Jump to content

Slightly unique solution required - input appreciated


Hannes7212
 Share

Recommended Posts

Hi all, I will appreciate if anyone can help me with a little advice on this one.  I will keep it as short as possible.

Solution required: Provide battery backup power for a 24h operation and utilise solar to extend backup period during daylight hours.

Challenge:

  1. Batteries should remain basically full at all times, so normal off grid mode with grid backup not an option.  Yes you can set the back to grid voltage very high but then it is going to flip flop between battery and grid with every cloud that passes during the day.
  2. For efficiency reasons need to always run in grid mode (bypass mode) when grid is available, even if solar is available.
  3. Need solar array to contribute to power consumption and to continue working when grid is down.

Options:

  1. Online UPS with grid tie inverter:  No solar when grid is down.
  2. Off grid with grid backup solar solution: Potential flip flopping between solar and grid on patchy days (load is very stable so if it was not for patchy days this would have worked)
  3. Inverters configured in online UPS mode:  Double the inverters plus solar chargers required to keep bank full at all times and invert DC to AC at all times.  Very expensive and not very efficient but it will work.
  4. AC coupled solution like SMA Sunny Island / Sunny Boy combination:  This is theoretically the best option (not considering cost...) BUT I don't have experience with SMA so I am wondering if one can meet the objective of always leaving the battery charged (using grid power at all times that it is available) and using the Sunny Boy to feed in any available solar to the AC side and keep doing so if the grid goes down to minimise or eliminate draw on the battery bank when solar is available.

I know there are many ways to skin the AC coupled cat with other products but as far as I can see only SMA does it properly.

OK so that was not that short.  Any comments will be much appreciated!

Cheers

Link to comment
Share on other sites

First things that caught my eye.

Most online UPS'es do not have the battery capacity for extended periods. So if Eskom goes down, SMA goes down and then you have a few minutes of online UPS backup.

I did something along your lines. Correct me if I am wrong:

Sun shines, use all available solar power.
Sun sets or there is insufficient sunlight, use Eskom, no batteries.
However if Eksom is off and there is no sun, only then use batteries till Eskom comes back on.
Battery charging is only done using solar.

EDIT: Made some refinements.

Link to comment
Share on other sites

Hi TTT, thanks for replying.  Yes to all points apart from charging with solar only.  The system must use any available source to charge the batteries to ensure maximum protection against power failures.  Backup is the first priority.

PS a large UPS is an option (we need 15kW constant power delivery with a 60kWh usable backup capacity).  BUT I do not like the pre-packaged battery solutions for these systems and the efficiency is a real problem over the long run (it gets costly).  If you want to go for a UPS with solar function you really get ripped off.  Also, of the 15kW we need, only 1.5kW truly need an online UPS, so I will put a small on in.  The rest is just normal stuff.

Link to comment
Share on other sites

Both Axpert (and I am under correction) and Victron Multiplus (if the client has a lot of cash) are able to do it.

What is the load that needs to be powered, average and peak load?

Link to comment
Share on other sites

11kW constant load, may peak a little but not more than 10%.  I cant see how Axpert and Victron can do it and meet all the requirements though.  The problem is that if we want to leave the batteries at 100% all the time (apart from when the grid goes down), how do I do that and also utilise the solar power produced without going grid tie?

I might be missing a simple trick which is why I am reaching out so to speak!

Link to comment
Share on other sites

Axpert was first a UPS, then they added a MPPT controller and then more software to make it for both worlds. Victron is similar. Eskom goes off, it powers the load.

I am used to 10ms switching for a UPS, or as you mentioned, online with no break between Ekom and UPS. Axpert is about 15-20ms and Victron 10-15ms depending on settings.

 

But before we go there. Few more things I thought of.

Inverter: 11kw - call it 13kw with spikes so round it to 15kw for you don't get 11-13kw inverters that I know of, is a LOT of power and a lot of batteries and a lot of panels. A LOT.

Panels: In winter, if you are Cpt, you need a lot more panels for the rain comes in winter as you know.
Rest of SA winter is sunny and cool so you will need less panels than Cpt.

So to spec the panels, keep the above in mind for now.

Batteries: How long do you want to power the load with no Eskom / solar?

