Thinking this through again you could speed the control loop up by enabling the feedback to the grid. Our utilities appear to not be too fussed if you do but they aren’t going to pay you for this. The new PAYG meters aren’t as sensitive to tripping so no problem there.
This will provide a process variable of the amount of excess power available and then will allow a much faster conrol loop (PID) to power the geyser element.
They won’t be fussed BUT you will be very fussed when your postpaid/prepaid meter goes into error mode and you are locked out (with no grid at all)… Be very careful.
You will still pay for exported power. And then what happens when the geyser is at temperature? All the excess will be exported, adding to your bill.
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You then heat the neighbours geyser!
Sure! But the amount of export power will be small.
Using a PID controller (see: Proportional–integral–derivative controller - Wikipedia) the proportional will start the process due to the difference between the exported power and the solar inverter power (the error)
Once the diverter starts feeding power the exported power will drop immediately due there being more consumption of solar power. This reduces the proportional output of the PID controller and thus reduces the output to the diverter.
However the integral control starts taking over and will ramp up and add to the output as long as there is still an error.
So the exported power will be small as the PID control gets the output to achieve control.
(Forget about derivative for now…)
Yes, I understand your proposed scheme. The problem is that when you cannot divert anymore power, like when the geyser is at temperature, you will be forced to export the rest. There is no practical way for you to heat your neighbours geyser.
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I think that a forum like this one is a great platform to exchange ideas. Not only to simply punt what you installed but to check out what the options are and what they offer.
From the word go I couldn’t see how inverters could divert their excess PV power to heating your geyser.
Recently I discovered this diverter which does this but to do the job properly you need to buy a device as expensive as your inverter. (And I haven’t seen them here in SA)
So I’m consequently happy with my 4 dedicated PV panels powering my geyser with 1kW approx as long as the sun shines and also with no inverter hammering away!
PS: Apologies to the OP but I hope these twists and turns in the discussion will encourage those trying to get the most out of their systems.
Herewith more on this topic: https://www.youtube.com/watch?v=w8_qTUR5v5Y
I didn’t look at the link you posted. But I know the OpenEnergy project has had two different diverter designs for some time. They have plans on line for building one. At one point you could order one (the guy is from the UK), but it was rather on the expensive side.
It essentially sees the reverse energy and uses that drive a variable amount of energy (probably some PWM technique) into a resistive heating element.
I think it is an excellent solution for people who have slightly oversized arrays (for winter), who then end up with too much energy in summer (when you usually also don’t care about all the hot water).
It is not a replacement for a grid meter/limiter. AS @_a_a_a said, if the water reaches the optimal temperature, then the rest has to go to grid.
If roofspace is not an issue, then maybe heating water with dedicated PV modules is an option, for the simple reason that it is a less expensive than a heat pump, takes less maintenance, and will probably last longer.
A lot of this discussion, I suspect, hinges about the systems people already have. If you are starting from scratch, then you can plan as you see fit. If you have to fit into an existing system, where you already have a bunch of panels on the roof, a diverter might be far cheaper than rearranging everything.
Agreed! We tend to keep on going on the same track regardless.
This isn’t smart. If you can stand back and get a perspective on a number of options it will prevent you landing up in a difficult situation. We need to be able to be critical but this often threatens people.
This is not so much in SA yet… but it will come. There will be questions about how to most optimally use the energy you generate.
At any moment in time, you could:
- Charge the house battery, if it isn’t full yet,
- Feed your excess energy into the grid, if it is profitable (it might not be),
- Heat water for sanitary use later,
- Charge the EV, if it is home,
- Throttle the PV since the car is charged, the water is hot, the house battery is full, and the grid prices are negative.
Imagine the complexity that arises in such a system, because you may well want to charge the EV with those negative grid prices too. It becomes a hectic planning exercise.
The point I am heading for, however, is that in such a system, it is good if the energy is in some kind of standard form. Something that can go in either direction, into the car, out to the grid, wherever.
For this kind of thing, having the water heating on a separate PV string… not the smartest move. On the other hand, this is just another example of how we have to make existing systems fit into what we already have… which usually does not include the crystal ball.
Hi kan jy mail asb. I need info on the whole build. smith.edwin@gmail.com
Three geysers on a 5kva with a 17kWh bank, 9 people, yeah, I see that daily.
Easily solved.
Throw more money at the “problem” … but then you have a oversupply in summer, the next “problem”.
Grid tied inverter with battery backup does mitigate the frustration to “wife is quiet”.
She showers with scalding hot water.
Her problem is solved, mine is not.
Respect to Eskom to keep this balancing act working for decades.
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Just a point on this water heating issue: This stand alone system doesn’t switch on/off. Only if it’s a scorcher of a day will it reach the 75°C cutout. So it’s on all the time.
So if you incorporate this load into your intelligent load control mix you still have to provide for it. and it’s probably your biggest load.
If you don’t have variable billing from the utility (which I reckon is a long way off) I can’t see any benefit.
Yes, not criticising your setup. I’m saying in a new build, where I envision that there will be an EV and the opportunity to make money from feeding into the grid, I would not design it that way. I’d put the extra panels in a place that gives me a choice. For most saffers this is a few years off in the future… so do as you please for now 
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