Introduction

In order to ensure the energy supply and reduce the environmental impact of the most advanced non-renewable energy sources, there is an urgent need for scalable, sustainable energy sources with high energy densities. According to the International Council on Clean Transportation (ICCT), the transportation industry must decarbonize as it is responsible for around one-fourth of all worldwide CO2 emissions. According to the ICCT, worldwide transportation-related CO2 equivalent emissions will increase between 2019 and 2022. 11% of these global transport emissions, which must be severely cut to address climate change-related problems, come from the marine industry. Since there are currently no infrastructures in place to store and distribute hydrogen, implementing a new fuel like hydrogen in the maritime industry will present a number of unique challenges. Ships will need to be retrofitted with hydrogen tanks, fuel cells, electric motors, or internal combustion engines that run on hydrogen.

Why Hydrogen

Of course, there are other alternative fuels besides hydrogen. Another is biofuels, which are fuels produced from plant or animal waste. However, despite their low capacity for sustainable production, they have a wide range of proposed uses in other industries. The majority of hydrogen is now produced using fossil fuels; in fact, 6% of global natural gas and 2% of coal are used for this purpose. Since it may be used in conjunction with renewable energies and help fulfil Green Deal objectives, hydrogen will play a significant part in the energy transition period. For instance, one of the repower Eu plan’s projects has set a target of 10 million tonnes of domestic renewable hydrogen production and 10 million tonnes of imports by 2030.Hydrogen storage for maritime applications is different and shows different challenges than hydrogen storage for stationary or automotive applications. Compared to hydrogen storage for stationary or automotive uses, hydrogen storage for maritime applications is unique and faces unique obstacles. The storage of hydrogen for maritime shipping is more difficult than these other situations, though. aboard the one hand, vast amounts of hydrogen are kept aboard an isolated ship, but in large-scale stationary storage, the hydrogen might be released from the carrier or kept in optimum storage conditions with the addition of external energy. On the other hand, compared to the automotive sectors, the storage on a ship is less constrained by weight and volume limitations, and more extreme temperatures can also be employed for hydrogen storage on a huge ship than in a much smaller passenger vehicle.

Colour coding

Hydrogen can be categorised as grey, blue, or green depending on the starting ingredients and the synthesis process. It is possible to produce grey hydrogen by reforming fossil fuels. More than 95% of the hydrogen used in the world is created by reforming fossil fuels, with natural or shale gas making up nearly half of that total. Hydrogen is coloured blue when carbon emissions are removed, stored, or utilised (such as during dry reforming). The use of a renewable feedstock and a renewable energy source for the conversion of raw materials and the operation of the processing plant are also examples of green hydrogen. The creation of grey and blue hydrogen has a significant negative impact on the environment, with annual global CO2 emissions of over 830 million tonnes CO2eq. On the other hand, a variety of renewable energy sources, including wind, solar, nuclear, hydropower, geothermal, and biomass, can be used to create green hydrogen.

Hydrogen bunkering
After it is produced, there are a number of ways to use hydrogen to power ships.

It can be burnt in an internal combustion engine, as Hydro Ville is currently doing. One downside to this is that burning anything in air, which consists largely of nitrogen, inevitably produces some level of nitrogen oxides – which are major air pollutants.

These emissions could be tackled by fitting some kind of after-treatment device, says Mao. But hydrogen can also be used in a fuel cell – a device which chemically converts the fuel into electricity without the need to burn it, and the only emission is water. The main challenges of making that work on a ship are just to make it big enough

But the real challenge for using it in long-distance shipping is how tricky it is to store. Hydrogen cannot simply replace bunkering fuel in the current system. To store it on board a ship as a liquid, it needs to be frozen using cryogenic temperatures of -253C (-423F),

Process of hydrogen bunkering

Bunkering facilities can have different configurations depending on how the fuel is transferred. There are four main types of bunkering:

• Truck to ship (TTS).
• Ship to ship (STS).
• Bunker station.
• Swappable containers.

