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attic exhaust

Discussion in 'Photovoltaics' started by Dodo, Aug 10, 2003.

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  1. Dodo

    Dodo Guest

    I'd like to use a solar panel to run two AC 3.4amp attic exhaust fans. What
    sort of components would I need to do this? I know I'd need the panel and an
    inverter, but I'm not sure if I'd need anything else. I'd also appreciate
    advice on exactly what models of equipment would suit my needs.
     
  2. Kevin M

    Kevin M Guest

    That's a question I've been wanting to learn about, too, but most of
    the fans seem to be 24VDC rather than AC. There are commercial
    systems available for 3-400 $US, and I haven't been able to see a way
    to build one for less than $200. Assuming I could figure out the
    amps, volts, etc.
     
  3. Joe Fischer

    Joe Fischer Guest

    : That's a question I've been wanting to learn about, too, but most of
    : the fans seem to be 24VDC rather than AC.

    Which makes sense for the following reason;

    AC motors will be damaged by voltage less than rated,

    while DC motors will just rotate slower at lower voltages
    without overheating or damage. (There are many types
    of motors, but DC motors commonly have brushes and either
    a slip ring with small brushes carrying the field current
    to the rotor, or have large brushes carrying the armature
    current).

    There are many low cost motors, mostly 12 volt
    in auto junk yards, they are the heater and air
    conditioner fan motors.
    They are attached to squirrel cage blowers,
    which may not be high volume fans.

    Some of them have permanent magnetic stators
    with rotating field coils and small brushes.
    The squirrel cage blowers do have a disadvantage
    of being noisy.

    There are also DC motors used for radiator
    fans for many newer cars, and that have regular
    fan blades.

    Any of these, 12 volt or 24 volt from trucks
    or RVs, could be used with just diodes and solar
    panels provided they are fused to protect in case
    of bearing failure.
    The most destructive problem with motors is
    that the bearings bind up and lock the rotor, which
    often means the motor overheats, smokes badly, and
    burns the windings out if not fused.

    The good thing is that 12 volt panels can
    be used with either 12 or 24 volt motors if wired
    correctly.
    The permanent motor in my cars draws a lot
    of amps, at least 10 or more. This is not near
    the equivalence of 3.4 amps on 120 volts, but it
    is considerable.

    The radiator fan motors would be easier to
    use for ventilation because they have the right
    type of fan blade already installed and may have
    the motor support bracket.
    But more than one or two may be needed,
    although any ventilation on a hot sunny day
    will provide some comfort, lower cooling bills
    and help protect the roofing materials from
    drying out.

    Joe Fischer
     
  4. SpiderG

    SpiderG Guest

    Do a Google search for solar attic fans - I got dozens of hits for both
    through the roof setups and gable end versions. Some can be outfitted with a
    klixon temp switch too (makes sense for winter use).

    I personally put a through the roof unit in a few weeks ago. Although a
    little pricey, it only took 30 min. on the roof - and no power cords to run.

    Good Luck!

    SpiderG
     
  5. Nick Pine

    Nick Pine Guest

    There's also much to be said for low and high vents, with no motors.

    Nick
     
  6. It seems to me that using an AC motor with an inverter is wasting a
    good percentage of the power. I would use the 12VDc off the panels
    with a DC motor such as the fan motor from a front wheel drive car, or
    maybe one even more powerful than that. Also, if the panel isn't
    getting full sunlight, then the DC motor might run at half speed,
    where the inverter might not run the AC motor at all.

    Oh, one other thing. It seems that a convection system might be the
    way to go with this attic ventilation. And also there are those wind
    turbines that run off convection.

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  7. I'm just up the road, a few miles from Dizzyland, and I know what you
    mean. But I still see houses with bags over the wind turbines in
    "winter" around here. Probably doesn't make a lotta diff on the
    heating bill.


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    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  8. Joe Fischer

    Joe Fischer Guest

    : It seems to me that using an AC motor with an inverter is wasting a
    : good percentage of the power. I would use the 12VDc off the panels
    : with a DC motor such as the fan motor from a front wheel drive car, or
    : maybe one even more powerful than that. Also, if the panel isn't
    : getting full sunlight, then the DC motor might run at half speed,
    : where the inverter might not run the AC motor at all.

    And there are millions of them junked every year,
    even with the shroud just like an attic fan.
    Some of them have a Sprague bearing clutch on
    the fan blade that lets it freewheel if the wind blows,
    reducing power consumption.

    Joe Fischer
     
  9. Kevin M

    Kevin M Guest

    That does sound like a great idea -- get a fan assembly from a junked
    car (assuming it was rear-ended, of course :) ).

    For those of us who are total solar neophytes, can you help me with my
    math:

    example fan motor from http://www.spal-usa.com/pdfs/10Fans.pdf (an
    after-market fan manufacturer):

    12V
    10 - 14.7A

    Watts = V x A

    Watts = 12 x 14.7 = 176.4 W

    As this thing would run primarily during the daylight, a battery
    doesn't seem as logical as a direct-powered system. How much solar
    cell would you need to drive a 180W motor?
     
  10. Steve Spence

    Steve Spence Guest

    as you are interested in running this only when the sun shines, and don't
    need battery storage, the panel wattage has to exceed the motor wattage, so
    anything over 200 watts of pv will work fine. one issue is cloud cover. the
    roof will still gain heat, but solar generation will be off. maybe 350ah
    battery storage, four 100 watt panels and a thermostat are not a bad idea.
    pretty expensive compared to a turbine, or soffit vents with a ridge vent.
     
  11. Kevin M

    Kevin M Guest

    That's what I was thinking -- they sell commercial versions at Lowe's
    for around $300 -- I'm sure there are ways to find more efficient
    motors, etc., but PV doesn't seem economically viable if you have any
    other source of power. More's the pity -- an attic fan seems like the
    perfect application of PV technology.
     
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