Ammonia SOFC Feasible For Deepsea Container Ships

A research team at Nanyang Technological University has evaluated various propulsion options for a deepsea container ship, including one based on an SOFC fuel cell with integrated ammonia cracker, and concluded the SOFC-based system has a comparable cumulative cost to that of LNG-fuelled or HFO-fuelled combustion engines when projected over a period of 20 years.

PhD student Shengwei Wu, who is affiliated to Maritime Energy and Sustainable Development – Centre of Excellence, conducted the study under the supervision of Dr SH Chan, Professor in the School of Mechanical & Aerospace Engineering, and Dr Bin Miao, research fellow at Energy Research Institute @ NTU (ERIAN). The vessel chosen was a 2018-built 20,000TEU container ship with a conventional 2-stroke engine generating 56,000kW output power and trading between ports in Europe and the Far East Area.

The researchers estimated CAPEX (including the propulsion system and fuel tank), OPEX, annual fuel consumption, projected fuel costs, and CO2 emissions and compared them to the same vessel powered by an LNG-fuelled combustion engine, an ammonia-fuelled combustion engine and an ammonia SOFC. The financial accounting also included costs that would result from the EU ETS scheme and an industry-wide carbon levy.

SOFC are high-temperature fuel cells, and their high-temperature operation can be used for to crack ammonia to produce hydrogen as feedstock. This maximises waste heat recovery. Cracking is required because SOFC using ammonia directly as feedstock are not yet commercially available, and the researchers predict that their efficiency would be lower than hydrogen SOFCs.

SOFC have been demonstrated to have high efficiency and availability. They are noiseless, have low maintenance needs and are both dependable and durable. It has been proven that the latest technology of the SOFC system is able to achieve a system efficiency rate of 60% and a total combined heat and power efficiency of more than 90%, compared to 50% and 53% in four-stroke internal combustion engines and two-stroke internal combustion engines respectively, state the researchers.

However, SOFC is highly recommended to be integrated into a battery hybrid system to ensure that power is available to meet a vessel’s dynamic demand, as they are relatively slow on start-up and load changes.

The analysis included energy demands based on historical voyage data, and the calorific value of each of the fuels was factored in. Although SOFCs have been demonstrated to have a lifetime of over 10 years, the researchers based their calculations on five years with a 25% increased rate of lifetime and a 42% decreased rate of cost for each replacement over the 20 years. The combustion engine calculations were based on a 20-year lifetime.

Yearly fuel consumption was estimated to be: 22,062,000kg for an LNG combustion engine, 68,578,000kg for an ammonia combustion engine and 50,291,000kg for an ammonia SOFC. The cumulative cost of the various propulsion systems over the 20-year period was analysed, and a blue ammonia fuelled SOFC propulsion system was estimated to be 6.8% cheaper than an HFO fuelled combustion engine-based propulsion system and 5.2% more expensive than an LNG-fuelled system with consideration of price and carbon factors. A green ammonia fuelled SOFC propulsion system was estimated to cost 17.5% more than an HFO propulsion system and 32.5% more than an LNG propulsion system.

While green ammonia was the most expensive option, the researchers indicate that application of a carbon tax would make it cost competitive with LNG and HFO fuel prices in long run. IMO regulations such as the Carbon Intensity Index (CII) would also adversely impact the suitability of these fossil fuels. The initial CAPEX of an ammonia SOFC and ammonia combustion engine was less than that of an LNG combustion engine, largely due to the need for cryogenic storage tanks for the LNG option. Although the CAPEX of an ammonia SOFC was estimated to be more expensive than an ammonia-fuelled combustion engine, the overall accumulative cost of the SOFC was cheaper due to its higher system efficiency.

The low volumetric power density of an ammonia SOFC could make it a challenge to fit the system into the existing machinery compartment without compromising cargo space, warn the researchers. “At the current stage, the technology bottleneck of ammonia cracker integrated SOFC such as volumetric power density, lifespan, and NOx emission would still be the main challenges for the industry to commercially adopt the ammonia fuel cell technology.” They therefore conclude that further research is needed to extend the lifetime of the technology.

The study was published in the International Journal of Hydrogen Energy.