International maritime shipping—powered by heavy fuel oil—is a major contributor to global CO2, SO2, and NOx emissions. The direct electrification of maritime vessels has been underexplored as a low-emission option despite its considerable efficiency advantage over electrofuels. Past studies on ship electrification have relied on outdated assumptions on battery cost, energy density values and available on-board space. We show that at battery prices of US$100 kWh−1 the electrification of intraregional trade routes of less than 1,500 km is economical, with minimal impact to ship carrying capacity. Including the environmental costs increases the economical range to 5,000 km. If batteries achieve a US$50 kWh−1 price point, the economical range nearly doubles. We describe a pathway for the battery electrification of containerships within this decade that electrifies over 40% of global containership traffic, reduces CO2 emissions by 14% for US-based vessels, and mitigates the health impacts of air pollution on coastal communities.
Transporting 11 billion tonnes annually, the maritime shipping industry handles nearly 90% of global trade by mass1,2. The industry’s meteoric growth has been underpinned by access to cheap, energy-dense heavy fuel oil (HFO). The shipping industry consumes 3.5 million barrels of low-grade HFO annually, produces 2.5% of total anthropogenic carbon dioxide equivalent (CO2e) emissions in 20182,3, and engenders enormous damages from marine eutrophication and ecotoxicity, air pollution, and climate change impacts4. By 2050, maritime shipping emissions are projected to contribute as much as 17% of global CO2e emissions5,6. The industry’s outsized contribution to criteria air pollutants—12% and 13% of global annual anthropogenic SO2 and NOx emissions, respectively—caused an estimated 403,300 premature deaths from lung cancer and cardiovascular disease in 20203,7.
Mounting political pressure has prompted the International Maritime Organization (IMO) to take regulatory action to reduce GHG emissions consistent with the Paris Agreement. Actions include resolution MEPC.302(72), which aims to reduce annual CO2e emissions by 50% by 2050 from 2008 levels8, and recommended amendments to the International Convention for the Prevention of Pollution from Ships (MARPOL)—whose members cover 99.4% of world shipping tonnage—to prohibit using or carrying HFO in Arctic waters after 20249,10. In concert, IMO’s 2020 emissions standards reduced the allowable marine fuel sulfur content from 3.5% to 0.5% by mass11.
Faced with this tightening regulatory landscape, the marine shipping industry is racing to identify commercially deployable zero-emission alternatives to HFO at a pace sufficient to substantially curb the sector’s emissions and avert catastrophic climate change. Optimistic outlooks for zero-emissions alternatives for marine applications suggest that electrofuels (e-fuels) would increase the total cost of ownership for bulks carriers by 200–600% relative to HFO12. Such analysis prompts additional research into which existing propulsion technologies could achieve parity with HFO in the near-future, particularly battery-electric propulsion. Maersk, the largest shipping company by volume, is already piloting battery hybridization on a containership operating between East Asia and West Africa13. A fully electric 80 m containership, the Yara Birkeland, is expected to begin autonomous operation in Norway in the early 2020s. Similar battery-electric vessel projects are underway in Japan, Sweden and Denmark14,15. However, systematic analysis of the adoption potential for battery-electric containerships has yet to be conducted. With the exception of these initial pilot projects, battery-electric propulsion has been underexplored as a potential low-emissions alternative in the marine shipping sector despite: its considerable emissions reduction potential; recent decline in battery costs; improvements in battery energy densities; increasing availability of low-cost, renewably generated electricity; and its substantial efficiency advantage over e-fuels such as green hydrogen and ammonia.
Using the best-available battery costs and energy densities, we examine the technical outlook, economic feasibility and environmental impact of battery-electric containerships. We define two scenarios: first, a baseline scenario using today’s best-available battery costs, HFO costs, battery energy densities and renewable energy prices; and, second, a near-future scenario that tests the impacts of projected 2030 improvements in these variables. By contrast to most previous studies, we treat the volume repurposed to house the battery energy storage (BES) system as an opportunity cost instead of a fixed technical constraint. We specify eight containership size classes and model their energy needs, their CO2, NOx and SO2emissions, and total cost of propulsion (TCP) across 13 major world trade routes—creating 104 unique scenarios of ship size and route length that can be compared with almost any containership operating today. We focus on battery-electric containerships and briefly explore the implications of our results for electrifying other ship types. Our results suggest that over 40% of global containership traffic could be electrified cost-effectively with current technology, reducing CO2 emissions by 14% for US-based vessels, and mitigating the health impacts of air pollution on coastal communities.
