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hotwater
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Joined: 29/08/2017
Location: United States
Posts: 120

Posted: 09:01pm 20 Oct 2017

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Even if the PV DC buss is electrically floating, there is always some leakage to ground. If we are splitting hairs on electrolysis, one leg of the PV DC buss will have some potential to ground. That means the element will always have a DC potential. Kinda makes any pules reversal pointless.

Replacement elements are only $9USD if you can get the old one out. Tanks are still just crap and don't last anyway. Pull out an anode after 3 years and see how much is left of it. If you do replace anodes regularly, the tank can last 40 years.

In a pulsed DC setup an off time can always be included to kill an arc. A FET shorting is a problem, but the over temp limit has to be manually reset. One time events should not be a problem.

The big advantage of an H bridge is if one FET fails shorted it will likely short another and open some over current device. But certainly there are other failure modes that could occur.

Solar Mike
Guru

Joined: 08/02/2015
Location: New Zealand
Posts: 1164

Posted: 12:14am 21 Oct 2017

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>> Replacement elements are only $9USD
I wish.. my last element cost >$120, special type, not standard screw in.

Can you still purchase vitreous enamel lined tanks requiring a sacrificial anode, all the solar tanks I have seen are either duplex stainless or heavy head copper.

>> A FET shorting is a problem, but the over temp limit has to be manually reset. One time events should not be a problem.
A single shorted mosfet will not necessary blow any fuse, the panels are constant current sources and will quite happily run into a dead short. What can then happen is the cylinder overheat and the safety cutout open or the AC thermostat open, then they are turned to molten metal by a 2KW dc arc.

Mike

Edited by Solar Mike 2017-10-22

hotwater
Senior Member

Joined: 29/08/2017
Location: United States
Posts: 120

Posted: 03:24am 21 Oct 2017

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I've seen fiberglass "solar" tanks that can take about 7 heaters at an unbelievable cost. Marathon fiberglass tanks are available here for really not that much more (50%)than the glass lined tanks. With the added life and better insulation it is a deal. Just saw an AU video where element was slipped through the lower drain port giving an extra element for off grid. I had never seen this thin element before that could pass through a 1 inch pipe fitting.

We also have some expensive ones with copper sheathing and fuse-able elements for upper elements that could lose water. My water heater works as a dump for excess power not used. Short current would much higher than the element would ever see. My tanks were used with the anode completely gone. Severe rust comes out when they are first filled each year at the camp. Waiting for the day of the big leak. At home I have a standard tank with external heat pump. Adding solar to the unused element will have payoff in energy saved and extended life of the heat pump. A couple years ago I did some testing with a 100W panel to see typical site energy creation. A boost converter powered the element at power point. That panel went to camp later that year. Now looking at getting a single 280ish watt grid tie panel. Just tank loss is about 80W/H. These small add on solar systems that can never provide all the heat needed are the best payoff you will ever get with solar. Every possible watt is always used. This seems like the perfect item for the big box stores. Go solar in a weekend for the DIY.

LadyN

Guru

Joined: 26/01/2019
Location: United States
Posts: 408

Posted: 10:25pm 05 Feb 2019

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Solar Mike said
My proposal is a simple H-Bridge with variable pulse width modulation to adjust the load on the PV array and allow alternating the polarity of the dc voltage across the element to prevent electrolysis

Madness said Obviously, this does meet your regulations or the over come electrolysis

I have a few questions about this to both of you:

1. What's the concern about electrolysis? The elements are enclosed within an isolated metal sheath/shroud and thus the water never comes in direct contact with either the + or - of the electrode, right?
2. Is the metal sheath/shroud around the coil not actually isolated as I ASSUME but somehow connected to either electrode?

Solar Mike
Guru

Joined: 08/02/2015
Location: New Zealand
Posts: 1164

Posted: 08:04am 06 Feb 2019

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Electrolysis isn't an issue if all parts of the system are functioning correctly, however if the element starts playing up and exhibits increasing leakage then there can be an issue, powering it from a DC sourced H-Bridge is a simple means to avoid any potential problems and present an AC square wave power source that will allow use of the existing thermostat (providing the cylinder is open vented; NZ Regs)

Mike

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