diy solar

diy solar

DC water heater element?

Photos of what i have done so far. 3 panels on the north east side of the house to catch solar gain to feed one side of the water heater element. 3 panels to feed other half water heater element on the south west portion of the roof. 4 panels on south west x 2 feeding 2 separate solar setups. 10 circuits feeding 90 percent of the 110 power for a 2200 sq ft home. Things working great. Still have to pretty things up and add outside disconnects but pretty well through for now.
 

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Mine is working great.
Yes. PV panels and heating element, suitably matched,with only the cable between the two is the way to go.
I think marketing is beginning to intrude with sellers pushing their products under the 'mppt' mantra.
 
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I just started playing with this. Yesterday, in full sun, I hooked a 48 volt 600 watt element directly to a 100 watt HQST 12 volt panel. Just a quick test, It was about 30-45 seconds and I could barely hold on to the element. Unhooked it and had to go do other things. Will try to get it in some water maybe today and actually start testing.

 
There are lots of DC heating elements out there, and you can choose AC elements that will work too.
First, doesn’t matter ac or dc, it’s a resistance. I personally use 1500 and 1000w 120v water elements. You can derive the resistance, but on 12v they won’t make a lot of heat. I use extra array power out of the inverter After the battery charges to heat a preheater hot water tank and send that to my regular heater which is a propane heated tank. Both are 30 gallon.
 
First, doesn’t matter ac or dc, it’s a resistance. I personally use 1500 and 1000w 120v water elements. You can derive the resistance, but on 12v they won’t make a lot of heat. I use extra array power out of the inverter After the battery charges to heat a preheater hot water tank and send that to my regular heater which is a propane heated tank. Both are 30 gallon.
The problem with trying to use AC elements is that the OHM's are too high therefore not generating very much heat at all. 120/240 volt elements start out around 8 ohms on up where 12-24-36 volt DC elements start at 1 OHM (12 volt - 400 watts) Match your element with your panel output and everything works great.
 
I just started playing with this. Yesterday, in full sun, I hooked a 48 volt 600 watt element directly to a 100 watt HQST 12 volt panel. Just a quick test, It was about 30-45 seconds and I could barely hold on to the element. Unhooked it and had to go do other things. Will try to get it in some water maybe today and actually start testing.

With just a 12 volt source you need to be using a low OHM element like a 12 volt. You will see a very noticeable heat jump from a 48 volt to 12 volt. My first test was with a 12 volt / 400 watt element. Within a a few seconds it melted through a 5 gallon plastic bucket it was sitting on.
 
First, doesn’t matter ac or dc, it’s a resistance. I personally use 1500 and 1000w 120v water elements. You can derive the resistance, but on 12v they won’t make a lot of heat. I use extra array power out of the inverter After the battery charges to heat a preheater hot water tank and send that to my regular heater which is a propane heated tank. Both are 30 gallon.

Power dissipated by a resistor is V^2/R
if you apply 1/2 the voltage, you get (1/2)^2 = 1/4 times the power.

A "12V" panel puts out about 17V, more or less depending on current draw.
(17V/120V)^2 = 0.02

A 1500W 120V element connected to a "12V" PV panel would put out 1500W x 0.02 = 30W
Make that two "12V" panels in series and you would get 120W. Three in series for 270W (a good load for 3, 100W 12V panels)

The problem with doing this is that the thermostat, also the over-temperature safety switch, are only designed to interrupt AC. They will probably fail on DC. Especially at lower voltage I might be able to implement a "snubber" to stop the contacts from burning. Better would be to wire the thermostat into coil of a relay rated to handle the DC voltage and current, have it control application of PV DC to heating element. With the thermostat switching an inductive load, I would add an RC snubber to prevent arcing and burning of the contacts.

The DC elements with thermostat built in are designed to switch DC reliably.

An easier and safer way to use the water heater as a dump load for excessive PV power is to feed it off your inverter. The thermostat and safety devices built in to the water heater will work reliably then. Either use the water heater at its rated power, or connect a 240V heater to 120V supply for 1/4 the power draw. Many have two elements, so you can use one or both to adjust power draw. If you wire the two elements in series, that will draw half the power of a single element.

Because temperature of water will likely vary depending on surplus PV, you can install a tempering valve to blend hot and cold, delivering constant temperature. Then you can jack the thermostat setting up higher than typical, for more energy storage in the same tank while the tempering valve provides scald protection.

If you don't use a tempering valve, you can at least put a "scald stop" on the shower head. People do receive severe burns from showers when water heater is too hot. I have a shower with tempering valve built in, which was very nice (doesn't go hot when someone flushes), but became gummed up with minerals after many years.
 
