From the previous post, I’ve started the investigation for using hydrogen as fuel, looking initially at its production methods. Next it is imperative to examine the actual process of this energy storage method. I will try to capture all facets from bunkering, storing on board the ship till it reaches consumption. As in this context I will refer to the energy density, I mention at the very beginning that I am referring to the energy stored in a system per unit volume.
Hydrogen is a very flammable gas, unique in ignitability at a very broad range of concentration levels, from as low as 4% till up to 77%. Moreover its flame velocity can reach up to 346cm/s, compared to a mere 43cm/s of methane for example. Luckily it auto-ignites at 585°C and it requires a minimum 0.02MJ energy to ignite, in other words it is not something that can happen so easily.
Going back to the system and its format, presently two forms of gas are developed for vehicles, the compressed gas and the liquefied gas, as hydrogen in its natural format takes too much space. In order to better visualise the concept, it is estimated that a car in order to achieve a range of a 100km would need a tank of 11m3. Even compressed 1ton of H2 would have 6x the volume of 1ton of diesel.
Most people are aware of hydrogen being used in fuel cells, which is basically compressed gas stored in specially designed and manufactured tanks, made of carbon. For cars it is presently pressured at 700bar, as we require the minimum possible volume in order to fit in the size of a car and it will only become financially viable when the required pressure drops at 100bar. That’s the case for two main reasons. One, compressed hydrogen has very low energy density, lower even to the one of liquefied H2 and there is inverse linear correlation between pressure and energy density, the higher the pressure, the lower the energy density. Two, in order to compress it to this level, extra energy has to be consumed. Obviously ships are less restricted in space and the present volume of the fuel tanks can be shifted to hydrogen, permitting a much lesser pressure than that of 700bar. Nonetheless the engineering of the tanks and the rest of the piping system will have to meet the requirements for hydrogen. Tanks will necessitate specific shape and materials as hydrogen has also very high diffusibility that can cause a metal to be brittle. For the piping, austenitic steel is to be used or metal coated with diffusion barrier layers. In terms that accountants understand, the infrastructure bares a very high price. And all the aforementioned when the fuel cells already introduced in marine industry can’t live up to the range given from batteries.
Another option heavily researched is the liquefied hydrogen, as then the present infrastructure on a ship that is using LNG could seamlessly change into liquefied hydrogen. The global commercial fleet, including cruise ships, have turn to LNG as a friendlier option to the environment. Shipowners and operators are familiar with the particularities of liquid gas running such ships. Yet hydrogen is liquefied at -243°C, while LNG at -162°C, there are only two plants for this liquefaction in Europe and no bunkering facility. Furthermore the tanks holding the LNG will have to be removed and new ones fitted need to meet the properties of the H2, for which the proven insulation for this low temperatures alone will bring the price for these tanks at very high levels. If you top to the aforementioned the low energy density (although higher than that of compressed gas as previously mentioned) and the loss of energy during the liquefaction, it is easy to understand that there is some work ahead of us till materialisation.
There is also the possibility to stored ammonia onboard and produce hydrogen from a plant in situ, consuming it immediately after, but then again is ammonia environmentally friendlier?
In research base there is a third, very intriguing option, storing the hydrogen in metal hydrides, through a solid chemical reaction. It is interesting to mentioned that for every ton of H2, we would have about 20ton of metal. For a 50m yacht that would not be acceptable but for a larger ship it might even improve the stability as it can bring the vertical centre of gravity lower. Presently it is not effective enough, the bond of the hydrogen is not strong enough and heat is needed to release the hydrogen from the metal, namely another energy loss. But it is matter of funding and development, as long as there is interest and profit at the end, it will happen.
As a summation of the aforementioned, the industry is experimenting with four options to integrated hydrogen in the energy storage plan for ships.
- Compressed hydrogen, proven from the space industry in fuel cells. The system has a high price and presently low range, yet it can and will progress as there is a market for it;
- Liquefied hydrogen, which is supported from the experience stemming from the present fleet of LNG ships. It also has a very high price and several open aspects to resolve, but the range would be sufficient and shift easier for cruise ships;
- Producing hydrogen onboard, which needs space and is sourced from fossil products;
- Metal hydrides that we look forward to hear more about in the near future, as presently it only exist in laboratories;