Idea for logic controlled relay to make use of excess generation

Hi, I’ve been messing about with home generation and monitoring for a while but only in a very analogue wa so far. I have an idea for a project that I think will be useful but, unless I buy a load of rather expensive control relays, I can’t get it to work (even then, doubtful). But I think that, if I can figure out enough, it should be doable with the hardware used here. Maybe it’s already been done?

Basically, I’d like to control power to a 240v AC appliance based on what excess generation is available.

I’m my mind I’d, monitor export to grid via a CT (I see there’s an Arduino - CT interface diagram available) and use that current reading to make a decision about whether or not to close a relay.

Something like;
IF; exported power, minus a preset figure, (set either within the software or via external dipswitches) that’s based on the power of the item to be run, > 100W (or some sensible number close to 0), then close a relay.

This would let an appliance, like a heat pump for example, that has to be run at full power, run only when excess generation meets its demand. Or, if you chose, more than half of its requirement would be available, or whatever was appropriate.

There would have to be some hysterisis built into the system, probably including a minimum run time would be simplest. Again either programmable from with the code or via external dipswitches.

Could this be applicable?

Thanks in advance

I made a mistake in my proposed calculation. It’d have to be;

IF; (measured appliance power consumption - nominal appliance power rating) + measured exported power > 0; close relay.

Or something like that

Welcome, Gareth, to the OEM forum.

I think if you look/search for Robin Emley’s Mk2PV Router, or go to his website, you’ll find that something very similar, or something that at least embodies the central concept, has been on offer here for a good few years. I can just remember working on a similar concept - but this was concerned with limiting the maximum demand over a 30-minute period - in the late 1960s-early 1970s when I was a student apprentice.

But you already appreciate the principal problems: many if not most appliances that could be amenable to that sort of, lets call it ‘demand sharing’, require whatever power they require over however long they require it, and the problem is once started, any interruption is likely to cause whatever it is to abandon its operation. So the problem comes down to the average, and most likely even the most tech-savvy, user knowing the potential power demand and the period it’s going to be demanded for, and then programming this knowledge into the system. This of course is why water and space heating are popular “dump loads” for excess generation (notably solar but also wind and hydro) and equally why the limitations imposed by EV chargers are causing so many problems - witness the topics and discussion here on openEVSE.

I think it’s eminently doable, whether it’s viable and turns out to be useful - or even usable, is where I have doubts.

Hi Robert,

Thanks for the detailed reply. I have been vaguely aware of the MK2 PV router for a while, but unfortunately didn’t hear about it until after I had already bought and installed 3 off the shelf units (solic 200 and iboost). I’d assemble one of his kits if I get the chance another time

However, I think (unless I’m mistaken) his products are focussed on diverting to a resistive load, as are most other diverters I’ve seen. I’m sure for the reasons you mentioned. I think I came across one that would close a relay for a set period if measured export exceeded a set value (immersun or Eddi I think) which is a good way towards what I’m trying to do and may be the only compromise solution.

I’m not trying to make a product, just a proof of concept that, if it works at house scale, I can implement on a much bigger setup to, hopefully, bank ice when there’s excess PV generation available onsite.

Hi Gareth, yes that is true, for the reason that Robert mentioned.

If you have 1000W being exported and your relay energises your heat pump (as in your example), several things will happen - 1) the heat pump will take whatever it wants - i.e. NOT 1000W as long as the replay is shut and 2) you’re going to destroy the heat pump in pretty short order as you can’t just “switch them off at the mains”. They have inbuilt heaters to keep the compressor oil warm and don’t like having power cuts esp. when running at full tilt.

If you use a 1000W fan heater, that’ll be fine, as long as it’s bolted to a wall so it doesn’t get moved and burn your house down. Same for an oil filled electric radiator, or a convector heater. Or better yet a storage heater.

There are some washing machines / dishwashers that can carry on their programmes after power cuts, but again you can’t tell them how much power to use.

Do you see where this going? Dumb resistive heating is easy to control. If you want to control a heat pump, a relay won’t cut it, you’ll need some sort of PC to HP comms to talk to the HP and ask it heat/cool to a certain level, but even then startup currents will go higher than your 1000W.
If you have an EV, there are programmable car chargers, but they usually need at least 1400W and many cars don’t take kindly to be cut off too often.

So in summary, yes, the Eddi can switch a relay, but it’s not much use.

This isn’t actually for a domestic heat pump. I don’t have one. It’s an idea I think could work. If it does, I’d potentially fit it in an industrial setting to bank ice. (Ok, that is a heat pump but probably not what you’d imagined if I said heat pump)

It should work absolutely fine with a domestic heat pump. Clearly, with something that complicated you couldn’t just fire a big contactor and start the motor. But it’ll have a “call” signal just like any boiler. My idea might need a separate “call” setup, so as to leave space to bank sunshine but not fill stores with bought electric in the nighttime. But this isn’t complicated stuff. And, like I said, it’s not something I personally can do right now anyway.

Either way, it isn’t a straightforward project to my mind, despite the relatively simple principles. But that might simply be my inexperience with things digital