09 JUNE 2020
A unique configuration of wind turbine that rotates on a horizontal transverse-axis to wind direction offers promise to sail a vessel directly into a headwind. The layout was originally intended to be installed atop tall buildings to quietly convert wind energy into electric power. There is scope to evaluate the Magnus/Flettner rotor for installation on maritime vessels.
Canadian physics Professor Dr. Brad Blackford set a precedent when he first sailed a small windmill powered boat directly into a headwind, competing with sail-powered small boats in a race across Port of Halifax. While 3-bladed horizontal-axis axial-flow wind turbines mounted atop are widely used to generate electric power at inland and coastal locations, there is a physical size limit to installing such technology on ships that roll and pitch in heavy seas, producing dynamic stresses that risk collapsing a tall windmill tower. Vertical-axis wind rotor assemblies are being built to withstand ship dynamics.
Tilting a vertical-axis wind rotor through 90-degrees produces the transverse horizontal-axis rotor, a concept pioneered by Broadstar wind turbines and providing the precedent to operate a boundary-layer Magnus or Flettner rotor installed on a transverse horizontal axis above a ship deck. The main purpose of such an installation would be to produce propulsion while sailing directly into a headwind, such as would occur on a westbound voyage across either the North Pacific Ocean or North Atlantic Ocean. Such operation would restrict such wind-assisted ships to ports that offer very generous overhead clearance, or air draft.
Winds in several locations internationally blow essentially in one direction over areas of ocean, with very little variation in wind direction. Such is the case for both the Southeast trade winds and Northeast trade winds in both the mid-Atlantic and Pacific Oceans. While there is some directional variation in the Westerlies that occur around the Antarctic region, the north Atlantic and North Pacific Oceans, these winds blow in the same generation direction. Such wind directions allow for possible application of a fixed wind turbine mounted aboard each ship that sails directly into headwinds in these regions.
While sailing in one direction, headwind energy could reduce ship fuel consumption, with potential to occasionally provide all of the ship’s propulsion energy requirements. For sailing with the wind, airborne kite sails offer a potentially viable method of reducing ship fuel consumption.
The installation of a transverse-axis Flettner Rotor could be achieved by using triangular support structures installed on either side of the ship deck and even at mid-deck, at the stern area and designed to withstand extreme wind conditions. A wind-assisted container ship could combine a low-level forward bridge located near the bow with a high-level transverse-axis wind rotor placed above the stern, with turbine location allowing cranes easy access to containers. Sailing the ship at 20-degrees angle from headwind direction would, using cosine from trigonometry, allow for 93.9% of headwind energy to rotate the turbine.
Even at 30-degrees to headwind direction, 86.6% of wind energy could still activate the rotor due to the boundary layer effect produced by the spinning rotor. That effect could actually “steer” prevailing incoming wind energy to a more perpendicular angle ahead of the rotor. Planetary reduction gearing could allow a fast spinning rotor to drive a power transfer mechanism involving upper and lower crankshafts linked by tension rods enclosed inside a casing, driving electrical generators installed on the ship deck. Rotor physical location allows for a large-diameter transverse-axis rotor and twin rotors in close proximity.
Rotor Support Structure:
There is scope for a very rugged support structure to carry a transverse-axis rotor placed well above a ship’s deck. Such a structure could be built to withstand the dynamic stress loads caused by ship roll and pitch movements. Triangular members on either side of the deck as well as at the midpoint area could carry the weight of twin coaxial rotors, with diagonal cross-bracing absorbing dynamic stresses caused by ship roll. The support structure would occupy space outside of rotor width and well below each rotor’s transverse working surface.
There is scope for the support structure to carry a transversely mounted air deflector mounted ahead and below the rotors, deflecting lower elevation headwind energy upward toward the rotors. It would reduce the amount of air that would flow below the rotor while increasing wind velocity over the boundary layer working surface of each rotor. It would also impose a refraction effect on approaching diagonal wind, partially steering the wind stream toward the rotors. A downstream airfoil would assist in maintaining fast wind speed over each rotor’s highest point.
Vessels carrying high-mounted transverse-axis wind rotors cannot sail under bridges and would be restricted to sailing between ports that offer unrestricted overhead clearance. The stern mounted transverse rotor would be suitable wind-assisted propulsion for container ships built with a forward bridge placed behind an X-bow. Such container ships could sail across the North Pacific Ocean, between some Asian and South American ports, also on the trans-North Atlantic service between selected ports. Suitable North American ports would include Ports of Virginia, Los Angeles/Long Beach, Quebec City and Halifax south terminal where overhead rail container transfer could operate.
Suitable European and Mediterranean ports would include Algeciras, Tangier, Hamburg (Euro-gate) and Barcelona. Generous overhead clearance is available at Asian ports such as Tanjung Pelepas, Pasir Panjang (Singapore), Shenzhen and Yantian (near Hong Kong).
Increasing eco-tourism could attract future clientele to travel aboard carbon-free tourist vessels. While 3-bladed turbines produce a sound wave every time a blade passes the tower, vertical-axis and transverse-axis Flettner rotors remain silent, enhancing their desirability on passenger vessels. While vertical-axis rotors would occupy deck space, a stern mounted transverse-axis rotor would leave most of the deck space to passengers and passenger services. A passenger vessel assisted by a transverse-axis wind rotor would likely be built using catamaran twin hull configuration, reducing roll in heavy seas and reducing roll-induced stresses imposed on the rotor support structure.
Increasing the physical size of the wind rotor increases its power output. While the largest stationary wind towers carry 3-bladed turbines rated at 12,000MW, it is unlikely that such technology would be developed for mobile operation generating power for ship propulsion. There may however be scope to develop a large-scale, transverse-axis wind rotor to be carried on a wide-base twin-hull or even triple hull catamaran to be coupled to the stern of a ship, with levers attached to the ship’s hull to reduce catamaran pitching. Maximum possible rotor diameter is presently unknown.
The transverse-axis wind time is merely an adaptation of the existing vertical-axis wind turbine (VAWT) tilted 90-degrees and installed on building roof edges. It is a well proven technology. Operating the wind rotor in transverse-axis mode offers the possibility of building a very rugged and robust support structure capable of carrying the turbine above the stern in very severe wind conditions. Ships equipped with such rotor turbines would sail into trade winds that incur minimal variation in direction, using the wind rotor to generate energy that would reduce ship fuel oil consumption.