PV water heating.....again, but wait!

I had a thought today, that seems a little too obvious and I'd like to see if anyone can set me right.

There's been a lot of talk these days about using PV to heat water. Seeing as the price of panels have come down, depending on your particular cirumstance it can make some sense. However the biggest technical obstacle that I can see is that any MPPT still needs a battery which leads to concerns about trashing a vital off-grid battery bank should anything go wrong, adding an addition complication of a SSR system controlled by the charger or the need for an expensive battery in a situation where none is needed elsewhere.

That being the case, my question is this.

Is it possible to modify a MPPT charger so that the controler side is electrically isolated from the output? This would enable the charger to output to a heating element directly without drawing on the same battery that is powering the controller. The advantage is that it would be impossible to draw too deeply on the main battery pack, the disadvantage would be that that MPPT controller would now be dedicated to heating, so ideally one would use a cheaper controller without all the bells and whistles.


Can anyone with a deeper understanding of the technical issues send me in the right direction?

6x Hanwha Solar G4 305watts
Midnight Solar Classic 150 w/ wizbang jr
4x 6v crowm 395 ah
Xantrex SW4024
location - Lanaudiere, Quebec, Canada

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 5,576 ✭✭✭✭✭

    If you are sending DC to the water heater, throw the charge controller out of the equation.

    For me the problem is having a viable DC thermostat. Having the thermostat triggering a relay off of house current would work though.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • northernsunnorthernsun Registered Users Posts: 6 ✭✭

    I routed a 12v signal from my Midnite classic through the AC thermostat that came with the hot water tank and put it in line with the 100A DC SSR that I have connected to the battery bank and the DC heating element. I run a 900W 24v heating element, works fine. I suppose I could connect the panels directly in that manner. You'd just have to carefully match the voltages of the panels and the element. In my neck of the woods the temperature swings annually from 32C to -40C, making it difficult to find a sweet spot on the voltage.

    6x Hanwha Solar G4 305watts
    Midnight Solar Classic 150 w/ wizbang jr
    4x 6v crowm 395 ah
    Xantrex SW4024
    location - Lanaudiere, Quebec, Canada
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭

    Search for below on ebay. A MPPT charge controller that doesn't need batteries to drive a water heater.

    Solar Water Heater EZ DIY Save $$$ No Pipe Changes Hot Water System PV MPPT !

    I am available for custom hardware/firmware development

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin

    Not attesting to the quality and function of the controller Jonr typed about... Just the website for it (and more information--sort of):

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • NANOcontrolNANOcontrol Registered Users Posts: 197 ✭✭✭

    Is it possible to modify a MPPT charger

    If you have the ability to do that, Why not just build one. Only takes a dozen cheap parts.


  • cow_ranchercow_rancher Solar Expert Posts: 117 ✭✭✭✭

    I love your wiring techniques.

    Rancher

  • NANOcontrolNANOcontrol Registered Users Posts: 197 ✭✭✭

    I was hoping to en bold someone into thinking they could do something at least that good. I woke up one morning deciding to get hot water. Being in the middle of nowhere, I went out to the garage and picked apart a couple of old computer and UPS power supplies. That actually worked for a couple seasons. I had a piece of bubble wrap over it for IP67 environmental protection.

  • SaiproSaipro Solar Expert Posts: 74 ✭✭
    Been heating mine for a while now direct from PV. My home essentially runs on 230V (asides a dedicated isolated 115V line). I have a standard AC water heater switch sitting right above the circuit breakers from the PV. That way, the heater draws power before the CC. My PV array is x2 required (cheaper to have extra PV for the cloudy day/extra to do the occasional heavy job than stack batteries and babysit them). Wifey slips the switch around 11am and switches off the heater around 4pm. At any, the water temperature rarely rises to a point where the thermostat has to kick in thus the thermostat is safe. The Midnite CC is protected by two pairs of diodes thereby never sucking power from the Midnite (one pair of diodes is sufficient but added a second for redundancy - to avoid stories that touch the heart).
    Semi off-grid

