Direct Solar PV water heater

firewick

New Member
Joined
Sep 5, 2021
Messages
8
Hello, I am new here and looking for some information / advice on heating water (and air) from PV panels. I believe that I searched and looked through all the threads and could not find the information I was looking for so that is why I am posting a new thread.

I have a few tanks / elements that I have connected directly to PV panels and they all have the same kind of tendencies. Example:

If I hook up two 250W panels in series to a (standard) 208V 5000W element I get about 70V, 8A at the element. If I hook up two more,
Four 250W panels in series the voltage remains the same even though it should be close to 140V. I have done this with slightly smaller wattage (standard) 240V elements as well and it does the same thing.

I am wondering why I can not get any higher voltage out of the elements? If I have a 2000W element I would like to be able to know how to hook up 2000W (or close to it) of PV directly to it.

Any help would be appreciated, thank you.

Edit: I think I found the answer. It has to do with resistance and “matching” the source with the load. It is why MPPT charge controllers are more efficient. The PV panels (source) are constantly changing and constantly need matched to the load to put out full power.

I’m still not sure how to match the resistance between the source and load, but feel free to leave any comments, I’ll probably still learn something.
 
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pierre

Somewhere down South
Joined
Dec 21, 2019
Messages
808
Just a silly idea : Are all the panels connected in proper series string i.e. + - + - + - + - ? Have you measured the total voltage over the string with a DMM without a load connected ? Have you tested the total Isc of the string ?
I am doing the same thing but more info later.
 

firewick

New Member
Joined
Sep 5, 2021
Messages
8
Update: It is definitely all about resistance just like David Poz said. Even though I watched that video (from David Poz) before my post, I didn’t quite understand exactly how (I blame it on my head cold). With some more searching and digging, I did come across a few articles that were quite helpful, here’s one of the best so far:


The panels do need matched resistance to the load (heating elements in this case). That’s it. Now I just have to find the best / most cost effective way to switch high voltage DC.
 

pierre

Somewhere down South
Joined
Dec 21, 2019
Messages
808
That is why I have opted for an MPPT and storage battery / boost converter system.
See my post here :
 

firewick

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Joined
Sep 5, 2021
Messages
8
That is why I have opted for an MPPT and storage battery / boost converter system.
See my post here :
MPPT is definitely the way to go. We have been off grid for over 6 years now. Our entire farm is (and has been since the start) solar powered. I used to heat our water with solar thermal in the summer and our wood stove in the winter, but last year when I was winterizing the solar thermal set up, I thought, maybe I can just use PV panels and run the electric directly to a heating element? I wasn’t sure of the difference in efficiency, but with no moving parts and no winterizing I figured anything above 50% would be worth it. So we have been testing various set ups and it is quite effecint. Not as good as solar thermal, but it is very, very, close. And like I said, no moving parts, no plumbing, no winterizing, etc. and way less expensive. So it was totally worth it and we are sticking with it.

This year I am going to try to add some heaters in our house to help supplement the wood heat and just keep the chill off if we are gone for an extended time... I would like to have it switch when the hot water tank comes to temperature to the house heat, but I can’t find any high voltage DC thermostats. So I am going to need to use relays or setup something a bit more expensive I think.

Either way everything is working great and just getting better. Good luck on your system. Looks great.
 

pierre

Somewhere down South
Joined
Dec 21, 2019
Messages
808
Very interesting projects you have going on. Being on a farm you do not have the same constraints as I have living in a complex with limited space. At the moment I still feed the PV energy directly into the geyser until the other components arrive. I am very pleased with the results so far and only use the grid 230v at around 7pm to bring the temp up to 55 deg C , if it is not on temp yet from the PV charging during the day. As we are moving into summer , I had the best day since installation a few weeks ago and measured around 1,5kW going into the element out of a possible 1,68kW.
Regarding the DC thermostat : you can rig up a relatively cheap control board where you use the existing thermostat contact to switch a low current AC or DC contactor / relay. This way your geyser thermostat will last indefinitely.
Good luck and keep posting.
 

