PV to DC Water Heater Elements
- Thread starter OnIsland.OffGrid
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Al Coppa
New Member
I don't know how you can match anything when using those elements. 600 / 12 = 50 Amps and 600 / 50 = 12 Ohms.
The 400 watt panels produce about 45 volts and that exceeds the 12 volt rating of the elements. No matter how you wire the panels you exceeded the rating on the heating elements.
Al Coppa
New Member
Let me explain something that I accidentally discovered about 3 years ago when I was testing some 3 year old panels. It was not my goal in life to use solar electric to make hot water. I headed down a rabbit hole not knowing where it was going to take me. It order to understand this it is necessary to be able to let go of something that I call " Grid think mentality "
Solar panels were designed to charge batteries and water heaters were designed to connect to the grid. They almost have nothing in common with each other. That is what you need to let go of.
I will explain to you what people are doing, and in doing it, they fail to make hot water or they make very little.
I will explain this using your numbers that describe your panels. The panels must be pointed directly at the sun.
Your panels produce 42 volts at their maximum power point. And 9.5 Amps. That means that they are 400 watt panels.
When you wire 4 panels in series then you get 1600 watts. The number that you will use is 85% of the 1600 watts when the panels are either brand new or up to 3 years old. That means that we use 1360 watts when making calculations.
I am using Ohm's Law to get the Ohmic value of the element that must be used.
4 X 42.1 = 168.4 volts. And 85% of the wattage = 1360 watts.
( 168.4 X 168.4 ) / 1360 = 20.85 Ohms of resistance is required at the maximum power point.
A 240 volt 3000 watt element has a resistance of ( 240 X 240 ) / 3000 = 19.2 Ohms.
When the element gets hot it will probably increase to just below 20 Ohms.
You decided to connect two 4500 watt element is series and that would be 12.8 X 2 = 25.6 Ohms and that is higher than what the maximum power point is looking to see,
You would need to buy two 240 volt 5500 watt element because each element has 10.5 Ohms resistance. And two in series would be 21 Ohms.
Al Coppa
New Member
When the solar panels produce about 40 volts each then your voltage is 160 volts. With a load on the panels that is going to drop to about 150.
The resistance of a 4500 watt element is 12,8 Ohms and increases to about 13 when it gets hot. That would be 26 Ohms of total resistance.
Ohm's Law states that the voltage squared divided by the resistance equals the wattage.
That would be ( 150 X 150 ) / 26 = 865 watts of electrical output that gets turned into heat.
( 865 / 1200 ) X 100% = 72% output. That's not enough of an output for me to want to do as a project.
I am looking to get a number closer to an 85% output.
240 volt 5500 watt elements are 10.5 Ohms and probably 10.7 when hot. That would be 21.4 Ohms.
The output voltage of the panels will most likely drop to about 145 because now they have more load on them.
( 145 X 145 ) / 21.4 = 982 watts and a gain of 117 watts. ( 982 / 1200 ) X 100% = 81.8% output.
982 watts is almost 1 Kilowatt and 3350 Btu's of heat going into the water.
I was able to boil a gallon of water in about 10 minutes only using 450 watts coming from two 265 watt panels wired in series.
I was able to do that by matching the heating element to 85% of the panels maximum power point.
Vmp at 50°C panel temp should be 39V at 10A load, and with 4 series panels + 26 ohm resistor load would be < 10A and voltage will be > 39V, likely 41V = 164V²/26 = 1034W in full sun. That may not be good enough for you but that should generate 22°C of heat gain into 50 gal tank per day (assuming no thermal loss). That's enough for 2 people to take showers and do dishes.
efficientPV
Solar Wizard
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If you want better than 85% output, you shouldn't do direct connection for water heating. The person who came up with this label IDEAL RESISTANCE should be shot. You only get that for an hour of the day at best and under ideal weather conditions. Go back and do some math at 70%, 50% and 30% current. It is the power of squares that kills you with direct connect. When I get these currents with an array I'm feeling good. There is a great penalty for going ideal resistance and under, very little penalty in daily power for going over ideal resistance. 85% is an optimistic value to shoot for. 70% is more realistic. Those elements in series will produce a lot of hot water in this situation. Series will not be ideal as the top will get too hot. Better to put a higher wattage element in the top (lower watts in series) and lower wattage in the lower section (that produces more watts). Absolutely he will need thermostat control with SSR.
