How do you install the EmonEVSE with solar diversion

mpooley · 2022-01-27T14:31:39.009Z

Hi I’m thinking of building the emonEVSE kit and I’m just wondering how I connect it to use the excess Solar or is this set up in software?

Thanks for any advice
Mike

JJC · 2022-01-27T16:46:18.613Z

Hi,

you need something to feed MQTT data to it. The easy way is to use the emonPi solar version

John

Robert.Wall · 2022-01-27T17:57:49.304Z

Not all that long ago, I added a page to the “PV Diversion” section of ‘Learn’ explaining the problems associated with having two systems competing for the excess PV. It is obviously not specific to any device, but even so it might be worth looking at.
Learn→PV Diversion→Using Multiple Diverters

mpooley · 2022-01-29T09:19:01.494Z

Thanks I am not familiar with the emonPi so will have a look.

mpooley · 2022-01-29T09:36:55.005Z

Thanks I am using a mk2 Diverter for my hot water and I am not sure that I can physically mount a contactor to the output side of the mk2 as the wiring is so awkward. Also when the solar power is low the mk2 is constantly switching on/off through the triac inside it.

Also I would want to give priority to my Hot water but I assume that’s easy by swapping over the wires through the CT

I’m very rusty with my technical knowledge as I’ve been retired for quite a while now.

thanks
Mike

Robert.Wall · 2022-01-29T13:27:39.831Z

I think using a contactor to control power to the EVSE would be doomed to failure. My thoughts went along the lines of taking an analogue output (suitably smoothed) from the Mk2 and using that to control the EVSE current - but I’ve absolutely no knowledge at all of the inner workings of the EVSE, so whether it’s easy, possible or impossible I wouldn’t care to say.

Putting the Mk2 “upstream” of the EVSE would be guaranteed to work, but it would give priority to the EVSE, the hot water would mop up any excess PV.

However, the other way round with priority given to the hot water, you’d need to ask the EVSE designers how it would cope when the Mk2 was running in burst mode and the power flow was rapidly reversing with a nett zero energy flow. You’d want the EVSE to shut down, and only respond when there is sustained export. That’s the question you’d need to ask.

mpooley:

I’ve been retired for quite a while now.

Join the club

mpooley · 2022-02-12T15:53:16.772Z

I had another thought! I could use a relay on both the CT’s wires with an open cct they would record zero flow so stop working. So could switch them that way?

With your point about burst mode on the mk2. I am now thinking that the Zappy wont start until it gets a current flow of 1.2 kW as per their tech details: so the mk2 will start first and the current flow would never get to the Zappy till there was over 4.2 kW flowing as the mk2 would absorb all the current up to 3kw.?

Thanks

Mike

Robert.Wall · 2022-02-12T16:40:35.964Z

mpooley:

So could switch them that way?

Definitely NOT. A c.t. is a current source, so open-circuiting a c.t. is equivalent to short-circuiting a v.t. - the sort of transformer most people are familiar with. Damage is likely to follow. Even if the c.t. is internally protected with zener diodes or equivalent, it’s still not good practice to open-circuit it.

You can safely short-circuit a c.t. - then it’s working under no-load conditions. The resistance of the short circuit must be small in relation to the burden resistance, because the current will share between the two paths in inverse proportion to the resistance.

mpooley:

so the mk2 will start first and the current flow would never get to the Zappy till there was over 4.2 kW flowing as the mk2 would absorb all the current up to 3kw.?

… or until the tank thermostat opened because the water is hot. So depending on the average starting temperature of the water and the tank capacity, probably the first 3-5 kWh (or so) below 3 kW would go to heating water, until then only the surplus above 3 kW would be available to the EV charger. After that, all the surplus would be available to the EV charger.

(This of course is if the EV charger sees the Mk2 as part of the house load - and assuming the immersion heater rating is 3 kW.)

mpooley · 2022-02-14T10:38:09.250Z

Thanks again for your help Robert.
I might just try leaving it simple and see how it works. I could adjust how much energy is diiverted to the tank by adjusting the thermostat down. I don’t really want to have to adjust the thermostat manually though.
As for the ct’s and a relay thanks for that info. I Don’t’ want to blow anything up!
So my next question is would using the relay to short out the CT’s be acceptable?
This would give me much greater control if it would be ok.

Mike

Robert.Wall · 2022-02-14T11:04:31.032Z

mpooley:

would using the relay to short out the CT’s be acceptable?

As I wrote, shorting a c.t. is quite acceptable. If you have (say) 0.5 Ω of contact resistance, that’s equivalent to having a 0.5 Ω burden, so the c.t’s sensitivity will be reduced to - for the standard Mk2 - 0.33% of what it was, effectively it will see zero current.
Your relay must have volt-free contacts, of course, because the c.t. input is like the emonTx’s and floats above ground.

mpooley · 2022-02-14T15:56:37.645Z

Robert.Wall:

Your relay must have volt-free contacts, of course, because the c.t. input is like the emonTx’s and floats above ground.

Just to be sure ! I’d be just shorting out the leads to the ct so no voltage as far as I can see ?
If that’s ok then That has solved my problem Thanks!

Robert.Wall · 2022-02-14T16:09:15.862Z

By “volt-free” (it’s a common term), we mean that the equipment containing the relay makes no connection to the contacts you’re going to use, so it does not apply any voltage to them (and that includes a voltage of zero (earth).

mpooley · 2022-02-14T17:21:59.453Z

OK I understand. I’m not intentionally going to do that