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Author Topic: Earthing Solar Panels on a Steel Barge, with Victron and SMA?  (Read 6157 times)
al barge
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« on: March 31, 2015, 06:27:48 PM »

Hello,

1st post from a long time lurker and digester of Navitron information, thankyou forum. (think I made an account 5 years ago-ish, but forgot it)

I'm sussing out the safest way to install PV on our Steel Barge's Aluminium roof hatches. We can't get FIT's as the government make it impossible for boats, and in any case Grid connection where our mooring is will be 20kish.

A bit of reading here and on the IET forums suggest Class II panels with an SMA HF GTI would be appropriate?
I'm thinking 8-10 Schuco mpe 235 ps 10 and an SMA2500HF-30 backfeeding our Victron Multi, which could be a simple and cheap self install system for me.. in theory!


There are a few caveats for us though:

Mainly i'm concerned the panels could easily be touched, whilst in contact with steel/aluminium of the Barge.
Currently power is taken from a shoreside 30Kva perkins generator, and we use an Isolation Transformer to protect from galvanic corrosion / safety, then to the Victron Multi 24/3000/70-16 with a 690Ah (C20) 24v FLA Bank. Earth is bonded to hull and re-linked to neutral on board before RCD
We also have an 1Ph Inverter drive to power the 3Ph winch motor on our crane, which dumps a little on the earth, but it's ok. so for various reasons I'd like to keep the main RCD 30ma, which I think rules out the quieter SMA TL inverters (problem excess earth dumping and tripping)?

I'd also like to utilise the frequency shifting Victron to tell the SMA how much power it is allowed to backfeed into the Barge mini grid during daylight baseload and to minimise conversions, then allow the Victron to keep on doing all the charging with the excess. I'll eventually install an Immersun type thing and DHW tank, for excess.

The Inverter could be other brands like Fronius, but ideally it'd be easy to set the parameters and possible to monitor online, as I already have the Victron which looks after the batts nicely, it makes sense to use clever frequency shift jiggery pokery unless convinced otherwise.

So the main issue is, what would be best practice for earth bonding the panels? Should I bond the frames or not? If i did i'd have to run a seperate earth back to the main bond point. I've seen some frameless Class II poly panels very cheaply, and can make my own mount/frame, but not sure if it's a good idea yet, with around 500v DC floating about, are there other concerns or options i've not thought about?

Bit of a long rambling 1st post  Embarrassed

cheers

al






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rogeriko
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« Reply #1 on: March 31, 2015, 08:13:33 PM »

Why on earth would you use a GTI just connect the panels directly to the batteries. Make sure you buy 28/29v mppt panels they will be perfectly matched to your batteries. Connect them all in parrallel through a simple cheap PWM controller, job done. GTI's are too lossy and a waste of money.
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marcus
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« Reply #2 on: March 31, 2015, 08:46:31 PM »

leaving aside Rogerlko's very valid question (unless it's a really BIG barge); then if you've got a 600v string of panels on your barge that could be touched, I would opine that the frames should definitely be earthed.

I'm not sure why you'd have to run the cable back to a 'main earth point' on a steel barge though;- surely the whole hull and structure are electrically a single piece which is bonded to your main earth point, so you could bond the frames to the steel structure locally (unless there's a maritime reg that specifically prevents that).
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al barge
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« Reply #3 on: March 31, 2015, 09:11:06 PM »

Why on earth would you use a GTI just connect the panels directly to the batteries. ....

Thanks Rogeriko, I'm not sure,

I figure a 94% efficient second hand SMA GTI etc to take the daytime 100w-300w baseload and daytime Barge workshop activities on-top, and then use the excess to charge the batteries only, would be more efficient than increased battery cycling with possible headache of an mppt/pwm charger fighting with the victron to charge, then the victron inverting it back to AC for the baseloads.

Separate PWM would give more redundancy, but they're not that efficient either, and decent MPPT's for 2.35kw of panels are quite expensive too, I'm also a little dubious letting a cheapo pwm charger loose on a 1,200 battery bank. Are there any products you would suggest, so i can figure the differences?

But either way, I still wonder what the best method for earthing/grounding panels that can be touched?



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al barge
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« Reply #4 on: March 31, 2015, 09:24:44 PM »

leaving aside Rogerlko's very valid question (unless it's a really BIG barge)

If 38mx5m (127'x17') are big, then yes Smiley

It's more the norm nowadays to keep all circuits off the hull apart from earth bonding, and to avoid galvanic corrosion it's best to keep potential stray voltage from travelling through the steel and turning the boat into a giant battery, so best practice to have a single bond point, which gets much harder on a larger boat.

If the panels were to be bonded to earth, I assume most GTI's would sense any problems and disconnect, but what would a simpler PWM do? Maybe a separate DC sensing RCD before PWM inputs, but that doesn't seem to be the norm?

