Fathom Spotlight: The Potential of Wind Power

The paper addresses the existing 'knowledge gap' in relation to the potential of wind power technology.

Wind is a renewable energy source that is generously available on the world's oceans. As shipping faces the challenge of reducing its dependence on fossil fuels and cutting its carbon emissions, there have been a growing number of studies exploring the potential for harnessing wind power for shipping.

In this week's industry spotlight we assess the angle of attack and provide a summary of a paper authored by Michael Traut and colleagues at the Tyndall Centre, the internationally renowned climate change hub.

What is the Paper About?

This study seeks to address the existing 'knowledge gap' in relation to the potential of wind power technology.

In the study, numerical models of two wind power technologies, a Flettner rotor and a towing kite, are linked with wind data along a set of five trade routes. Similar studies conclude that significant savings of up to 15% at a speed of 15 knots and up to 44% at 10 knots can be made for some of the analysed ship types. They suggest the technology is cost-effective, given rising fuel costs and environmental pressures, and argue the need for more detailed analyses.

Therefore, this study provides the aforementioned thirst for deeper analysis.

The paper introduces a numerical performance model of a Flettner rotor and one of a towing kite. They are combined with wind data from the Met Office's Unified Model with a high spatial resolution of less than 1°. A methodology for assessing the wind power contribution of the technologies towards ship propulsion is provided and applied to five different shipping routes, representing different trades, served by ships of different types and size, in different environmental conditions. Furthermore, the methodology stresses simplicity and modularity, highlighting potential power savings, rather than scoping percentage savings that depend strongly on external parameters such as ship size and type.

Finally, this paper affords appropriate attention to the variability in wind power contribution, which is examined in relation to the kite and Flettner rotor's different dependence on wind velocity.

The methodology comprises of three components: a numerical model of the wind power technology; trade routes; and wind data along those routes. From these components, the potential wind power contribution is calculated.

As a result, the methodology stands ready to be applied to any other route and to incorporate other wind power technologies such as fixed wing sails, and it serves as a starting point for more detailed studies, thus contributing towards building up the knowledge that is needed for shipping to meet the low carbon challenge.

The Technologies

Flettner Rotor

Flettner rotors are controlled via a single parameter, their rotational speed. It is assumed that the rotors are incorporated into the ship structurally as – by design – large forces are at work, and to ensure the hydrostatic and dynamic stability of the vessel.

They, however, take up deck space and very likely increase the overall height of the ship, presenting potential barriers to their installation, depending on the ship's type and intended operational profile. It is noted that these assumptions and issues need to be assessed further when considering implementing Flettner rotors on a specific vessel. All results were calculated for a single rotor.

Choosing an optimum number of Flettner rotors would depend on many factors, such as vessel specifics, which are outside the scope of the study, but assumptions about the number of rotors need to be taken into account when putting results into context.

In 2010, the 10,000 dwt cargo vessel E-Ship 1 was completed, equipped with four Flettner rotors of 27 m height and 4 m in diameter, demonstrating the design feasibility of the technology.

Kite

Compared to other wind power technologies, kites have some advantages: they may operate at higher altitudes where wind speeds are often greater; they fly in front of the ship and therefore do not take up any deck space or change any of the ship's maximum dimensions; finally, they are in motion themselves, leading to higher apparent wind speeds, and consequently to higher thrust. The ideal case is that of cross-wind motion: in tail-wind conditions, the kite flies in a direction vertical to the true wind.

At a certain kite speed, which is a function of the kite's lift-to-drag ratio, the sum of the lift and drag force points in the direction of the rope. As the lift and the drag force go with the square of the apparent wind speed, a large thrust is generated in this ideal case. More generally, the kite flies a pattern in front of the ship. In favourable wind conditions it is deployed and controlled to fly along its circular trajectory. If wind conditions become unfavourable it is hauled in. Both processes are computer-controlled and fully automated. The model rests on the assumption of a towing kite that fulfils these operational criteria, without affecting the ship's stability adversely.

The Routes

Five routes were selected for analysis: Yantian to Felixstowe, Tubarao to Grimsby, Varberg to Gillingham, Dunkirk to Dover, and London to Milford Haven. All routes follow the shortest possible path, which includes passing the Strait of Malacca and the Suez Canal on the way from China to Europe.

What Were the Results?

The kite and the Flettner rotor showed very different behaviour with respect to wind direction. The kite worked well with a tail wind, in particular when the wind speed was large compared to the ship speed. The Flettner rotor, on the other hand, worked particularly well for sideways winds, with the output becoming very low or even negative for a straight head or tail wind as the lift force points in the direction perpendicular to that of the ship, and the drag force dominates.

Results varied depending on the route analysed and potential savings may be viewed in relation to power requirements by the ship serving the route. To consider an example, on the route from Yantian to Felixstowe, representative of the Far East unitised cargo trade, the average power delivered by a kite would be of the order of 1–2% of the main engine power required by a slow-steaming container ship of 30,000 dwt, or 2–3% for a single Flettner rotor. If more than one Flettner rotor were installed, the contribution would be expected to increase linearly until significant interference effects set in. However, on a container ship, there is also the barrier of limited deck space availability, and other markets may be more natural entry points for initial uptake.

