Is it possible for PV diverting the opposite way round

[Gone West]

I currently have a Solar iBoost which feeds excess PV output to an immersion in the vented DHW tank. Is it possible to have a system where all PV generation goes to an immersion in a thermal store, first, and then excess generation goes to the rest of the house?

Edited June 25, 2022 by Gone West

[SteamyTea]

Yes, but what I think you are asking for is a ready made device that does it 'out of the box'.

That probably does not exist because of the way immersion heaters work and the varying output from PV

[Dillsue]

Isnt that what your diverter does now? Diverts excess PV to the immersion and when the stat cuts out all PV goes to the house and then to the grid if the house doesnt use it??

[Gone West]

17 minutes ago, Dillsue said:

Diverts excess PV to the immersion

That's right, it does, but what I'd like is all output going to immersion first, then excess going to the rest of the house.

[Temp]

Trying to get my brain around this. Thinking aloud..

 

So let's say the PV is generating 1.7kW and the house has a demand of 1kW.

 

With the Iboost

 

Tank Cold... tank uses 0.7kW, house uses 1kW, Export is 0kW.

Tank Hot... tank uses 0kW, house uses 1,kW, Export is 0.7kW

 

With the proposed system...

 

Tank Cold... tank uses 1.7kW, house Imports 1kW.

Tank Hot... tank uses 0kW, house uses 1,kW, Export is 0.7kW.

 

So the overall effect would be to import energy to heat the tank faster? 

 

 

 

Edited June 25, 2022 by Temp

[PeterW]

It’s quite complicated to do it as the inverter normally needs to “see” the mains to get its phase signal so using a switching isolator to dump to the immersion first without going via the CU will be problematic. 
 

What may be more use is twin immersions - set the top higher than the bottom and use the bottom as secondary dump load. That way you’re only heating a smaller part of the tank but it’s the “useful” part unless you’ve run a bath and drained the tank. It’s also usually above the boiler thermostat so won’t let the boiler trigger if small amounts of water are drawn off. 

[Dillsue]

31 minutes ago, Gone West said:

That's right, it does, but what I'd like is all output going to immersion first, then excess going to the rest of the house.

But youll import from the grid if theres demand from the house whilst ALL output is going to the immersion??

[Gone West]

2 hours ago, Temp said:

So the overall effect would be to import energy to heat the tank faster? 

 

1 hour ago, Dillsue said:

But youll import from the grid if theres demand from the house whilst ALL output is going to the immersion??

But won't there be situations when I would be using the PV to heat the water rather than burning oil? This is making my head hurt 😁.

[TW9]

What would happen if you put the CT clamp for the iboost on the inverter output? Would that result in the iboost attempting to divert to the immersion whenever you're generating electricity? The immersion will take as much of the solar electricity as it can leaving the rest to go to the house.

 

Or is that utter nonsense that won't work?

[kommando]

The diverter I use is a DIY build from a kit called a MK2 Diverter. I uses 2 CT clamps, one on the input to the meter and uses this to diverter any excess to a 3KW immersion heater and a second CT on the feed wire to the immersion heater to record the excess it sends to the immersion heater. It can handle up to 6kw so you can add a second immersion heater and the whole kit is controlled by a Arduino chip. So it can be reprogrammed to a different diversion strategy as long as you can keep to 2CT clamps.

 

Here are the list of programs already available. 

 

https://mk2pvrouter.co.uk/downloads.html

 

I have never used mine for anything other than the standard setup ie feeding a second immersion heater fed from the bottom of the hot water tank so its unlikely ever to trip the thermostat to stop it heating water. 

 

However I did read a thread recently which may include the answer to your need.

 

https://community.openenergymonitor.org/t/arduino-based-mk2-pv-diverter-being-resurrected/11713/2

 

To quote from the thread

 

 The user whom I helped had just two systems - I think battery and water heating, and his problem was the two systems oscillated, stealing power from each other alternately. I suggested that he ran the wire feeding his immersion heater backwards through the grid c.t. of the other system, so its current in the main cable was cancelled out. So it knew nothing about the immersion heater, while the immersion PV system saw the battery charger as part of the normal house load, which automatically had priority. That solved the problem.

[RobLe]

It is a strange thing to want to do.

PV diverters normally have a current clamp around the mains feed, and optimise for zero power flow average in that cable.  If you could (get a sparky) fit live PV and live immersion through the clamp(same way through), then all PV production will be sent to the immersion, until it is full.  If your lucky  the wires will reach, they will fit ok as they’ll be thinner than the main tail.

Have to say, I wouldn’t advise it, I expect on average it will not replace as much grid elec as a normal diverter install, so will cost more.  Diverters are mostly about saving money not co2, as elec/oil/gas are largely similar carbon intensity.  

[Gone West]

Thanks for all the comments. It seems it would be a complicated solution to a less than good idea. I think the idea of two immersions is simpler so will aim for that, plus thinking about it, at the moment, oil is a third of the price of electricity.

[Wil]

Wouldn't you just 'boost' the immersion as soon as  the sun was shining to do this? Then Immersion is running at full chat, so tank heats up fast. Any PV produced is self consumed and you then import any electricity needed? Why mess about with diverting if your intent is to get the tank hot as fast as possible? Then once finished 'boosting' to say 65, let your diverter do it's thing for the rest of the day?

 

Beats me why you'd want to do this, except if speed of getting heat in is the priority, but it's possible. 

[Gone West]

1 hour ago, Wil said:

Beats me why you'd want to do this, except if speed of getting heat in is the priority, but it's possible. 

What I was hoping to do was, to put all the PV output, whenever it's available, into a thermal store, rather than run other things in the house and only use the excess to heat the thermal store as a Solar iBoost would normally do. When the thermal store is fully charged the PV output would go back to powering other things in the house. This might reduce oil consumption was the reasoning behind the idea.

[Wil]

So you need something measuring the inverter output and then putting exactly that much power to the TS immersion? 

 

You could do as above and put the diversion CT around the inverter output +ve wire so that it tries to divert exactly the solar amount. Problem is what then happens to any excess PV- this wouldn't then allow you to go back to 'normal' diversion without manually moving the CT back to your main incomer.

 

But as you're effectively choosing to import energy to get your tank hot sooner, I go back to my previous point about just boosting the immersion when the sun is shining and getting the TS hot ASAP. Or if you don't need the TS hot immediately, turn off the oil boiler until the end of the afternoon. Might struggle if you're calling for heating from it and only using an immersion though.

[JohnMo]

Could you use a Shelly EM plus 50A clamp on your inverter output.  Once output equal to xx, switch power to immersion, when below xx switch off immersion.