Floating solar array to power land-based loads

[AndyEich]

Before I get too deep into my questions, I wanted to first check whether this post is new and correctly categorized in the "marine" sub-forum.

I'm looking to construct a small solar array on a floating raft. Most of my questions are around the environmental factors, particularly in terms of how to ensure safety when sending the power to shore. Voltage, choice of submerged cable, and fusing approach would be key, as will how to manage the system's transition between summer and winter (when the pond freezes).

I've been unable to turn up any similar threads, but I'm a new member, so please speak up if those of you who have been here longer have a better sense of where this belongs or whether it duplicates prior discussions. If, after a couple of days, this *does* seem like a new topic and it's in the right place, I'll follow up with some more background and specific questions. Thanks!

 

[Suijkerbuijk]

Wel you need cables that it can handel it .
The cable will not be the problem but the connector you gone use .

Those cables and handel a lot abuse.

 

[AndyEich]

Thanks, that is an interesting cable spec! "They are suitable for submerged installations with a maximum cumulated immersion
period of 6 months / year." I wonder how many years one could do that.
For connectors, so far I figured the Amphenol SurLok Plus ones that EG4 uses in their outdoor batteries were a safe bet. I presumed those would be suitable since the connectors won't be underwater. I also figured it would be good to have a breakaway connector at the barge end, but I haven't found any yet that look appropriate.

 

[hwy17]

It's ok cause PV voltage is already a floating system.

- Not actual advice and at least half joking, but also half serious

 

[James Getman]

Well pump wire? Good for ac, don't know about max dc voltage.

 

[IGBT]

What are you using for a raft? I had the idea of using queen size air mattresses lol

 

[AndyEich]

I live on a small wooded lot beside a pond. I have few options for solar on land, but I'm allowed to have boats/rafts in the pond. There is not much wave action, but it does freeze every winter. There are few other users of the pond, so not much worry about people messing with it.

I've built some docks over the years based on 12'x4' wooden sections, and my plan was to put 3 of those together (to make a 12x12 platform) on top of a couple rows of the plastic drums ("tight-head" version) you can typically find free/cheap on craigslist. One of the fun side-projects would be figuring out if I can control the rotation of the platform on its mooring well enough to track the sun in azimuth, but I digress...
The wood frame is being done mostly out of familiarity--I'm sure there are longer lasting / lighter options such as aluminum, but I don't know how to do them in a cheap/flexible way.

Thanks for the suggestion of submersible pump wire. My dad knows about that, so I'll ask him.

I will probably look to get a "hello world" version going with a baby array first, but ultimately I'm thinking of 6x 400W panels. I think that will generate a DC voltage around 250V. What's the safest way to get that power to shore? Do I want to put the MPPT on the raft and bring ~50V to shore? Would it make any sense to have a "buffer" battery on the raft to absorb the peak generation and send it to shore more slowly, over a smaller gauge wire? I may put a battery on the raft anyway, as I plan to eventually loft the array up on a pergola of sorts and build a tiki bar under it that would need power for a fridge, blender, etc.

 

[Brucey]

I would just use normal PV cables, making sure that the connections don't get near the water. If you have the SCC with the panels on the water you are probably looking at fairly hefty gauge of cable to being the higher amperage ~50V back to the inverter.

 

[AndyEich]

Thanks for the advice!
How can I protect anything in the water if something were to go wrong and the cable shield was compromised? My understanding is that freshwater is not as conductive as skin, so people (and I presume pets/wildlife) will get shocked and drown if they're in the water nearby when the water is energized. Do you put a GFCI and/or fast CB on it? Wouldn't there still be a brief period where the water is electrified, and is there any way to prevent that entirely?

 

[wme]

Anchor the raft, make 2 pylons and use a messenger wire to keep the power wires out of the water.

 

[AndyEich]

Yes, that's an option, but doesn't the worst case end up the same--if the power line falls in a storm, it electrifies the water until a breaker pops...?
The disadvantages are that it would be more conspicuous, and it would be challenging to maintain consistent tension given that the barge would need to be so firmly anchored that it couldn't move around. I saw it as much more desirable to have the cable be underwater, so it would be possible, if desired, to disconnect the cable, leaving the end secured to the buoy, and move the barge.