Link to comment
Share on other sites

HI TTT, jip it is a nice system.  We need 4 hrs backup so we are working on 48kWh usable storage.  We are suggesting 15kW of solar panels because the will only assist to feed this system (still assuming I am staying away from grid tie inverter feeding the whole place).  Putting more panels up is a waste as it will just be throttled if not used.  The solar is just a sweetner for the company to show some payback on a grudge backup investment, not to run the whole system all day long (though it will come close on sunny days).

Inverter power not a problem.  New Axperts coming out this month can do 9 units of 5kW (not kVA) so 45kW single phase and Victron can go many 10kW Quattros in parallel.

None of that is the problem.  The problem is to keep the solar coming when the grid is up without dipping into the batteries and then to keep it coming when the grid is down as well.  I suspect a good AC coupled system can do this but I am not 100% sure.

cheers

Link to comment
Share on other sites

@plonkster can explain in detail how you can use a Victron Multiplus or Quatro's (Multiplus is fine).

From what I know, the 5000va Victrons in parallel (4000w continuous at 25deg Celcius) can charge the batteries via either Eskom or a separate charge controller, all programmed to do as you need for what I have gathered. Would add a BMV for then they can use say 10-20% of the batts daily before going back to Eskom.

Very expensive yes, but if the client is touchy I would not go cheap.

Batteries - I would decide the batteries first, for 12kw for 4 hours = 48kwh with inefficiencies more like 62kwh needed. Yes, I know people don't want to know this but wire losses, battery efficiency, battery age, DOD, temp all comes into play with large loads running on the brink. So one has to add that factor to not disappoint and only get like 2-3 hours after a while instead of the planned 4 hours.

The battery bank is going to be huge as you probably know already. So the chargers would need to be quite strong.

Link to comment
Share on other sites

If you can cut the consumption that is required for backup -- ie identify a few items you can do without if the grid is out -- then a 10KVA Quatro can do the job. You run it grid-interactive in ESS mode and use the BatterySafe self-consumption program with a very high minimum SoC set (eg 90%). That way the top 10% of the battery is used to smooth out the occasional cloud passing in front of the sun, leaving the other 90% for backup. So best of both worlds, you avoid the switching back and forth and you get backup.

Of course a 10kva Quatro isn't cheap, and it needs extra anti-islanding and MPPTs, or a Fronius PV-inverter.

Only other thing I can think of is InfiniSolar. Not that the inverter is much cheaper, I think in that size you're also talking >50k.

It's late and I'm tired. ask me again tomorrow. You can hammer out a solution with the Victron, that much I can tell you right now :-)

Link to comment
Share on other sites

Hi Hannes

I am not sure I understand your opposition to gridtie. The way I see it you would want under normal circumstances to be using solar or a mixture of solar and grid and only in the case of a power failure to be using batteries with solar assisting so that you have a longer period of autonomy. Gridtie does not necessarily mean feeding into the grid although most hybrids are capable of feeding back.

9 hours ago, Hannes7212 said:

 New Axperts coming out this month can do 9 units of 5kW (not kVA) so 45kW single phase and Victron can go many 10kW Quattros in parallel.

It all comes down to tom. If the budget is large then Victron with 2V cells. You going to need roughly a 1300Ah battery bank. If you cannot afford that and start having to cut corners then the new Axpert (called Infinisolar V series if we thinking of the same inverter) will do the trick. You will however have to look critically at your battery bank to see whether they match the Axpert's  charging capabilities.

Might I ask who is importing the 5kW Axpert? FCS are importing the 4kW but when I spoke to them they were not planning to import the 5kW.

Link to comment
Share on other sites

Hi Chris / Plonkster, thanks for the input.

I am not opposed to grid tie at all I just need it to stay on when the grid goes down as you mentioned. In fact I think the best solution is a form of grid tie (island mode).  What I am trying to figure out is how to NOT use the batteries at all (they must just sit and float) while the grid is up, day and night.  They must only work when the grid is down and if that happens during sunshine hours they will be assisted fully or partially by solar.

So I meant I do not want to do pure grid tie with a UPS (or bi-directional inverter running independently in UPS mode) as I need the solar assistance when the grid is down.

Hope that makes sense.

