Charlie Bartlett, author of “Polluter to net zero: How to decarbonise shipping” in Marine Professional of August 13, 2021, only tells us half of the relevant story regarding the decarbonization of shipping. Accountable CO₂ emissions for ship propulsion are not limited to the on-board combustion of fossil fuels.

Diesel fuel as well as Bunker (heavy shipping fuel) are both high density energy carriers, surpassed only by liquid pure hydrogen and nuclear fuel that are not as easily handled as these carbon-rich fluids. Alternatives are wind in the ship’s sails, on-board nuclear power, biofuels, synthetic fuels, or electrochemical battery storage. Electrical power is required as a first step towards obtaining the two latter ones. To evaluate whether a propulsion method is decarbonized, the full path from the energy cradle to the ship’s wake must be accounted for. This includes the construction of all required equipment and supporting infrastructures, and the operation thereof. However, not only CO₂ emissions come into play but also the consumption of other resources such as other materials and energy. If sails are added to the ship, she may stochastically require less additional propulsion power.

1.   Today’s liquid fossil fuels:

The processing path is quite straightforward and efficient for liquid fossil fuels: from well to the ship’s tank, 8% to 9% of the delivered energy will have been consumed for exploration, extraction, and refining, with their associated CO₂ emissions. Once stored as bunker fuel in a ship, there is a direct thermal process from the stored energy to deliver thrust to the propeller and to other systems with an energy yield of approximately 35% (diesel engine).

2.  Synthetic gas or liquid fuel:

For any alternative synthetic fuel, complex and inefficient processes are involved.

To be decarbonized, such fuels must be produced with the help of electricity, generated either in hydraulic or nuclear power plants, or in a delocalized way by windmills or photovoltaic (PV) panels. In the PV case, the technology is gathering solar irradiation for the equivalent of 15.2 % of the time (World average according to BP statistics, in Europe it is rather 10 to 11%). For wind turbines, this capacity factor is ~27% (~20% for onshore wind parks). Losses in the order of 15 to 25% are incurred for transportation and intermediate storage of this intermittently produced electrical current.

To further convert this electric power to any combustible fuel, gas or liquid, and back to (propulsion) power, an overall energy yield of about 20% is generally reported albeit no industrial validation is yet available. Furthermore, the material recovery is much less than 100% which implies that the conversion must take place within large recycling loops and wastes are expected that will need disposal.

This means that for one energy unit delivered to the ship’s tank, 6 to 8 will have been lost in translation, in other words wasted. The ship will not sink but it is still an unaffordable energy and money sink, even though the solar input is free.

3.  Electrochemical storage with batteries:

First, the energy density in a battery is quite low, 0.14 kWh per kg of the best and costly Li-ion batteries, as compared with 11 kWh/kg for diesel fuel. This means that a Panamax container ship with a load capacity of 14 000 TEU (twenty-foot equivalent units) or a deadweight capacity of 155’000 tonnes would need batteries initially loaded with 55 GWh (energy yield assumed up to the propeller shaft and other systems: 90 %) to get the same two months autonomy as today. This would imply a battery weight of 400’000 tonnes – twice and half what the ship could displace! No further comment is needed on such a conjecture, as for example considerations for the loading time for such batteries and the required number of Tesla superchargers, with at least one large nuclear power plant on standby in every port. 

Don’t even think covering the whole ship with solar panels. These two hectares would only deliver a maximal power of 3 MW at noon by clear sky, and zero at midnight.

4.  Biofuels:

Biofuels are obtained from agricultural crops in the form of transesterified oil (biodiesel), mostly palm oil, or fermented sugars (bioethanol) from maize, sugar beets or canes. They have two fundamental shortcomings: first their low return of energy because they need a lot of processing and other raw materials, and second their poor use of land area. The purpose of nature is neither to produce biomass efficiently nor to supply energy. The use of wood as heating agent was discontinued as soon as other means (coal, then oil and gas) were made available and the dramatic pre-industrial deforestation could be significantly curtailed. As a simple reference, the ethanol yield from a sugar beet field would be of the order of 6700 litres per hectare with a heat of combustion of 160 GJ/ ha. The energetic value of the tank capacity of the container ship will be 55% of that with Bunker fuel since ethanol is lighter and has a lower heating value. The harvest from 2000 hectares (20 square kilometres) planted with sugar beet would be required to fill the ship’s tank. If converted to bioethanol, the whole Swiss production would barely be capable of filling 8 or 9 such ships every year. Thus, we only can be glad not having any seaport. Data for biodiesel would be even worse as the hectare yield is about half that of ethanol and additional methanol is required for the transesterification. Here too, additional comments are superfluous, in particular on the supply of Switzerland with sugar.

In today’s Ecologistan, everything is deemed possible because it is desirable. The sheer size of the problem gets conveniently forgotten because positive thinking is demanded. Besides large nuclear-powered vessels, the fossil fuel substitution with alternative means will require a technological breakthrough for an efficient and affordable “power to synthetic liquid fuel” process, would it be for shipping, land, or air transport. This is not something that can be planned on music paper or agreed upon in treaties.

This article was sent to Marine Professional for publication in response to the original piece.