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March 18, 2021

How wind assisted ship propulsion can transform marine transport

The global shipping industry carries approximately 90 percent of world trade by weight, accounting for nearly 3 percent of human-induced CO2 emissions. Together with the Delft University of Technology (TUDelft), the engineering consultancy Blue Wasp wants to change the course of things.

If left to business as usual, the industry’s greenhouse gas emissions are expected to increase by 50-250 percent by 2050 compared with 2012 levels. Moreover, most of the world’s fleet is still burning Heavy Fuel Oil (HFO), which is an unrefined residual oil that has a high percentage of sulphur content. 

Due to this, the shipping sector is responsible for around 13 percent and 15percent of the global sulphur and nitrogen oxides emissions respectively due to human activities. To put it into perspective, the 15 largest ships alone emit more of these noxious gasses than all the world’s cars put together.

Although slowly, things have begun to change. In April 2018, the International Maritime Organization (IMO) has identified levels of ambition that aim to lead to a reduction in total annual greenhouse gas emissions by at least 50 percent by 2050 compared to 2008 and to pursue efforts towards phasing them out. 

Certain regulations for improving the efficiency of newly-built and existing ships have already been enforced, but several academic studies have indicated that these efforts are insufficient - especially if the shipping industry has to be aligned with the goals outlined in the Paris Agreement. Several measures are currently under discussion and it is likely that stricter regulations, that will have a considerable impact on the shippings’ business-as-usual operations, will be enforced in the short term. 

Alongside political environmental policies, various private initiatives have also made their appearance. For example, the Getting to Zero Coalition, an alliance of more than 120 major players in the maritime, energy, infrastructure and finance sectors, has committed to “getting commercially viable deep sea zero emission vessels” within the next 10 years. This is a big statement, which needs radical and urgent interventions in order to be materialised.

To sustain the decarbonisation of the shipping sector, numerous technical solutions have been proposed and several projects are underway to show their feasibility. The large majority of these include new fuel types like biofuels, ammonia or hydrogen that promise to fully eliminate any noxious and greenhouse gas emissions when burnt. Although very promising, in case these fuels are sustainably sourced, these solutions are possibly still a decade away, as they require a whole new infrastructure to be produced and distributed.

Among the technologies proposed for the energy transition, wind-assisted ship propulsion (WASP) stands apart. Wind propulsion, in fact, is a so-called primary renewable. It does not require new infrastructures or storage as the wind energy is directly turned into propulsive power on the spot. This drastically reduces the costs of its application and avoids the efficiency losses associated with energy storage and transformation. Another considerable benefit is that wind is a cost-free source of energy and several WASP technologies are already available and ready to be adopted at a large scale.

It might sound like a return to the old days, when large clipper ships, solely moved by the power of the wind (and by tens of seamen) were employed for global trade. But, in fact, things are very different nowadays. As the name suggests, wind-assisted propulsion is a hybridisation for ships, where the vessel propulsion is split between a ‘sail’ system and a main propulsor. Ship operation is not reliant on wind availability, as may be thought. Instead, if there is a shortfall in wind along a given voyage the ship must rely on the main engine to make it to the next harbor on time.

These ‘steel sails’ (some actually being aluminum or of composite construction) contribute to the propulsion of the ship, thereby saving fuel and improving transport efficiency. Modern wind-assist propulsion devices can be of different types. There are the wing sails (like rigid, highly efficient sails), the kite sails that take advantage of stronger high-altitude winds and active devices such as the Flettner rotors (tall spinning cylinders) and the Ventifoils (aspirated wings). The latter two are high-lift devices that require some energy input to function, but in return they can provide much larger aerodynamic thrust compared with the square-rigged sails of yesteryear. This is good because the ships have also gotten larger, where the powering requirement of the largest commercial vessels is similar to the electricity demand of a small town.

This extra aerodynamic thrust can be applied directly to move the ship forward (in contrast with the transmission losses involved with the main engine), but there are some complications. Like any wing profile, the lift and drag properties of a WASP sail system will produce a beneficial thrust, but also a transverse heeling force. The vessel hull will need to balance this new transverse force, thereby arriving at a new vessel operating condition. 

Being otherwise optimised for a very specific and symmetric operating condition, the ship resistance will increase and this increase may in fact counteract the aerodynamic thrust. 

As a final point, the new operating condition may severely impact the controllability of these vessels, especially if large installations are considered. 

Still, existing vessels with a WASP retrofit may expect savings on the order of 10-20 percent, depending on the wind conditions and how the ship is operated. On the other hand, a new-build WASP vessel designed for full utilisation of the available wind-power promises savings in excess of 50 percent. This a remarkable result that can be achieved only if the wind-assist devices are properly integrated with the vessel from its early design stage.

Although the time schedule of these hybrid ships will not depend on wind availability, the amount of fuel that can be saved during a voyage is certainly influenced by it. Therefore, active vessel routing to seek out the most favorable wind conditions is an important part of the story here. 

To take full advantage of beneficial wind conditions, the ship should be operated differently than it is today. It should be able to change course and change speed, while keeping the time of arrival unchanged with respect to business as usual. With the 10 WASP ships currently in operation this is not the case, yet. A change in mindset is necessary first, but it will be worth our while, as active vessel routing is proven to drastically increase the amount of fuel that can be saved.

Answering the call for a greening of the shipping sector, a new generation of engineers and entrepreneurs is developing the technical and practical feasibility for several efficient WASP devices and the full integration with vessel trading operation and financing models. 

It is time to embrace the promise of wind-assist. It's a compelling short-term intervention alongside the phase-out of fossil-fuelled engines, and, going forward, will be an essential part of the power mix for the new zero-carbon fleet.

Blue Wasp is an engineering consultancy specialised in wind-assisted ship propulsion that offers independent technical advice to support the decision making of stakeholders across the shipping industry. Growing out of a 10-year research experience specific to the topic at TUDelft (in The Netherlands), the company helps realise the unique economic and environmental benefits that wind assistance delivers.

Giovanni Bordogna holds a PhD on the Aerodynamics of wind assisted ships from TUDelft. He is a goal-oriented engineer with a strong passion for green technology and entrepreneurship.

Nico van der Kolk is an experienced engineer with a broad background in experimentation and numerical simulation and a strong interest in environmental policy and regulation. He holds a PhD on the sailing behavior of wind-assisted ships from TUDelft.