Solar to Hot Water Heater Hookup

[John1397]

If one hooks up solar panel to bottom element after removing AC line seemed like good idea then I was wondering if the wires going to the element will get hot instead of the element see drawing.

 

[Rive]

If that solar panel really has just 200W maximal power you don't have to worry, it won't make anything hot in that setup.
Especially your 4.5kW heating element...

... but I would advise against trying this with a matching 4.5kW solar panel array. DC voltage at that power is not a joke.

 

[berkeman]

If one hooks up solar panel to bottom element after removing AC line seemed like good idea then I was wondering if the wires going to the element will get hot instead of the element see drawing.

Hooking a "200W" PV panel to a "4500W" heater element in a water heater is a total mismatch in impedance, and you will be wasting lots of power. Do you understand what the "200W" rating of that PV panel means? Do you understand what that "4500W" heater element rating means? (What voltages and currents apply to each, for example?). Have you learned about Maximum Power Point Tracking (MPPT) for PV panels yet?

 

[OmCheeto]

If that solar panel really has just 200W maximal power you don't have to worry, it won't make anything hot in that setup.
Especially your 4.5kW heating element...

According to my calculations, a 24 volt, 200 watt panel, @ 4 hours solar input per day would supply about 10% of the energy required for a 100°F/38°C, 10 minute long, 2 gpm(7.5 lpm) shower. (60°F[16°C] cold water supply temperature)
But if a DC-DC step up device were installed, to up the voltage, such that the panel could supply its full potential of 200 watts, that would jump to 40%.
Not too shabby!

At 24 volts, the panel is limited to 50 watts, with the heating element's 11Ω resistance.
Actually, a 48 volt, 200 watt panel would supply 200 watts without an inverter, in this case.
Yay! No fancy circuits to burn up.

... but I would advise against trying this with a matching 4.5kW solar panel array. DC voltage at that power is not a joke.

I agree with you there.
But this begs the questions: How often people actually run out of hot water? Should a solar PV auxiliary system be designed for the worst case condition?
Or would a better question in deciding how to optimally design the system be: How much hot water do people use on an average day?

 

[OmCheeto]

... I was wondering if the wires going to the element will get hot instead of the element see drawing.

For a 48 volt panel, I get about 4 amps for your system.
I believe 12 gauge wire will handle that.
So, no. The wires should not get hot.

(Compared to the element that is. Anything with resistance is going to have an associated temperature rise when you run a current through it. So if you are cleaver enough, you might be able to thermally insulate your wires, and start a fire.)

 

[John1397]

Hooking a "200W" PV panel to a "4500W" heater element in a water heater is a total mismatch in impedance, and you will be wasting lots of power. Do you understand what the "200W" rating of that PV panel means? Do you understand what that "4500W" heater element rating means? (What voltages and currents apply to each, for example?). Have you learned about Maximum Power Point Tracking (MPPT) for PV panels yet?

200 watt panel actually only puts out 100 watt against 4500 watt will not be much but I will get 1KWH per day should take chill out of water is what I am thinking. Battery has different characteristics than fixed load do not know if MPPT would do much good.

 

[berkeman]

200 watt panel actually only puts out 100 watt against 4500 watt will not be much but I will get 1KWH per day should take chill out of water is what I am thinking.

I'm an engineer at heart (as well as at work), and the idea of throwing away over half of the available energy (probably more in your case) is unpleasant for me.

Battery has different characteristics than fixed load do not know if MPPT would do much good.

I'm having trouble parsing what you said there, but putting a basic MPPT converter between your PV and your water heater element will make it work much better. On the flip side, your water heater element is built for an AC Mains power input, so your MPPT converter circuit would need to output an AC Mains level voltage. If you buy an off-the-shelf unit, that should be fine (it should come with safety certifications), but if you were going to try to build your own, I'd have to recommend against that.

 

[Rive]

But this begs the questions: How often people actually run out of hot water? Should a solar PV auxiliary system be designed for the worst case condition?

You are asking the wrong guy: once the price of used (scrapped?) solar panels hits the mark I'll just buy them - to be used as roof tiles

But till that, I would expect reasonable return for any investment, and right now that means some minor details like decent design, inverter ready to handle PV panels and such

 

[gmax137]

when I lived in south florida many houses had solar hot water systems installed. very effective, but you'd have to do plumbing. It is more work to install than a PV panel and wires, but you might get a much bigger bang for your buck. the house I lived in had the electric hot water heater turned off for 8 or 9 months out of the year.

 

[John1397]

The only problem is I took a used 4500 watt heater X 240 volt and applied 12 volt supply sure it gets a little warm but it only draws 1 amp that's not going to do much and it has 12 ohm resistance what I need is element that draws 8 amps when applying 12 volts any ideas?

 

[berkeman]

The only problem is I took a used 4500 watt heater X 240 volt and applied 12 volt supply sure it gets a little warm but it only draws 1 amp that's not going to do much and it has 12 ohm resistance what I need is element that draws 8 amps when applying 12 volts any ideas?

Do you not see a fundamental problem with this approach?

 

[John1397]

I think if I take the measurements of the element dived by 12 ohms this will give me resistance per inch then using ohms law I would need 8 times shorter wire or so. Then instead of 12 inch long element might end up with two or three inch long element as less ohms should get hotter.

 

[Opera]

I build these MPPT systems to make hot water and they are quite simple to build, but far too complicated for most people. The fundamental principle can be found in this paper https://www.mdpi.com/2071-1050/13/18/10270/htm As an example, my garage has a 40gallon hot water tank for laundry with two 4500W 240V heating elements in parallel at 60V DC from solar panels. This provides enough hot water to do all my laundry loads with hot water. Tall tanks are preferred because of stratification. The top third heats easily and the lower part provides pre heating. Less than $20 besides panels. Just a fixed voltage setpoint is sufficient if adjusted seasonally. I use mine in parallel with a MPPT charge controller to harvest excess solar which would otherwise be wasted.

[Reference to user's YouTube channel removed by the Mentors]

 

[sophiecentaur]

If one hooks up solar panel to bottom element

This would depend on your practical skills but did you consider replacing the heating element with a low voltage element? This link has a 12 / 24 V element that could match your DC supply and I'd bet that there are adaptors to allow it to fit the existing Immersion Heater thread. I appreciate that plumbing can be a bit scary but don't reject the idea without a thought.

 

[Opera]

There are many battery voltage heating elements available. Still, 300W is about the practical limit for 12V as wiring loss becomes a factor. A heating element is only rated for so many watts per inch and that heat density should not be exceeded. Higher volts is a better way to proceed with grid tie panels in the 30V range. Above that voltage mechanical thermostat contacts will arc and burn. Direct connect with solar panels is inefficient most of the day requiring twice as many panels. Direct connect only has 2 hours of usable solar energy a day. Panels are current sources. Lower sun produces less current. Power is current squared X resistance. At half panel current, power is 1/4. Electronics to maintain power point voltage is needed. One KWH only heats 6 gallons of water. Pictured is an efficient water heater control I made for less than $20 just to show how simple they can be.