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Water heating without charge controller or batteries

Rikki

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May 16, 2024
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Scotland UK
Simple question from a very amateur newbie:

Would it harm a solar module to be connected directly to a large resistance load like a water heater? - entirely without a charge controller or battery bank?

I am considering testing the idea with one 120W solar module feeding directly into a 12/24V DC submersible water heater that is designed for connecting to car batteries for boiling small quantities of water.
The aim is simply to warm a large volume of water (a well insulated 240 litre tank) to around 50-60 deg C, and there will be a cut off thermostat switch.

The heater is rated at 600 Watts so may not do very much from a 120W input but I would just like to try it and see, and I guess there is no danger of an overload.

Would this project be a bad idea and if so why?

Many thanks for any constructive input,
Rikki
 
With a resistive load there should be no problem with a direct connection between the solar panel and the heating element. A mechanical t stat should work.

Just for fun lets say the panel produces 100W (assuming good solar conditions and the resistance of the element is matched well to the PV to optimize current) for 5 hours a day which is 0.5kWh = 1700 Btu
Which will raise the temp of 240 liters (528 lbs) of water by 1700/528 = 3.2 degrees. Some of which will be lost at night depending on ambient temp and tank insulation.

Doesn't sound like a very practical way to heat water.
 
I'd suggest that one little 120W panel is not likely to heat a whole tank of water any significant amount. I think you'd be better off wiring four 250W, 30V residential panels in series to make 120VDC, and run that through a regular heating element. Set it up with a DC rated temperature regulator, which will cut off the current once the desired temperature is reached.

Most electric elements today are designed to run at 240VAC, so maybe eight residential panels in series for 240VDC.
 
With a resistive load there should be no problem with a direct connection between the solar panel and the heating element. A mechanical t stat should work.

Just for fun lets say the panel produces 100W (assuming good solar conditions and the resistance of the element is matched well to the PV to optimize current) for 5 hours a day which is 0.5kWh = 1700 Btu
Which will raise the temp of 240 liters (528 lbs) of water by 1700/528 = 3.2 degrees. Some of which will be lost at night depending on ambient temp and tank insulation.

Doesn't sound like a very practical way to heat water.
Thanks, some useful numbers to work with
 
I'd suggest that one little 120W panel is not likely to heat a whole tank of water any significant amount. I think you'd be better off wiring four 250W, 30V residential panels in series to make 120VDC, and run that through a regular heating element. Set it up with a DC rated temperature regulator, which will cut off the current once the desired temperature is reached.

Most electric elements today are designed to run at 240VAC, so maybe eight residential panels in series for 240VDC.
I believe 240V water heaters have (2) 120Vac elements in series. You could parallel the elements for 120VAC or 120V DC.
AC voltages are measured as RMS and equivalent to DC voltages for the same watts.

120Vrms x 1.414 = 169.68 Vpp (peek to peek) of AC. If you looked at an oscilloscope of the 120Vac in your receptacle it would really be ~170Vpp
Smart people a long time ago realized this and came up with Root Mean Square for DC equivalent.

Short answer all of our AC voltages are RMS and equivalent to DC voltage to create the same power. Dumb resistive load doesn't care if you put AC or DC in it. Motors require the AC so totally different.
 
The problem is the thermostats are not usually rated for DC.

Otherwise the panel and the heating element will be fine with each other.
 
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For resistance heating the math is simple and its exactly the same for AC & DC.
Volts x Volts divided by ohms = Watts
You will need volts, watts/amps, and ohms to make an accurate selection. Frequently the ohms are unknown.
Say you saw a heater labeled as 3,000W @ 240V that you want to use but that's all you know about it. To determine its ohms, then 240v x 240v divided by 3,000w=19.2 Ohms.
Also, for heat 1 watt = 3.41 btu
 
I would suggest that a high voltage DC contactor (Littelfuse or equal) would be superior to a DC SSR in a DC water heating application.
Why? He was looking for to use one 120W 12/24V panel. Common AC relays are rated for small DC voltages, normally 24-30V. It's even written on the relay body, for example "10A 250V AC / 30V DC"
 
Thanks, some useful numbers to work with
You will have two problems with this setup, one worst than another.
First, 600W 24V heating element is VxV/P =0.96ohm. Your panels Vmpp=20V and Impp=6A (wild guess),
so at 20V that 0.96ohm resistor would ask I=V/R =20.8A from panel, which is not able to supply because it has max mpp 6A. This would make panels voltage to collapse and as a consequence power would drop down next to nothing.

