Sunday, 3 August 2014

E-Ship 1: Magnus Sailing

Today's shipping industry contributes to about 5% of total carbon dioxide emission and the GHG emissions have shot up to 2.7% according to the studies conducted by International Maritime Organization. Oil prices have frenetically hit the ceiling with increment rates almost three times higher than those of the nineties. In such a scenario, the stratagem of ship designers has been to attain the optimum benchmark between prevailing fuel economy and environmental safety standards. Visions have shifted towards unconventional propulsion systems and use of renewable resources of energy is rapidly burgeoning. 

Realizing this, Enercon, with its 20 years experience in wind power engineering, has taken one of the most innovative steps in the design of its wind turbine carrier, E SHIP 1. The fact that it is designed to carry offshore wind turbines to their sites, is just like another story. But what will probably baffle you is that they have used the same physics to propel the ship, that a footballer uses while doing the banana free kicks! (Watch video) It is called the Magnus EffectTo understand this effect, imagine a fluid medium (like water or wind) moving at some velocity in some direction. Now let's place a stationary cylinder or a ball (let's take a cylinder here) in the medium and mechanically induce a rotation in the cylinder about its axis. Now refer to Figure 1. and tally what you're going to read.

Fig. 1: The Magnus Effect
(Image Courtesy: Google Images)
The fluid will obviously pass around the boundary of the cylinder, but note the difference in the pattern of flow on both the sides of the cylinder. On the top side (as per the figure) the velocity of the cylinder opposes the fluid velocity (thanks to friction) therefore reducing the net fluid velocity on this side. Recall the famous Bernoulli's Equation, and you'll feel the pressure on this side increasing. On the lower side, the scenario is very much the opposite. Supported by the direction of rotation of cylinder, the fluid on this side has an increased velocity and therefore, reduced pressure. The pressure difference between the two sides of the cylinder results in a force that is vectored along the direction shown (from high pressure to low pressure), which now induces in the stationary cylinder, a translation motion along its direction. This effect is used to swing the balls in football, cricket, golf, tennis, table tennis and baseball. Now take a look at Figure 2 and we'll soon see how the designers used a 90 year old technology that was rejected long time back, to slice through fuel price issues.

Fig. 2: The ENERCON E-SHIP 1
(Image Courtesy: Google Images)

The 90-Years Old Rejected Technology

The ship is powered by a diesel electric propulsion system consisting of one Controllable Pitch Propeller and three rudders. But what is remarkable in the ship, is the set of four vertical cylindrical structures on the main deck. They, in operation along with the diesel-electric propulsion system actually help the ship save about 20% of its fuel consumption when there are beam winds, also reducing the load on the propeller.  And they do that exactly by the principle of Magnus Effect. When there is a beam wind, these cylinders are rotated about their axis by an electric drive so as to direct the resultant magnus force along the direction of required surge.The concept of these cylinders were developed by Anton Flettener, and hence received the name: Flettener Rotors. The direction and speed of rotation of all the four are automatically set by the electronic wind sensing and control system, which involves less crew work as in case of sail powered propulsion, also leaving less room for human errors. The vectorial representation of this is as shown in Figure 3.

Fig. 3: Magnus effect applied to a ship
(Image Courtesy: Enercon)

The Question You Missed

These flettener rotors are rotated by means of an electric drive. So the obvious question is, Why spend that energy on electricity when it could have been spent on the conventional diesel-electric propulsion system? This obviously seems like a paradox, as in, we are just reducing consumption of diesel and ending up generation some extra electric energy which would power rotors that finally use the wind energy. We have seen the fuel consumption drop by 20% when the rotors are used along with the diesel-electric drive. But have we dropped the total energy consumption? The answer is Yes. The hot exhaust fumes of the diesel engines that power the conventional propulsion don't go waste. They are led to run a Siemens steam turbine that generates the electricity used to spin the flettener rotor sails. Figure 4 shows the CFD estimated power savings with varying directions of wind (24 knots / 6 Beaufort Scale) at the design speed of 16 knots. The yellow dot shows the attained savings at the trials, which was successfully above the estimated levels.

Fig. 4: Power Saving (%) vs. Wind Direction
(Image Courtesy: Enercon)

The Green Hull

Enercon seems to have left no stones unturned to optimise the ship for minimum environmental effects. With a 20 year global experience in wind power engineering, they designed the hull of E-Ship 1 to aerodynamically offer less resistance, therefore cutting fuel consumption even more. Also, to protect the for'd placed superstructure from green waters in rough seas, the wave breaker was developed and tested. The resistance of the ship was further decreased by using a low-resistance hull coating below the waterline. 

Fig. 5: The break water in front of the superstructure

Oh, and one more fun fact: She uses her hear from the engines to cool her interiors. In case, you know, 20% wasn't much of a reduction to set new standards. LSD

Article By: Soumya Chakraborty

Recommended Readings: Dynamic performance of Flettner rotors with and without Thom discs. (University of Manchester, UK)


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