Smokin

Solar Enthusiast
Joined
Sep 8, 2021
Messages
130
MPPT is definitely the way to go. We have been off grid for over 6 years now. Our entire farm is (and has been since the start) solar powered. I used to heat our water with solar thermal in the summer and our wood stove in the winter, but last year when I was winterizing the solar thermal set up, I thought, maybe I can just use PV panels and run the electric directly to a heating element? I wasn’t sure of the difference in efficiency, but with no moving parts and no winterizing I figured anything above 50% would be worth it. So we have been testing various set ups and it is quite effecint. Not as good as solar thermal, but it is very, very, close. And like I said, no moving parts, no plumbing, no winterizing, etc. and way less expensive. So it was totally worth it and we are sticking with it.

This year I am going to try to add some heaters in our house to help supplement the wood heat and just keep the chill off if we are gone for an extended time... I would like to have it switch when the hot water tank comes to temperature to the house heat, but I can’t find any high voltage DC thermostats. So I am going to need to use relays or setup something a bit more expensive I think.

Either way everything is working great and just getting better. Good luck on your system. Looks great.
Standard top element hotwater heater thermal switchs do that. They call it quick recovery. Top element runs first until it is satisfied the it diverts power to the bottom element. How will these switches behave with DC arcs??
 

GLC

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Joined
Aug 26, 2020
Messages
320
Location
Texas
Here is what i found out.
 

MattiFin

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Joined
Dec 31, 2020
Messages
284
Hello, I am new here and looking for some information / advice on heating water (and air) from PV panels. I believe that I searched and looked through all the threads and could not find the information I was looking for so that is why I am posting a new thread.

I have a few tanks / elements that I have connected directly to PV panels and they all have the same kind of tendencies. Example:

If I hook up two 250W panels in series to a (standard) 208V 5000W element I get about 70V, 8A at the element. If I hook up two more,
Four 250W panels in series the voltage remains the same even though it should be close to 140V. I have done this with slightly smaller wattage (standard) 240V elements as well and it does the same thing.

I am wondering why I can not get any higher voltage out of the elements? If I have a 2000W element I would like to be able to know how to hook up 2000W (or close to it) of PV directly to it.

Any help would be appreciated, thank you.

Edit: I think I found the answer. It has to do with resistance and “matching” the source with the load. It is why MPPT charge controllers are more efficient. The PV panels (source) are constantly changing and constantly need matched to the load to put out full power.

I’m still not sure how to match the resistance between the source and load, but feel free to leave any comments, I’ll probably still learn something.

In your 2x vs 4x series panels the problem is that you are hitting on the panel max current limitation. Your panels can supply about that 8A no matter how many you put in series. Double voltage would need double current and you don't have that available. 4 series and 2 parallel would be different matter as then you would have 140V and 16A.

Besides of that solar panels are difficult to match with resistor load, you easily lose a huge amount of power as soon as you are outside of the "sweet spot" (sun angle, clouds, temperature)
IE: lets assume 140v 16A panel setup in full sunshine connected to 9 ohm heater (approx same as 5000W 208v) = you get roughly 140V and 15.5A =2170W
add a bit of angle and clouds so that you get 50% of sunshine of previous example and your current drops to 7.75A. Now 7.75A over 9 ohm heater makes 69.75V and 540W. Properly set up MPPT would be still churning over 1000W out of the same situation.
 

efficientPV

Solar Addict
Joined
Sep 24, 2019
Messages
446
Nobody ever posts the results they get with direct connect. I'd like to see some total weekly power readings that can be compared with a solar calculator. Just ambiguous stuff like stuck my hand in the bucket and it was hot. It will be raining all this week and I still need hot water. I use a power point controller and it works on those days.
 

Rednecktek

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Joined
Sep 8, 2021
Messages
475
So here's a hypothetical for y'all...

If I had a 200w solar panel with the following specs:
Voc 36.3V (open circuit)

Vmp (voltage at maximum power) 28.71

Imp (amps at max power) 7.25A

And I picked up a 24v 900w DC element...

And the tank on the top of my camp shower had a threaded hole in the side...

Could I just wire the panel directly to the element via an on/off switch and help warm up the water? The tank being flat black helps, but I figure any free heat is a good thing, and I can't see it ever overheating with the ground water being as cold as it is. The sun can only do so much when it's cold outside. I'd just flip off the switch when I drained the system to leave so it's not burning in the air for months at a time.

I'm assuming that by only using 1 panel, or maybe 2 in parallel, that them being the same make/model/specs would alleviate the resistance-balancing-tweaking-perfect-pairing issues?
 