If someone posted charging a battery with PV and a knife switch, everyone would respond with righteous indignation. But do stupid direct connect water heating and everyone says, meeee tooo!
Al Coppa
New Member
Your mistaken, you put 3 panels on a tracking mount and you get 85% 12 hours a day in the summer where I am located.
A 675 watt output is enough power to heat 44 gallons of cold water to 125 degrees in the summer, each day, for about 5 months.
Using an electronic device keeps people in solar groups from learning how electricity works. They use a plug and play method of snapping connectors together and then hoping for the best. It not an optimistic value because its been tested and it works.
It will not work in the winter because there is a reduced number of hours in the day and clouds.
The assumption in solar groups has always been that if I can't do it then no one else is smart enough to do it either.
My background is not in solar. It is in electrical experimentation.
Solar is a very small part in the entire electrical industry.
Battery charging is what needs MPPT controllers because the controller has a charging program that it uses and it decided whether it wants a 50% output coming into the batteries or 100%. It controls the charging program and not the panels.
Lavarock7
New Member
While not Solar electric, have you considered either of the following 2 ideas?
A cheap Propane on-demand hot water heater instead. Propane for this would probably cost $22 a tank at Walmart/Lowes/HD and would last a month or two for showers, etc. You would need either an A/C supply for the burner or some igniters use a battery.
Throw a long garden hose on the roof and let solar do it's thing. Well, maybe a couple hoses. I get scalded from hose water left in the sun for a while.
Luckily here in Hawaii the temp of water is either 70 degrees or so minimum where mainlanders could have 55 degree water supply to their house.
A cheap Propane on-demand hot water heater instead. Propane for this would probably cost $22 a tank at Walmart/Lowes/HD and would last a month or two for showers, etc. You would need either an A/C supply for the burner or some igniters use a battery.
Throw a long garden hose on the roof and let solar do it's thing. Well, maybe a couple hoses. I get scalded from hose water left in the sun for a while.
Luckily here in Hawaii the temp of water is either 70 degrees or so minimum where mainlanders could have 55 degree water supply to their house.
Al Coppa
New Member
You said that you are in Hawaii. Can you tell me how they are charging you for electricity now? Is it so many Kilowatt hours at 40 cents each and then the rest at 55 cents? Do people get $800 electric bills?
Lavarock7
New Member
Yes it is but I don't have those numbers handy at the moment. I am a very light user. With the exception of stove and dryer and perhaps a light use of an appliance or two which are still on the grid, my consumption is something like 400 watts. While not yet fully set up, my usage from the utility is dropping.
I will double my solar PV input soon from 1600 watts to 3200 watts and have my 3rd battery online. That should help greatly.
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Lavarock7
New Member
Each County in Hawaii charges different rates. Our rates will probably go up because the electric company was sued over the Maui wildfires.
One problem with rates is they usually state the kWh rate. Hawaii has lots of extras and also includes our G.E.T. (General Excise Tax) which is a progressive tax on goods and services (not a sales tax). It is like seeing an ad for a vehiucle that when you get the 'out the door' price it is much higher. So I give you the total bill is just over 46 cents per kWh.
The best I can determine, these are the new rates. I am on Hawaii Island. Daytime is 9am-5pm, Evening Peak is 5pm-9pm and Overnight is 9pm-9am. Note that in the rates below, the rows are not in time order.
This is out solar setup which is supplying solar to part of the customers here on island. https://www.aes-hawaii.com/waikoloa-solar-storage-project#Project overview
One problem with rates is they usually state the kWh rate. Hawaii has lots of extras and also includes our G.E.T. (General Excise Tax) which is a progressive tax on goods and services (not a sales tax). It is like seeing an ad for a vehiucle that when you get the 'out the door' price it is much higher. So I give you the total bill is just over 46 cents per kWh.
The best I can determine, these are the new rates. I am on Hawaii Island. Daytime is 9am-5pm, Evening Peak is 5pm-9pm and Overnight is 9pm-9am. Note that in the rates below, the rows are not in time order.
