Water heating without charge controller or batteries

[Rikki]

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

 

[BentleyJ]

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.

 

[MichaelK]

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.

 

[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.

Thanks, some useful numbers to work with

 

[BarracudaBob]

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.

 

[hwy17]

The problem is the thermostats are not usually rated for DC.

Otherwise the panel and the heating element will be fine with each other.

 

[Gold Country Russ]

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

 

[Gold Country Russ]

AC water heater thermostats will melt, weld, or catch fire on DC power. Don't do it.

 

[BarracudaBob]

AC water heater thermostats will melt, weld, or catch fire on DC power. Don't do it.

you could use a solid state DC relay for the element and control it by low voltage & current from the thermostat

 

[BarracudaBob]

I like to tinker so I thought the idea was interesting.

Here is some resources: http://waterheatertimer.org/Convert-AC-water-heater-to-DC-using-low-voltage.html

 

[Gold Country Russ]

you could use a solid state DC relay for the element and control it by low voltage & current from the thermostat

View attachment 216113

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.

 

[kmin]

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"

 

[400bird]

I believe 240V water heaters have (2) 120Vac elements in series. You could parallel the elements for 120VAC or 120V DC.

Nope
Nearly all run 240vac to each element, but only run one element at a time.

 

[kmin]

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.

 

[kmin]

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.

 

[Zwy]

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.

 

[kmin]

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.

 

[efficientPV]

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.

 

[BarracudaBob]

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:

Heat with DC Water Heating Elements | Missouri Wind and Solar

Heat water directly using a DC water heating element together with a wind turbine generator or solar panel (with or without a battery bank).

windandsolar.com

There are a couple on youtube too

 

[JRH]

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.

 

[BarracudaBob]

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.

 

[JRH]

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..

 

[BarracudaBob]

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.

 

[Gold Country Russ]

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.

 

[BarracudaBob]

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 love that episode