Goods and people have been transported by sea throughout human
history. In the modern world, shipping remains the most efficient
method for moving cargo where speed is not a major concern. Large
quantities of material can be moved using relatively little fuel
compared to alternatives such as road haulage, trains, and
aeroplanes. Shipping makes up approximately 80% of the EU’s
imports and exports by volume, and the International Chamber of
Shipping estimates that 1.5 tons of cargo per person per year are
carried by sea worldwide.

Commercial shipping is almost entirely powered by fossil fuels.
As a consequence, maritime traffic is estimated to produce
approximately 3% of global carbon emissions. Significant work is
therefore being focussed on technologies that can reduce emissions
from the maritime sector as part of wider efforts to decarbonise
the world economy.

Wind power

For much of human history, ships have been propelled by the wind
using sails. Steam and internal combustion engines finally
displaced sail for commercial shipping in the 20^th
century due to their greater reliability. However, wind-powered
ships may be making a comeback.

The Flettner rotor, also known as a rotor sail, is a tall
cylinder with a flat end cap fixed upright onto a ship’s deck,
appearing much like a funnel. The cylinder is spun around its long
axis by an onboard power supply. Wind blowing past the rotating
cylinder produces a force on the rotor perpendicular to the wind
direction due to the Magnus effect. This force can be harnessed to
help propel the ship.

A Flettner rotor still requires an onboard power supply to spin
the rotor, but fuel efficiency can be greatly increased by reducing
load on the ship’s engines. In addition, the rotors can be
retrofitted to existing ships, avoiding the cost associated with
replacing a ship entirely.

Battery-electric ships

Current batteries store much less energy per unit volume
compared to fossil fuels and take longer to recharge than
refuelling a fossil-fuel vehicle. The increased downtime and
reduced range mean that current battery-electric vehicles cannot
compete with fossil fuels for long-distance commercial applications
such as road haulage, railways, and shipping.

However, battery-electric ships may still be suitable for
short-range travel or applications where ships are expected to have
relatively high downtime that could be used for recharging. A
prototype electric boat has already been tested in Belfast port as
a workboat for transporting pilots and crew to and from larger
ships in the port. It is hoped that similar boats could be used for
short-range ferry routes around the UK.

Alternative fuels

Just like other transport sectors, development in clean maritime
is also being focussed on engines that can run on alternative
fuels. A variety of different technologies are being investigated
including biofuels, hydrogen combustion, and hydrogen fuel
cells.

Ammonia has also been considered as a fuel source for shipping.
Similar to hydrogen, ammonia can be used in either an internal
combustion engine or in a fuel cell to drive an electric motor.
However, ammonia has a higher energy density than hydrogen and can
more easily be stored and transported in liquid form due to its
higher boiling point. This has advantages compared to hydrogen for
shipping, where range is a significant concern. In addition,
ammonia is already used as a fertilizer worldwide. This means that
infrastructure for storing and transporting ammonia already exists
in many ports around the world.

Despite these advantages, ammonia-fuelled shipping will only
produce carbon reductions if the ammonia itself is generated using
renewable energy, just as for other alternative fuels. The vast
majority of ammonia is currently produced from natural gas using
the highly energy-intensive Haber process, and so changes to the
supply chain will be needed as well for decarbonisation.

Green corridors

The international nature of much global shipping creates
additional difficulties with decarbonisation. Many novel shipping
technologies such as alternative fuels require new infrastructure
such as electric charging points or depots to store alternative
fuels. It is no use leaving your home port on a hydrogen-powered
ship if there is not guaranteed to be hydrogen for refuelling at
your destination port.

To facilitate uptake of cleaner maritime technologies, a number
of countries have started initiatives to form so-called “green
corridors”. These are specific shipping routes along which the
countries commit to promote the implementation of the supporting
infrastructure for low-carbon shipping technologies. At COP26 in
November 2021, 24 countries signed the Clydebank declaration
committing to establishing green corridors between their ports.

Conclusions

Shipping is an important sector that still has a long way to go
in terms of decarbonisation. An additional difficulty is that many
ships are registered in small jurisdictions with relatively little
regulation. This may make it more difficult to promote uptake of
green technologies. Encouragingly, the International Maritime
Organisation, a branch of the UN whose members include most
countries in the world, recently agreed unanimously to achieve net
zero shipping by or around 2050.

The exact pathway to this goal remains unclear, but nonetheless
the agreement represents an important step. Research and
development in the sector are only likely to accelerate in the
coming years as efforts to decarbonise the world economy
intensify.

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