21.5 Renewable Options For Shipping

Introduction

Shipping is the backbone of global trade. Most goods cross oceans in vessels powered by heavy fuel oil, one of the most polluting fuels used anywhere. As climate policies tighten and customers demand cleaner supply chains, the shipping sector must cut greenhouse gas emissions, air pollutants, and its dependence on fossil fuels. This chapter focuses on renewable and low carbon energy options that can be used in shipping, and the specific opportunities and challenges they present.

Why Shipping Is Hard To Decarbonize

Ships travel long distances, carry heavy loads, and operate far from fuel stations and repair facilities for long periods. They need dense, reliable energy and simple, robust systems. Any renewable option must respect strict safety rules, fit within limited space on board, and be available in ports around the world. These conditions make shipping harder to decarbonize than many land transport modes and explain why a mix of solutions will likely be needed.

Efficiency First, Then Fuel Switching

Before considering fuels, it is important to recognize that cutting energy demand is often the fastest way to reduce emissions from ships. More efficient hull designs, better propellers, air lubrication, improved logistics, and slower steaming all reduce fuel use. Once efficiency options are applied, renewable fuels and auxiliary systems can target the remaining energy demand more cost effectively.

Biofuels For Shipping

Biofuels are currently among the most practical renewable options to reduce emissions from existing ships because many of them can be used as drop in fuels with only limited modifications to engines and fuel systems.

Conventional biofuels such as fatty acid methyl ester biodiesel and bioethanol are already blended into road fuels in many countries. For shipping, more advanced biofuels are of greater interest. Examples include hydrotreated vegetable oil and other hydrotreated biooils that can resemble marine diesel, and biofuels derived from lignocellulosic biomass or waste streams.

A key attraction of marine biofuels is compatibility with existing engines and infrastructure, especially when fuels meet standards that allow them to substitute for marine gas oil or very low sulfur fuel oil. However, their sustainability depends strongly on the source of biomass and land use. When produced from wastes and residues, biofuels can offer substantial life cycle emission reductions relative to fossil marine fuels. When produced from crops grown on land that could otherwise support food or high value ecosystems, the climate and biodiversity benefits can be much lower and may even be negative.

For long term large scale use, the availability and cost of sustainable biomass are critical limits. Shipping competes with aviation, heavy industry, and power generation for the same resource. Therefore, even though biofuels can play an important role, especially in the near and medium term, they are not expected to be the only solution for decarbonizing shipping.

Renewable Hydrogen And Ammonia

Hydrogen produced from renewable electricity and water is a key building block for several future marine fuels. In shipping, it can be used directly or converted into hydrogen based fuels such as ammonia. These options are important because, in principle, they can provide very low life cycle emissions if both hydrogen production and related processes are powered by renewables.

Using hydrogen directly as a ship fuel faces several technical constraints. Hydrogen has a high energy content per kilogram but a very low energy content per cubic meter at ambient conditions. To be stored on ships, it must be compressed or liquefied, both of which require energy and specialized equipment. Compressed hydrogen at high pressure uses large storage volumes, which can reduce cargo space on board. Liquefied hydrogen is much denser but needs storage at very low temperatures, which increases system complexity. These issues are especially challenging for long ocean voyages that demand large amounts of energy.

Fuel cells can convert hydrogen into electricity with high efficiency and low local emissions, and can drive electric propulsion systems. For short sea shipping and ferries with relatively predictable routes and frequent access to refueling points, hydrogen and fuel cells are already being demonstrated. However, the global infrastructure to supply green hydrogen to major ports is still at an early stage of development.

Ammonia is another hydrogen derived fuel that is attracting attention. It contains no carbon and can be synthesized from renewable hydrogen and nitrogen from air, using renewable electricity. Its volumetric energy density is higher than that of compressed hydrogen, and it is already traded worldwide as a chemical and fertilizer, which provides some existing infrastructure and handling experience.

