Can 30v PV be put on a 12v system?

help!help! Registered Users Posts: 9
This is an off-grid, remote south-central Colorado mountain-top location with an RV and storage building, which we currently only use occasionally....to become a full-time home when we finish building.

We have a 1040w, 12v system on the storage building roof, but need double the power capacity (another 1000+ watts). It currently has an Outback FM80 FLEXmax MPPT charge controller, a Sunforce 4500w12v pure sine inverter, and 8 SunXtender 305ah 6v Deep Cycle AGM batteries. The system also has a transformer to run the 220v water well pump. The RV has 2 deep cycle batteries (not AGM). Later, we'll probably buy cheaper charge controllers for the storage building and use our two Outbacks on the house, and possibly convert this 12v system up to 24v -- but not now.

We'll soon build a house with a 48v system of about 4000 watts PV on the roof plus a 1500-2000w wind turbine. We plan to divert or "dump" the excess wind load (when the batteries are full) into an electric water heater with a heating element changed out to 48v, but would need more "dump" if no electricity or water were being used, and resistors seem such an expensive waste.

Questions:

1) I'm considering panels that are 36.76 Voc open circuit/29.38 Vmp maximum power voltage for the house 48v system. CAN THESE PANELS ALSO BE USED IN THE 12V SYSTEM (charge controlled down to +12v-), or must I buy lower voltage panels to add to the storage building system?

2) Is there a website showing different ways to wire a PV/wind system? I know the turbine needs its own controller and can be wired so that only the wind wattage (not the panel wattage) needs to be diverted, but it's unclear how to trigger the flow to the water heater element when the batteries are full, and how to trigger the flow to go to resistors if the water has reached top temperature.

3) Can wind turbines be "disconnected" from the system once installed, when no electricity is being used?

4) Would an "overflow" or second water heater take any potential extra "dump", or could it, too, reach capacity if no electricity is being used?

5) Should resistors be mounted inside, next to the turbine charge controller, or outside?

6) Is it safe to plug in an electric space heater (in lieu of resistors) to take the excess "dump" if the water heater has reached top temp?

Comments

  • bmetbmet Solar Expert Posts: 630 ✭✭
    Re: Can 30v PV be put on a 12v system?

    I can only assist with Question 1.

    Yes, with the right charge controller. I am not familiar with the Outback products, but this is one I am considering.

    http://www.solar-electric.com/mosumpsochco.html

    I am purchasing a 24v panel to charge a 12v Power Pack. Later, that panel could be used for well pump duty. This controller is very flexible for smaller systems like mine.
  • BB.BB. Super Moderators, Administrators Posts: 29,712 admin
    Re: Can 30v PV be put on a 12v system?
    help! wrote: »
    1) I'm considering panels that are 36.76 Voc open circuit/29.38 Vmp maximum power voltage for the house 48v system. CAN THESE PANELS ALSO BE USED IN THE 12V SYSTEM (charge controlled down to +12v-), or must I buy lower voltage panels to add to the storage building system?
    As Bmet says, you can use most MPPT charge controllers to take higher voltage/lower current Solar Array Vmp and efficiently down converter to the lower voltage/higher current used to charge a battery bank.

    More or less the DC equivalent of an AC step down transformer (technically a buck type switching power supply).

    There are a lot of different MPPT controllers with differing input and output ratings... You still need to match the array to the controller to the battery bank in the end.
    2) Is there a website showing different ways to wire a PV/wind system? I know the turbine needs its own controller and can be wired so that only the wind wattage (not the panel wattage) needs to be diverted, but it's unclear how to trigger the flow to the water heater element when the batteries are full, and how to trigger the flow to go to resistors if the water has reached top temperature.
    More or less--just think of the battery as the heart of your system and just focus on each device that hangs off the battery (appropriate fuse/breaker connected to the battery common bus point(s)).

    You hook up the solar PV charge controller like there is no other battery / controller connections.

    And you connect the wind turbine dump controller the same way--just to the same two battery bus points. Does not matter if there is a solar PV controller, an AC charge controller, etc...

    Programming wise, you will have to make some choices... You could program the Solar PV array controller to always charge at 14.8 volts, and the Wind Turbine Controller to charge at 14.4 volts. That way, both the "excess/unused" solar PV array energy and the wind turbine energy can be sent to heat your hot water.

    Otherwise, you just program the solar controller as normal (14.4 volts / 13.6 volts float) and the dump controller at ~14.7 volts--so when there is excess wind turbine power, the controller will dump only that to the water heater load.

    With diversion/dump controllers--the NEC recommends (requires?) that two parallel controllers be used... So if one fails, the other one is still working and keeping your battery/turbine from failing from overcharging/overspeeding.

