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Running geyser element on +/- 100vdc?


Elbow

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Hi,

What thoughts on running an ordinary 230v / 3kW geyser element directly off 3 panels in series?

I have 3 extra 350W panels with Vmp 39.2v, Imp 8.94.

So 3x39.2 = 118VDC.  

I work out the element resistance as 230v/(3000w/230v) = 17.6 ohms.

So with 118v it would draw 6.7 amps.  Which comes out at 788 watts.

So that's about 75% efficient.  (It would be more efficient if the element resistance was a bit less)

With less solar energy the power would drop but its a very simple way to harvest some heat into the water.

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 A pv module is a constant current source, so don't count on the 118V... count on the 8.9A Imp, let's say 9A to make the math easy.

9 * 17 = 153V.

But you only have 118V, so it's going to run at a voltage slightly higher than Vmp, and a current slightly higher than 6.7A. Can't say exactly, but the 700W you calculated is probably not far off.

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13 minutes ago, plonkster said:

 A pv module is a constant current source, so don't count on the 118V... count on the 8.9A Imp, let's say 9A to make the math easy.

9 * 17 = 153V.

But you only have 118V, so it's going to run at a voltage slightly higher than Vmp, and a current slightly higher than 6.7A. Can't say exactly, but the 700W you calculated is probably not far off.

Thanks,

A challenge with the idea is that I can’t think of a safe / compliant way to disconnect the element from the panels and connect to the mains if there isn’t enough sun to heat the water by a deadline time.

I suppose it would have to be two 2P contactors, interlocked with some sort of current sensing so I am 100% sure the one is disconnected before opening the other.  Or some other expensive interlocked changeover thingy.

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1 hour ago, Elbow said:

2P contactors, interlocked with some sort of current sensing so I am 100% sure the one is disconnected before opening the other

Normal two-contactor changeover with auxiliary NC contacts (for electrical interlocking), and a physical lockout installed between them. It's going to cost a bit though... 🙂

Also, contactors (at least one of them) have to be rated for DC.

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On 2019/03/20 at 8:39 PM, Elbow said:

...

What thoughts on running an ordinary 230v / 3kW geyser element directly off 3 panels in series?

...

That's not a good idea. Really.

The DC current likes to create an arc whenever you try to turn it on/off. And the thermostat in the geyser is designed exclusively for the AC current (assuming the arc dies as soon as the sinewave passes thru zero). If you apply DC to this circuit after a day or two the thermostat will fail and there are only 2 possible scenarios what will happen next:

A ) The thermostat and it's casing will start a fire.
B ) The thermostat will be fried in the ON position, efectivelly turning your geyser into a ticking "Steam-bomb".

 

Edited by Youda
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