Intelligent switch for water heater.

Discussion in 'The Projects Forum' started by Energetic, Jul 9, 2011.

  1. Energetic

    Thread Starter Member

    Jan 3, 2011
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    I would like to build something that can sense current from a PV generator on the DC side and switch in an immersion circuit when the pv output is above 3kW (nominal immersion rating) or 1kW (retrofitted smaller immersion element)

    Any advice on the best way to approach this or referral to anything published would be great, electronics is not my forte...

    Thanks in Advance.
     
  2. Energetic

    Thread Starter Member

    Jan 3, 2011
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    sorry meant to post this in the projects forum...
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    I moved your thread to the "Project Forum" per your comment.

    hgmjr
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I don't really understand the switching decisions. What is the load before you switch on the heater? What sort of voltage and current is there in total, and how much is the immersion heater?
     
  5. Energetic

    Thread Starter Member

    Jan 3, 2011
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    0
    Apologies for incoherant ramblings...

    A standard immersion heater here for Domestic Hot Water is 3kW @ 230v mains AC supply. I want this to switch on then the PV generator is doing above 3kW so i am able to effectively store the excess power from the PV rather than exporting it to the public grid. I don't want it to be drawing from the grid when a cloud comes over and the output from the PV drops below 3kw.

    The DC output of the panels is variable dependant on the conditions. Due to this variation it may be better to do the sensing part after the inverter on the AC mains side where the voltage is at least reasonably constant.

    Another option is to use a smaller 1kw immersion element in the same manner but not all storage cylinders can be retrofitted with these and i'm not sure mine can yet...

    Hope this makes more sense.
     
  6. RFactor

    Active Member

    May 1, 2009
    33
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    Is the heater the only device on the PV or is it supplying other devices?
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Are we talking about a common water heater that usually includes two heating elements, upper and lower, with basic adjustable thermostats for each? The reason I ask is, unless I'm mistaken, a severe under voltage condition can't hurt them, as the elements are resistive. If the thermostats are electronic, that must see ~240V, then that's a separate issue.

    Does your inverter have an input lower limit before it will turn on?
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Sorry to be dense but I don't think we understand yet: What is your panel doing at 100W, at 500W, at 2900W? I understand that you want to heat water to dump "excess" power, but you haven't defined what "excess" means. What are the other loads? You can't suddenly kick on a huge current load (heater) without affecting them.
     
  9. Energetic

    Thread Starter Member

    Jan 3, 2011
    33
    0
    Hi Guys, thanks for sticking with me on this, i live and breathe PV at the moment and forget that not everyone else does.

    The PV feed from the inverter feeds into the standard domestic consumer unit via a standard MCB. Typically the daytime base load in the rest of the house is around 300w or less so at 3500w from the PV it would cover the whole house and the immersion with a bit to spare.

    The immersion heater is a typical 13A element used in the uk, it's controlled by a stat and has a thermal cutout. See link below for an example:
    http://www.screwfix.com/p/incolloy-immersion-heater-11/66532
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    Gee, I made a few comments but asked only one question, which is still unanswered. A standard MCB means nothing to me. Yes, I could google up on it, but I won't.
     
  11. Energetic

    Thread Starter Member

    Jan 3, 2011
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    Miniature Circuit Breaker, standard circuit protective device hereabouts. As for the inverter cut in voltage it is 175V.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Are we talking about the same thing? I'm referring minimum inverter DC input before the inverter will turn on and produce X voltage AC output. Actually, it would be very useful to see a curve of inverter input vs. output.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Is it not practical to dump excess power BEFORE it gets run through the inverter? That would make a lot more sense, to not run all that juice thru an expensive (and less than 100% efficient) inverter just to then heat water with it.

    If you can dump on the DC side, then you need to sense whether the inverter has what it needs, and dump the rest. I'm not saying I have a solution, just that maybe I'm beginning to grasp the question.
     
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