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efficientPV

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Sep 24, 2019
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446
That would be an extremely bad match for one panel, even two in parallel. Put two panels in series and use a 2,000W 120V heating element. Only about $10 at any big box store.
 

Rednecktek

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Messages
475
That would be an extremely bad match for one panel, even two in parallel. Put two panels in series and use a 2,000W 120V heating element. Only about $10 at any big box store.

So in layman's terms, because I haven't ever dealt with it, why would it not work?
 

efficientPV

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Sep 24, 2019
Messages
446
Panel 28.78 / 7.25 = 3.96 ohms element 900W / 24V = 37.5A 24V / 37.5A = .64 ohms

Panel at 7.25A * .64 ohms = 4.64V 4.64V * 7.25A = 52.5W ON AN ABSOLUTELY PERFECT DAY FROM A 200W PANEL!

The same battery voltage elements are NEVER a good match for panels.

element 2000W / 120V = 16.7A 120V / 16.7A = 7.2 ohms Two panels in series is 8 ohms. Best to go with a higher resistance.

That ideal resistance is good for less than 2 hours each day. It is better to design for a much lower current. If you often get haze or clouds, a 5500W 240V element would be 10.5 ohms and give better daily performance with two panels in series. Even the 2000W 120V element would work well with one panel. Save that extra money for another panel later.
 
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Rednecktek

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Sep 8, 2021
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AAAaaaahhhh, OK. I was figuring trying to get everything matching up voltage wise was the best option but it sounds like higher voltage would be the better way to go. Thanks!
 

sunshine

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Joined
Apr 24, 2020
Messages
455
I was figuring trying to get everything matching up voltage wise was the best option but it sounds like higher voltage would be the better way to go.
The general consensus is double the number of panels to give double the wattage of your matched element to get a good result on a sunny day.
For your 900w24v element you need 9 of your 200w panels.

There has been a direct to water heater tank released by a company recently that is only feasable, (re efficientPV reply) because of the tanks rather small volume

 
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efficientPV

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Joined
Sep 24, 2019
Messages
446
I use this 50L 13 gallon horizontal tank at my camp which has a 1600W 120V element which only cost $150 shipped on my 60V array. The other tank is an ECOsmart 6 gallon which I used in series with another 10 gallon tank till that started leaking. Much better insulation but twice the price. This 50L has served my needs this summer with showers and a dishwasher and with that element it can produce 450W. Nice setup for the camp. The digital display can be made to work on 12V. I also have a 40 gallon tank in the garage which provides all the hot water for laundry. It runs only on excess panel voltage from the camp 60V array. 1-2KWH a day is enough to keep it at 110F. Ready hot water is nice to have and being free makes it better. I removed my propane heater years ago.
CampHW21.jpg
 
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indalecio

New Member
Joined
Oct 26, 2021
Messages
22
I do manufacture the solar powered water heater. The research is done. But need to do market issues which is not my advantage. For now I have designed 40L, 80L and 100L types. Also this system could do hybrid system with wind for winter time room heating systems.:)
 

Solarboyandog

New Member
Joined
Nov 8, 2021
Messages
2
Hello, I am new here and looking for some information / advice on heating water (and air) from PV panels. I believe that I searched and looked through all the threads and could not find the information I was looking for so that is why I am posting a new thread.

I have a few tanks / elements that I have connected directly to PV panels and they all have the same kind of tendencies. Example:

If I hook up two 250W panels in series to a (standard) 208V 5000W element I get about 70V, 8A at the element. If I hook up two more,
Four 250W panels in series the voltage remains the same even though it should be close to 140V. I have done this with slightly smaller wattage (standard) 240V elements as well and it does the same thing.

I am wondering why I can not get any higher voltage out of the elements? If I have a 2000W element I would like to be able to know how to hook up 2000W (or close to it) of PV directly to it.

Any help would be appreciated, thank you.

Edit: I think I found the answer. It has to do with resistance and “matching” the source with the load. It is why MPPT charge controllers are more efficient. The PV panels (source) are constantly changing and constantly need matched to the load to put out full power.

I’m still not sure how to match the resistance between the source and load, but feel free to leave any comments, I’ll probably still learn something.
Hi...you can find a pdf of how to match element to panel output...look for David Poz on youtube. He has a vid on heating water by his solar array.
 
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