So to make anything usable you need 4 x 120W panels with that 600W/24V heater, or for one panel, a heater with 4ohm resistor.
 
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Second problem is that this calculation would be valid only on moment of full production. Vmpp of a panel is almost constant, but Impp varies from zero to max, so in practice even with perfect resistance matching, you still would be able to pull out of your panels maybe 50% of your max production.
That's why we use MPPT regulators. It takes care,that voltage doesn't collapse, keeping it always near to Vmpp, by regulating current flow.

Anyway, for low voltage and low power like these, it would not be difficult to build MPPT controller with programmable buck converter and microcontroller.
 
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As mentioned, do not use a thermostat rated for AC on DC power. You will need a DC rated contactor to switch the DC power.
 
As mentioned, do not use a thermostat rated for AC on DC power. You will need a DC rated contactor to switch the DC power.
No you don't. As mentioned normal AC relays are rated up to 24-30V DC.
 
12V heating elements will NEVER match a 12V solar panel no matter how many panels are put in parallel. As said before the heating element resistance has to match the panel. Divide Vmpp by Impp to get ideal resistance. Ideal resistance is not ideal as most of the day sun is less. Double ideal resistance for higher average heat production. Heating water is complicated and direct connect is the worst way. The only reason people here can heat water is they have way too many panels, far to big a battery and way too much money. I use a power point controller so I can still have hot water on poor days.
 
One 300 watt 30 Volt panel would have the impedance of 3.6 ohm, 3 in parallel would be 1.2 ohms, 4 would be 0.9 ohm. Using heavy gauge wire to keep the resistance low you would have the power to drive a 600 watt 24v 0.96 resistive load. Obviously I am speaking theoretical but putting panels in parallel decreases the total resistance/impedance by a factor 1/Rtotal = 1/panel1 + 1/panel2 + 1/panel2 + ...
I would prefer to keep the voltage hi and the current low. There are plenty of DC contactors or solid state relays that could handle the DC voltage & current. Since the invention of money never say never.

There is a link for a DC thermostat on this page:

There are a couple on youtube too
 
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The problem is the thermostats are not usually rated for DC.

Otherwise the panel and the heating element will be fine with each other.
Just asking…. Why if the power is no where near enough to overheat anything would one need a thermostat… let’s say you only wanted a moderate boost in temp ( for what ever reason ) ,,?

Why not just let the panel(s) just produce all they can and then let it cool a bit over night and start again the next day….

I have wondered this too but never dug in deep enough to find out the detail..

Experiment and find the amount of panels needed to reach a pre determined temp in perfect sun conditions with the right size heater element for the water volume and then let it roll …simple rig… maybe adjust the number of panel seasonally …

It’s Gona turn off anyway every afternoon… if it maxes out at ,(let’s say 130F,) why regulate it .. it’s nowhere near dangerous

I can think of a few uses for moderately hot water in a shop …

I’m sure I’m missing somthings, but what…!
J.
 
Just asking…. Why if the power is no where near enough to overheat anything would one need a thermostat… let’s say you only wanted a moderate boost in temp ( for what ever reason ) ,,?

Why not just let the panel(s) just produce all they can and then let it cool a bit over night and start again the next day….

I have wondered this too but never dug in deep enough to find out the detail..

Experiment and find the amount of panels needed to reach a pre determined temp in perfect sun conditions with the right size heater element for the water volume and then let it roll …simple rig… maybe adjust the number of panel seasonally …

It’s Gona turn off anyway every afternoon… if it maxes out at ,(let’s say 130F,) why regulate it .. it’s nowhere near dangerous

I can think of a few uses for moderately hot water in a shop …

I’m sure I’m missing somthings, but what…!
J.
When a switch opens DC it want to arc longer than a AC would. Mechanically they usually switch faster and contact travel farther away. Look what inverters are doing. They switch DC high current all the time. I can be done just don't use a AC device for DC. There are also ways to do it what a "snubber" capacitor to take the surge. For safety use a fuse and some sort of thermostat.

Here is another link to components: https://www.hurricanewindpower.com/dc-water-heating/
Amazon also sells DC water heater elements. I guess it is a thing.. I also watch a couple that use a solar dump voltage controled relay. When the batteries reach a certain voltage they dump the solar into the water heater. This way you can use the same panels for two different purposes.
 
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When a switch opens DC it want to arc longer than a AC would. Mechanically they usually switch faster and contact travel farther away. Look what inverters are doing. They switch DC high current all the time. I can be done just don't use a AC device for DC. There are also ways to do it what a "snubber" capacitor to take the surge. For safey use a fuse and some sort of thermostat.

