Heating water with PV solar

[FredB]

Hello all,

I was tiring to figure out how to easily and cheaply control the DC element with the existing AC water heater thermostat. I apologize if this has already been discussed elsewhere.

How about using a stock 120VAC water heater thermostat controlling a "wall wart" 120 VAC to 12VDC transformer wired to the control side of a DC SSR with the output of the SSR in line with the DC element. Too simple? I am probably missing something. ( I usually am  )

[ClassicCrazy]

I think what you describe might  work.
I took a different route and did not use the AC heating thermostats. I instead used inexpensive thermostats that have a remote sensor and stuck that in along the side of the heater ( or under the existing thermostat if you can make it fit ) .
You didn't mention what DC voltage you are planning to use.
I have 240v or more DC and I needed to use a IGBT transistor block instead of SSR because of the higher voltage there aren't many SSR's that can handle that.
I made some videos of my prototype DC water heating adventures.
This summer I redid most of the plumbing using PEX instead of garden hoses, and also put in a differential solar hot water heating controller to turn the circulating pump on and off. It has been working very well using just solar .
Take a look here
https://youtu.be/s7trVpHLts4
https://youtu.be/pFsO8_1QbKY
https://youtu.be/j9B9mOV-YLQ

Larry

[FredB]

Hi Larry, thanks for your videos, I ordered the Toshiba MG75Q2YS1 IGBT and a large heat sink for it as it looks like a good solution. I didn't know such a thing existed! I'm just an old mechanic and not schooled in electronics. We finally got some sun today and so far heated the water from 79 deg. to 131 deg. at 3pm with partly clouds. I have a old 1000 watt array that is shaded till about 10am wired direct to a 120 VAC 1650w element in a old 15 to 20 water heater. I getting max 70v at 8.4a (588w) (max i saw) from the array to the element. I would say the experiment was a success! Now I just have to 1 - get a matching DC element, 2 - make it automatic, and 3 - plumb it into the cold water inlet of my existing propane water heater.

[ClassicCrazy]

Hi Larry, thanks for your videos, I ordered the Toshiba MG75Q2YS1 IGBT and a large heat sink for it as it looks like a good solution. I didn't know such a thing existed! I'm just an old mechanic and not schooled in electronics. We finally got some sun today and so far heated the water from 79 deg. to 131 deg. at 3pm with partly clouds. I have a old 1000 watt array that is shaded till about 10am wired direct to a 120 VAC 1650w element in a old 15 to 20 water heater. I getting max 70v at 8.4a (588w) (max i saw) from the array to the element. I would say the experiment was a success! Now I just have to 1 - get a matching DC element, 2 - make it automatic, and 3 - plumb it into the cold water inlet of my existing propane water heater.

Great -  did you take note of the fact that you will need to put some pull down resistors for the IGBT control ?  If you watched all the videos I talked about that.  With only 70v dc you could use a SSR - it probably wouldn't get as hot ( meaning it would be more efficient ) as the IGBT . But may not be that much difference and with the IGBT you could add more PV no problem.
This is the one I used until I got above 220v and burned it out. ( if you get one and it doesn't work you need to switch the output wires around the other way since they are polarity sensitive).
https://www.aliexpress.com/item/4000085227082.html?spm=a2g0s.9042311.0.0.4fc44c4dIJZKe7
You can just use dc 12 to 24v from batteries to run the control circuit.
This is the type of thermostat I am using - I have the dual one since I can use both sides if I need to .
https://www.aliexpress.com/item/33003600571.html?spm=a2g0s.9042311.0.0.27424c4dDYD3Vu
I would recommend putting a circulation pump to circulate your small DC water heater with your larger gas heater in case you have extra hot water you can heat more water. But with your array maybe not worth it.
For plumbing if you haven't worked with PEX watch some youtube videos and get the crimp tool - well worth it - makes plumbing so much easier and easy to change something - just snip the pipe , rearrange however you want,  and you can reuse the same fittings.
This is the crimp tool set I got but you can get the other stainless band type crimpers for less and they work well too ( this one crimps the copper rings)
https://www.amazon.com/gp/product/B01FJA96X2/ref=ppx_yo_dt_b_asin_title_o06_s01?ie=UTF8&psc=1

