How to gauge solar panel energy without turning on your load?

Discussion in 'The Projects Forum' started by crs831, Sep 18, 2013.

  1. crs831

    Thread Starter New Member

    Sep 18, 2013
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    Hello guys,

    I have an interesting design challenge. I need to power a 1 HP motor from a solar panel array in 5 minute increments. Here's the challenge... I'm not supposed to turn the motor on if I can't be sure it will run for at least 5 minutes. It's acceptable to dial down the duty cycle to prevent the motor from stalling in a condition of low light, but I don't it to stall. We want to avoid using batteries if possible.

    This is for a high-end, niche product so we can afford to implement the best solution. Does anyone have experience with this issue?

    The solar panel array has been sized appropriately for the load. The motor requires 90V DC and draws about 10A continuous current (25 amps peak).

    Thanks in advance!
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Under what sky conditions? Without a storage system like batteries, you would have to be able to predict 5 minutes into the future... even if it were way over spec'd.

    Ken
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Any panel array will have a I-V (voltage versus current) curve, allowing you to predict the current available at a given voltage or vice versa. That curve moves depending on available light, but the shape is more-or-less the same. With a bit of data collection, you can model the panel performance given any lighting and load.

    To predict whether you have 5 minutes of operation, you'd need to predict light level. At any moment you could challenge the panel with a smaller load and estimate the light intensity available at the moment.

    How you predict whether a cloud is approaching is another problem altogether.
     
  4. crs831

    Thread Starter New Member

    Sep 18, 2013
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    Assume the solar array is large enough to hold up the motor load on a cloudy day. This way we can rule out the condition of clouds passing by and interrupting our flow. Perhaps the bigger concern is morning and night. I don't want the system to keep trying to turn on the motor during early dawn and through dusk.

    I could perturb and observe the incoming panel power then decide to go for it or delay 10 minutes... but that seemed too obvious and dirty. Besides, I don't want to perturb the system using the motor and I'm not sure how accurate of a reading I could get using resistance as a load. Maybe this would work but I was hoping for something more innovative.

    Thanks
     
  5. wayneh

    Expert

    Sep 9, 2010
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    If cloudiness doesn't matter, then you just need to program in sun angle. Basically you could use a clock that adjusts for changing times of sunset and sundown. Maybe shorten the day a bit when cloudy.
     
  6. #12

    Expert

    Nov 30, 2010
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    The way I see it, you can either hire a full time meteorologist so you can blame him or add batteries. There is just no way a person that thinks like an engineer is going to try to run numbers on whether a cloud is going to happen in the next 5 minutes.
     
  7. crs831

    Thread Starter New Member

    Sep 18, 2013
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    I thought about using time at one point. Then I switched to the possibility of using a second smaller, more sensitive solar panel. I thought about continuously testing the smaller panel's power performance and use the readings to predict how the larger solar panel array would perform under the same light conditions. To be honest, I'm not sure if this is a good idea because I don't have that much experience with solar panels. I don't if or how the two panel setups will drift from each other over time. Thoughts? Ideas?
     
  8. #12

    Expert

    Nov 30, 2010
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    Measure them and find out how much they "drift" from each other.

    The solar panel industry is changing so fast that nobody can guess what is going to be available tomorrow or what you bought last week, especially considering the fact that you have not provided brand name, model number, or size. We can only measure what we have and run the math.
     
  9. ErnieM

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    Apr 24, 2011
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    That is not a simple problem. A motor has a high starting current to generate the torque to get it going (and some other things too... I'm no motor expert just know this situation). Now a solar cell is basically a current source, so when you such out a large slug of current the voltage drops... net power will also drop so it leaves your motor stuck in place but drawing lots of current.

    To make this work you need a way to store the energy for this kick.
     
  10. ErnieM

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    Apr 24, 2011
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    Is there suddenly something wrong with obvious? What is your specfication for the maximum allowable "dirty"?


    You will get a reading directly proportional to the accuracy of the resistor.

    Why not make it wiffy capable or add a blue fang so it can automatically call the weather channel and check the cloud cover.

    Or just use a solar panel that's not being used for anything else but IS pointed in the general direction of the sun to indicate just how much power that very same panel can presently generate.
     
  11. ErnieM

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    Apr 24, 2011
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    Is there suddenly something wrong with obvious? What is your specification for the maximum allowable "dirty"?