Link to comment
Share on other sites

O yes on the 5kW Axpert (Voltronic) inverters.  I am an installer of off grid systems and solar pumping solutions and I have a very good relationship with the guys from SolarMD in Cape Town (get my Lithium batteries from their sister company MyPower24).  They are going to bring the 5kW inverters in.  They buy directly from Voltronic.  I hope they don't mind me saying that...  I am already installing the new inverters which have the upgraded 4kW MPPT.

Cheers

Link to comment
Share on other sites

1 hour ago, Hannes7212 said:

What I am trying to figure out is how to NOT use the batteries at all (they must just sit and float) while the grid is up, day and night.

But, I assume you still want to use any generated PV, in other words you want to do self-consumption?

If you are okay with a shallow-cycle, then I think you can run off-grid (aka with an Axpert), the occasional high load or cloud is handled by the batteries. You then switch back to the grid if the batteries goes below say 90%.

But I sort of suspect that is not what you want to do, you really want them to stay floating at all times.

Then I'd think of two options:

1. The Multiplus ESS setup I use myself. Set the Mode to "Keep Batteries Charged". Then it never cycles them at all. How you get the PV into the system is a separate matter, but it can be either via PV-inverter (aka GTI) or via MPPT controllers onto the DC bus. For your use case (immediate use) the GTI will be better.

2. Similar to the above: Use an off-grid inverter and feed them with a GTI. You can then use any inverter, and simply run it like a UPS. The GTI supplies the power in the day. A grid limiter will be required if you don't want to feed back. I know people who run Axpert+Sunnyboy combos like this.

If you go with the first option, I'd advise using a Fronius GTI as it integrates seamlessly and the CCGX knows how to grid-limit it.

Much as I love the blue stuff, I suspect you can get away with option 2 and it will be cheaper.

Link to comment
Share on other sites

2 thoughts:

Grid tie must be done according to regulations and costs involved - my question on that, with Victron able to do as needed, is grid tie worth it?

And if I have a 1300Ah battery bank, I would want to use some of it to try and recoup some of the investment and not throw it away after X years never being used. Therefor, as Plonkster said, I would use 10% of the bank.

As a matter of fact, if there are no power failures, I would go as low as 20% DOD (SOC of 80%) every day and recharge with solar the next day. IF there is a failure at THAT time, I mean, what are the chances?

And when power failures return, I would switch and run with batteries back onto continuous float until failures are over. 

Link to comment
Share on other sites

38 minutes ago, plonkster said:

But, I assume you still want to use any generated PV, in other words you want to do self-consumption?

If you are okay with a shallow-cycle, then I think you can run off-grid (aka with an Axpert), the occasional high load or cloud is handled by the batteries. You then switch back to the grid if the batteries goes below say 90%.

But I sort of suspect that is not what you want to do, you really want them to stay floating at all times.

Then I'd think of two options:

1. The Multiplus ESS setup I use myself. Set the Mode to "Keep Batteries Charged". Then it never cycles them at all. How you get the PV into the system is a separate matter, but it can be either via PV-inverter (aka GTI) or via MPPT controllers onto the DC bus. For your use case (immediate use) the GTI will be better.

2. Similar to the above: Use an off-grid inverter and feed them with a GTI. You can then use any inverter, and simply run it like a UPS. The GTI supplies the power in the day. A grid limiter will be required if you don't want to feed back. I know people who run Axpert+Sunnyboy combos like this.

If you go with the first option, I'd advise using a Fronius GTI as it integrates seamlessly and the CCGX knows how to grid-limit it.

Much as I love the blue stuff, I suspect you can get away with option 2 and it will be cheaper.

Thanks for this.  You totally get my challenge.  I think grid tie is the only option as I think it is is the only way to avoid the inverter ever switching to inverter mode (as apposed to staying in bypass mode) apart from when there is a power outage.  So then it is a case of setting the GTI up to work in island mode with the Victron and yes I believe Fronius would be the way to to.

Is the Fronius / Victron integration quite intelligent and "smooth"?  In other words, does it get rid of the all PV on until batteries full then all PV off for x minutes etc. like Scheider works for instance becuase that is just bad.  SMA does this very well and were the first to do so properly which is they came to mind.

On the cost:  Yes Victron is expensive but somehow for this kind of application I just prefer a "proper" transformer inverter.  

