I have been trying to find out more information on the following without success:

Is there a way to share a PV array between directly heating the geyser and a solar inverter. Basically, the PV array to be connected in parallel to both at the same time. The geyser will have a DC element and I think, I need a solar charge controller between the PV and the geyser. So, when the sun comes up in the morning, the energy from the PV array is shared between heating the water in the geyser and charging the inverter batteries. When the geyser is hot enough, the full energy with the utilized to finish charging the batteries.

I think this way will be more efficient, than redirecting excess energy from the inverter to the geyser element.

Please let me know of your opinions on the above idea and if you know of a working installation like that. 

On 2021/07/13 at 6:45 PM, PowerUser said:

The geyser will have a DC element

The element isn't the problem usually, it's the thermostat. These are usually simple affairs, which work fine with AC, but won't like switching higher voltage DC.

You may find that the idea, while good on paper, is more trouble than it's worth.

A 6uF capacitor of 350V across the thermostat should solve this problem. 

Solution provided by @Abe53

11 hours ago, Scorp007 said:

A 6uF capacitor of 350V across the thermostat should solve this problem. 

Solution provided by @Abe53

Can you point me to that solution please. 

18 hours ago, Scorp007 said:

A 6uF capacitor of 350V across the thermostat should solve this problem.

? I don't think that a capacitor across the contacts will magically allow it to quench a DC arc. Plus, the capacitor will short circuit every time that the contacts close electrically. Yes, this will "wet" the contacts, but I don't believe that it will overcome the severe pitting that will result from breaking high voltage direct current.

I can just go with Abe53 that has been using panels to directly power his geyser ement with this method for more than a year without welding his thermostat. 

If a 0.22uF cap prevented all cars up to the late 1970's from pitting their points while quenching the spark of over 200V at over 10 000 times per minute it sounds right to me. A capacitor only discharges when making a short circuit and causes no harm AFAIK. 

Even if it does not work the cap is cheap enough to try. 

We not breaking a high DC voltage as we talk about 150-200V.

Hi guys, just confirming,  AC thermostat is still reliably switching 200VDC protected by a 6uF cap. In its favour: It is a capilliary mercury oven-type thermostat, the 2kW geyser element only draws 8A max and a pilot light draws down the cap charge between switching. When thermostat switches off, PV voltage rises and charges 200V battery bank - through a diode to avoid batteries heating water. No charge controller. (Manual battery management, no battery balancing, yet only one recycled 18650 cell out of 1802 needed replacing in the last 6 months.)

Busy installing several more solar PV systems in anticipation of EKSDOM load-shedding.....

6 hours ago, Abe53 said:

AC thermostat is still reliably switching 200VDC protected by a 6uF cap.

I wonder how long it will last. The capilliary mercury oven-type thermostat still has ordinary contacts as far as I know. I would check its temperature when on from time to time.

Edit: From Wikipedia article Contact Protection :

One method is to add electronic components such as: capacitors, ...^[5] However, this is the least effective method as these neither significantly influence the creation of nor suppress the arc between the contacts of electromechanical power switches, relays and contactors.^[6]

Reference [6] seems to be a commercial site promoting a particular brand of arc suppressor (note: I don't believe that these are designed for DC switching). So their impartiality may be in question. However, I believe that the facts still stand.

Edited July 20, 20215 yr by Coulomb

56 minutes ago, Coulomb said:

I wonder how long it will last. The capilliary mercury oven-type thermostat still has ordinary contacts as far as I know. I would check its temperature when on from time to time.

Edit: From Wikipedia article Contact Protection :

One method is to add electronic components such as: capacitors, ...^[5] However, this is the least effective method as these neither significantly influence the creation of nor suppress the arc between the contacts of electromechanical power switches, relays and contactors.^[6]

Reference [6] seems to be a commercial site promoting a particular brand of arc suppressor (note: I don't believe that these are designed for DC switching). So their impartiality may be in question. However, I believe that the facts still stand.

Interesting.... Why were condensers then used for decades on vehicles ignition systems. 

@Abe53, do you have a picture or a diagram of your setup? What happens if the capacitor fails?

5 hours ago, Scorp007 said:

Why were condensers then used for decades on vehicles ignition systems. 

Because they do help. But note that ignition systems switch at the 12 V level, with only of the order of an amp flowing, so DC arcing is of comparatively little concern. Granted, there is a hell of an inductive kickback from the primary of the ignition coil, but I don't think that compares to the arc of switching 200 VDC at many hundreds of watts. I seem to recall that contacts were basically a consumable item, or at least items that are expected to fail fairly regularly.

I agree 100%. Yes points did fail and had to be replaced. It is not like a geyser would have to switch more in it's 5 year life than what points had to switch in a minute during the period before failure. 

Lets agree a cap is not a solution. Replacing the AC thermostat with a DC rated version is a save swop. 

I started this tread with the intention of finding reliable working solution and not just a proof of concept. A simple capacitor, might work with the AC element but this is not something which should be installed in a properly designed electrical system. The solution should be fully compliant to the standards and be able to pass official CoC. I'm all for DIY because I'm willing to do the work and save on cost and not because I cut corners on safety.