Design of off-grid system

gactrxgactrx Solar Expert Posts: 72 ✭✭
Hi, I think I have most of my gear sorted and sized. I have a wind generator to though in the mix as well. I have drawn up a plan which will obviously need some refinement but it's a start. I want to use the wind generator mainly to heat water as I have enough panels for everything else. I have attached the drawing. It's possible I can use the Midnite auxiliary output and a 100A Crydom SSR I have but the SEA440 dump load relays look pretty robust. Attachment not found.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    A couple of quick observations...

    First, you can let the MPPT controller charge to >26.0 volts. Add the solar panel output to the wind turbine output to heat water (and run other loads).

    Second, be careful about the Silicon relays... Apparently there are a lot of counterfeit products out there.

    And mount the SSRs on good heat sinks. They tend to run hot and will fail early if not properly heat sinked.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gactrxgactrx Solar Expert Posts: 72 ✭✭
    BB. wrote: »
    A couple of quick observations...

    First, you can let the MPPT controller charge to >26.0 volts. Add the solar panel output to the wind turbine output to heat water (and run other loads).

    Second, be careful about the Silicon relays... Apparently there are a lot of counterfeit products out there.

    And mount the SSRs on good heat sinks. They tend to run hot and will fail early if not properly heat sinked.

    -Bill
    Hi, do you mean actually connecting the DC output of the wind generator to the output of the solar panels before the Midnite controller? How would that affect the MPPT tracking? Would it turn the wind generator into a windmill on a sunny day?
  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    gactrx wrote: »
    Hi, do you mean actually connecting the DC output of the wind generator to the output of the solar panels before the Midnite controller? How would that affect the MPPT tracking? Would it turn the wind generator into a windmill on a sunny day?

    No. Not physically. In general, you want:
    • solar solar panels -> solar charge controller -> battery bank.
    • Wind turbine .-> rectifier -> battery bank.
    • Battery bank -> dump controller -> resistor bank
    I am suggesting that if you want to (for example) make the resistor bank your water heater--You want both the wind turbine (excess) power heating the water, and the solar panels also heating the water (when the sun is up). It sounds confusing, but you want the solar panel power+wind power to both go to heating water (if your intent is to get lots of hot water).

    Note that there are "issues" with using a water heater as a dump load... If you have lots of wind and your 40 gallons of water are hot--Then what happens. If you continue to "heat the water", you will eventually generate too hot of water and generate steam. So--You need a way to either shut down the wind turbine or a second resistor load bank to turn on when the water heater is turned off.

    And you cannot turn a typical wind turbine into a powered fan. The standard wind turbine these days use an Alternator and Rectifier (AC to DC diode assembly). This is a "one way" power conversion. You cannot feed DC power "backwards" through the diodes and cause the alternator to turn (like a motor). An alternator is really a type of three phase motor--And you would need to feed it 3 phase AC power (without the diode rectifier) to turn it into a running motor.

    There are (typically) older wind turbines that use DC Generators--These are brush type (universal) motors--And if there was a battery connected to a generator output (or solar panels)--It could turn the wind turbine into a (very poor) powered fan.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gactrxgactrx Solar Expert Posts: 72 ✭✭
    I see thanks. Looks like I'll need to get another relay to run from the hot water thermostat. One with a 24v coil. I have updated my drawing which I think will work. I have a coil resistor that came with the Soma 1000 for a dump load. The wind generator will default to the resistor in case the coil burns out or a wire drops off.
    I'll get an SEA-440 plus another of the contactors they use. Pity the $NZ is a bit low at the moment. The Crydom SSR should be a good one as I bought it from Digi-Key.
    Cheers,
    On second look at the diagram, I haven't accounted for when the hot water is hot and thermostat opened but the batteries are not fully charged and it's a windy day. I could put a manual over ride switch for the dump contactor that is a risk as I could forget to switch it back. I'll check in the Midnite forum as I think I should be able to do something with the aux output on the controller.
  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Interesting idea on how to add a dump load to the wind turbine... I have not seen anyone do that before. Should work OK.

    Let us know how it works out for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    BB. wrote: »
    Interesting idea on how to add a dump load to the wind turbine... I have not seen anyone do that before. Should work OK.

    Let us know how it works out for you.

    -Bill

    It is the same relation to the battery that a standalone pure shunt dump controller would use.
    Not having ever used one, I cannot comment from experience, but I would worry about the high cyclic current out of the battery (microcycles) eventually taking a toll on its life.
    if you size the load so that even with low turbine output it does not draw enough to drain the battery, you might have a working idea. My other concern is that you would have to also size the load so that an overvoltage from the turbine in a high wind does not end up overcharging the battery.
    Those are among the reasons that a standard dump is often connected on the turbine side of an isolation impedance of some sort. The other factor is that there are not any other uncoordinated sources of charging current.
    SMA SB 3000, old BP panels.
  • gactrxgactrx Solar Expert Posts: 72 ✭✭
    inetdog wrote: »

    It is the same relation to the battery that a standalone pure shunt dump controller would use.
    Not having ever used one, I cannot comment from experience, but I would worry about the high cyclic current out of the battery (microcycles) eventually taking a toll on its life.
    if you size the load so that even with low turbine output it does not draw enough to drain the battery, you might have a working idea. My other concern is that you would have to also size the load so that an overvoltage from the turbine in a high wind does not end up overcharging the battery.
    Those are among the reasons that a standard dump is often connected on the turbine side of an isolation impedance of some sort. The other factor is that there are not any other uncoordinated sources of charging current.