Here is another link to components: https://www.hurricanewindpower.com/dc-water-heating/
Hmmmm , not sure if I wrote it wrong or you misunderstood my question.… in my scenario there is no switch’s or inverters or anything but some panels wired for the need - 2 wires - a heating element - a tank .

That’s all…

yes I would probably wire in a dc fast action switch just to disconnect if need be…

If it never got over the desired temp running wide open all sunny day , why would I need a thermostats at all…

This is a total off grid distant barn concept where there is no other power available, that’s why I’m asking… why would I regulate somthing that is happy doing what it does 6-7;hours a day..be it 100 degrees orv140; degrees it would be very usable..
 
Hmmmm , not sure if I wrote it wrong or you misunderstood my question.… in my scenario there is no switch’s or inverters or anything but some panels wired for the need - 2 wires - a heating element - a tank .

That’s all…

yes I would probably wire in a dc fast action switch just to disconnect if need be…

If it never got over the desired temp running wide open all sunny day , why would I need a thermostats at all…

This is a total off grid distant barn concept where there is no other power available, that’s why I’m asking… why would I regulate somthing that is happy doing what it does 6-7;hours a day..be it 100 degrees orv140; degrees it would be very usable..
It was a new cool subject for me to look into. I didn't know it was a 'thing' but now I do. Just sharing some though.
 
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Just asking…. Why if the power is no where near enough to overheat anything would one need a thermostat… let’s say you only wanted a moderate boost in temp ( for what ever reason ) ,,?

Why not just let the panel(s) just produce all they can and then let it cool a bit over night and start again the next day….

I have wondered this too but never dug in deep enough to find out the detail..

Experiment and find the amount of panels needed to reach a pre determined temp in perfect sun conditions with the right size heater element for the water volume and then let it roll …simple rig… maybe adjust the number of panel seasonally …

It’s Gona turn off anyway every afternoon… if it maxes out at ,(let’s say 130F,) why regulate it .. it’s nowhere near dangerous

I can think of a few uses for moderately hot water in a shop …

I’m sure I’m missing somthings, but what…!
J.
With no demands you may make steam, melt plastic piping, suffer steam or hot water burns or with the failure or lack of a T&P valve have an explosion. Overheated water heaters were the MythBusters favorite episodes, very impressive and even more dangerous.
Search on YouTube for MythBusters exploding water heater episode.
 
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With no demands you may make steam, melt plastic piping, suffer steam or hot water burns or with the failure or lack of a T&P valve have an explosion. Overheated water heaters were the MythBusters favorite episodes, very impressive and even more dangerous
 
I replaced my water heater a few years ago. I regret not getting a high efficiency heat pump design. Especially now that I could have run it on solar AND cool the room where my batteries are. We are in a miserable heat wave in Florida right now. I had to put a fan in that room to keep them in the mid 80's.
 
Goodness gracious… stop complicating this 🤣……
it’s a damn 55 gallon barrel with a loose lid…. It is not a water heating tank… there will be no steam, there is no valving, there is no plastic piping to melt….

It’s a cauldron of warm water that’s heated ….it has a 3.5 GPM Shureflow pump picking up water when I need to use it through a pick up port…near the bottom…

It’s meant to generate enough warmish water to spray cow shit off the floor…or some shovels …boots …etc.etc etc..

It’s not a new Victron unit I’m thinking about bringing into mass..production…

All I was asking was why would i regulate somthing for safty when there is no danger of hot water exploding or doing anything anything…

Some things in life are simple … just simple…

J.

 
I didn't know it was a barrel. However, I certainly don't want anybody to convert an AC water heater to DC without understanding that the thermostats will not work and that a suitable high amp DC contactor or relay must be used to stop, start, and control temperature. Whenever I see a similar post, I will check to make sure they know the hazards.
 
in my scenario there is no switch’s or inverters or anything but some panels wired for the need - 2 wires - a heating element - a tank .
With a random heating element it would not work, for a reason I wrote in post #14.
Matching your heating element carefully with your panel it would kind of work, but not getting out the full potential of your panel.
In case, that energy was wasted anyway, it could be acceptable result.
 
AC water heater thermostats will melt, weld, or catch fire on DC power. Don't do it.
No it won't.
Those things are ratted to switch 3000 watts at 240 volts.
Those contacts will not be harmed by 12 or 24 volt DC switching 300 watts.

but I would be inclined to heat water directly with the sun through a home brew copper heat ex changer and pex pipes well insulated.
Search Aliexpress for 12 volt ball valves and pumps as required.
 
