Grid tie inverter parallel with MPPT chargecontroller on the same PV panel

[billi]

I wonder , what would happen , if i connect a MPPT charge controller  available  upto 600 Volt Voc  in parallel  to a Gridtie Inverter to a say 4000 Watt PV ?  So the Gti is connected to the utility Grid to feed in  , and the MPPT  charger connected to the Battery  of a (semi) offgrid house ....

Would it work , that once the battery is full the GTI would start more and more to feed the Grid  or would the GTI use all power to feed the grid  and the charge-controller gets nothing ?

Or is this a total nuts idea ? 

Thanks Billi

[clockmanFR]

Billi my hearty,

Go on Billi make our Day.

Please do try.

Oh yes, and to-days topical comment, "Ensure a responsible adult is present".

[billi]

   sure i am on  a "mission impossible " , to try  to figure out  future Grid/off Grid/ FiT harvesting ideas ....

Actually , for fact , i am crazy ! to even spent one minute   of thoughts about this  .....

But my feeling , tells me , that  FiT payments in Europe will  be limited or knocked out  in the near future  for us small producers , sure  for those people that have their 20-25 years  contract agreed today , its fine ....

But for newcomers ,   it might  be be not beneficial  any-more to install  a PV on their roof and feed the Grid only

Will sleep over it

Not now , but later 

Billi

[pdf27]

SMA are already trying the same sort of thing - http://www.sma.de/en/products/backup-systems/meter-box-for-sunny-backup.html

[billi]

sure ,  but not  too impressed ,

What the world needs (for households) is  a self supply  first and a FiT  for export  


Some  countries differ to the FiT  of  the UK

[stephendv]

Would it work , that once the battery is full the GTI would start more and more to feed the Grid  or would the GTI use all power to feed the grid  and the charge-controller gets nothing ?

I think the problem will be the MPPT algorithms which will get very confused, imagine if the charge controller MPPT has found Pmax at 50V and is drawing current from the panel, now the GTI does its MPPT sweep, where is it going to find Pmax- might be at a different voltage completely because the charge controller is already drawing current from the panel.

For what you want to do, I think an AC coupling idea with some clever relay switching based on battery SoC will be easier to achieve.

[Justme]

Outback inverter chargers do exactly what you want.

You can set the export (sell) to what ever you want.

[billi]

For what you want to do

I want to help Germany    , because its so  hard there  to setup a  battery based  PV  and as well simple export  the surplus ...
So many   Battery -Grid tied ideas come on the market , but all have one in common - very pricey- , ok the Nedap powerrouter seems to be ok isch

Yes Justme , i saw that,  but are these exporting Outback Inverter/chargers available in 230 Volt 50 hz ?

Thanks

[nowty]

To reply to "stephendv"

"For what you want to do, I think an AC coupling idea with some clever relay switching based on battery SoC will be easier to achieve."

That is pretty much what I do, I have got both on grid GTI and off grid (AC coupled) systems which automatically switch house circuits via a big relay based upon night / day and battery state of charge.

I have mainly done it this way because I have more PV than 4kw without permission from the DNO to connect to the grid so I have to have my PV excess capacity as off grid.

If I had permission to add more PV to the grid I would probably switch it all to the grid and find ways of diverting surplus grid power to batteries and hot water heating. The immersun is an obvious choice for water heating and I am sure it would not be that difficult for another immersun type device to be invented to charge batteries.

[sbchapman]

Nowty - I am in a similar situation to you. The only difference is that I have export meter and called my FIT provider to check the metering procedure with installation of personally installed additional (non MCS) microgeneration. I was told in no uncertain terms that anything I added, if not FIT registered should not even be connected to AC. If it was fitted by a MCS certified company and registered for FITs they would only meter export from one installation - which begs the question, when everyone is on export meters (which WILL happen eventually with SmartMeters) how do they differentiate between different contributions of different technologies?

So for the moment I was wondering about running a standalone DC battery charging setup, with an inverter on a timer to meet demand when the PV isn't generating. Is yours "smart" i.e. meets demand that would come from the grid even during the day? I suspect the financials don't make it worthwhile for me as I'll only save the differential between imported and exported units, and as exported rates should rise inline with energy costs (or at least FIT rates?) the differential would stay approximately the same ... or have I got my assumptions wrong?

