Direct Solar PV water heater

firewick

New Member
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.
 
Last edited:

pierre

Somewhere down South
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
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
That is why I have opted for an MPPT and storage battery / boost converter system.
See my post here :
 

firewick

New Member
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
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

New Member
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??
 
Top