DNV has published a whitepaper on wind-assisted propulsion systems (WAPS) explaining its fuel saving potential and how it can play an important role on the GHG regulatory landscape.
Wind-assisted propulsion systems (WAPS) such as rotor-, suction-, wing- and soft sails, as well as kites, are gaining renewed attention as the shipping industry is facing increasingly stringent greenhouse gas (GHG) emission regulations to support net-zero emissions by 2050.
As wind is an inexhaustible and direct source of energy at no cost, WAPS can be an attractive solution for shipowners to meet new regulations that put a price on emissions and mandate the use of more expensive low GHG intensity fuels.
Moreover, wind-assisted propulsion has already delivered annual fuel savings of between 5% and 20% for certain ships, according to shipowners, operators, and technology makers, resulting in reductions in GHG emissions in the same ratio.
An increased adoption of WAPS by larger cargo ships in the deep-sea segment, which are responsible for approximately 80% of global shipping emissions, would contribute significantly to achieving IMO GHG reduction goals.
The GHG regulatory landscape
Key takeaways
- IMO regulations: The EEDI and EEXI indices set CO2 emission requirements for new and existing ships, with stricter requirements over time. WAPS can help meet these requirements through a correction factor. Introduced in 2023, the CII measures annual CO2 emissions per transport capacity work, with ratings from A to E. WAPS can improve CII ratings by reducing fuel consumption.
- EU Emissions Trading System (EU ETS): From 2024, ships over 5,000 GT must buy and surrender emission allowances for CO2 emissions. WAPS can reduce these costs by reducing annual fuel consumption.
- FuelEU Maritime Regulation: Effective from 2025, this regulation sets GHG emission intensity requirements for ships, with reductions required over time. WAPS can help achieve compliance by reducing GHG intensity through a specific wind reduction factor.
Upcoming regulations
The IMO is now also developing new regulations to support the ambitions in the 2023 IMO GHG Strategy, with planned adoption in 2025 and entry into force around mid-2027. These regulations will include a technical element, which will mandate the use of lower GHG intensity marine fuels and will also include a GHG emissions pricing mechanism.
As for the existing regulations, the use of WAPS on ships could include a reward towards the GHG intensity and will also support shipowners with compliance costs through reduced fuel consumption. It is not yet clear whether the reward will be based on design criteria or operational reporting on the use of WAPS.
WAPS technologies: Fuel saving potential
Sails generate direct propulsive power by producing aerodynamic forces and providing additional thrust. The generation of aerodynamic lift and drag forces depends on wind direction and speed, which also influence the relative propulsion contribution and potential fuel savings. Sail power can either increase the ship’s speed with the same engine output or reduce the engine load, allowing for throttling down and achieving the same speed with lower fuel consumption.
Wind-assisted propulsion has already delivered annual fuel savings of between 5% and 20% for certain ships, according to vessel owners, operators, and technology makers. Under given operational conditions, the potential is large, and DNV has verified WAPS reaching peak values of about 30% reduced energy consumption per nautical mile in favourable conditions.
The fuel-reduction potential of vessels utilizing WAPS depends on factors including:
- Ship size and displacement: WAPS developed or commercially available today have a limited size. Even the largest available systems have a limited relative impact on additional thrust and fuel savings for large and heavy vessels. For smaller and lighter vessels, these same systems have a higher relative impact, meaning that achievable relative savings can be higher.
- Number and size of WAPS: The number and size of the WAPS units installed on a vessel affect the aerodynamic thrust generated, and thus the potential savings. The available deck space and airflow are among factors that influence the optimal choice of number and size. Interaction effects between sail units is also an influencing factor, as it affects the aerodynamic performance of the total number of WAPS units.
- Trade routes: The geographical area and the meteorological season in which a ship operates significantly impact the potential gains from WAPS. While wind and weather conditions are inherently stochastic, they can, with statistical confidence, be considered relatively reliable for certain locations and times. The Global Wind Probability Matrix published by the IMO (IMO MEPC 62/INF.34) serves as a source for wind statistics and is also used to derive the Energy Efficiency Design Index (EEDI).
- Type of WAPS: The systems available and described above are not only different in their appearance and working principles but also differ in their performance-related characteristics. The pure aerodynamic lift that a device generates, and its drag, are both important. Where the lift/drag ratio provides indications about the upwind performance of the system, which might be beneficial on faster ships, the pure maximization of lift force, irrespective of the drag, might be beneficial on slower ships. Moreover, the choice of the system type may also be impacted by the trade route and the prevailing wind conditions, for the same reasons.
Weather routing
Weather routing is a strategic approach used in maritime navigation to optimize the efficiency and safety of commercial shipping operations. By leveraging advanced meteorological data and predictive models, weather routing enables vessels to chart courses that maximize the use of prevailing winds (and waves and currents), avoiding adverse conditions while maintaining schedules.
Furthermore, harnessing favourable winds lets ships cut both fuel consumption and greenhouse gas (GHG) emissions as well as improving overall operational efficiency. Modern weather routing systems integrate real-time data and sophisticated algorithms to provide adaptive route recommendations, ensuring that ships achieve the best balance between speed and energy efficiency. This innovative application of technology underscores its growing importance in sustainable maritime transport.
On open oceans, ships have more flexibility in choosing their route to follow favourable weather patterns and optimize routes. Thus, the greatest potential for fuel savings can be realized in intercontinental trades. Compared to a direct route (great circle), weather routing can easily double the fuel-saving.