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Hot Water by Solar Electric direct via MPPT ?

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  • #61
    Originally posted by bcroe View Post
    The efficient design will manage to feed the heaters without inverters. I see the first challenge as matching
    the impedance of the heater to keeping the panels somewhere near MPPT. If there is plenty of sun keeping
    the panel output current fairly constant, it may avoid an MPPT device. That leaves some series/parallel
    combination of panels to match up to varied series/parallel heating elements. I doubt anything in the 12V
    realm would be practical.

    As for the control contact arc problem, there are devices to handle the switching, to be controlled by
    the contact(s). We had some pretty hefty relays in phone offices, and now we have cheap FETs that
    may easily be paralleled for this sort of thing, I have used 20. Inductive spikes may kill them if no
    protection is used. Keep in mind, the panels can be shut down by shorting the output, maybe just
    while a fragile switch is operated. Bruce Roe
    Tricky to accomplish since morning and evening will always see a drop in current.
    So what can I expect to happen if I power a resistive DC load on a non-MPPT CC when the current drops, assuming I have matched the DC heater resistance (impedance) as you suggest?

    Tgriff: Have you considered going back to a 24VAC T-stat energizing the coil of a power relay which would switch your DC voltage?
    As BRoe said, there are some pretty robust power relays but they are AC coil. I just don't believe those SSR's are going to last no matter what the rating says.
    You may need one power relay per PV panel b/c the following are only rated up to 28VDC:
    https://www.automationdirect.com/adc...ad-pr40_series)

    As for Solar Thermal, it works great if the system is designed properly and maintained well; most systems I've seen are neither.
    i would recommend a DrainBack system to avoid many headaches associated with high temp and high pressure.
    Control is the tricky part. Resol controllers are very reliable.

    Comment


    • #62
      Originally posted by jflorey2 View Post
      At least two companies I know of offer MPPT trackers for this purpose. (DC out to heating element) One of them also provides batteryless 120VAC for emergency use.
      interesting, can you tell their names?

      Comment


      • #63
        Originally posted by psablo View Post
        Tricky to accomplish since morning and evening will always see a drop in current.
        So what can I expect to happen if I power a resistive DC load on a non-MPPT CC when the current drops,
        assuming I have matched the DC heater resistance (impedance) as you suggest?

        Tgriff: As BRoe said, there are some pretty robust power relays but they are AC coil.
        I have managed to use a few AC coil relays on DC. Using a power supply the relay can be expected
        to pull in at a much lower voltage. I then boost that about 30% and call it the DC coil rating.

        Certainly an MPPT control with an electrical on/off would be ideal, almost all require panel Vmp to be
        greater than the load voltage at peak power. Without that, careful matching of components could
        achieve decent efficiency but an on/off method is needed. Yes when sun intensity or angle change
        you move away from MPPT. One point of view is, when power drops below 40% the efficiency does
        not matter any more.

        Here is an old school load line to see how a 12V vmp 20A imp performs with varying sun and different
        loads. It can be scaled to your working numbers. My first attempt would be to set up for panel
        voltage somewhat above MPPT at best sun, shifting toward and somewhat past MPPT with reduced
        sun and current. Going way down, does MPPT still matter?

        Another thing going on here, is to have multiple paralleled strings facing rising, possibly mid day, and
        setting sun. This will tend to flatten out daily power as in the second curve, but costs many more
        panels. It has the additional benefit of staying closer to your MPPT in good sun, AND bringing in
        more power under somewhat dispersed (thin clouds).

        Just what is the overall plan, is this totally stand alone? Is cold water OK when weeks go by with
        very poor sun? We had 27 days in a row one Dec at 61084, no sun seen. Is the PoCo to be a
        backup, with a complete rewiring? Bruce Roe

        Last edited by bcroe; 01-05-2019, 10:54 PM.

        Comment


        • #64
          Originally posted by bcroe View Post

          Going way down, does MPPT still matter?
          That's what i was thinking; thanks for input.

          I can't speak for Tgriff but I'm just trying to keep pipes from freezing in a well insulated pump control house which will probably never freeze, but just in case...
          And, I'm in SW Co at 8500 ft where we rarely have more than 2 days without crystal blue skies and intense sun.

          Re: jflorey2, correct me if I'm wrong, but don't Diversion Load Controllers require a battery bank? I was under the impression this discussion was based on not having any batteries.

