2 Array 2 Classic 250s diversion wiring question

[WillEert]

I am considering an upgrade to my system in order to permit PV direct DC diversion  into my domestic hot water tank. There will be two identical arrays operating at Vmp of 235v @-35C. Each array will have its own Classic 250. Rated PV power of each array is 3150w. Each array is identical to the other one. I believe that I read that each array must have its own separate charge controller which makes sense. The question is : If the 2 array charge controller systems must be separate from each other how would one tie the two DC feeds to the HWT from each array together. Once they are interconnected then the arrays are no longer separate from each other. The intent is to control diversion with DC SSR's driven by a suitable controller. Possibly tieing the two separate array / Classic systems together after the SSRs would work? Suitable sized diodes somewhere?

Thanks

Will

[CDN-VT]

Will Let me ask a few
Hot water is nice , but im sure you would want to hit the first harvest into the battery cells .
Then I would use the Aux float trip to a Relay or whatever / however your feeding the DHW.
I have a solar collector for my DHW , the odd cool shower , but the wife says in keeps the stallion at bay.


VT
Edit after a re-read
I read it incorrectly to first time , One thing is to always have a thermostat and safe guards on these systems.
Years ago the learning curve on mine was safe but messy . The speed that a collector can get the fluid to is impressive , and with that I used a tier tank system 3 tanks . So in sunny days , lots of morning & evening showers keeps us cool & the grey water for the crops . I first had blow offs of high temps from the safety valves .
VT

[dgd]

Willert,
This is a similar scenario to a small industrial system I helped install earlier this year.
Two Classic 250s each with an array of 3600 watts pv (12 by 300watt pvs in two strings of six making near 230v mpv)
The total 7200 watts charges a 2000Ah 48V bank.
Diversion of excess power is to a hot water cyclinder.
This, like most modern cyclinders has two (top,bottom) or three (top,middle, bottom) locations for heating elements.
This enables seperate 230vac elements to be used for diversion from each PV array using an SSR for two of the four strings - one from each controller.
A lot of effort was put into tuning this setup as the original 200 litre tank was heated up to 65C too quickly leaving hours of available power being wasted. It was changed for a 400 litre tank with 3 element positions and 2Kw 230vac elements used - these were just element types with no temp controlled ac switch included as this would probably die quickly with DC power
One of the elements could be manually switched out for wintertime.
The SSRs used are pretty expensive having 300Vdc and 40A rating.
Initially the AUX2 on each C250 was used the control these, settings were different so that the elements were powered up offset from each other.
Now I'm sorting out an Arduino DUE control system both for web server (using the large text displays examples I posted in Arduino area) and PWMing the two or three SSRs using modbus data from the two Classics and using Wbjr, temperature sensors on tank as inputs and eventually outputs to control switching of an LPG water boost heater.

Although the various water heating diversion schemes are interesting to read about I'm definitely in favour of the diversion direct from PVs and not having the Classic or battery bank having to power the water heating.

dgd

[WillEert]

Hi dgd,

PV direct DC diversion at a suitable array voltage looks like the way to go to me. Paul mentioned it as an idea and the more I thought about it the better I like it. The trick is wiring the array to get a suitable voltage for the heating elements and then having a reliable DC thermostat that can handle the amount of current safely.

I think what you are saying to me is keep the arrays separate by using two or more heating elements. I can do this as my tank has two elements in it. I am planning to use an arduino uno one wire ds18b20 thermostat system to control the  dc SSRs and regulate the tank temperature within limits. I was hoping to tie the two arrays together and then use a lead / lag system for controlling the elements based on the amount of diversion wattage available. This way if I had 3000w available from the two arrays in total and two 3000w elements I would then have one element at maximum power making maximum heat with the other not making anything. I am uncertain how much heat an element actually makes when you do not supply it with maximum wattage - I do not think heat created is linear with wattage but I do not know. So in a way the question becomes - how much heat might a (for example) 3000w element make if it only had 1500w going into it. Would the amount of heat created at 1500w be 1/2 the amount of heat created at 3000w? I think I would get more heat out of one 3000w element at maximum wattage than two 3000w elements at 1500 watts each. I was thinking of using one 3000w element in the lower and leading position and then a 2000w in the upper and lagging position.

If not is there a way to tie the two diversion wires together? It seems such a waste to divert power into oversized elements and not get maximum heat out the elements. On the other hand if the elements are undersized too much for average peak wattage available then again power is wasted.

Thanks

Will

[dgd]

Yes, thats what I am suggesting, keep the two arrays separate and diverting to their own heating elements.
When you are using two or more Classics to charge the same battery bank then dealing with diversion gets a bit more complicated.
Its very unlikey that you would have the same diversion capability from each Classic or its input array because there is no charge balancing between the Classics. Ok, full power from both in bulkmppt but on reaching Absorb its most likely that initially both will be contributing power to the bank but this will soon diverge so that one Classic is doing most or all of the charging and the other is either idle or contributing very little.
In Float then again one will providing the power the other near idle
This assumes one array is near capable of making sufficient power to maintain later Absorb and Float stages. (ie in good weather as in poor weather both arrays will probably be powering the battery charging with nothing or very little left to divert to water heating)

Your suggestion of 50% or near, diversion capability from each PV array is therefore probably never going to happen
most likely 90%+ from one array and next to nothing from the other.
So no advantage to be gained from commoning the outputs from the arrays for balanced diversion purposes.

dgd

[Westbranch]

Based on what dgd is saying , I can imagine setting one Classic to go to Float sooner that the other (lower threshold value) using a couple of WBjr to determine the End Amp value you want to use.

eg. one set at 20 A for a 1% (of Capacity) EA and the other at 22A so one stops charging sooner than the other.

[WillEert]

I think the advantage to be gained by commoning the output from the arrays is that the elements would operate at closer to their maximum wattage more of the time. Certainly the lower or lead element would. If I could supply a (possibly) 3K watt element with the commoned excess ( if any) of a 6.2K watt array I think that after battery charging demands I would have a pretty good chance of running it close to maximum wattage for a lot  of the time. If there was more excess power then this would go to a lower wattage upper element that I would tune to try to get to maximum or close to some of the time. I presently divert from a 4.4 kw array using an AC system into a 2K watt element and it runs a max on a sunny day for a large portion of the time that the system is in absorb and float.

It seems to me that anyone who has fooled around with diversion systems to make hot water likely has a collection of elements of various sizes - I know I do. 

So for me the question still is - Is there a way to tie the two arrays together on the DC side? Would the dc ssrs give enough separation that the Classics could still operate correctly? If the PWM duty cycle is at 100% then essentially the ssr is on all the time and it is like a closed switch  so the net effect is the arrays are tied directly together.  I guess the problem is by commoning  the array outputs you end up with the controllers being unable to manipulate each array voltages as they need to. Likely they would end up fighting each other and it would not be good. I still think there should be way to common the outputs in some fashion however.

Thanks for all and any replies

Will