Larry 

[FredB]

Hi Larry, thanks for your videos, I ordered the Toshiba MG75Q2YS1 IGBT and a large heat sink for it as it looks like a good solution. I didn't know such a thing existed! I'm just an old mechanic and not schooled in electronics. We finally got some sun today and so far heated the water from 79 deg. to 131 deg. at 3pm with partly clouds. I have a old 1000 watt array that is shaded till about 10am wired direct to a 120 VAC 1650w element in a old 15 to 20 water heater. I getting max 70v at 8.4a (588w) (max i saw) from the array to the element. I would say the experiment was a success! Now I just have to 1 - get a matching DC element, 2 - make it automatic, and 3 - plumb it into the cold water inlet of my existing propane water heater.

Great -  did you take note of the fact that you will need to put some pull down resistors for the IGBT control ?  If you watched all the videos I talked about that. 

Yes I saw that, what resistor do you recommend?

With only 70v dc you could use a SSR - it probably wouldn't get as hot ( meaning it would be more efficient ) as the IGBT . But may not be that much difference and with the IGBT you could add more PV no problem.
This is the one I used until I got above 220v and burned it out. ( if you get one and it doesn't work you need to switch the output wires around the other way since they are polarity sensitive).
https://www.aliexpress.com/item/4000085227082.html?spm=a2g0s.9042311.0.0.4fc44c4dIJZKe7


Done, ordered 4

I have a 300VDC SSR I was going to try for now.

You can just use dc 12 to 24v from batteries to run the control circuit.

I have a 48v system (brand new!) But am planing on using the "Waste-Not-High" from one of my Classic 150's to switch the array from the controller to direct PV. Maybe using a NO and a NC SSR?

This is the type of thermostat I am using - I have the dual one since I can use both sides if I need to .
https://www.aliexpress.com/item/33003600571.html?spm=a2g0s.9042311.0.0.27424c4dDYD3Vu

Ordered 2!

I would recommend putting a circulation pump to circulate your small DC water heater with your larger gas heater in case you have extra hot water you can heat more water. But with your array maybe not worth it.

I was thinking I need to do this, Tee hot from the propane to the cold tee of the DC heater?

For plumbing if you haven't worked with PEX watch some youtube videos and get the crimp tool - well worth it - makes plumbing so much easier and easy to change something - just snip the pipe , rearrange however you want,  and you can reuse the same fittings.

I have both PEX A and B tools...

Larry

[ClassicCrazy]

I forget what I used for the resistor - pretty sure I mentioned it on video.
If you can't find that I can go look to see what I have out there.
If you look at my video again you will see that how I plumbed the circulation pump is that I used the drain of the water heater but part of the reason I did that was so I could use partially full water heater that wasn't under pressure.  You want to take the hot water from top and send it to the cold of other tank .  I can't remember what I did on my newest version  with a closed system- will have to look again. You don't get good circulation if you dont' do it right. Pretty sure  I  have the water coming out of the drain of my big tank - then going to the cold input of small tank , then the hot water from that tank goes to a tee on top of big tank - will have to look and see if I have the tee in cold input or the hot . But my system is kind of different than a truly pressurized system . For the summer now with lots of energy I keep both tanks full , the small tank has a DC heating element and the bottom of the large tank has DC element. The two DC elements are wired in series because that gives me a pretty good match for my DC solar array. Those two elements are different sizes but it is the total resistance that matters for transferring the most heat. I still have an AC heater element in the top of my large tank and that one is on 120v which is controlled by the second thermostat .  The solar hot water differential controller I have is what I use to run the pump - it turns on when the small tank is hotter ( by 3 or 5 degrees )  than the large tank and shuts off when the temperatures equalize.   