This is out solar setup which is supplying solar to part of the customers here on island. https://www.aes-hawaii.com/waikoloa-solar-storage-project#Project overview
I like trying ideas out.
This past weekend, I hooked a 3500 watt element up to 4ea. 460 watt bifacial panels in series.
Cold clear weather, got 163VDC under load, 9.4 amps, 1,532 watts from the element.
3S2P, same panels would get around 2100 watts from the element.
Either will definitely heat water if it is on for long. As long as it gets sun, it will produce some heat.
You can get a 30A 900VDC rated contactor with a 12VDC coil for less than $60 online.
The coil draws around 0.5 amps, so I would not be afraid to run a $10 12VDC plug-in transformer through the water heater
thermostat to power the coil.
I seriously doubt 6 DC watts would destroy the thermostat.
PV power to the lower element, household power to the upper element, in case the PV isn't enough.
This past weekend, I hooked a 3500 watt element up to 4ea. 460 watt bifacial panels in series.
Cold clear weather, got 163VDC under load, 9.4 amps, 1,532 watts from the element.
3S2P, same panels would get around 2100 watts from the element.
Either will definitely heat water if it is on for long. As long as it gets sun, it will produce some heat.
You can get a 30A 900VDC rated contactor with a 12VDC coil for less than $60 online.
The coil draws around 0.5 amps, so I would not be afraid to run a $10 12VDC plug-in transformer through the water heater
thermostat to power the coil.
I seriously doubt 6 DC watts would destroy the thermostat.
PV power to the lower element, household power to the upper element, in case the PV isn't enough.
Or get 500A 900V 1.5w 12v coil contactor for $15.
Al Coppa
New Member
I don't think that you are going to have any problems.
I was going to use a 25 Amp solid state relay and run about 6 volts at 40 milliamps through the thermostat. These solid state relays require 3 to 32 volts DC to trigger them. I am going to mount it on a test board that has no battery charging on the board.
I thought about using 3.7 volt solar light batteries to power the relays. In an emergency I can charge them using the solar light that they were originally part of.
efficientPV
Solar Wizard
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There was a nice video where panels were aimed every 15 minutes and he got amazing consistent results thru ought the day. I thought that was misleading to many who would think the same results with fixed panels. I have some east facing panels and they supply 70% of my daily energy. 99% don't track as that is costly and limited to those who have a lot of space. It still doesn't account for clouds, something you just give up on. I have certainly known for years there is very little technical talent in the solar world. Actually, resistance PV heating needs power point far more than battery charging the solar world doesn't understand that. Maybe you should experiment more and see what panels produce in a fixed setting.
The following is a post I made elsewhere for a guy with chickens wanting to melt ice. I think it is a good example.
All solar is local. This morning it is 8 degrees, overcast and there is a slight dusting of snow on the panels. At home I have
a 450W 60V array for supplemental heat to my heat pump water heater. This is mostly for product development, my summer home is all solar.
At this time the system is producing 0.25A @ 60V or 15W. This is lower than usual but in winter getting 40 to 70W (15%)is not.
I removed the snow about an hour later and current increased to 0.96A (58W). The current rating of these panels is similar to yours.
You design for the bad days. Good days take care of themself.
Suppose your panels and element can only produce 1A on a winter morning. This data can be verified by measuring element voltage in the morning.
1A X 1.5 ohms = 1.5V Multiply that by 1A and it produces 1.5W.
2A X 1.5 ohms = 3V Multiply that by 2A and it produces 6W.
3A X 1.5 ohms = 4.5V Multiply that by A and it produces 13.5W.
That 200W panel hasn't provided a lot of power but it was needed to get that much current. Panels are current sources.
This range of current is typical for winter. Days with more sun likely wouldn't need a heating system. This may struggle in winter.
Let's substitute a 6 ohm heating element
1A X 6 ohms = 6V Multiply that by 1A and it produces 6W.
2A X 6 ohms = 12V Multiply that by 2A and it produces 24W.
3A X 6 ohms = 18V Multiply that by A and it produces 54W.
This system will peak at about 70W because resistance will limit voltage to about 23V, the open circuit voltage. My guess is 20W is needed to melt ice.
This looks fairly successful.