    You will get a reading directly proportional to the accuracy of the resistor.

    Why not make it wiffy capable or add a blue fang so it can automatically call the weather channel and check the cloud cover.

    Or just use a solar panel that's not being used for anything else but IS pointed in the general direction of the sun to indicate just how much power that very same panel can presently generate.
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    If you don't like batteries what do you think about super caps. You could then size the capacitor bank to charge to some level before turning the motor on and use that same voltage to change the duty cycle if the voltage starts to sag.
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Both ErnieM & ronv's comments are a good starting point for a workable system. A small slave panel can be used to indicate power capability of main panel before a load is attached & when load should be removed. I used this approach with a one million BTU thermal system to control the pumps. Maybe you can find some information on a " Maximiser ", which allows a solar panel to start a motor which requires more starting current than panel can provide. http://www.voltscommissar.net/minimax/minimax.htm provides inf. on a small system.
     
  14. ErnieM

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    Apr 24, 2011
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    No where did I say add a slave panel.

    The only detail of the maximizer that matters is the 940uF of capacitance across the solar cell. The rest is just a switch. How much cap is needed depends on the panel and the loading motor.
     
  15. Bernard

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    Sorry, I misread your statement, "Or just use a solar panel that's not being used for anything else---".
     
  16. ErnieM

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    Apr 24, 2011
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    No I apologies. I was getting a little "playful" with the OP who seems to have a desire to overly complicate his project, and it does get frustrating to see someone take a simple idea and think adding 20,000 extra parts makes it "better."

    Note the underline (was in italics in original). <grin>

    As project was defined the task is to sense available power BEFORE load it turned on. Therefor it is possible to turn on a dummy load to the same panel as a test. If the power measures is sufficient, turn on the motor.

    Seems simple enough to me.
     
  17. tindel

    Active Member

    Sep 16, 2012
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    crs831,

    I had a similiar problem to yours a while back. What I settled on (after some advise from a more senior engineer) was to dedicate a single string tied hard to a battery and load that string... as the power available goes up, so does the voltage. I compare that voltage to a precision reference and turn on/off based on how much power I believe is available on the SA.

    Now, you don't want a battery, but I'm assuming you have a supply of some sort, even a SA will suffice... if you power your comparator and reference from it, then you can load the SA and compare from it. A couple things to keep in mind though - you don't want your comparator to signal a false positive as the power is failing, also you have to mind your maximum output voltage of your solar array when selecting parts (a zener can help with this).

    As far as the 5 minute requirement - that's a tough requirement to satisfy... as has already been mentioned, but I don't think it's impossible... Have you considered a PID (proportional, integral, derivative) controller? I just barely know about them, so I'm not sure it will predict very accurately that there absolutely won't be any cloud cover in 5 minutes, but will probably give reliable data to shut-down in time. They do use them in airplane and boat auto-pilot routines.

    Also, if your not familiar with IV curves and the characteristics of solar arrays, be sure to read my blog post...
     
    Bernard likes this.
  18. wayneh

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    Sep 9, 2010
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    Me too, that's what I suggested back in #3.
     
  19. ronv

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    Nov 12, 2008
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    That would be the neat thing about super caps. All you would need to do is measure the voltage and you would know if you had enough stored to run for 5 minutes. Expensive but clean. :)
     
  20. massimj

    New Member

    May 5, 2013
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    What if you used super caps, they can start and run the motor, but not for long. The solar array you would need to stay ahead would be massive. That would be because of what has already been mentioned, the panels would not always be at peak current, so you would have to have way more than just what you would need for optimal conditions. In any case, without batteries, your only other option is the super caps. They would mimic batteries, but would be short lived. What they could do, is average out your power. When you produce more than you need, the caps charge, and when the panel falls off a little, the caps discharge. if you get clouds, your project fails, and there is no way around that without batteries. Oh, and to know what your panels can put out without measuring them directly, you could use a very small solar panel, and have a microcontroller read it. Calibrate it by putting both in full sun, when your panels are in full sun, so is your very small sample panel. You would record that output with your microcontroller and use it as your gauge to know that your array is in full sun and should be turned on. The turning on and off of the array is all handled by the controller. Good luck, Joe
     
    Last edited: Sep 21, 2013
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