Connecting solar panels to hot water heater

alexdaviesalexdavies Posts: 3Registered Users
So, I'm sure I'll have some folks shaking their heads at this one...

I don't know what I'm doing here, and I know just enough about electrical and solar to be dangerous.

But, I had this idea the other day... 
I bought a bunch of solar panels a while back for cheap which I planned to hook up to our off grid cabin. They're old, morphus 42 Watt panels that were bought and never used for a project. Upon receiving them, I found out that they put out around 100 V. 

I never ended up investing in a charge controller that can handle this higher voltage, and kept our super small, and simple 12V, 200W solar array for our LED lights etc... 

I've been trying to figure out the best option for hot water at the cabin... Tossing around the idea of propane on demand, or maybe solar hot water panels etc...  THen,  I got to thinking that maybe I could try running these old panels of mine directly into an electric hot water heater, and when it's sunny, maybe I'll be able to slowly heat up my water enough for a warm shower at the end of the day. 

So, I did a little experiment with a hot water heater element that I had kicking around. I hooked up to 5 of these 42W panels - direct wired, no breaker, no switch into the element. Just a note; This heating element is a conventional 3000W, 240V element... But... Sure enough, when I connected the panels to the element, it immediately got hot. I dipped the heating end into a 5 Gal pail of water for an hour, and sure enough, the water heated up significantly. 

So, here I start thinking that I've come up with some brilliant idea, and I'm dreaming of a warm shower. I go and buy a hot water tank, install 8 panels on the roof, fill the tank up and plumb it all in. I decided to install a regular 120V 15amp on/off switch on the negative wire before the heater, then wire the panels direct into the red and black wires of the hot water heater (240V, 3000W conventional heater). I'm a little nervous at the warning signs about only connecting 240v ac etc... but I'm thinking that it's only a heating element and a thermostat which to me seems like a glorified switch. nothing electronic etc, so I think that 100V dc isn't going to hurt anything... especially in Low watt (only 336W out of these 8 panels)

I turn the switch on, and all is good. I then turn the switch OFF, and the switch burns out (crackles, turns green for a second).
Curse a bit... Then disconnect panels from the water heater, and try wiring panels into my test element that I tried the day before in the 5 gal pail with a new switch. Turn the switch ON - element heats up... Turn it OFF switch burns out same as before. 
Switch of course also says 120V AC only... But 336W of these panels at 100V is only pushing 8 or so amp. So switch should handle that sort of power...

So, that's where I'm at now... I don't want to burn my cabin down, and need to seek some advice before moving forward. Not sure if I've burned out the brand new thermostat on the hot water heater... 

Maybe I need some sort of a controller... Or a DC 24V water heating element (but then of course my panels are too high voltage).
I noticed there's a lad selling dc water heating elements with built in thermostats online... Maybe I plug this into the water heater tank and bypass the ac thermostat and heater... 

HELp?!

Comments

  • EstragonEstragon Posts: 2,607Registered Users ✭✭✭✭
    Switching DC at 100v is not the same as switching AC at 120v. AC averages out to 120v, but crosses zero volts as it cycles, making it harder to sustain an arc. DC has level voltage, so arcs much more easily. Any switches/relays, including the thermostat should be made for DC, and rated for the kind of voltage you're using.

    The element itself doesn't care about AC or DC.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • NANOcontrolNANOcontrol Posts: 58Registered Users ✭✭
    It is very difficult to switch DC over about 35V as you get those results.  Your contacts were probably brass which is about the worst to use.  That said, I heat water with PV and love it.  I do it with about 60V because I use grid tie panels in series.  I also operate the panels at power point for maximum efficiency.  I do this switching electronically, so it really isn't a disconnect.  There are switches and relays that can handle this DC voltage.  If I advised  you I would get a lot of crap, but any switching should be fast and have a wide space. There are videos doing this on youtube.  Take that for what it is worth.
  • alexdaviesalexdavies Posts: 3Registered Users
    Right...
    So, If I just hardwire direct to my heater element, bypass thermostat and don't install a switch, then I'm maybe in the clear...? I can just experiment with adding or dropping panels on a really hot sunny day so that the water doesn't get so hot that it blows the pressure relief valve... 

    In this configuration, there's no possibility of short circuit? 

    Thanks
  • MrM1MrM1 Posts: 218Registered Users ✭✭
    REC TwinPeak 2 285W 3S/3P 2.6kW-STC / 1.9kW-NMOT Array
    MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware
    Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system
    Grid-Assist Off-Grid / Need 3200Whs Daily Off-Grid
    System went Online Oct 2017
  • MrM1MrM1 Posts: 218Registered Users ✭✭
    REC TwinPeak 2 285W 3S/3P 2.6kW-STC / 1.9kW-NMOT Array
    MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware
    Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system
    Grid-Assist Off-Grid / Need 3200Whs Daily Off-Grid
    System went Online Oct 2017
  • MrM1MrM1 Posts: 218Registered Users ✭✭
    I do water heating with solar panels but opted to use my Inverter as an AC source and down sized my element to 1000W 240v as to not overload the inverter or battery. 
    REC TwinPeak 2 285W 3S/3P 2.6kW-STC / 1.9kW-NMOT Array
    MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware
    Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system
    Grid-Assist Off-Grid / Need 3200Whs Daily Off-Grid
    System went Online Oct 2017
  • jonrjonr Posts: 1,029Solar Expert ✭✭✭✭
    You can see here for a pre-built product that I think can hold the panel approximately at Vmp (using fast PWM and some filtering).

    https://www.ebay.com/itm/Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT
  • alexdaviesalexdavies Posts: 3Registered Users
    jonr said:
    You can see here for a pre-built product that I think can hold the panel approximately at Vmp (using fast PWM and some filtering).

    https://www.ebay.com/itm/Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT

    Hi Jonr, This page wasn't available for me to load... Maybe it's because I'm in Canada?


  • westbranchwestbranch Posts: 5,084Solar Expert ✭✭✭✭
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge, Hughes1100 Sat Modem
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mike95490mike95490 Posts: 7,790Solar Expert ✭✭✭✭
    >  I turn the switch on, and all is good. I then turn the switch OFF, and the switch burns out (crackles, turns green for a second).

    You can use a DC rated breaker from Midnight Solar.  They are pretty cheap  $20  150VDC  switch usage rated..
    http://www.midnitesolar.com/productPhoto.php?product_ID=197&productCatName=Breakers&productCat_ID=16&sortOrder=13&act=p


    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • EstragonEstragon Posts: 2,607Registered Users ✭✭✭✭
    > @alexdavies said:
    > Right...
    > In this configuration, there's no possibility of short circuit? 
    >
    > Thanks

    Any circuit can short... a breaker like the one @mike95490 suggested would be a good idea.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • oil pan 4oil pan 4 Posts: 703Solar Expert ✭✭✭✭
    A hot water heater element is a short circuit.
    You may also try a dpsp switch and wire the contacts in serries.
    It might break the dc arc.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • mcgivormcgivor Posts: 2,052Solar Expert ✭✭✭✭
    edited July 16 #14
    Use a DC rated switch or breaker, there is a reason they're made, to properly extinguish the arc and prevent welding of the contacts or worse causing a fire. An alternative would be to use a DC relay see attached pdf, and use the thermostat and switch to control the relay coil.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

Sign In or Register to comment.