Finally a power point system where winter panel voltages will be about 21V and stay there regardless of current.
1A = 21V Multiply that by 1A and it produces 21W.
2A = 21V Multiply that by 2A and it produces 42W.
3A = 21V Multiply that by 3A and it produces 63W.
This is magnitudes more successful. The new problem is a temperature control system to prevent the water from becoming a bacterial stew.
With 200W you could boil the water. A power point control can operate with a 1.5 ohm element. Pulses are always the maximum current of
15A. It operates much like a light dimmer using duty cycle to control power. The electronics is just easier to build with higher voltages.
Al Coppa
New Member
I only do experiments in the summer because there is not enough power in the winter to do anything with. It would be like buying a race car and then testing it to see whether it can go 30 miles an hour. I am interested in going 150 miles an hour.
Where I am, in the summer, the sun produces an 1120 watt output per square meter so that increases the output power by about:
( 1120 / 1000 ) X 100% = 112%. I use a special meter that tells me the value of solar power coming down to the earth.
When we got into winter I used my meter to measure the solar output and it read 30 watts per square meter.
In my case, with my 265 watt panels that would be ( 30 / 1000 ) X 265 = 8 watts.
In the summer, I did not test panels for a maximum output, only to see whether I could get an 85% output.
I need 85% to be happy doing electrical experiments and Will Prowse needs 80% to make him happy. The panels were only 3 years old so they had to be called used panels. I wanted to buy them new but they got discontinued just before I was able to buy any.
Al Coppa
New Member
I accidentally discovered how to make this all work when I was testing some used solar panels.
One thing that you asked about was arcing. Use solid state relays that are powered using a 3 to 32 volt DC source to power the relay or relays. They draw about 40 milliamps of current. Then you are able to use a standard water heater thermostat because it can handle 40/1000th of an Amp.
I gave myself a graduation party and I rarely do anything with 12 volts and I do not use 12 volt heating elements that use 600 / 12 = 50 Amps.
That is an insane waste of current just to make hot water.
I had two panels that were 265 watts each and I was testing them to see whether I could get an 85% output and this is how I did it.
The MPP voltage squared / 85% of the power rating = The resistance of the heating element that is required to be used.
That is used on panels that are either brand new up to about 3 to 4 years old.
My panels produced 30 volts so 30 + 30 = 60 and ( 60 X 60 ) / .85 X 530 = 8 Ohms.
I put an 8 Ohm element in a gallon of cold water and I had the water boiling in about 11 minutes. That was one gallon of cold water and 450 watts.
I was going to do something on Youtube about heating water and I was going to use 3 panels wired in series and solve the equation for an 88% power output.
The math for the 3 panel water heater is MPP volts equals 90 volts and 90 X 90 = 8100
8100 / ( 88% X 795 watts ) = 8100 / 700 = 11.6 Ohms of resistance.
A 240 volt 5000 watt water heater element has a resistance of ---- ( 240 X 240 ) / 5000 = 11.5 Ohms and that is the one that I am going to use.
mrnudie
New Member
i have a 7 panel string (240vdc) (400w bifacial panels) going to a 2400w (24ohm) hot water storage service. The thermostat uses a/c and a contactor with 3 contacts in series and a 50watt light bulb. That way there is no dc ark to extinguish as a lamp when “cold” takes care of that. The contactor also has 1 normally closed contact that i will use to run more power (mainly cloudy days) back to the battery bank via a high voltage mppt when the heater is hot.
Warpspeed
Solar Wizard
Connect a relatively large capacitor across the solar voltage source (thousands to tens of thousands of uF recommended).
That will be charged continuously by current from the solar panels.
Allow the combined voltage to rise above the rated maximum power panel voltage (MPP shown on the solar panel ratings plate).
When the upper threshold voltage is reached, connect both the combined solar source and capacitor to the heating element by switching on a mosfet.
The heating element will then discharge the capacitor, pulling the solar voltage down.
Allow the voltage to fall to a lower voltage below the rated peak power voltage.
Turn the mosfet off, allowing the voltage to again rise.
The voltage across both solar panels and capacitor ramps up and down between two set voltage thresholds.
The system cycles or oscillates at a relatively low frequency. In effect very slow motion pulse width modulation.
