12V heater from solar water pump panels

mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
I have a design in mind that I would like to get some feedback on. I have an existing well at my off-grid cabin, and I'm preparing to install a Grundfos SQFlex solar pump in the well (I currently have a 110V pump in the well that runs off my generator). To run the pump, I'll use (3) 175W panels wired to get a combined ~100V to run the pump (via a Grundfos control box). However, I'll only need to pump water Mar-Nov, so I'm looking for a way to utilize the solar panels during the winter (when not needed for pumping water). My plan is to have the panels operate a small 12V heater to leave in the cabin, just to help keep the temperature from freezing inside the cabin during the weeks between visits. My thought is that I would run a line from the panel combiner box to both the pump control box as well as to the cabin for the 12V heater. The DC wiring from the combiner box to the 12V heater in the cabin would run through a MPPT charge controller (to step down to 12V). The heater would be then connected to the charge controller via the load control "out" terminals on the charge regulator. My plan would be that the heater would be always on, but would only run when the panels are putting out enough power to turn it on and run it. It wouldn't put out much heat, but it's a small cabin (~1000 sq ft) so my hope is that it would be enough to help keep the cabin above freezing. I've attached a schematic of the proposed system and wiring. Can any of you see any flaws in my logic or proposed set up? Thanks.

Comments

  • mike95490mike95490 Solar Expert Posts: 9,010 ✭✭✭✭✭
    Re: 12V heater from solar water pump panels

    Leaving a window shade open to allow sunlight in, will heat the cabin more than 300W of electric heat would.
    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 ,

  • BB.BB. Super Moderators, Administrators Posts: 31,038 admin
    Re: 12V heater from solar water pump panels

    Where you planning on having a 12 volt battery bank with the MPPT charge controller and heater?

    I believe practically all MPPT charge controller need a working battery connected to the DC charge outputs before they will function correctly with your solar panels.

    I don't think any MPPT will work without a battery connected.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: 12V heater from solar water pump panels

    You seem to be going about it in a way that wont work.
    Just connect the output of the panels to a 110?v 1000w ?heater.. no charge control losses. wire wound heaters dont care if its AC or DC ..An MPPT controller wont work without a battery as already told to you.
  • mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
    Re: 12V heater from solar water pump panels

    Thanks for the feedback - it's all good to know. I wasn't aware the MPPT controller wouldn't work without the battery connected. I'll have to look at another option to utilize the watts generated during the winter months.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: 12V heater from solar water pump panels

    gave you the solution..it simple and as effective as you are going to get.
  • mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
    Re: 12V heater from solar water pump panels

    John P - Sorry - just getting back to this post after being away for a while. Can you give me an example of a 110v 1000w wire wound heater. I may be showing my ignorance here, but I'm not familiar with what kind of AC heaters can be wired directly from DC input. I'd love to have some way to utilize the 450W of panel output from Nov-Mar to heat the cabin, but my primary concern is no risk of fire, etc. During Nov-Mar, I'll only be visiting the cabin every 6 weeks, so I need the solution to be maintenance free and completely safe. But it sure would nice to be able to have something that could heat the place up (even if it's slightly) - when we show up, the inside of the cabin can be 40 degrees, and it takes a long time to warm it up to 65 degrees with our wood stove.
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: 12V heater from solar water pump panels

    Why do you care if the cabin freezes? If you have a system that "might" keep the cabin from freezing, then you have to cover the precaution that it "won't" keep it from freezing, by draining the water etc.

    It is been my experience in over 50 years of living with freezing cabins, that most things will take a freeze just fine. If you drain the water and the drains etc, then figure out what else shouldn't freeze.

    A bit of our food stuffs don't like to freeze, but most can goods can take a freeze quite well. Our rule is one winter, and after that the quality suffers. The only thing I have had freeze and get damaged is my distilled water jug, that I forgot in a battery box one time when I went away.

    Tony

    PS. I think that you proposing a solution in search of a problem. Where I live, 300 watts of part time heat might raise the ambient 1 degree, even with only 600 ft sq, well insulated.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 12V heater from solar water pump panels

    there are ways of doing just what you propose, but 1> i would be leery due to possible fire hazards and 2> it is still dependent on solar energy so why not do a more efficient thermal collector? it will not guarantee the cabin won't go below freezing because i can't guarantee the sun will shine all of the time or enough on those really cold days. if you have windows facing south then leave them unobstructed (ie, open the curtains or shutters) so as to passively collect solar energy.
    if you still feel you would like to utilize some of the solar collected from the pvs then about the safest way i can think of doing it is to wire up many 100w or higher 120vac incandescent lights bulbs in parallel connected directly to the 3 pvs in series and you can optionally use some kind of mechanical thermostat that can handle the currents being passed at that voltage. the thermostat is a challenge and remember that light bulbs generate enough heat to cause fires. do this at your own risk and i will not be held liable for anything that may happen in your doing this.
  • mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
    Re: 12V heater from solar water pump panels

    I'm not worried about keeping the cabin from freezing - just trying to help keep the temperature up a bit so that when we do go up there every few weeks in the winter, it will take us less time to get the temperature up to 65-70. The origination of the whole issue is that I have 525W of PV panels that aren't used in the winter - I use them to drive a solar well pump for irrigation in the spring through fall, but I shut the pump system down in the winter. More than anything, I'm trying to find a use for the watts generated in the winter so they're not wasted - I though a heater was a possibility, but maybe not.