This means that in the TTS, either a truck tanker (for liquid fuels) or a tube trailer (for gaseous fuels) is connected to the ship to fill its tanks. This bunkering configuration is more flexible and initial costs might be reduced compared to a bunker station since a fixed storage container is not required. A high level of flexibility can be reached also by the STS method. The ship can be filled even when not docked. Moreover, if the vessel is at the berth, the refuelling can occur on the side opposite to the shore. In this manner, there are no issues with simultaneous operations, and only the infrastructure necessary to fill the bunker vessel is required. On the other hand, costs cannot be lessened since the bunker ship must be built, operated, and maintained. The same conclusion can be drawn for the bunker station. In addition, significant flexibility can be attained with a permanent and fixed infrastructure. The main advantage of this solution is that large amounts of fuel can be stored at the harbour. Finally, swappable containers offer great flexibility and can reduce bunkering time. However, this solution is not feasible when large amounts of fuel are needed on board.

Lately, an increasing interest has been shown in hydrogen fuelling ships, as demonstrated by the handbook for hydrogen-fuelled vessels published by DNV in 2021. The main difference between LIQUID HYDROGEN and the other types of bunkering fuels is its extremely low temperature. Different phenomena may occur beyond the fast vaporization of hydrogen, and all the substances except helium will condense or even solidify in contact with liquid hydrogen. For these reasons, liquid hydrogen transferring equipment must be extremely well insulated. Furthermore, any air content in pipes and tanks must be removed by purging with helium. It should be noticed that the EIGA standard 06/19 foresees purging with nitrogen with the subsequent removal of this latter with cold gaseous hydrogen for liquid hydrogen transfer. Cooling down and warming up the equipment will avoid the formation of an unbearable thermal gradient with consequent mechanical stresses which might lead to the rupture of the hardware. DNV defined in a recent report three methods to transfer liquid hydrogen to the fuel tank of a ship:

• With a cryogenic pump.
• By pressure differential.
• A combination of the above.

The first method is used when large amounts of liquid hydrogen have to be transferred, as reported by Peschka already in 1992 Suitable materials for ultra-low temperatures must be employed to construct cryogenic pumps. The piston pump developed by Linde is designed to be immersed in liquid hydrogen. The liquid hydrogen has initially 3 bar and 24.6 K. The pump can vaporize it and increase its pressure up to 875 bar with a temperature range of 30–60 K. For this reason, cryogenic pumps are used in hydrogen refuelling stations with liquid hydrogen storage to fill hydrogen vehicles with 700-bar pressurized tanks. Moreover, the HRS footprint can be reduced since there is no need for refrigeration or high-pressure storage when cryogenic pumps are employed. However, the pressure should not be increased by two orders of magnitude when transferring liquid hydrogen between two storage containers, so centrifugal pumps are adopted in this case. If the cryogenic pump will not be included in the bunkering facility design, then the liquid hydrogen must be transferred by generating a pressure differential between the two tanks. In particular, the pressure of the supply tank must be increased using a vaporizer. This forces the evaporation of a fraction of the liquid hydrogen_, thus intentionally generating BOG. In this fashion, the tank pressure is built up and the transfer can occur. Both methods can theoretically be implemented in the TTS, STS, and bunker station configurations previously described.

The TTS configuration for liquid hydrogen would not only need the truck but also a very well insulated loading arm system (LAS), also called a bunker boom, installed on the quay. The LAS is a complex system composed of flexible liquid hydrogen hoses, fixed pipelines, valves, dry break couplings, and safety devices such as an emergency release system. Long flexible hoses as a replacement for the LAS are discouraged since these are affected by a large heat flux compared with the LAS. liquid hydrogen could also be offloaded in an intermediate double-walled insulated storage vessel to avoid refuelling from the road tanker. However, this solution recalls the permanent bunker station. Bunker rates for the TTS solution vary between 1000 and 4000 kg/h. According to Pratt and Klebanoff, considering the bunkering of 1000 kg of liquid hydrogen, the times for inserting and cooling down equipment (e.g., pipes, tank) before liquid hydrogen transfer, and for purging and warming up afterwards, are 40 and 30 min, respectively. The bunkering time of these operations could be reduced if innovative transfer procedures are considered such as the ones proposed for the aviation sector by Mangold et al. The same issues would be manifest for an STS configuration. This is a suitable option for liquid hydrogen bunkering since it provides flexibility.

