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  • PV water heater

    I purchased some panels awhile back for around $1500 bucks and they are sitting in my garage collecting dust. I was going to add them to my system but after realizing my battery bank was to small for the added power I have just been sitting on the panels....for about a year. I have debated selling them but have also been looking for ways to use these panels.

    The panel specs are 195w 17.6 v. I have 6 of them.

    An online store has PV direct powered heater elements. I see one for 48vdc rated at 1000watts.

    I want to touch on three points and please feel free to answer one or all of the points.

    1. From what I understand, the element can be run at a higher DC voltage...ie the panels run in series as long as the string does not exceed the wattage of the element. SO....how about if I ran 5 of the panels in series. 88vdc x 975watts 11.07a? I was thinking I could run my SunDanzer fridge BF105 off the other panel and still have a pretty and symetrical solar array.

    2. My roof has a fixed angle of 45 degrees facing south. The cabin location sees a 1.98 solar radiation day in December. This would produce about 46kwh on my worst solar day of the year. My best would be in March, July, and August at 103kwh. I would have to size the water heater for my worst day...which would give me a blazing hot steam bomb in the summer unless I find a way to put a thermostat in the mix. So I ran some number to see what would happen if I installed the panels on the side of my cabin at 74 degrees. This would produce the same 46kwh day in December and would also reduce the summer Kwh down to 89kwh for a couple weeks in March. The benefit of this angle would be no snow accumulation year round and protection from possible hail storms due to the increased angle of the panels.

    3. I am kinda liking the idea of no batterys, charge controllers, inverters and additional wiring for this set up and thats why its an intriguing use of the panels. What size tank should I invest in for a 48v 1000w DC heating element considering my poor winter sun availablity? Not sure of the math on this one.

    PS I am totally aware I will have no hot water on cloudy days so no need to try an burst my bubble there.

  • #2
    Originally posted by 894tom View Post
    I purchased some panels awhile back for around $1500 bucks and they are sitting in my garage collecting dust. I was going to add them to my system but after realizing my battery bank was to small for the added power I have just been sitting on the panels....for about a year. I have debated selling them but have also been looking for ways to use these panels.

    The panel specs are 195w 17.6 v. I have 6 of them.

    An online store has PV direct powered heater elements. I see one for 48vdc rated at 1000watts.

    I want to touch on three points and please feel free to answer one or all of the points.

    1. From what I understand, the element can be run at a higher DC voltage...ie the panels run in series as long as the string does not exceed the wattage of the element. SO....how about if I ran 5 of the panels in series. 88vdc x 975watts 11.07a? I was thinking I could run my SunDanzer fridge BF105 off the other panel and still have a pretty and symetrical solar array.

    2. My roof has a fixed angle of 45 degrees facing south. The cabin location sees a 1.98 solar radiation day in December. This would produce about 46kwh on my worst solar day of the year. My best would be in March, July, and August at 103kwh. I would have to size the water heater for my worst day...which would give me a blazing hot steam bomb in the summer unless I find a way to put a thermostat in the mix. So I ran some number to see what would happen if I installed the panels on the side of my cabin at 74 degrees. This would produce the same 46kwh day in December and would also reduce the summer Kwh down to 89kwh for a couple weeks in March. The benefit of this angle would be no snow accumulation year round and protection from possible hail storms due to the increased angle of the panels.

    3. I am kinda liking the idea of no batterys, charge controllers, inverters and additional wiring for this set up and thats why its an intriguing use of the panels. What size tank should I invest in for a 48v 1000w DC heating element considering my poor winter sun availablity? Not sure of the math on this one.

    PS I am totally aware I will have no hot water on cloudy days so no need to try an burst my bubble there.
    Hey Tom you have made two huge errors in your assumptions.

    1. A resistive heating element is a fixed passive resistance. So if you have a 1000 watt element designed to operate at 48 volts, it has a fixed resistance of 2.3 Ohms. If you were to apply say 96 volts or twice the voltage, the element will burn 4000 watts or 4 times than its maximum safe rating. It would burn up almost instantly and likely catch fire.

    2. Next error is you state December insolation is roughly 2 hours which I agree is about right for your area in NY. However for you to generate 46 Kwh on a December day with only 2 Sun Hours would require 46 Kwh / 2 h = 23,000 watt solar panel. I do not think you have 23,000 watt solar panel do you?
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Hey Tom you have made two huge errors in your assumptions.

      1. A resistive heating element is a fixed passive resistance. So if you have a 1000 watt element designed to operate at 48 volts, it has a fixed resistance of 2.3 Ohms. If you were to apply say 96 volts or twice the voltage, the element will burn 4000 watts or 4 times than its maximum safe rating. It would burn up almost instantly and likely catch fire.

