Energy saving measures

--> Recent decades have brought a host of green ship technologies such as less resistant hull shapes and coatings, more efficient engines and propellers, waste heat recovery and many others, altogether resulting in substantial fuel savings and carbon emission mitigation. However, mainly aimed at new builds, most of the innovations cannot be implemented cost effectively to existing vessels. So, for the majority of the about 40,000 merchant ships around today, slow steaming remains the main route towards meaningful fuel savings.  In any case, when all is said and done a vessel relying only on its engine for propulsion still needs to burn substantial amounts of fossil fuel to move cargo on the high seas. At this point the only way to further savings and emission mitigation remains the harvesting of renewable energy.

To that effect the last twenty years has seen many efforts towards reviving wind power as a means of propulsion for merchant vessels. Notably, the “Dynaship” and the Japanese “UT Wind Challenger”, solid-sail full rigger concepts, the “SkySail” kite sail system, and the Enercon E-ship Flettner rotor vessel. Also, many new ship designs include arrays of PV solar panels.

The jury is still out on contemporary sailing concepts, but beyond any doubt some will be utilized in cargo ships within the next few years. Many marine engineers agree that Flettner rotors represent the closest approach to a safe, push-button operated sailing system. It is a well -known, proven technology, which can readily be implemented in new builds. However, current concepts require several, deck-mounted rotors in order to obtain meaningful fuel savings, are expensive to install, and tend to impede loading and unloading and daily operation of the vessel

The Monorotor® (patents pending) is a self-contained wind propulsion system, specially designed for easy retrofit onto existing bulk carriers and tankers. The term Monorotor signifies a single rotor, which is able to match the performance of the multiple slim rotors historically used and currently being proposed for cargo vessels. To accomplish this it needs to have a large diameter, typically 8 to 20 meters, and a height of 20-25 meters. With these dimensions the rotor system would be a significant obstacle if placed amidships on the main deck, besides obstructing the view from the bridge. As a consequence they are located at the forepeak above the forecastle, or aft of the superstructure, straddling the stern. Due to the distance from the helm to the forepeak, on most vessels the blind sector represented by the Monorotor, as viewed from the bridge, will be within the maximum 5 degrees specified  by the IACS.

The main objective behind the Monorotor concept is to provide a modular, self-contained system which can be easily customized to fit a wide variety of cargo vessels.

Even though Monorotors can be used individually, a common installation will comprise two modules, with one rotor on the forepeak above the forecastle deck and another straddling the stern, elevated above the aft deck. The typical support bases are tripods comprising two rear legs which are angled symmetrically outward, and one foreleg pointing forward at an angle of about 45 degrees. The legs are tubular steel fabrications, 0.5 to 1 meter in diameter, supported on heavy footplates which connect with hull strong points or wide box girders for distributing the weight load and working forces over a large area of deck and underlying hull frames. The top of each leg is exactly contoured and welded onto the tubular casing of the drive shaft assembly, thus defining the top of the rigid pyramid-shape base structure.

The accompanying drawings serve to illustrate how the Monorotor support bases may be customized in order to minimize the need for modifications to the vessel receiving the retrofit. Another objective is to avoid re-locating essential equipment like bollards and anchor- and mooring-windlasses.

Monorotors will be made available in several models with diameters ranging from 8 to 24 meters featuring one of three standard driveshaft assemblies. The “Configurable Specification” table lists variables, which may be customized to comply with client requests.

Flexibility of design is accomplished by means of changes to the support legs such as their length, attachment angle, diameter, wall thickness and the way they are welded onto the driveshaft assembly, foot plates and box girders.

·         Rotor clearance, typically 3-4 meters may be increased when required.

·         The Offset distance from the rotor axis to the forefoot, in order to keep clear of deck equipment. The rotor axis may be cantilevered out from the stern if necessary in the case of a short aft deck.

·         The Spread, as wide as possible while keeping clear of bollards and cable guides.

·         Box girders, optional, may mitigate the need for internal reinforcements.

·         Mast support base: Bow rotor option.

The customization process is discussed with the client, and the custom design is subjected to FEM. Further evaluation by class, as well as stability and sea-keeping analyses to be performed as required.

