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  • #76
    Hi inetdog. That is a very concrete example and clear explanation. Thank you so much. Got the MPPT relevance to resistence elements good but ask that you check me on this conclusion below cause it seems to violate common sense.

    I.e--in this example the only way to increase power output is to increase resistance. 120 volt panel, 1000 watt, vmp of 30 volts. If I hook to 120 volt, 2000 watt element I've got a resistance of 7.2 ohms. Now if we wanted the perfect element to dump excess juice from a 120 volt battery this would be perfect match because would get full 2k output. (16.67 amps x 120v). But, Imp of panel only 8.3 amp so according to your instruction you multipy 8.3 by 7.2 ohm=60 volts. Then times 8.3 again and only get 500 watts from 1k panel at full solar.

    But if use 120 volt 1000 watt element the resistance doubles to 14.4 ohms and you get almost full 1000 watt flow thru.

    * So assuming you say this is correct would it be right to say that the only way you can ever get a perfect match and no loss at noon is to have the element and panel the same in voltage and watts?


    Been ages since I actually tried to measure this, but my feeling is I'd get about 90% of maximum power for 4 hours or 9 out of the 10 amps in your example before I started rapidly dropping to 5 amps. If anyone can contradict this please do. So 90 amp/hrs plus whatever dribbles you get after 4 hours ain't too bad but the mppt problem is a bummer for sure. Probably a deal killer.

    The general consensus is there is no mppt fix to this strictly dc direct connection? Like can't put a mppt controller in between? Believe I got that impression when read this whole thread a week ago but will review. Don't need to know why. Would be beyond me I'm sure.

    Got the Thermostat issue covered in this increasingly metaphysical system. High dc volt Relays

    Fear I'm asking too many questions at once but at the moment maybe you are willing to linger in beginners corner but maybe not next week so thinking ahead and maybe being a little too grabby.

    Thanks again for the illumination and good day.



    Originally posted by inetdog View Post
    OK, here is a nice concrete example. For the sake of argument we will assume that you can find any resistance heating element you want in any power rating you want. That is not true in the real world and is another reason for putting active conversion equipment in between source (panels) and load (heating elements.)

    1. Assume a panel array that will produce 10A at 100V at full noon sun. For any other condition it will produce a lower current, but approximately the same voltage at the maximum power point.
    2. Assume a matched resistive load (1000W heater element with a resistance of 10 ohms). At full power the 100V from the panel will cause the heater to draw 10A, so a perfect match.

    Now look what happens when the solar input to the array drops to 50% (a couple of hours either side of solar noon).
    The voltage, Vmp, will still be 100V but the available current will be only 5A.
    But a current of 5A into a 10 ohm resistor will correspond to a voltage of only 50V. That means that you will be getting only 50 x 5 = 250 watts out of the array into the heater. If you could cause the resistance of the heater to change to 20 ohm instead of 10 ohm you would get 100V at 5A for a power of 500 watts.
    Since a fixed resistance is not going to be able to extract full power from the panels at any point other than full design output, you need some active circuitry such as an MPPT input and output stage which will convert the panel output of 100V at 5A to 70.7V at 7.07A. That will be a perfect match to a 10 ohm resistor and will give you the full 500 watts of power.

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    • #77
      Originally posted by lark View Post
      Hi inetdog. That is a very concrete example and clear explanation. Thank you so much. Got the MPPT relevance to resistence elements good but ask that you check me on this conclusion below cause it seems to violate common sense.

      I.e--in this example the only way to increase power output is to increase resistance. 120 volt panel, 1000 watt, vmp of 30 volts. If I hook to 120 volt, 2000 watt element I've got a resistance of 7.2 ohms. Now if we wanted the perfect element to dump excess juice from a 120 volt battery this would be perfect match because would get full 2k output. (16.67 amps x 120v). But, Imp of panel only 8.3 amp so according to your instruction you multipy 8.3 by 7.2 ohm=60 volts. Then times 8.3 again and only get 500 watts from 1k panel at full solar.

      But if use 120 volt 1000 watt element the resistance doubles to 14.4 ohms and you get almost full 1000 watt flow thru.

      * So assuming you say this is correct would it be right to say that the only way you can ever get a perfect match and no loss at noon is to have the element and panel the same in voltage and watts?

      * It's been a real long time since dabbled in this so please refresh me on this---Does one stay pretty close to 100% for that 4 hour time window you mentioned and then starts dropping or is one down to 50% 2 hours after/before noon. everything else perfect of course. My feeling is the former, that I can count on 90% for 4 hours anyway?

      * The mppt problem is a bummer tho. The general consensus is there is no mppt fix to this strictly dc direct connection?, Like can't put a mppt controller in between? Don't need to know why. Would be beyond me I'm sure.

