Solar Diversion to Water Heater - No Grid Export

[tssd]

I have a currently ~3kW Solar array supplying an Off Grid inverter. (which does have a mains charger, but can't export to Grid)
I would like to heat water with excess Solar.
I have a CT on the Mains Charger supply, so I can see when there is no Utility draw (or very little, the AEOTEC Home Energy Monitor I use reports 6W, when there is no demand)
I can't measure Grid export as the Load controller, like a normal Solar diverter would, Eddi, Immersun etc. so probably need to fabricate something

I would like ideally to control the heater so it uses all the spare Solar (within reason!)

At the moment my Solar inverter just idles the Panels, when there is no AC demand, and the batteries are full.

So I would need some kind of PWM Speed/Load controller that increase the load until the Mains charger starts feeding, then back the load off a bit, so realistically, it would need to maintain a Small AC Charger current, as it won't go to zero or below. Say, 20W +/- 10W as the control range.

Anyone know of something off the shelf that could do this?

thanks

 

[SamDeleted]

Would it be possible to replace ADD to your solar charge controller something like a morningstar tristar PWM , set on load division , diverting excess DC directly to a DC hot water element

 

[SamDeleted]

Installing and configuring a Tristar controller for a wind system | Hugh Piggott's blog

I'm not 100% sure how it works


It is possible you could add a tristar to your current system , ?


It looks like it senses when your batteries are full and starts to divert the load as appropriate

 

[tssd]

Been doing some research and a Thyristor load controller seems to be what I need.
There are some on Amazon that proport to control 5kW loads, but are they the same Chinese kW as the Chinese Lumens in flashlights?

Found these fairly local to me https://www.ppi-uk.com/wp-content/uploads/2020/07/S1-3DR-Dec-15.pdf
Going to order one to try out.
0-10V input to control 0-3 kW output
Been testing loads on my Solar Inverter, and the PV input voltage does rise from VMPP towards VOC when not all the solar is being used, so I can monitor that to provide the control for the thyristor, with bit of simple electronics
I'll post back when I've got it running

 

[tssd]

Got the Thyristor board, connected it to a 5 to 20V step adjustable power supply for testing.
5V gives 1.3kW load from the 3kW Immersion.
The bottom Immersion I am using is a mechatronic type, and I'm not sure how the electronics will behave, with lower duty cycles, might have to swap it for the plain old mechanical stat on the top immersion.
Be nice if someone made a changeover stat so I could run either immersion based on the top stat status.

Got an OP Amp comparator PCB on order, hopefully be able to modify that to do the PV voltage tracking and control the thyristor.

 

[Mart Hale]

I started experimenting with using solar direct to a hot water heater element Connection is simply solar panel then connect to heating element, with a 55 gal barrel of water.


I started with 2 panels then worked up to like 9 panels each 260 watts ratted but give me 220 watts in practice.


After I was comfortable with this, I then want to my present solar system which has over 2000 watts of solar going to an inverter to batteries.
To this system I added a Y connector to both Positive and Neg to the power coming in from my solar panels directly before the charge controller.

Now from here I added an electric car relay so I could control the connection, and ran both positive and neg directly to my hot water heater.


Once my solar charged battery is full around 2 PM each day, I then trigger the relay set on a 2 hour timer to direct power from the panels to the hot water heater element, the 2 hour limit prevents a boil over of the tank, it is manually triggered, I test the temp of the water coming out of the tap before I trigger it so I know it will not over heat.

Heating element is rated at 3800 watts, so I am well within spec.

All this time I could of been using that extra power, I just got brave enough now to try it.

 

[Grakat]

Hi there

In Australia there is a device called a catch relay, which kicks in when your solar generation or export gets over a certain amount. Its simple and doesn't require a lot of work. There must be something similar overseas?

 

[efficientPV]

I've been doing that for years except I look at PV voltage and divert just enough energy to bring the array voltage back to just above power point. I pulse power from the array avoiding any extra strain on inverter, charge controller or battery. No need to invent anything, this is proven technology. This is one of my three water heaters. This tank only supplies water to the clothes washer and it has lowest priority. I have three controllers on the same array and each has a different priority. All cycles use hot water. That is so nice when you have a front loader. You don't get mold stink with hot water. I can't believe more people aren't doing this. Look how simple it is. All automatic, no wifi or computers needed.