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  • Solar panel directly connected to immersion heater?

    Hi experts,
    I am planing to buy a 250/500 watt solar PV panel and connect it directly to my 2kw immersion heater attached to hot water cylinder without any convertor/inverter in between. (pure DC to heating element). I believe this should work in principal and should raise temperature of water by 10-15 degrees in one day.

    My question is - will this work? Are there any problems connecting this way? Is there a better way of achieving the same result without additional cost?

    Your views/comments will be greatly appreciated
    Thanks

  • #2
    So you are going to short circuit the panels with the electric element? Where will that put you on the panels IV curve, which tells you how much power you will be giving the element?

    Comment


    • #3
      Thanks adoublee, so is there a way to avoid the short curcuit? I guess adding some sort of resistance?

      Comment


      • Wy_White_Wolf
        Wy_White_Wolf commented
        Editing a comment
        No. You need to figure the resistance of the heater and plot that on the IV curve of the heater to tell if it's going work at all.

        WWW

    • #4
      Well if you are talking about using a Grid Tied Panel say a 250 watt panel with a Vmp= 32 volts and Imp = 8 amps and say connect it to a 12 volt 500 watt immersion heater with 8 amps from the panel generates 8 x 8 x .576 = 37 watts from your 250 watt panel. So you tell us, does that work for you?
      MSEE, PE

      Comment


      • #5
        Originally posted by Nathan251145 View Post
        Thanks adoublee, so is there a way to avoid the short curcuit? I guess adding some sort of resistance?
        No, you need a power converter which none exist to do what you want. Easy enough to build for a designer.
        MSEE, PE

        Comment


        • #6
          Originally posted by Sunking View Post
          Well if you are talking about using a Grid Tied Panel say a 250 watt panel with a Vmp= 32 volts and Imp = 8 amps and say connect it to a 12 volt 500 watt immersion heater with 8 amps from the panel generates 8 x 8 x .576 = 37 watts from your 250 watt panel. So you tell us, does that work for you?
          No, I was thinking of a Non-grid tied systemsystem. So are you saying that the power from the panel will be too less?. All I really want is to raise the water temperature by a few degrees in one day. Can a 500w panel work with a 500w immersion element? Obviously with some sort of resistance to limit the current to avoid short circuit

          Comment


          • #7
            Originally posted by Nathan251145 View Post

            No, I was thinking of a Non-grid tied systemsystem. So are you saying that the power from the panel will be too less?. All I really want is to raise the water temperature by a few degrees in one day. Can a 500w panel work with a 500w immersion element? Obviously with some sort of resistance to limit the current to avoid short circuit
            A solar panel is a current source. A 250 watt panel has a Isc of roughly 8 amps.

            A Resistance Heater is a fixed amount of resistance. A 12 volt 250 watt heater is a resistor with a value of .576 Ohms

            So you have a current source of 8 amps flowing through a fixed resistance of .576 Ohms. Ohms Law for Power = Current x Current x Resistance. Now use 8 amps and .576 Ohms.

            8 amps x 8 amps x .576 Ohms = 37 watts. @ 4.6 volts

            You need a power Converter, a Buck Converter, to change the panel output from 32 volts @ 8 amps to 12 volts @ 21.3 amps = 250 watts.

            So your idea will work, just not work worth a damn because you are changing your panel wattage from 250 to 37 watts of heat. In other words only 15% efficient. You loose 85% of your power. Which is perfect for democrats taking 85% of your money. If you are a democrat, that is the way you should do it because it is the right thing to do.
            MSEE, PE

            Comment


            • #8
              Originally posted by Nathan251145 View Post
              I am planing to buy a 250/500 watt solar PV panel and connect it directly to my 2kw immersion heater attached to hot water cylinder without any convertor/inverter in between. (pure DC to heating element). I believe this should work in principal and should raise temperature of water by 10-15 degrees in one day.
              Sure, it will work. But you have to match the resistance of the coil to the output impedance of the solar panel array. To do that we need to know what solar panels you have chosen, and then you will have to choose a heating element accordingly.

