?).

    My unit will have to be a wall mounted one -- not roof.
  2. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    I've designed & built several thermal air panels, both active (with blower) and passive (convective.) Key to making the convective versions work well is the quality of the backflow prevention flaps- they need to be light enough to open at very low pressure, but tight enough to prevent backcycling at night. Sizing the openings sufficiently large is also important for convective designs. If they're too restrictive you get a high delta-T, but low flow.

    Also note, high temp output air == high loss when the outdoor temps are low. On active designs I try to keep the output temps under 110F (but over 80F.) Convective versions are prone to this loss. If after building the thing you find it's putting out 130F+ air when it's under 30F outdoors (a 100F delta-T between collector & outdoors) you may want to consider putting a small blower on it. You can control the with a simple snap-disc type thermostat switch mounted directly on your heat exchanger plate/downpout. This is a nice cheap version with the right temp range (turns on at 110F, turns off a 90F):


    http://www.grainger.com/Grainger/WHITERODGERS-Snap-Disc-Fan-Control-Switch-2E245?Pid=search


    It's enough of a switch to run a substantial blower, but they don't need much. The size & type of blower depends on the size of the collector and the cross sectional area of the air path through the collector.

    See also:

    http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm


    Wall mounting is absolutely the right approach for an Indiana latitude- better convective forces, and better heat rejection during the summer. Roof mounting at an optimal angle for space heating would require a VERY steeply pitched roof (50 degrees or higher). With a wall mount the performance is undercut only slightly, but allows the use of overangs to limit summertime gains, and the mid-day summertime sun angles cause much of the direct sun to be reflected by the oblique angle to the front face of the glazing, a phenomenon of optics called "total exterior reflectance". The angle at which that happens is a function of the difference in refractive index of the air vs. that of the glazing material.

    In a New England climate the annual offsetting of the heating bill on the collectors I've measured in-situ with a good heating-fuel history have been everywhere from 1 therm to 3.5 therms per square foot of glazed area (which is equivalent to ~0.75-2 gallons of heating oil.) That offset performance is a function of both the efficiency of the heating system and that of the air collector. The more efficient heating system, the less difference it'll make, but with an unobstructed southern exposure with an ~80-85% AFUE system in decent shape, 4x8 (32') decently built thermosiphoning collector will offset better than 50 therms, but probably not 100. With a blower resulting in a lower operating temp will usually improve performance by 20-50%, for a small cost in power use.

    While I generally advocate active systems if designed well, there are fewer code & maintenance issues with a thermosiphon. The most recent version I helped design & implement was a ~80square foot building-integrated thermosiphon built into the wall of a rec-hall type building. I don't have good heating fuel performance data on this intermittent-use building to compare but I've measured 130F output air when it was 15F outside, 68F indoors, which is not a mark of good efficiency. I'm sure even at a buck a therm gas (current rates) and a 80% efficiency gas furnace they use to heat the place it'll still pay off the ~$800 in materials expended (teenage sweat-equity went into the assembly) in less than a decade.
  3. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    well I just replied and it didn't show up.

    anyway -- thanks -- yes, I will be using a fan at the top to circulate the air. turn on at 80 degress min, correct?

    how well do you think a 32 sq ft unit will perform? only 4 - 5 hours of 100% sun where I want to mount it -- but every little bit helps.

    I can buy dampers that will automatically "flap" off when the fan is off, correct?

    what cfm do you think I need for 32 sq feet? --- THANKS !
  4. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    I doesn't have to be very much flow for a 32 square foot collector. A 15-20W electronic chassis cooler (75-100cfm) would be enough if you don't rest. Find one that's not a screamer (<<50dbA) if it's going into someplace where the noise would be an issue. See:

    http://www.surplussales.com/fans-blowers/FansBlow-3.html

    Using lightweight plastic-film flaps at the bottom as the backflow check valve works. It doesn't have to seal perfectly. There are multiple approaches detailed on the BuildItSolar.com site if you start digging there. (DO check them out for useful tips.)

    Partial shading output estimates of performance would be just a wild guess, but you can get a surprising amount of indirect heat out of snow-reflected sun too. Even due-east or due-west mountings aren't bad from a production point of view (if a PITA from an unwanted summer gain point of view.) If it's mostly mid-day sun and 4+ hours you'll likely get at least 2/3 of what you might have if completely un-obstructed.
  5. mar3232

    mar3232 Member

    Messages:
    108
    Location:
    Indiana
    thanks Dana -- gonna start working on it.

    seems to me the place best not to "skimp" is the polycarbonate cover. I may go with a nice full sheet of 1/4" -- well sealed.
  6. Mladen

    Mladen New Member

    Messages:
    2
    Location:
    Serbia
    Solar air heater - made of pop cans

    Hi all,

    have you considered making solar air heater by yourselves? I have make one by flowing step-by-step instructions on net. It is quite simple and it works well. This way you can help recycle cans and heat you home. Pop cans are used to collect heat. It looks great and works also great.


    regards,
  7. Mladen

    Mladen New Member

    Messages:
    2
    Location:
    Serbia
    kolektor.jpg

    here is what it looks like, mounted on my house :)
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