off-grid 1.2 kW solar system need help with dump load

program.coordinator
program.coordinator Registered Users Posts: 13 ✭
in Off Grid Solar & Battery Systems #1
Another situation, This is a 1.2kW solar off-grid system, inverter is unknown at the moment, Charge controller is Outback MX80, and the battery bank is 4 12V 220Amp-hour batteries connected in series/parallel making it a 24V 440Amp-hour battery bank. The dump load is a water heater. I have never talked a person through the steps on what to add. I thought that you would need another charge controller to do different functions, I'm not positively sure. Another thought was to program the controller to cut off the array when the battery bank is full. Any advice? Thanks.
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  • offgrid me
    offgrid me Solar Expert Posts: 119 ✭✭
    #2
    Re: off-grid 1.2 kW solar system need help with dump load

    Generally you do not need or use a dump load with solar. The CC will regulate how much current at what voltage goes into the batteries and will reduce these levels down to the floae value when the batteries are fully charged. If you want to use excess pv energy after the batteries are full some charge controllers have an aux relay control that can be programed to trigger when float is entered so you can then feed the water heater.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: off-grid 1.2 kW solar system need help with dump load
    program.coordinator wrote: »
    Another situation, This is a 1.2kW solar off-grid system, inverter is unknown at the moment, Charge controller is Outback MX80, and the battery bank is 4 12V 220Amp-hour batteries connected in series/parallel making it a 24V 440Amp-hour battery bank. The dump load is a water heater. I have never talked a person through the steps on what to add. I thought that you would need another charge controller to do different functions, I'm not positively sure. Another thought was to program the controller to cut off the array when the battery bank is full. Any advice? Thanks.

    A wind system needs a dump load to maintain a power draw from the turbine at all times to keep its speed under control. When no current is needed for the batteries, it goes to the resistive dump load.

    Since a solar PV system is perfectly happy and healthy providing any output from maximum down to zero, depending on what the battery needs, you are not talking about a classic "dump load."
    Instead, you have an "opportunity load". Something which does not need to be powered from the panels, but might as well be whenever there is power to spare.

    A Charge Controller intended for PV use will, as you suggested, just "cut off the array" when the batteries are full. However if you want to make some use of that energy, many CCs have an auxiliary relay output which will be turned on when the CC is in Float. You can then use that to drive a larger relay which connects the panels directly to the water heater resistors. You should not need any kind of extra charge controller for this purpose, since as long as the panels generate less power than the heating element is rated for, it will not damage the heater.

    However, if you really want to get the most power to the heater, you do need to match the resistance of the heater at least approximately to the MPP of the panels.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: off-grid 1.2 kW solar system need help with dump load

    In addition to what offgrid me said, this is a backwards way to design a system. Off grid system need to start by knowing what the load requirements are. That dictates the size of the inverter and battery bank, and the size of the battery bank determines how big an array and what charge controller to use.

    As it is the power output has been limited by the battery bank size to a maximum of about 5 kW hours. The array at 1.2 kW is slightly small for the battery size but would work.

    In this case the controller would be an Outback FM80, which does have the AUX function that offgrid me mentioned.
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  • program.coordinator
    program.coordinator Registered Users Posts: 13 ✭
    #5
    Re: off-grid 1.2 kW solar system need help with dump load

    Thanks for the info, another engineer is now in control of this project because its all the way in Africa, but he will be doing this kind of set up just using the right size resistive heating element for the water heater. The solar array does not generate that much power over the capacity of the battery bank, but the client does not want to waste any energy.
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