DNV concluded that swappable containers should not be adopted for liquid hydrogen bunkering due to safety reasons such as the risk related to hoisting liquefied gas containers. Interestingly, in 1994, Petersen et al. considered mobile containers the best solution for shipping liquid hydrogen. They validated their choice by demonstrating that

• A high degree of redundancy can be achieved because a failure of the tank would not affect the carrier,
• Costs can be reduced by using the same tank for storage and transport of liquid hydrogen,
• Tank inspection and survey would be independent of the ship ones, and
• Loading arms
• High-rate liquid hydrogen pumps can be avoided.

The authors of the Zero-V study consulted the technical representatives of Linde and Air Products gas suppliers to verify the liquid hydrogen bunkering feasibility. They considered simultaneous bunkering of two road tankers to reduce the bunkering time. A total time of 9 h was calculated to fill the ship tanks with a capacity of 5840 kg each. The trucks should be hooked up to a dock stationary fueling stanchion that connects the trailers with the tanks of the ship.

Challenges

Environmental aspects must be always considered even when using hydrogen on board vessels. As demonstrated hydrogen has virtually no emissions when used to generate electricity on ships, but the environmental impacts of its entire lifecycle can be very different depending on how the hydrogen is produced and even transported. For complete decarbonization of the maritime sector, black (or brown), grey, blue, and yellow hydrogen must be avoided. Moreover, both scheduled (venting) and unintentional (leakages) releases of hydrogen into the atmosphere must be limited to prevent the intrinsic indirect GHG effect of hydrogen. Furthermore, the release of hydrogen can have consequences in terms of costs and safety since precious fuel is lost and flammable atmospheres are created during releases.

Many technical challenges are met with when storing Liquid hydrogen both in the harbours and on-board vessels due to the cryogenic nature of this substance. Extremely well-insulated (e.g., vacuumed MLI) storage devices (tanks, pipes, valves, etc.) Must be employed to keep liquid hydrogen in the liquid phase, i.e., at a temperature close to its boiling point. This means that appropriate tanks must be built to store liquid hydrogen both onshore (ports) and on-board ships. The current challenge is represented by the low availability of this type of components since few suppliers exist worldwide. Therefore, this low availability has a negative implication for the cost of these devices. Moreover, the cryogenic nature of liquid hydrogen affects the BOG formation with consequent venting. This process is needed to keep the pressure of the liquid hydrogen tank within a safety threshold and avoid its rupture. As previously mentioned, the release of a flammable gas has a negative influence on environment, costs, and safety.

Future Ahead

The optimal use of hydrogen still needs to be determined for the most suitable applications where lower cost or more efficient alternatives do not exist. A comprehensive assessment is needed of the interplay between hydrogen demands and electrification, evolutions of the energy grid (including in supply of clean baseload power, grid reliability, and rates of effective CCS), biofuels, and sectors that use hydrogen as a feedstock or fuel. A detailed regional approach, informed by the availability of resources and end-uses, and bolstered by the funding available for hydrogen hubs, will inform how best the hydrogen ecosystem can evolve to enable maximum benefit. All these challenges will need to be addressed in the most efficient, effective, and comprehensive manner.

The hydrogen market is still in its infancy, but the potential and momentum for market development have never been greater and we believe it has excellent possibilities to replace the current MGO. Because the price, the cost of the technology and the availability, Hydrogen can play a role in the future but not in the present. We need to take advantage of the resources we have now and use them. For example, biofuels are the present solutions to help to decarbonize the sector because they can be used directly in the current technology and it won’t require enormous investments

We can foresee three stages over the next 15 years for hydrogen-derived fuels in shipping: an increase in trials and first-of-a-kinds from now to 2025; an uptake of hydrogen by early movers in the industry by 2030; then a wider scale rolls out after 2030, as costs come down and the infrastructure for refuelling becomes more widespread. Already, shipping companies are talking about ordering “zero ready” ships that are ready to easily retrofit for ammonia.

The result for shipping emissions depends on a whole range of factors, from regulations to the uptake of other technologies. But there is a growing body of evidence of how fast the shipping sector could decarbonize if it set its mind to it, with one major report finding it could almost completely decarbonize by2035 using currently known technologies, including alternative green fuels such as hydrogen.

– Anuja Singh