No you don't. As mentioned normal AC relays are rated up to 24-30V DC.
Do it and let me know how it works out for you long term.

Only takes once to make the potential rocket ship. Enjoy the ride.
 
Goodness gracious… stop complicating this 🤣……
it’s a damn 55 gallon barrel with a loose lid…. It is not a water heating tank… there will be no steam, there is no valving, there is no plastic piping to melt….

Loose lid and 55 gallon barrel with no insulation- good luck with holding heat. You might as well go pee against the wind, it would be about as useful. :ROFLMAO:

Water evaporates and takes heat with it. Raise the temp of water and the rate of evaporation increases. Without a sealed insulated lid, good luck.

It’s a cauldron of warm water that’s heated ….it has a 3.5 GPM Shureflow pump picking up water when I need to use it through a pick up port…near the bottom…

It’s meant to generate enough warmish water to spray cow shit off the floor…or some shovels …boots …etc.etc etc..

Paint the barrel black, let it sit in the sun. It will work much better. I guess I'm just an old guy that learned a few things along the way.

Or use the electric power to run a pump that circulates water thru a 50 foot black hose to and from the tank. I have a water tank on the work truck with a short hose. I use it washing my hands and cleaning heat exchangers. The water in the hose is always nice if the sun is out.

It’s not a new Victron unit I’m thinking about bringing into mass..production…

All I was asking was why would i regulate somthing for safty when there is no danger of hot water exploding or doing anything anything…

The issue is there is not anything safety related in place if an electric hot water heater is used. If there isn't any hot water use over several days, the tank temp will increase each day as hot water heaters do not lose heat very fast. There has to not only be a system in place to control temp but also pressure. As temp rises, pressure will increase until a point of criticality is hit.

I already stated what I thought about using a 55 gallon uninsulated open lid barrel. :)

Some things in life are simple … just simple…

Safety comes first, regardless. It can be simple and safe.
 
No it won't.
Those things are ratted to switch 3000 watts at 240 volts.
Those contacts will not be harmed by 12 or 24 volt DC switching 300 watts.

but I would be inclined to heat water directly with the sun through a home brew copper heat ex changer and pex pipes well insulated.
Search Aliexpress for 12 volt ball valves and pumps as required.
The thermostats have short throw AC contacts that have no arc quench, on DC when the contacts open, they can continuously arc, and you won't ever know it until too late. You will need something like a Littelfuse DCN high voltage DC contactor. To effectively use an AC water heater on DC may involve a string of 3 or more panels with voltage well over 100. Note that most arc welding is at about 24 volts.
 
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In Nam the army had canvas showers.
They were black colored and installed on a framework to hold them up.
I would guess they held 250-500 gallons.
With only the sun to heat them they worked great, however I don't know how good they would work at 10 below.
Greg
 
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In Nam the army had canvas showers.
They were black colored and installed on a framework to hold them up.
I would guess they held 250-500 gallons.
With only the sun to heat them they worked great, however I don't know how good they would work at 10 below.
Greg
The $10 camp showers at Walmart hold a bit less.
 
Loose lid and 55 gallon barrel with no insulation- good luck with holding heat. You might as well go pee against the wind, it would be about as useful. :ROFLMAO:

Water evaporates and takes heat with it. Raise the temp of water and the rate of evaporation increases. Without a sealed insulated lid, good luck.



Paint the barrel black, let it sit in the sun. It will work much better. I guess I'm just an old guy that learned a few things along the way.

Or use the electric power to run a pump that circulates water thru a 50 foot black hose to and from the tank. I have a water tank on the work truck with a short hose. I use it washing my hands and cleaning heat exchangers. The water in the hose is always nice if the sun is out.



The issue is there is not anything safety related in place if an electric hot water heater is used. If there isn't any hot water use over several days, the tank temp will increase each day as hot water heaters do not lose heat very fast. There has to not only be a system in place to control temp but also pressure. As temp rises, pressure will increase until a point of criticality is hit.

I already stated what I thought about using a 55 gallon uninsulated open lid barrel. :)



Safety comes first, regardless. It can be simple and safe.
 
Gee, what a wonderful comment… thank you…… 👍…….have a nice day…

J.
 
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Do it and let me know how it works out for you long term.

Only takes once to make the potential rocket ship. Enjoy the ride.
I was writing about relay, not thermostat. If a relay is rated for 250VAC / 30VDC, either I can use it for <30VDC, or I can't use it even for <250VAC.
Even a thermostat at AC is going to fail after switching x times. That's why you can't setup a water boiler without pressure valve, better even two of them.
 
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