How to / DIY > Everything else
Easy PV water heating
BruceM:
"What would be the simplest system that would take the power from PV panels / a wind turbine / a pico hydro plant and convert it to 220v ac so I can feed a 1000w heating element directly?"
OK, so while you wrote 120 amps I assume you meant your Victron makes 120VAC. If you pitch your electrically leaky elements (which should never have been sold to you or anyone) for some decent 120V elements, then you could manually switch on the element direct from the Victron. Or if you like to live dangerously, add a 1000W toroidal transfomer and use your leaky ones. It will take hours, as your physicist friend noted correctly. The standard element in the US is 4500W, and while dual elements are used, only one is on at a time. So multiply the normaly recovery time times 4.5. For automation, two things can be done to make it idiot proof (something I increasingly find I must have for myself). One, you could add a relay to the element and thermostat on the tank so it turns off when hot. Second, you could add some electronics to turn off the element if the battery voltage dropped below full float voltage, or even better, based on both PV voltage and battery voltage. You'd like to not have the water being heated when the battery is being charged with all the available PV. So turn off when PV voltage is too low (measure it when the battery is below and after bulk charging). A time delay before on relay could prevent rapid cycling if that is warranted.
You didn't mention your PV configuration regarding nominal (marketing) string voltage and amps. Since you wrote about 1000W heating element I am assuming you have more than that for PV power. I could find an appropriate voltage sensing relay if I knew the PV voltage. Many systems today use higher voltage series strings of PV to reduce wiring losses, which the PV charge controller buck converts down to your nominal 12V battery voltages. Depending on the actual voltage you are seeing on the PV side of the charge controller while charging and once the batteries are near or after full, I can help pick some appropriate hardware. Voltage sensing relays seem to be in the $14-30 range.
Other possibilies are to use dedicated PV panels for water heating, with a DC solid state switch rated above the maximum open circuit voltage of the panels. 4- 24V, 240-270W panels will operate a 1000W, 120V element nicely. Only a thermostat would be needed to make it fool proof, self regulating.
Best Wishes,
Bruce
mihit:
If you're sticking with AC elements, I'll chip in with "micro inverter" - these clip/bolt/glue on the back of the panels and invert panel output to your line voltage AC, then you run AC wiring from the panels to the load.
Simplest I think. Probably not cheapest.
Also knocks your panels out for battery charging unless you either plug in a battery charger, or otherwise rectify your AC and through a controller of some ilk.
mihit:
--- Quote from: mike90045 on March 27, 2022, 06:26:30 PM ---
--- Quote from: mihit on March 27, 2022, 06:54:45 AM ---The most efficient solar water heating is vacuum tubes. ........
--- End quote ---
Yes, but the 5+ year failure rate on the tubes is abysmal. I see way too many folks looking for replacement tubes, developing schemes to recharge the tubes with acetone or something ( I love sweat soldering a tube with acetone in it ) to even want to bother with any of the vacuum tube systems. Often, many tubes have failed, but the rest of them keep the system going for a while.
--- End quote ---
Really? Interesting... I think here they're warranted for 10-15 years.
I haven't gone there yet, but it was definitely my plan...you've got me thinking twice now. What kind of climate are you talking? What are the failures?
BruceM:
Alas, the microinvetrters I'm aware of are for grid tie only and not stand alone. But I like the idea!
There are a couple all in one inverterr/PV charge controller units that will operate without batteries that could do the job, but not cheaply.
One thing I forgot to mention is that a (US) standard 4500W/230V water heater element is about 1200W on 120V. So perhaps the leaky element can be replaced cheaply.
veggie:
dieselspanner,
Have you looked into the CyboEnergy hot water inverter.
Might work well in your case.
https://www.cyboenergy.com/products/offgridHmodeloverview.html
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