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On Sat, 2 Mar 2019, Tom via list wrote:
It seems that FIT is no longer paid for new PV so electricity companies will be getting free power on sunny days. As such I'm wondering how to stop this happening and this of course comes with a more generalised power control (you want to turn on your immersion and/or storage heaters when you are not using all you generate. I've had a look around and there are devices that come close but they seem ludicrously expensive. Anyone got any experience of these at all - I was thinking of making something with a PiZero to opensource to try and stop this madness!
This is a solved solution - there are power switches used in e.g. data centers that will take 2 feeds in and produce one out. (Search for Automatic Transfer Switch) If one input fails then they automatically switch over to the other, so make the PV power source the primary and the incoming mains as the secondary.
Same idea as automatic generator backup too.These switches are not cheap - especially as the amps increase, however it might give you something to go on.
That doesn't solve your wastage issue (where waste here is defined as excess PV that you can't use or sell).
Also feed PV into a battery then into the switch. The downside here is that the switch is working at 230v, so there will be a small inefficiency in the conversion from PV DC to 230v AC, but you need to do that anyway. That also gives you a whole house UPS, assuming you have enough storage - lead acid is still cost effective if you have the space for them, same for NiFe cells.
The switch also provides the isolation needed to stop you back-feeding your PV into the grid. This is essential when the mains incoming fails - e.g. in a deliberate situation when people are working on it....
And you don't even need a Pi to do this, although a good MPPT solar controller to do the battery charge and feed to the inverter would be good.
If you want to DIY the transfer switch then (much better than a Pi for many reasons) you could use an ATmega (Arduino) and a great big 2-pole, 2 throw relay/contactor to switch from one to the other. The down-side there is that you'll lose a few cycles during the changeover period - that may be acceptable though. You power the ATmega from the inside power via it's own small supercapacitor or LiPo cell with the normally closed position of the contactor being the incoming mains.
Or, a fix for this might be to put a double conversion mode UPS unit after the switch to soak up the transfer time. A down-side then is that the feed from the mains or solar would need to be double if trying to charge flat batterys after an extended outage (or you accept a much longer charge time). Double conversion units ar not cheap, nor are they that efficient - I used a 2KVa one at home for a while some years back and it idled at 60w with no load )-:
I'd stick to little UPSs on sensitive kit on the inside. Gordon
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