The Naval Architect: November 2019
Marine fuel cells have finally been accepted as a reality. Questions of if they’re possible have dissolved under the weight of mounting research and evidence; now it’s more a matter of when and how we will begin to see their widespread introduction into the maritime industry.
Proton exchange membrane (PEM) fuel cells, which convert hydrogen and oxygen into electrical energy for power generation, have stood out as the most popular choice for the shipping industry. When powered by renewably generated hydrogen, the technology is cast as the ideal solution for an emissions free future. But pilot projects are difficult to get off the ground and one major technical roadblock is still under debate: What is the best method of storing hydrogen – liquified or compressed?
The problem with using compressed hydrogen is that you need a huge volume in order to travel long distances, says Dr Joachim Hoffmann, fuel cell expert and product manager at Siemens, which has been researching and developing fuel cells for over three decades. Liquified hydrogen, meanwhile, must be stored at extremely low temperatures and is liable to losing some of its hydrogen in the form of boil-off gas.
Siemens has partnered with Germany-based Hydrogenious, who are exploring a newer, promising method of hydrogen storage – Liquid Organic Hydrogen Carriers (LOHC). The concept uses thermal oil, which has a high storage density, to transport the hydrogen. Through a chemical process, the hydrogen is bonded to the organic oil. There is no need for refrigeration or pressurisation, meaning one of the biggest advantages – in terms of cost saving and ease of construction – is that you can simply use an ordinary diesel tank for storage.
Hoffmann explains the oil is typically used in industry for heat storage of temperatures up to 300-400°C, so its risk of ignition or explosion is low. “This makes it interesting as a storage system for hydrogen because pure hydrogen is always risky if it gets mixed with air.”
To dehydrogenate the liquid for use in the fuel cell, it must first pass through a reactor. Once the hydrogen is released, it is cleaned from any leftover by-products of the LOHC and then enters the fuel cell. Therefore, “the only hydrogen in the system is between the reactor and the fuel cell. That’s a tube system of around 5-10m, nothing more,” says Hoffmann, effectively making the potential danger quiet low. The liquid is also recyclable, as it can be re-charged as often as required at an on-land site with more hydrogen.
Although the system’s risks are low in comparison to other storage concepts, LOHC lacks in efficiency. “You’ll consume some hydrogen when producing the heat that is required to release the hydrogen from the oil,” says Hoffman. This disadvantage can be viewed as a necessary comprise, however, if hydrogen fuel cell powered vessels are to meet potential safety requirements.
