Summer cottage pv - use of excess pv in heating?

samppasamppa Registered Users Posts: 20
Hello!

I have an offgrid cottage, it's on an island. Solar pv system is 2x135w panels, 200Ah battery, mppt charge controller. It's mostly used for led lights, laptop charing, mobile phone charging etc.. I go to the cottage only in summer and early autumn, and often only for weekends. So most of time the battery is in float.. when I'm not there, there are no loads.

I'd like to use all excess pv power to provide a bit of heat. What would be the best way to do this?
Say get a 250w 12v ceramic car heater, and so that when battery is full, pv power is diverted to this heater. How do you do this diversion?

I know solar thermal would be more efficient, but I'm not interested in installing that right now.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Summer cottage pv - use of excess pv in heating?

    Several ways... The simplest would be to get a diversion/shunt/dump controller and have the controller "turn on" when the battery is full.

    A Xantrex/Schneider Schneider Electric C35 Solar Charge Controller configures as a diversion controller should work.

    C35 User Manual

    Note, the other solar charge controller should be set with a higher setpoint voltage--You want that controller to continue to dump 100% of its current into the battery bank and you want the diversion controller to "dump" the excess charging current into the heater.

    Note--If you have a PWM charge controller for the main solar charger, you could bypass it, but then you would have a safety issue. If the heater fails (the thermal fuse popping is not on common, especially if heater gets blocked by something), the battery would be over charged by the solar array. This can cause the battery to over charge or worse.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Summer cottage pv - use of excess pv in heating?

    In the real world, ith only 270 watts of panel, by the time you factor in any excess power time, you are likely to get next to no real useable heat out of such a system, some thing on the order of (best case) About 600 BTUs/hour. How many hours of excess APV do you have?

    You might simply try to put some heat in to domestic hot water preheat, with a diversion load into a water heater element.

    Tony
  • samppasamppa Registered Users Posts: 20
    Re: Summer cottage pv - use of excess pv in heating?

    Thanks! I've thought that using a diversion controller would be a good solution. C35 or C40 is quite cheap too.
    I'm there mostly on weekends, so the batterys are full most of the time.. there is no running water, water is heated by wood in a separate sauna for washing. So no electrical water heating. Cottage is heated by wood.
    Even a small heat source would make a difference over winter here: keeps the place and structures drier. Wintertime there is cold, but quite sunny with lots of snow.

    If this seems to work, I could always install more panel capacity too. It's quite cheap now.
    Problem with solar thermal is that I don't want to install air ducts.. and installing a liquid system is way too invasive here.
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Summer cottage pv - use of excess pv in heating?

    The approach mentioned by Bill with two independent systems will work, and work with what you have. If you decide to add some more solar, and need to upgrade the primary charge controller, then choosing one that has aux ports that can take care of/properly coordinate such "oppurtunity loads" will be a good idea. Some of the better ones even can control a solid state relay in pulse mode so that even partial spare power can be utelised.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • BB.BB. Super Moderators, Administrators Posts: 29,854 admin
    Re: Summer cottage pv - use of excess pv in heating?

    The Midnite Classic family of MPPT charge controllers appears to have one of the more advanced "waste not/want not" functions. Hook that up to a relay/solid state relay.

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

    But they are not cheap...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jebattyjebatty Solar Expert Posts: 56
    Re: Summer cottage pv - use of excess pv in heating?

    Apart from the switching control, why not just use an incandescent light bulb to soak up the power/heat?
  • ILFEILFE Solar Expert Posts: 364 ✭✭
    Re: Summer cottage pv - use of excess pv in heating?
    jebatty wrote: »
    Apart from the switching control, why not just use an incandescent light bulb to soak up the power/heat?

    I would imagine that wouldn't be the best choice to make, due to the fact that he stated:
    samppa wrote: »
    I go to the cottage only in summer and early autumn, and often only for weekends.

    I am under the impression he wants a more reliable diversion load?
    Paul
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