 

[wme]

Using the 6 month submerged cable, dip in the water would not be a problem.
Think of the suspended cable as a opportunity for Christmas lighting

 

[SolarFlares]

Do you have a dock that needs shading? Make a roof for the dock and put the panels on the roof?

 

[HarryN]

Probably will need to use a rapid shut down unit per panel.

Maybe use a low voltage setup - for instance 12 or 24 volt solar array voltage.

Really ground things well to the bottom of the lake, including the panels. That way any leakage is carried through by the wires vs the water or person.

Non conductive raft or boat.

Might be worthwhile to consult with a marine electrician on how to avoid stray currents.

 

[wpns]

Rapid shutdown and ground fault and arc fault in the inverter will promote safety and then it’s just submersion rated solar wire.

High voltage is going to be easier/cheaper from the panels on the lake to the inverter in the house.

 

[wholybee]

Thanks for the advice!
How can I protect anything in the water if something were to go wrong and the cable shield was compromised? My understanding is that freshwater is not as conductive as skin, so people (and I presume pets/wildlife) will get shocked and drown if they're in the water nearby when the water is energized. Do you put a GFCI and/or fast CB on it? Wouldn't there still be a brief period where the water is electrified, and is there any way to prevent that entirely?

In order for this to happen, there needs to be a few conditions met. You need to have an energized conductor in the water. That conductor needs a path back to ground. That path needs to go through a person. And that current needs to be AC.

Of those, for you only one of those conditions would be met. (energized conductor in the water) As humorously pointed out, the voltage from the panels is floating. There would only be a path to ground if you grounded the negative wire to earth ashore. And the voltage from the panels is DC.

For installations with A/C, an ELCI breaker is used. If there is an imbalance in current (more current leaving through the hot conductor than is returning through the neutral) then that current must be taking some other path so the breaker trips.

ESD & FAQ

WHAT IS ELECTRIC SHOCK DROWNING? Electric Shock Drowning (ESD) is the result of the passage of a typically low level AC current through the body with sufficient force to cause skeletal...

www.electricshockdrowning.org

 

[AndyEich]

I appreciate all the replies!

HarryN suggested rapid shutdown per panel. Is the Tigo TS4 (https://www.tigoenergy.com/product/ts4-a-2f)? That looks like it requires a pair of wires per unit (3 pairs to support the 6 panels I'd have in series), and I presume these need to run the whole way to the inverter? Since the inverter will likely be several hundred feet away, with a nontrivial effort needed to bury all that conduit, I'd *really* like to avoid this. Have I misunderstood how those are used? Would a solar disconnect switch be almost as good, or is there a critical failure mode that requires the per-panel shutdown?

When it transforms from just a solar raft into a floating tiki bar/boat, I was considering having a smaller inverter onboard, so I'll read up on ELCI. It is intriguing that most of the electric motors (Torqeedo, Mercury Avator, etc) use 48V, as that makes it a little more reasonable to reuse the same array for onboard use, say with an old EG4 3k, vs having a separate panel and new inverter just to work at lower voltages. I'd previously assumed that 48V DC onboard was a bad idea, and this perception was reinforced by my current motor of choice being 36V (https://newportvessels.com/products/nk300-kayak-motor), but apparently that's not the case.

I can accomplish the ground to bottom of the lake and non-conductive raft, though I do wonder if that has any negative safety implications by creating a path where my floating DC voltage (essentially a free safety feature, as I understand it) can get a good ground right near the raft, creating a potential hazard. Wholybee, the three conditions discussions was very helpful!

Mechanical aspects of the design are progressing. As an aside, I think I now understand that MC4s for the connections to shore would be appropriate, and the SurLocks are only needed for high current going into the battery.

 

[HighDesertOffgrid]

PRO TIP: Add ziplines for ease of maintenance.