Here's some back of the envelope math for batteries and ships:

- Weight is less of a limitation than you would think. Ship size is measured in tonnage. 40K-60K is a medium sized cargo carrying ship. So lets assume a ship like that.

- Battery weight calculations are going to be key. If you assume 170 wh/kg, 6 tonnes of battery equals about 1 mwh of battery.

- Energy usage of ships is speed dependent and it's a non linear relationship. You can save a lot of energy by going a bit slower. Going about 15 knots, a ship like this might use 15mw of power.

So the math becomes something like 6 x 15 = 90 tons of battery per hour. 5000km is about 2700 nautical miles (1 knot == 1 nautical mile/hour). So, you need about 180 hours of battery. Or about 16200 tons for a total of 15mwh x 180 = 2.7gwh of energy. That's a big battery.

The real limitation here comes from the cost of the batteries, which is dropping fast with sodium ion. The reason CATL is bringing this up is because they've been doing similar math with some informed $/kwh math. If they can get it down to around 20$/kwh, a mwh would cost 20K, and a gwh would cost 20M. So the battery would cost 54M$.

The key here is that this is still assuming 15knots. Energy usage might drop considerably if you drop it to 10 knots or even lower. You might only need 7200 tons of battery at those speeds.

The ship can handle the weight either way, though you are sacrificing useful load of course. The real constraints here are cost and speed. You pay a fat premium for a fast ship. Of course ships this size aren't cheap. A few tens of millions is normal. And they burn through many millions worth of fuel per year too. So, even though that amount of battery is expensive, the math might actually work out to these ships being cheap enough to operate that they'd earn back their battery.

You'd have to be pretty bullish about cost and performance of batteries. But CATL clearly feels that way. They have several battery chemistries at their disposal with higher densities (and cost). Over time, batteries might get cheaper and more dense. Ship designs might be optimized for batteries (e.g. structural hulls with battery). There's a lot of wiggle room here. But it's not an impossible proposition.

Those are some big numbers. It makes me think of a crazy thought experiment:

How many MW could a container ship carry by literally shipping energy stored in batteries?

As in they fill up entirely with batteries, sail to a desert, plug into a cable to charge on cheap solar, charge up, sail to a population center, plug in to discharge. Repeat.

If your average solar array gives 2.5MW per hectare then 15MW would require 6 x 100m x 100m to run, or a beam of 150m for a 400m long vessel (eg Evergreen G-class.)

That’s only ~4x wider than the current big classes of ship. Maybe we will see twin hulls with a solar field slung in between?

The downside of course is you can no longer romantically sail by starlight (or at least, technically, by the starlight of non-Sol stars.)

I think another problem is how fast you will be able to charge such a huge battery and how expensive such ship battery charger will be and if you expect to have such charger at most ports (ideally for every docked ship). Will you even need a power plant at every port?

The upper weight limit for both 20ft and 40ft shipping containers is around 28,300 kg. Since SodiumLion batteries in containers will be weight limited, it makes sense to use 20ft shipping containers. One container can probably store around 3MWh of energy. For a 2700nm trip at 15 knots with 15MW of power usage, you need 2700Mwh divided by 3MWh per container gives us 900 20 ft shipping containers (TEU) for the battery.

OTOH if we look at popular shorter range routes like within Asia or Europe the calculation looks a lot more favorable.

The one technology that I thought might work really well here is flow batteries. You need a couple of sets of tanks, but then you can do charging by pumping out the old electrolyte and pumping in charged electrolyte. But these seem to have stagnated, possibly because they're dependent on expensive short lived membranes.

I wonder if there's significant scope for offsetting electricity consumption by adding deck renewables. Not for container ships, but maybe for tankers .. which are only needed because of fossil fuel consumption elsewhere. Hmm.

What you are suggesting in the best case is completely uncompetitive with current ships both in terms of weight and cost. Even the charging costs for 1GWh is absurdly high compared to Heavy Fuel Oil. An orders of magnitude more expensive.

Not to mention dedicating 15-20% of deadweight tonnage (and a higher percentage, maybe 30-40% of its gross tonnage) would make a ship instantly uneconomical, especially as the batteries must be laid along the keel for stability, meaning the ship loses the ability to carry many cargoes.

What's possible in the medium term are Heavy Fuel Oil/Electric hybrids that use battery power in regulated Emission Control Areas instead of Low Sulfur Marine Fuel Oils or Diesel, and using HFO in blue waters and to charge batteries.

Transoceanic battery-powered cargo vessels are probably 100 years away - fusion will arrive first.

If deployed with hybrid sails [1] this could be even more competitive to Bunker fuel for bulk/container carriers and even RoRo car carriers.

IMHO a hybrid approach would achieve similar to superior perfomance to the current state viz a viz cost/perfomance and enviromental impact.

Additionally as has been deployed for some longhaul trucks battery swaps can also cut down on the time to redeploy after offloading the cargo.

[1] https://gcaptain.com/berge-bulk-launches-worlds-most-powerfu...

> If you assume 170 wh/kg

I've wondered if thermal storage might be superior. Lower round trip efficiency if one uses resistive heating, but I think that would be ok.

Lithium hydride, heated to melting, stores something like 4 MJ/kg, more than 6x the specific energy of your assumption there (admittedly with loss on discharge due to the losses in the turbine.) If that is too expensive, graphite is another possibility, at even higher temperature.

If we could only complement that battery with a small nuclear reactor, then we'd be in business. Come to think of it, shipping would be quite interesting for a SpaceX style disruption - there is a market for many many thousands of units - enough to actually get good at building them in a repeatable fashion. Of course there are considerable engineering and political challenges, to put it mildly.