    Hi, thanks for your reply. Couple of things I'm not sure about. Why would I get high cyclic currents? I'm guessing this may be because each time the contactor closes to supply the element, the bank voltage would drop below say 24 volts and the SEA440 sensor would open the contactor again then repeat the cycle?
    The element I have is a 3 x 2 ohm so in parallel that's 36 amps or 860 w at 24 volts.
    What do you mean by an isolation impedance?
    Not sure what you mean by an uncoordinated source of charging current.
    Thanks,

  • jackamofinajackamofina Registered Users Posts: 1
    Hi, I am just starting my solar project so I may be clueless but in looking at your design I see that the charge controller will stop charging at 26 volts but your dump does not trigger until the bank hits 27 volts. So your dump will in effect never trigger on a no wind, sunny day. Since you have a dump load to protect your batteries why would you need to limit the solar input at all?



  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    You are correct... Normally a "series" type charge controller is used--The PWM/MPPT controller between the solar panels and the battery bank. Because solar panels can simply be turned on/off like a light switch (the controller can use less than full available current from the solar panels)--It turns out that the battery banks are more accurately recharged and will last longer with a series type charge controller.

    Shunt/dump/diversion controllers are connected from the battery bank, to the dump controller, then to the dump load (typically a resistor bank). In general, the battery bank is either being "charged" or "discharged" (i.e., full current into battery bank minus any loads, or full dump current from battery bank plus any loads). So, while the charging source + dump controller is operating, the battery is being either "over charged" or "discharged" -- Never just sitting at "float" charge while the load current out equals the charging current in.

    When you have an "opportunity load" (say a water pump or or other "useful" load like an electric water heater)... You would want the series controller (i.e., typically solar panels) to be charging a full on along with the other charging sources (wind/water turbines, etc.). Then you can run the opportunity load for a longer period of time (because all charging sources are working together).

    Hybrid AC inverters (ones that are capable of off grid battery operation or GT inverter operation)--They put the hybrid inverter in "GT Mode" and vary the power through the inverter (back to the utility lines) as a "variable" dump load--I.e., the MPPT solar charge controller to battery bank is running 100%, and the GT inverter mode is varying the power feed to the utility... And the battery bank (on average) sites at "float voltage".

    As you can see, this stuff gets complicated pretty quickly. And there how you configure your series and dump controllers may differ--Depending on if you have a Resistor Bank (unused power goes to "waste" heat) or if you have a "useful load" (water pump or water heater)....

    And you have to worry about conflicting programming (what if your solar panels+wind turbine can supply more energy than your water heater load can take), or what if something fails (your grid power goes down and cannot act like a dump load, a thermostat fails, a resistor/heating element fails, etc.). If you have a diversion/dump load+controller--Good design would require a second redundant dump controller system just "in case".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gactrxgactrx Solar Expert Posts: 72 ✭✭
    Hi, I am just starting my solar project so I may be clueless but in looking at your design I see that the charge controller will stop charging at 26 volts but your dump does not trigger until the bank hits 27 volts. So your dump will in effect never trigger on a no wind, sunny day. Since you have a dump load to protect your batteries why would you need to limit the solar input at all?


    Hi, thanks for your comments. I haven't quite worked out the finer details yet and the voltage probably need more thought. I'm hoping to design something robust with out using an SSR if possible. Worst that can happen hopefully is I burn out the cylinder element. The cylinder is vented to atmosphere.I have everything in the drawing except the contactors. Just working out how to fit them all together without having to buy anymore expensive electronics.
    Cheers,
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Hi, I am just starting my solar project so I may be clueless but in looking at your design I see that the charge controller will stop charging at 26 volts but your dump does not trigger until the bank hits 27 volts. So your dump will in effect never trigger on a no wind, sunny day. Since you have a dump load to protect your batteries why would you need to limit the solar input at all?



    The dump load is just there to protect the wind turbine against finding itself without a load when the batteries are full. If you put a CC between the wind turbine and the dump controller you prevent that safety feature from working. That is why the diagram shows the turbine output (rectified) being connected directly to the batteries without either a PWM or MPPT CC.
    SMA SB 3000, old BP panels.
  • gactrxgactrx Solar Expert Posts: 72 ✭✭
    inetdog wrote: »

    The dump load is just there to protect the wind turbine against finding itself without a load when the batteries are full. If you put a CC between the wind turbine and the dump controller you prevent that safety feature from working. That is why the diagram shows the turbine output (rectified) being connected directly to the batteries without either a PWM or MPPT CC.

    Hi, I've made a change to my diagram. With this, I can switch the wind generator in and out pretty much like a solar panel. The controller using aux 2 will make sure the wind generator and panels are in circuit with charging potential when ever the bank is not fully charged. I'll still need the SEA-440 controller to make sure I'm not pulling the bank capacity down too far with the heating element.I can't use aux 1 as I have a WizBang Jn which uses that but I may sacrifice this if it seems a better idea to control the element contactor with aux 1 which is a PWM output.Maybe another question for the Midnite forum.
    Cheers,

    Attachment not found.
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