ONBOARD CRYOGENIC STORAGE OF NEW FUELS MAY BE FEASIBLE


The University of Southampton is pursuing fundamental research into the feasibility of fully composite cryogenic tanks for marine applications, according to Charlie McKinlay (pictured). The University of Southampton is pursuing fundamental research into the feasibility of fully composite cryogenic tanks for marine applications, according to Charlie McKinlay (pictured).



Researchers at the University of Southampton have evaluated the potential for cryogenic storage of hydrogen and ammonia as bunker fuel and are now looking to composite tanks as the way forward

A study undertaken by Charlie McKinlay under the supervision of Professors Stephen Turnock and Dominic Hudson determined the energy requirements of a long-haul 135,000m3 capacity LNG carrier from over three years of voyage data. The maximum, 9,270MWh, and typical, 6,350MWh, were used as the basis for calculating the potential fuel storage requirements if new fuels were used instead. The researchers concluded that hydrogen and ammonia could be feasible to use for long distance shipping despite their relatively low volumetric density.

The study assessed the engineering challenges of installing storage tanks for new fuels onboard the tanker, replacing the fuel oil tank that is already onboard to supplement the LNG used as the primary fuel source. This tank holds enough fuel oil for even the longest voyage.

The volumetric density of ammonia is similar to that of LNG, and only a small increase in fuel tank size would be required. However, the mass of ammonia is greater than LNG and could increase the total mass of the vessel by over three percent. It would also likely require the use of NOx abatement equipment onboard. The mass of batteries also made them a less desirable option than hydrogen from the study’s perspective.

Storage of liquid hydrogen onboard the tanker would involve a maximum 242 percent increase in tank size compared to the fuel oil tank, which is 2,700m3 and currently takes up about two percent of the vessel’s capacity. For the more typical energy requirements, an increase of 66 percent would be sufficient, although space would also be required for cryogenic equipment. The required volume was 6,550m3 for liquid hydrogen storage and 11,040m3 for pressurised hydrogen gas storage.

“Hydrogen is frequently dismissed for mobile applications due to low volumetric density, however these volumes are not unrealistic,” state the researchers. However, McKinlay says that subsequent work has shown that pressurised hydrogen gas storage is a less attractive alternative to liquid hydrogen due to the weight of the storage tanks that would be required. As a pressurised gas, hydrogen can be stored at around 700 bar, but a steel casing is typically used, which could pose a significant weight penalty on a ship.

Onboard storage of hydrogen also poses engineering challenges as leaks are difficult to detect, and the gas is explosive. Even at low concentration, it is flammable in air. As a cryogenic liquid, a volumetric density equivalent to 800 bar can be achieved, but the energy required to maintain temperatures of around -250oC, increases total energy demand by 30 percent and storage costs five-fold.

The researchers believe that fully composite crygenic tanks could be developed for marine applications, and the University of Southampton is pursuing this in future work. Such tanks could represent a reduction in weight compared to existing storage tanks by 30 percent and be cheaper to manufacture. Composites offer good insulating properties and would eliminate the risk of corrosion and associated maintenance.

The university is planning research aimed at overcoming potential concerns about composites’ permeability, impact resistance, fire resistance and susceptibility to micro-cracks caused by fatigue. An inner hybrid composite layer could form a braided sleeve that is resistant to cracking over a wide temperature and pressure range. An outer lining could consist of a wound fibre-resin composite for stiffness and strength.

In partnership with Shell Shipping and Maritime, further work is planned to investigate potential composites and to assess the practicalities of manufacturing them. At this stage it is not certain whether or not this fundamental research will result in a method that is actually feasible, says Hudson.

However, success could have major implications as shipping aims for zero emissions by 2050. Hydrogen could be commercially available at a competitive price by 2025, and the development of electrolysis technology fuelled by renewable energy would make it an carbon neutral fuel. Additionally, despite it’s low volumetric pressure, hydrogen has a relatively high heating value which makes it feasible for use in internal combustion engines as well as feedstock for fuel cells.

Already around 50 billion kilograms of hydrogen is consumed worldwide each year, primarily by the fertiliser and oil refining industries. As demand for oil drops, this hydrogen could become available for other uses.  

Historically, there was a direct correlation between global demand for LNG and its increased use as a bunker fuel. Hydrogen could follow the same trajectory.

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