56 minutes ago, Richard Mackay said:

The way in which the available controllers do this is by measuring the excess power. In a grid tie system this would be the power that is fed back into the grid. (this is an analog value) The system diverts power to the geyser element also by means of a variable (analog) control.  

The standard control algorithm for this control is called PID. see below..

Hi Richard - thanks for that.  I do in fact already use a PID controller algorithm in my software.

On 2020/09/26 at 8:16 PM, Vassen said:

Just an update. Managed to install the sonoff sensor into the geyserwise pocket. I cut the cable, fed through the geyserwise pocket and soldered, and used heat shrink over the join. It seems to be working fine afterwards. Will hopefully swap the element tomorrow and then need to a relay or contactor and a bigger enclosure

As a matter of interest, the stock temperature probe on the Geyserwise is a 10K NTC thermistor with a B value of around 3950.  Should be readily usable as is in most home-brew projects.

Feedback from Kwikot...

They say that the highest temp any commercial thermostat uses is 75°C. From the geyser standpoint, the lowest temperature limit is the safety valve, which will start opening at around 92°C.  The safety valve is cycle limited to 2 operations, so it is best to stay well away from 92°C.  There will be some vertical stratification of temperature inside the geyser, so it is difficult to tell what the maximum safe temperature is at the thermostat pocket. So anything above 75°C is unknown territory.

11 minutes ago, Vassen said:

After replacing the element with a 2kw one, I’ve been heating the geyser to 80 degrees but it’s difficult to see how the stratification affects the water within. The element is really tiny and sits on one side of a fairly large tank. In my opinion, vertical mount geysers make more sense in storing hot water with the element being at the bottom. 

Thanks.  I am also planning on targeting 80°C.  Not quite within Kwikot's recommended range, but still far enough from any safety limits. Every 5°C is an extra 0.9kWh of 'storage' in a 150L geyser.

Finally got my smart thermostat running (although after dark, so no solar boost yet).

All the bits and pieces (including reverse engineering a Geyserwise TSE to use for the thermostat) are fairly well documented in the code:

https://github.com/justinschoeman/ModbusThermostat

Basically performs 3 functions:

1) turn element off when system load is too high, and only turn it on again when load is fairly low (it does rely on most inverters ability to sustain temporary overloads, as it takes about 1.5 seconds to turn off the element).

2) act as a normal thermosts.

3) heat the water as hot as safely possible when there is excess solar electricity available (since this is a bang-bang controller, it probes by turning the element on for a few seconds and if battery draw is too high, turn it off again).

Well done Justin

Had the first day of sunshine yesterday, and have some test results...

My geyser is a newish Kwikot 150L. No geyser blanket, but hot pipes are insulated.

I initially heated the geyser to 75°C.  Confirmed that there was no temperature overshoot, and that there were no vents or overflows from the geyser.  Then heated it to 80°C and all was still good.

Last element activation was at around 5PM at which point the geyser ended at 80°C. By the time I showered at 11PM , it was down to 67°C.  After a fairly long shower it was down to 62°C. And this morning it was down to 50°C (still without heating).

So the basics seem to work quite well.  It will probably require a geyser blanket for similar results in winter, but an off-grid electric geyser is certainly practical.

The software does require some work though, as the boost dropout on battery current is too quick (it takes longer than I expected for the MPPTs to ramp up and take over the load).

7 minutes ago, Vassen said:

From my research on the topic, the geyser blanket doesn’t really add much value in terms of keeping the water warmer as the geyser already has a thick layer of protection. 
 

The pipe lagging on the other hand is an absolute must as a lot of heat escapes the pipes. 
 

The difference in winter is that the temperature of water entering the geyser is a lot colder

Fully agreed.  Most modern geysers are pretty well insulated. 

I did a test by first wrapping my geysers in a 150mm insulation and then covered it with the geyser blanket foil.  Ran it for 2 days and didn't notice any major differences in standing loses.

I then covered the pipes using thermal lagging, this reduced the standing loses by 3-5 degrees over a 24 hour period.  (Tests were concluded in the winter)

1 minute ago, DaveSA said:

Also cover the valves and anything warm to the touch

That is pretty much what I have done.  Only warm spots left are the end plates (flanges) of the geyser, which are largely uninsulated.  So maybe not a full geyser blanket - just tape some 'nappies' over the ends of the geyser?

Hi geyser guys, sorry, but all of this thread just do not compute.

Storing energy in hot water is a losing investment. It will pay off in only certain, rare occurrences, like on a cold cloudy morning after a sunny day. Heat is leaking away all the time and can easily be replaced when the sun shines again.

I just stringed 5 x 40V, 360W panels to supply 200VDC to a standard 230VAC 2kW element in my existing geyser. Wired a 6uF capacitor over thermostat to keep it from arcing and welding shut. By about 11:00 AM thermostat opens, voltage rises, when more than battery voltage, 210V battery bank charges through a diode. (Which keeps batteries from feeding geyser.) 

We have hot showers and by sunset full batteries for the admittedly inefficient but cheap Eaton 5KVA UPS to see us through to bedtime. No charge controller, manual battery management when bored. I just do not see complications improving my utility. 

(I also have a VIctron-Pylontech system with less capacity, at three times the capital cost.)

What am I missing?

 

 

 

 

7 minutes ago, Abe53 said:

Storing energy in hot water is a losing investment. It will pay off in only certain, rare occurrences, like on a cold cloudy morning after a sunny day. Heat is leaking away all the time and can easily be replaced when the sun shines again

Between dishes at night and showers morning and night, I was using about 3kWh of battery each night to keep the geyser at a usable temperature.  Make it hot enough during the day means that I have 3kWh more battery for other loads.

I would say that ~R800 and a few days work is a worthwhile investment for 3kWh of extra battery capacity.

While sunlight is readily available during the day to heat the geyser, no one uses hot water during the day.  Pretty much only used before or after sunset. So I am pretty sure that in many cases, it makes a lot of sense to put extra energy into the geyser.

Hi Justin Schoeman

Did you finally manage to solve your problem? I am looking for a solar PV diverter solution in South Africa. I installed solar PV when I lived in France and I bought a diverter from a chap on the web who constructed them. Essentially, it was a modified electricity meter on his circuit breaker board. All unused electricity for the PV panels was directed to the geyser, and only once the geyser was up to temperature did it export to the grid. For example, if only 350W was unused, it directed the 350W to the geyser. It did not have to wait for 2kW (or whatever the element rating was) to be unused before switching the immersion on. He has all the instructions on the web on how to do it. It looks easy if you can do the electronics and program the flash memory with the code he supplies. I bought a fully assembled unit and it worked perfectly. Essentially, I am looking for the South African equivalent. I can link to the website with all the instructions.

Thanks

Peter