    And generally speaking... Wind / Diversion controllers are not that accurate at charging and maintaining/floating a battery bank accurately.

    From a battery point of view--you should plan on the Solar PV charge controller as your main battery charging device. The wind diversion controller is just there to drain off excess energy when the winds kick up.
    3) Can wind turbines be "disconnected" from the system once installed, when no electricity is being used?
    I assume it depends on the exact brand/model of turbine--But in general, you have a switch that simply shorts the output of the turbine to itself (or three switches if three phase output). The dead short will normally keep the turbine from spinning very fast and keep the blades stalled. In some cases, it has happened that a shorted/shunted turbine will overheat and cook the windings.

    If you have the option, furling, mechanical brakes, feathering the blades are all good to have in case one of the shut down options fails.
    4) Would an "overflow" or second water heater take any potential extra "dump", or could it, too, reach capacity if no electricity is being used?
    Always plan for the worst case (and nobody home to know that something happened). You could use a second temperature sensor in the water heater to switch on a resistor bank. There is all the T&P (temperature and pressure) relief valve set to around 210 degrees / 150 PSI (vent valve water outlet to someplace safe).

    And there should be a tempering valve on the dump tank if the water temperature an exceed 140F (as I recall, I am not a plumber) to reduce the chances of scalding.
    5) Should resistors be mounted inside, next to the turbine charge controller, or outside?
    I would install them in a large room/garage/out building to prevent water/weather damage. I would not mount them underneath the battery bank, charge controllers, or inverters (don't want to overheat your electrics). But it all depends on how your bank is rated.

    6) Is it safe to plug in an electric space heater (in lieu of resistors) to take the excess "dump" if the water heater has reached top temp?[/QUOTE]
    Lots of questions here--You want the dump load to be reliable (and redundant) to prevent equipment damage/injuries.

    Could you find a XX volt dc space heater and use it as a load--Probably... Again, a real space heater has over temperature devices to limit the chances of fire if some clothing falls on it or the heater gets knocked on its face. All potential points of failure...

    How often will your wind turbine generate excess energy--probably not that often (few hours during stormy weather?)... But you know your winds better than I.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Can 30v PV be put on a 12v system?

    the fm80 will downconvert to 12v so this is a possibility. the battery voltage you settle with will need the turbine to output for that same battery voltage. there's no mix and match as both need to have the same battery voltage no matter how versatile an mppt is at being able to go with other voltages.

    the dump load needs to handle all of the turbine's output or it could burn up the load set up for it and when that goes the turbine is in danger. now turbine/solar mixes can get a bit tricky as others have posted on in the past, but it is doable with the right planning.

    the best dump loads going right now are resistive heating elements be it for air or water, but it must be rated for all of the turbine's potential output.
  • help!help! Registered Users Posts: 9
    Re: Can 30v PV be put on a 12v system?

    This location is a meadow 600 ft below the mountain peak, and winds vary from nothing to 40/50mph (code minimum for buildings is 90mph), day and night, year round. A turbine at the peak would give us constant wind, but much electricity would be lost in the wiring distance down to the meadow.

    I'm finding, the more I research, that wind is a different animal from solar, and complicates everything because excess load must be diverted. That said, it's too valuable to ignore, especially at night and on overcast days. We really would like a vertical turbine for its silence, but will probably settle for a much, much cheaper horizontal -- IF we can find a quiet one. Installed home wind turbines are so scarce, I have to rely on YouTube to see how noisy a particular model is.

    We've been doing most of the construction ourselves, using vacation time during good weather, so I'm concerned about wind electricity buildup on the future house between our visits (especially in winter, when water tanks will be drained.) The best solution seems to be PLANNING for a turbine in the system, but not actually INSTALLING the turbine until someone's living there full time.

    Because of the cold weather, a solar hot water heater system wouldn't be terribly efficient, and we've been investigating heat pump, on-demand propane, and on-demand electric water heaters. We're planning as if propane isn't an option for heating, hot water, or cooking (survival mode, and plenty of free firewood available), but the on-demand electric water heater uses too much electricity. I'm hoping an electric water heater used as a "dump" for the turbine (and using excess solar power) will do the trick.

    What I don't know is if a heat pump could/should be used instead?

    The maintenance room (with cistern, pressure tank, pumps, water heater(s), solar controls, and batteries will be indoors, on concrete slab, next to an indoor laundry/gardening/hot tub room, and close to the garage, so backup "dump" resistors could go in one of these rooms.

    I know the turbine voltage must match that of the PV system it's connected to; I'm just unsure how to do wind, and information is scarce.
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