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excess energy for chargin batteries

I see a few threads about what people do with excess energy from solar and most seem to heat stuff up 

In my case I have a house with 4Kw solar array and with grid tie in and FITs contract, meaning my system can not change with out screwing up the FTIs so fitting a new inverter with a powerbank charging and discharging feature is not possible.

I have a solar diverter for my hot water but still I have excess energy, i could just turn up the temp on the heater and make the water hotter, but ppl would hurt themselves with hot water taps so that's not practical.

I would like to store the excess energy in battery bank using it from the AC plug. 

I would need to know when im exporting and how much and to use that to regulate a charge that's connected to a AC battery charger, also a battery charger that could be controlled like this.

The battery bank will be a Li-ion chem. between 10-20kwH

Does anyone know of such a device on the market or a selection of devices that can be cobbled together to work this way?

Also then using this power I could convert back to AC and power the same water heater so that when ppl use the shower at night I don't rely on Gas to reheat it for a morning shower?

I'm sure i will find other applications for the power but starting small and a good use case etc.

(09-21-2019, 08:59 AM)boysie Wrote: so fitting a new inverter with a powerbank charging and discharging feature is not possible.

I'm not sure why you you've come to that conclusion - fitting a bidirectional inverter/charger will allow your existing system to remain untouched. I'm assuming you are on the 50/50 deemed export deal ?

correct in the uk we get a FITs 50/50 deal meaning the system is locked under contract and any changes would break that contract.

the bit being

It allows for time shifting power, charging from solar, providing grid support, and exporting power back to the grid. <<<

you are not allowed to export back to the grid from a battery system because that would mean your powerwall is grid tied and there for the system has changed

there for i need to store and use the power i can not export for battery.

i have to store 240 side and use it some other way of which i have a few ideas floating but not finding many devices that will help me achieve this option.

I can't feed back into the 240(grid) from battery and there for i can not use the house 240v power rings to distribute the power unless I take the house off grid first to do it or some of it say a ring, switch to battery and not grid, never should they be on the same power circuit. You are also not allowed to touch the ring or change the ring circuit in the uk with out proper accreditation or your insurance.

so with some ideas using IoT, zwave etc etc im sure i can pull a custom system together and use that extra power without sending to the grid. its not about money its just about learning and thinking out the box and making sure everything i make i use letting it disappear down the road seems pointless as i get paid for 50% anyway so sod um its my power i want it Smile

I'm in the UK.

You might like to have another read of what you can and can't do with regard to adding a storage system to an existing FiT claiming installation - so my original response still applies.

But given that you appear intent on building some or or all of the storage system yourself, what can or can't be added to an existing FiT claiming installation, assuming you are concerned about falling foul of the terms of that contract,  is completely irrelevant.

A 10 to 20kWh hour storage system that can just be cobbled together does not exist, however any competent person can carry out electrical work in their own property, so if you deem yourself competent enough to cobble together a 20kWH storage system you should be more than able to connect it to your house wiring.

But, unless you have the means and equipment to comprehensively test and commision your cobbled together system, and have doubts about negatively affecting your FiT agreement you'll likely be better off employing the services of a proven competent and suitably qualified company.
You could divert into hotter water… this is why thermostatic mixing valves exist & you could always retrofit one into your home installation. Hotter water and a thermostatic mixing valve basically increases your capacity without increasing the tank size.

As to your power solutions… there really are multiple ways to cobble something together to make it work. Since this is a DIY forum, most any if not all engineering would be done by you. Not much around here is a turn-key solution but I have seen various devices that address these sorts of thing, I just don’t know much about them.

You could put a clamp meter in your panel to sense when you are exporting, at that threshold you could trigged an AC charger to consume more from your own outlets into a battery bank, and terminate the charge accordingly. Perhaps a Arduino or similar logic contraption.

I personally don’t think they would know if you were exporting back into the grid from battery, not if you are using a current limiting GTI and using it as a peak shaver. Its all synced, and it would just appear that you have a low load at home. Maybe during a power outage this would be a problem if your GTI kept the solar GTI online.

That said, if you really wanted to be extra sure, just power something off-grid using the battery pack which you charge from excess mains power. Perhaps an EV.

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