Many thanks again.

Link to comment
Share on other sites

3 minutes ago, Hannes7212 said:

Is the Fronius / Victron integration quite intelligent and "smooth"?  In other words, does it get rid of the all PV on until batteries full then all PV off for x minutes etc. like Scheider works for instance becuase that is just bad.  SMA does this very well and were the first to do so properly which is they came to mind.

Yes, it is smooth. Look at the release blog post for Venus 2.0, they have a zero feed-in option specifically for Fronius that avoids the usual methods. The usual methods is to either switch the GTI off completely (go into Island mode and push the frequency out of range), throttle back the GTI (go into island mode and use GFPR). These work, but there are always problems, flickering of lights and so on. The new method talks to the Fronius over the network (I assume using Sunspec) and tells it to back off, thereby getting smooth backfeed limiting. Must have a CCGX though (or beaglebone, or Rpi), though at this price point I'd just get the CCGX. No point in saving a few bob on that when the inverters are 10 times more expensive init?

With a Multiplus/Quatro in the system you can also do things like prioritise solar charging, that is if you were running from the batteries during an outage while there was insufficient PV (eg at night), then it will avoid using grid power to recharge the batteries, limiting charging to what is available from the GTI (or MPPTs). Probably not a big issue in this application, but in theory you want to use your cheap/free power to do inefficient things like charging batteries.

You might even throw some Lithium batteries at it (that's one of your things right?) and do a tiny bit of self-consumption. Perhaps not now, but you have the flexibility to do that in future.

(Only, the batteries you use don't have BMS integration with the CCGX yet. I sort of talked to MP24 about it but there is no way I'm going to have time for that now... :-) ).

Re "proper transformer", the Multiplus does use a high-frequency design like almost all modern inverters, but it has a very heavy toroidal transformer in it (unlike many others). This was apparently done on purpose :-)

Link to comment
Share on other sites

The Infini (3 x 5kW in parallel) can do this really well. It's a couple of clicks of the mouse.

1. Set charging of batteries from PV and AC so batteries will always be charged.

2. Set load supply priority with PV available: PV > Grid > Battery

3. Set load supply priority without PV available: Grid > Battery.

There is it!

Link to comment
Share on other sites

Hi cvzyl,

Thank you for that.  Sounds simple enough.  I don't know the Infini inverters so can you please confirm that in normal operation with the sun coming and going and the grid staying on it will never "switch" from bypass mode to inverter mode, ie no 15ms dips in the supply.  Happy for this to happen only in cases where grid power drops (will it happen then?)

Thanks

Link to comment
Share on other sites

The Infini is a true hybrid, so no switching with dips. Though I hinted last night that they are about the same ballpark price, reality is that the Infini comes with a built-in MPPT, so you save on some equipment: The additional MPPT or GTI you would need for the other setup we discussed.

Link to comment
Share on other sites

Just a thought.

Ensure that whatever equipment you decide on has the amps to charge the battery bank at 10% of Ah of the bank.

 

IF you need to charge the bank, you would need to be able to do it via MPPT or built in charger.

As Plonkster pointed out, if you have the panels, it is preferable to charge from solar.

Link to comment
Share on other sites

2 hours ago, Hannes7212 said:

Hi cvzyl,

Thank you for that.  Sounds simple enough.  I don't know the Infini inverters so can you please confirm that in normal operation with the sun coming and going and the grid staying on it will never "switch" from bypass mode to inverter mode, ie no 15ms dips in the supply.  Happy for this to happen only in cases where grid power drops (will it happen then?)

Thanks

Hannes, as plonkster also mentioned, the Infini is a true hybrid and seamlessly mixes AC (grid) and DC (PV and battery) sources. In the configuration above it will mix PV and grid and only utilize batteries when grid is unavailable and PV not sufficient to sustain load. Even then it will use whatever PV is available and only the balance will be drawn from the battery bannk. Each Infini can accept 7.5kW panels, has dual MPPT controllers built in as well as battery charger rated at 60A. That means each Infini should be good for banks up to 600Ah (10%).

They are pricey (around R50k each last time I heard) but offer a huge amount of flexibility. There has been reports of issues by one forum member but others have reported no issues (except that they are not waterproof).

C

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...