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Comment by joering2 | Hacker News Reader

What these ships are mostly surrounded by are waterfields that go as far as you can see. Can they come up with some sort of rolled flexible solar panels that once ship goes into full sail they release their “solar tail” behind that floats on the water surface and captures sun and turns it into electricity for ships’ engines?

I mean 300 meters square of solar panel should be sufficient to push the ship at its nominal speed. For free!

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I think the issue would be salt water. Salt water destroys everything, I can’t imagine that a PV array floating in water would have a good life expectancy.

On the plus side, if the insulation fails you could turn container ships into accidental fishing boats.

> Salt water destroys everything

This is the statement I hear relating to all kinds of ocean-based renewable energy ideas. Sources are never provided.

The problem must have to do with economic viability and the replacement/maintenance costs exceeding savings.

Could you supply sources to elaborate on where the break even point of solar tail (or any two other ocean renewable energy) systems would be as relates to costs and benefits?

Edit: I don't really want to argue whether or not steel rusts in saltwater. My argument was that the break-even point of a "solar tail" is dismissed out of hand without sources because of land-based technologies being considered as the only ones.

> Sources are never provided.

We also do not need to provide sources for statements such as 'water is wet'.

Anything on a boat, or even shoreside near to the ocean is subject to corrosion and corrosion protection is a serious maintenance issue. I've lived 2 km from the sea and even there the nails would rust right out of the building due to the salt that the wind would carry in.

Yes, salt water is corrosive. And yet, there are metal ships with viable lifetimes. This sub-thread is a bit less helpful than I am asking for.

Edit: Come on. It's not impossible to engineer for ocean use.

Wave power generation: https://www.bbc.co.uk/news/uk-scotland-scotland-business-250...

Longer term: https://en.wikipedia.org/wiki/Submarine_power_cable

Salt water is so corrosive that there is a whole branch of materials science dedicated to coatings and all kinds of other anti-corrosion measures such as galvanic protection using sacrificial material. The evidence is overwhelmingly on the side of statements such as 'simple systems that work well on the shore need extensive rework for ocean deployment assuming they can work at all'.

Even a simple scratch is enough to lay all your carefully produced work to waste.

Agreed that the environment is vastly different than on land, that a scratch through a coating is akin to no coating at all, that anti-corrosion measures are a constant vigil in oceans.

But I asked for sources treating the proposal in the original comment, and got told that "water is wet".

It’s obviously not impossible, it’s just expensive, which is a problem in the given context.

How big is the check and why? That's what's so hard to get an answer for when people are determined already to not to try. I mean, I guess I have Google and a long-weekend. I was hoping for someone to drop an URL that can explain this and provide key terms.

Throw out a speculative idea, get a speculative answer.

Dunno why you’re expecting me to go all civil engineer on you, nor do I understand why you’re acting like I’m the barrier to the idea being tried.

Go do the math yourself, it’s your idea.

It wasn't my idea. My idea was that it shouldn't be dismissed for the reason you presented without a citation.

What can be asserted without evidence can also be dismissed without evidence.

As there's a well known consensus that some factor (e.g. saltwater caused maintenance issues in this particular case) tends to be a game-breaker, then it's perfectly reasonable to dismiss 'without a citation' ideas that encounter this factor if the idea has no mention/citation of how they're going to fix, avoid or tolerate this.

If there's a plausible-seeming way that the idea can work despite the obstacle, then by all means we can discuss if that way will be sufficient or not; but if the idea simply ignores major problems, then there's nothing to discuss.

Point taken regarding your first line, though that sword would cut both ways I think. See https://news.ycombinator.com/reply?id=17886400 regarding the second and third.

There’s a reason why pictures of “beautiful” beach front houses are usually from a long distance away: they are usually in pretty rough shape.

My request was not intended to be asking for sources about corrosion, but for sources as to why corrosion should be the limiting factor. I would edit it to lead with the question it asks in the third line, if it weren't now too late.

Here's a source for you that mentions that even some typical formulations of stainless steel are susceptible to corrosion in saltwater:

https://www.clintonaluminum.com/304-stainless-steel-in-seawa...