So is there a consensus on if we can do this safely yet? I will say the day in coming that we will agree on how to do it.
 
We don't do "consensus" here. :)
Hello Hedges, you are one that has great feedback and I gather from your comments in the past that you have some reservations on the things being done to take panel power directly to a energy supply, like direct electric water heating. Is my suspicion correct?
 
Hello Hedges, you are one that has great feedback and I gather from your comments in the past that you have some reservations on the things being done to take panel power directly to a energy supply, like direct electric water heating. Is my suspicion correct?

Yes. The heating element will be fine operating on DC but the thermostat designed for AC won't. The thermostat and overtemperature cutout need to be operated with AC or low-current "pilot duty" use.
 
Yes. The heating element will be fine operating on DC but the thermostat designed for AC won't. The thermostat and overtemperature cutout need to be operated with AC or low-current "pilot duty" use.
OK, that is interesting and helpful.
 
First, doesn’t matter ac or dc, it’s a resistance. I personally use 1500 and 1000w 120v water elements. You can derive the resistance, but on 12v they won’t make a lot of heat. I use extra array power out of the inverter After the battery charges to heat a preheater hot water tank and send that to my regular heater which is a propane heated tank. Both are 30 gallon.
Can you post photo of your setup? Have you considered the sbms0 and the dssr20 with diversion of you have a 12v or 24v system?
 
First, doesn’t matter ac or dc, it’s a resistance. I personally use 1500 and 1000w 120v water elements. You can derive the resistance, but on 12v they won’t make a lot of heat. I use extra array power out of the inverter After the battery charges to heat a preheater hot water tank and send that to my regular heater which is a propane heated tank. Both are 30 gallon.

What is the source of control signal you use to enable the heating elements?

During bulk charging, there may or may not be surplus PV production depending on whether battery charging is regulated to a target charge current.
During absorption, battery is held at constant voltage and controls accepted current itself.
During float, very little current goes to battery.

In general, surplus PV watts may be less than heating element will consume so might need to toggle on and off cycling batteries.

I have available programmable SoC based relay switching by Sunny Island. Also battery voltage.
The best signal is AC frequency - it ramps up and Sunny Boy curtails production: 100% output up to 61 Hz, linearly decreasing to 0% output at 62 Hz.
Optimum control of load might be 0% up to 60.5 Hz ramping to 100% at 61 Hz. That could drive a dimmer switch with horrible power factor.
I think my transfomerless GT inverters were upset by a VFD which would also have poor PF. I'm changing those to transformer type.
Clean but complicated is a switcher to draw sine wave current and feed heater with variable Vrms. SMA used to sell one:


Similar scheme could vary motor speed for water pumping, heat pump and A/C loads.
 
Ok, so i said that i would keep you in the loop on my results. Scenario that i am using is replacing lower 240 volt water heater element in my water heater with DC current pulled off of my solar controller from the line feed when the batteries went on float mode not contributing anything to collection of solar energy.
Re-capping: I tried multiple different dc heating elements that did not work very well and will concentrate on the progression of best results.
So I did a comparison test on DC water heater elements. First tried single element 24 volt 400 watt with a dc controlled thermostat. In this first test i attempted to use energy direct from solar panels not going through the solar controller. Panels used were 305 watt , VOC 45.6 volts, ISC 8.921 amps. Tried different panel combinations in parallel and in series, best results were 3 panels wired in parallel. Using 5 gallon bucket of water, the 400 watt element heated water from 75 degrees to 145 degrees. Readings off of the element with 3 panels were 32 volts with 22 amps or 704 watts from a 400 watt element. I installed this unit first in my lower hot water heater bung. Worked great for about 4 hours and then the dc thermostat fried. Not good to push 700 watts through a 400 watt thermostat. Thermostat was still ok and working but I pulled the element thinking that it was going to fry and cause a leak to happen. Next i tried a 24 volt 900 watt single element. It took 1 hour to raise the water temp in a 5 gallon bucket of water from 75 to 125. This was ok but did not do as well as the 400 watt element. Back to the books to see what I could try next. I had someone on here mention a dual probe dc water heater element, next chapter.
We're did you buy the 900 watt 24V heating element. I'm looking for one. Thanks
 
900 watt 24V heating element.

Thats insane. Almost half ohm. Wire and every connection will add almost that much and reduce heating by half.
 
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What wattage inverter is recommended for 24V system?

Now consider that inverter, running off 24V, powering a 900W 120V water heater element.

Amps is amps. 900W / 24V = 37A, not a big deal, except IR loss is 5x larger percentage of 24V system vs. 120V system.
 
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