      Got the Thermostat issue covered. Relays

      Fear I'm asking too many questions at once but at the moment maybe you are willing to linger in beginners corner but maybe not next week so thinking ahead and maybe being a little too grabby.

      Thanks again for the illumination and good day.
      The Imp and Vmp of the panel are only applicable at rated conditions. MPPT is there for all the time when things aren't perfect (a majority of the time). At any given set of conditions (light flux on the panel and panel temperature) there will be a single voltage/current relationship that maximizes the power production. That point is always changing. A good MPPT controller will adjust it's control point continuously to see if it is better or worse then before, and adjust towards the direction that made more power. A cheap pseudo MPPT controller will regulate just to maintain a set panel voltage, which, while not ideal, may be good enough for water heating.

      Comment


      • #78
        Originally posted by lark View Post
        Hi inetdog. That is a very concrete example and clear explanation. Thank you so much. Got the MPPT relevance to resistence elements good but ask that you check me on this conclusion below cause it seems to violate common sense.

        I.e--in this example the only way to increase power output is to increase resistance. 120 volt panel, 1000 watt, vmp of 30 volts. If I hook to 120 volt, 2000 watt element I've got a resistance of 7.2 ohms. Now if we wanted the perfect element to dump excess juice from a 120 volt battery this would be perfect match because would get full 2k output. (16.67 amps x 120v). But, Imp of panel only 8.3 amp so according to your instruction you multipy 8.3 by 7.2 ohm=60 volts. Then times 8.3 again and only get 500 watts from 1k panel at full solar.

        But if use 120 volt 1000 watt element the resistance doubles to 14.4 ohms and you get almost full 1000 watt flow thru.

        * So assuming you say this is correct would it be right to say that the only way you can ever get a perfect match and no loss at noon is to have the element and panel the same in voltage and watts?

        * It's been a real long time since dabbled in this so please refresh me on this---Does one stay pretty close to 100% for that 4 hour time window you mentioned and then starts dropping or is one down to 50% 2 hours after/before noon. everything else perfect of course. My feeling is the former, that I can count on 90% for 4 hours anyway?

        * The mppt problem is a bummer tho. The general consensus is there is no mppt fix to this strictly dc direct connection?, Like can't put a mppt controller in between? Don't need to know why. Would be beyond me I'm sure.

        Got the Thermostat issue covered. Relays

        Fear I'm asking too many questions at once but at the moment maybe you are willing to linger in beginners corner but maybe not next week so thinking ahead and maybe being a little too grabby.

        Thanks again for the illumination and good day.
        It seems paradoxical at first, but when dealing with a current limited power source you often can utilize more of the potential panel output by increasing the load resistance.

        You are trying to maximize the product of V and I subject to the fact that the available I is both strictly limited and dependent on the amount of light hitting the panels, while the V (both Voc and Vmp) is close to constant independent of light on the panels, but will drop like a rock if you try to pull too much current.

        You can actually see a similar effect when trying to use a primary battery to produce heat (electric lighter, heat socks, etc.). If your only goal is to get the highest possible power for a short time, without caring about battery life, you want your load resistance to be so low that it is equal to the battery's internal resistance. But not any lower than that!
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

        Comment


        • #79
          Hi Crazi. --Have you tried this in any way? Have others? Where?

          What, out there, is a "cheap pseudo" controller you would try if you were feeling Crazi?


          Originally posted by CraziFuzzy View Post
          The Imp and Vmp of the panel are only applicable at rated conditions. MPPT is there for all the time when things aren't perfect (a majority of the time). At any given set of conditions (light flux on the panel and panel temperature) there will be a single voltage/current relationship that maximizes the power production. That point is always changing. A good MPPT controller will adjust it's control point continuously to see if it is better or worse then before, and adjust towards the direction that made more power. A cheap pseudo MPPT controller will regulate just to maintain a set panel voltage, which, while not ideal, may be good enough for water heating.

          Comment


          • #80
            Originally posted by lark View Post
            Hi Crazi. --Have you tried this in any way? Have others? Where?
            Tried what, specifically, an MPPT controller? Just about any controller out there worth purchasing has some level of MPPT control on it. What sort of logic it uses to determing where the peak point is at any given time might vary. Some will occasionally sweep over their entire range and remember where the highest point was (the sweep method). Some will continuously shift off of their current piont a bit and see if they are better or worse, and use that to choose which way to shift next (the perturb-and-disturb method).
            Originally posted by lark View Post
            What, out there, is a "cheap pseudo" controller you would try if you were feeling Crazi?
            No idea on an off the shelf item that would work best for driving a heating element - usually they are built as battery chargers, so their logic is a bit more complicated, in that they are also controlling/limiting charge rate as well.

            Comment

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