HWS Diverter

I now have got Node Red exporting the "Live" power data to my home control system so I can now look at lashing up a hot water diverter - I have got the data flowing into the controller that does the hot water and that controller has a spare analogue output that I can drive a thyristor to vary the power into the immersion heater on the cylinder. The thyristor controllers are only about £80 - needs a supply directly off the board probably through a contactor - though that is probably overkill for this application as there is no real way of fitting a high limit thermostat that would be in a sensible location.

Sounds interesting. Keep us posted

I will - yesterday was the first time we completely filled the battery - and had to go into emergency anti export mode - and put some washing on!
And today the battery is already 45% full and we aren't at peak solar yet.

We have been washing today. So doubt the battery will get to 100%. Yesterday we too filled the battery at 16:20. We then started exporting 500W

Methinks that you will need to modulate the drive to the immersion heater in order to limit the export to "nominally zero" instead of "output +0 -3kW".

So, variabble phase, or PWM (or a motorized variac 😀 )

Yes modulating is what I said.

Something like this. I’ve used their gear before and it’s ok. Been going a long time have axio.
https://www.i-acs.co.uk/store/ax-mpr1-4-axio-single-phase-4-0kw-thyristor.html

Ok. I'd be a pedant and say that that's a bang bang controller, with controllable on cycle. rather than a "thyristor". It might have a thyristor/triac in the output stage, but it's also got the control circuitry to drive it built in. so you don't have to.

mrand31 Not 100% sure what you mean by bang bang controller? It switches on a zero volts and off somewhere along the cycle (Which does produce some noise) but that is what thyristors do - they are only a switch at the end of the day - its not an inverter which reproduces the sin wave but at a lower peak to vary the voltage.

Thyristors tend to be switched on at some point in the cycle, then cease conduction when the current falls to zero. My reading of the data sheet for the controller is that, for a given control input, it switches the output on at the zero crossing point, then keeps it on for a whole number of cycles. With the number of cycles dependent on the control input. the process is repeated every four seconds. Thay's why it's a bang bang controller. The load goes Bang! Bang! every four seconds. 😀
[https://en.wikipedia.org/wiki/Bang%E2%80%93bang_control]

That way, you get zero current switching at both the start and the end of the pulse and virtually no noise.

I'm not sure how the 4 second cycle interacts with the SMETS2 meter, because the load will be swapping from import to export every cycle. I don't know how often the meters integrate instantaneous port to generate the import/export figures. I guess, since they're being made and sold for this use, that it works as one would hope.

[https://irp.cdn-website.com/e7c2a7a0/files/uploaded/AX-MPR1-4%2C6%2C9%20300321.pdf]

mrand31 Your Bang Bang description sounds a bit like "burst control" as used in my microwave where 50% power means it's banging out 100% power for 50% of the time (say 5 seconds on and 5 seconds off) and 25% power means it's banging out 100% of power for 25% of the time, 2.5 seconds on and 7.5 seconds off.

That's how thyristors work, on off, the RMS (Average) power is the sum of the On's and off's. As this happens at a fast rate, there is no perceived on off.

See how LED lights dim.

TheDragon (GivEnergy) I thought LEDs were dimmed using Pulse Width Modulation of a DC voltage with the brightness determined by how long the voltage is high for. It's also the way a brushless DC motor's speed is controlled; by switching the DC on and off quickly for a varying period of time. If a sinusoidal mains voltage is connected to a load as it crosses zero and off again four seconds later when it crosses zero, it will be drawing full load current for that 4 second period. That means a 3kW immersion heater will be pulling 12A for four seconds, then switching off for four seconds, then back on and so on. With an effectively unlimited supply from the grid, this would cause blips on the local distribution network but wouldn't otherwise cause a problem. If you were trying to feed that from an inverter, I'm not sure it would be able to respond quick enough to prevent the device from pulling the 12A from the grid until the inverter output ramped up and then exporting 12A when it switches off, until the inverter output ramps down.

I'm not sure how the myEnergi Eddi works but that is able to divert varying amounts of energy to heat water.

mrand31 I'm not sure how the 4 second cycle interacts with the SMETS2 meter, because the load will be swapping from import to export every cycle.

My understanding is that smart meters have two integrators - one for import and one for export. Whichever is happening, only one counter is incrementing, they don't decrement.

I think it's down to Angelfire to build it and then report back to us all as to what happens. :-)

The inside of a Solic 200 should that be any inspiration to you here

mrand31 @TheDragon (GivEnergy) @anglefire Ahem 😅 (This is an embarrassed emoji) I've now actually read the data sheet and it does use look like it uses PWM on each cycle, so it should work exactly like a variable speed DC motor controller or variable brightness LED. Since the mains AC output is directly proportional to the 0-10Vdc control signal, you would just need to input a 1Vdc signal for each 300W of heating. (there's no humble pie emoji).

Read the second page of the data sheet I linked to.

mrand31 I think you are right - I've never considered it - because it doesn't matter with what I do usually - but 2 seconds on and 2 off is going to be crap!
I'll see what else I can find that is cheap. Ot sack it off as a bad idea! But whatever, I'm glad I posted it on here!

mrand31 Now I've read the second page, I'm just going back into my cave where I will contemplate my navel for a little while.

Tim it’s quite nice in here though 🤣

If you still want to go the homebrew route, the MK2 PV Router kit might be an option.
It's driven closed loop from an arduino & CT but it wouldn't be beyond the realms of possibility to additionally control it from GIVTCP with some tweaks I guess?

Here

In the meantime, how do the commercial diverters do it without blanking out every radio in the neighbourhood?
Expiring minds want to know.

They PWM at low khz rates. Not seconds. Hence no visible flicker. But total RMS load is modulated according to the on/off ratio.

I have an old Solar iBoost, which is built for this purpose, running off a CT clamp and pushing excess export into the immersion. Anyone played with one of these to integrate the control side? I'd like to dynamically set the power modulation based on Home Automation, rather than a CT clamp

Britkat I’ve not but would be interested to try at some point. Though I’m not sure the speed would be quick enough over comms. That is the biggest worry I have doing it with a thyristor controller as the polling from the inverter is only 10seconds or so.

hoggy the shop is effectively closed at the moment due to high demand.

I do have a proportional controller on my old immersion, but thats not actually my likely end outcome.
I will probably swap out my 3kw heater and replace with 2kw version ( there is a 1kw version too ), but this method still works pretty well even for a 3kw when there is reasonable PV.

Use GivTCP ( I have on 5s refresh ) to see what SOC, PV and load ( excluding immersion ) is. If SOC > 95% and PV > 1kw and baseload <1kw switch on immersion for 15 mins. No switching etc. Worst case is the battery covers all the demand for that period.

If the sun goes in, the battery runs the immersion. If it doesn't PV runs it. Cycle resets every 15 mins so if SOC has depleted it shuts down and PV restores it. If all criteria still active then another 15 mins cycle starts

I am changing the immersion smart switch to a load measuring one soon. That will give more control as I can measue power through that. When the immersion thermostat switches off, or I know 5kw has been passed, I will switch off the immersion divertor and switch on another smart switch will switch on for a 2.2kw charger on the EV using similar logic

PV destination order load->battery->hw->ev

Really simple and very cheep. Using the battery as a buffer just makes the logic a lot easier. Fast reaction times are not essential for this to work. I accept its a little less efficient but I feel its good enough. My export is minimal, my hot water is very hot.

Yikes I just posted on the same subject in the node red thread but didn't see this post until now. I took an approach similar to @Seye, just to add a smart switch to control power to existing immersion heater.