These voltage thresholds are set to be either side of the maximum rated power voltage, thus always maintaining the solar voltage fairly close to the maximum peak power point from twilight to full sun.
The heating element resistance should be kept "reasonable" but is not absolutely critical.
It will discharge the capacitor pretty much regardless of the actual resistance.
Power transfer efficiency will remain high over a very wide range of solar, and a reasonable range heating element resistance.
In gloomy conditions, the cycling rate will become very slow, but it will still pulse power into the heating element without overloading the solar panels.
Some kind of transient voltage suppression across the mosfet will be required, quite a few different approaches to that.
An avalanche rated mosfet will work very well (at a low enough repetition rate) so will a fast diode back to the solar capacitor, or even a simple snubber.
If the controller is located fairly close to the heating element there should not be much series inductance there to cause a problem.
If the capacitor is made large, the system will cycle slow enough (in the order of seconds) that any switching losses will be negligible.
While the system is cycling on and off, the capacitor charging current from solar remains constant.
Quite a few of these have been successfully built, and they work amazingly well.
*hint* A standard garden variety 555 timer chip contains two voltage comparators connected to the set and reset inputs of a flip flop.
This can be run in normal astable mode, using the high voltage solar capacitor, and two separate adjustable voltage dividers.
Output of the 555 will need to be inverted to drive the mosfet gate.
That will be charged continuously by current from the solar panels.
Allow the combined voltage to rise above the rated maximum power panel voltage (MPP shown on the solar panel ratings plate).
When the upper threshold voltage is reached, connect both the combined solar source and capacitor to the heating element by switching on a mosfet.
The heating element will then discharge the capacitor, pulling the solar voltage down.
Allow the voltage to fall to a lower voltage below the rated peak power voltage.
Turn the mosfet off, allowing the voltage to again rise.
The voltage across both solar panels and capacitor ramps up and down between two set voltage thresholds.
The system cycles or oscillates at a relatively low frequency. In effect very slow motion pulse width modulation.
These voltage thresholds are set to be either side of the maximum rated power voltage, thus always maintaining the solar voltage fairly close to the maximum peak power point from twilight to full sun.
The heating element resistance should be kept "reasonable" but is not absolutely critical.
It will discharge the capacitor pretty much regardless of the actual resistance.
Power transfer efficiency will remain high over a very wide range of solar, and a reasonable range heating element resistance.
In gloomy conditions, the cycling rate will become very slow, but it will still pulse power into the heating element without overloading the solar panels.
Some kind of transient voltage suppression across the mosfet will be required, quite a few different approaches to that.
An avalanche rated mosfet will work very well (at a low enough repetition rate) so will a fast diode back to the solar capacitor, or even a simple snubber.
If the controller is located fairly close to the heating element there should not be much series inductance there to cause a problem.
If the capacitor is made large, the system will cycle slow enough (in the order of seconds) that any switching losses will be negligible.
While the system is cycling on and off, the capacitor charging current from solar remains constant.
Quite a few of these have been successfully built, and they work amazingly well.
*hint* A standard garden variety 555 timer chip contains two voltage comparators connected to the set and reset inputs of a flip flop.
This can be run in normal astable mode, using the high voltage solar capacitor, and two separate adjustable voltage dividers.
Output of the 555 will need to be inverted to drive the mosfet gate.
doing it different
Solar Enthusiast
Any schematic to work from?
Warpspeed
Solar Wizard
SM-12 is one of those small boards that are the guts from a Chinese 12v dc wall pack.
These provide a fully isolated and well regulated 12v dc from any dc input voltage from about 35v to 400v.
Available from e-bay or Ali from many different sellers.
https://www.ebay.com.au/itm/1268489...10sfOZXyWbVfV5qgrbAwAvBg==|tkp:Bk9SR57Vk8amZQ
There is a thread here somewhere with a lot more detail, cannot find it right now.
Many of these have now been built and they are all working really well according to the builders.
These provide a fully isolated and well regulated 12v dc from any dc input voltage from about 35v to 400v.
Available from e-bay or Ali from many different sellers.
https://www.ebay.com.au/itm/1268489...10sfOZXyWbVfV5qgrbAwAvBg==|tkp:Bk9SR57Vk8amZQ
There is a thread here somewhere with a lot more detail, cannot find it right now.