·         Each rotor generates the same amount of thrust as 2-4 slim rotors of similar height.

·         Is less complex with few moving parts.

·         The supporting structure may be designed more efficiently within the ample space inside the rotor.  Less reinforcement of the vessel structure is required since forces may be spread over a larger area. The main support legs, which connect with hull plating or wide-base deck support structures may be placed 10+ meters apart, only taking up a small amount of deck space.

·         Rotors are located at the extreme stern and bow, well clear of hatches, cranes and anchoring and mooring equipment. Interference with gear and daily operations is minimized because Monorotors are elevated 3-4 meters above deck, also raising crew safety.

·         A free fall lifeboat may be located and launched under the shelter of the aft Monorotor.

·         In most cases few modifications are required. During installation the one-piece drive and support module is lifted on board first and welded in place, and then the rotor is added and bolted on to the drive shaft top flange. Only two lifts are required. The entire process including power and control cabling can be performed within a week, possibly combined with scheduled maintenance.

·         The Monorotor drive shaft is supported by two large capacity roller bearings. They are oil lubricated and oversized to provide decades of lift. The 1 -2 ton  gear motor is backed by a 5 year warranty. In the unlikely event of a bearing- or drive failure the vessel may proceed as normal under engine power until repairs can be performed.

·         Maximum rotation speed is low 40-80 RPM depending on diameter.

·         Operating parameters are vessel course and speed, wind direction and velocity, rotor RPM and generated thrust. The variables are monitored and the system is operated automatically by a microprocessor control module located in the wheelhouse. No specialized crew is required.

About 50 KW of electric power is required to spin the Monorotor.

The rotor is designed to rotate at maximum of 60 RPM. The table above shows how the thrust can be kept within the 55 ton design limit by adjusting the rotor RPM. Thrust is monitored by strain gauges located on the central support at the point of maximum stress. A PLC program continuously adjusts and maintains the Monorotor RPM within safe limits while maximizing thrust. The program also monitors wind speed and angle of roll in order to stop and park the rotor in extreme weather. No specialized crew is required to operate the system.

When evaluating the performance of the rotor system it is important to keep in mind that it contributes thrust and propulsion horsepower. On the other hand, when calculating propulsion horsepower of a marine engine, its rated power must be adjusted for propeller efficiency (0.65-0.70). For example: 2,000 Monorotor horsepower equals about 2,900 engine brake horsepower.

Many owners are looking at solar collectors to supplement the electric power on board. However, dependent on the trade, panels tend to dirty up and often suffer damage due to weather and crew activities.

The top surface of Monorotors, located high above decks and holds  is an ideal platform for solar power generation. P/V solar panels installed flat on the 18 meter diameter top surface of a model 16/22 Monorotor may contribute 20-25 kW during daylight hours.  The DC power is routed down through the rotor center and on to a DC/AC inverter via a brush and slip ring assembly.  Offered as an option under the patent pending Monorotor system.

Experiments indicate that when comparing the thrust of two Flettner rotors of different heights but with the same projected area and rotating at the same spin ratio, the efficiency of the taller and slimmer rotor is higher than that of the shorter and stubbier rotor.

The difference in output is caused by boundary losses due to air flowing from the high pressure to the low pressure zones over the ends of the cylinder. The end areas of the shorter cylinder represent a relatively larger part of the total projected area, causing higher losses and lowering its efficiency. The short circuit of the air stream near the ends can be mitigated by adding larger diameter discs (Boundary effect fences) at the cylinder ends.

Conversely, since the pressure gradients and conditions near the ends of equally tall rotors are the same at similar surface velocities and wind conditions, the efficiency remains the same as well, irrespective of the diameter and aspect ratio.

This interesting characteristic of Flettner rotors is their ability to become “invisible” in strong winds. The effect manifests itself as a distinct drop in drag as the surface to flow velocity is approaching 1. This means that the braking effect of the rotors on a vessel traveling against the wind can be mitigated or drastically reduced by lowering the spin ratio (3-4 to generate thrust)  to match the combined velocity of the wind and the speed of the vessel.  Besides improving the general efficiency of non- collapsible rotors the Barkley effect can also be utilized to improve the safety of the vessel in inclement weather conditions.