Because floating solar panels add drag proportional to their area, and it takes a lot of area of panels to power a motor that is sufficient for a cargo ship even without the added drag of the panels. Also, because oceans and the things one runs into in them aren't easy on solar panels being dragged along by cargo ships.
i wish there was more talk about this. it seems i heard a lot about making hydrocarbons from co2 in the air + solar or algae a couple years ago. if your hydrocarbons are made this way it seems they would be carbon neutral.
i'm guessing there's more research to make it feasable since i haven't seen "carbon neutral gas alternative" at the local Chevron.
There has been quite some buzz about ammonia, as it is fairly easy to turn electricity into hydrogen, and hydrogen into ammonia. It has a reasonably high energy density, is not too nasty to handle, and already has a huge industry built around it.
My understanding is that drag is more about the "front-on" view of a craft than how long the craft is.
Since solar panels are very thin and aimed up, it feels like they add minimal cross-sectional area to the craft. Your assertion seems trivially incorrect to me?
Oceans can be extremely rough, but even mild waves make it inappropriate to approximate PV as thin.
The requisite area to power a ship is huge, something like 1.4km^2 (ballpark estimate for 20% cells, reasonable capacity factor guess, 60 MW consumption requirement). If a ship is about 30m wide, it's trailing about 45 km of PV. You're not even into 4 digits of cargo ships before the combined length is longer than the circumference of the planet.
> My understanding is that drag is more about the "front-on" view of a craft than how long the craft is.
Drag (fluid mechanics generally) is... ludicrously complicated. For the typical shapes of ships, I believe you are correct that the main factor is cross sectional area perpendicular to the direction of travel, but that’s not universally true. i think that for a floating raft of panels, it would be proportional to the panel area, similar to how for winged aicraft its the wing area and not the cross section perpendicular to direction of travel.
There's a pressure drag and skin friction drag. Friction drag is supposedly a majority component unless you sail a brick. But I don't have sources to prove that.
Interesting idea, but that would require more than a square kilometer (or a 100m strip 10km long) of solar panels (not accounting for the additional power required to tow the panel array).
Solar power being useful doesn’t require 100% of propulsion to come from solar panels.
You see solar panels added to a wide range of boats because even bunker fuel isn’t free and panels are light for the power they provide over even a few days. A current 399.9 * 61.3m container ship doesn’t need panels everywhere to benefit, but the potential savings is significant if they do.
This is unfortunately not true because of the dynamics of diesel engines: there is by design surplus energy relative to requirements from running them at efficient operating points. Otherwise the ship is not a good ship.
You can always scale design to fit reduced demand. Also, loss of efficiency is more than made up for with vastly lower energy demand.
“Lowering speed reduces fuel consumption because the force of drag imparted by a fluid increases quadratically with increase in speed. Thus traveling twice as fast requires four times as much energy and therefore fuel for a given distance.”
“Container ship Emma Mærsk in Aarhus, 5 September 2006
Mærsk Line's E-class container ships such as the Emma Mærsk can save 4 metric kilotons of fuel oil on a Europe-Singapore voyage by slow steaming.[5] At typical fuel prices of US$600-700 per tonne,[4] this works out to a saving of US$2.4-2.8 million on a typical one-way voyage. Maersk's Triple E-class container ships were designed for slow steaming and have less powerful engines than their predecessors.[5]”
I'm not an expert, but I've worked close to some of the engines that power those ships. My gut feeling is that you're vastly underestimating how much power those ships consume (and therefore produce).