The search for low-emissions pathways for maritime shipping
In the short term, most ship operators have turned to energy efficiency measures such as slow steaming (deliberately reducing a ship’s cruising speed to reduce fuel consumption), route optimization and hull fouling management to meet IMO mandates16. However, the 10–15% emissions reductions achievable through these measures are not sufficient to comply with forthcoming IMO efficiency regulations17,18. Hybrid battery technology has been explored a viable short-term solution to reduce—but not eliminate—emissions from fossil-fuel energy sources. One study suggests a best-case scenario for hybrid systems is only 14% reduction in emissions for dry bulk carriers (comprising 2% of global fleet emissions)19, not substantially better than the existing energy efficiency measures. Small modular nuclear reactors, which have been used in military and submarine applications for decades20, are a viable alternative, but are unlikely to achieve wide-spread deployment in commercial vessels given the regulatory challenges surrounding nuclear proliferation, safety and waste disposal. Marine gas oil, liquefied petroleum gas, liquefied natural gas, methanol and their bio-derivations have received substantial attention as medium- to long-term options, but recent research has questioned the potential of these fuels to reach cost parity and considerably reduce lifecycle GHG emissions21,22,23. Not all transport modes are viable candidates for immediate and direct electrification; commercial jet planes cannot reasonably be electrified until battery pack specific energy increases to three to ten times their current values24. It is within this context that propulsion technologies generated with renewable power have received the most attention. For example, blue hydrogen (hydrogen produced from natural gas with carbon capture and storage) is expected to reduce GHG emissions by only 20% compared with burning natural gas25. Although renewably produced ammonia and hydrogen provide operational emissions reductions, the inefficiency of the production process relative to HFO makes them unlikely to become sufficiently cost-competitive to displace fossil fuels26,27. By contrast, direct electrification is typically five times more efficient than e-fuels in the transportation sector, exclusive of losses from e-fuel transport and storage27.
By contrast to other modes where battery weight dramatically reduces payload capacity or range, such as light-duty vehicles and planes, the sheer size of containerships means that the additional weight from the battery can potentially be offset with a smaller percentage forfeiture of cargo. Past work has suggested that battery electrification of marine vessels is unfavourable given the low energy density of batteries relative to hydrocarbon fuels28,29,30,31. However, their assumptions about battery energy density and cost are outdated, differing in some cases by one to two orders of magnitude from today’s best-available figures of 210 Wh kg−1 specific energy32 and US$100–134 kWh−1 (ref. 33). Furthermore, these studies assumed that the maximum battery capacity is limited by the existing onboard space dedicated to mechanical propulsion systems and fuel storage, so their findings suggest that battery-electric ships would require several recharges to traverse even short routes.
Technical feasibility of battery-electric container shipping
The key technical constraint for battery-electric container shipping is the volume of the battery system and electric motor relative to the volume occupied by a vessel’s existing engines, fuel storage and mechanical space. The extra weight of the BES system is, however, non-trivial in determining a vessel’s power requirements. Operationally, containerships can increase their carrying capacity by increasing draught (that is, the vertical distance between the waterline and the keel) on the basis of the Archimedes principle. A higher draught increases the hull resistance, and thus more power is required to achieve the same speed. On voyages less than 5,000 km, we find that the necessary increase in power is less than 10% of the original power requirements. For example, for a 5,000 km range small neo-Panamax ship, we estimate that a 5 GWh battery with lithium iron phosphate (LFP) chemistry, with a specific energy of 260 Wh kg−1 (ref. 34), will weigh 20,000 t and increase the draught by 1 m—a small fraction of the ship’s total height and well within the bounds of the vessel’s Scantling (maximum) draught. For voyages longer than 5,000 km, the increase in draught exceeds the vessel’s Scantling draught.