White bread is bad in saltwater too, that doesn't mean there aren't better materials for the application.

https://www.quora.com/Does-aluminium-react-with-salt-water

https://en.wikipedia.org/wiki/Organic_solar_cell

Salt water destroys everything so don't build boats guys

That's the point of the source you requested and I provided: there are some formulations of stainless steel that are actually kind of ok in salt water despite most being poor.

No one is arguing that it's impossible to build a bear with solar panels. The point is that you can't slap a cheap panel from Alibaba on a float and have any assurance that it will work reliably. Thus costs go up and the whole idea may become infeasible.

I don't have a source, but as an engineer working for the US Navy, we are frequently told that the Navy spends an enormous amount of money on corrosion control. Like, in the billions to tens of billions of dollars per year. As a consequence, corrosion control is a significant fraction of the Navy's R&D budget.

https://www.gao.gov/assets/650/647926.txt

https://www.onr.navy.mil/en/Science-Technology/Departments/C...

Thank you.

How would you keep them clean enough to produce optimal output? Every drop of salt water would quickly dry and leave a salty residue. How do you clean that while at sea?

I am not the author of the original comment, but I really like the concept. From what I understood this would be floating at surface level and would be continuously washed over by surface water.

There are many good objections. It may be that the original comment does not illustrate the promise of this energy platform very thoroughly.

300m^2 only gets you a couple hundred kW on the brightest days at peak efficiency. These ships require powerplants providing on the order of 50,000 kW.

A 40-acre floating solar farm would add a LOT of drag. Better to make the solar farm stationary, use it to synthesize fuel, and run the ship with an internal combustion engine for power density. Maybe someday batteries could be used.

I believe you. The difference between required and available power is large.

Can you provide sources that derive/explain/summarize the figures you present so that others may better understand the reasoning?

Sure, sorry.

The rule-of-thumb max for solar irradiance is 1 kW/m^2 (noon, clear day, low latitude, etc):

https://en.wikipedia.org/wiki/Solar_irradiance#Irradiance_on...

The rule-of-thumb for the electric power output of a solar cell is about 20% of the irradiance.

https://en.wikipedia.org/wiki/Solar_cell_efficiency

And because this drops in the morning and evening and falls to zero at night, you realistically only get a bit more than 1kWh/day with a 1 m^2 solar cell.

And these ships require huge powerplants. Maersk's biggest ships use this engine:

https://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA9...

which outputs over 80 MW.

No citation for the drag caused by a floating mat of solar cells. The fact that these enormous ships burn this much fuel should be evidence that dragging a much more enormous mat should show that dragging an even more enormous and more draggy mat would be a non-starter.

Thanks!

My acceptance of the idea that drag might be manageable has to do with volume displacement and not surface area.

A long shallow "tail" could exert orders of magnitude less drag (vis a vis surface area) than a deep heavy hull pushing up a big wake. (Related to why boats are long and narrow...)

https://en.wikipedia.org/wiki/Wave-making_resistance

Edit: The fact that you see that as a clear non-starter has me wading into Google results for hydrodynamics. It is definitely over my head, but I am curious. Is there a rule of thumb for drag in water that I'm not finding?

https://www.boatdesign.net/threads/wetted-surface-area-appro...

http://www.engr.mun.ca/~bveitch/courses/r-p/Assignments/ITTC...

http://www.fade.up.pt/cifi2d/files/effect-of-wetted-surface-...

My assertion that it's a non-starter is based on the area of an 80 MW solar farm, not hydrodynamics. 80,000 kW x 24 hours / 1 kWh/m^2 = 2,000,000 square meters of solar cell area.

That's a lot. In contrast, the ship has an area of 24,000 m^2. A light-weight solar system might weigh 10 kg/m^2, so the whole thing weighs 20,000 tons. The ship has a staggering capacity of 200,000 tons.

So yes, it can carry it, but the 'long shallow tail' is really long - literally 20 miles, if it's the same 60m width as the ship - and really shallow. Maybe that's low drag, but if so, why wouldn't they build ships like that?

That is an interesting result. A 20 mi long piece of electronics floating in the open ocean would have [:deadpan:] more prohibitory issues than drag. Even if it were more optimistically estimated, this has obvious maneuverability (not to mention production) problems. They build ships the way they do for smart reasons, but I do like to explore the possibility space.