              Comment


              • #9
                To make this work better without a Converter is to use lower voltage Battery panels of 18 volts. Example you could use 125 watt panels, two of them wired in parallel. Each panels has a Vmp = 18 volts and Isc of 7.35 amps. When two panels are parallel you have 14.7 amps of current. Using the same 12 volt 250 watt heater means you know have 14.7 amps x 14.7 amps x .576 Ohms = 124 watts from 250 panels. Only 50% efficient, but a batter match of resistance.
                MSEE, PE

                Comment


                • #10
                  In addition to the good advice above, here is an old time GRAPHICAL solution to performance of
                  a PV panel source connected to a resistance heater load. With a 0.3 ohm heater 3V gives 10A
                  of current, 6V gives 20A, and so on. Plotting these point gives a straight load line from 0,0.

                  Then plot the power curve of a 12Vmp 20Amp 240W panel. 15Voc, 25Asc. These 3 points give
                  a rough curve as shown. That gives a max power point at A, 12V X 20A = 240W.

                  However, conditions hardly ever give the ideal maximum, so lets plot a more achievable PV curve
                  at 80%, or 192W. Vmp will hardly vary at 12V, Imp drops to 16A. At B power is 192W. The
                  heater load line intersects at C 5.8V, 19.5A, this is where they would operate if connected
                  together. But only 113W, or 59% of the available power is delivered.

                  100% efficiency could be achieved if the load line ran right through the panel MP point B. 12V
                  divided by 16A (ohms law) gives a load line of 0.75 ohms. If the heater ohms are off by a
                  factor of 20 (in either direction) the efficiency might plot out around 5%. Not the way to use
                  resources.

                  What about when the sun drops to 40%, point D of 96W potential? The curve intersection
                  at E shows our "ideal" heater drops to 7.4V, 9.7A, or 72W. At 75% efficiency some
                  adjustment would help. The 0.3 ohm heater efficiency is in the toilet.

                  So plot out the panels and loads under consideration. If the efficiency isn't above 60%, it
                  will likely be a big disappointment. A severe mismatch can easily get into single digit
                  efficiency, a total waste of time.

                  Electronic means of improving the match are possible. But off the shelf devices pretty
                  much work with some sort of power reserve (grid, battery), not direct load connection.
                  Bruce Roe
                  Attached Files
                  Last edited by bcroe; 08-02-2016, 04:24 PM.

                  Comment


                  • #11
                    Originally posted by Nathan251145 View Post
                    Hi experts,
                    I am planing to buy a 250/500 watt solar PV panel and connect it directly to my 2kw immersion heater attached to hot water cylinder without any convertor/inverter in between. (pure DC to heating element). I believe this should work in principal and should raise temperature of water by 10-15 degrees in one day.

                    My question is - will this work? Are there any problems connecting this way? Is there a better way of achieving the same result without additional cost?

                    Your views/comments will be greatly appreciated
                    Thanks
                    arent you better off getting a solar water heater since heating is what you wanna achieve

                    Comment


                    • #12
                      Originally posted by VTSE View Post

                      arent you better off getting a solar water heater since heating is what you wanna achieve
                      He is educated in the USA. They have no clue what you are talking about. That is how they got stuck with Chump or Billiary as Potus
                      MSEE, PE

                      Comment


                      • #13
                        The other issue is the heater thermostat. It isn't designed to break a DC power source; could arc over and burn up. Bruce Roe

                        Comment


                        • #14
                          Hi

                          I do exactly what the OP suggested, I have two 175W 35V MPP panels connected directly to my original 3kW immersion heater. The immersion had only ever been used once in 20 years just to see if it worked.

                          I have a relay that switches parallel to series at about 38V and back to parallel at approx. 35V. It was only done for a bit of fun, not expecting it to pay for itself or even work very well but in the summer it is actually pretty good. In full summer sun the impedance match is very good, it often gets up to about 72V (280W) in series and on bright cloudy days is often sat at around 36V (70W) in parallel.

                          Output on dull days is virtually nothing but like I said it is just for fun.

                          Not too worried about the possibility of the heater thermostat arcing, I have never know mine to operate even when the water got really hot. OC voltage is just over 80V so it may be ok anyway.

                          Feel free to criticise this inefficient use of the panels but I have had a lot of laughs with it, mostly by blowing up control circuits I was experimenting with!

                          Comment


                          • #15
                            Building and evaluating control schemes is a huge step forward from blindly connecting things together. Maybe the thermostat
                            could be wired in to control your DC rated relays. Bruce Roe

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