      2. Next error is you state December insolation is roughly 2 hours which I agree is about right for your area in NY. However for you to generate 46 Kwh on a December day with only 2 Sun Hours would require 46 Kwh / 2 h = 23,000 watt solar panel. I do not think you have 23,000 watt solar panel do you?
      1. What your saying makes sense, but I am just going by what he was claiming...he said as long as the input wattage did not exceed the 1000watt rating...voltage did not matter.
      2.. Thats one month....not a day. Sound better now? I'm guessing your responce was sarcasm? Of course you know what I ment?

      Comment


      • #4
        Originally posted by 894tom View Post
        1. What your saying makes sense, but I am just going by what he was claiming...he said as long as the input wattage did not exceed the 1000watt rating...voltage did not matter.
        Here is another way to look at it:

        If your element is designed for 240 volts AC, then applying 120 AC or DC volts to it is unconditionally not a problem (unless it is not in water, of course.)
        At 240 volts, it will pull about 4.17amps.
        Successfully applying 480 volts AC to it will destroy it.

        But if you have a panel whose Vmp is 480 volts and whose output power is 1000 watts, its Imp will be 2.28 amps, and its Isc will not be much higher, lets say 2.5 amps.
        Now if you connect that panel to the heating element, you will not be in any danger of burning out the element, because it will be able to pull at most 2.5 amps from the panel, and the panel voltage will be less than 240 volts at that time.

        It is a somewhat odd statement, but absolutely correct that if the panel maximum output power does not exceed the power rating of the heating element, then there is no combination of panel current and panel voltage that can damage that element. However, different combinations of panel specifications and heater specifications will determine whether most of that potential maximum power actually ends up in the heater or whether it is wasted because the panels are running far from their MPP.

        Just don't try it with batteries, generators, or power supplies because, unlike solar panels, they can be damaged by attempts to draw more than their rated power from them and they could damage the heating element while trying!!!
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

        Comment


        • #5
          Originally posted by 894tom View Post
          1. What your saying makes sense, but I am just going by what he was claiming...he said as long as the input wattage did not exceed the 1000watt rating...voltage did not matter.
          That is true but leaves out a lot of details because a solar panel is a current source, not a voltage source. For example lets just say you use 4 -250 watt, 24 volt panels wired in seried to generate 96 volt Vmp and Imp = 10.4 amps. If you connect the panels directly to the heating element will deliver 10.4 amps into the heating element thus developing 21.2 volts and 220 watts out of 1000 possible.

          Same 4 panels in parallel will apply 36 volts and supply only 17.6 amps or 633 watts out of 1000 possible.

          Originally posted by 894tom View Post
          2.. Thats one month....not a day. Sound better now? I'm guessing your responce was sarcasm? Of course you know what I ment?
          I did not know for sure what you meant because you stated 46 Kwh day which I knew was an error. So it was not sarcasm, just bringing it to your attention.
          MSEE, PE

          Comment


          • #6
            What about this
            1 strings of 6 with an MPPT charge controller feeding the 48V element.
            Or one string of 5 with the CC and the sixth panel set for the fridge or whatever.
            At 48V you could probably use the internal thermostat in a standard water heater to limit the temperature. I would still add a mixing valve to be sure and allow higher temperatures in the tank
            That element will heat about 4 gallons an hour so don't expect much in the winter.
            NABCEP certified Technical Sales Professional

            [URL="http://www.solarpaneltalk.com/showthread.php?5334-Solar-Off-Grid-Battery-Design"]http://www.solarpaneltalk.com/showth...Battery-Design[/URL]

            [URL]http://www.calculator.net/voltage-drop-calculator.html[/URL] (Voltage drop Calculator among others)

            [URL="http://www.gaisma.com"]www.gaisma.com[/URL]

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            • #7
              Heater resistance is temperature dependent

              The current and voltage computations appear to assume that a heater rated to consume 1000 Watts when supplied with 48 volts will always have 2.3 ohms of resistance. Essentially all materials used for heating elements have a positive temperature coefficient. Hence the resistance of the heater is likely to be significantly lower than 2.3 ohms when cold. Maximum transfer of energy from the solar PV panels to the heating element will occur when the impedance of the source (V/I) matches the resistance of the load, the heater.

              Somehow, using high technology PV panels to heat water seems like a terrible waste. Might some black garden hose work just as well?

              Comment


              • #8
                In the same situation, but with only a few watts laying around.
                Thinking of using a salvaged 50 gallon water heater tank, inline, with bypass as to isolate and experiment, to the cold feed of the energized main tank.
                Seems to me like the standard warp switch tstat, manual reset limit, and p&t valve would handle the overheat end.
                Looked at purchasing a dc element, but was going to try the salvaged elements first.
                My thoughts are any energy gained would be of more value than storing the surplus parts.

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