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I need your help for my solar power greenhouse fan project

  1. May 15, 2014 #1
    Hello everyone,

    Firstly thank you for taking the time to read this! I am a complete electrics beginner, so forgive anything stupid I say. I'm also typing this on a phone, so sorry for the odd nonsensical word!

    So here is what I am trying to do: I have a small greenhouse, the plastic kind. I live in the UK, and the weather is pretty unpredictable. When its sunny and I'm not home to open up the flap my greenhouse steams up, which isn't good for the plants. I want to make a small fan, that sits in a hole in the plastic to act as an extractor to ventilate, but only when it's sunny.

    I bought a solar light and took it apart, connected it directly (no battery) to a 12v laptop fan. In direct Sun there wasn't enough power to start it moving, but enough to keep it spinning if I started it off manually. So I tried connecting another panel. I tried it in series and paralel circuit, and just got the same outcome.

    I have just bought a more lightweight fan, that is only 5v. I connected the two panels in the same way, but can't get it to do anything. Tried series and paralel, nada. Hooked it up to two AA batteries just to test it works, which it does.

    I believe the panels are 2v each, it doesn't say on them, but I have found very similar one a online that are 2v.

    I have so far spent only about £6 on this, it's not worth spending lots of money on. So I'm open to any and all bodging suggestions!

    Thank you for your help!
  2. jcsd
  3. May 15, 2014 #2


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    Welcome to PF.
    You need to measure a couple of things when your solar cell is in the sun, the open circuit voltage and the short circuit current. That will give us the information we need to work out what you should do.

    A fan will have little effect on a greenhouse unless the greenhouse is very small. An alternative way to adjust ventilation is to roll up the plastic side on a steel tube. Attach the tube with tape to the bottom of the sheet, with a large diameter pulley at one end. Pulling a string wound on the pulley will raise the side of the greenhouse without dragging the plastic or flapping in the wind.
  4. May 16, 2014 #3

    Thanks for the reply! Unfortunately I do not have any instruments to help me measure the current, I was just hoping for some insight as to why I can power the 12v fan a small amount, but the 5v fan not at all?

    Yes the greenhouse is very small! It is like this one:

    All I need is to encourage the hot air to flow out of the top, creating a current of cool air that can enter at the bottom. I usually roll up the door flap during periods of sunny weather and close it at night when it gets much colder. But if I am not home to open it, I would like the sun to activate the small fan to help circulate the air and prevent fungus problems that like to live in the steamy dampness!

    I hope that makes sense!
  5. May 16, 2014 #4


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    Your 5V fan might need significantly more current than the 12V fan.
    The fans may have brushless DC motors inside that require more than 4V to begin operation.

    It does make sense but I think the fan is unnecessary. A hole, (maybe with a small tubular chimney), at the top would permit hot air to rise, probably more efficiently than with a small fan.

    ? With a fan, would you have a plastic flap to close off the fan hole when it was dark?
    Without a fan, the hole could be covered with a light weight flap of plastic that is opened by the rising hot air inside.

    You could make the plastic equivalent of a bimetallic strip that would open the upper vent when temperature inside rises above a set point. That is a more reliable lower technology. http://en.wikipedia.org/wiki/Bimetallic_strip
  6. May 16, 2014 #5
    ventilation has a huge impact on greenhouses. the whole purpose of a greenhouse is to keep an area warm by restricting convection.

    Your fans are probably failing because you typically have to do more with a solar circuit than just connect the panels to a load. A PV cell inside its operating range acts like a constant current source. Whatever voltage your system demands, the PV will deliver a fixed current. This holds until it hits its maximum power point (MPP). Then the current from the PV drops to 0 very quickly.


    The picture (assuming it uploads properly) shows the problem. If your fan draws 5 or 12 volts, the PV supplies 0 amps and 0 power. If you get a fan for a voltages less than the MPP, it will work fine. Or you could create a circuit that forces the fan to only take a voltage less than the MPP. The 5 or 12 volt fan would operate in a diminished capacity, but it would work (at least most low power fans would). The concept of circuitry that keeps the voltage at the MPP to get the most power possible out of the cell is called Maximum Power Point Tracking

    Attached Files:

  7. May 17, 2014 #6
    Also, if your panels really are 2 volts, then just adding another panel and putting all 3 in series should let you run the 5 volt fan perfectly.
  8. May 17, 2014 #7


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    Imo, you just need more area of PV cells (for the current) than you get from a few garden lights. eBay has some small panels for not much money.
    You have a big advantage in that you will only need the fan on when there is plenty of Sun.
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