SSR? a Dump Load for PV array
 

http://i46.photobucket.com/albums/f1...psb7b37684.jpg

I had two PV arrays connected to the heating elements in my 40 gal water heater.

Recently, we had some really cool but sunny days and it caused the tank to get
a lot hotter than we needed. Moving into the unsafe temperature range.

So, I disconnected the 500W array and now we have a cooler tank..
On bad solar days, the temperature might not get up to 110F.. :(

So, I tried to build a simple controller using a DC SSR.. And failed..


Now, I'm looking for another path.. Maybe Dump-load like the windpower guys use..?.
If the wind is really blowing and the battery bank is at full charge,
they use a very large relay to switch the windpower into a high wattage resistive load..
The excess wind power is now heating the air..
(Sometimes they don't waste it, they heat up a hot water tank). :thumbup::thumbup:

http://www.cleanenergybrands.com/sho...sionwiring.jpg

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I once experimented with controlling DC current using an AC SSR..

http://www.electronics-tutorials.ws/io/io27.gif
The AC SSR uses a device called a Triac to switch the AC line into a load.
The Triac is kinda like two SCRs tied together, back to back.
Depending on which way the AC is flowing on that cycle, one of the SCRs does the work (conducting the current).

What I found out about the SSRs that I was using in my lab project, was:
They would conduct DC current when turned on by the control input,
but they would not turn off when the control input was shut off..

So, thinking about my requirements, I realized that once my tank was up over 160F, I should somehow shut off the PV heating..
I don't want to use a mechanical relay, since they arc with high DC voltage..
(I've already fried the standard hotwater heater (230VAC) controls).

But, if I used an AC SSR, I could only turn it ON, and never OFF.. :(

So, if I connected SSR terminals across the PV +&- power lines, turning ON the SSR would short out the panels..
Up to almost 8 amps would flow through the SSR and it would latch ON..
And could not be turned off.. (using the control terminals).

Would that be so bad? Why would I need to turn the heat back on?
If I took a 3 hour shower?

Then it dawned on me.. When the sun when down, the PV DC would
disappear, and the AC SSR would turn off..
The next day, (if the control terminals weren't powered), the SSR Triac would be OFF..
Only when the water got too hot, would the SSR be turned on, for the rest of the day..

I figure that a 20A 240vac SSR should be able to handle 7 or 8 Amps of DC current from the panels.
Since the Triac voltage drop is low (it will have very low resistance when ON),
it shouldn't heat up very much.. My guess would be just a few watts..

What do you guys think? Will it work? Or will it smoke the SSR?

115 VDC at 7 amps into the 13 ohm heater is typical operation..
So, short it out with an SSR and get 0.8 volts at 7 amps (5.6w)?
That should be a piece of cake.. :p Maybe?

If it worked, the input power to the 13 ohm water heater elements
would see less than a volt.. And all the excess power would be
dissipated as heat in the wiring of the panels and feed lines..

http://www.bimblesolar.com/image/cac...el-500x500.png

When shorted out with very low resistance, a solar panel puts out a lot of current.
It will warm up your amp meter a little.. But, because it's a short ,
the panel voltage is super low and the total wattage is peanuts..
Current x Volts = watts..
My guess is, the panels will be a good bit warmer when connected to a dead short..
Seem logical anyways..

Here's some triac info..
http://makezine.com/projects/make-36...s/try-a-triac/

You can get DC SSR that have mosfets in them, they still use the opto isolator and are specifically designed to switch DC sourced loads on and off, readily available on aliexpress for similar prices to the AC versions.

I have a similar situation, I am have recently installed a 12 volt 400AH Lifepo4 battery (4 x 3.3v 400 AH cells), these are charged by a 60amp mppt charge controller, powered by 6 x 140 watt panels mounted on a frame above the porch leading into the house.

The battery powers a 1500 watt sine wave inverter, that runs everything in my home office, 3 pcs, stereo, lights etc. If Im not using the office much, then the battery is full and the sun is shining; the controller just sits drawing zero current from the panels. I thought about switching the panel outputs direct to an extra low voltage HWC element fitted in the bottom of the cylinder and using the DC SSR to do the switching. A voltage sensor can turn this off when the battery voltage drops back with load, although this is a bit tricky with lifepo4 cells as the voltage curve is virtually flat for all loads, perhaps a current sensor would be more appropriate.

Cheers
Mike
NZ

Seems like the 40 amp 220v DC SSRs that I get from China aren't able to handle 8 amps..
I've killed two so far.. I'm gonna give up on them for now..
http://i46.photobucket.com/albums/f1...psc5151ccc.jpg
This one worked for an hour or two, then died on a partly sunny afternoon.. It never even got warm..

But, I have a large stockpile of AC SSRs.. :D

Maybe use some series resistance to limit the peak current? Like a space heater?

Is it possible to convince the 500w tracking array to turn away from the sun when not required?

Or use a DC SSR to switch in a 500w floodlight or two to dump some power.

The internal resistance of the SSR will be in parallel with the 13 ohm heater.
I would have to use a very low (1 ohm?) resistance to keep the 13 ohms from adding more heat.
The max current of the PV is self-limiting at 8 Amps. (due to a weak sun). :p
Too much current is never a problem with series PV arrays.

What I'm looking for is to reduce the water heating by at least 95%..

At this point, I'm looking to turn off Both arrays. If the tank temp gets too high,
I don't want even a small amount of BTUs coming in. If it's early in the day,
even a little extra BTU will accumulate and cause problems.


I would like to use a DC SSR, but DC SSRs don't seem to like me..
For some reason I can't keep them running.. Maybe it's just bad luck.
But, I've never had any AC SSRs suddenly go belly-up for no reason..:)

Have you monitored the heat of the SSR? Most of them require a heat sink if you're putting any decent power through it.

True about the heatsink although I would think it,s more likely the inrush current when trying to short out 110v.
The photo of the blown SSR shows a minimum of 24v DC so it could well be oscillating because as soon as the SSR closes the voltage would be well below 24v DC and switch off again.
This constant switching with very high inrush and poor heatsink can't be good.

As the SSR is easily capable of switching the load of the water heater, could you not just switch of the heaters when the tank is at temp?