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« Reply #5 on: March 31, 2015, 09:34:21 PM »

Just use a morningstar controller http://www.navitron.org.uk/store/off-grid/chargers-load-controllers/diversion-load-controller-45a-prevents-overcharging They are among the best. Dont believe anything about a 94% efficient GTI charging system thats a load of **** by the time you have converted panel DC into AC at a different voltage and then converted the AC back into DC at a different voltage we are talking about 60% efficiency. I know its what I do for a living. In the UK you need every amp you can get if you are off grid that is why its sooooo important to get 28v panels at a high amperage. Its the amps that charge a battery. On a Barge the batteries are not far from the panels so no problems with the wiring. As far as earthing is concerned is the cabin top metal? In which case make sure the bolts that hold the panels are bolted onto clean metal, and then just strap all the panel frames together. The inverter is designed to run for years it will just silently do its job baseload or full power. I have many offgrid inverters running over 7 years now never been off for even one minute.

If you only have 24v system on the panels then you dont need to worry about earthing at all. If I remember correctly 50v is the lower limit for earthing, I am sure someone will correct me if I am wrong.
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marcus
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« Reply #6 on: March 31, 2015, 09:50:05 PM »

Oh I see - if you're talking about prioritising efficiency of PV to daytime AC loads and are less concerned about efficiency PV to battery then GTI tie makes more sense.

As Rogerlko says 50v a.c. and I think 60v d.c. (IIRC) are safe touch voltages so if you want to avoid earth worries you may be better off with d.c. connecting the PV.

Quote
It's more the norm nowadays to keep all circuits off the hull apart from earth bonding, and to avoid galvanic corrosion it's best to keep potential stray voltage from travelling through the steel and turning the boat into a giant battery, so best practice to have a single bond point, which gets much harder on a larger boat.

I see, but then this IS earth bonding, and if there was any significant leakage to earth the GTI would shut down, and I'd have thought you would need to run quite a lot of amps through the earth bond & hull to get a galvanically significant voltage gradient on the hull - rather more amps than a 600v PV array would produce IMHO.
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al barge
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« Reply #7 on: March 31, 2015, 11:25:10 PM »

Marcus, yes i'm kinda more interested in minimising battery cycling, as they're the most expensive consumable part. Assuming charging and using energy stored in batteries is the most inefficient part of being Off Grid my thought is it's most efficient to minimise how many times the batteries are charged and drained as the chemical process wastes energy.

It may be that Parallel 24v panels is the safer way to go, but i'm still trying to get my head around the efficiencies, I think Victron advocate GTI+Victron using frequency shift control being more efficient and better on batteries than a separate MPPT and inherent battery cycling.

Rogeriko's suggestion:

System 1.

24v Solar panels>PWM>battery bank>Victron>outlets

Pros: safer low voltage, cheaper PWM
Cons: Expensive lower power panels, less efficient PWM (90%?), more Battery cycling, daylight base-load also needs conversion from stored battery to AC (90% eff' Victron), 2 Competing chargers, 60% efficiency after charging battery.

or:

System 2.

Standard solar panels>GTI>Outlets (daylight)
                                    >Battery Bank>Victron>Outlets (night + when excess sun after base-loads)

Pros: Cheaper more powerful panels, 94% efficient mppt harvesting direct to AC in daylight, less battery cycling, less DC-AC conversion.
Cons: Higher voltage DC possibly not as safe, more expensive GTI, battery charging has extra AC-DC 90% efficiency loss, only 60% efficient when using stored battery energy (when it's dark).

If you can follow that, I'm still not sure how a PWM controller system can be more efficient, unless I've got some assumptions way off?



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al barge
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« Reply #8 on: March 31, 2015, 11:44:26 PM »


Or to put it another way,

What's more efficient?

2.35Kw of panels with a (80-90% eff'?) PWM charge controller feeding DC direct to a battery (60% eff'?) then being converted @ 94% by Victron Multi to AC

or

2.35Kw of panels with a 94% eff' MPPT GTI directly feeding loads in daytime, and excess charging batteries through Victron Multi @ 60%? (can it be that bad?) then converting only in night-time @ 94% back to AC
  
And then what's best to use for safety on a metal boat, or is that a moot point if GTI can sense DC earth fault with frames bonded.


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« Reply #9 on: April 01, 2015, 01:06:15 AM »

it's going to make pretty much naff all difference tbh

if you use a GTI it will convert the DC you feed it from the panels into a set DC voltage before it converts it to AC

which is basically what your battery charger is doing anyway



it will make no difference at all the the batteries.... you're generating X amps, you're using X amps

doesn't matter if you use a GTI or a battery charger, the batteries will see exactly the same amps in/out

(if you feed the batteries 10 amps and take 12 amps out, the battery only supplies 2amps out etc. etc.)
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« Reply #10 on: April 01, 2015, 02:04:52 AM »

Dont believe anything about a 94% efficient GTI charging system thats a load of ****

Well it's true in the lab with a perfect regulated square wave, when it's climate controlled, forced air cooled on conductors made of 24K gold.  bike

50v a.c. and I think 60v d.c. (IIRC) are safe touch voltages so if you want to avoid earth worries you may be better off with d.c. connecting the PV.

Safe-ish. Unpleasant to adults, children might be more sensitive.