    255W Canadian Solar × 12, 200AH 48V US 185 XC2 bank, Victron Bluesolar MPPT 150/85, Victron CCGX, Victron MultiPlus 48V/5kVA/70A inverter (primary system) Victron Phoenix 48V/375VA inverter (backup for critical loads)

    300W Yingli × 2, Midnite Brat, 200AH 24V bank (powers DC LED security lights)

  • MrM1MrM1 Registered Users Posts: 487 ✭✭✭✭
    Or you could set up a DC heater like this using and AC source for the thermostats 
    http://waterheatertimer.org/Convert-AC-water-heater-to-DC-water-heater.html
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2017 / System, Pics and Discussion
  • NANOcontrolNANOcontrol Registered Users Posts: 197 ✭✭✭
    Once upon a time people used to build things.   The board on the right is a $6 board from ebay modified to heat water and connect directly to solar panels that also supply a battery charge controller.  This is connected to 60V, two grid tie panels in series. The meter is reading about 60W to the water heater on a very cloudy day. With a direct connect like the above, that would only be a watt if it could even turn on.  Any direct connect is a really bad idea. With this power point controller, any extra power when the panels go over the power point voltage goes to heat water.  That can be as little as 5W up to the full panel power. No need for the wife to switch anything, it is all automatic. Can't believe people in solar are still using crappy diversion methods that waste energy. This is actually a great system for a camp (battery charging and hot water) that costs less than $40.


  • SaiproSaipro Solar Expert Posts: 74 ✭✭
    @NANOcontrol: Could you give pointers (a link or something)? I'm having mysterious issues viewing your uploaded picture.
    Semi off-grid

    255W Canadian Solar × 12, 200AH 48V US 185 XC2 bank, Victron Bluesolar MPPT 150/85, Victron CCGX, Victron MultiPlus 48V/5kVA/70A inverter (primary system) Victron Phoenix 48V/375VA inverter (backup for critical loads)

    300W Yingli × 2, Midnite Brat, 200AH 24V bank (powers DC LED security lights)

  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    Saipro, the photo should show... You can try this link (direct to photo storage):

    https://us.v-cdn.net/6024911/uploads/editor/bk/l7k3tob1cunk.jpg

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SaiproSaipro Solar Expert Posts: 74 ✭✭
    edited May 2019 #14

    Thanks. The problem was with Internet Explorer. Showed fine on Firefox (the link you gave appeared too on Firefox).

    Back to the diagram. I see a bunch of stuff. A display sitting above a  buck converter which in turn sits on a PWM controller (I used the exact type {I think} in a recent solar streetlight project and a couple of them failed within two weeks; I noticed the same single Zener diode had died in them all, right above the battery inputs - weirdly had a 7.2V Zener voltage on normal ones). On the right is a PCB with FETs, capacitors and other stuff but notably an IC. If the device works for you, I'd like to have one too. Can I get the eBay link and the modification schematics?

    Semi off-grid

    255W Canadian Solar × 12, 200AH 48V US 185 XC2 bank, Victron Bluesolar MPPT 150/85, Victron CCGX, Victron MultiPlus 48V/5kVA/70A inverter (primary system) Victron Phoenix 48V/375VA inverter (backup for critical loads)

    300W Yingli × 2, Midnite Brat, 200AH 24V bank (powers DC LED security lights)

  • NANOcontrolNANOcontrol Registered Users Posts: 197 ✭✭✭
    Do you mean the water heater schematic?

  • SaiproSaipro Solar Expert Posts: 74 ✭✭
    Wonderful. Will set to it by next weekend. Thanks!
    Semi off-grid

    255W Canadian Solar × 12, 200AH 48V US 185 XC2 bank, Victron Bluesolar MPPT 150/85, Victron CCGX, Victron MultiPlus 48V/5kVA/70A inverter (primary system) Victron Phoenix 48V/375VA inverter (backup for critical loads)

    300W Yingli × 2, Midnite Brat, 200AH 24V bank (powers DC LED security lights)

  • NANOcontrolNANOcontrol Registered Users Posts: 197 ✭✭✭
    What array voltage do you have and would this be a dedicated 2 element water heater or auxiliary on the lower?
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