Marine engines can be adapted to burn ammonia alone or in combination with other fuels. The main advantage is the potential to operate with near zero carbon emissions if the ammonia is produced from green hydrogen. However, ammonia introduces new challenges. It is toxic to humans and marine life, so strict safety measures are needed to prevent leaks and accidental releases. When combusted, ammonia can produce nitrogen oxides, so specific combustion strategies and exhaust after treatment are required to control air pollution. Unburned ammonia slip from engines also needs to be minimized because ammonia is harmful and contributes to indirect climate effects.

For both hydrogen and ammonia, the cost and availability of green molecules are closely linked to the pace of renewable power deployment and electrolysis capacity. Large scale bunkering facilities, updated port safety regulations, and crew training will be needed before these fuels can be widely adopted in international shipping.

Renewable Methanol And Synthetic Fuels

Methanol has emerged as another promising alternative fuel for shipping. It is a liquid at ambient conditions, easier to store and handle than many other alternative fuels, and has been used in industry for decades. When produced from renewable sources, methanol can provide significant greenhouse gas reductions.

There are two broad types of low carbon methanol for shipping. Biomethanol is produced from biomass or biogenic waste gases. E methanol is produced synthetically from green hydrogen and captured carbon dioxide, using renewable electricity. Both can be blended with, or in some cases fully replace, conventional marine fuels in specially designed engines.

Because methanol is less energy dense than conventional marine fuels, ships need larger storage tanks or more frequent refueling to travel the same distance. Its lower flash point means different safety rules than conventional fuels and requires adapted fuel handling systems. However, compared to gases or cryogenic liquids, liquid methanol is relatively simple to integrate in ship designs and port infrastructure.

Synthetic diesel and kerosene derived from renewable hydrogen and captured carbon dioxide, sometimes called e diesel or e fuels, are another class of renewable options. When they are produced with fully renewable energy and genuine additional carbon dioxide removal or recycling, their combustion can be close to carbon neutral on a life cycle basis. The main advantage of these e fuels is compatibility. They can often be used in existing diesel engines and existing fuel distribution systems with minimal changes.

The main constraint for e fuels is cost. Producing them requires large amounts of renewable electricity for hydrogen generation and for the synthesis process. There are also energy losses at every step. For these reasons, such fuels are currently much more expensive than fossil marine fuels, although costs are expected to fall as technologies scale and renewable electricity becomes cheaper.

Onboard Renewable Power: Wind And Solar

Apart from renewable fuels, ships can also harvest renewable energy directly while operating. The most traditional option is wind power. Modern wind assisted propulsion does not mean simply returning to old sailing ships. Instead, it involves technologies such as rotor sails, wing sails, kites, and other devices that use aerodynamic principles to generate thrust and reduce engine load.

Wind assistance can significantly cut fuel consumption on many routes, especially where consistent winds are available along major trade corridors. These systems usually do not fully replace engines but act as an auxiliary power source that reduces the amount of fuel burned. Retrofitting existing ships with wind assisted technologies is an active area of innovation and commercial deployment.

Solar photovoltaic panels can also be installed on ship decks and superstructures. The power they provide is relatively small compared to total propulsion needs of large cargo ships, but it can cover part of the electrical demand for onboard systems such as lighting, navigation, and auxiliary machinery. For smaller vessels and ferries, especially those that spend time in port or operate short distances, solar combined with batteries can make a more significant contribution.

Both onboard wind and solar power help reduce fuel use and emissions without needing new types of fuel. However, their output depends on weather and available surface area, and they do not solve the entire decarbonization challenge for large deep sea ships. They are better seen as important elements in a broader portfolio of efficiency and fuel solutions.

Hybrid Ships And Electric Shipping

For certain shipping segments, particularly short sea routes, inland waterways, and some ferries, full or partial electrification is a practical renewable option. Battery electric ships store electricity, ideally from renewable sources on land, and use it to drive electric propulsion systems. Hybrid ships can combine batteries with internal combustion engines that run on renewable or low carbon fuels.