Regarding the Classic and using waste not - if using AUX one it will be just on off. If using AUX 2 it will be PWM .  I used PWM for AC element which is from the batteries - but the DC elements are on different array so I can't use the PWM from AUX 2 for that.  Make sure you check the jumpers for you AUX so they are supplying 12v instead of being in the dry contact mode. I don't know how you have your system wired up - if you are using an independent PV array or the same array you charge batteries with ?

I have thought before that it might be worth just getting an inexpensive cheap 220v modified sine wave inverter and use that for AC to a regular heating element. It would save on all the DC rated SSR's , switches, etc. But since I have a dedicated PV array I use that for water heating.

Let us know how your system turns out.

Larry

[FredB]

Hi Larry, I should mention I am a installer for several of the off-grid families living here on the mountain, I always experiment on my own system here before recommending stuff to my friends / customers and I think because of the substantial price drop on PV panels there is a lot more we can do with PV solar.  I think the ultimate PV water heating system is a standard 240VAC water heater with a PV DC direct or "Dump Load" lower DC element with the top AC element run off the battery / inverter / generator backup with "Waste Not High" and this is what I am recommending to my friends / costumers that have the excess power. Most of them including me have switched to the Simpliphi 48V 3.8kWH batteries, and I have to say I am loving these. They take mush less solar to charge and they are fully charged (with full sun) by 10 or 11am. The rest of the day, what to do with all that power? So, water heating, jacusie sp?, pool pump, electric vehicle charging etc.

[ClassicCrazy]

I talked about the resistors I used in this thread - and Bob had also commented on a different circuit that would work too.
http://midniteftp.com/forum/index.php?topic=5064.0
Larry

[ClassicCrazy]

Hi Larry, I should mention I am a installer for several of the off-grid families living here on the mountain, I always experiment on my own system here before recommending stuff to my friends / customers and I think because of the substantial price drop on PV panels there is a lot more we can do with PV solar.  I think the ultimate PV water heating system is a standard 240VAC water heater with a PV DC direct or "Dump Load" lower DC element with the top AC element run off the battery / inverter / generator backup with "Waste Not High" and this is what I am recommending to my friends / costumers that have the excess power. Most of them including me have switched to the Simpliphi 48V 3.8kWH batteries, and I have to say I am loving these. They take mush less solar to charge and they are fully charged (with full sun) by 10 or 11am. The rest of the day, what to do with all that power? So, water heating, jacusie sp?, pool pump, electric vehicle charging etc.

I got free reject damaged PV panels for a commercial solar project. I have also seen Will Prowse on youtube had video about getting inexpensive used PV panels. I think the future is here already with lots of used PV from commercial solar plants that have been upgraded already. If someone has the room to put up an array that is good news for direct heating of hot water.

Larry

[FredB]

I forget what I used for the resistor - pretty sure I mentioned it on video.

I will look again.

If you can't find that I can go look to see what I have out there.
If you look at my video again you will see that how I plumbed the circulation pump is that I used the drain of the water heater but part of the reason I did that was so I could use partially full water heater that wasn't under pressure.  You want to take the hot water from top and send it to the cold of other tank .  I can't remember what I did on my newest version  with a closed system- will have to look again. You don't get good circulation if you dont' do it right. Pretty sure  I  have the water coming out of the drain of my big tank - then going to the cold input of small tank , then the hot water from that tank goes to a tee on top of big tank - will have to look and see if I have the tee in cold input or the hot .

Ya, I will really have to think about this, because my system(s) will be pressurized.