Steve

[Justme]

Check the Outback International range.

[nowty]

Steve - I fully understand your FIT provider saying that you cannot connect any additional non FIT system to AC as long as they mean the “AC grid”. That makes sense as even without DNO issues you would be being paid for metered export from both a registered and non registered installation.

If however they mean you cannot have any non MCS installations whatsoever on the same site as a site registered for FIT, then I would be ripping my off grid installation out pronto. I wonder if any of the senior guys on here could clear that up.

To answer your query about whether my system is “smart”. Its only smart in that load circuits are switched to it at 4pm until 8am (GMT) but only if the battery is at 90%+ charged. And it disconnects if the charge gets less than a pre-set value of between 60% and 80% depending upon time of year. The number of circuits I switch are also dependant on the time of year, more in summer less in winter.

For example at the moment its winter so minimum circuits (lighting and central heating pump), it will use around 4 units (kwh’s) per day which is the amount my batteries can store from 100% down to 80%. I choose 80% this time of year because the most my off grid PV can deliver on a sunny day is about 5 units, just enough to get back from 80% to near 100%.

In summer I have additional loads, TVs, sky boxes, computers, internet, fridges and that bumps up the usage from 4pm to 8am to around 10 units.  In summer my off grid PV generates from 5am to 7pm (BST) the AC coupling powers about 3 units worth with the other 7 coming from the batteries. I choose a battery cut off of 60% because 100% down to 60% is about 8 units. In summer the average daily generation is around 15 units or so which is more than enough to get the batteries back to 100%, in fact they are usually in float by early afternoon.

And of course during the day 8am to 4pm everything is switched to the grid where its powered by the grid backed up with my normal FIT registered 4kw GTI.

One thing I find is that I waste tons of potential off grid power in summer as the PV generation backs off generation as the batteries fill up. As I already have an Imersun on the grid tie heating water, I just have no use for the lost power. Maybe an electric car is needed !

[sbchapman]

Nowty - thanks for the response.

Just to clear things up ... They're not saying I cant install anything, just that anything I install should not be AC connected and therefore with the potential to export. As you rightly point out, if it did, I would be exporting from a non-FIT registered installation. So I believe your setup is OK if you manually enable the battery system at nightime, provided the battery system cannot produce sufficient power to export if local demand is low. (That was my take anyway)

So the only way I see it working for me is charging batteries, which I could then use to offset grid demand, ideally at any time but realistically like you have setup, at nighttime. This is why I was interested in "smart" switching.

Regarding your summer config and wasted energy, I guess you could investigate a DC immersion, however if you are on deemed export right now and have an immersun there is problably no justifiable benefit. I found it interesting that you limit the loads off battery in Winter due to the inability to charge the battery the next day - I'd not really thought about that I just expected to use as much as possible within the realms of maintaining battery life/condition, even if it meant the battery couldn't be fully recharged next day.

Steve

[nowty]

OK, sounds like I assumed correct that your FIT provider ONLY was against another unregistered system being connected to the AC grid, not to an off grid AC system.

To clarify I don’t manually enable my battery system, it is fully automated on a timer and SOC of the battery. If its night and there is a good battery level, selected circuits are switched with a heavy duty double pole relay. The grid AC live and neutral are disconnected and the battery system AC live and neutral are switched in, so there is no possability of exporting any power back to the grid. If the battery gets low, or fails, or is daytime again, or I manually switch it off for maintenance the relay simply drops out and returns circuits back to the grid. And this happens so quick there is hardly a light flicker and computers, internet, etc run normally.

There is some one off manual intervention throughout the year to set the battery % SOC cut off level and selection of circuits (via extension lead plug and sockets) to balance the load verses the average available PV power available. Also I have a boost facility from the grid to give extra charge to the batteries but I only use this for emergencies or doing the occasional equalisation charge.

[sbchapman]

Can you briefly explain how you do the automated / timed switching - and where does the SunnyIsland fit in? I vaguely remembered an earlier post of yours, and searched, finding this: http://www.navitron.org.uk/forum/index.php?topic=16866.30

However it mentioned your Victron Multiplus....?

Thanks