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The shipping industry is looking at ways to halve its greenhouse gas (GHG) emissions in line with the International Maritime Organisation’s (IMO) target for 2050. Batteries and marine fuel cells have shown promise on smaller vessels but the flexible approach needed to use these technologies as a power source for larger, oceangoing ships is only now beginning to emerge.

Through its extensive research, Foreship has helped to determine the benefits and limitations of batteries, or ‘energy storage systems’ (ESS). ESS are ideal for shortsea routes and for fulfilling peak power needs, and cruise ship owners have adopted them for deployment in emission-control areas. The technology is experiencing rapid improvements in energy density and price per kilowatt-hour but, used in isolation, is not capable of propelling large ships over long distances.

Batteries based on lithium-nickel-manganese-cobalt-oxide (NMC) chemistry are an affordable, tried-and-tested form of ESS, and their market dominance makes it difficult for other chemistries/technologies to gain momentum and economies of scale. Lithium-titanite-oxide (LTO), for example, outperforms NMC at higher power and is better suited to high cycle life applications. Unfortunately, its progress has been hindered by pricing: it costs more than double the comparable NMC ESS.

A newer alternative chemistry is a hybrid of LTO and lithium-manganese-oxide (LMO), which offers a greater cycle life than LTO at a similar cost to NMC. However, LMO is not expected to be commercially viable for six to eight years. For now, then, NMC remains the favoured form of ESS.

FUEL CELL OPTIONS

The high electrical efficiency of fuel cell technology makes it a promising means of propulsion for a broad spectrum of vessel types. Research into fuel cell systems with an output of 3 MW is already being conducted, while several installations are planned in the 100-kW range. Foreship anticipates that fuel cells generating stable loads will be an accepted method of marine propulsion within a decade – perhaps supported by ESS for peak loads.

The more mature and power-dense form of fuel cells are based on polymer electrolyte membrane (PEM) technology. Solid-oxide fuel cells (SOFC) are more expensive and offer lower power density but can deliver electrical efficiency of up to 65 to 70 per cent – at least 20 per cent higher than that of PEM. In addition to its lower efficiency, PEM is highly sensitive to impurities in the hydrogen it uses as fuel; if other fuels are utilised, it requires a complex water management system. Nevertheless, it remains the fuel cell technology of choice for the time being.

CALCULATING POWER

Foreship’s statistics show that there are 283 ships in the entire merchant fleet with a gross tonnage of 3,500 or less, for which electric propulsion appears feasible. The vast majority of these are coastal cargo vessels, while a small number are container feeders that operate over short distances.

For the purposes of illustration, Foreship developed an analysis of a notional coastal cargo ship trading on an undisclosed route with a normal service speed of 12 knots, requiring 1,450 kW of propulsion power with a service load during transit of 120 kW.

A turnaround time of 168 hours means 52 return trips per year, with the vessel operating on a clear and simple route. Using this baseline concept, the ship’s weekly energy consumption is 156 mW-hours (mWh), corresponding to 34 tonnes of marine gas oil (MGO). With the price of MGO at $680 per tonne as of June 2019, the annual fuel bill is $1.22m. Using the IMO carbon factor, 3.206 for MGO, yearly carbon dioxide emissions are 5,734 tonnes.

For comparison, Foreship developed a separate analysis of the same ship using battery power in sensitive areas. In this case, weekly energy consumption is 147.4 mWh from onboard production, including a 2.5 per cent saving from peak-load shaving and 5.7 mWh from electrical shore power. Weekly onboard energy consumption corresponds to 32.4 tonnes of MGO fuel, resulting in a yearly fuel bill of $1.15m. Shore electricity with a kWh price of $0.1 adds $30,000 to this figure, yielding a total energy cost of $1.18m per year. Annual CO₂ emissions are 5,405 tonnes.

With fuel cells powered by sustainably sourced hydrogen, weekly energy consumption is 163 mWh, corresponding to 11,993 kg of liquid hydrogen. At the higher end of its price range, liquid hydrogen costs $8.30/kg, meaning the yearly fuel bill is $5.18m. Carbon dioxide emissions are, though, zero.

COMPARE THE MARKET

In comparison to the baseline, powering the ship with an ESS is moderately cheaper and reduces yearly carbon dioxideemissions by 9.6 per cent. Fuel cell technology using hydrogen from renewable sources cuts emissions entirely but costs around four times more than the baseline. When the hydrogen for the fuel cells is produced with natural gas, yearly cost comes down by roughly half compared to when sustainably sourced hydrogen is used; emissions are greater, but still 25 per cent lower than the baseline.

Our investigation demonstrates that if all 283 merchant ships below 3,500 gt were to install ESS, a total of 164,239 tonnes of carbon dioxide emissions would be saved. Were they to implement fuel cell systems using hydrogen captured from renewables, that figure would jump to 1,693,921 tonnes, equating to 0.18 per cent of global CO₂ emissions from shipping on an annualised basis.

Commercial factors represent an obstacle to the industry-wide adoption of electric propulsion technologies, particularly fuel cell systems. Consequently, we will likely see an increase in the use of hybrid propulsion systems in the coming years. However, as alternative energy sources mature and benefit from further research and development, cost will gradually come down – and the IMO’s 2050 target will look increasingly achievable.

*Jan-Erik Räsänen is head of new technology at Foreship, an independent, employee-owned naval architect and engineering firm based in Helsinki, Finland. More details on the company’s work can be found at www.foreship.com.

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