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  • Minnesota is getting cold, solar heat?

    I am interested in how to get some solar panels to generate heat in a small well insulated shed that I have.

    I would like to start with two or four 315 watt panels that I already own combined to simply make heat in the shed. I am interested in something as primitive as using no controller to convert the raw current from the panels to BTU's thru wire to the shed, a short distance away.

    Wiring something like a heating element coupled to thermal mass.

    Ideally I will get something going that would be scale-able to add panels/elements to be able to keep it above freezing in the shed over winter.

    Any advice on where to start would help, Thanks for reading.

  • #2
    You're putting the cart before the horse. Start by calculating a first approximation of the shed heat loss rate per hr. per degree of temp. difference between inside and outside temp.
    Then, estimate what your panels will supply per heating season using your chosen or available panel orientation. Without batteries to store the gathered solar energy you'll need to consider how to store the heat generated by the PV panels. That will most likely involve some form of thermal mass (and perhaps some form of added insulation and weatherstripping to tighten the shed. Do the weatherstripping first. Insulation and weatherstripping is much cheaper and initially more effective than thermal mass. Also, thermal mass works best in conjunction with a lot of insulation and weatherstripping, and for your application I'd suggest not using something besides water which would probably make resistance heating elements ineffective.
    Also, thermal mass by itself is much less effective without a reduced building heat loss coefficient. What you want is a long thermal time constant roughly expressed as: (effective thermal mass/effective building heat loss rate). So, a lot of insulation will mean a lower denominator of that equation and so a longer thermal time constant as well as a lower heat loss rate.

    FWIW and IMO only, for all the hassle and inefficiency of using PV to directly heat a shed without battery storage, you might consider that you'd perhaps be money, hassle and complexity ahead by adding a good size south facing window with moveable insulation for nighttime use and/or when the sun isn't shining. Another alternative is a DIY air heater with a blower motor. Either one will harvest about twice as much or more net heat per square area as any PV panels. The air heater is also easily scalable. The PV not so much. Then, get s bunch of cement blocks, paint them flat black and expose them to the sun as it shines through the window.
    Most effective solar energy applications are not rocket science.

    Using PV to generate relatively low entropy and very versatile electricity and then turn that electricity into dumb heat is like using a chain saw to cut butter or gold bars as paper weights - a waste of something called availability.

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    • #3
      Thanks for the advice.

      I was concerned that when I posted here I would only get answers to questions I did not ask.

      Now back to the question: Could someone suggest a heating element that I could direct connect to the panels to produce heat even if it was wasteful. Maybe an old toaster sitting next to a brick?

      Imagine the shed is two cubic meters and super insulated to start. Also imagine that I have unused panels and could use them with no additional cost, so for this situation let's think of them as free.

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      • #4
        Originally posted by kengineer View Post
        Thanks for the advice.

        I was concerned that when I posted here I would only get answers to questions I did not ask.

        Now back to the question: Could someone suggest a heating element that I could direct connect to the panels to produce heat even if it was wasteful. Maybe an old toaster sitting next to a brick?

        Imagine the shed is two cubic meters and super insulated to start. Also imagine that I have unused panels and could use them with no additional cost, so for this situation let's think of them as free.
        Disregard the information I provided. My apologies for offering assistance in areas you're ignorant about but that I believe can be at least informative and perhaps helpful.

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        • #5
          Anyway now that we have my welcome to the board out of the way does anyone know of a heat element that could be directly wired to panels to produce heat?

          Comment


          • #6
            You could use some electronic power resistors from a supplier like
            Jameco, the power rating and resistance need to be matched up to
            a relatively efficient operating point of the panels. If you do not work
            out a reasonable match, most of your PV generated energy will be
            lost, this has been discussed here before. Incandescent bulbs work
            pretty well connected to the line, but have about the worst response
            to solar curves.

            The way most of us solve that problem, is to bury the water lines
            deeply enough to protect them from the coldest weather.
            good luck, Bruce Roe in oft frozen northern IL

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            • #7
              I searched and found some good info here about matching the panels to the load when someone had a similar question. Just ignore the part about it being for a water heater.
              https://www.solarpaneltalk.com/forum...mersion-heater
              Dave W. Gilbert AZ
              6.63kW grid-tie owner

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              • #8
                Azdave,
                Thanks for help with my question. Your information is a step in the right direction.

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                • #9
                  Ideal resistance is only good for 2 hours a day and only with ideal sun. In winter you seldom have that unless in the desert. Studies have shown that the ideal resistance can almost be doubled and the average daily production of heat will be more than with the "ideal resistance." Panels are current producers and power is a square function. Drop the current a little and power will drop dramatically unless a power point controller is used. Although power point controllers are quite simple, they are way beyond your capability and commercial products are quite expensive. In selecting a heater, use one which is considerably higher in resistance. Below is an example of how element miss matches dramatically reduce power and a picture of a 1500W power point controller. Using low voltage heater elements like 12V elements is insane. 12V panels never match a 12V panel. Wire losses are considerable GWH22828.jpg so try to stay under 10A. Put your panels in series for the highest voltage you are comfortable with.

                  % Rated Increase over
                  Panel Amp Direct Connect

                  100% 0%
                  90% 10%
                  80% 25%
                  70% 50%
                  60% 67%
                  50% 100%
                  40% 250%
                  30% 333%
                  20% 500%
                  10% 1100%

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                  • #10
                    PNPmacnab and others, thanks for helping me understand the benefit from matching components to get the most out of a small slice of current. This is just a hobby application until I get something that works. I like to think of "fail early, fail often" when working on projects and from this discussion, I can see failures that I would have needed to experience if not for the advice I received.

                    I started youtuubing and am building an understanding of the variables involved to get the most out of the panels.

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                    • #11
                      PV solar has a lot of exciting areas to work with. It is unfortunate that so many never get beyond Buy It Now mentality. I see solar as pretty much a "hobby" because they spend a lot of money and not get much for that. I have specialized in PV resistive heating because it has so much potential for very little cost. I heat water with my device by operating it on the same panels as a MPPT charge controller and extracting any excess power not being used. It is amazing how much energy is just wasted when it could be harvested. People don't value free energy. The nice thing about resistive is it can be added to existing systems with minimal cost and can harvest instantaneous bursts of energy. Below is just a half hour of energy harvesting with scattered clouds.
                      DIVERSION_21-09-22.png

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