This article was first published in the Spring/Summer 2019 issue of Spring/Summer 2019 issue of International Cruise & Ferry Review. All information was correct at the time of printing, but may since have changed.
The International Maritime Organization’s (IMO) 2020 cap on sulphur content in marine fuel is now close enough to be a concern for marine fuel purchasers and ship engineers. To ensure compliance, they must offset their high-sulphur fuel oil with an exhaust gas scrubbing system, or they must switch to low-sulphur fuel oils, marine gas oil (MGO), LNG, methanol or another alternative.
However, the IMO’s broader commitment to halve the shipping industry’s carbon dioxide emissions by 2050 looks to be beyond the reach of even the most energy-efficient combustion engine – regardless of whether it is supplemented by carbon capture. Assuming that there are also limits to carbon offsetting, ship operators will have to rely on supplementary, or even replacement, ship propulsion technologies to meet this target.
Multiple ship operators have already started projects involving battery power, which have made the potential and limitations of energy storage systems (ESS) clear. ESS is beneficial on short sea routes, or for peak power needs. Cruise ship owners crossing sensitive waters have also been early adopters of ESS technology but, despite rapid increases in energy density and falls in the price per kilowatt hour, using batteries alone to propel a large ship over long distances is still unfeasible.
The same is not true of less mature fuel cell technology, which converts hydrogen-rich fuel into electrical and thermal energy via electrochemical oxidation. The direct nature of this conversion achieves high electrical efficiency and has broad potential as a ship propulsion technology. It also offers operators the promise of meeting the IMO’s carbon dioxide-busting target – although owners should not consider investment based purely on short-term returns.
Research work on marine fuel cell systems with output of up to three megawatts (4,000 horsepower) is already underway, while several sea-going installations are planned in the hundreds of kilowatts range covering commuter ferries and research vessels. If not commonplace, fuel cells generating stable loads are expected to be part of accepted marine propulsion technology within a decade, perhaps supported by ESS for peak loads.
Certainly, Foreship has recently seen rapidly accelerating interest from owners seeking guidance on the way fuel cells can work in parallel with combustion engines to improve fuel efficiency and reduce emissions. In light of our investment in expertise and consultancy services in this area, Foreship has been directly involved with some far-reaching feasibility studies into the use of fuel cell technology onboard ships on behalf of different owners.
Two fuel cell technologies are maturing in the marine context: polymer electrolyte membrane (PEM) technology and their solid oxide fuel cell (SOFC) counterparts. To date, the PEM technology used in the automotive industry has led the way, where its relative maturity has brought lower pricing and higher power density than SOFCs. PEM fuel cells use platinum-based electrodes and a humidified polymer membrane as the electrolyte. The SOFC electrolyte is a porous ceramic material, while the anode is a nickel alloy and the cathode is normally made of lanthanum strontium manganite.
However, some of the advantages claimed for PEM fuel cells may not be realised in the marine context. For example, where hydrogen is the primary fuel, the PEM is highly sensitive to impurities. Hydrogen has a boiling point of -253˚C at 1 bar and would need to be stored on a ship as a cryogenic liquid, a compressed gas or would need to be chemically bound. Using liquid hydrogen (LH2) for a PEM fuel cell to drive a ship would require roughly four times more fuel space than for MGO. Plus, as it stands, no supporting infrastructure exists to bunker hydrogen.
Today, almost all commercially-produced hydrogen is derived from natural gas or coal gas, with independent classification society DNV GL recently commenting that the levels of carbon dioxide created in the refining process means that its well-to-propeller carbon dioxide emissions are actually higher than those from heavy fuel oil. By comparison, hydrogen produced using renewable electricity for water electrolysis is almost emission-free, but it’s very expensive.
One solution is to use fuel reformers to convert an original fuel, such as natural gas, methanol or low-flashpoint diesel, into hydrogen-rich fuel. For example, an LNG bunkering network has been developed and, unlike the internal combustion engine, a fuel cell using LNG would not experience methane slip and would therefore achieve significantly smaller greenhouse gas emissions compared to internal combustion engines. However, while LNG can be reformed in the case of the SOFC, an external reformer is needed in the case of the PEM solution. This would take up space and also require complex and potentially short-lived water management systems.
In the case of PEM, the best alternative primary fuel to hydrogen is methanol. Not only is methanol less mature as a marine fuel than LNG, it is toxic and it is primarily produced from natural gas. Converting natural gas to methanol involves an extra step, so it’s less efficient, and also requires a sizeable reformer external to the fuel cell.
Some believe that these practical obstacles will be overcome by determination. Nevertheless, it is a simple fact that the energy conversion efficiency from hydrogen to electricity in a ‘conventional’ PEM cell is 40-45%, while the SOFC process has potential for 65-70% electrical efficiency. Furthermore, the combination of zero methane slip and high efficiency achieves the required 50% reduction in greenhouse gas emissions. If it is true that the reforming process generates carbon monoxide and hydrogen, SOFC plants can use both as fuel. In addition, the higher operating temperature of the SOFC (750˚C versus 160˚C) creates the potential to increase efficiency further by exploiting waste heat recovery.
Despite this, it is fair to point out that SOFC’s relatively high operating temperature, together with its smaller power density and higher cost in the short to medium term, may restrict the use of this technology onboard ships.
Foreship believes that PEM technology will make a valuable contribution to ship propulsion in the years ahead. However, over time, the relative advantages of PEM technology will be eroded and, like ESS before it, the technology’s potential will be most fully realised in shorter transits where onboard fuel storage is not an issue.
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