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Can fuel cells powered by LNG be used for propulsion?
By Wendy Laursen
Two technology development streams are converging toward a point that makes fuel cells attractive as a power source for marine propulsion. They are the growing adoption of hybrid-electric powertrain systems and the projected growth in the number of vessels burning LNG as fuel.
Professor Zuomin Dong of the University of Victoria, Canada, has led multi-million dollar research programs on hybrid-electric and fuel cells for ground vehicles and, most recently, for green ship propulsion and power system development. This encompasses the development of various advanced plug-in hybrid-electric vehicles, low-speed fuel cell vehicles, and the system design of a hybrid propulsion system involving diesel generators, hydrogen fuel cells and a plug-in battery system for a medium-sized research vessel.
Energy Conversion Factor
“The energy conversion efficiency of internal combustion engine vehicles on the market is low, probably less than 20 percent,” says Dong. “The ideal internal combustion cycle efficiency is around 40 percent, but issues such as heat loss reduce it to around 20 percent no matter how well you try. A fuel cell is a different story. It does not use a combustion process, rather an electro-chemical process. The theoretical efficiency is about 60 percent and the practical efficiency around 40 percent, twice that of a diesel engine.”
LNG is an important future fuel source for fuel cells because it is more readily available than hydrogen. There is already a certification process for installing LNG technology onboard ships with certified fuel transportation and storage products. The growing demand for dual-fuel and LNG engines for vessels means that the trading fleet will become increasingly familiar with the technology and more will be carrying LNG as fuel.
“Ten or twenty years from now, you will for sure see some of the larger marine vessels operate on fuel cells. The reason is that, today, there is a lot of interest in burning LNG as a fuel. So in a few years’ time people will know how to effectively carry LNG as a fuel. And if the fuel is there, you’re better able to change to a power plant twice as efficient, which is a fuel cell.”
More Research Needed
While the development of sodium-oxide fuel cells that can take LNG directly as fuel still needs work, there is another propulsion trend putting the other building blocks in place for fuel cell propulsion. The use of diesel-electric propulsion with battery energy storage systems is growing as the integrated system can provide greater efficiency and fewer emissions than a purely diesel-propulsion system due to the ability of the diesel engine to operate at its optimal speed. This is more efficient than variable speed operation, and it is a cleaner option for time spent in port.
“Very soon you will see more electric propulsion systems with a diesel generator because electric motors are continually being improved,” says Dong. “Today it is difficult to buy large battery energy storage systems, but they will be available soon. There are at least six major assembly plants making them available and affordable in larger quantities in North America,” he adds.
Fuel cells, like diesel engines, are designed to operate at a specific power output so it is the combination of appropriately sized batteries that expands their suitability for a range of vessel types. Toyota’s electric car, the Prius, runs on a battery with around 2kWh of energy storage. An extended-range vehicle might have a 20kWh battery, but a ship would require a 200kWh battery system if it were to be used for propulsion. Therefore suitable battery technology must be readily available if fuel cells are going to fill more than just a small niche in the industry.
From LNG to LNG Fuel Cells
Dong sees a transition period of 10 to 20 years where fuel cells are used as auxiliary generators to improve the efficiency of diesel or diesel-electric systems. “I think for a while you will have the two systems co-exist, but I would say that the importance of the diesel generator will gradually be diminished.” Beyond that, LNG-fueled engines could become the stepping stone for the wide adoption of LNG-fueled fuel cells as prime movers. – MarEx
The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.