The application that no one will ever pay for might be a permanent, autonomous, very large, plastic garbage consuming vessel in the Pacific Gyre.

The form factor is less a limitation (in theory) than the will to allocate resources in such a way.

Somebody made a way to start cleaning this up, and a proposal to pay to do it (front page 3 days later):

https://news.ycombinator.com/item?id=17899395

I think you're vastly overestimating the power output of solar cells.

Using 20% efficiency panels gives us 0.2KW/m^2/hr.

According to this Quora answer [1] a small container ship uses 15,000kW: for one 24 hour day that means it needs 360,000KWH in energy.

If we assume 10 hours a day of full sunlight then we need 180,000m^2 (equivalent to 18 hectares or 44.48 acres or 33.7 football fields) of solar panels.

This of course overly optimistic as it assumes no drag nor increase in power needed to haul these solar panels (and batteries for use overnight, etc.).

[1] https://www.quora.com/How-much-does-it-cost-to-fuel-a-cargo-...

Nitpick, but 0.2KW/m^2/hr would be an acceleration, not a constant rate. You mean 0.2KW/m^2 - this is constant power output. W already includes a per-unit-time component. I'm nitpicky on this topic because of how many professionally-written articles I've seen make similar mistakes.

I think you're under-estimating the power of the square. 180,000m^2 is 2 x 300m x 300m.

I'm guessing that you're underestimating the cost of drag ... tailing a 300 sq meter "boat" would probably require more energy than it could produce. I have no data, and haven't done any back of the napkin calculations to back up my stance ... but just look at the speed difference between a monohull vs a catamaran vs a foil sailboat. Reducing the contact area with the water (and thus reducing drag) is _everything_.

Drag increases with the square of velocity [1]. Some of the ships might be able to travel much slower? I keep asking for sources because dismissals out of appeal to obviousness are so ubiquitous.

[1] https://www.lmnoeng.com/Force/DragForce.php

Edit: Admittedly, you may be right, but without even back-of-the-envelope calculations, and with the assumption that these boats must go as fast as possible, I don't know why.

I too freely admit that you might be right as well (if the assumption of speed can be dismissed) ... lol, this feels like it would be perfect fodder for one of those physics youtube channels that answers channels, or perhaps XKCD's https://what-if.xkcd.com/

A typical 250W PV panel is 1.6m^2. 300m2 of these will give a peak power of about 47kW which is about 63hp.

for comparison a Wärtsilä RTA96C is rated for over 80MW which means you'd need over 0.5million m^2 of PV panels to match this. (about 85 football pitches).

Feasible but expensive

The idea that it would need to be 1000X bigger is helpful. The original poster's figure of 300m^2 does not make much sense.

Rereading with the principle of charity in mind, I think the original poster meant (300m)^2, though it was not clearly so.

To entertain the strongest interpretation of the idea, for purposes of understanding the reasoning: consider a very large, very slow ship trailing a towed PV array not made of steel [1], additionally harnessing sea-surface energy, and the extension/retraction of the tow cable [2, 2.5], made from non-corrosive plastic or plastic-coated materials, that costs hypothetically nothing to maintain, but must be completely replaced every 10 years.

What are the specifications of such a system that it would generate sufficient power to cost-effectively augment or replace internal combustion, and what would the viable price-point for installation be considering costs of bunker fuel? Other comments [3, 4, 5] added the kind of information that might help to answer that. I am interested in sources, and discussion which isn't immediately dismissive of something unproven.

Put another way, how much would low-grade petro-fuels have to cost before this became a viable alternative, and what sources support your conclusions?

[1] https://en.wikipedia.org/wiki/Organic_solar_cell

[2] https://en.wikipedia.org/wiki/Wave_power

[2.5] https://www.tugtechnologyandbusiness.com/news/view,hybrid-pr...

[3] https://news.ycombinator.com/reply?id=17886124

[4] https://news.ycombinator.com/reply?id=17886177

[5] https://news.ycombinator.com/reply?id=17886190

Edit: spelling

Edit 2: added a reference

For free, when there are no waves, when the sun is shining, when there is no other vessel that crashes in to them and when you convince someone to make it all and then gift it to you.

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