Battery electric ferries are already in commercial use on fixed, relatively short routes, where they can recharge at terminals equipped with high power chargers. This configuration suits frequent, predictable operations and allows the use of very clean energy if electricity grids are decarbonized. Onboard batteries also support regenerative functions, such as capturing energy during braking or maneuvering, and can improve operational flexibility.

The main limits of full electric propulsion are energy density and weight. Even with modern lithium ion technologies, batteries store much less energy per kilogram than liquid fossil fuels. For long distance deep sea shipping, the size and weight of batteries required would consume too much valuable cargo space and would increase ship weight excessively. However, for shorter routes, tugs, harbor craft, and certain coastal operations, electric and hybrid propulsion is already viable and expanding.

Hybrid systems also allow optimization of engine loading, which can improve fuel efficiency and reduce emissions. Batteries can cover peak power demands during maneuvers or port operations, allowing engines to run at more efficient steady loads. If the primary fuel used in the engines is renewable, such as sustainable biofuel or renewable methanol, the overall system can significantly reduce life cycle emissions.

Infrastructure, Standards, And Safety

Renewable shipping options are not only technical questions on board ships. They require extensive changes on land and at sea. New fuels need production plants, storage terminals, bunkering facilities, and safe handling procedures. Ports must invest in infrastructure for alternative fuels, such as ammonia, methanol, renewable hydrogen, and high power electricity connections for shore power and battery charging.

Standardization plays a key role in enabling global adoption. Ships operate across jurisdictions, so common technical standards, fuel specifications, and safety regulations are essential for interoperability. International organizations, particularly those focused on shipping, develop and update codes that cover ship design, fuel systems, crew training, and emergency response for new fuels.

Safety is central when alternative fuels are introduced. Many renewable fuels have different hazards from conventional fuels. Hydrogen is highly flammable and diffuses quickly, ammonia is toxic and corrosive, methanol is toxic and burns with an almost invisible flame. Battery systems introduce risks of thermal runaway if not designed, operated, and monitored correctly. Addressing these risks requires careful design, robust operational procedures, and dedicated crew training.

Comparing Options Across Shipping Segments

Different parts of the shipping sector will likely adopt different combinations of renewable options. Deep sea container ships, bulk carriers, and tankers, which travel long distances between continents, need fuels with high energy density and global availability. For them, renewable ammonia, methanol, or advanced biofuels, combined with efficiency measures and possibly wind assistance, are often seen as leading options.

Short sea shipping, ferries, and inland waterway vessels have more flexibility. The shorter routes and predictable schedules make electric or hybrid propulsion, green hydrogen, or renewable methanol feasible. These vessels can refuel or recharge at the same ports frequently, which simplifies infrastructure needs.

Specialized vessels such as offshore support ships, tugs, and harbor craft can often rely on local infrastructure in a few ports. This makes them attractive early candidates for demonstration of new fuels and technologies, because the supply challenge is more limited and controlled.

The choice of renewable option in each case depends on multiple criteria including total cost over the vessel lifetime, fuel availability, ship design flexibility, safety, regulatory compliance, and alignment with decarbonization targets. Many operators are also concerned with the risk of investing in a fuel pathway that may later become less favored by policy or markets. For this reason, fuel flexible engines and modular designs that allow changes over time are gaining interest.

Looking Ahead

Transitioning shipping to renewable energy is a long term process. Existing ships have lifetimes of several decades, so large parts of the current fleet will still be in operation for many years. Retrofitting measures, such as wind assisted systems, efficiency upgrades, and partial fuel switching with biofuels or renewable blends, help to reduce emissions in the near term.

At the same time, new ship designs that can use low carbon and renewable fuels at scale will need to be ordered and built. The timing of these investments must align with the deployment of fuel production and port infrastructure so that vessels can find reliable supplies along their routes.

Collectively, the renewable options for shipping, from biofuels and advanced synthetic fuels to hydrogen, ammonia, methanol, and onboard renewable energy, provide a diverse toolkit. No single solution is universally optimal. Instead, different combinations will be chosen for different ship types and routes, guided by technology progress, fuel costs, international regulation, and the global pace of renewable energy deployment.