    I have 400 watts of other panels dedicated to some DC lights, DC booster pump for water pressure in the cabin. Maybe I can route the 525W of separate panels into my other 400W and battery bank, but the two systems are different voltages, etc. Could that work? Can a larger charge controller take two separate inputs with different voltages (one at 95V and the other at 48V)?
  • BB.BB. Super Moderators, Administrators Posts: 31,038 admin
    Re: 12V heater from solar water pump panels

    The solution for the two different Vmp arrays is to use two MPPT charge controllers connected in parallel to the same battery bank.

    Regarding the two separate systems--It may be a good idea to eventually combine the two separate systems (separate battery banks?) into one central system.

    But that does depend on your needs--if the pumps are quite a distance from the cabin--then you have the issue of sending lots of low voltage current long distances (uses lots of copper wire); or sending medium voltage DC (solar panels at Vmp=100 vdc or so) to the charge controller at the cabin; then sending AC power back to the remote site where the pumps are (expense of AC inverters plus possibly changing to AC pump costs)...

    To a degree--you have to back up a moment, say where (and how much) power is being used at each location (if physically separated) and how much sun/solar panel space/existing panels you have available to build out an updated system.

    Many of us here are a believer in designing the system on paper first before spending money on hardware that may not get you closer to your final goals.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: 12V heater from solar water pump panels

    Just for grins,,, the BTU value of 1 kwh of electricity is 3340.

    So your 300 watt heater would under ideal circumstances put out ~1000 BTU/hour.
    Over an 6 hour winter period, that would be ~6000 BTU/day. My wood stove is supposed to put out ~ 30,000 BTU/hour, a Rinnai propane heater will make better than 35k into the room per hour.

    You have to wonder how much a solution in search of a problem you are trying to create.

    Tony
  • mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
    Re: 12V heater from solar water pump panels

    It seems like a likely option would be to wire the panels to two separate controllers, and wire them in parallel to the same battery bank (and then figure out how to use the combined watts during the winter). Can someone give me an example of specifically how to wire the two controllers in parallel? I'm not familiar with how that wiring would work. Thanks.
  • BB.BB. Super Moderators, Administrators Posts: 31,038 admin
    Re: 12V heater from solar water pump panels

    Just wire up each charge controller as normal, and attach both directly to the battery bank (bus connection, +/- connection point, etc.).

    What you don't want to do is run one set of wires to charge controller A then carry on to charge controller B... That can cause both charge controllers to read "higher" voltage because of the shared current--Plus the two controllers could interfere/confuse each others (say one is PWM and the other is MPPT).

    If both charge controllers are the same brand/model (like Outback)--then some have the capability to network together and operate in sync (transition from absorb to float, share a single remote battery temperature sensor, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mjp24cohomjp24coho Solar Expert Posts: 104 ✭✭✭
    Re: 12V heater from solar water pump panels

    Thanks, Bill. I currently have a morningstar MPPT 15A controller with (2) 200W panels. My plan was to get a Morningstar Tristar 45A MPPT controller for the (3) 175W panels. Even though they're both the same brand, I imagine they wouldn't be able to communicate as you mentioned. The alternative would be to purchase to Morningstar TriStar 45A MPPT controllers (or any Outback), but I'd prefer not to have to boy two new $400 controllers.
  • LucManLucMan Solar Expert Posts: 223 ✭✭✭
    Re: 12V heater from solar water pump panels
    john p wrote: »
    You seem to be going about it in a way that wont work.
    Just connect the output of the panels to a 110?v 1000w ?heater.. no charge control losses. wire wound heaters dont care if its AC or DC ..An MPPT controller wont work without a battery as already told to you.

    Is there any chance of overloading the panels with this type of connection?
  • BB.BB. Super Moderators, Administrators Posts: 31,038 admin
    Re: 12V heater from solar water pump panels

    No, solar panels are current limited (current sources)... Their output current is a function of the amount of sunlight hitting the panel. The solar panel outputs about the same amount of current operating at Vmp as it does near Zero Volts (dead short). So, the panels cannot be overloaded by just adding a passive load (connect a battery/power source backwards to a solar PV panel--the panels will be toasted).

    In general, unless you have some sort of MPPT type device (perhaps a linear current booster type device) between the solar panels and the resistive load, the overall efficiency of such a setup will be very low (low single digits?) per watt of sunlight.

    Would be much better off with solar thermal (hot water, hot air) which will easily be in the 50-80% efficiency range.

    For a similar amount of heat from a solar PV system--you are probably looking at 10-50x or more the square footage of solar PV panels vs solar thermal. Let alone the huge increase in system costs for all of those solar PV panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 12V heater from solar water pump panels

    concerning this,
    Originally Posted by john p viewpost.gif
    "You seem to be going about it in a way that wont work.
    Just connect the output of the panels to a 110?v 1000w ?heater.. no charge control losses. wire wound heaters dont care if its AC or DC ..An MPPT controller wont work without a battery as already told to you."

    the idea is sound that it will just dump into a wire resistance load like a heater, but the voltage or waveform will not suffice to operate the fans integral to many of these heaters and in some of them it is important that the fan operate. this is why i thought of just dumping into a light bulb as it is a resistive load too that would not need a fan. it may give off a bit of light as well, but most times i believe it would be dim. yes, bulbs can burn out, but they are cheap and if a heater burns out it is down for the count. 1 bulb out won't stop the others from still giving off heat and the slightly lower voltage would allow the bulbs to last a bit longer than most of us normally see.
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