Using a solar panel for the first time

Discussion in 'The Projects Forum' started by Aqlor, Mar 26, 2011.

  1. Aqlor

    Thread Starter New Member

    Mar 26, 2011
    3
    0
    Hello guys,

    After lurking the internets for a while I've found this forum and since it seems pretty active and users are getting answers quickly I decided to register and ask my questions directly ^^


    I am planning on a project that consists on using a 20Watts solar panel I have here collecting dust instead of solar power( I got into some chemistry contests before and it was given to me and I need to find something useful to do with it - not that hard is it? )

    I live in Portugal and in the summer the house will be pretty hot, we don't have AC here and my father uses the attic to make some of his paintings, so what I though of doing was getting some computer fans ( I got 5 or so for free ) and run them off the panel by making a attic fan to remove heat and humidity.

    Each one of these fans runs at 12V at about 0.15A (~1.80Watts) so they should run fine from the panel:

    Assuming the panel runs at 70% its max rate here during the summer I am expecting about 14 Watts or so
    The fans would take about 9 of these 14 Watts (5 fans * 1.80 Watts) so I am thinking of lighting something up that consumes about 5 Watts ( maybe some High Powered Leds - near the fans of course, I want to make sure the leds don't get us sweating even more :p )

    However there are some particularities about this:
    Sometimes there isn't enough power to run the fans so I want to use something similar to a Solar Engine that charges up some capacitors and discharges once the required voltage is needed.
    I also do not want to use batteries for this, there is no need for the fans in late evening


    Before asking specifics questions about the project, I would like to know if you people think it is doable and if it is if you could give me some ideas on how to build a 12volts solar engine and how to keep the fans from burning on a very sunny day.

    You should know, I know very little of electronics and I am trying my best to learn it in a useful way :)

    Thanks in advance,
    Manuel
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    20 watts is a very modest amount of power. a circuit to switch the output of panel to a dump load is certainly possible.

    Using the panels directly to power the small fan motors would require a small amount of components as well.

    Using both modes together on a sunny day might be possible using a PWM switching control and H bridges, OR with only twenty watts of total power you could dissipate excess power in a simple transistor power supply(large heatsink required) if the panel outputs get to high in full sun.
     
  3. Aqlor

    Thread Starter New Member

    Mar 26, 2011
    3
    0
    Thanks for the reply

    Could you tell me what components would I need to run the fun motors from the panel directly?
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    If you used enough fans to load the panel to about 10 watts or more, you would not need a single solitary thing

    Open circuit the panel might reach 17 volts or more. So, if too few fans are connected, during full sun they might suffer damage if allowed to run very long that way.

    Using a small value power resistor in series with the panel output might be wise in that case.
     
  5. Aqlor

    Thread Starter New Member

    Mar 26, 2011
    3
    0
    I see...however, if I used that much fans on cloudy days none of them would run , they need a certain amount of power to start...

    Isn't there a simple way to limit the panel output power? I don't care too much if that is wasted

    Btw, do you have any ideas on making a 12 volt solar engine?
    For cloudy days I would need something like this this but with higher voltage
     
  6. russpatterson

    Member

    Feb 1, 2010
    351
    16
    I've done a lot of this kind of stuff with small solar panels, mostly for water pumps, but also for some 12V fans. The issues you need to be aware of are:

    1) The fan motors draw a large current spike at startup in order to overcome friction etc. So your panel may not start them because it may not supply enough current. You can just try it out and see but the current the panel will deliver varies greatly by the amount of direct sunlight it's getting. So you have to put some smarts in your device to handle this. It's important to know that if you run power to your fans and they do not start spinning they essentially act as a resistor and dissipate the power via heat which will probably burn through the motor windings. So avoid this case.

    2) You may indeed damage your motor if the panels put out so much more power than the fans use that the voltage gets in the high teens.

    3) In the evening or cloudy times your fans may stop spinning but still be getting enough power delivered to burn them out like described above.

    I have a friend that just connected his 15 watt panel to some 12V fans for his greenhouse, it just runs when it's sunny, and hasn't yet self-destructed due to the reasons above so I guess it can be that simple. (but your luck probably won't hold out).

    What you really need (I've been at this for a few years) is a microcontroller that can sense voltage and make decisions to start the fan when there's enough power (yes you can use some big caps to store the startup power like you mentioned) and to stop running when the voltage gets too low (say 10 Volts). MOSFET transistors seem like the best way to switch the power on/off from the panels/fans.

    You could start with an Arduino and build up the components if your looking for a project. Or your welcome to my experimenting board. I have my schematic up here:

    http://backyardsolar.blogspot.com/

    You can order the blank PCB board (of that schematic) from SparkFun BatchPCB. Let me know if you're interested and I'll send you the info.

    Have you tested your panel for maximum open circuit voltage and current yet? Let me know if you don't know how to do that.

    Have fun experimenting.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    One way to approach this would be to map out the power curve of your panel. By that I mean, under full sun, place different resistive loads on the panel and measure the voltage. Calculate power to the load, V^2/R, and plot it against R. You'll learn the loading that can produce peak power - it'll be the same as the internal resistance of the panel. Unfortunately, you'll need several big resistors to collect this data.

    You need power to move air, and designing for efficient transfer of power will give a superior result. It'll also allow you to estimate voltage and current under any scenario (at full sun).

    I don't know much about fan efficiency, but I'm guessing one large fan is more efficient than several little ones.
     
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