When ships moved from muscle- and wind power to burning coal and other fossil fuels for their propulsion, they also became significantly faster and larger. Today’s cargo ships and ferries have become the backbone of modern civilization, along with a range of boat types. Even though tugs and smaller pleasure vessels are a far cry from a multi-thousand ton cargo or cruise ship, one would be hard-pressed to convert these boats back to a pure muscle or wind-based version. In short, we won’t be going back to the Age of Sail, but at the same time the fossil fuel-burning engines in these boats and ship come with their own range of issues.

Even if factors like pollution and carbon emissions are not something which keep you up at night, fuel costs just might, with these and efficiency regulations increasing year over year. Taking a page from alternative propulsions with cars and trucks, the maritime industry has been considering a range of replacements for diesel and steam engines. Here battery-electric propulsion is somewhat of an odd duck, as it does not carry its own fuel and instead requires on-shore recharging stations. Yet if battery-electric vehicles (BEVs) can be made to work on land with accompanying low ‘refueling’ costs, why not ships and boats?

A recent study by Lawrence Berkeley National Laboratory (LBNL) researchers Hee Seung Moon et al. as published in Nature Energy claims that a significant part of US maritime traffic can be electrified this way. Yet as a theoretical model, how close does it hit to the harsh realities imposed by this physical world which we live in?

Different Scales

Justine McAllister (IMO 8107878), a tug boat in New York Harbor. (Credit: Eric Baetscher, Wikimedia)
An important aspect with any battery-powered craft is matching the battery capacity with the expected range. For BEVs like cars, the goal is to put as much battery capacity into the vehicle as possible, constrained mostly by factors such as the cost per kWh and how much physical volume is available in the vehicle for batteries without intruding on the driver and passengers. This is how we ended up with a range of BEVs that can cover a sizeable chunk of daily usage cases, as well as specific cases like buses where the daily range requirement is planned in advance and thus very easy to optimize for. Even so, a number of road-bound vehicles are hard to electrify with just batteries, such as cross-country trucks due to the sheer weight of the batteries required in most scenarios. Unlike a fuel tank, these batteries also do not lose weight as they become more empty.

In the case of boats, these smaller vessels tend to have pretty limited range. For example: tugs put in a lot of work, but either remain bound to a specific harbor or slowly follow a set watercourse like a river with a gaggle of barges in tow. Here you can have recharging infrastructure set up and charging points ready to go with relatively little difficulty in the harbor or at mooring spots along the route. More challenging are vessels with more erratic routes, not to mention ships with routes that are so long that no reasonable amount of batteries could power it without recharging or swapping batteries. The main case in point here is container ships.

In a 2022 study by Jessica Kersey et al. in Nature Energy it was found that for routes of less than 1,500 km electrification would be economical, assuming a battery price of $100 per kWh. At that point the main question remains how many batteries you can fit into the ship without negatively impacting the cargo load that it can carry. A container ship can travel around 540 km per day at its average cruising speed, with a shipping route like Los Angeles to Yokohama of 7,792 km (4,207.6 nautical miles) taking over two weeks:Putting enough batteries on cargo ships to allow them to travel these routes without recharging in between is too much to ask. This is why the focus with battery-electric propulsion for ships and boats is on these shorter routes, where the total volume of batteries combined with electric motor(s) does not significantly exceed the volume (and weight) previously taken up by the diesel engine(s) and fuel tanks. As modelled by Kersey et al., for a small neo-Panamax container ship this would be the case if the route is kept below 3,000 km. Yet if the route is extended to something like 20,000 km the batteries would take up 32% of the containership’s carrying capacity.

Using batteries with higher energy density could help here, but as seen with today’s favorite battery chemistries using the higher density Li-ion comes with fewer charge cycles and worse stability, while LiFePO₄ with its common use in especially BEVs and grid-storage and solar-storage batteries has much better longevity and safety record, at the cost of more weight per kWh.

Removable Batteries

Currently a number of battery-electric boats and ships are in service, with ferries being one of the first to be outfitted with such propulsion, case in point being the Norwegian Bastø Electric ferry. This 600 passenger and 200 car ferry uses its 4.3 MWh battery as well as a diesel generator to travel the 10 kilometer route between Moss and Horten. While docked the batteries are charged up when a charging point is available. This makes it not a pure battery-electric boat, but rather a hybrid.
The Zhongyuan Haiyun Lu Shui 01 battery-electric 700 TEU containership. (Credit: COSCO Shipping)
More interesting are the two battery-electric containerships owned by China’s COSCO Shipping which recently began carrying shipping containers along the approximately 500 km route between Nanjing and Shanghai’s Yangshan Port. The Lu Shui (Green Water) 1 and 2 vessels are 700 TEU container ships that can travel at 10.4 knots over the Yangtze river. Perhaps most interesting about them is that they don’t have a battery bank integrated into their hulls, but rather take swappable batteries, with a standard capacity of 57.6 MWh, but with optional connection points for additional battery packs.

In the aforementioned LBNL study by Hee Seung Moon et al. the assumption was made that existing vessels would be retrofitted with batteries and electric motors, which would place a range of restrictions relative to newly designed and built vessels like COSCO’s newly commissioned ones. Being able to swap out battery packs along with shipping containers allows freshly charged packs to be ready when the containership docks and avoids the hassles of quick charging after each trip and replacing batteries after their approximately decade-long useful lifespan, for LiFePO₄.

Practical Within Limits

It’s clear that for shorter routes the use of battery-electric propulsion can make sense. Depending on the local grid this can also be less polluting than burning low-sulfur diesel fuel, and conceivably be cheaper, though it all has to be worked out on a case-by-case basis. In the case of COSCO the reasoning appears to have been that these custom container ships are perfect for such a shorter route, with cost savings to be expected over the use of direct-driven diesel or diesel-electric propulsion. Ultimately the success of battery-electric propulsion will come down to simple economics, especially in the cut-throat shipping business.

Featured image & thumbnail: Containership MSC Texas. (Source: Wikimedia Commons)

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