HWC Diverter - without CT

Hi All,
A while ago I had a solar diverter running well with Robyns code using a SSD and a EmonTX shield. Ive since moved and have a completely different home automation / solar metering setup with the cylinder powered from the sub board.
I would like to build a device to control the consumption of my hot water immersion heater (like with Robyns diverter) using an MQTT topic to specify the consumption. I could then adjust its consumption with nodered to follow my grid meter and battery setup.
Im thinking the hardware could be based off an ESP device.
Has anyone considered or built this kind of thing? Not sure how much code I would need to modify of the original diverter or if I could start fresh.
Many thanks,
Mike.

You must mean Robin Emley.

Hi Mike,
The Mk2 PV Router should be suitable for all grid-tied premises. In all cases, the primary CT needs to go around one of the incoming cables which pass through the meter. The Mk2 controller will then adjust the consumption of its load(s) such that the energy state of the premises is constrained to a near constant level. This method of control ensures that no surplus energy leaks away to the grid, and none of the energy that is consumed by the Mk2’s load(s) is chargeable.

Similar control could no doubt be performed in other ways. With an earlier arrangement that preceeded the Mk2 Router, the flow of energy was monitored for a few seconds and an external switch was adjusted so that energy would be consumed by the “dump-load” at a certain rate. Because the response time was slow, the system would take a short while to find its new balance whenever the conditions changed, and its hardware was more costly than the Mk2 version.

To amplify Robin’s comments, speed of response – hence the likelihood of unwanted export or import (‘leakage’) occurring – was my fear with the system you envisage. Robin’s diverter responds within 2 mains cycles, and is capable of operating with absolutely zero leakage in either direction.

Your proposed system would appear to suffer at least 4 delays: the interval between a change and the time your grid meter makes the new consumption (or otherwise) available, the delay in nodered, the response time of the ESP device and finally a wait until the next zero crossing to turn on the switching device. And of course, you need to be aware that any delay in a closed loop control system can lead to instability.

I’d suggest your first move would be to find out how frequently your grid meter makes the required information available (or can be requested to provide the information) and evaluate the anticipated performance of your scheme based on this.

If you’re not concerned with leakage, then my concerns are not justified.

Thanks guys,
Yea I understand that latency can allow leakage and or unwanted import and that this is not ideal in most situations. I have used the diverter in its standard form in another home and it was very accurate, a fantastic solution by the designer Robin!
My home battery is run through a victron inverter and im running the feedback loop for that in node-red due to limitations on the type of grid meter aloud in that system. It’s working pretty well as I get meter updates from Fronius fairly quickly (over UDP modbus). I wanted to use this system for to control my cylinder as my hot water cylinder is powered off a sub main a long way from my meter. We also get several hours of free power in the evenings so I was wanting to use this one device to heat using excess during the day (solar) and also heat to a high temperature to get the most of the free energy in the evenings.
Regards, Mike

While I completely agree with @Robert.Wall, we should be pragmatic about diversion.

My own solar powered “micro immersion heater” is just 600W and is controlled by a relay drivenby Node Red which is only updated every 5 secs from EmonCMS. If there’s more than 650W being exported turn on the relay, if there’s less than 650W turn it off. Yes, there is some leakage, and yes there is some import, but it was cheap to set up, and in the last 10 years it’s diverted over 6,000 kWh into the immersion tank, and that’s the point. No carbon, no grid loss.

Don’t let perfection be the enemy of good enough. (My apologies to Voltaire.)

I’ve also got to point out that some suppliers in some countries insist that there must be absolutely NO export at all, and in this situation only absolute perfection (albeit in the export direction only) is ‘good enough’.

So yes, having made my living in engineering for many decades, I totally agree that ‘good enough’ is the objective, how good that is will be totally subjective. Hence my final sentence above.

What’s the story behind that? In most places, the evenings are when things get most expensive.

And as an example of just how rough and ready that can be at the other end of the control spectrum, a popular strategy in these parts where it’s sunny more often than not, is just to put a simple timer on the element to ensure it only runs when the sun is likely to be out.

On a sunny winter day, with a 5kW PV system and a 3.6kW DHW storage tank, that looks like:

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