Solar Panel Cut-Off Mechanism?

SystemSystem Posts: 2,511 admin
Hello, I represent the Engineers Without Borders chapter at the University of Maryland, College Park.

Currently we are designing a solar powered water pump to be implemented in Africa. The setup is abnormal however, instead of inserting our own submersible pump we will be outfitting an existing mechanical water pump (shown in picture) with a belt drive.

pump-overview.JPGpump-closeup.JPG

A 1/3 horsepower, 90 volt DC Brushed motor will be powering the belt drive.

The water will be pumped into a 10,000 liter tank. The tank will have an overflow and other proper piping. The entire system will also have an overall cut-off switch in case of emergencies. We will most likely also have a float switch, although this is still being debated.

Currently we are planning to attach three 160 watt, 24 volt (BP3160) panels (wired in series). The panels would not be directly attached to the motor but rather to a solar pump controller (link to the controller). We are using the PCB-90 model. The point of the controller is to boost current in the mornings and evenings so that the motor can start earlier and stay running later (even if it is at a slower speed).

The problem we are encountering is that at peak sunlight the three panels have the possibility of putting out 450 watts (they have a PMAX warranted power of 152 watts). This of course would not be healthy for the motor (which needs approximately 3-4 amps to run, this includes the resistance of the mechanical pump).

BP3160-characteristics.JPG

We considered going with just two panels, however, we would not be able to pump the required amount of water per day (used for small garden agriculture). Three is a must if the motor is to turn on as early as we want it in the morning.

How much harm can this cause the motor?

Is there are type of device or simple circuit that could cut out one of the panels after a certain power is reached? Unfortunately the device/circuit would have to be relatively simple and durable. The system will have little to no maintenance once we leave.

Thanks very much for reading through all this! Any comments/critiques/help is welcome and appreciated.

Comments

  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Solar Panel Cut-Off Mechanism?

    Once again, the experts will weigh in with a more learned opinion but,,

    The motor doesn't care how much wattage the panels produce, it only cares if there is the PROPER voltage with enough current to start and run. For a simple example, the fan motor on a vehicle doesn't care it it is being run off the 90 amp alternator, or the battery, as long as it has the right voltage and enough amps.

    So there should be no need to "cut off" one panel. If you have excess panel capacity over a good part of the day, you could consider a battery bank and controller to A: Give you a starting amperage boost so the pump could start earlier, B: let the pump run later in the day, and C: Allow you to pump on less than perfect days.

    You should do the calcs regarding starting vs running loads for the pump(s) size the PV accordingly and consider adding a battery bank. If a battery bank doesn't make sense, too much panel capacity is not a problem (IMHO).

    Good luck,

    Icarus
  • BB.BB. Super Moderators, Administrators Posts: 31,085 admin
    Re: Solar Panel Cut-Off Mechanism?

    Basically, solar PV panels are a relatively unregulated supply... The output voltage is dependent on temperature and load (and, obviously, the amount of sun). Solar panels have a relatively wide output voltage range (roughly two:one between open circuit cold and Maximum Power Point Hot).

    You would have to review your motor's (and controller's) specifications to know if it can survive on your solar panel load.

    The answer, is probably yes--but then the next question is how long do you expect the motor to last between brush changes and commutator resurfacing... Maybe a couple thousand hours between overhauls or 400 days (at 5 hours per day)? So, probably 1-3 times per year the DC motor would need servicing.

    If you could go with a nice motor designed for long service life in harsh environments (like "AC" pump motors) might be better for long term operations.

    I don't have any well experience, but the Grundfos SQFlex series of Pumps look really nice. Designed for both AC and DC input, wide range of voltages/power, and ability to (with the same pump) run solar panels, batteries and AC generator.

    I would stay away from DC Brushed motors unless there is some reason to use them locally (i.e., supporting local motor manufacturer, available supply, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Solar Panel Cut-Off Mechanism?

    Told ya smarter minds than mine,,,,,! Trust what Bill says!

    T
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: Solar Panel Cut-Off Mechanism?

    I think your issue is you won't have enough panels, not to much.

    The motor probably need more like 6 amps as a brushed DC motor is not that efficient. Also, since you have a pump controller, it will limit the amps to the motor, so it should prevent over powering the motor, check with the vendor if the amps out is programable. The Controller will probably also use 5-10% in running the converter, another loss and the heat will reduce the panels probably 20-25% in that type of climate.

    I ran for 9 months a 180V brushed DC motor, 1/2 HP direct off a 1.6kw array for 12 hours a day for a pool pump Eventually the brush wires failed ( excess current ), but if should give you an idea of margins here.

    With only 500watt ( peak ) , there isn't a chance you could harm a 1/3HP motor.Also, I second not using brush motors, there are inefficient and require service often
  • System2System2 Posts: 6,290 admin
    Re: Solar Panel Cut-Off Mechanism?

    Thanks for all the help, I am going to review this information with my team and come back with some more questions :D
  • mike95490mike95490 Solar Expert Posts: 9,042 ✭✭✭✭✭
    Re: Solar Panel Cut-Off Mechanism?

    The photo did not appear in your original posting.
    My question is this:
    is it a piston (positive displacement) pump, that can run slow
    or
    a vane/turbine type pump, that has some minimum speed requirement
    before it moves water

    Also, on a cloudy day, what will happen when the pump does not run? Will a backup generator run the same motor, or will you swap drive belts to another source ?

    An old style windmill can move a fair amount of water, over a 24/7 period
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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