The distribution of additional weight also impacts the hydrodynamics, aerodynamics, stability and energy consumption of a vessel35. Internal combustion engine (ICE) vessels use a ballast system whereby water tanks charge and discharge depending on the cargo load to distribute weight and counteract buoyancy. Case studies of fully electric or hybrid propulsion systems suggest that ballast systems can be partially or fully replaced by BES systems without substantial impacts to symmetry (trim) and balance by distributing battery components throughout existing void, mechanical and ballast spaces35. Furthermore, BES systems do not need to be arranged around a central drive shaft and can be more flexibly configured within the vessel’s interior12,36. The volume of an onboard BES system depends on the ship’s power requirements, cruising speed, voyage length, electrical efficiency and battery energy density. Containership energy consumption can be approximated with the Admiralty Law, a version of the propeller law that is widely used in first-order estimations of ship power requirements and fuel consumption37,38. Although a bottom-up approach to estimating energy requirements would incorporate additional terms, our objective is to capture the relative changes in energy requirements between the two propulsion methods. Assuming an identical vessel and operational profile, the energy needs of ICE and battery-electric ships differ only by the engine efficiencies and mass, which directly changes the vessel draught.
Equation (1) describes the energy needs of a ship with a low-speed, two-stroke marine ICE fed by IMO-compliant low-sulfur HFO, where PSMCR is the maximum continuous power rating (where SCMR is the specified maximum continuous rating), Vaverage is the average cruising speed, Vmax is the maximum design speed, tvoyage is the time to traverse the route and ηICE is the ICE tank-to-wake efficiency.
Equation (2) describes the energy needs of an equivalent battery-electric ship, which includes a correction for increased draught due to battery system weight, where Tloaded is the draught when loaded with the battery energy system, Treference is the typical operating draught, and ηmotor and ηinverter are motor and inverter efficiencies, respectively.
Nickel manganese cobalt oxide, LFP, nickel cobalt aluminium and lithium titanate oxide are commercially available lithium-ion chemistries with the requisite cycle life, specific power, charge rates and operating temperatures to support container shipping applications39,40. The choice of battery chemistry depends on specific operational characteristics. Vessels with shorter, more frequent voyages, lower power requirements, and charging time constraints would favour the high charge rates and long lifecycles of LFP batteries41,42. For ships with longer ranges and less frequent battery cycling, the relatively low cycle life and high energy density of nickel manganese cobalt oxide batteries may be more suitable. Given that electrification will probably be limited to small, short-range vessels until battery costs are further reduced, we model the use of LFP batteries.
We find that minimal carrying capacity must be repurposed to house the battery system for most ship size classes and along short to medium-length routes. For a small neo-Panamax containership, representing an average containership in the global fleet, the volume required by the battery system is less than the volume currently dedicated to the ICE and fuel tanks for routes under 3,000 km. For the longest modelled route of 20,000 km for this ship class, the battery would occupy 2,500 twenty-foot equivalent unit (TEU) slots or 32% of the ship’s carrying capacity. Supplementary Table 1 provides the baseline values used for each ship class. Figure 1 shows the percentage of ship carrying capacity forfeited to the BES system for the eight modelled ship classes across routes from 0 to 22,000 km, with current and near-future battery energy densities. We find that as carrying capacity increases, the percentage of total carrying capacity volume occupied by batteries decreases because larger ships typically have lower energy requirements per unit of carrying capacity43,44.
A number of contact-based options are already commercially available for the shore-to-ship interface, including manual and automated plugs from ABB, Cavotec, Mobimar, Zinus and Stemmann–Technik, with non-contact inductive charging solutions currently under development46. Charging stations can be deployed at port terminals or offshore to allow ships to charge while queuing for berth allocation.
The optimized and high-throughput nature of port operations (average berth utilization rates typically exceed 50%) support high charging infrastructure utilization and associated cost reductions45. Adapting methods used for trucks40 and trains47 we estimate the levelized cost of a 300 MW charging station interconnected at the transmission level to be US$0.03 kWh−1 at 50% utilization, inclusive of hardware, installation, grid interconnection, and annual operations and maintenance costs across the system lifetime48.
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Source: Nature Energy