søndag 14. juli 2013

Wind Assisted Propulsion of Ships


av Zhenju Chuang, ph.d., recently graduated from Institute of Marine Technology, NTNU

Reducing CO2 Emissions from Shipping

SkySails towing kite
Climate changes have been given a large attention in the media lately. The general opinion is that the recent climate changes are mainly caused by human activities. This together with fluctuations in the oil prices has led to a great focus on energy conservation in the marine industry [1]. Shipping is responsible for more than 90% of world trade, creating   greenhouse gases, like COx, NOx, SOx. Fig 1 shows global CO2 emissions. It seems that emission from shipping is not as big as other manufacturing industries. However, if we do nothing from now, according to EU prediction, by 2020, SOx will increase 40%, NOx will increase 50% and even more to PM2.5 being 55%.

So now we are facing a big opportunity to explore renewable energy. Wind is the best solutions for shipping since it is sustainable, easily accessible, cost and emission-free.

Wind assisted ship propulsion is becoming more and more popular now. Several main devices will be introduced here. Like sail (soft sail and rigid sail), Flettner rotor, wind turbine and kite.


Figure 1 Global CO2 emissions

 













Soft Sail

Figure 3 (a) Square rig 
Figure 3 (b) Fore-and-aft rig


Soft sail was originated from ancient times, and its history is as long as human civilization. Two main categories of softsails are shown in Fig 3. They are square rig and fore-and-aft rig. The rigs are made of soft material, such as canvas, cloth which are rigged across masts [3]. Performance comparisons between two kinds of soft sails are shown in Table 1.

Table 1. Performance comparison between square rig and fore-and-aft rig

Rigid Sail

Figure 4 shows two examples of rigid sails. They are all consisted of self-rotating aero foils standing on an end to meet wind direction. They are made of hard material, so structure reliability is higher. Flaps can be included to increase its efficiency. Also rigid sail can reduce ship motion and thus reduce ship resistance. [4]
Figure 4 (a) rigid sail on deck
Figure 4 (b) rigid sail with flap

Flettner Rotor

Anton Flettner made the first comprehensive research on modern lift generating devices for auxiliary ship propulsion in the early 1920s. He came up with the Flettner rotor, which is a rotating cylinder that makes use of the Magnus effect to create large cross-forces in the direction of propulsion. These forces can be very large. [5]
Figure 5 E-Ship 1 – first voyage with cargo in 2010
The Flettner rotors are compact and made of high-tech materials. The technology used by these rotors is familiar to ship operators. They are suitable for retrofitting to slow speed vessels working in side winds. They may, however, have higher maintenance costs and they require an external energy source. If the external energy source breaks down, the rotors will not contribute to any thrust. When they are sailing windward, high drag forces are developed. There is also no way to reduce the aerodynamic force from the rotors in strong winds. These drawbacks concerning this wind assistance solution can be critical at unfavorable wind directions or if problems with the external energy source occur, and there are not many ships equipped with Flettner rotors today.

Wind Turbine

It is also possible to create propulsion by using wind turbines. The power generated by the wind turbines do not need to be used directly, but can be stored and used to drive for instance a propeller. Two categories of wind turbines, namely horizontal axis wind turbine and vertical axis wind turbine are shown in fig 6. [2]
Fig 6 (a) Horizontal Axis wind turbine
Fig 6 (b) Vertical Axis wind turbine

Kite


The German company SkySails has developed an example of a kite system for cargo vessels. The system is used parallel to the main engine if the wind conditions are favorable. It consists of three main components:
· Towing kite with a rope
· Launch and recovery system
· Control system for automatic operation
A kite can be actively controlled in order to create its own flying speed and thereby increasing the apparent wind speed. This leads to increased traction force per sail area. [6]

Conclusion

Wind assisted ship propulsion has great potential. Practical driven from international polity are more and more focus on green shipping and dwindling oil reserves. In order to develop more advance technology to investigate wind assisted ship propulsion, several uncertainties should be taken into account. For example, sea surface effect, atmospheric turbulence and wind vertex shedding of superstructure. Even more frame work for optimal combination of merchant ship type, wind assisted system and weather routine should be worked out.

References

[1]  Angvik, I., Wind Assisted Ship Propulsion, master thesis, NTNU, 2009
[2] Bøckmann, E., Wind Turbine Propulsion of Boats and Ships, master thesis, NTNU, 2010
[3]C. T. Nance. Outlook for wind assistance. Journal of Wind Engineering and Industrial Aerodynamics, 19:1-17, 1985.
[4] Silvanius, M., Wind assisted propulsion for pure car and truck carriers, master thesis, KTH Center fro Naval Architecture, 2009
[5] Craft, T. J., H. Iacovides, and B. E. Launder. Dynamic performance of Flettner rotors with and without Thom discs. Proc. 7 th Symp. on Turbulence & Shear Flow Phenomena. 2011.
[6] Naaijen, Peter, and Vincent Koster. Performance of auxiliary wind propulsion for merchant ships using a kite. 2nd International Conference on Marine Research and Transportation. 2007.


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