Considering the route from Varberg to Gillingham, for a typical, slow-steaming general cargo carrier of about 5500 dwt, the average power delivered by a kite would be of the order of 20% (outgoing) to 45% (returning) of the required main engine power, or 20% (both directions) for a single Flettner rotor. Assuming an installation of three Flettner rotors, the average wind power contribution is then more than half of the main engine power demand.

The average wind power contribution on a given route ranged between 193 kW and 373 kW for a single Flettner rotor and between 127 kW and 461 kW for the towing kite.

The average power contribution from the kite ranged from 127 kW to 461 kW; it is more volatile, both over time and geographic location, than that from a Flettner rotor and, in comparison, the transient power is lower than that from two or more Flettner rotors. However, it has the advantage of taking up very little deck space, and an automated kite, subject to availability and favourable economics, is certainly a low-carbon technology option worth further consideration.

The variability of the power output from the Flettner rotor is shown to be smaller than that from the towing kite while, due to the different dependencies on wind speed and direction, the average power contribution from a Flettner rotor was higher than that from the kite on some routes and lower on others. While for most forms of international cargo shipping wind may not be suitable as the sole source of propulsive energy, a comparison of average output to main engine power requirements of typical vessels serving the routes indicates that it could deliver a significant share. For instance, installing three Flettner rotors on a 5500 dwt general cargo carrier could, on average, provide more than half of the power required by the main engine under typical slow steaming conditions.

However, it was noted here that a few steps are needed to more realistically translate wind power contribution into fuel savings, when conducting studies focusing on a specific vessel with a given operational profile. Side slip and rudder losses introduced by any mis-alignment of the ship course and the wind-generated thrust were not considered here and, in comparing propulsive power and main engine power, transmission and propeller losses were neglected in this study. Furthermore, the main engine efficiency may change as a function of power output so its efficiency profile could be factored in. These effects partially cancel each other but need to be accounted for in more detailed analyses.

Implications for Decarbonisation of the Shipping Sector

This paper raises significant points in regard of policy development and technology road mapping. It demonstrates that even for specific technologies, the process of providing a definitive CO2 saving per technology measure should be approached with caution.

This study provided a range of 2–24% and 1–32% main engine fuel savings for a single Flettner rotor and a towing kite, respectively. If the sector is to decarbonise commensurate with the 2 °C target there is a need to define the applicability and the costs, both direct capital expenditure costs and more indirect costs such as reduced available deck space, and savings, both on the fuel bill and in emissions, of a technology measure – in this case Flettner rotors and kites – for a given vessel and route, within appropriate timeframes and scales. While this is not possible for the costs, a rough estimate of the financial savings is instructive: Considering the route from Varberg to Gillingham, a fuel price of USD 650, and a specific fuel consumption of 180 g/kW h, fitting a Flettner rotor could save about USD 600, the towing kite about USD 900 per day at sea.

On the subject of time frames, the routes and vessel types that look set to benefit the most from wind technology in the forthcoming years – relatively slow and small ships with a potential to meet a substantial share of their power requirements with wind – should be the focus of further research. From the analysed cases, a general cargo carrier serving the route between Varberg and Gillingham should be the type of example to be explored further to exploit wind power. The output of this research takes a first step towards informing deeper and wider market penetration, including other routes and ship types.

In terms of scale, this research demonstrates that considerable savings can be made on specific routes. Considering the climate change mitigation challenge, wind technology should be viewed as part of a wider shift towards a decarbonised shipping sector. Along with slow steaming and other incremental efficiency improvements, renewable propulsion technologies reduce the overall demand on the main engine. This in turn makes alternative fuels, such as sustainable biofuels or renewable synthetics, more attractive. From the more narrow view as a technology measure providing a reduction in emissions to the wider view as an element of a transition to a decarbonised sector, wind power has a step drop mitigation potential for shipping.

From a practicality perspective it should be noted that this analysis was completed along existing shipping routes – which are not necessarily optimal with respect to wind gain. With this in mind, there is arguably further potential to explore historic shipping routes and to optimise travel speeds along them. Finally, the combination of wind technologies requires further attention. The positioning and function of Flettner rotors and kites are not mutually exclusive and hence in unison could harness tail and sideways winds – resulting in increased propulsion power and CO2 savings.

The Conclusion

This paper demonstrated the significant opportunities for step jump emissions reductions that wind technologies have to offer. It outlined next steps towards realising the potential, highlighting a demand for more detailed studies on socio-economic and technical barriers to implementation, and providing a basis for research into step-change emissions reductions in the shipping sector.

Results showed that, although the transient power contribution is too low and variable for the industry to consider wind as the sole driver of ships typically serving the selected routes, wind can in some cases provide a major share of required propulsive power. The next steps needed in order to tap into this potential are highlighted, including modelling of the implementation of a kite and a Flettner rotor on a particular vessel with a specific pattern of operations. These studies will have to account for practical barriers and the complex integration of thrust contributions from both the main engine and the wind power technology to estimate fuel savings.

The methodology presented here provides a step towards closing the existing knowledge gap and stands ready to serve as a basis for further studies towards grasping the emission reduction opportunities presented by wind power, both as a technology providing a step drop in emissions and as an element of a wider transition to a decarbonised shipping sector.

For the full conclusions please refer to the original paper via Science Direct.