I'd have thought you would need to run quite a lot of amps through the earth bond & hull to get a galvanically significant voltage gradient on the hull - rather more amps than a 600v PV array would produce IMHO.

I don't think so. There's other problems that arise too like if you have multiple earth/ground bonds the potential difference between them give a lower reference to one and instead of plating the cable to the hull one plates the hull to the cable. Bare in mind some boats live in salty estuaries and that acts as an electrolyte and a corrosive.
Also if you moor beside other boats with transients travelling through the hull then you've turned the entire marina into a weak battery.

Seeing as the electrons are DC at the generator (PV module) odds are there's one less conversion happening most of time. Even if you are using AC in the daytime you've already inverted it.


I'd bond the earth and ground to the same point. It's not ideal because of transients but thems the rulez.
http://www.smartgauge.co.uk/earthing.html

Would money saved on a PWM controller buy you more PV and offset the efficiency numbers? Or is that all the space used?
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al barge
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« Reply #11 on: April 01, 2015, 02:33:47 AM »

Thanks Knighty,

I may have to go the separate MPPT route, as a bit more digging around SMA tech notes, and it looks like the HF GTI's have a fault, so can't be used in off grid mode, only the TL series are recommended, can anyone confirm real world use?

I think TL's dumping the Panel Capacitance on the earth upstream of the main 30mA RCD will cause nuisance tripping, all circuits come after a 30mA RCD for safety issues, but seems bonding the frames is a good idea whatever.

I was hoping to use Victron and a GTI's clever gubbins together, but not so sure now..
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al barge
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« Reply #12 on: April 01, 2015, 02:59:23 AM »


I'd bond the earth and ground to the same point. It's not ideal because of transients but thems the rulez.
http://www.smartgauge.co.uk/earthing.html

I've spent a fair time reading the smartgauge site, and went by recommendations on there and wiring matters for marinas and boats.


Would money saved on a PWM controller buy you more PV and offset the efficiency numbers? Or is that all the space used?

Maybe, but although the space is large, we get some focused shading from dock walls/boat crane, and we'll have less roof/deck area when the unshaded area is covered in panels, so 8-12 maximum probably, depending how I can mount them..

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« Reply #13 on: April 01, 2015, 04:22:59 AM »

Why on earth would you use a GTI just connect the panels directly to the batteries. ....

Thanks Rogeriko, I'm not sure,

I figure a 94% efficient second hand SMA GTI etc to take the daytime 100w-300w baseload and daytime Barge workshop activities on-top, and then use the excess to charge the batteries only, would be more efficient than increased battery cycling with possible headache of an mppt/pwm charger fighting with the victron to charge, then the victron inverting it back to AC for the baseloads.

Separate PWM would give more redundancy, but they're not that efficient either, and decent MPPT's for 2.35kw of panels are quite expensive too, I'm also a little dubious letting a cheapo pwm charger loose on a 1,200 battery bank. Are there any products you would suggest, so i can figure the differences?

But either way, I still wonder what the best method for earthing/grounding panels that can be touched?





Using a chargecontroler , does notconsequentially  mean  that power from the Panels  has to go first into the battery ,  and out again to the AC consumers ...., the PV power  wanders via the battery poles to the Victron to direct supply  the AC loads ...  so it will not cycle  the battery

The only benefit i see in a GTI  is some more efficiency during the day when  directly  satisfying the 100w-300w baseload   , ....

I would get a MPPT chargcontroller      for the panels  wired 2 in  series    , possibly later  a small GTI and 500 watt PV  AC-coppled

Billi
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al barge
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« Reply #14 on: April 01, 2015, 11:08:35 PM »


Using a chargecontroler, does notconsequentially mean  that power from the Panels  has to go first into the battery, and out again to the AC consumers ...., the PV power  wanders via the battery poles to the Victron to direct supply the AC loads... so it will not cycle  the battery

The only benefit i see in a GTI  is some more efficiency during the day when directly satisfying the 100w-300w baseload   , ....

I would get a MPPT chargcontroller for the panels  wired 2 in  series, possibly later  a small GTI and 500 watt PV  AC-coppled

Billi

Thanks Billi, I spose yer right that DC coupling wouldn't really cycle the batteries as such. Our base load can vary a bit and in daylight hours i build speakers in my workshop, which can add quite a bit to the consumption with power tools etc.

I'm still feeling it would be better just to have one charger on the batteries looking after them, as it's possible with future proofing an MPPT (of either type), we could have up to 12 x 250w panels eventually, meaning 125a peaks of charge at the same time as the Multi putting 70A into the batts if the genny got turned on, ignoring load diverters etc.. sh*tfan

But a GTI will Backfeed AC to the Multi, letting it do the charging and just be told to throttle back output when necessary, we could add load diverters to either system eventually though.

If i went DC couple route, i'd be inclined to go for a clever MPPT like the Outback, Victron is a bit overpriced, but they're basically similar prices to a GTI..

So are there any other GTI's that are happy off grid? Looks like Fronius all are, and the Galvo model may be better for a boat, and simple wifi datalogging, any other Navitron shop GTI's that can be configured for frequency shift?

Or of bonding thoughts on a steel barge?




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