Basics of Diversion to HWT

[HighlandSolar]

So we made it thru our first winter offgrid, quite happy with our system!
We are seeing a consistent excess of solar power and now we are preparing to make use of the excess thru diversion to a HWT. My understanding from different discussions from great contributors here, that with the Classic that possibly Aux 2 set to Waste Not Hi, a SSR or two, heat sinks and DC heating elements are the way to go. Where would I find the "directions" to safely put this together? Any direction would be appreciated.

[ClassicCrazy]

Did you read this discussion ?
http://midniteftp.com/forum/index.php?topic=3457
and this one too

http://midniteftp.com/forum/index.php?topic=3595.0

Larry

[HighlandSolar]

Thanks Larry, that is definitely the direction I was hoping for. I will read, and re-read these links. I will also update on how it goes.

[ClassicCrazy]

Yeah - I reread those discussions and learn more that didn't sink in the first time ! 
Let us know what your plans will be. There certainly are a lot of options.
Larry

[HighlandSolar]

This week i'm getting between  3.5 hrs to 5.5 hrs of float time so I should move forward now with the diversion project.

My plans after reading these posts are basically use Aux 2 on my Classic on Waste Not Hi, to switch a SSR to 2 x 48 volt 2,000 watt HWT elements on a 50 gal tank. My array is 3.6kw and the DC element can be reduced to 1,000 watts each if needed. I was thinking a MidNite DC breaker in there somewhere as well.

I think I just need to figure out whether or not to a) reduce elements to 1,000 watts each and b) whether to run one line, one breaker, one SSR to the elements or a separate line to each element.

Reading the other posts and the heat going thru the SSR's heat sinks, caution says run two, keeping the amps down? 2,000/60v=33 amps or 1,000/60v=16amps.....

Last thing was that the solar control room (inverter, batteries etc.) are 85 feet away from the HWT, does that need ant consideration in my Plans?

Thoughts?

[ClassicCrazy]

Will you have a thermostat in your control circuit ?
Also you probably saw in those discussions the importance of using good quality SSR and use proper diode for them too.
I don't have a lot of hands on experience with SSR myself , other than what I have been reading. Though last summer I was using a cheap SSR with one of those cheapy temperature controllers turning it on. I manged to melt the case on the temp controller before I realized yes I need a heatsink !

Larry

[HighlandSolar]

I hadn't considered a thermostat or a diode. Found a CDN manufacturer of SSR's and Heat Sinks called Omega out of Montreal (not made in China). And two 48 v elements from Missouri Wind and Solar (got nailed with Duty and Customs tax crossing the border, that kind sucked). I will look up the diode thingy though.

[ClassicCrazy]

I hadn't considered a thermostat or a diode. Found a CDN manufacturer of SSR's and Heat Sinks called Omega out of Montreal (not made in China). And two 48 v elements from Missouri Wind and Solar (got nailed with Duty and Customs tax crossing the border, that kind sucked). I will look up the diode thingy though.

You would want a thermostat in the control to the SSR coil circuit so that if water gets too hot it will shut off power to the element.
dgd had mentioned inexpensive thermostats in one of the prior posts - they are only couple bucks on ebay .

If you read through those SSR threads you will see a bit of discussion about the need for diodes mainly for inductive loads. But I think there was some thoughts that even a resistive circuit will sometimes be a bit inductive just from the wiring. 

There were some links posted to Crydom SSR company info on when and how to use diodes. But maybe the company you are sourcing your SSR from also have that info.

Larry

[HighlandSolar]

Thanks again Larry, I will look for the thermostats and as well the diode thingy on crydom's site.

[dgd]

This week i'm getting between  3.5 hrs to 5.5 hrs of float time so I should move forward now with the diversion project.

My plans after reading these posts are basically use Aux 2 on my Classic on Waste Not Hi, to switch a SSR to 2 x 48 volt 2,000 watt HWT elements on a 50 gal tank. My array is 3.6kw and the DC element can be reduced to 1,000 watts each if needed. I was thinking a MidNite DC breaker in there somewhere as well.

I think I just need to figure out whether or not to a) reduce elements to 1,000 watts each and b) whether to run one line, one breaker, one SSR to the elements or a separate line to each element.

Reading the other posts and the heat going thru the SSR's heat sinks, caution says run two, keeping the amps down? 2,000/60v=33 amps or 1,000/60v=16amps.....
 

I assume you have done some calculations on the energy in kwHrs you need to heat your tank to help you decide on element ratings?
If not yet then using the  standard formula for energy needed in one hour to heat a volume of water by a specified temperature  is...
Water volume in litres * 4 * temp increase needed in oC / 3412
So assuming a 35oC temp increase  (say from 15 to 50C) then the numbers for you are   250litres (50gal) *4 * 35 /3412 = 10.26kw/hr
Since you have 3.5hrs float then that would work with a 3kw element in the tank
A 5 hour float would look good for 2kw element. Thats where I would probably start, with two 1000w 48v elements. If the don't get the 6 hours then it just means the water will not get up to the end temperature.
You could experiment and just wire the elements directly via a 25amp breaker each to the battery bank and then just see what happens when you manually switch in one or two elements during float, observing temp increase over time and how the battery bank handles the element load.
Once you have some performance data then is the time to try to automate the heating using the Classic AUX and SSRs.
dgd

[HighlandSolar]

Great idea, thanks for the input!
I will try that manual switch first.

[HighlandSolar]

I am curious about the diodes.

If I am hooking up a SSR direct to the 48 v battery bank, using the charge controller Aux 2 to switch it, then do I really need the diode?

I understand if I am connecting direct to the panels that there is potential for voltage spike but from the battery after the charge controller there should not be that potential correct or not?

[TomW]

Seems I read someplace that even resistance loads may have that inductive kick back?

Something to do with them being coils of nichrome wire?

Not even sure they use nichrome these days in these things?

Either way, a protection diode will be cheap insurance to save the SSR from over voltage from an inductive spike in a high power DC circuit.

It would be a sensible thing to include in the circuit.

For those that do not know, when the magnetic field collapses when an inductive circuit is shut "off" it creates a voltage spike into the SSR. Snubber or flyback diodes let it conduct around the device safely.

You can look here for some details:

https://en.wikipedia.org/wiki/Flyback_diode


Just from here.


Tom

[HighlandSolar]

Thanksfor the input Tom. I will read that.