But my system is kind of different than a truly pressurized system . For the summer now with lots of energy I keep both tanks full , the small tank has a DC heating element and the bottom of the large tank has DC element. The two DC elements are wired in series because that gives me a pretty good match for my DC solar array. Those two elements are different sizes but it is the total resistance that matters for transferring the most heat. I still have an AC heater element in the top of my large tank and that one is on 120v which is controlled by the second thermostat .  The solar hot water differential controller I have is what I use to run the pump - it turns on when the small tank is hotter ( by 3 or 5 degrees )  than the large tank and shuts off when the temperatures equalize.   

Regarding the Classic and using waste not - if using AUX one it will be just on off. If using AUX 2 it will be PWM .  I used PWM for AC element which is from the batteries - but the DC elements are on different array so I can't use the PWM from AUX 2 for that. 

Something else to research.. I don't really understand the difference between on/off vs PWM control.

Make sure you check the jumpers for you AUX so they are supplying 12v instead of being in the dry contact mode. I don't know how you have your system wired up - if you are using an independent PV array or the same array you charge batteries with ?

Not sure yet, probably both in different systems. Mine will be different than the others I do (always!  .

I have thought before that it might be worth just getting an inexpensive cheap 220v modified sine wave inverter and use that for AC to a regular heating element. It would save on all the DC rated SSR's , switches, etc. But since I have a dedicated PV array I use that for water heating.

Worth thinking about!

Let us know how your system turns out.

Will definitely. Thanks for your thoughts / advice

Larry

[FredB]

I got free reject damaged PV panels for a commercial solar project. I have also seen Will Prowse on youtube had video about getting inexpensive used PV panels. I think the future is here already with lots of used PV from commercial solar plants that have been upgraded already. If someone has the room to put up an array that is good news for direct heating of hot water.

Larry

Ya, I wasn't going to say but you can get 250w + used panels off ebay for $40 each... shush don't tell anyone. 

[ClassicCrazy]

I got free reject damaged PV panels for a commercial solar project. I have also seen Will Prowse on youtube had video about getting inexpensive used PV panels. I think the future is here already with lots of used PV from commercial solar plants that have been upgraded already. If someone has the room to put up an array that is good news for direct heating of hot water.

Larry

Ya, I wasn't going to say but you can get 250w + used panels off ebay for $40 each... shush don't tell anyone. 

PWM switches the load on and off real fast so it can control the current.
So waste not with PWM keeps the voltage at  Absorb or Float but can vary the extra power to the load by switching on and off faster or slower. It works really well. But if you have Whizbang you can't use AUX 2 for that and AUX 1 doesn't do PWM.  Since I have two Classics in follow me mode , I can use the AUX 2 on the second Classic to do the PWM.

Larry

[FredB]

I talked about the resistors I used in this thread - and Bob had also commented on a different circuit that would work too.
http://midniteftp.com/forum/index.php?topic=5064.0
Larry

In the video it was a 20K resistor.

[FredB]

PWM switches the load on and off real fast so it can control the current.
So waste not with PWM keeps the voltage at  Absorb or Float but can vary the extra power to the load by switching on and off faster or slower. It works really well. But if you have Whizbang you can't use AUX 2 for that and AUX 1 doesn't do PWM.  Since I have two Classics in follow me mode , I can use the AUX 2 on the second Classic to do the PWM.

Larry

OK got it (I think ) So I take it this "fast switching" doesn't bother the SSR? Or is it a special type of SSR?
Thanks for all your help Larry,

Fred

[ClassicCrazy]

PWM switches the load on and off real fast so it can control the current.
So waste not with PWM keeps the voltage at  Absorb or Float but can vary the extra power to the load by switching on and off faster or slower. It works really well. But if you have Whizbang you can't use AUX 2 for that and AUX 1 doesn't do PWM.  Since I have two Classics in follow me mode , I can use the AUX 2 on the second Classic to do the PWM.

Larry

OK got it (I think ) So I take it this "fast switching" doesn't bother the SSR? Or is it a special type of SSR?
Thanks for all your help Larry,

Fred

Yeah no problem switching SSR or IGBT .