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          • #65
            Originally posted by bcroe View Post
            If an MPPT control is to be used, it could also handle the job of turning power on and off. Bruce Roe
            Exactly what I was thinking. (True of MPPT _and_ PWM.)

            Comment


            • #66
              Originally posted by jflorey2 View Post
              At least two companies I know of offer MPPT trackers for this purpose. (DC out to heating element) One of them also provides batteryless 120VAC for emergency use.
              I hadn't done a search in a while but WOW, did I find some on Alibaba:

              https://www.alibaba.com/showroom/sol...e-heaters.html

              Comment


              • #67
                Originally posted by Tgriff View Post

                I hadn't done a search in a while but WOW, did I find some on Alibaba:

                https://www.alibaba.com/showroom/sol...e-heaters.html
                Sorry, guys - most of these are cheap controllers for charging batteries, nothing to do with water heaters. Those that do have incomplete/contradictory specs.

                Yes - I'd be interested in the names/models and any experience with any *real* such devices.

                I saw the one from TechLuck and it appears to be real, though I have concerns over their heat sync (a wall-mounted electrical box).

                Comment


                • #68
                  Originally posted by Tgriff View Post

                  1. It has two elements: the upper is 3800 W and lower 5500 W, with a max of 5500 W (wired so only one element on at a time)

                  2 & 3. I didn't log water temperature or electrical performance for the few weeks I was testing it. I was just glad to have hot water at night and again in the morning - and it did that much well.

                  4. Actually, I used it for weeks without connecting the water heater to POCO - and it always had hot water for me (until the SSRs overheated). This whole discussion has me more interested in resuming where I took off ...

                  5. I know the reliability of using a PV solar system, but have read of problems with pumps & leaks in other systems. Does anybody here have personal experience living with one?

                  Also: I'll have to put in a plug for my YouTube video on a solar irrigation system: Solar Irrigation
                  I work on some HVAC equipment that uses SCR controllers for reheat (480V AC electric resistance heating.) These heaters pulse to maintain setpoint, and I have not had any failures yet. I don't know if they would work with DC line voltage but it's worth a check.
                  The control circuit is usually DC .

                  I have a 84 sq ft flat plate solar thermal HW system that has been in operation since 2008. It is a drain back system and has had 0 issues. No glycol so minimum maintenance required. I change the anode rods in the storage tank every 3 years.
                  The first 15 ft of piping from the collectors is copper, the rest back to the storage tank is Pex.

                  Comment


                  • #69
                    Originally posted by LucMan View Post
                    I work on some HVAC equipment that uses SCR controllers for reheat (480V AC electric resistance heating.)
                    These heaters pulse to maintain setpoint, and I have not had any failures yet. I don't know if they would work
                    with DC line voltage but it's worth a check. The control circuit is usually DC .
                    SCRs are tough, but they can't be used to control pure DC line. Once they are on, they stay on until the
                    line power is removed. Bruce Roe

                    Comment


                    • #70
                      Originally posted by Tgriff View Post


                      I saw the one from TechLuck and it appears to be real, though I have concerns over their heat sync (a wall-mounted electrical box).
                      Thanks for finding that, it looks interesting I might try one.

                      Comment


                      • #71
                        I have been reading the TechLuck FAQ's and he says:

                        Q: Why can't I hook up my solar panels directly to the hot water heater?

                        A: If you are using more than 150 watts of solar power the AC thermostat contacts can arc, weld, burn up or melt together. The thermostat will no longer function properly to prevent overheating of the tank, creating a dangerous condition.

                        The design of the controller's special CPU is programmed for Maximum Power Point Tracking (MPPT) and also makes the DC into a form of high frequency AC and stops any arcing within microseconds, just like the zero crossing of AC does. The rapidly changing waveform inhibits arcing of the thermostat contacts. Unlike any other MPPT controller, this one is specifically for use in solar water heating using the original factory thermostat.

                        As for heat sink, he says, use a bigger junction box to mount the controller if you get above 750W PV.

                        Comment


                        • #72
                          Originally posted by psablo
                          The design of the controller's special CPU is programmed for Maximum Power Point Tracking (MPPT) and also makes the DC into a form of high frequency AC and stops any arcing within microseconds, just like the zero crossing of AC does. The rapidly changing waveform inhibits arcing of the thermostat contacts. Unlike any other MPPT controller, this one is specifically for use in solar water heating using the original factory thermostat.
                          Sounds like a cool product, but I wonder if it causes radio interference? That could be contained. Bruce Roe

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