Many of these have now been built and they are all working really well according to the builders.
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Al Coppa
New Member
I am not sure what the fascination is with 12 volt heating elements. Your 12 volt 600 watt element has a resistance almost equal to zero.
I would not use that element to make hot water. 12 / 50 Amps = .24 Ohms. You are basically shorting out the panels.
A 400 watt solar panel probably has an open circuit voltage of about 45 volts and a MMP of about 40 volts at about 10 Amps.
Two panels wired in series have an MPP that is at about 80 volts.
I use what I call the 85% rule and this is what it comes up with: ( 80 X 80 ) / (85% X 800 watts ) = 6400 / 680 = 9.4 Ohms of resistance required to make this work.
A 6000 watt element that is rated at 240 volts has a resistance of: ( 240 X 240 ) / 6000 = 9.6 Ohms. I probably own an element like that and use it when testing solar panels.
I am using the Ohm's Law formula that states that the voltage squared divided by the wattage equals the resistance.
I had two 265 watt panels, and I needed to get an 85% output of 450 watts. I had a gallon of cold water boiling in about 11 minutes.
I got this idea while I was testing some used panels to see whether I could get an 85% output. I did not expect the water to boil. That was a surprise.
efficientPV
Solar Wizard
- Joined
- Sep 24, 2019
- Messages
- 1,764
I'm pretty surprised he has posted that schematic as he had abandoned trying to resolve the design issues with that circuit and left everyone to fend for themselves. Without a doubt, that is the worst hot water design out there in terms of blowing up, inefficiency, and inconvenience of use. That design is a total joke.
Warpspeed
Solar Wizard
That sounds like a challenge 
Tell me about the design issues that you see.
And where exactly is the power loss and inefficiency ?
Tell me about the design issues that you see.
And where exactly is the power loss and inefficiency ?
One thing to find out before putting much effort into solar water heating is how much it actually cost you per month to run the hot water heater on the grid.
I have been using $2 a day to run the house with solar. Everything is on solar EXCEPT the stove which we don't use much, microwave, airfryer and a few overhead lights I haven't changed over yet, an 80 gallon electric hot water heater and a 30 gallon electric hot water heater.
So the most it cost to run two hot water heaters a day is $2 but it reality its more like 70 cents or maybe $1.10 since the other loads are there too.
Now with that out of the way I have a house that has a dc setup solar heated hot water system. Its a 24v solar setup (first one I did) and it doesn't work that great. It can maintain the temperature fine but it never gets around to getting the tank back up to temp without manual intervention.
I have one of the original heating elements in it and a 24v dc 600 or 800watt can't remember which element in it too.
I was able to make it work like wanted at one point by dedicating a set of batteries to it. The solar charged the batteries and the batteries power the element. This actually worked since it could run longer but it still was veeery slow.
I would run the hot water heater off an inverter before doing the dc method. The water panels on the roof or ground mounted is what solved it finally. Well that and having a swimming pool to dump excess heat from the water panels into. VERY important to have somewhere to send the excess heat
I have been using $2 a day to run the house with solar. Everything is on solar EXCEPT the stove which we don't use much, microwave, airfryer and a few overhead lights I haven't changed over yet, an 80 gallon electric hot water heater and a 30 gallon electric hot water heater.
So the most it cost to run two hot water heaters a day is $2 but it reality its more like 70 cents or maybe $1.10 since the other loads are there too.
Now with that out of the way I have a house that has a dc setup solar heated hot water system. Its a 24v solar setup (first one I did) and it doesn't work that great. It can maintain the temperature fine but it never gets around to getting the tank back up to temp without manual intervention.
I have one of the original heating elements in it and a 24v dc 600 or 800watt can't remember which element in it too.
I was able to make it work like wanted at one point by dedicating a set of batteries to it. The solar charged the batteries and the batteries power the element. This actually worked since it could run longer but it still was veeery slow.
I would run the hot water heater off an inverter before doing the dc method. The water panels on the roof or ground mounted is what solved it finally. Well that and having a swimming pool to dump excess heat from the water panels into. VERY important to have somewhere to send the excess heat
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