Connecting solar panels to hot water heater

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?!
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
The element itself doesn't care about AC or DC.
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
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
https://www.ebay.com/itm/Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT
I am available for custom hardware/firmware development
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,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
http://www.midnitesolar.com/productPhoto.php?product_ID=197&productCatName=Breakers&productCat_ID=16&sortOrder=13&act=p
|| 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 ,
> 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.
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
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.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
That's quite a mouthful do digest, one way to add panels without wasting potential energy when the batteries are fully charged would be to use a diversion controller such as Morningstar, for the Schneider 60-150, adjustable current sensors could be used to divert energy whenever loads are light, anything is possible but much thought has to go into design to ensure ballance.
As far as DC is concerned with AC rated contacts, this is not good practice, at very least use the AC thermostat to drive a coil of a DC rated relay/contactor, this way the current is kept to a minimum. SSR's are nice, but when dealing with high current they generate heat, or losses, which need to be compensated for with heat sinks or fans.
Suggestion, start a new discussion for a single query, this will focus on one issue rether then going off in all directions, which leads to confusion. Just a friendly suggestion, hope you appreciate.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
Use half the power for the same amount of hot water.
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.
The existing thermostat can be used with a low (like 3.3V) signaling voltage.
Agreed, panels operating above Vmp is a good indication that the charge controller isn't using all of the available energy (and some could be used for water heating).
I am available for custom hardware/firmware development
|| 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 ,
I am available for custom hardware/firmware development
When the SSR is "on", you have p=i^2*r...
R is at its lowest value when on. Note that heating goes up with the square of the current, but only 1x change in resistance.
2x current = 4x power (heating). 2x resistance only 2x heating.
And as the devices switch from off to on, the parts are in the linear mode. P=VxI. So as the device switches, you have VxI loses.
If this happens 1x second, usually not a concern. If this happens 1,000x per second, then it can be s big concern (more time in Lynett linear mode, more VxI heating. Big power FETs tend to be slower, so more linear heating.
Also big power FETs tend to have more capacitance, high frequency means more energy charging and discharging the gate capacitance. There will be some maximum practical switching frequency (slower devices can only switch every xxx mseconds). Should be in the source specifications somewhere.
I am not a power engineer, but this is roughly how I understand the issues.
Bill
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,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
-Bill